Bad, bad, off-idle stumble

[aircap]

Hi, all:

This is my first post here. Sure hope somebody can steer me in the right direction.

I have a 1982 Yamaha Maxim 650, which I just bought a few months ago. I've had a number of bikes in the past ('75 Yamaha RD-350B, '80 Yamaha 400 Special, '70 Honda CL-450, '78 Yamaha XS-500, '77 Honda GL-1000), but this one's got a problem I've never had to deal with before.

Once the bike warms up properly, it has a terrible problem coming off idle, like say at a traffic light. I'll be sitting there at idle, then when the light turns green, I go to accelerate away and there's just no power there at all. I have to feather the clutch and twist the throttle until the RPMs slowly ramp up just to get going.

Once I'm up to speed, it's no problem (in fact, the bike is fast and responsive)...it only really does this after I've come to a complete stop, and the engine has returned to idle speed. Also, it only does this after it's warmed up all the way...it behaves very well when it's only a little warm in the morning.

I just replaced all four of the rubber intake manifolds, as the old ones were cracked as hell...I hoped that replacing them would eliminate this annoying (and dangerous) problem. It's making me mad at my motorcycle, and we all know that's no way to be.

Any ideas on what to look at next? My spark plugs are shiny clean, by the way...someone suggested that they might be fouled, but that is not the case.

Keep in mind that I'm not an accomplished gearhead. I am reasonably intelligent, and can do basic mechanical tasks, but please explain things gently and simply if you suggest any procedures.

Thanks, folks...I hope to hear from some of you soon!

With best regards,
Michael Carmody
Wichita, Kansas USA
Air Capital of the World

[txbadboy]

hmmm...definitely a weird problem considering that bikes, especially of that year, dont have nearly as many complicated systems as say a newer model car. i would say fuel problem or maybe something with your ignition, but only when its warmed up....have you tried checking your vacuum lines? are there any sort of temperature sensors on this bike? (especially if it has a radiator)

[kar_the_terrible]

my shot in the dark is: possibly the carb diapraghms??

[aircap]

Hey, all:

Had all four of my cracked, horrid old intake manifold boots replaced, and the problem continues.

It's annoying.

Carmody
Wichita, Kansas USA
Air Capital of the World

[TechTMW]

Does it make a popping sound through the carbs?

I'm thinking it might be clogged idle mix passages or a bad diaphragm. Either way , I think it's carb related.

[aircap]

You know, that's a pretty good question...but I don't really recall any popping or backfiring through the carbs. It's something carb-related, though, I'd betcha money. Probably having them all rebuilt would do a world of good. Any idea of a ballpark figure on how much that might cost?

Thanks again,
Carmody

[TechTMW]

Have no idea on the price of rebuilding - it varies by area. Just call your garage and ask them, Most have a flat rate for carb cleaning that they can quote over the phone.

I also notice that you're the same person who's having the issues w/ missfire on cyls 1 & 4. (I'm brilliant)

If you are going to have the bike taken in for the electrical testing, just have them to solve your problem. Your carbs may not need a cleaning at all, because the symptoms you describe could also be due to an electrical missfire. It's tough to make a diagnosis w/out being there to see/hear the bike.

[stormme66]

Just an idea, keep an eye out on E-Bay or the breakers for another complete set of carbs. Can be cheaper than taking the bike to a garage to get someone else to look at it. If you can find some reasonable, cheap ones, you could swap them out and see if that helps?

[aircap]

Hey, TechBMW and Stormme66...

It would be nice if everything could be solved in one fell swoop...guess I'll holler at my wrenchman and see what he says.

Thanks for all your responses, folks.

Best,
Carmody

[NORTY]

Sounds like fuel starvation. Unless it ignition related. Let's see, a 24 year old moto with drivability issues... Buy a new bike to ride and just wrench on your "project bike." Quite buying old pieces of "other people's problems."