Help with ?jetting? problems. At my wit's end

Hey Rich, I stripped my top end last night and found everyting 100% Ok in the piston department. Piston & rings were in 'new' tolerances...these motors are amazing.

Carb slide intact as well.

The only thing I did find which was 'wrong' was a frayed throttle cable which was hidden until I actually removed the carb. Dont want to teach granny to suck eggs but I guess you have checked that ?

I did find something weird though: The top of the piston was covered in sticky black stuff: burnt oil I guess, but i could remove by wiping & contact cleaner and it came away real clean.

Also the three intake valves are charcoal black whereas the the two exhaust valves are a nice grey metallic colour.

I know previosuly I was running rich because I was an air filter monster...I hadn't cleaned the filter in months after riding in some pretty dusty conditions (2 x 3 hr races where I had to wear a wet bandana to avoid suffocating on the dust cloauds and visibility 0 at some points of the race.)

But whats with the difference in colour intake vs exhaust ??

Having fitted a new pipe recently and my first run in months I could smell burning oil whenever I stopped, but ZERO smoke out of the exhaust. When I cracked the head cover found pooled oil on top of the front right valve bucket. Smelt of burnt oil too when I opened her.

Could I have overfilled the oil ? I remember checking, and it was 'high' on the mark but didn't seem overfilled.

A long list of questions but if you can help...that would be great.


One of the first things I did was replace the throttle cable because it was frayed. But it's still doing the running up. I also meticulously adjusted the cable tension to make sure it wasn't hanging up when I turned the handlebars but that didn't fix it either. I hope the frayed throttle cable fixes yours. Don't forget to inspect the carb slider. The one thing about yours and my problem is that it doesn't start to happen until it is fully warmed up. If it were just a throttle cable, why wouldn't it happen cold, too? :)

I'm not an expert on this by any means, but I think everything you described regarding the carbon in the combustion chamber is normal. I don't know why the valves are a different color from each other but mine looked the same way.

On my bike, if I overfill the oil, I get a lot of oil coming out the breather tube. If you overfilled yours, that could explain the oil pooling at the bucket because of the head design. It would sit up there until it's blown out.

I wouldn't worry about the oil smell too much if you don't see any abnormal smoke coming out of the exhaust pipe.


so the rings are where it's happening?

i was just about to say two things rich

first, the cables-let 'em have more slack.

two, the twistgrip gap to the throttle housing.

however that wouldn't explain the 20 minutes bit though.

you can get this overrev if the bike runs very lean at low revs and you close the throttle. by whacking the throttle closed you infact now have the correct mixture and it doesn't want to come down!

good luck coz i'm stumped as well. pleased to see that after many tests you came back to the old ones.



Rich, for an extremely good price, you could have bought a 420 cylinder and Wiseco piston kit from Eric Gorr. I think the whole thing runs ~ $350.00 COMPLETE...


Yep, it sure is a head scratcher. I too thought the throttle tube had worn out when this first started. I bought a new throttle tube, but I compared it to the old one before I installed it and they were identical. So I took it back. What has been the most confusing about the whole thing is that I have had the same jetting for a very long time and suddenly it's too lean??!! Has to be something else. (Could the new carb slider have anything to do with it? Seems unlikely to me.) I made sure the throttle cable was adjusted properly and have taken the carb apart for cleaning/inspection more times than I can count. We'll see if the new piston/cylinder/rings does the trick. My mechanic friend is sceptical and thinks it's jetting but we know for sure that the piston/cylinder/rings are shot.


Believe me, I've considered that option. The big bore kits are a little cheaper than stock. And I haven't gotten the new parts yet. But I did a lot of searching on TT and it looks like the big bore kits on these older WRs will likely lunch the connecting rod. I'd give up a little power and spend a little more for the reliability factor. My dealer will sell me the stock piston/cylinder/rings for about $370.

there is one more straight forward test you can do and that is to swop carbs with somenone for the weekend.

it would all stop there. the slide? well from what i could see they fit and that is that? anyway, why no problems from the start?

i've always wondered why we all feel that blowing [@#$%&*!] further INTO a carburrettor is called 'cleaning' it!!


Agreed with Taffy you need to replace your carb. All other attempts will have no sucsess. Somehow your carb no longer holds vacuum consistently and leaks at various rates as you are riding. :) A new carb is expensive but it must fix your problem.

...i've always wondered why we all feel that blowing [@#$%&*!] further INTO a carburrettor is called 'cleaning' it!!

I guess because it's called carb CLEANER :)

A new carb is expensive but it must fix your problem.

THAT ain't gonna happen!

swapping the carb with a buddie is probably the best way to narrow the search but still wont fix your carb, if that's the probablem. does your carb have the air cut valve? another diaphragm, another o-ring. if i understand it's purpose correctly, it creates a slightly lean condition at off throttle to keep the engine from dropping rpm's to qiuckly and throwing us over the bars. i believe it can be blocked off. good luck.

I don't have anyone to switch carbs with. But you guys are right about ruling out the carb if I could switch with someone. Carb was deoctopussed years ago.

Interesting prediciment. My pilot screw has vibrated enough to lean it out on repeated rides, after about 30 minutes of riding. My idle adjust will also periodicly vibrate loose. Probably of no help, but it baffled me to no end for a long time.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now