plug chop

Ok, i understand the principles and how to do it, but if i do a plug chop and the plug shows being too rich or too lean, do i adjust the idle mixture, or do i have to change the main jet.

How do you know when you have the correct ratio at idle.

Also after plug chopping, can i just stick the same plug in, adjust the mixture and try again.

Re: plug chop

Firstly, plug chops are generally done at wide-open throttle.. at top speed. The main jet has no influence on idle mix. The carb has a separate idle system.

That being said, factory bikes are carefully set up to run very well.. Components are chosen with a balance of reasonable power and good reliability, low maintainance and long life in mind.

A plug chop on a stock bike will tell you if something is not right.. Perhaps you're using too much oil in the mix.. or your air filter is dirty .. or the points have worn and ignition timing is off.. Or the weather got hot and a cooler heat range plug should be used, etc, etc.

There's no reason to change main jets to correct some fault with air:fuel mixture on a stock bike. What would be called for is a thorough tuneup that will return the bike to as-new running condition.


Plug chops for high performance on a modified bike are a whole different animal. These chops are aimed at extracting maximum power by leaning the mix to the absolute minimum safe ratio.. or as a way to select or modify different major components.. Squeezing out some extra power reduces reliability and jeopardizes the engine in general.


It's just about pointless to do a plug chop with a used plug.

The idea is to get a single, accurate snapshot photo of exactly what is happening at this very moment under these immediate conditions. Reading an old plug is reading obsolete information. Anyway, there's nothing wrong with having 4 or 5 (practically) new spark plugs laying around..


You have the correct idle air:fuel ratio when the bike idles well.. it idles at a low rpm whether the engine is cold or hot, any time 24/7/365 ..

Again, all this is set up back at the factory. Idle jets are not adjustable or removable on the simpler carbs.

The few (if any) available idle adjustments are all you need IF the fuel system is clean and the bike is in perfect factory tune..

Re: plug chop (for joew)

Ok, what i'm trying to establish is the air:fuel mix at idle. How do i know when i have it set correctly, i followed your (joew) instructions on another thread which was (unscrewing the throttle stop screw til the engine is just about to die, then adjust the air mixture screw til rpm increases, continue doing this til no difference is noticible.

When i tried this i get no increase change in rpm adjusting the air mixture screw. Any idea why this would be. Have read elsewhere that ideal position is about 1 3/4 turns. (its a pa50)

Another question joew, read some of your post regarding timing on the pa50, i've been using a timing light to get some acurate readings. but i am having trouble getting a spark right on the fire line on the flywheel. can only get it within a few degrees. How accurate have you managed to timing?

Re: plug chop (for joew)

Air/fuel at idle is generally the optimum rpm obtained at idle when engine is warmed up totally. When you have top rpms, turn screw in a little `til you hear rpms drop slightly. Now it's correct. don-ohio

Re: plug chop (for joew)

I've done work on the engine and then readjusted idle on the Keihin carb many times.. the proceedure i use has never failed me. Only thing i can think of is maybe your carb could be cleaner. These butterfly carbs are extremely sensitive to the slightest bit of dirt or fuel varnish. The mixture screw should have a profound effect on idle.

1-3/4 turns?

Right now my mix-screw is set 1/2 (one half) turn out. This bike idles really well.. sometimes I come to a stop and cant hear the engine (but it' true, my ears are ringing), the idle is so smooth and low . I'm thinking the engine died or is about to die but it doesn't.


An inductive timing light works great to check it. When setting it i use a little battery powered stop lamp bulb with the engine off.. the lamp brightens (or does it dim?) just as the points open. The tiny screw and groove on the points that adjust them is just hard to manage.. a lot of care and patience is required.

If you can't get timing on the mark no matter what, it could just mean the point's body (where it contacts the cam) is worn down and can no longer reach the proper timing.

But if the trouble is just overshooting or undershooting the timing mark keep on trying .. move the points very slightly, tighten the screws, start it up and use the inductive light to check it... and do it again and again till it's right.

Re: plug chop (for joew)

i think the carb may need a good clean out, i turned it about five or six turns tonight and it made no difference.

adjusted the timing again and i'm getting closer, i'll persevere til i get it.

many thanks

Re: plug chop (for joew)

decided to give the carb a clean tonight, but had some trouble removing the main nozzle. Any tips, tried tapping against a rag, then a piece of timber, didn't want to go crazy, so i've left it. Tomorrow i need to find somewhere who will let me use their compressed air.

do any of the small tubes in the air intake side need to be removed or do they just need air blown throw them.

on the idle side again, am i correct in thinking that screwing the idle screw in is increasing the air in the mix.

also, managed to get that timing just about right, but is it normal to have a the spark fire just of every once in while. using a stobe light, and its bang on the fire line, except every 5 - 10 secs i might get it firing a little early, any ideas.

Re: plug chop (for joew)

On my bike the timing light shows a pretty steady spark, even at idle... the flywheel mark is slightly shakey but not more than a degree or two. Maybe the problem is the cheap-o timing light.

I'm not sure what would cause the strobe to show no flash. I think that even if the spark plug misfires the electrical energy is still passing through the plug wire and will be detected by the strobe's pickup coil.. But maybe the ignition itself is not producing energy (dirty points?)


The carb's main nozzle.. i guess you mean the brass emulsion tube above the main jet.. is held in by mild friction (and lots of dirt?).

You can start it on it's way out by pushing it's tip downwards .. the tip sticks up in the carb throat. Then it should come out with some coaxing.. (pound the carb into your fist, etc.) Perhaps an "L" shaped piece of thick wire can get in there and push it down even further.

If it refuses to fall out, compressed air into one of the air-horn brass tubes should blow it out. Otherwise maybe stick something into the main jet opening and wiggle the tube's end around a bit .. soak the area in carb cleaner.. pound and fiddle with it some more..

_do any of the small tubes in the air intake side need to be removed or do they just need air blown throw them_

no need to remove the short tubes.. probably a very bad idea to try it..

_on the idle side again, am i correct in thinking that screwing the idle screw in is increasing the air in the mix_

No.. its not that simple.

The "air" screw is actually an air/fuel screw. It allows more or less pre-emulsified fuel/air to combine with pure idle air.. this mix-of-mixture :) then enters the carb's bore through tiny idle holes near the throttle plate.

The idle mixture's emulsified fuel:air comes from the emulsion chamber above the main jet.. the pure idle air enters through one of the brass tubes in the air horn.

The mixture screw actually has nothing to do with idle rpm.. its just a fact that when the air:fuel mix is more correct more power is produced and idle rpm increases.

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