Running Rough

[ajhbike]

Suddenly the 57 is running rough and is putting out soot at the tail pipe. I put some sea foam in the gas and that doesn't help. Thoughts on primary places to start? I did a superficial check to make sure the coil wires were connected, pulled the air infeed (which sucks alot of air in), made sure the jet was cleaned. I pushed the kill switch to stop and the ammeter was pinned on the positive side. Regulator is new from Napa 3 months ago (polarized the generator).

[ntrenn]

Suddenly....
Soot.....
Dead cylinder...leaving unburned fuel to make soot.
I would start with new plugs. I like the 3116 autolites...never had one foul.
Then consider points and condenser.

[ajhbike]

Hood off to change plugs I guess

[LRiddle]

Hood off to change plugs I guess

You can do it with the hood on.

[staninlowerAL]

You can do it with the hood on.

Yeah, a box end wrench works better than a socket plug wrench.

[Urbish]

You can do it with the hood on.

Yeah, a box end wrench works better than a socket plug wrench.

I use an offset 12-point box end on all of my tractors. You can wear heavy gloves and pull the plug wires, one at a time, while it is running. The one that makes no difference is your culprit. I'd remove, inspect, clean, and gap that plug first. The condition of the plug may help indicate why it fouled. If you put new plugs in, you may be hiding an underlying cause and they'll foul too.

[ajhbike]

what size offset wrench...it has been 20 years since I pulled a plug

[Don McCombs]

7/8

[Jim Becker]

I usually use the box end of a 7/8 combination wrench. Use it from the carburetor side with the handle angled down. The fit on some is a little close if you have underslung exhaust.

[ajhbike]

okay, thanks, will check out the wires while running and check out the plugs. Yes, i have the underslung exhaust

[Eugene]

I pushed the kill switch to stop and the ammeter was pinned on the positive side. Regulator is new from Napa 3 months ago (polarized the generator).

Question. Was the amp meter pegged/maxed out after the engine kill switch was in the off position and engine completely stopped?

[ajhbike]

yes

[ajhbike]

Also and I must admit I never noticed before, but that air intake takes in alot of air. I actually heard it over the idle engine noise. I removed the cap and put my hand over it and it was about as strong as a house vacuum. Is it normal that the engine consumes that much air? Maybe it is normal and it was another learning moment.

[Urbish]

Also and I must admit I never noticed before, but that air intake takes in alot of air. I actually heard it over the idle engine noise. I removed the cap and put my hand over it and it was about as strong as a house vacuum. Is it normal that the engine consumes that much air? Maybe it is normal and it was another learning moment.

A Cub has a 60 cubic inch engine (.0347 cubic feet). The engine draws in roughly that much air in two full revolutions given that it's a four stroke engine. So, if a Cub is idling at 500 rpm, the equation for air consumption is 500 rev per minute * .0347 cubic feet / 2 revs = 8.7 cubic feet per minute. The units of cubic feet per minute are known as a volumetric flow rate. A typical consumer grade shop vac will pull 50 to 100 cubic feet per minute.

Note that volumetric flow rate is not the same as the amount of vacuum (in psi or mm of Hg) that the engine will pull. If you cover the intake, it will suck on your hand just as hard as a vacuum cleaner (or perhaps even harder), it just won't happen near as quickly. I'm thinking that an engine will pull even harder since it's a displacement pump whereas vacuum cleaners use a turbine fan. Once a certain level of vacuum is reached, a vacuum cleaner will cavitate and won't pull any harder. A perfect displacement pump will pull all the way down to a total vacuum: -14.7 psi (atmospheric pressure).

[ajhbike]

Thanks Jim, I didn't think about it but you are right...it is like a positive displacement pump so it will be much more than a vacuum....and since I still have a ring on the palm of my hand I must have been approaching 29mmHg....it certainly didn't cavitate. So, it looks like i don't have a compression issue!!!!