Complicated carb problem

rezae80December 26, 2013

I bought a JD 285 tractor and was told carb is flooding an needs a kit.

When I first blew in fuel inlet while float was closed, I found out it was passing air so I ordered a kit and replaced the nozzle. Now it holds ht blow pressure.

When tested on the machine, I found that I still have the flooding issue.

This time I tried to pressure water in the inlet when empty and something strange occurred. As soon as the water was getting in the fuel inlet it started coming up the nozzle and pilot channel causing the flooding.

The only thing that makes sense to me is that there should be a hole or something at the top of reservoir to allow air come our while the reservoir is filling. It seems like this hole is clogged and the pressure is pushing the water up the fuel nozzle as soon as the reservoir starts to fill. To check this theory, if I pre-fill the reservoir and then try to pressure water in, it stops the fuel since the flow blocks it as expected, but if the bowl is empty and I start to let the water in, it starts to pour inside the carb.

When I opened the bowl and looked above the float, I can see a hole which makes sense to be a hole that depressurizes the bowl while filling up, but when looked inside it does not make sense to be connected outside anywhere.

Any of you guys have any experience with it? Is there supposed to be something like a capillary hole to allow air out while fuel is filling up the bowl?

Any other thoughts?


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You seem to have talked yourself into some wrong conclusions about the operation of the carb.
Yes, most carbs have a vent feature for the bowl, but this vent is not simply to facilitate filling of the bowl by venting out the pressure head of air above the rising fuel when the bowl fills.
The vent is also there to assure that the carb float bowl has only atmosheric pressure above the fuel level in the carb.
Fuel is "drawn" out of the carb and into the engine when venturi vacuum (negative pressure) is developed in the intake throat of the carb. The carb bowl vent permits atmospheric pressure to push on the fuel, causing it to flow out of the bowl and into carb throat when the throttle plate of the carb opens.
The vent also allows entry of air that may be used in some carburetors to begin mixing air with fuel as an emulsion before entering the carb throat, this emulsion of air and fuel helps the fuel to atomize better before being whisked into the combustion chamber.
Does this carb have a metal float or a foam composition float?
If it has the old school metal float, the float may be perforated and filling with gasoline......causing the float to sink and thus opening the needle valve.
It may also be that the float height is not properly adjusted, preventing the needle valve from pressing tightly enough against its seat to effectively stop the fuel from entering the bowl when it is full.

    Bookmark   December 27, 2013 at 1:14AM
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Thanks for reply mownie.

You are correct on the negative pressure, in my case though when the bowl is empty and fuel starts to fill it up, it does not even get to the float level and starts to come up the nozzle as soon as it reaches the holes on the nozzle. That is why I suspected the vent was the issue. Anyhow during cleaning I found the vent was clogged and cleaned it, tested the carb with water and reinstalled onto to machine today. Surprisingly it started to run but o boy gas was spraying out of exhaust making the wall beside it dripping wet!!! The engine didn't shut off or lose power, just spraying gas while running. I got kind of scared and shut if off in few seconds. Restarted it like this for couple of times again and saw the same thing so shut if off.

Took the carb out and took out the bowl, did not see too much gas. It seemed like the needle valve was doing its job and shutting off the gas, so I readjusted the float to lower level and restarted, same thing.

My tractor does not have manual fuel valve so I cannot reduce the flow, but is there anything else that is responsible for this?

From my understanding the bowl gets filled to a certain level and that determines the pressure to which the negative flow will suck the fuel.

Any suggestions?

To answer your questions, the float is not metal and I did the sink test, no issue. The float is supposed to be sitting level (horizontal) when flipped up side down which is what I did but not changed to lower to reduce fuel level due to spraying.


    Bookmark   December 27, 2013 at 3:58PM
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You bought the tractor used. Someone else may have damaged the carburetor in some way that seems to be causing fuel to get into the main fuel nozzle without being metered by the main jet.
If you are certain that the needle valve is seating tightly enough that fuel is not overfilling the bowl, then there is certainly something that is allowing fuel to pass freely in large volume (relatively speaking) into the fuel delivery main nozzle.
You should not be applying any great amount of pressure to the fuel inlet. You really should only be applying air pressure by using your mouth. To leak check a needle valve it is best to turn the carb upside down with bowl removed and very lightly press the float to seat the needle.
Spray or bush a mixture of dish detergent and water all around the needle and seat.
Blow into the hose attached to the inlet nipple and watch for bubbles to appear. That is all the pressure you should apply.
I don't know how you are applying water pressure but too much pressure of air or anything is apt to unseat the needle because at high pressures the needle simply behaves like a piston or servo and unseats itself.
Needle valves in carbs are only intended to resist something around 2 to 5 PSI.

    Bookmark   December 28, 2013 at 12:37AM
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Yes I bought it used and very possible to have damages already.

I have done both tests you mentioned:
1- Held the carb up side down and pushed slowly on the float and did the bubble test, no bubbles.

2- Put the bowl back on and held the fuel inlet pipe under falling water from the tap and pushed slightly in, held for a while. No water came up the nozzle but when opened up the bowl found water in it to the level of the flow which meant water was stopped.

I am also doubting that fuel might be getting into the main nozzle in larger volume, need to find how to test it.. perhaps I could use a blower into the main air flow path and see the fuel injection.

But on another point, I have changed the pilot screw position and not sure it could cause this much fuel flow. I might need to tighten up the screw pretty much all the way and watch what would happen!
Other than the pilot screw, is there usually some other mechanism to meter or restrict the fuel flow in the main nozzle? Shouldn't there be a screw to reduce fuel into the main nozzle?

Thanks for your time again

    Bookmark   December 28, 2013 at 2:02AM
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The tests you have done to determine whether the needle is seating effectively, and that the fuel level is reaching the correct height in the bowl........are only one facet of having the stage set for proper carburetor function. All the things you see during those tests are being produced under STATIC conditions (static, so far as air flow through the carburetor throat).
Most currently manufactured carburetors do not feature ANY means of "fine tuning" the metering of fuel through the main jet.
Old school carburetors utilized an adjusting screw on the main jet to permit optimizing the metering of fuel through main jet. Federal EPA rules have eliminated that item and so the fuel flow is determined arbitrarily by the orifice diameter of the main jet.
If a defect or damage is present that allows fuel to somehow bypass the main jet to enter the main fuel nozzle, proper metering has been defeated and the carburetor can easily "overfuel" the engine.
As to the "pilot screw".....the pilot screw affects only the amount of venturi vacuum that is applied to the PILOT jet fuel passages. Once again, the volume of fuel provided for idle RPM is arbitrarily limited by the pilot fuel jet orifice diameter, but EPA does permit a pilot air screw (aka idle air screw) so that idle RPM mixture can be fine tuned. The pilot air JET sets an arbitrary MAXIMUM volume of venturi vacuum that may be applied to the pilot fuel circuit, the pilot air SCREW allows for fine tuning the air volume lower than the maximum volume.

So, if your needle is seating effectively, and the float height is correct, and the main jet size is correct, that fairly well indicates fuel is able to find its way to the main fuel emulsion nozzle by an additional path and not just through the metering orifice in the main fuel jet.

You have not stated that you have had any problems that are usually associated with leaky needle valves. Leaking needle valves typically result in a gravity fed fuel system EMPTYING the fuel tank while the engine is off during storage. Our usual complaints in those events are that the engine combustion chamber is full of gas and that gas has also gotten into the engine crankcase lube oil.
So I am presuming you have not had any of those troubles.
All your woes are while the engine is actually trying to run, is this correct?

    Bookmark   December 28, 2013 at 12:15PM
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Yes, pay close attention to the pilot jet- which is on top of the carb. It should be removed and confirmed that the tiny hole is open and then resecured. It is not adjustable. The idle air screw at the rear of the carb , which is adjustable -won't create your flooding effect. You may want to verify the nozzle assy (out of the stem). In many cases, they can't be assembled the wrong way, but on occasion either end of the large nozzle will fit into the venturi and the jet will fit in. In either event, you guys are way to technical for me before my morning joe... :)

    Bookmark   December 28, 2013 at 12:41PM
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The weather is nicer today I'll be playing around to see if any improvements.

Other than oil contamination with gas, is there any other danger to let it spray gas from exhaust while testing it? like fire or anything else?

Also, is it safe to remove the air filter while testing or it is not worth letting unfiltered air to get in the engine even for testing?


    Bookmark   December 28, 2013 at 12:51PM
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Correct mownie, I started having the problem of not starting since the needle valve was worn and gas was dripping from carb. After replacing it and opening the vent, it runs now but spray gas from exhaust while running.

    Bookmark   December 28, 2013 at 12:54PM
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"Other than oil contamination with gas, is there any other danger to let it spray gas from exhaust while testing it? like fire or anything else"

Fire is pretty dangerous but the anything else would be an explosion which is at least one notch higher on the dangerous scale.

    Bookmark   December 28, 2013 at 1:05PM
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Really? darn it, but there is no gas leakage from anywhere within the engine or carb, only exhaust is spraying it. Maybe I should drain the duel out of gas tank and put little bit at a time only. makes sense?

    Bookmark   December 28, 2013 at 1:10PM
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"...only exhaust is spraying it"

Atomized gasoline is a big bang waiting to happen. Way more dangerous than a gasoline puddle on the floor and that is dangerous enough as the gas evaporates.

With respect, if you don't understand fundamental safety practices when working around flammable liquids and explosive gases then get your tractor to someone who does.

Regarding your carb there's an old joke... the word carburetor comes from the Latin root carbueratum meaning "not to be messed with" although the way it was told to me 60 or so years ago messed wasn't the word used if you get my drift..

Carbs of all levels of complexity either make perfect sense to some or are like string theory to others. While you seem comfortable trying to diagnose your carb problem it is not that complicated and we can't see what you've got or exactly what you're doing. Unless you can move it closer to the monitor so we can see it we can't give you the detailed help you need.

I can tell you this... almost every time a I've seen a DIYer suspect a float level problem they end up adjusting the float so the carb won't work.

In the pilot circuit the pilot jet meters the fuel while the pilot air screw meters the air.. In the main circuit the main jet and nozzle meter the fuel while the throttle plate or carb slide (depending on carb style) meter the air. The float simply maintains a gross level of fuel in the bowl

Until you know that every air and fuel pathway is clean and clear and all the nozzles and jets are clean and clear stay away from the float unless it is obviously damaged like full of gas or bent o hell.

    Bookmark   December 28, 2013 at 1:41PM
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I believe this is a water cooled engine?
See link below-
IF so, I suspect it is spraying water out the exhaust.
If gasoline was spraying, I'd expect it would ignite at least some of the time and give you a fireball.

With it running, hold a piece of cardboard/metal plate or similar to "condense" some of the moisture on it. As soon as you have some, smell it. Does it SMELL like RAW gasoline?

Here is a link that might be useful: Link below

This post was edited by bill_kapaun on Sat, Dec 28, 13 at 14:48

    Bookmark   December 28, 2013 at 2:43PM
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Thanks for the advice justalurker,

I would not say I understand everything about the carb as I have been reading up on it for a week only and this is not even comparable to people spending years experiencing with it.

I did most of my tests at home with water and blowing only to see how the theory ties up with it. I could atomize water with two straws and a bowl of water. There is also this bug in me to find out the problem.

I will try to take some photos or even videos if I can and post it here. You guys are really great it has been a fantastic learning curve so far.

To try to simplify the problem, here is what is going on in my head: Assuming the needle valve is closing with the float properly, the pressure with which gas comes up should depend on these factors in my mind:
1- Level of gas in the bowl
2- Speed of air passing thru
3- atmospheric pressure of outside (since vent is fully open)

Assuming cylinders are vacuuming the air properly, I will ignore option 2

I am thinking mostly about option 1 which depends on the float adjustment. While testing outside today, I did lower the float level even though recommendation was much higher (it should be level when holding up side down and needle valve just get closed). I also drained the gasoline and poured about 1 litre only in the tank. But now my battery died. I need to get some booster cables and connect to car to get it going.

Regarding the question about water vs gas, I am pretty sure it is gas since it is spraying on a piece of metal beside it and it is sooo much that it starts dripping within a second!!! Yes IT IS THAT MUCH!!! Smells like raw gasoline.
Perhaps the reason I have not seen a fireball is I ran it for few seconds only and shut off when I saw gas dripping.

Another thing I am doing which might be a cause is that I have disconnected the governor and choke. I do my live tests with choke closed and throttle open manually.

Again thanks for all the input. I could always go and buy a new carb for $180 ish from JD Dealer but I still think this is not that complicated and should at least stop spraying gas if the bowl fuel level is set properly.

BTW, what pictures would help? I will take some of the carb opened up showing the float level...

    Bookmark   December 28, 2013 at 3:04PM
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Looking at the exploded view of this Mikuni carburetor, I see no less than 5 items, which if damaged or defective, might cause unmetered fuel flow to reach the carb throat.
1.....Main jet
2.....Main jet holder
3.....Main nozzle
4.....Main nozzle
5.....Carburetor casting bore where all the above items are stacked
A scratch or gouge or mismatch of the parts fit is all it takes to lay waste to the designed intent.

    Bookmark   December 28, 2013 at 3:28PM
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You are right mownie, the reason I did not mention them is when I took them out to clean them, did not find anything strange. I also blew in the main jet while blocking both main nozzle and hoke#6 in the attached photos, I would say very small leak when blew hard (could be since blockage was not that great), but nothing that shaky.

Anyhow I also attached some photos of the carb and marked them up with what I knew (seems like I need to upload one by one)

The float is currently set lower than where it should be to lower pressure. I have picked up jumper cables and will retest tomorrow.


    Bookmark   December 28, 2013 at 6:34PM
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photo 2

This post was edited by rezae80 on Sat, Dec 28, 13 at 23:31

    Bookmark   December 28, 2013 at 6:35PM
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photo 3

    Bookmark   December 28, 2013 at 6:36PM
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photo 4

    Bookmark   December 28, 2013 at 6:37PM
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Photo 5

    Bookmark   December 28, 2013 at 6:38PM
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Photo 6

    Bookmark   December 28, 2013 at 6:39PM
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#3 screw referenced as a vent, is not supposed to be there. AT least to my recollection.An internal vented carb on these typically will vent in the front of the venturi. You see 2 brass jets, then referenced a plain bore w/ #5 Pilot air jet?. That I believe is the vent. Externally vented carbs use a steel tube which is casted in the top when the carb is made. It is possible to switch #s 4 and 6 on that same picture and this can contribute to runability issues, but I doubt to an overflow issue. Someone has been having fun.....

    Bookmark   December 28, 2013 at 10:36PM
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Thanks for the insight tomplum,

Yes, I just took out the pilot air screw and blew through it, found that air is coming out from #4, so in real life, #4 is the pilot air jet. I took another look at the diagram (have service manual of the engine) and thinking again, #6 is the main air jet.

I updated the second pic, also uploading the carb page of the manual in next post.

Regarding the vent, I agree with you and #3 screw was not supposed to be removed. The reason I took it out was no other vent was working in the carb when I tested. If I put the screw #3 in and blow inside fuel inlet while blocking jets, no air gets out!!! The only unknown hole at this point is #5 that makes sense to be the vent, but when I looked inside there seems to be a ball at the end of it. Tried carb cleaner spray inside, no luck. Put a soft wire in, but it got bent. So either this hole is clogged, or not supposed to be open at all!!!

But regardless of this, could having the backup vent open cause fuel flooding to the extent of spraying out of exhaust???

Will do more live testing tomorrow :)

Thanks for all your help so far

    Bookmark   December 28, 2013 at 11:28PM
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Updated photo 2 and replaced, this edit post option is cool

This post was edited by rezae80 on Sat, Dec 28, 13 at 23:33

    Bookmark   December 28, 2013 at 11:29PM
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Remove the pilot fuel jet and inspect it and inspect the bore that it mounts into for signs of damage.
It has occurred to me that if something is amiss in how that pilot fuel jet fits and seals into its designated bore that maybe the pilot fuel delivery circuit could be where the extra fuel is coming from.
Also, rotate the carb 180 degrees and get a shot from the opposite side as suggested by the red arrow in my photo edit of your pic.

    Bookmark   December 29, 2013 at 12:40AM
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Hi mownie,

Based on the part catalog, there are 4 parts in below order as unscrewed:
1- Main jet
2- Main Jet Holder
3- Main Nozzle
4- Main Nozzle (Emulsion Tube)

I did remove the first 3 earlier, but after your note, I tried to remove the last one (believe this is the one sticking out of carb when looking into the main air flow, but realized the screw is stripped. Seems like someone had been trying to remove it and could put excessive pressure. Attached a photo here, is this the photo you were referring to?

Thanks for calling it out, not sure if this is the issue for overspraying, from looking into it with a light it seems like the grip for unscrewing it has been chipped off, if I tried to screw it in further, it would have grip. sorry I took like 20 photos to get this one. Might have to use an extraction set to take it out.

Do you think from the photo this could be a cause?


    Bookmark   December 29, 2013 at 1:26AM
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photo of carb components, it is item 9

    Bookmark   December 29, 2013 at 1:37AM
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The damage that I can see looking into the MAIN JET BORE would certainly cause the mating part (main nozzle) not to seal against it.
There also appears to be some damage to the threaded bore as well.
You misunderstood what I was wanting when I indicated I would like to see the carb from 180 degrees and you must also have missed that I identified the PILOT FUEL JET in the photo I borrowed from you. To clarify just a bit what I wanted to see, in the photo I used......I was wanting you to turn the carb so that side marked A faces away from the camera and side B faces toward the camera.
Basically I wanted to see if the cast tube at B is "open" or if it is just an unfinished casting segment.
And regarding my suggestions about the PILOT FUEL JET:
The pilot fuel jet is the item you labeled as 1 in your very first photo post.
The pilot fuel jet threads into the carb casting in such a way that it intersects the pilot fuel riser gallery that is cast into the carb body. I was hoping to learn if there was any sign of damage to the pilot fuel jet, or the bore and the seating surfaces where the jet must seal in order for proper metering to be done by the pilot fuel jet.
I forgot to address your concerns about the gallery that you correctly determined to be plugged with a steel ball.
This is a technique the carb OEM (Mikuni in this case) uses to permanently close off a drilled passage in the casting. This method is used in lots of manufacturing practices where there is a need to create complex circuits of intersecting holes drilled from different points of access. Once the holes have been drilled to size and the circuit is established, one or more of the drilled hole starting points will be drilled to a slightly larger diameter than the first drill size. Then the access holes are closed up using a hard steel ball pressed into the hole right up against the smaller drilled passage. Usually the OEM will lightly "stake" the edges of the ball plug hole to further prevent it from loosening or being tampered with.

    Bookmark   December 29, 2013 at 11:27AM
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See the photo below and answer this question based on your comments about the screw in the vent.
You say you took the screw out because with the screw in place....fuel flows straight from the main inlet to the main nozzle.

Does this happen with the engine stopped? Or only when the engine is running (trying to run)?

With the screw removed, does fuel run out of the screw hole when the engine is stopped?

    Bookmark   December 29, 2013 at 11:47AM
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I think you're beating a dead horse here. The upper part of the passage was compromised, the nozzle is apparently spun. Not all of these upper nozzle bodies thread in however, some will just push out from the top. If the 1 venturi passage is set, then this carb must rely on one of the passages w/ the brass jets as relief. I could peek in my deere manuals for thoughts on Monday. This carb series is generally a good carb, but if you have to disassemble the nozzle- one needs to use a well fitting screwdriver blade and snap it out like you mean it. Be thankful it is only a 150$ carb...

    Bookmark   December 29, 2013 at 12:42PM
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Hi mownie,

It only happens during cranking or if engine starts to run. If I leave the screw in and crank the engine, then remove the air intake, gas drips all over!!!

With screw removed, no fuel comes out of the hole with engine stopped or running. But engine CAN be started with screw removed, however gas is sprayed from exhaust.

Regarding your previous comments, thanks for clarification. Yes I missed the markup in the photo #1 regarding pilot fuel jet. And to reconfirm, the hole you marked as fuel riser gallery seems to be closed (I blew in it, nothing goes through). The reason I marked #1 as connected to main fuel jet was that if I blow inside the fuel jet and open screw #1, air comes out. Attached is the photo you mentioned. Hopefully got it right this time.

Hi tomplum, I would definitely buy a new carb if this one does not work at the end. I guess I am just trying to pin point the problem and if it is not repairable I will get a new one. Yup, the price is $170-$180 with tax and everything.

    Bookmark   December 29, 2013 at 1:40PM
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Drop AM123578 into an Ebay search and you can have a used one for $65 or a new one for $139 shipped to your door.

    Bookmark   December 29, 2013 at 1:58PM
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OK, I can see that the hole is definitely not an open vent passage (as was suggested by the Kaw manual I have been referring to).
But now that I look again at the Kaw manual, I see that the vent hose it refers to does not lead to the item I marked as B in the photo. Sorry for all that confusion.
It appears as though your carb does not in fact feature an external vent at all (as tom already pointed out earlier) nor has the casting been finished out with that aim as an option.
And the final rendition by Mikuni of your particular carb may not utilize the pilot fuel riser in the casting to source the fuel to the pilot fuel jet.
So I stand corrected on that as well.
Mikuni and others commonly use a rather generalized casting mold to produce "raw examples" of a carb, but it does not actually become a specific carburetor until they finish it out during the machining processes.

    Bookmark   December 29, 2013 at 2:27PM
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@ rezae80,

IF this was your tractor from day one and you knew that no hack had ever attempted to repair the carb, or if it ran right when you got it and then developed a problem, then your attempt to diagnose and correct this problem would be validated... cause we all like to understand what and why it happened.

BUT, since the tractor running badly when you got it and not knowing the service and repair history and having tried all/most of the sensible steps to diagnose the problem with no success IMO it is time to buy a new carb and move on to another project.

    Bookmark   December 29, 2013 at 4:02PM
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I guess you guys are right. No matter what I do it still keep spraying gas from exhaust. I checked the ebay ones but I live in Toronto and shipping/Import costs make it almost same price as dealer. Time to buy a new one, thanks for all your help, will post the results when I get it and replace, cheers

    Bookmark   December 29, 2013 at 4:23PM
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Just wanted to close any loose ends on this post. The new carb fixed the problem of gas leaking from the card, but the interesting thing was the fact that new carb came with no screw (the #3 in the photos).

The problem of gas coming out of exhaust still stayed, but it was also due to no spark in one of the cylinders which I rooted to ignition module. It is followed in a separate post.

Thanks all for support

    Bookmark   May 30, 2014 at 10:42AM
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