Help with carb Issues. | GTAMotorcycle.com

Help with carb Issues.

peterm15

Well-known member
99 Yamaha Vstar 650

So my bike sat for a few years, carbs were drained before hand.

Beginning of the season, no issues with starting or running, a little popping because of a gutted exhaust. So I went ahead, changed the air box out for a pod system, cleaned and jetted the carbs to the recommended jet for the gut/pod. (I can't recall the size off hand)

Replaced the intake boots and o rings as they were cracked. Synced the carbs, adjusted the mixture, replaced fuel filter, plugs, all oils etc . Getting it ready to do.

Now after 300km the bike has massive hesitation at lower RPM, starting to move etc. I've played with the mixture screws with no luck or improvements. I have noticed that the idle fluctuations depending on the lean of the bike. On the stand is a little low, increases as you go from left lean to right side lean.

Any suggestions on what it could be? Floats? If that's my next test any informative write ups on how to test the floats without the Yamaha fuel gauge?

Thanks for any help.
 

Trials

Well-known member
Yes it sounds like you have a fuel level problem in the float bowls, and I would be amazed if you can ever get CV carbs to work well with directly connected pod filters ;/ they just don't usually work well together, the intake path is too short and it creates turbulence that messes up the vacuum operation of the carburetors. Root cause of the problem is that CV carbs are designed to operate as lean and fuel economical as possible and you have freed up both intake and exhaust enough that now the carbs can't keep pace with the fuel metering requirements.
 

peterm15

Well-known member
Yes it sounds like you have a fuel level problem in the float bowls, and I would be amazed if you can ever get CV carbs to work well with directly connected pod filters ;/ they just don't usually work well together, the intake path is too short and it creates turbulence that messes up the vacuum operation of the carburetors. Root cause of the problem is that CV carbs are designed to operate as lean and fuel economical as possible and you have freed up both intake and exhaust enough that now the carbs can't keep pace with the fuel metering requirements.
The thing is that these pods are tried and tested by many members on another forum.

The kit itself is tailored to your exhaust type/modification so I didn't really see that being an issue. I'm not expert however.

I'll give a go to the floats before removing the jet, and pods otherwise I'll have no choice but to stock out the bike until I can figure out the issue.

Any suggestions on dealing with the floats? That how to is long gone from my memory. And I don't have the tool to measure the fuel level. I do however have access to high precision measuring devices (I'm a machinist, obviously not for bikes)

Thanks.
 

Trials

Well-known member
If the carbs have the ability to stick a piece of clear plastic tube onto the bottom drain on the float bowl it is super easy, you just improvise a sight gauge and you can see the fuel level from the outside.

... that bike has those down-draft jobs, I'm not familiar with them enough to know if that trick will work or no.
 

peterm15

Well-known member
If the carbs have the ability to stick a piece of clear plastic tube onto the bottom drain on the float bowl it is super easy, you just improvise a sight gauge and you can see the fuel level from the outside.

... that bike has those down-draft jobs, I'm not familiar with them enough to know if that trick will work or no.
Down draft job?

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Trials

Well-known member
Down draft job?
Isn't it fitted with 2 carbs where the air flow drops vertically down through the intake tubes? ... I refer to that as a down-draft carburetor.
... float bowls hang off the side of the carburetor instead of off the bottom.
 

Trials

Well-known member
Too bad the float bowls aren't clear plastic :| That would be a nice touch wouldn't it !
 

Trials

Well-known member
You have an original part, cast one in clear, fuel proof plastic ;)
Or figure out a way to MacGyver a sight tube on there, that is still probably the cheapest easiest solution to reliably set the float bowl fuel level.
 

Trials

Well-known member
OP describing the issue as "massive hesitation at lower RPM" would imply one or more of the lower speed fuel circuits as being a culprit rather then a fuel supply starvation issue which would be most profound at wide open throttle. The leaning from side to side making a difference in the bikes operation does indeed point to the floats as being a problem.

Popping sounds can be the result of over-lean carburetor lean fuel misfire, what happens is, there is not enough fuel in the fuel air mix to fire so a small amount of raw fuel is expelled into the exhaust and then the next time it does fire that fuel ignites in the exhaust system, pop!
 

peterm15

Well-known member
Fuel pump working ? Fuel filter in good shape ? Don't overlook the obvious.
Filter is new, less than 2 tanks of gas has gone through it.

Fuel pump seemed fine, I will test again though.
Thanks.

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peterm15

Well-known member
you get alot of conflicting information with that. (well I have with Google searches)

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boyoboy

Well-known member
What make of carb kit? pods are good for a race bike - but make for poor drive ability on a street bike. imo.

1999? Would be reasonable to replace the float needles and seats while the carbs are apart... its req'd maintenance with carbs. A steel ruler can be used to set float height.
 
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boyoboy

Well-known member
Filter is new, less than 2 tanks of gas has gone through it.

Fuel pump seemed fine, I will test again though.
Thanks.



What was the fuel pressure on your first test?
EDIT and since the fuel filter was changed recently - were you having problems at that time? (2 months ago)? - before the jetting/pod changes?
 
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peterm15

Well-known member
No pressure check, just visual, sucked fuel quickly through clear tubing into carbs.

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