motorcycle carb advice

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beebeeri000

beebeeri000

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hi, im picking up a motorcycle and the guy said when he was riding it in low revs it ran fat but in higher rpm would run dead lean. what is this a sign of? ive never had an rpm based carb problem before.
 
I use to work at a suzuki shop, if it's stock I would just pull them out and clean them out , check float levels, make sure tank and petcock are clean and you have clean air filters to start.
 
beebeeri000 said:
hi, im picking up a motorcycle and the guy said when he was riding it in low revs it ran fat but in higher rpm would run dead lean. what is this a sign of? ive never had an rpm based carb problem before.
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It's a sign of the wrong carb for the bike :D

If it's the stock carb follow the cleaning advice above.
It's either REALLY dirty inside or someone screwed with it badly.
 
Ill wager it has a modified air box, or no box and /or KnN type filters on it. There is almost no way in the world for a novice to tune that set-up. Heck, even I sometimes give up.lol. If Im right, dont buy it unless it comes with the factory air box included. Trying to find a used one can be challenging, and a new one may cost a fortune.
 
We'll need to know the make and model

Generally:
Idle to 1/4 throttle is the float orifice and the pilot jet
Passed 1/4 moves into the main.

some tips IF you buy this bike.

First off check synchronization on the existing setup (adjust and make a note of result)

Then remove the carbs and look for an after market jet kit ( like dynojet)

Let's presume the jets and needles aren't modified,
. clean them well, (when you think you have them clean clean em again)
- carefully look over the slide valve diaphragms for tears
. adjust the floats
. button everything up and install
- look for any air box modifications, filter pods or a K&N filter. (make a note of the changes. If it's a modified Air box try to revert it back to normal as much as possible by taping up holes etc)
- look for an after market exhaust (Make a note)
- Make sure fuel filter and petcock are clear (repair as necessary)
- Synchronize the carbs

If you find a jet kit was installed strap in for some tedious tuning.
 
The problem is most likely throttle position related rather than rpm. If the carbs are clean and all rubber parts are in good shape, including intake boots and diaphragm, if fitted, then it is in need of tuning. It's not rocket science. Up to quarter throttle on the pilot jet, from there needle height and profile, and WFO on the main jet. Mark your throttle so you know exactly where you are and address one component at a time. Also check needle valve and float for proper function to rule out flooding or starvation. What kind of bike is it?
 
well i was wondering why i hadn't got any e-mails of people replying to this and then i found that it didnt get set to instant updates. it's not the stock carb,. its a yamaha xv535 and the gut swapped out the original intake and dual carbs for a custom made one with a carb from a honda cb500 i think ill upload some pics
 
rirv4n.jpg
 
Okay , that looks like a kiehen cv carb . A common "upgrade" was to drill the slide and use a very light spring in the diaphragm cover.This is an irreversible modification. You can probably pick up a used virgin o eBay for cheap. When i had my sporty, i tried S&S super carb and a mikuni flat slide. Both great carbs but neither was as smooth as the kiehen. Easy to jet. Easy to adjust. What size is this twin you're looking at? What other mods does it have? BTW that intake looks abnormally long, good for WFO but rough for city driving.
 
Should be a slow jet and a fast . Believe it or not , it kinda works like half of an AFB. Take the bowl down and there's your jets . I really liked the one i had. Very smooth.
 
I would check the valve lash first and do a leakdown on it before you waste a ton of time on it.That set-up is very sensitive to pressure pulses coming in from the back, or low vacuum signals.
But then ..........................................see post #5.
 
Keihin CV carb. Loading at idle/pullover usually bad/leaking needle seat. Unable to take throttle bad pump/ clogged passages in bottom of fuel bowl or bad slide diaphragm. Like these guys said clean clean the carb. The little orfice/passage in the bowl can be a bear.

Also like has been said, vacuum slide carbs and long intakes don't play well together. That air filter is the worst setup there is as it allows air across the face of the carb at speed and jacks up the vacuum signals the carb sees. You need an airbox on that carb. Homemade intake sealing is suspect and no telling what gaskets. Check for intake leaks. Is that jb weld piled all over it? Probably fighting a cracked intake. You'd be better off with a butterfly carb on that intake. Often those carbs need the airbox to hold the carb on. Only thing I see is the boot. Is it banded? Long intake, jb weld, and no carb support coupled with some vibration is a recipe for intake leaks. I 'd try and source the proper intake,carb, and airbox and get rid of that mess that's on there.

You asked for advice on another thread and have disregarded everybody's and most of it thus far.
 
GeorgeH said:
Also like has been said, vacuum slide carbs and long intakes don't play well together. That air filter is the worst setup there is as it allows air across the face of the carb at speed and jacks up the vacuum signals the carb sees. You need an airbox on that carb. Homemade intake sealing is suspect and no telling what gaskets. Check for intake leaks. Is that jb weld piled all over it? Probably fighting a cracked intake. You'd be better off with a butterfly carb on that intake. Often those carbs need the airbox to hold the carb on. Only thing I see is the boot. Is it banded? Long intake, jb weld, and no carb support coupled with some vibration is a recipe for intake leaks. I'd try and source the proper intake,carb, and airbox and get rid of that mess that's on there.
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x2
 
funwithfuel said:
Okay , that looks like a kiehen cv carb . A common "upgrade" was to drill the slide and use a very light spring in the diaphragm cover.This is an irreversible modification. You can probably pick up a used virgin o eBay for cheap. When i had my sporty, i tried S&S super carb and a mikuni flat slide. Both great carbs but neither was as smooth as the kiehen. Easy to jet. Easy to adjust. What size is this twin you're looking at? What other mods does it have? BTW that intake looks abnormally long, good for WFO but rough for city driving.
Click to expand...

its a 535cc yamaha virago and this is the only mod on it. im gonna see about getting a different air cleaner for it but it stis out fat cause the carb is to one side instead of being in the center
 
AJ/FormS said:
I would check the valve lash first and do a leakdown on it before you waste a ton of time on it.That set-up is very sensitive to pressure pulses coming in from the back, or low vacuum signals.
But then ..........................................see post #5.
Click to expand...

yeah im gonna try to mess with this and get it working after i get it, but the guy said he still had the original dual carbs and intake they just got put in a box so ill try to get those working too if i can.
 
Step 1: Find the original carb and intake combo with airbox and shitcan that mess.
Step 2: Tune the proper carbs and intake using a new air filter
Step 3: Report back to us how well it runs.
 
ok so i was talking to him more and he said that the previous owner ruined the stock carbs by messing up the metering block, so im going to see about hooking up the carb thats on it to the stock air cleaner AJ ;P and ima see about picking up a jetting kit for it. well ill do all of this until i can pickup a replacement pair of stock carbs for it. i'm picking it up around the 13th so after i get it i'll upload some more pics and videos, its gonna look a little funny in the back of my friends sports chevy colorado with toyota rims lol
 
well while it will be a bit of work to get this right, the bike is running (not great) with the title in hand and only 6810 miles for $300, so to me it seems worth it to me atleast.
 
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