I've been living with this issue for lack of time to troubleshoot for months. I have some time now to hopefully get it resolved so I am seeking the wisdom of the group. I am getting a pronounced bucking, almost a jackhammer feeling particularly at 5k and to a lesser degree at 7k. This ONLY occurs if I am coming off throttle and try to cruise at those rpms. If I am just revving up through the RPM range, I pass through 5 and 7 with absolutely no issue. It's only when holding steady throttle at 5 or 7, particularly if I am coming down to either from higher rpm. I'm not close to an expert, but this does not seem like a jetting issue per se to me, partly because I did not touch the jetting prior to this starting to happen and it revs up fine throughout the rev range, with none of that bogging, strangled feel that one feels when jetting is way off. Also, the plug color is fine. The only possible diagnostic clue I have is that it seems to me that this started after I had run a tank of av gas that had been in the tank for about 3-4 weeks before running. I would really appreciate some help. I can live with it as is, but I would rather get it resolved if possible.

Jim
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-Jim
'93 MC21

You have to start somewhere.

If your sure its not carburation I'd pull your generator cover off and check your crank. Hold the fly wheel firmly in one hand and try and move it up and down, front to back. Rotate the crank a little and try again.

Any movement and its time to have the crank out.
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If I have to take the carbs off once more...

I am pretty sure that GeezaGillard had a similar problem to this.
In his case it wasn't so much a bucking/jackhammer as just a bogging over-rich feel.
But I'm pretty sure it was happening under the same conditions.
Basically he would be in top gear doing about 80mph and would ease off the throttle down to about 60 mph and then on re-applying the throttle it seemed to bog down when throttle was applied again, but only for about 4 seconds and then it would clear and be fine if you kept the throttle on. The bike would always accelerate fine except for just after coming down from higher revs.
Jim, if you keep the throttle on and "power through" the bucking, does it clear up in a few seconds?

In GeezaGillard's case he was able to track it down to the carby needle valves not sealing very well in their seats.
He used a bit of brasso on the end of a cotton bud to clean up the brass seats and put in new rubbers on the tips of the needle valves and the problem was gone. Magic!

I guess the float bowls were just momentarily slightly overfilling somehow when coming down from higher revs/throttle, causing bad/rich carburation?
Seemingly due to the needle valves not quite sealing off soon enough?

Perhaps in your case the momentary slight over rich condition is just enough to be on the verge of stopping the fuel igniting?
This would make the firing intermittent causing a bucking effect?

Just thinking out loud. I have no idea if this is your problem, but worth investigating?
It is possible I guess that GeezaGillard actually unknowingly did something else during the needle valve refurb that was actually the fix, but I seriously doubt it.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

thanks guys...since I am not certain I can rule out carburation, and the crank is not very old, I'll probably start by getting into the carbs and try to clean things up, as suggested. Is there a FAQ for carb cleaning procedures by any chance?

yes, once it is bucking, if I increase the gas a bit, it will power through in a few seconds. If I don't increase the gas, I think it would just keep bucking, but I've always just gassed it to get out of it, so I don't know that for a fact. Thanks for the leads, guys.

Jim
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-Jim
'93 MC21

Jim wrote:I've always just gassed it to get out of it

I think we can relate to that
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

have you done any mods to hte bike? It would be worth getting your carbs apart and checking your jets and emulsion tubes aren't gummed up.

Since you are a titanium member, it is worth talking in there with someone who can discuss jet sizes.

It sounds like carburettion problems of some sort, a key piece of information isn't presenting itself to resolve the problem though.

all mods done (wire splice and pipe, stock air box) were done long before this glitch started happening, so that's why I was thinking maybe something got gummed up with the av gas sitting in there too long. In any event, I agree I need to get into the carbs and try to make sure everything is clean. Never done it before, sad to say, so if there is any FAQ or description of how to remove and clean carbs etc. that someone can point me to, that would be great. Thanks.

Jim
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-Jim
'93 MC21

Here is a link to Geeza's post with a similar problem:

http://nsr250.net/forums/viewtopic.php?p=75354#75354
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

Hey, thanks, I always like when pictures are included!

Jim
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-Jim
'93 MC21