\"Plug Chop\" test

I have searched the archives. I find references to doing a 'Plug Chop Test' but can not find an 'exact explanation' of how to perform one. Can someone please explain to me what exactly is a 'Plug Chop Test'?

I have read (twice) the 'great' article link that 'Ron C' provided recently on Reading Plugs. I suspect from this article and other comments made on the forum that this test is basically running 'new' plugs for a 'short time' at 'full' throttle and then inspecting the plugs for indications as described in Ron's article link. Am I far off?

Not far off, Plug chop test is just as you assumed, running the bike in the circuit ie: pilot, main, needle jet. area of throttle range for an amount of time then killing the bike in that range and coasting to a stop and pulling the plugs and reading them. I usually do about 1 mile in each range to get a good reading.

Full warm up the engine by running it for about 10 minutes... Then make a top speed full throttle run for a half mile or more, then kill the motor and stop and pull the spark plug on the spot.

It is important to pull in the clutch lever, chop the throttle and turn the engine off with the engine kill switch and pull over to stop. You don't want the engine to come to an idle, that will change the plug color.

Some people will tell you to use new spark plugs, others will say that you should use used, but cleaned plugs. I use used, cleaned plugs.

I was always a firm believer on equipment engines to never shut an engine off while it's running full bore. Instruction manuals for small engines always tell you to let the engine idle for a minute before shutting it down otherwise damage may occur. Is this not the case with a motorcycle 4 stroke engine as well?

Post edited by: BeeGee, at: 2005/11/03 19:56

BeeGee wrote:

I was always a firm believer on equipment engines to never shut an engine off while it's running full bore. Instruction manuals for small engines always tell you to let the engine idle for a minute before shutting it down otherwise damage may occur. Is this not the case with a motorcycle 4 stroke engine as well?<br><br>Post edited by: BeeGee, at: 2005/11/03 19:56

Sounds like something that might be recommended for a 2 stroke engine, but I dunno.

Beegee, You asked for help on a plug chop. I suggest you buy a new set of spark plugs. Drive the bike to a road where you can run at higher speeds without endangering anyone or losing your license. Pull to the side and put the four NEW plugs in the bike and then run at wide open throttle in 4th or 5th gear for about 1/2 mile. Hit the kill switch and coast to the side of the road. The burn on your spark plugs will reflect ONLY main jet air/fuel mixture burn. If you warm the bike up with the plugs in it, you won't know if the condition reflects idle circuit, mid-range or wide open throttle. It won't hurt the bike to do this test. Anyway, remove your plugs and examine them after doing a "plug chop" and you will KNOW if your main jets are too small, correct or too large. The plugs will be white=small, tan=correct or black=too large or somewhere between. The secret to doing a meaningful plug chop is to NOT RUN on the plugs till you are ready to do the actual wide open throttle run.

That all now makes sense. I do have one question however:
Can a 'Plug Chop' test be done with the bike on its centerstand in your driveway and simply running the bike through its gears at say 3/4+ throttle (or) does the bike need to sense a load at 'full' throttle such as running on the street to get proper results?

Post edited by: BeeGee, at: 2005/11/04 18:26

Don't do it - under any circumstances. It would be useless for a chop test (since the engine won't be under load), and very dangerous. A guy I know has no thumb because of running his bike on the centerstand through the gears...:S

The bike must be run for a half mile or so with WIDE OPEN THROTTLE. I don't think you could keep the throttle wide open on a bike on its centerstand that long and if you put the bike in gear, as you suggested, and something slipped, you would have a good story to tell... I have explain a plug chop. You could also go to a dyno tuning place and they will have an exhaust gas analyzer hooked to their dyno and when you run full out, you won't need to monkey with spark plugs... they will tell you the air/fuel mixture and can make suggestions based on the size of your main jets. I mention this since you seem hesitant to do a plug chop as I described.

Yes it certainly would make for a spectacular home video if the bike came off the centerstand while going through the gears. You've convinced me - I'll stay safe - and find a quiet stretch of highway some early Sunday morning. If stopped by a cop we'll just hope he's a biker as well and shows sympathy for an old man chopping his plugs. Thanks for the help guys - your the greatest!

You don't have to do the runs in top gear. 4th gear will yield a short jail term and 3rd gear only fines bwhahahaha It is the only way, short of a dyno testing run as I suggested. These guys can also give you good guidance on the other ranges of operation and jetting as well. One of these dyno sessions can cost upwards of $100 but is well worth it if they allow you to dial in jetting perfectly. Of you can wait till Sunday AM!