1996 Speedster carb problem

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I have a 1996 Speedster and one engine won't consistently rev up all the way. It bounces between 4k and 7k. If I just go straight (turning causes it to lose rpm's) and am patient, it seems to stay at 7k more often (80% of the time). I KNOW it is a carb issue because I switched carbs between engines (and nothing else) and the exact problem followed. I have replaced fuel lines, rebuilt carbs twice (once quickly, once very very very carefully), and nothing changes. The problem started two years ago, but only occasionally at first. Now it is consistent.

I don't know what else to even check on the carbs. The pop-off is good, all the adjustments are correct, and I was very careful to follow the Mikuni guidelines.


First... stop running the boat until you get it sorted out. Running a 2stroke lean will kill the engine.

These carbs run off of vacuum... and the real issue is... people take the pop-off test as gospel. The problem is with that is... you are using pressure to make a correlation to a vacuum system. AND... not everyone runs the test the same. Not to mention... you don't know if your fuel pump is producing enough pressure to keep the carbs filled.

OK.... so lets start with a few questions...

1) How are you running the pop-off test? Are you wetting the needle? Are you ramping the pressure quick, or slow?

2) Are you using ALL OEM PARTS !!!! The aftermarket parts kind of suck. The Mikuni hard parts are stamped with what looks like a little window, and the mikuni diaphragm has a red nipple.

3) Have you checked compression in that engine? If your compression is low... and you have leaking rings... the carb response will be wrong.

4) Have you verified 100% that every internal port is clear. When I had my shop open, I would get carbs in, where people said they were clean, but I would find the bypass ports were plugged up, or the backflow reed was not sealing.

ANyway... let me know, and we can move forward.
OK, to answer the questions:

I'm running the pop-off test the way I saw it done on a few videos - wet and pretty fast. I've actually tried it at different speeds to see how the result varies out of curiosity, but once I got it in the right range, it has stayed there.

I didn't use OEM parts. I bougt a four pack of cheap carb kits last year. I know this might be a problem, but having changed out everything twice in both carbs, and seeing no change whatsoever, I have trouble believing every crappy part I put in has the exact same issue. Also, the problem started before I rebuilt the carbs, and nothing I have done has made any difference, good or bad.

I did check the compression - 145 on all four. But remember the problem followed the carbs when I swapped them from one engine to the other engine - I did that to confirm it was an issue with the carbs.

I was very thorough as I cleaned the carbs this last time. Everything is clear.

Obviously, I'm missing something. The rpm's don't gradually move between 4k and 7k - they jump and are always at either 4k, 5.5k, or 7k. It's never a gradual drop or rise.

My obvious next step is to rebuild again with OEM parts, but, as I said, I am obviously missing something which I would like to try and figure out before I tear them apart again.

It’s the carb parts... 100% absolutely the carb parts. I know it’s hard to fathom that every manufacturer making carb parts other than Mikuni can get away with selling garbage that doesn’t work, but that’s exactly how it is. I’ve been down that road along with countless others that have come and gone on the forum, and the aftermarket kits NEVER work right when they’re installed. You’re going to have to rebuild them again with OEM kits, and I’d go ahead and do the other motor as well if you used the same kits on it previously. Otherwise you’ll start having issues with that one as well... The gasket thickness is not the same on some of those kits, and the rubber compound that the diaphragms are made of is not the same. Once you get a Mikuni one in hand, you’ll be able to tell the difference in stiffness immediately. Get those swapped out and I’d be willing to bet your problem is gone.
I've got a not so shocking update - it was the carb parts, and my incompetence. I rebuilt using Mikuni parts, and it didn't work at all, so I decided I would just change one part at a time until it worked. The first change I made was putting a different spring in for the pop-off pressure, and now the boat works perfect. So, apparently I don't know how to properly check the pop-off pressure, or more likely, my pressure gauge is not calibrated correctly - I actually belive this to be true.

I think I actually know exactly which carb part was causing the problem. There is a check valve in each carb, and the little piece of plastic that comes with the carb kits was about three times thicker in the junk kits than in the Mikuni kits. It makes sense to me that if those are way too stiff, fuel flow would be an issue.

Thanks for the help!!
Glad you got it figured out! It’s tough to track down exactly what the problem is with those things.
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