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Discussion Starter #1
Hello,

I need your help.... for a friend of mine.

He has a VN800, 1997. Stock carb, stock intake, half stock exhausts... With this setup, the bike were running well, no issues, till last year November, when this symptom apper : when accelerating, the engine die.

He told me these answers to my questione :
1. bike starts well in both cases, cold & hot.
2. engine idles well
3. when accelerating, engine dies, even in neutral.
4. if he pull the chock, the engine don't die and he can drive the bike.

He told me that he has cleaned the carb and checked the membrane inside and the rubber connections between carb and cylinders and they are ok.

Thanks
Marcel
 

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Hi, marcelm!
If it runs on choke and doesn't without, it's a clear indication that there is not enough fuel. The choke just enriches the mixture and gives more gas to the engine.
When did he clean the carb? Was it before or after these problems?
 

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once i had to pull choke on full to keep my bike running, the problem the mechanic found was a crack in the rubber air intake manifold letting in too much air, the bike was running lean so it needed fuel to keep running, i put in a new intake manifold and the bike ran fine thereafter with no need for the choke
 

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Will it accelerate with the petcock in the prime position? My bike did something similar and turned out to be junk vacuum hoses. Although it wouldn't idle for very long in the ON position.
 

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My bet would be something to do with the carb or jets. It's just too easy to miss something when you clean your carb(s) to rule it out even though he already cleaned it. Also, as you twist the throttle, pay attention to when it dies. The jets, the needle, needle clip, etc. all affect different throttle positions, from closed throttle at idle to WOT, it could help you troubleshoot. Also, you could check for vacuum leaks with carb cleaner. Spray it on the intake manifold behind the carb when the bike is idling and see if the RPMs drop. If it does, you've got a vacuum leak.
 

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yes with the petcock set on (PRI) the gas will flow freely without the need of a vacuum pulling open petcock plug (ON Position)
 

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4. if he pull the chock, the engine don't die and he can drive the bike.

pulling the chock gives more gas so it sounds like the gas in carb fuel bowl is too low and needs the choke, or there is a crack or hole in the rubber intake manifold letting in air making bike run lean thus needing the choke for more gas, or the petcock is not allowing enough gas to flow to the carb bowl
 

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Most common cause as other have stated is a vacuum leak...either hoses or possibly the petcock itself.
 

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Discussion Starter #9
Guys, thanks to all of you !!

Sunday I will go to my friend's garage and will check the bike.

First I will put the petcock in PRI position and see if engine will die or not.

NOT die => the issue is Petcock / vacuum hose.
WILL die => the issue is not petcock. Need to check more deeply the carb (membrane, main jet) and intake rubber manifold.
 

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After re-reading the issue it's more likely the accelerator pump has failed. The diaphragm is probably cracked.... I would pick up an DIAPHRAGM SET 18-0245. Should be be about $30
 
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