Problems with compression test.

[Joed756]

Why did you do this compression test in the first place?
I wouldn't worry too much, if each of your four cylinders is reading about the same you most likely have a bad gage even though you replaced it. You go to any well known auto supply store and borrow a compression gage or better yet, your engine is running on all four cylinders, don't go looking for problems you don't have.

 

[cdestuck]

Why did you do this compression test in the first place?
I wouldn't worry too much, if each of your four cylinders is reading about the same you most likely have a bad gage even though you replaced it. You go to any well known auto supply store and borrow a compression gage or better yet, your engine is running on all four cylinders, don't go looking for problems you don't have.

I have come to that conclusion. I just have to figure out why my number two cylinder is not firing. I have spark, and I had all four carbs off to rebuild them and was relatively sure they were all OK. But this week I’m gonna pull them back off and open up that one just to go over it. I’m sort of baffled over this whole deal.

 

[Joed756]

When did number 2 stop firing? It has spark, so the issue has to be fuel. Why would you take all the carbs off if only one cylinder isn't firing? I think you should check valve gap on number 2 before anything else.

 

[RevB]

When did number 2 stop firing? It has spark, so the issue has to be fuel. Why would you take all the carbs off if only one cylinder isn't firing? I think you should check valve gap on number 2 before anything else.

Why if they're all 65psi?

 

[VRR.DYNDNS>BIZ]

I've always been taught that you should have a fully charged battery, all plugs out of the engine and wide open throttle.
That method has never failed me.

Lets get to the basic reason of plugs out and battery charged. rotational rpm increase wil increase the reading significantly. A quick test we perform on recoil start engines is a slow pull and at tdc if little resistance, the rings and cylinder or valves are leaking a bit to maybe to much. Adding a few drops of oil then will confirm rings as the source if an increase of at least 10%. What I suggest is plugs out and you will see a rise in the reading. This is all due to the dynamics of leakage does exist and at what level over time which is the rate of revolution.

 

[Hammermechanicman]

I have come to that conclusion. I just have to figure out why my number two cylinder is not firing. I have spark, and I had all four carbs off to rebuild them and was relatively sure they were all OK. But this week I’m gonna pull them back off and open up that one just to go over it. I’m sort of baffled over this whole deal.

If you had the carbs off and apart have you synced the carbs?

 

[Red Good]

Motorcycle mechanic here and you must have all the plugs out and stick them in the plug caps and lay them on the head to not fry the ignition then with the throttle wide open spin it over till the needle stops bouncing . record and go to the next hole . Best of luck

 

[TobyU]

Put the comp tester back into the toolbox.
It's just not needed.
I have fixed 1000s of mower engines and have never screwed one into one.
Well, not at the first time when I realized they are not like cars.
The only tester you need is your finger
If it blows it out nicely, it has enough compression to run

 

[Red Good]

is the compression tester a rubber tipped one or a screw in one ? Rubber tipped ones are hard to get accurate readings off of with all the other things you are doing . Make sure you have spark on #2 by testing it by itself . The fact that it runs makes me believe it does have sprk and it is a carb related problem . I would use the screw on the bottom of the bowl and drain that carb with the petcock shut off , when done draining turn the petcock on and let some fuel drain again and then shut the screw off and see what it runs like then . Those 650 4's were little rippers when new .Good luck .

 

[jviews12]

I also doubt you have 65 lbs on all 4. Sounds suspecious.