| Author | Message | ||
     Modrummer |
Ok so I've been having an issue where at higher RPM wide open throttle the bike bogs/acts like its hitting a rev limiter. Works fine at half throttle and low rpm. I am running a velocity stack intake and custom 1 3/4" exhaust with 45/175 jetting and needle shimmed 0.050. I just discovered today that if I cover the float bowl vent on the front of the carb, the bike accelerates smoothly and goes wide open throttle all the way to redline. What gives? I'm pretty sure from the factory this vent is supposed to be open. | ||
| Gearheaderiko |
Are you running an air filter? Float bowl vent? Got a picture? When did this start happening? | ||
| Modrummer |
I am running an outerwears filter. With no filter, it actually revs a little higher. Also, with no muffler (straight pipe) it revs higher as well which made me think the bike was running rich (since both removing the filter and removing the muffler should be leaning it out) I tried a 170 and even a 165 main jet that I still had with no luck (and obviously those should be way too lean). But as soon as I cover the float bowl vent with my finger I can run it all the way up to redline. See pic below:  I made a few changes all at once this winter so it's hard to nail down the culprit. I changed to a 1 3/4" header into a muffler (with very little baffling so fairly free flowing), a velocity stack with outerwears filter, and upped my jetting to 45/175. Idle mix screw is set at 2.5 turns out. Timing has been checked (multiple times). Running dans super boot. Float level checked last year, but I'm thinking I should check it again just in case. Basically I'm just wondering why blocking the float bowl vent would allow the bike to rev all the way up? | ||
| Gearheaderiko |
Are you running the correct Outerwears filter? Using the wrong filter will cause problems like you are experiencing. Are you running a gasket between the stack and carb? In theory, if you’re bblocking the float bowl vent you are leaning out the mixture. If you’re using the wrong Outerwears filter, you’re running rich. If you’re at see level, you may need to go richer on the jetting. But right now you need to swap out the air filter. STOP changing things until you find the problem. It’s more than likely a problem that you’ve caused, so “retrace, don’t replace”! | ||
| Modrummer |
I can verify the filter # when I get home. But I've tried running with no filter, and while it does run BETTER it still hits a point where it spits a bit at high RPM before it pushes through. So I guess that says the filter is at least PART of the problem. I am running a gasket between the stack and carb. I think we are on the same page, that the carb is running too rich at some point, because everything that leans it out helps (removing the muffler, removing the filter). So the thing I cant figure out is why, because I've tried a 165,170, and 175 main jet (a 175 should be right for my mods, but I tried the others to see if a leaner jet would make it better) and it made almost no difference. I've tried no shims on the needle and .050" shims on the needle. I double checked my float level and it is at .460" So basically either removing the velocity stack or removing the muffler makes it run better because it leans the tune out a bit. Ug. | ||
| Gearheaderiko |
Stop. You’re fighting the outwears filter. Any further tuning with that on is pointless as it will not be consistent. Your best bet, if you have one, is to put on a traditional air filter until you sort out the issue. A stack will not make that much of a difference in running. it should be noted that it is very easy to cover the carb inlet or disrupt the flow to the carb with your leg, given the placement of the carb. | ||
| Modrummer |
Sorry I didn't see your message earlier. I'll remove the outerwears filter again tongight and do an A to B test to make sure. It's hard to imagine the outerwears is the problem, I just double checked my email, its part # 12-1267-01 and I got it from D-C Parts (Dan) who deals to all the Blast guys on Buell Rider's Online. I'll see if the filter itself has a part # on it. | ||
| Gearheaderiko |
If it ran fine before the mods, then you’ve fouled something up. Retrace everything you did. A list of everything (everything!!) you did would help. | ||
| Modrummer |
Ok so here's the list: -Velocity stack with outerwears filter. -Sportster 1 3/4" header. -Rejetted from stock to 46/175 -shimmed Needle .050 -replaced Auto Enricher (bench tested mine and it wasn't closing up) -drilled out idle air screw cap and set to 2.5 turns out. -adjusted timing via static method (barely needed any adjustment, was almost exactly in spec) -verified float level (currently at .45") That's all I can think of at the moment. Basically it comes down to this: If I remove the the filter, the bike runs a LITTLE bit better (still hesitates at the RPM that it normally acts like it hit a rev limiter, but then pushes through. If I remove the muffler and run it straight piped, it has zero hesitation. With a muffler on (even a very non-restrictive muffler) it hits that point at high RPM where it sounds like it is banging off the rev limiter. Or, if I cover the float bowl vent with my finger, the bike runs perfectly even with the outerwears filter on and the muffler on. So to me the big mystery is: what is covering the float bowl vent doing that is making the bike run perfect? Also, I am DEFINITELY running rich (plug was sooty as was inside of header pipe) but I tried jetting all the way down to 165 and still had the same issues. | ||
| Gearheaderiko |
Did you take the carb apart? | ||
| Gearheaderiko |
It should noted that the Blast is not that sensitive to changes in jetting, mixture, timing, intake or exhaust-it’ll still run fairly well (you’ll notice changes in available power though). If it has a running problem, it is more often due to a real problem, not to a “tuning” problem. | ||
| Gearheaderiko |
Did it have this problem before you shimmed the needle? A common mistake is not using the correct washers, leading to a needle that is effectively shimmed .25” not .050”. | ||
| Modrummer |
Yes took the carb completely apart (Bike had been sitting for many years when I bought it and was completely gummed up.) I've owned and worked on a few carbed bikes before, and I have a 79 Corvette with a Quadrajet carb that I rebuilt as well, so I'm not a complete novice, but I still have plenty to learn. I've checked every passage in the carb now and made sure I could blow compressed air through them. I made all of the mods at once over the winter (velocity stack, outerwears filter, exhaust, jetting, shimming). But I think you were on to something with the outerwears filter. I did some A-B testing last night and running the velocity stack with no filter, the bike has a slight hesitation around 29MPH in 1st gear and then pushes through to the redline. If I put the outerwears filter on, the bike just acts like it's htiting the redline right at 29MPH and won't go any faster in 1st gear. So I'm confused because TONS of people run velocity stacks with an outerwears filter... | ||
| Modrummer |
Ok I just talked to Dan (D-C parts, the supplier of Blast parts who frequents Buell Riders Online). He is POSITIVE he sent me the right filter as he only stocks the one for the Blast. So unless the outerwears filter is the problem, whether it's the correct one or not... | ||
| Gearheaderiko |
Blast redlines at 32 mph in first gear (that can vary by 2-3 mph). So I’m not sure if you really have a problem. |
