Engine miss

Tue Nov 05, 2013 11:32 am

I have a miss that I cannot fix. I installed new plugs (gapped .023), new wires, new rotor, new cap, and timed it to TDC. What could be causing a miss? It starts right up and has good throttle response.

Re: Engine miss

Tue Nov 05, 2013 11:37 am

Valves? Check the valve adjustment.
Ed

Re: Engine miss

Tue Nov 05, 2013 11:56 am

did you check compression on the engine?

Re: Engine miss

Tue Nov 05, 2013 1:00 pm

I'm not sure I have the skills required to check the valves. Yes, I checked the compression. 1, 2, & 4 were all good. 3 was a little low. I think 3 was around 80. This cub had been sitting for a long time. It seems like the more I run it the better it runs. I'm wondering if the miss will work itself out over time too.

Re: Engine miss

Tue Nov 05, 2013 1:02 pm

When does engine miss occur? Constant or intermittent miss?

Probably an ignition fault, could also be a sticking valve.

Ignition check. Need timing light and a piece of chalk. Mark pulley at some place about 180 degrees opposite TDC mark.

Looking for missing, intermittent or moving flash from timing light.

Flash for cylinders 1 & 4 will occur at or near the timing marks on the pulley. TDC mark at low idle.

Flash for cylinders 2 &3 will occur some where close to your chalk mark on the pulley. Doesn't have to be close to the mark, you are checking for the fault.

Vacuum gauge test will tell you a lot about engine condition and possible fault location.

Edit: You have the skills to adjust the valves.

Re: Engine miss

Tue Nov 05, 2013 4:02 pm

Eugene,
It misses at all ranges of engine speed and it's intermittent. I had a timing light on it last night and it seemed like the light would miss a flash every now and then. I just thought I was blinking or something so didn't think too much about it. I will try the chalk and timing light tonight.

What can I do with a vacuum gauge?

Another question, off topic, when the engine is at approximately 1700 - 1800 rpms should it advance to the 16 degree mark?

Re: Engine miss

Tue Nov 05, 2013 5:28 pm

The problem is with the ignition system, some place
Jason_Coffey wrote:It misses at all ranges of engine speed and it's intermittent. I had a timing light on it last night and it seemed like the light would miss a flash every now and then.
Determine if the miss is isolated to one cylinder or more than one cylinder, or all cylinders and which ones?

What can I do with a vacuum gauge?
You probably won't need one to diagnose and solve this problem. However, a vacuum gauge is an excellent diagnostic tool.

Another question, off topic, when the engine is at approximately 1700 - 1800 rpms should it advance to the 16 degree mark?
Yes, at or close to the 16 degree mark, the spark should advance as the engine speed increases. Since you are asking the question. Under the rotor, center of the post is a wick. This wick is supposed to be lubricated.

If the advance is stuck, remove distributor from tractor. Sit distributor upright in vise. Add solvent to the center of the rotor post. Let free up, then lubricate. Fairly common problem.

When was the last time you changed the points and condenser?

Nother edit. I don't know where the problem is located. One problem could be that the distributor is not timed to the engine. Ignition firing at the right time, but spark going to wrong spark plug.

Re: Engine miss

Tue Nov 05, 2013 6:10 pm

Check your battery connections. I had symptoms that indicated a failing coil or condenser, but it turned out to be corrosion on the post and inside of the negative cable connection at the battery.

Re: Engine miss

Tue Nov 05, 2013 6:58 pm

And another thought. Mark distributor base directly under the #1 plug tower and the #4 and #2 plug tower on the distributor cap.

Roll engine over by hand until the rotor points to the #1 mark on the distributor base. Match up the pointer to TDC mark on the pulley. Rotor should be pointing directly at #1 mark on distributor base. Back engine up until the pointer matches the advance mark. Again the rotor should be close to pointing at the #1 mark.

If rotor tip comes close to either the #4 or #2 mark, distributor is not timed to engine. Cross firing.

Re: Engine miss

Tue Nov 05, 2013 10:09 pm

Eugene wrote:The problem is with the ignition system, some place
Jason_Coffey wrote:It misses at all ranges of engine speed and it's intermittent. I had a timing light on it last night and it seemed like the light would miss a flash every now and then.
Determine if the miss is isolated to one cylinder or more than one cylinder, or all cylinders and which ones?

What can I do with a vacuum gauge?
You probably won't need one to diagnose and solve this problem. However, a vacuum gauge is an excellent diagnostic tool.

Another question, off topic, when the engine is at approximately 1700 - 1800 rpms should it advance to the 16 degree mark?
Yes, at or close to the 16 degree mark, the spark should advance as the engine speed increases. Since you are asking the question. Under the rotor, center of the post is a wick. This wick is supposed to be lubricated.

If the advance is stuck, remove distributor from tractor. Sit distributor upright in vise. Add solvent to the center of the rotor post. Let free up, then lubricate. Fairly common problem.

When was the last time you changed the points and condenser?

Nother edit. I don't know where the problem is located. One problem could be that the distributor is not timed to the engine. Ignition firing at the right time, but spark going to wrong spark plug.


I timed the engine with a light. It is dead on TDC. It does not, however, advance to 16 degrees at full throttle. It might advance 1/4" past TDC. I didn't get a chance to mess with it tonight. I will check it out tomorrow with a timing light as you suggested earlier. I don't have points or condenser anymore. I installed the petronix kit. Everything is new. Battery, coil, plugs, wires, cap, and rotor. I recently converted it to 12v.

Re: Engine miss

Tue Nov 05, 2013 10:33 pm

I put the light on it and checked all four cylinders. The only thing I noticed was the rate the light flashed on #3 would flash so fast at times it was almost a solid light for 1-2 seconds then it would go back to a normal rate of flash.

Re: Engine miss

Tue Nov 05, 2013 10:38 pm

I may have missed it, getting old and decrepit, but did not see where you checked the points. check them for being burned, or for the gap not being right, also for the distributor shaft having sideways play.

Re: Engine miss

Tue Nov 05, 2013 10:58 pm

Don't have points John. Converted to petronix.

Re: Engine miss

Tue Nov 05, 2013 11:01 pm

Jason_Coffey wrote: It does not, however, advance to 16 degrees at full throttle. It might advance 1/4" past TDC.
I installed the petronix kit. Everything is new. Battery, coil, plugs, wires, cap, and rotor. I recently converted it to 12v.
I put the light on it and checked all four cylinders. The only thing I noticed was the rate the light flashed on #3 would flash so fast at times it was almost a solid light for 1-2 seconds then it would go back to a normal rate of flash.
Pull the distributor. Check mechanical advance and condition - repair. Then reinstall the points and condenser - see if the condition persists.

Re: Engine miss

Wed Nov 06, 2013 7:28 am

Check the inside of the distributor cap for a carbon short from one tower to another.