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post #1 of 19 (permalink) Old 07-11-2008, 02:02 PM Thread Starter
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Engine running rough

I hope this is the correct area to post this, if not, sorry. My 2002 has been running like a Swiss watch since I bought it a year ago. I put K&N filters on it, iridium plugs, Vance&Hines exhaust, and rr relocate since. Rode it last week about 100 miles, no problems. Filled it with gas and parked it in my garage for about a week.

Today I start it up and it's running pretty rough (more than normal after a cold start) so I figure it needs to warm up. It was also backfiring pretty bad when I released the throttle, mostly out of the back cylinder. Rode it about 50 miles like this, never did run smooth unless the throttle was half open or more. I also put some SeaFoam in before this ride to see if that would help. It did not. The backfiring was at epic levels when I slowed down from highway speed. I think I even caused a horse to stampede that was in a field next to the road. Sorry horse.

I'm starting to think I got some bad gas when I filled up last, but I'm not sure if these symptoms are the result of that. About half the tank is gone now, should I drain the rest and try another fill up? Also, how do you drain the tank? Thanks for any suggestions.
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post #2 of 19 (permalink) Old 07-11-2008, 05:11 PM
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Since the petcock is a vacuum feed type, you need to provide suction on the vacuum line in order for fuel to flow. When I drained my tank, it was off the bike, so I had more options. With it on the bike, I've heard of 2 different methods, and I'm hoping that someone who has actually tried this jumps in! The first was to siphon the gas out of the tank. The second was to set the petcock to RES, disconnect the 2 fuel lines, and attach 2 hoses to the petcock nipples and route them into a fuel container, and then use a manual pump on the vacuum line to provide suction and get the fuel to flow into the container. I would also drain the carbs and use about a half can of seafoam in the next fill up to clean out any residual stuff from the system as well.

I am thinking that you can also be experiencing a problem somewhere in the ignition/fuel system. Have you checked the plugs to see if they are fouled? In general, the way I understand backfiring, there is a problem in the spark/air/fuel balance. Bad/no/poorly timed spark could be to blame. Clogged air filters, or a blockage in the air intake, could cause you to run too rich. Finally, a problem with the fuel delivery, like a stuck float, could be the issue. Since the bike is running poorly, I am assuming that excessive air getting into the exhaust is not causing the backfire.

I would try a new fill up to eliminate bad gas as the culprit. Then work through the ignition and fuel system until you find the trouble.

I am pretty new to this stuff, so I hope some of the more experienced folks chime in here! Good Luck!

Chris
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1989 VN750-A5
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post #3 of 19 (permalink) Old 07-11-2008, 05:47 PM
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Chris,
from my end you gave a pretty clear and concise explanation of the best way to go about it, with out the use of power tools.

It's what I would have suggested. However, neither draining method is going to remove the last dregs from the bottom, but a good fill of nice clean fresh gas will help that. I don't know about the seafoam thing. its not common up here.

Check the plugs. They are the first thing to check when diagnosing tuning issues

85 VN750 basketcase ( yes its a true 750, and no smog controls, its good to be Canadian)
It will have
a rebuilt 96 top end, rebuilt 85 bottom end
degoated, Eared
stripped, lowered, raked, bobbed,
reed valve boxes shaved
ported and polished
balanced
right side dump exhaust
heads and cylinders shaved for compression
looking for smoothbore sidedraft carbs
possibly fat bob tanks
modified vn800 or sportster drag pipes or I'll just make some

Chopperfix
Duck's Garage
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post #4 of 19 (permalink) Old 07-11-2008, 06:12 PM Thread Starter
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I checked the plugs and they seem ok, pretty clean and gap is correct. Air filters are ok too. I'm starting to think it's not the fuel now. I just went out to run it again and it seems like the back cylinder is not even firing at idle. If I choke it about half way the back cylinder fires up. It also does if I throttle it more than a quarter turn. But once I let off the throttle or choke, it dies off again. I'm afraid my wallet is going to lose some weight here in the future, I really don't have the skills to pull apart carbs or whatever is causing the back cylinder to be starved of fuel. Bummer. Thanks for the responses.
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post #5 of 19 (permalink) Old 07-12-2008, 12:32 PM
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I think I am having the same problem... at idle, it seems like the back cylinder isn't firing (judging by rich smell and low output or air in the left exhaust opposed to the right). I have done the ear shave, vance and hines, and plugged the reed valves. I have NOT rejetted the carbs yet.

I have tried re-adjusting the fuel/air screws and that doesn't seem to help. After about 1/4 throttle, it seems like its firing... if i'm cruising at 4k rpm's it seems like its not firing, but if I cruise around 2-3k it seems like it is. Last night you could see little puffs of fire out of the left exhaust as well...

The left rear plug boot is partially broken... but I'm not so sure that is having any kind of adverse effects on it (still need to check plugs).

Could it be a stuck float? Where is the best place to buy a carb rebuild kit?

Lonewolf, it sounds like we have the same problem.
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post #6 of 19 (permalink) Old 07-12-2008, 12:43 PM
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Quote:
Originally Posted by LoneWolfOH View Post
I checked the plugs and they seem ok, pretty clean and gap is correct. Air filters are ok too. I'm starting to think it's not the fuel now. I just went out to run it again and it seems like the back cylinder is not even firing at idle. If I choke it about half way the back cylinder fires up. It also does if I throttle it more than a quarter turn. But once I let off the throttle or choke, it dies off again. I'm afraid my wallet is going to lose some weight here in the future, I really don't have the skills to pull apart carbs or whatever is causing the back cylinder to be starved of fuel. Bummer. Thanks for the responses.



If it runs with the choke on but not with the choke off then your carb is plugged up someplace or you have a vacuum line off. You may be able to clean it out with some carb cleaner sprayed in the inlet of the carb. And make sure your vaccum lines are on and not split or cracked.

85 VN750 basketcase ( yes its a true 750, and no smog controls, its good to be Canadian)
It will have
a rebuilt 96 top end, rebuilt 85 bottom end
degoated, Eared
stripped, lowered, raked, bobbed,
reed valve boxes shaved
ported and polished
balanced
right side dump exhaust
heads and cylinders shaved for compression
looking for smoothbore sidedraft carbs
possibly fat bob tanks
modified vn800 or sportster drag pipes or I'll just make some

Chopperfix
Duck's Garage

Last edited by Duck; 07-12-2008 at 12:46 PM.
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post #7 of 19 (permalink) Old 07-12-2008, 12:48 PM
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Quote:
Originally Posted by natxbrotha View Post
I think I am having the same problem... at idle, it seems like the back cylinder isn't firing (judging by rich smell and low output or air in the left exhaust opposed to the right). I have done the ear shave, vance and hines, and plugged the reed valves. I have NOT rejetted the carbs yet.

I have tried re-adjusting the fuel/air screws and that doesn't seem to help. After about 1/4 throttle, it seems like its firing... if i'm cruising at 4k rpm's it seems like its not firing, but if I cruise around 2-3k it seems like it is. Last night you could see little puffs of fire out of the left exhaust as well...

The left rear plug boot is partially broken... but I'm not so sure that is having any kind of adverse effects on it (still need to check plugs).

Could it be a stuck float? Where is the best place to buy a carb rebuild kit?

Lonewolf, it sounds like we have the same problem.
Variations on the same problem I think, Dirty Carbs.

This one could be a stuck float making the cylinder flood, and misfire. Flame in the pipes could be from extra un burnt fuel being burnt in the exhaust.

85 VN750 basketcase ( yes its a true 750, and no smog controls, its good to be Canadian)
It will have
a rebuilt 96 top end, rebuilt 85 bottom end
degoated, Eared
stripped, lowered, raked, bobbed,
reed valve boxes shaved
ported and polished
balanced
right side dump exhaust
heads and cylinders shaved for compression
looking for smoothbore sidedraft carbs
possibly fat bob tanks
modified vn800 or sportster drag pipes or I'll just make some

Chopperfix
Duck's Garage
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post #8 of 19 (permalink) Old 07-12-2008, 01:14 PM
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Quote:
Originally Posted by natxbrotha View Post
I think I am having the same problem... at idle, it seems like the back cylinder isn't firing (judging by rich smell and low output or air in the left exhaust opposed to the right).
Don't the exaust cross in the goats belly and the rear cyl. come out the right side, front cyl. come out the left...???? I always thought that was the way it was, not sure though, could someone correct me on this......
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post #9 of 19 (permalink) Old 07-12-2008, 01:36 PM
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the vines and hines pipes have independent exhaust on the left and the right... no H pipe or goats belly.

Duck, when I took apart the carbs and cleaned them all out a couple weeks ago, it kind of seemed like on of the float needles was not real smooth in the hole it slides into... so if that little needle thing got stuck, that would cause the float to get stuck?
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post #10 of 19 (permalink) Old 07-12-2008, 01:41 PM
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Quote:
Originally Posted by natxbrotha View Post
the vines and hines pipes have independent exhaust on the left and the right... no H pipe or goats belly.

Duck, when I took apart the carbs and cleaned them all out a couple weeks ago, it kind of seemed like on of the float needles was not real smooth in the hole it slides into... so if that little needle thing got stuck, that would cause the float to get stuck?
Absolutely. Anything in there can make a needle stick. Even if the needle gets a wee bit off-center in the orifice, it will leak, so any crud or damage can cause problems.

85 VN750 basketcase ( yes its a true 750, and no smog controls, its good to be Canadian)
It will have
a rebuilt 96 top end, rebuilt 85 bottom end
degoated, Eared
stripped, lowered, raked, bobbed,
reed valve boxes shaved
ported and polished
balanced
right side dump exhaust
heads and cylinders shaved for compression
looking for smoothbore sidedraft carbs
possibly fat bob tanks
modified vn800 or sportster drag pipes or I'll just make some

Chopperfix
Duck's Garage
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