PLUG CHOP PROCEEDURE

Can anyone tell me what the proper procedure is for doing a plug chop test to read spark plugs.I've read about this many times and there seems to be a lot of verying opinion about the subject.I would like to know your opinions on how long to run the test.Some people say one pass is enough,others say up to 3miles,wich is dificult because you run out of rpm.So you would have to hit the kill switch ,back off the throtlle pull in the clutch ,slow down,then repete the test a few times.Unless there is a better way?Also some guys say you want new plugs each time,and others say you need to put some time on the plug first.I guess if you put enough miles on during your test it wouldnt matter,the plugs should recolor.:side:

No need to repeat the test, once will do it. No need to use 5th or 6th gear, it will work in second or third. Do not put in the new plugs and then ride to the test area, it kinda defeats the whole purpose.

This is the way I do it, and it is really the only way I ever read about how to do a plug chop.

Get new plugs and put them in your pocket. Ride out to your test area, which should be long enough to let you run about a half-mile. Let the engine cool off and remove the old plugs and install the new ones. Start it up and get right to it.

Full throttle in second or third gear is fine, as long as you are into the main jet. If you can go up a slight incline that's better than downhill. When you get about a half mile full throttle run, hit the kill switch, shut throttle, pull in clutch, all in one smooth act. Coast to a stop and pull out the plugs after the engine cools off. That's it.

I kinda hate to step up to bat here, because I haven't done this, but some things strike me as obvious.

The point is to run the engine for some period of time in the load range that exercises the jet you are testing, then immediately stop the engine to read the plug color. the engine must be stopped abruptly (the "chop") as operating the engine at any other RPM would bring other jets into play, and contaminate the test results. Only one jet at a time should be tested, so you can tell if that one jet requires adjustment. Since different jets are used a different RPM ranges, the RPM should be held constant during the test.

I think the plug should re-color, yes, if it is given that the plug is in the proper heat range so that it is self-cleaning properly, and the rings or valve seals aren't spitting oil at it.

How long does it take? I don't know, I've seen one mile listed a few times. Room for a voice of experience to speak here.

Full throttle in 2nd or 3rd for 1/2 a mile? I think my engine would be way past redline (and probably would have self-destructed) by the time I got to the end of the run.

OKC_Kent wrote:

No need to repeat the test, once will do it. No need to use 5th or 6th gear, it will work in second or third. Do not put in the new plugs and then ride to the test area, it kinda defeats the whole purpose.

This is the way I do it, and it is really the only way I ever read about how to do a plug chop.

Get new plugs and put them in your pocket. Ride out to your test area, which should be long enough to let you run about a half-mile. Let the engine cool off and remove the old plugs and install the new ones. Start it up and get right to it.

Full throttle in second or third gear is fine, as long as you are into the main jet. If you can go up a slight incline that's better than downhill. When you get about a half mile full throttle run, hit the kill switch, shut throttle, pull in clutch, all in one smooth act. Coast to a stop and pull out the plugs after the engine cools off. That's it.

What about doing the test while the bike is not on the move, I mean just put neutral and give gas while you hold a bier with your other hand? Now serious: would it be possible? This way one would not have to worry about traffic interfering.

antokan12 wrote:

What about doing the test while the bike is not on the move, I mean just put neutral and give gas while you hold a bier with your other hand? Now serious: would it be possible? This way one would not have to worry about traffic interfering.

I think you are just the candidate to try it. Let us know how it works out.

Well let me reach for an industrial fan to cool the engine down while trying! :whistle:

antokan12 wrote:

Well let me reach for an industrial fan to cool the engine down while trying! :whistle:

Since the test depends on throttle opening and not engine rpm, testing it with no engine load is rather pointless

That is a point! Although RPM and Throtle position are normally directly related, load makes a big difference to the test. I think this is interesting:
hmfengineering.com/forums/showthread.php?t=32
It Remind us how wheather can affect the procedure.
Hope it is helpful.

the engine must be under load.If you do this without load you could blow it up.I canot go a 1/2 mile in 2nd gear . If I go through all the gears it is only 1/4.I ran it through all the gears three times and I broke the key on the rotor. NOT GOOD. So i put in another one and tried again .Same thing spun the rotor again.????

I'm gonna play devils advocate here....

Thinking out loud, not gospel!

If bike is stock, it will run with stock jets if everything else is correct. Exception being that the pilot may be a tad on the small side to make emisisons compliance on 80's bikes.

If bike has a less restrictive intake, the pressure differential between the inlet and outlet of the carburetor is greater. This increases the mass flow rate which in turn increases the velocity in the venturi which LOWERS THE PRESSURE at the NEEDLE JET opening. Lower pressure in the jet opening and the same atmospheric pressure in the float bowl results in a greater pressure on the fuel and more flows.

If bike has a less restrictive exhaust the cylinder pressure will be lower when the intake valve opens. This further increases the pressure differential and speed through the venturi.

At first glance it would seem that the jetting need not change. After all the vacuum changes all the time anyway and we get more fuel flow with more vacuum. That's what the tapered jet needle is for!

The mass flow of air and fuel is pretty much proportional to their respective pressure differentials for a fixed opening size. BUT the pressure in the venturi is roughly proportional to inverse square of the carburetors inlet and outlet pressure differential(Bernoulli equation). So, the increase in pressure to push the fuel is not linear with the increase in pressure to push the air according to the physis as well. The fuel mass flow increases more slowly than the air mass flow!
This is WHY we need a bigger jet with pods and open exhaust.

Aside: A skinnier venturi needs a larger jet than a fat one to obtain the same mixture.

OK. so now we know what's happening in the carb. The math is way to complicated and there are too many other variables affecting the flow to calculate the required jet size.

So what to do?

1)You can't run a big bike at WOT for very long unless you're on a dyno. If on the dyno, you don't need to be reading plugs.

So for us it's iterative work.

Know what the stock jet sizes and jet needle position should be. If pipes and inlet air system is stock and nothing else is wrong, she'll run just fine. If she doesn't, fix what's broke before adding hot rod parts.

....Proposed scheme for tuning with hot rod parts.....
Open the throttle all the way. If she pulls good to redline in as many gears as you dare, it's OK the way it is. The stock jets were either too big or all your expensive mods didn't change anything. If pull is 'weak' or engine pings then try a larger main jet. When she pulls for all shes worth you have the right size main. If you keep going up in size you'll feel no difference and just burn more fuel.

2) Open the throttle half way. If she pulls good to redline then your needle is OK. If she's weak and you have a tapered needle, move it up so a skinnier part of the taper corresponds to the same throttle opening. If it's a straight needle except at the tip(and it may be for some carbs on some engines) try the next skinnier needle. If you run out of clip, try a skinnier needle but be mindful of the taper and set the clip accordingly.

3) Open throttle 1/4 way and do a chop here .

Finer tuning is possible by messing with different taper rates on the needle and different needle jets. IMO at this point dyno time is well worth the $.

So, no plug chopping except at 1/4 throttle. At idle you adjust by the max RPM method. At other 1/3, 3/4, and WOT you adjust by needle clip position and needle taper. At WOT you adjust by main jet size. Remember, the main jet gets done first because it affects all the needle settings.

Does this make sense??? Did I miss something???

The little old CB350Four is running 90MPH at WOT in top gear. The CB400FK will redline in 5th gear and bang the rev limiter at 3/4 throttle. It will bang the limiter in 6th but that's too fast for me. I can't imagine being able to run the kz1000 at WOT for any length of time. Maybe cut off one coil and do two plugs at a time

-Duck

Excellent post Duck, please put it in pdf format and stick it in the Filebase.

Then I'll link to it when I get around to creating the forum FAQ