Having experimented unsuccessfully with plug-chops, I am beginning to wonder if it isn't just a big myth.

I am lead to believe that the procedure is to run it at 12,000rpm in top gear for at least ten seconds, then pull the clutch and kill switch, then coast to a standstill. I do not believe this for two reasons:

Reason 1:
The recent post on "what's the top speed you've been" shows that the majority of people haven't ever been anywhere near 12,000 RPM in top gear, let alone held it for 10 seconds. I myself would be doing in excess of 210kph at that point with my gearing; admittedly I am geared for the circuit, but anybody who is geared for the circuit should perhaps hit 12,000 rpm in top gear, but certainly not be holding it for 10 seconds. On the street ... you have to question the wisdom of carrying out that procedure on all but the most deserted of motorways.

Reason 2:
WHy do you do a plug chop? To check your main jets are not going to seize the engine. What is the best way to seize an engine due to wrong main jets? Run it at 12,000 rpm in top gear for as long as you can ...

I realise that there are exceptions to all this, and that people have posted saying that you don't HAVE to do it in top gear. However I am still left with the overriding feeling that the plug-chop as described by most people must be largely a myth.

I personally have managed to jet by 'feel' at max throttle in middle gears, plus some dyno runs, plus checking the plugs after sessions at the track. I have repeatedly tried and failed to succesfully carry out a plug chop!! It is just not realistic.

WHo here has actually done the max throttle max revs top gear ten seconds plus plug chop? How many times have you done it, and do you recommend it?
_________________
MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-

Plug chops are (in my opinion) an essential way of setting the engine up.

Most of my hard riding is done on the track, so the engines must be set to run under those conditions. this means that max rpm is never held for more than a second or two at most. Using what would be safe track settings on the road would probably lead to engine failure.

An engine on the track is either under acceleration, or braking on a closed throttle. You're never holding the throttle constant for more than a couple of seconds.

On the road you're gonna be stuck in traffic, chugging down the high street or flat out for miles on your favourite stretch of tar trying to dodge those troublesome police men.

Setting the main should be done starting from rich. You can get a feel for what's going on inside, by the rate of acceleration from say 7~8 grand up to the red on WOT. If it won't rev out and won't stop smoking then you're probably too rich. Wild rpm (13,000), but bugger all torque to get you there means that it's way too lean and will nip before you get up to speed.

I do the main first, then try to dial in the bottom from closed throttle low rpm. This would be the slow jet and air screw.

Once it'll pull off the bottom nicely, and rev out at the top I'll then go about fiddling with the needle to clean up any mid throttle issues.

If I find myself going richer on the needle clip then I'd start to consider a drop in main jet, especially if it lost some rpm. opposite is also true of course. The straight diameter has more in common with the slow jet and air screw, so you might find a fraction of a turn here and there might also be necessary to get a smooth transtion.

What I like to end up with is a weapon that has max power and throttle response all through the range, but then from here I'd just sneak up a main jet size or two for protection in case I get carried away. I also set my personal bikes up at night 'cos it's a bit chillier (sometimes below 20 deg C) so that I don't have to hover my fingers over the clutch on my way home through the jungle on those particularly fresh evenings.

That's really helpful

Just one question ... I don't see a plug chop appear anywhere in your excellent setup description

I can guess that you are suggesting that for the circuit you plug chop at 12,000rpm in top gear for a split second? Or -what I observe most riders doing- you just check the plugs at the end of a session? Oh! ANd at which point in your procedure should it be done? At the very end? Or when first setting the main?

I am going to work harder at getting my jetting right, it is terrible in the middle which is OK for some circuits where wide open and fully closed are all you need, but some circuits require other throttle position occasionally.

Thanks again.
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MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-

The chopping would be done while setting the main at the beginning.

The plug colour gives a very good idea of what's going on inside the engine at the exact rpm you whacked the throttle closed and hit the kill switch

Make notes about engine feeling, rpm etc while your setting up and take a note of the plug colour. This'll help you set up faster in the future.

Don't be too scared to change a couple of things,. but if anyone's unsure of whether to go richer or leaner then go richer. If it makes the problem worse then just try going the other way instead. At least you'll be safe.

Cheers MAtt.
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MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-

invigiator wrote:
Who here has actually done the max throttle max revs top gear ten seconds plus plug chop?

StephenRC45 and I use a quiet (until we get there!) 1 mile stretch of road about 4 miles from where I live. With a rolling start it's ideal for plug chops as there is a slight incline just at the end of it - right before the 30mph limit signs!!

invigiator wrote:How many times have you done it...

Too many, but not enough!!

invigiator wrote:...and do you recommend it?

Yes, but not for 10 seconds! Especially if you're just starting out with your first base setting!!!
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Andy.
NSR-WORLD.COM

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Andy wrote:

invigiator wrote:...and do you recommend it?

Yes, but not for 10 seconds! Especially if you're just starting out with your first base setting!!!

Taken from the tuning section of NSR-WORLD under carbuerettor tuning, under jetting. I've been whining about the plug chop thing for ages and nobody has ever questioned my repeated use of the following quote until now!:

The easiest thing to do is a "plug chop". This involves running the bike flat out in top (6th) gear, near the "red-line", i.e., 12000rpm, for at least 10 seconds

_________________
MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-

If you intend to be running flat in top for ten seconds or more then you want to be setting up for this kind of running, so a ten second plug chop is feasible, but like Andy, on anything other than a stocker I'd resist.

this sort of use (abuse?) would require a bigger main jet than optimum setting as you'd want to keep the rev ceiling down and a bit of worthless fuel running through the pipelines will help cooling.

My 300 for a poor example, is setup to go from 1st to 6th from the u-turn near Mr. Moo's old shop to the turning leading to my jungle shack. It'll rev out to just over 12,500rpm over this short distance and hit about 130mph. At this point I'll hit the brakes, stamp down a few cogs with some fiercesome blipping just to keep the taxi boys alert, and then cruise slowly down the lane home.

A few years back ('94 'ish) I had a sreeter 21 tuned to reasonable numbers and I'd plug chopped it up and down the road during the day and had the jetting pretty spot on. It was running avgas and we put it on the brake at 74hp, so not a bad tool.

One evening Paul (on his MC28 in pretty much stock trim back then) and I were being tour guides for a couple of lonely girls, and I was hammering along full volume with the poor misses hanging on for dear life. As I slammed off the noise for the junction...Niiiiiiiiiiiiiiiiiiiiiiiiiiiiip... the thing thing locked solid.

Ooh err.

Of course the misses got the blame and I sent her packing. The rest of the night was spent in the work shop cutting another cylinder for my poor baby.

The moral here is to not only plug chop in the same sort of weather conditions that you'll be riding, but to also ask the wife politely to take a taxi. An NSR ain't no passenger vehicle.

Good call Invigiator! I'll ammend that!

There are a lot of little updates that could be done to the Main Site. Some of it was written 5 or 6 years ago when I edited nsr250.com and simply lifted straight across, some of it, like that particular part of the carburetion section, is still 4 years old!

I can read something time and time again and not see any glaring problems, but I can't ever get anyone to proof-read the pages, so it's only when someone spots a "glitch" that I can ammend it.

I never say NSR-WORLD is the be-all and end-all of NSR information however, but I am sure (without going back and re-reading the section) that I do say somewhere that there is no substitute for having your bike professionally set up on a dyno!

To give you an idea, I spend around 30-40 hours a week on the Site, and if you saw the post on Dave Booth's Le Mans report, that alone took around 4 hours to edit by the time I'd done the images! Invariably I don't often scan back through the older pages as I'm working on the new.

Next on the list is a revamp of Posers' World now that I'm getting moaned at for that!!
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Andy.
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Also... does it not say, at the beginning of the jetting section?

Everyone's opinion of the jetting process will vary, so as mentioned throughout our pages, this is only our guide!

_________________
Andy.
NSR-WORLD.COM

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I followed the tuning guild and held my 300 wide open for 10 seconds... it went pop. Should i sue? I fear it was more my fault though thinking i was on something that could stand being thrashed
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If I have to take the carbs off once more...

Hey Matt,

have you seen, or heard of a det counter for an NSR250? Would be nice so you don't need to worry about plug chops.

I still need to hook up the one on my RS. Just need to run through the plugs I got now, so I can order some longer ones to use with the det counter ring.
_________________
Charles Gallant

I can help you here Charles:
The best value data aquisition and det counter i've seen.
Real nice guys too.
http://www.sabresport.co.uk/home.html

Plug chops have nothing to do with engine RPM, gearing, or road speed. What they are used for is to read carburetor jetting (i.e. rich or lean) at various throttle positions.
For example when you adjust the idle jet (1st. thing) you will start the bike and run it for a few seconds and then shut it off. You will never touch the throttle because that would allow the carburetor to function through the slide needle and main jets thus fowling a true idle jet only plug reading.
For a slide needle reading you would not open the throttle farther than the stage of the needle you want to test. Also carburetor jetting is cumulative so if the idle and slide needle are off it will give you a false main jet reading.
Regarding main jet plug chops you will need a load on the engine to keep it from going pop. The only thing that matters is that the throttle slides are wide open. The actual load on the engine is doesn’t matter (gear, speed, whatever). Also be sure to turn off the engine before you let go of the throttle, and 10 sec. of WOT (wide open throttle) is a long time! You should only need 3 - 5sec. or so to get a good reading (works for me).
All of this is assuming you know how to read plugs, and you have the correct carburetor size, needle slide, slide needle, and slide cutaway; because all of those things will effect jet selection. Also altitude and barometric pressure will also effect jetting so when in doubt jet rich! Wiping splooge from your cans is cheaper than rings and cylinder plating.
When I do plug chops I jet it rich then ride with someone around town. I see what conditions cause hesitation and have a friend tell me when I am spewing smoke. Then, I will "fine tune” the jetting again. When properly done plug chops won't give you the exact jetting but they will get you close.
I hope this information helps.

From the point of view of logic rather than experience this POV makes more sense.

J
_________________
MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-