Plug chop

Clutch in - decompressor on!

Could somebody explain what a plug chop is and how it would check carburation? Sounds like something I should know.. Much appreciated.

A 'plug chop' is often used to check whether the carburettor jetting (normally the main jet) is correct for the engine. As you are probably aware, the colour of the spark plug 'firing-end' gives a good indication as to whether the fuel/air mix is correct - the problem is that the colour will primarily show the mixture ratio when the engine was last turned off i.e. usually when after idling for a few seconds. To check a main jet ratio the engine has to be ridden with the throttle virtually fully open (i.e. when the carb main jet is in-use) and then the engine has to be 'killed' at that point i.e. without slowing down to bring other carb jets into the equation - this is usually achieved by simply turning off the ignition hence John Hawden's question above. It should be remembered that performing this operation is a little risky as there's no engine braking and it's recommended that the plug-chop is performed on a quiet (preferably straight) road.

HtH

I was about to explain my understanding of it, John, but fund this much more precise explanation. Hope it helps.

1. Get new spark plug(s), but don't put them in yet. 2. Warm up motor to operating temp on old plugs. 3. Install new plugs. 4. Accelerate through all gears to top gear with throttle pinned. 5. When you hit the top of the top gear, hit the kill switch and pull in the clutch. 6. Remove the spark plug(s) and ride home on the old plug(s). 7. Hold the new plug in a vise and use a hacksaw to cut the threads away from the center electrode. 8. You should see a brown 2mm "smoke ring" at the base of the white center electrode if the main jets are dialed in perfect. 9. if the smoke ring is darker than chocolate brown or taller than 2mm, you're rich on the mains, but if it revs clean to full throttle, you can leave it there and be on the rich side. 10. If the smoke ring is smaller than 2mm or lighter than chocolate brown, you're lean on the mains, go up a size and try again.

George

OK, the idea of a plug chop is to check the carburation settings, in particular the main jet. Set the carb up according to the manual, satisfy yourself that all other settings, valve and ignition timing, valve clearances etc are all spot-on, and give it a good run. With the engine thoroughly warmed up, head out to your favourite long straight stretch of road, preferably one which has a slight incline, in order to ensure the engine is working hard. Downhill will not do. Wind it up flat stick in top gear, hold it there; hold it.... when top speed has peaked, whip in the clutch, shut the throttle and kill the spark; preferably; or pull in the valve-lifter. Coast to a halt, check the plug colour, which will tell you what the main jet is doing.

A tweak to that process, if your straight stretch is long enough, is when it will not go any faster, roll the throttle off slightly. If the speed remains the same, or even increases, then it is running rich. It should begin to slow down of course. Then, back up to top speed, throttle on the stop, and contInue as above.

Needless to say, only do this where and when it is safe to do so, obeying all speed limits etc.....

Ian

PS. It seems that three of us were typing an answer at once! I am sure something can be gleaned from each.

I'm not sure I'd want to whang my 63 year old bike up to top speed at her age! Could not the same effect be had by topping out in say 2nd or third? Surely it's a function of revs/load rather than speed?

You are not loading it up sufficiently in the intermediate gears, that is why top gear with a slight incline is best. To take your thinking on, why then bother to ride it? Just rev it up on it's stand.... It would not give the same effect, George. The faster it goes, the greater the effect of wind resistance, the greater the load.

TBH, your 63 year old bike would be no less happy flat in top, as in 2nd or 3rd. The only things that are turning faster are the wheel bearings and tyres, and I am sure they are not 63 years old! Unless you are worried about running out of petrol of course?

All great, in theory, but With the mag cut out under the dual seat (?54 Dominator), and no de-compressor, how do you hit the "Kill button" and de- clutch at the same time? I guess you could disconnect the horn, and wire the cut out to the horn button, does not seem worth the effort. Use the recommended plugs, adjust the carb, as per instructions, and there should be no need for a "Plug chop". Then, what would I know, I have only been riding since 1953.

Wire it to a kill button on the handlebars if you wish John, but if you don't want to do it, don't do it; simples.

You are unlikely to be far off if you have the book main jet. Much more likely is that the needle jet might need adjusting - these things wear. To check the needle jet, run it at 1/2 to 3/4 throttle in top as above and switch the petrol off. If it goes better just before running out, you are running rich and your needle needs to be lowered a notch. To check for running lean, as above but instead of switching off the fuel, shut the choke a bit. If it goes better, raise the needle a notch.

My thoughts on plug chops is that it was a way of checking fuelling when an engine has been tuned, or modified in some way, when original settings are inappropriate. Having said that, petrol has changed considerably since the original settings were determined, so if one does encounter an issue with fuelling, it is a valid way of checking it; cheaper than hiring the services of a dynamometer.