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Toro B&S 6.75 compression test

Posted by mvron 10 (My Page) on
Thu, Oct 10, 13 at 19:15

What would you say if I got 40 on my B&S 6.75 engine?


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RE: Toro B&S 6.75 compression test

  • Posted by baymee LehighValleyPA (My Page) on
    Thu, Oct 10, 13 at 20:21

It depends if it has an automatic compression release and if the throttle was wide open (probably was). A leak down test is more accurate and can pinpoint where the loss is.

Without ACR, 90# is usually needed to start.


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RE: Toro B&S 6.75 compression test

I'm going to get a better compression tester tool - threaded type and get another check. When I received this mower there was quite a bit of oil/grease on the deck right below the spark plug. That's concerning. I had another mower with similar problems earlier. No spark, no run etc. It turned out the head gasket was bad. Maybe this one also?


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RE: Toro B&S 6.75 compression test

  • Posted by baymee LehighValleyPA (My Page) on
    Fri, Oct 11, 13 at 13:52

I've never seen a bad head gasket on one of these. However, the head bolts come loose in the upper right quadrant. Put a half inch socket on all of them and moderately tighten them. Doubt if it will make a difference.


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RE: Toro B&S 6.75 compression test

Do you know what the torque setting is for those head bolts? This mower has been really trashed. I'll do that first and then wait for my new compression tester. Redue the compression test. If none of that works I might take it to my shop. The mechanic is really sharp with this stuff(like bayme) for a leak down test. Like I said there is no spark even when spraying starter fluid directly into the cylinder. It does seem to have some compression.


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RE: Toro B&S 6.75 compression test

  • Posted by baymee LehighValleyPA (My Page) on
    Sat, Oct 12, 13 at 12:51

It's possible to start an engine with low compression if you can make it spin fast enough. Is the plug dry? Spraying fluid down the cylinder is the best way to make the plug wet and then it won't start at all.

A very tiny squirt is all it needs through the carburetor, enough to fill a tire valve stem cap. If it's a dry plug, and with starter fluid, the next step to to look at the bowl nut or the flywheel key.


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RE: Toro B&S 6.75 compression test

I'll re-check the plug to see if it's dry. I'll also check the bowl nut and the flywheel key. What should I look for in the bowl nut? Be sure all the orfices/jets are clean? I'll report back - thanks again for ideas.


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RE: Toro B&S 6.75 compression test

  • Posted by baymee LehighValleyPA (My Page) on
    Sun, Oct 13, 13 at 20:47

Your jet has a small hole in the center. When you remove it, you gas tank will empty, so tip up the front of the mower while the nut is out or empty the tank first.

Try not to remove the bowl when you take the nut out and don't overtighten it or you can ruin the fiber washer beneath it.


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RE: Toro B&S 6.75 compression test

" Like I said there is no spark ....."

Maybe you should address that issue?


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RE: Toro B&S 6.75 compression test

I purchased a "real" compression tester. It read about 58 with the "start" handle pulled back. I did remove the float bowl nut - without removing the float - and cleaned both the jets. As I recall, with a low compression reading one can't expect a spark. Is the that correct? Haven't taken the mower apart to check the flywheel key yet. The spark plug seems reasonably dry. I haven't checked the obvious yet - the spark plug.


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RE: Toro B&S 6.75 compression test

If the flywheel key is broken would the mower blade still turn - because the blade is turning.


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RE: Toro B&S 6.75 compression test

  • Posted by baymee LehighValleyPA (My Page) on
    Wed, Oct 16, 13 at 15:23

Yes, the blade will turn unless the flywheel has broken free and lost contact with the crankshaft end. Compression has nothing to do with creating spark. It's easier to check for spark with the plug out. If you have fuel, compression and spark, your mower should start. Remove the nut on the flywheel and you should be able to see if the key is sheared. It only has to be off 1/16" and it won't run.

See this link:

http://forums2.gardenweb.com/forums/lmower/?11080


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RE: Toro B&S 6.75 compression test

Maybe you should address this issue ? Roflmbo Bill , Your mother sure didn.t raise no fools !


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RE: Toro B&S 6.75 compression test

After bringing in the spark plug - that said made in China on it - my mower guru told me that the motor was made by Tecumseh in China. Not his favorite. More fun ahead.


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RE: Toro B&S 6.75 compression test

You need to rule out spark issues (electrical) first. You should get spark regardless of compression if your magneto is good. Most people test for spark with the plug out of the cylinder, just grounded to the metal of the cylinder head. You can see a spark for every revolution of the flywheel if everything electrical is ok.


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