Engine missing/good spark

Fri Apr 26, 2013 3:36 am

Here's the rundown.....traded for the cub & it had a slight miss, kind of what I'd call a tick. Nothing steady and didn't affect RPM's or power. Put seafoam and Marvel mystery oil in the gas, ran it for 3 minutes and the miss went away and it ran smooth. About 1/2 way through the tank, the miss, or tick, returned. Annoying, but no problem.
Now the engine is missing badly and won't get up to full RPM's. Had a set of new plugs for another Cub I've got to go pick up so I changed the plugs. Same thing. Got out the voltmeter and started checking spark. Everything was ok. Went to pull the boot back on #2 spark plug and the plug wire pulled off by accident, but lo and behold, no change in the engine. Ran exactly the same. Replaced the plug, no change. Pulled each of the other 3, and the engine ran worseout after pulling each of the other 3. Pulled the #2 plug and it had a light oily film, but didn't appear to have been combusting. Ground the plug and turned over the engine and it had good spark. Is this a stuck valve or what? Please let it be something simple! Lol

Re: Engine missing/good spark

Fri Apr 26, 2013 3:41 am

Forgot to mention that I blew put the carb also. Didn't help.

Re: Engine missing/good spark

Fri Apr 26, 2013 4:12 am

Just laying here trying to go to sleep but can't. Lol. All I can figure is intake leak nit letting fuel into the cylinder since the tractor is running and turning over. The only other alternative I can think of is....well I don't want to think about it. Lol. Take your best guesses and I'll pull the manifold and valve cover this weekend.

Re: Engine missing/good spark

Fri Apr 26, 2013 5:04 am

dbboss wrote:I'll pull the manifold and valve cover this weekend.
Just the valve cover. Check and adjust the valve tappet clearance. This will also let you see if a valve is sticking.

Compression tests to confirm engine condition. Check and test for mechanical problem(s) first.

Could also be an ignition fault such as bad plug wire, spark plug, internal distributor problem.

Re: Engine missing/good spark

Fri Apr 26, 2013 5:05 am

Do a quick compression check on cylinder 2. Here are some partial guesses, don't know that I have the whole answer. SOmetimes that "tick" is a buildup of carbon on top of a piston, actually hitting the head. The Seafoam loosened it, and tick went away. Loosened some more, and a chunk of carbon now has a valve stuck open. The compression test will help determine if valve is burned or stuck open, as well as visual check when you take off the valve cover. There are other possible causes as well, I'm just going out on a limb here.

Re: Engine missing/good spark

Fri Apr 26, 2013 2:30 pm

Will an intake or exhaust valve being stuckback cause the engine to act exactly like that cylinder isn't getting fire even though it is? I checked the electrical and it is ok. Strong spark everywhere. It is acting like only the #2 cylinder isn't getting fuel.

Re: Engine missing/good spark

Fri Apr 26, 2013 2:59 pm

Yes. Try the previous suggestions in diagnosing the problem. Could be a number of causes for the problem.

Re: Engine missing/good spark

Fri Apr 26, 2013 3:01 pm

10-4. Hopefully the rain will hold off some this weekend and I can get some work done to it. It'll probably be next week before I can do a compression test though.

Re: Engine missing/good spark

Sat Apr 27, 2013 9:13 pm

Must be a valve sticking. Noticed it ran fine for the first 3-4 seconds after starting it. Filled the tank again, more Seafoam, more Marvel Mystery oil and after running at full throttle for 5-6 minutes and it leveled out and ran a lot better. Now only has the occasional tick(miss) that it had before. I'm guessing that is going to be the valve still trying to stick some. I'll pull the head, clean, lap & adjust the valves after I get #2 up and running. Just got it unstuck today, but loose rust and trash in the intakes plus one valve sticking. Got valve cover, manifold and carb off now. Gonna pull the head next week .

Re: Engine missing/good spark

Tue Apr 30, 2013 10:01 am

Pulling the head might be helpful, but before that why not investigate the valve lash itself -- open the valve cover gasket behind the manifold, adjust as required, spray some penetrating oil on the springs, keepers and the valve stems. Turn the engine by hand with the crank to see which valves may be sticking. If they don't move, then the head has to come off. If they move and free up, adjust, keep using SeaFoam and see what happens. May save the cost of a gasket... just a thought :idea: for what ever that is worth.