[deeto]

Hey folks, sorry for the general topic name but my buddy and I could really use some help.

I have an old suzuki dual sport I gave him with a Mikuni BS34SS carb on it. We've cleaned it, adjusted the mixture screw, and ensured all the fuel air and spark delivery is functioning OK.

The problem is that the engine sputters and misses if we whack the throttle open from idle. If we gently open the throttle the rpm's rise all the way to about 85-90% and then it starts sputtering again. No matter what we do we cannot get to the limiter.

We've played with the mixture screw, setting it both on the lean side and on the rich side. And we see the effects of both of those settings like we should, but we just cannot get rid of this stumble.

The float level is correct and the carbs are so clean you could eat off them. The bike never used to do this and we're totally stumped as to the source. It initially sounded to me like a rich setting issue, but we leaned the mixture out and still got the same problem with the addition of sneezing and backfiring that leaning it out causes.

All the carb boots and diaphram have been checked for leaks are are ok. Intake and exhaust are stock.

We are at our wits end here. Anyone have any thoughts what this could be?

[JAWS]

What bike are you working on?
Joe

[deeto]

1985 Suzuki SP250

[JAWS]

Don't know a thing about that, but most 4 strokes won't do well on the stand when you whack the throttle open. Try it while riding. Are you sure that the sputtering at top isn't the limiter? If it isn't then your main may be too rich. I am sure you could go to the Suzuki forum on Thumpertalk and get some good advice. Try there.
Joe

[Asmith]

Are you sure that's the correct carb???

A 34mm seems a bit big for an old 250 of that era.

It should have a VM28SS.

Does it have a choke that can be selectively deployed??

ie varying stages??? Try it with just a little choke if it does and see if it cleans up.

It may just be putting in too much air for that engine.

Increasing the fuel ratio to match is just too much for the engine to burn effectively.

[Smash]

Sounds like too much gas to me. Also try a fresh plug. If you've had that one soaked in gas forever they seem to get saturated and don't run as clean on the top. Take some fuel out and try again.

[deeto]

all the testing we've done has been while actually riding it up and down the street.

Everything is stock, the 34mm carb is OEM equipment.

When I used to ride the bike years ago, it never had this issue. I could whack the throttle and had no issue.

We're definitely not hitting the limiter.

The early limiter/stumbling is a classic rich condition isn't it? We've set the mixture lean to compensate and we start to get the typical sneezing/backfiring that can cause, but the issue still exists. We can't seem to make it go away.

Perhaps the mixture is fine and this is a symptom of another issue? We checked for air leaks and found none. Fuel and plug are new. Anything else it could be?

edit - we tried setting the choke halfway and it didn't change anything