Vintage Dirt Bike Q & A

I have an 82 CR480,the motor has had a full rebuild.It still has the same problem as when I bought the bike. It will not idle.It goes like the blazes through the rev range but when you drop it back to near idle it will stall.I've tried all available slow jets in the carby(3 options).It runs stronger near idle with the richest one but still no idle.In all cases there is no response from adjusting the air screw and stop(idle) screw.This tells me the problem is somewhere in the air/fuel circuit and not ignition.Could it have a vacuum leak? It also has a minor exhaust leak around the block to engine pipe(oily black muck-it has a new seal)Anyone have suggestions?

I am sympathetic...I recently rebuilt the carb on my CR250M and with the idle adjusting screw all the way in...it still can not maintain an idle...the carb has been ultrasonically cleaned...new jets and a new needle etc...but still no steady idle...I'll check in on this thread from time to time to see if anyone has an idea...I suppose that you could create an idle by simply adjusting the throttle cable a bit...but you would lose your freeplay.

Good luck

Mike
http://meditarivemachinery.blogspot.com/

I just went thru the same problem with my 75cr-250. I have rebuilt; tuned and modified carbs for over 30yrs and after countless hours I bought a 36mm Mikuni vm carb. I bolted it on and it purred like a kitten. I believe after 40yrs of service the stock carb body must have worn out on the inside.I dont give up easy; but nothing seemed wrong.Im glad I got the Mikuni. I was losing my mind.LOL.

I'd suggest checking out the entire idle circuit, from the air intake in the bellmouth to the two fuel outlets in the carb throat. Here's how I do it:

1) remove bowl & idle air screw/spring
2) turn carb upside down
3) spray carb cleaner into bellmouth idle air intake w/your finger over the air-screw hole

You should get cleaner spritzing out the pilot jet and both carb-throat holes (in the carb floor, near the engine end of the slide). You might get a bunch of crap instead, or you might see cleaner shoot out of a hairline crack or some other place it shouldn't (happened to me once). You might have to spray cleaner into every hole in both directions if it's a dirt clog; compressed air is also good after the cleaner.

I've only had one air leak (LH crank seal), but though it reduced the low end a lot under load the idle was still OK, too fast actually. Even if you suspect an air leak, I'd still perform the above checks.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I did the same thing but couldnt find any cracks.It still bugs me that I couldn't find the problem.

I just fought my MT250 with a simular problem. It ended up being the left crank seal.

Test by using a plumbers torch, unlit of course, and direct propane to the left side seal area with the cover off.
Have someone work the throttle to maintain idle as good as possible.
If the idle raises up or, hey, it actually idles on its own, the seal is leaking.
IDK why my 40 year old seal was leaking but it was. LOL

Rick

I feel like an idiot.I did as rayivers suggested and fortunately found nothing unusual.While it was apart I opened my eyes as to how the idle should be set.Turning the screw in will give the highest idle speed(I thought it was lowering).The previous owner had it set way too low(manual tells you little) and I used it as a starting point.No response either way I turned it.So with the carby back on bike,I set it to maximum(all the way in)and presto the bike now idles.Just lowered speed and adjusted air screw and away we go.Thanks for your help everyone.Your info may be helpful in future.