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2008 Bonneville Black
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Discussion Starter · #1 ·
My '08 carbed Bonnie isn't running perfect. She runs smooth in-town driving, and pulls well when I lay on the throttle. But when cruising around 3,500 rpm she runs a little rough. It feels like incomplete ignition in a cylinder or something. It's been an intermittent problem for a while, but it was bothersome yesterday. And my fuel mileage is down, I think because I'm laying on the throttle more often to get beyond the problem rpm range.

So, the problem is fuel, spark or air, right? I have Uni pod filters on it, air shouldn't really be a problem.

I've already trimmed the end of the vent hose to the fuel tank at an angle, but I'm going to disconnect that this afternoon and see what affect that has.

I'm going to look at the fuel filters to see if they are clean. I had some red clay-like stuff in there last year that I cleaned out and it seemed to be better after that.

I put in new plugs this spring. I'm going to inspect that ignition wires.

I'm going to make sure the carbs are balanced.

Anything else you can think of to try/test?
 

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Perhaps you have already given this thought: don't do everything at once so that you may know exactly what resolved the problem.
Sounds more like fuel/air than electrical. Good luck!
 

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"I've already trimmed the end of the vent hose to the fuel tank at an angle, but I'm going to disconnect that this afternoon and see what affect that has."

Now try relocating the vent hose from your carbs to a location with relatively still air such as behind one of the side covers. This hose T's off from a pipe between the carbs and just goes to atmosphere. Made a huge difference on my bike.

Chris
 

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90 percent of running issues end up being electrical. As they ay most carb trouble is electrical. I would swap out the coil. Number of those have gone out and the other thing to check is that the pick up coil is set to spec, check that with a feeler gauge. THe other common electrical issue is the igniter. I don't think this is a carb issue.
 

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I'ld take a close look at carb balance.

Since the problem seems to occur with "steady state" running at mid-range (3500 rpm) and not at high or low speeds the problem is very unlikely to be one of air or electrics.

My guess is that one of the carbs is lifting the needle slightly higher than the other and so, at mid-range, that carb is pulling more fuel because its working a thinner part of the needle.

Vacuum carbs suck in more ways than one.
 

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2008 Bonneville Black
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Discussion Starter · #6 ·
I'ld take a close look at carb balance.

Since the problem seems to occur with "steady state" running at mid-range (3500 rpm) and not at high or low speeds the problem is very unlikely to be one of air or electrics.

My guess is that one of the carbs is lifting the needle slightly higher than the other and so, at mid-range, that carb is pulling more fuel because its working a thinner part of the needle.

Vacuum carbs suck in more ways than one.
It was interesting. The balance of my carbs seems to be dependent on rpms. At 4,5000 rpms they are balanced perfect, at 3,000 rpm they are off.

Is this typical does anyone know?
 

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2008 Bonneville Black
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Discussion Starter · #10 ·
TORS, BBARKed with Uni Pod filters. 145 mains, no shims, stock pilot jets. Not sure how many turns out the idle screws are turned out at this point.

This problem has survived several changes in jetting. I had the issue with 140 mains and a couple shims in, for instance.
 

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I have a similar problem. bike runs great but often when cruising at 3000-3500 rpm it also feels a little surgey. (not miss firing)
have balanced the carbs and is ok thru all revs.

did you relocate the vent hose as suggested?

I'm going to try this on my way home from work

I would have thought if it was an ignition coil, it would play up through all the rev range
 

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2008 Bonneville Black
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Discussion Starter · #12 ·
The guts of my carbs look good and clean, I can say that. I pulled them apart and did general inspection/cleanup, but they look great.

I'm thinking the carbs get out-of-sync at certain rpms, through some subtle airflow issue. It's a subtle problem, so it would make sense.

I figured out the perfect way to re-attach the throttle cables: Fiddle with them for about half-an-hour, swear several times, and once they have decided you have suffered enough they will allow themselves to be attached. :mad:
 
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