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MUNK
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20 Posts
Discussion Starter · #1 ·
I would be grateful if i was offered some advice on this 5 month long problem.
I bought a crash damaged 1998 yzfr600r. Instead of replacing all panels i turned it into a street fighter. The only engine modification was to reconstruct the exhaust, (same internal design but slightly shorter) and removed the ram air ducts (which i imagine mostly comes into play when racing around). The existing vent for the carbs that used to connect to the ram air ducts terminates to open air.

It runs fine in the garage, revs as much as you want... perfect. When Starting to drive from a warm Idle, it initially accelerates quite well up to a few 1000 revs for the first few seconds, but having done so, it hangs for many seconds at about 3000rpm until i stop the bike, or leave the throttle long enough for the idle to come down to 1,100rpm. If i try to accelerate while it is hanging (reving high) its hopeless and hesitates so badly that it feels like it does not want to move forward. This happens until i stop the bike again. Its a continuous cycle.

I had the carbs professionally cleaned (ultrasonically?), changed the jets, the needles, the carb boots. Checked the float heights (fuel level 11 or 12mm below the mark), the diaphram, the sliders, the rubber seal. All carbs are syncronised. I tried to look for a vacumn leak... and even with the new boots i found one but think i managed to get rid of that problem by over-tightening them. But i still have this problem. Is it possible to have a vacumn leak from the shaft of the butterflies?

My mechanic could not figure it out and got fed up of me!

Help ???
 

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sickboy
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3,544 Posts
99% of the time a hanging idle is a lean condition. The other 1% is your throttle cables sticking.
Have you tried the old spray brake clean all around and listen for the idle to raise trick when looking for a vacuum leak?
If your sure theres no vacuum leak and that your cables are not sticking id say take a stab in the dark and go up a size on your idle circuit jets.
 

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MUNK
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20 Posts
Discussion Starter · #3 ·
thanks for the advice. Sticking cable it isn't. I drove it without the tank on to study that. I could still have a small vacumn leak and used a whole can of Starter Spray trying to find them. Initially it was coming from the top of the boots. Even though the boots looked fine i replaced them. They could be after market boots but not sure. Maybe that is a problem ?? Anyway they still leaked so i altered the jubilee clip to get a tighter seal. Not convince i have no vacume leak but the engine speed changed much much less when i sprayed them with Starer Spray. After all this it still runs the same and showed no signs of improvement.

I learned this evening that i made a mistake in not connecting the vent tube from teh carbs to the underside of the airbox. I wonder though how much it would influence the problem i have.
 

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Registered
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4,653 Posts
Not having that hose hooked up would cause a good vacuum leak did you check it after you hooked it up
 

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sickboy
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3,544 Posts
Not having that hose hooked up would cause a good vacuum leak did you check it after you hooked it up
No sir, that vent tube is just that, a vent. It just vents to atmosphere, the reason they put it to the airbox is so that it doesnt get dirt into it. Hooked up or not it will in no way cause a vac leak or make the bike run different. Worst case scenario, if there was no hose on the nipple at all it may let some dust into the carb but thats it.
 

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MUNK
Joined
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20 Posts
Discussion Starter · #6 ·
i believe i found the problem and thank you so much for your advice. After connecting the carb vent to the air box and understanding the importance of having a good airtight seal on the air box with this connection, and correct balance of air pressures, it runs very well by comparison. Acelleration is a little flat at 4000rmp but i'm not complaining because at least i have a street fighter that works!!!! Thanks so much.
 
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