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Discussion Starter #1 (Edited)
I just put a new engine in my 95 vn and got it running but it's running way to rich so much so that sitting at 1-2k you can hear it loading up and it will finally die. Restarting is almost impossible. I've been into the carbs 3 times now checking everything and don't find anything unusual. Here is what I do know:

Engine:
The engine is a from a 96 VN 750 and has 6,200 miles on it. It has a lot of vacuum at crank and idle. I have a Carbtune and it will pull the rods up far enought to click on the top of the gauge. My other engine would barely move them at idle. I don't know if that's an issue but it is different. I'm using the same carbs from the old engine where they were running fine. I have black smoke and the plugs are black indicating rich. The front cylinder is popping back through the intake and out the exhaust. I get a lot of backfiring from the front cylinder. It might be sticking valves because it's sat for so long. I had the same issue with sticking valves on the original engine and it cleared up after a few hundred miles.

When it loads up to the point of stalling I can see fuel on the pivot rods for the butterfly valve on the outside of the carb. As I said I've had them off 3 times now and gone through them. All passages are clear and all moving parts move freely. I'm going to check the float level tomorrow but I'm out of ideas. It seems to run fine once I get it out of the 3000k rpm range but as soon as I go below that it starts messing up. the idle mixture screw is 3.5 turns out and the choke is OFF. I have had the idle mixture screw out 2, 2.5, 3, 3.5, and 4 turns out at different times, there doesn't seem to be much difference after 2 turns. I don't think there is a way to set the ignition timing on these things. If anyone has any ideas where to go I would sure appreciate it.
 

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On His Lady Vulcan
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So its only at idle that your running rich correct?

Have you done an ear shave, if so check your needle, these CV carbs are pretty basic, kind of like a garden hose if you will. The faucet is the needle and the jets are the hose, so if youre getting to much gas, turn down the faucet. If that doesn't cure what ails her look into a smaller pilot jet perhaps.
 

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Discussion Starter #3
No ear shave on the bike, and as far as I know the carbs are stock with stock jets. If this was a car I would say the float is stuck and flooding the carb with gas. I need to look down the carb throat with the engine running to see if something is obvious.
 

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On His Lady Vulcan
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No ear shave on the bike, and as far as I know the carbs are stock with stock jets. If this was a car I would say the float is stuck and flooding the carb with gas. I need to look down the carb throat with the engine running to see if something is obvious.
OK then it should be stock jetting, look into the bowls and make sure that the float hadnt cracked and pull it apart to inspect the float needle as well.

When you installed the carbs to the new/used engine did you happen to take the carbs apart and clean them? If so heres a curve ball that happened to me a while back, the float bowl o ring gasket wasn't seated correctly and cause a sever rich running condition.
 

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The Professor
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Check your choke enrichment plungers and make sure they go all the way down. It sounds like the problem is in the enrichment circuit. Also Pull a vacuum on the petcock diaphram nipple and see if any fuel comes out , keep vacuum for about three minutes and see if the carbs leak any gas out of the vacuum ports.
 

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Discussion Starter #6 (Edited)
I have had the carb bodies apart and actually changed out the floats and needles this time around to a different set. I made sure the float needles were clean and the tips were pliable. There was no evidence of the old floats having a problem. I did use the old float bowl gaskets since it seemed like they still had some seal to them and they weren't leaking. I'll get new ones to go back with now.

Choke enrichment plungers are the ones that get pulled up when the choke is on? If so I checked, cleaned, and polished them and they move up and down freely and give a satisfying clunk when they hit the bottom. Do they seal against a seat when fully down?

I put vacuum on the petcock diaphram nipple to prime the carbs and fill the bowl after putting them back in. Fuel comes out when vacuum is applied but the diaphram doesn't hold vacuum. Should it? I figured it had a metered bleed off so it would stop fuel flow quickly when the engine wasn't running.

Does anyone have a description of the carb circuits and when each one is active and how they work? All I have at this point is the FSM and it doesn't provide that type of description.
 

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I'm wondering if the cam timing is off to pull that much vacuum...?? A restricted intake (filters and related plumbing) could do the same but I'm sure you would have noticed that.

Do you have a vacuum gauge to give an exact number?

There aren't any #4 washers under the mixture screws are there? That would richen the fuel and your 2-4 turns out with the washers would then be 3-5 turns out for example.

Just something to consider.
 

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Discussion Starter #8 (Edited)
The engine only has 6,200 miles on it but it sat for a number of years. As far as I know and can tell, it's never been opened up. Cam timing would not be my first thought with the engine as new as it is. There are no unusual sounds coming from it it's nice and quiet and runs smooth when it runs. If the timing had slipped there should be some noises to go along with whatever had failed. These engines use chains and they just don't slip like belts do. I figured the higher vacuum was just due to the newer engine, the old engine had 32k on it and was pretty worn out.

With the carbs off I sprayed carb cleaner into the intake around the valve areas and blew it out. I don't see any damage or foreign objects in there, it looks good, just varnished up. Additionally I pulled the ears off and let it run without the air filter restriction, it made no difference. I've also run it with the rubber elbows off that connect the carbs to the air box, no change. I have a vacuum gauge and will check that tonight.

When you talk about the #4 washers I assume you are talking about the main jet needle? The only mixture screw I know of is the idle mixture screw and the only washer on the idle screw is the very small one that protects the O-ring that seals the screw.
 

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Discussion Starter #9
Update:
I did find that the choke lever is sticking slightly. That is, the forks don't come fully away from the plunger when it's been released. I don't see any movement in the plunger when I push the choke rod into the fully retracted position but there is a possibility the choke could be unseated slightly. Would that be enought to cause this serious of a problem?

I went through the FSM with a fine tooth comb and found that I might have the needle holder covering the vent hole, I didn't know about positioning it so it doesn't block the hole.

Additionally, does anyone know the difference between the front and rear jet needles? The FSM says not to mix them, I don't think I have because I did one carb at a time but the only difference I can see is that the front needle has N53A on it and the rear needle has N53B or 8 on it.
 

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The Professor
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I have had the carb bodies apart and actually changed out the floats and needles this time around to a different set. I made sure the float needles were clean and the tips were pliable. There was no evidence of the old floats having a problem. I did use the old float bowl gaskets since it seemed like they still had some seal to them and they weren't leaking. I'll get new ones to go back with now.

Choke enrichment plungers are the ones that get pulled up when the choke is on? If so I checked, cleaned, and polished them and they move up and down freely and give a satisfying clunk when they hit the bottom. Do they seal against a seat when fully down?

I put vacuum on the petcock diaphram nipple to prime the carbs and fill the bowl after putting them back in. Fuel comes out when vacuum is applied but the diaphram doesn't hold vacuum. Should it? I figured it had a metered bleed off so it would stop fuel flow quickly when the engine wasn't running.

Does anyone have a description of the carb circuits and when each one is active and how they work? All I have at this point is the FSM and it doesn't provide that type of description.
Replace your petcock diaphram, it should hold vacuum and have no fuel comming out of the vacuum nipple. The fuel is going into the front cylinder causing your rich condition. :beerchug:
 

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Discussion Starter #11
That makes sense, I'll check that. I used a hand vacuum pump to produce the vacuum and there was no fuel in the vacuum line after I pumped it to fill the bowls. Looking at the diagram, there is an O-ring on the shaft that connects the diaphram to the fuel part of the petcock, so it could have a leak in the diaphram without sucking fuel. If the O-ring was bad also then it would certainly suck fuel. If it should be holding vacuum, that would be a vacuum leak which will certainly contribute to the problem. I'll get one on order.

On another note, is there a difference between the choke plunger for the front and rear carb? Ron Ayers shows the same part number for both front and rear carbs and for 95 and 96; however, the plunger on the 96 seems to be slightly longer and has a longer point at the end. I know I changed out one of the plungers and at the time didn't notice the difference. If the longer point keeps the plunger off the seat that would be the same as the choke being on wouldn't it?
 

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The Professor
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That makes sense, I'll check that. I used a hand vacuum pump to produce the vacuum and there was no fuel in the vacuum line after I pumped it to fill the bowls. Looking at the diagram, there is an O-ring on the shaft that connects the diaphram to the fuel part of the petcock, so it could have a leak in the diaphram without sucking fuel. If the O-ring was bad also then it would certainly suck fuel. If it should be holding vacuum, that would be a vacuum leak which will certainly contribute to the problem. I'll get one on order.

On another note, is there a difference between the choke plunger for the front and rear carb? Ron Ayers shows the same part number for both front and rear carbs and for 95 and 96; however, the plunger on the 96 seems to be slightly longer and has a longer point at the end. I know I changed out one of the plungers and at the time didn't notice the difference. If the longer point keeps the plunger off the seat that would be the same as the choke being on wouldn't it?

If no fuel came out of the vacuum port then that is not the problem, the leak down could have been connectoins or pump.
 

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Simple Solutions
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just a thought someone mentioned cam timing effecting vacume ... im curiouse how this would do this but if a lifter had an air pocket then could that effect the vacume? (im gonna say no but was throwing it out there ) another thing to try is pull the boots off the carbs and see if there is enough air to compensate... what about about a failed air mix screw? i cant fathum how to much fuel is getting pat the jets... did one fall out (j/k) im leaning away from choke cause even if i was running fulll choke it wasnt rich enough to foul the plugs so quickly... or is it running ...

by the backfire in both directions im leaning more toward the valve sticking or the lifter failing why did the motor have so few miles was the bike wrecked did u see it... im gonna bounce this one off lance ut after my rebuld and i had that pop/backfire issue... it was case the rocker slipped off the lifter could this have happened in this motor form either lifter driing out and or the bike crashed and or the motor was dropped when shipped?? just thowing some ideas out there
 

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Discussion Starter #14
I believe I may have found the reason but don't know why yet. Tonight I went to do a float check and following FSM procedure with the fuel level check line was above the carbs. When I opened the bowl vent and put vacuum on the petcock, the fuel kept on running out. The floats are not shutting off the fuel allowing it to flood. It's the same on both carbs. It was dumping into the cylinders. I don't know why yet because I made sure the floats were free and within the marks before assembly. I don't have time to tear them down tonight but will get them this weekend. This time I'm going to make sure the floats work before putting the carbs back on the bike.

Thanks for all the great ideas I was pretty frustrated with the whole thing, I know far more about the carbs and how they work now than I did before.

Just FYI - The bike was not wrecked, the original owner bought it in Cali, moved out here and then eventually quit riding it and sold it to a neighbor. The original owner had defaulted on the loan and the the neighbor was never able to get a clear title because the loan was so old and had been bought and sold so many times. He decided to sell it for parts, I think he made his money back.
 

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Discussion Starter #15 (Edited)
Update:
I went through the carbs tonight with a fine tooth comb. Found nothing wrong with the floats, they were free, sealing when tested, and within the marks. I polished everything with steel wool and made sure there were no burrs on anything. I tore everything apart and cleaned it, making sure it was all assembled correctly. The choke plungers are seating and everything looks and acts good.

I did find a problem with the vacuum piston on the front carb. What are the symptoms when a vacuum piston quits working? When I was moving the piston up and down after several time it would quit pushing air out, when I blew shop air near it, the piston would not come up. I didn't blow air directly into the piston hole which could blow the diaphram.

My thinking is that the piston quit working which cause high vacuum on the outlet side of the carb because the air flow was restricted. With the higher vacuum and the throttle valve opened, the cylinder was able to suck gas out of the main jet or other areas and without the higher air flow to lean it out, cause a rich mixture condition that eventually loaded up the plugs.

Put the carbs on the bike and it started right up. Has been running well as far as the original problem. Still getting a backfire from the front cylinder but will start another post for that. I think the problem ended up being the vacuum piston not opening up properly. I think that may have contributed to the float issue or I didn't do the float check with the right equipment. Thanks again for all the help.
 
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