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Discussion Starter #1
So I have a 91 vn750. Did some engine work an finally got it all back together. I have cobra pipes and earshave, with 38 pilots and 140 main jets. Carbs have been completely rebuilt, all new parts. The bike will start and run if holding the throttle slightly open. I am leaking an abnormal amount of fuel from the front cylinder carb from where the air filter connects. Looks like it may be starting to come out where the 3 holes on the bottom of the opening are. The rear cylinder also seems to not be firing completely. The front pipe gets very hot while rear cylinder only gets somewhat warm. Not sure what is causing these problems or where to go from here. Also, the rear spark plugs have gas on them so I think its getting fuel. The front cylinder plugs have plenty of gas on them and are fairly dark and wet (carbon buildup).

Things I have done...
Complete carb rebuild including choke parts, enrichers, needles, jets, etc
Cleaned spark plugs-checked good spark at all 4 cylinders
Checked timing...just fine
Petcock rebuild kit
Fresh gas and seafoam

I have done a few others things but other than the obvious i am not sure what to check. I am not sure if any one has had these problems or know what i should check next. Please give any and all suggestions. Thank in advance.
 

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romeobravo172
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842 Posts
Will let some tec people chime in here, but keep an eye on oil level, will almost bet you are getting fuel into oil!!
 

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So I have a 91 vn750. Did some engine work an finally got it all back together. I have cobra pipes and earshave, with 38 pilots and 140 main jets. Carbs have been completely rebuilt, all new parts. The bike will start and run if holding the throttle slightly open. I am leaking an abnormal amount of fuel from the front cylinder carb from where the air filter connects. Looks like it may be starting to come out where the 3 holes on the bottom of the opening are. The rear cylinder also seems to not be firing completely. The front pipe gets very hot while rear cylinder only gets somewhat warm. Not sure what is causing these problems or where to go from here. Also, the rear spark plugs have gas on them so I think its getting fuel. The front cylinder plugs have plenty of gas on them and are fairly dark and wet (carbon buildup).

Things I have done...
Complete carb rebuild including choke parts, enrichers, needles, jets, etc
Cleaned spark plugs-checked good spark at all 4 cylinders
Checked timing...just fine
Petcock rebuild kit
Fresh gas and seafoam

I have done a few others things but other than the obvious i am not sure what to check. I am not sure if any one has had these problems or know what i should check next. Please give any and all suggestions. Thank in advance.
Are you sure of your description? You say the front cylinder is leaking gas and runs hot. It is running very rich and should be cooler. You say the rear cylinder is running hot which would be from a lean condition, but you say the plugs are dark and wet which means running rich. I would think that you have a float problem in each carb. I would also put in new iridium plugs, check the plug wires and boots for leaks, and make sure the battery is strong. Also the air/fuel mixture screws should be adjusted to your new jets.
 

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Discussion Starter #4
Description is correct. Thats kinda why im stuck in what to do. Mixture screws are about 2.5 turns out. While troubleshooting yesterday had something interesting happen. Had the spark plug and wire removed from the left side on the rear cyclinder to check spark. Cranked the eng quickly and it started up and ran better than I had seen since rebuild with no sparkplug installed. Seems like the front cylinder might be doing a lot of the work. It is definitely the front cylinder running completely at the same time as leaking fuel though. I think i may have 2 separate problems. One being the leak and the other being the rear cylinder. The front cylinder is the one running rich (dark black plugs). Rear plugs look fairly normal. Have you guys been able to source plugs from a common store (autozone, napa, etc) instead of ordering. Thanks for the help. I really need to figure this thing out. Too much money already invested.
 

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I got my iridiums at O'Riley. Most stores can order them if they don't have them in stock.
 

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There should not be fuel leaking from either one of your carbs. If fuel is leaking from the carb inlet, then it is also most likely leaking into the cylinders as well. Either the float levels are not set right (if they can be adjusted, I don't remember) or the needle valves are leaking. There are a couple of other possible issues with floats. They could have a leak and filled up with gas, or they could be rubbing on the side of the float bowl (that one happened to me once)

Other than the carb or the ignition, there is nothing easy to fix that I can think of that could be wrong with the rear cylinder.

You can buy those plugs at most auto parts stores, and if they don't have them, any Japanese bike dealer will.
 

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Got my iridiums at Advance Auto for $6.99 each.
 

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Discussion Starter #8
I'll pick up some new plugs and double check my needles. Maybe they are seated right. To be sure, does the shorter needle go in front or back. Wanna make sure they are correct. I think I remembered seeing something about the yellow plastic clip going on top of the needle had to be in a certain way. Is this true. After that I'll go with the floats. May the rear cylinder is normal and the front is the one all screwed up.
 
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