Yamaha RD250 Plug Chop Attempt 

In the 80's the way I used to plug chop was to always use 'used' plugs with carbon covering the porcelain tip, warm the engine by riding medium revs for about 15 mins in warm weather, then slow right down, gun it in second, third, maybe fourth and then instantly, clutch in and cut the ignition. Whip one or both plugs out and read the colour on the porcelain, light tan = too weak, nice beige tan colour = good, dark tan to black = too rich. If you want to be ultra safe (to guard against siezures) then go for mid to dark tan (not too dark). To my understanding you can only test with used plugs, whenever I tested with new plugs I would never get an accurate reading. If you were way too lean it would hit a wall around 6000rpm and act like the main jet was fully blocked and probably be very fluffy over 4500rpm ish. If you were way too rich it would be like the choke was on and start 4 stroking and holding back. The way it sounded down the dual carriageway I would say it was very close, probably no more the 5 out on the jets so in the 135 to 145 range. If you've got some used good plugs put them in and go ride around, sub 6000 rpm to get some carbon on them, then gun it as above and you should get a good reading. Good luck, remember if in doubt go rich and work back! The sound of the RD is pure nectar.

i agree with other comments always better to be rich than lean u dont want a failure and knacker your newly rebuilt crank etc keep at it lad ul get there

I´ve never actually cut the plugs up to get a plug chop and understood it was chopping the ignition not the plugs themselves but I could be wrong. The key is looking at the colour of the insulator near the tip after running at load (but not necessarily top gear but WOT. The consensus is that modern fuels with additives for cleaner burn take longer to leave deposits on the plug that you can read. One other issue is that fuel with ethanol in burns more slowly than 100% petrol requiring the ignition to be advanced slightly to get the best results from an engine. I would run the bike (but if you are worried it`s maybe a little lean then maybe increase the main jet by one size and/or raise the needle (drop the clip by one notch) to get readable deposits on the plugs and periodically do a WOT pull, cut the ignition and just look at the plugs at the roadside looking for a light brown colour close to the tip. In an ideal world, take it to a dyno but otherwise the above approach should hopefully avoid any disasters.

If "feels" like it's on the lean side. I see no color change at all on the insulator. With you being wide open (ish) that would be the main jet doing all the work there. It would not hurt at all to err on the side of a bit richer so maybe go up on the main Jets. Maybe try a 145 and see if it bobbles and burbles too much. But I would definitely err on the side of caution and better to be too rich than to lean. Your needle jet and pilot jet seemed okay because it's not hesitating to accelerate and once you are cruising with the throttle mostly open, those Jets aren't much of a factor. Try a 145 and chop it again? Also, if you get one of those plug reading magnification lighting tools that look like what the doctor sticks in your ear, you don't actually have to hacksaw the plugs apart, you can just look way down deep in there with the lit up magnifying ability and see if that ring you are looking for begins to develop.

Looking at the top of the plug before you cut it up, I would say by the colour of the metal bit you're running lean, it should be a chocolate brown and not a creamy colour. Kick up the main jet size and run again but don't bother destroying the plugs, you don't need to, just look at the colour of the plug, you can always wire brush the top of the plug clean after each rejetting operation...

I’m just starting the jetting journey with my 1973 RD250. Mine has DG pipes and it’s running lean on 155 jets with the needles at their highest (richest) setting. I’m now going for 175’s to see what happens. I may be wrong, but I don’t think there’s anything to be lost by trying larger jets. You’ll use more fuel but it’ll be safer if you’re worried about running lean. Having said that, your bike looks and sounds just great 👍

Tip: Ditch the expansion pipes and fit originals. Jet to OEM setttings, you will have a better running bike and no worries of running lean.

You are only really interested in the main jet setting. Pilot jet and needle jet are irrelevant on full throttle. If the bike idles well and picks up well, as you open the throttle, then they are set about right. It certainly doesn't seem to have to be bogging down, as you accelerate. As for the main jet, it's better to start rich and work back, if you're worried. As already stated in other comments. Modern fuels are designed to run cleaner. I.e. less carbon deposits, so that plug chop isn't really going to show much. Don't cut the plugs, there's no need, it's just wasting a couple of perfectly good plugs. 2strokestuffing is trying to build a highly tuned engine, at the extreme end of a 2-stroke motor. Just look a the tips of the insulator, see examples in a Haynes manual.

Always in the back of your mind once you have blown an engine I'm afraid but better safe than sorry mate

I’ve never had definitive results with plug chops on modern fuel, plus it won’t pick up things like the bike going really lean when you roll off the throttle (which is the current issue with my 350). The only sure way is to check the AFR on the dyno.

Firstly, I'm not an expert on this subject, so treat these as questions, more so than advice. My understanding is that the plug cut test is more appropriately performed on older plugs to reveal any build up of carbon that may occur on the insulator. The shut down test on new plugs doesn't require the destruction of the plug but rather an analysis of the where the deterioration is on the plating on the outer electrode, which should ideally be on the bend (if memory serves me). Once again, this is my belief, don't take it as fact. That said though, that plug does look lean. I'm seeing a yellow to white colour through my monitor. I'm also wondering what your lubrication tuning is like. My old RD pumped more oil into the chambers than it did fuel, and boy, did it drink fuel. It was a pig of a thing. If I had my time again, I would modify it and run premix. You've got all of the opposite problems to the ones that I had. And no, I don't know how my bike was jetted. But the bike does sound to be running well, with plenty of power. You've got to be close. One last thing comes to mind. Do you know what the cylinder head temperature should be on these and have you measured it? These old air cooled 2 bangers got hot when they ran lean. They got really hot. That might be a test worth doing if you can find the specs. Hopefully I haven't thrown a cat amongst the pigeons. Thanks for the memories, and good luck with it. Beautiful bike that you have.

Don't think you had enough miles on those plugs to tell you anything. You don't have to be in top gear either you can do it in third/forth as long as its WOT and under load.

Just put the Ducati 900 through the CT. Got 5 years. Cool stuff. Have fun with Yammy

Where did you get the vape ignition system from