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Old 10-02-2010, 01:22 PM   #1
aartd
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Default 83 VT750 stalls above 3700 rpm

I have an 83 VT750 which sat for over ten years. I cleaned the carbs. replaced the fuel pump, fuel filter, battery, and air filter. It starts and idles
fine but will not rev over 3700 rpm. If you open the throttle too much it wants to die out and starts runnning rough & sometimes dies. Do you think this is a carb issue or a weak ignition problem. I also replaced the spark plugs.
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Old 10-02-2010, 01:40 PM   #2
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Did you put the cover back on the airbox? It's all sealed-up, no holes it didn't leave the Factory with, correct? The carb-insulators between the head and carbs are all sealed-up, no age cracks, rat-gnawed-holes, etc. - and the insulator-clamps are all properly-tightened, right?

If you answered "No" to any of the above, correct that.

If "Yes" to all the above, pull the slide-covers on the carbs and check for torn carb-slide diaphragms, bent needles, worn needle-guides, and sticking slides.
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Old 10-03-2010, 11:31 AM   #3
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also.......check to make sure nothing crawled up the exhaust and plugged it up. You said it idles, but can you spray carb cleaner around the manifolds and vacuum lines, without the engine speed rising? (indicating a vacuum leak and resulting a/f mixture being wrong)
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Old 10-03-2010, 11:52 AM   #4
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It sounds like when your carbs go to the main jet circuit, it starts to go over rich. Maybe it starts sucking straight gas right from the floatbowl into the venturi? The needle setting on the carb slide? Make sure the main jet isn't loose, has the appropriate o ring underneath it, or that o-ring holds the jet in the carb body (like Kehin CV carbs did on some models).

What do the plugs look like?
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Old 10-03-2010, 05:46 PM   #5
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Thanks for the tips. I took the carbs apart again. The main jets were tight. The slide diaphrams and jet needles look OK. The spark plugs are shiny black and wet with fuel. I think you are right about the carbs flooding the engine. When I first took the carbs apart one of the diaphrams was wet with fuel and had expanded in size. Only after drying out would it fit properly in the groove in the carb body. Anything else I should check before I put the carbs back on? I will do a vacuum leak test with some carb cleaner spray and poke a rod of some kind into the pipes
just to be sure about them. One other thing I noticed was that one of the carb slides was a little sticky in the bore so I cleaned both carbs a little better.

Last edited by aartd : 10-03-2010 at 05:49 PM. Reason: added info
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Old 10-04-2010, 08:25 AM   #6
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I took the bike out for a test ride today and it started to flood out again.
I shut off the fuel at the pet**** and after running bad for a little while ,
the excess fuel cleared out and I was able to rev the engine all the way to 8000 rpm in neutral for a short time.(fuel was shut OFF) Does this sound like a float height issue or
a float seat & float valve problem or something else?
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Old 10-04-2010, 08:34 AM   #7
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Maybe the float has a pinhole and is filling with fuel. This will cause the bowl to overfill regardless of the float height.
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Old 10-04-2010, 03:57 PM   #8
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Quote:
Originally Posted by seruzawa View Post
Maybe the float has a pinhole and is filling with fuel. This will cause the bowl to overfill regardless of the float height.
Hollow brass floats? Did I hit a time warp?
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Old 10-04-2010, 04:14 PM   #9
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Quote:
Originally Posted by aartd View Post
I took the bike out for a test ride today and it started to flood out again.
I shut off the fuel at the pet**** and after running bad for a little while ,
the excess fuel cleared out and I was able to rev the engine all the way to 8000 rpm in neutral for a short time.(fuel was shut OFF) Does this sound like a float height issue or
a float seat & float valve problem or something else?

Since you said you just rebuilt the carbs, have you checked your oil level gas contamination and being too high?

If your oil has a gas smell to it, your carbs may be leaking raw fuel down the venturi and into the combustion chamber, past the rings and into the crankcase.

An engine won't run right with a high percentage of gas in the oil because the mixtures too rich. (and you'll ruin the engine if you try too long) If you cut off the gas, eventually you'll go into lean-misfire and get your 8k rpm's, but with no power. Oh, and If there's gas in your oil, don't forget to fix the carb leak problem before you ride into the sunset!
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Old 10-04-2010, 04:15 PM   #10
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Quote:
Originally Posted by ranchwrench View Post
Hollow brass floats? Did I hit a time warp?
It's an '83.
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