No compression, ACR issue???

T

thedesertvette

Member
I acquired a generator with a GX340 clone. Would not start without a shot of starting fluid but once running it would run strong, even under heavy load. Thoroughly cleaned the carb and still would not start without starting fluid. Checked the compression and it was below 10 PSI. Pulled the engine apart, lapped the valves, light honing of the cylinder and new rings. Still no compression so I bought a leak down tester and at TDC leak down was only 15 percent, same at BDC. The valves were adjusted correctly to .006 IN and .008 EX. I loosened the EX valve up to .030 and compression came up to 50 PSI and would start right up without the starting fluid. I moved the rocker off the EX valve and there was so much compression I could barely pull the rope. I did verify the flywheel keyway was ok and timing was good. To me, it seems like the ACR is holding the valve up too high/long and releasing all the compression. When I had the engine apart, the ACR looked ok, no damage and spring was ok. What am I missing? What else could be causing this issue? This one really has me stumped. Thanks
 
You cannot do a compression test on these little engines without a starter box. Cannot get the engine spinning fast enough with the rope. Other than that it sounds like you have a good mechanical understanding of the engines. Only thing that comes to mind is setting lash at the wrong point on the lobe but you seem to understand where to set lash.
 
racinnut, I'm not really trying to do a compression test, just trying to get it so it will start without starting fluid. Needs the starting fluid since the compression is so low. This engine only has the rope to start it. Once it starts, it runs great. I;m pretty sure it's the ACR bumping the EX valve too much, releasing the compression. Any idea what the lift/duration of the ACR should be on this GX340 clone.
 
Try a little more lash on the exhaust.

More lash limits how much the valve is held open by the compression release.

The pull rope will tell you if you have too much lash.

15 percent is a lot of leakdown.

Did you test with rockers removed from pushrods?

Can you hear where the air is leaking?
If from crankcase, rings. Of course exhaust and intake are from valves.

If you know where the leakage comes from, you can address the problem.
 
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To set lash accurately, use the method I prefer.

Turning the engine in the direction it runs.
When the exhaust starts to open, set the intake valve lash.
Continue turning in same direction.
When the intake opens and starts to close, set the exhaust lash.

If it starts and runs, go back and check lash with the method you usually use.


The method I gave makes sure the compression release does not affect lash settings. You are always checking on the base circle of the cam.
 
95 shaw, I did open the EX clearance, see original post. At .030 I had 50 psi compression and it starts fine. As far as the leak down, I had honed the cylinder and put new rings in, have not run it enough to seat the rings. I will try your method of setting the lash. Thanks.
 
thedesertvette said:
95 shaw, I did open the EX clearance, see original post. At .030 I had 50 psi compression and it starts fine. As far as the leak down, I had honed the cylinder and put new rings in, have not run it enough to seat the rings. I will try your method of setting the lash. Thanks.
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Thats more than a old flat tappet SBC cam would suggest. Not sure if it would beat itself to death with all that lash as i've not run across this situation. Is this one someone has tinkered with in the past? Has correct pushrods and rocker arms and lifters? As someone else asked did you clean the emulsion tube and low speed jet with a small gauge wire to get all the holes good?
 
Guys, First off, I'd like to thank everyone that contributed to this thread. Last night, my buddy was in need of a generator and I let him have this one. He didn't care about having to use starting fluid to start it, he just needed power. It was a nice generator and ran strong once started. Again, thanks for all the comments/suggestions.
 
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