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Carbs and Fuel System
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post #1 of 25 (permalink) Old 07-01-2011, 10:32 PM Thread Starter
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Unhappy Starving for fuel

Hi guys, first post. Just want to say you all have helped me A LOT in working on my bike. Hoping someone can help me here, the dealership didnt help much. I have a 94' 750 and i shaved the ears, removed the stock pipes and put straight pipes on it. I rejetted the carbs and shimmed the needles. Right now it has 40's for the pilots (which i think may be too small because i have the mixture screws out as far as i can take them, almost 7 turns and i still get popping on deceleration) the mains are 142's and urrently have the needles shimmed up with 3 shims (seems excessive to me). Oh, and i do have coasters on it as well in case that comes up.

Here is the problem i'm having. At idle its fine, starts fine, even when cold. I dont even have to choke it. At wide open throttle it has plenty of power no hesitation at all. But at cruising speed (im guessing the needles take over here) it bucks and carries on like its starving for fuel (this is at 3500 to 4000 rpms, somewhere in there, it acts worse on windy days which makes me think its running lean. If i choke it, it instantly straightens up, of cousre indont want to have to run it that way. Origonally i had 2 shims under the needles and took it to the dealership and explained what it was doing to try to get some advice and he said it sonded like the needles were'nt shimmed out enough, so... I put another shim in, no change whatsoever, so does anyone have advice for me, i feel like i'm wading through mud here!

Last edited by Mattejacob; 07-01-2011 at 11:13 PM.
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post #2 of 25 (permalink) Old 07-01-2011, 11:36 PM
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It would be helpful to know things such as where you live @ altitude. the closer you get to sea level the richer the jetting usually has to be. It may that you have reched the limitations of the 142 main jet no matter were the needle is i have seen posts here of guys running up to 150's on mains. What color are your plugs?

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post #3 of 25 (permalink) Old 07-01-2011, 11:55 PM Thread Starter
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I dont think the plugs look too bad. They are brownish grey, not fowled or anythihng, and they are dry. I live in charleston wv so the altitude isn't too high, i would sa a couple hundred feet maybe, we live in the valley right next to the river. I was kinda wondering about the mains, but it accelerates so well i thought it would be starving if it was the mains. But i was going to order a couple of sizes of each anyway because of the pilots being too small. Thanks for the advice!

Should i order 145's and 150's try em and see how it does? I'm almost positive about the pilots being too small because of the popping on decel, but i can tolerate that until i get it to straighten up on cruising, but i'll probably swap both out while i'm into it anyway.

The dealerships around here don't carry many jet sizes so i have to order them every time.
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post #4 of 25 (permalink) Old 07-02-2011, 08:33 AM
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I've seen posts from other people listings similar symptoms and it turned out to be the carb vent line getting a bunch of air forced into it while riding. So just as an easy, no cost check, you could look it over to make sure it's not cracked anywhere, and the end of it isn't just laying exposed somewhere on top of the engine. Just a suggestion.
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post #5 of 25 (permalink) Old 07-02-2011, 10:29 AM Thread Starter
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Actually, I hadn't even thought of that. I'm assuming you're talking about that one that t's off and goes into one. I didn't know what to do with that when I rejetted because noone had really said anything about it. It's routes over top the engine, just like you said! What do you do with it, you can't block it off can you? I'll check it out today. If it's not the problem I happened to find some old mains laying around last night that are bigger. I'm glad you mentioned that. I never would have looked there.
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post #6 of 25 (permalink) Old 07-02-2011, 10:53 AM
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I believe the line in question is the one that was attatched to the right ear when you removed it. There are some post on here that ive read of how guys have delt with this same issue, Maybe on a good ear shave proceedure post.
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post #7 of 25 (permalink) Old 07-02-2011, 12:37 PM
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You can put a filter on that vent line, or point it towards the back of the bike and hold it in position with a zip tie. The problem seems to occur when the hose is pointed towards the front or side and air is forced into, or moving across the hose opening, which causes pressure variations in the carb float chamber.

Phil used Emgo crankcase filters for the vent on his earshave:

Somebody else mentioned using some $3 lawnmower filters for the vent hose.


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post #8 of 25 (permalink) Old 07-02-2011, 01:15 PM Thread Starter
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Ok, it's not that vent line, I have that routed to the back like you said and the other that was mentioned (the vacuum on the right) is plugged. Last night I put some bigger mains in and I really think this may be my problem. But I don't know how much bigger to go! I put 150's in and it's better but not where it should be. At wide open throttle it still acts like it's starving fir fuel. If I choke it at WOT it doesn't really do anything. But I can tell its not getting enough on the top end. What do you guys think, am I in the right track?
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post #9 of 25 (permalink) Old 07-02-2011, 03:12 PM
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150's sound too big.. what is your elevation... did you PM on this as well?
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post #10 of 25 (permalink) Old 07-02-2011, 03:43 PM Thread Starter
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Elevation us prob only a couple hundred feet. And no I didn't PM on this. Just hoping to see some light at the end of the tunnel here. I'm a mechanic and I've worked on all kins of things but this bike seems to be getting the best of me here. Keeps leaving me scratching my head!
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