Backfiring, erradic rear cylinder... No Idea
Thread Starter
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Tourer
Joined: Sep 2009
Posts: 309
Likes: 2
From: Tegucigalpa
Backfiring, erradic rear cylinder... No Idea
Hey there fellas,
I want to resurrect my old 1995 Fatboy and I have been running with an issue that I can't figure out. The bike was running great, until one day, while I was riding it started with the problem out of nowhere. The rear cylinder seemed to cut out. Brought it back home and started trying to diagnose the problem.
The symptoms are:
- Rear cylinder seems to be misfiring and puffing some white smoke
- Front cylinder is rather hotter than rear cylinder
- Hard accel makes the carb backfire heavily
- Cannot ride it because it seems just one cylinder is firing like it should
Tests done:
First, I thought it was an intake leak so I replaced the manifold gaskets and carb gasket and verified the flanges where still useable. Nothing improved.
Second, replaced spark plugs and tested spark, all was good, nothing improved.
Third, replaced the entire ignition system with an Ultima system thinking it possibly was the coil was dead, so decided to invest in a whole new system. Nothing improved.
Fourth, Rebuilt the carb and used fresh gas. Nothing improved.
Fifth, but a new cheap chinese CV carb; I know, they're not great but if it at least idled normally it was a cheap test. Nothing changed.
Sixth, Tested the VOES and works fine activating at -5psi and holding the vacuum.
Seventh, I Re-tested for intake leaks using smoke mthod and propane method and I concluded I don't have an intake leak.
After all the above, what else can I test without having to tear down the top end?
Thanks.
I want to resurrect my old 1995 Fatboy and I have been running with an issue that I can't figure out. The bike was running great, until one day, while I was riding it started with the problem out of nowhere. The rear cylinder seemed to cut out. Brought it back home and started trying to diagnose the problem.
The symptoms are:
- Rear cylinder seems to be misfiring and puffing some white smoke
- Front cylinder is rather hotter than rear cylinder
- Hard accel makes the carb backfire heavily
- Cannot ride it because it seems just one cylinder is firing like it should
Tests done:
First, I thought it was an intake leak so I replaced the manifold gaskets and carb gasket and verified the flanges where still useable. Nothing improved.
Second, replaced spark plugs and tested spark, all was good, nothing improved.
Third, replaced the entire ignition system with an Ultima system thinking it possibly was the coil was dead, so decided to invest in a whole new system. Nothing improved.
Fourth, Rebuilt the carb and used fresh gas. Nothing improved.
Fifth, but a new cheap chinese CV carb; I know, they're not great but if it at least idled normally it was a cheap test. Nothing changed.
Sixth, Tested the VOES and works fine activating at -5psi and holding the vacuum.
Seventh, I Re-tested for intake leaks using smoke mthod and propane method and I concluded I don't have an intake leak.
After all the above, what else can I test without having to tear down the top end?
Thanks.
HDF Community Team
Joined: Dec 2005
Posts: 5,394
Likes: 2,596
From: NC USA
Intake leaks cannot always be found by the common accepted methods of checking but bad as yours sounds, it should have shown up if that were the culprit. Try this - watch the voes light on the Ultima ignition module at idle. At approx. 750 or more rpm, the red light should be steady. If it's flickering at all, you have an intake leak.
That year has a vacuum operated petcock unless it's been changed. They can leak fuel back thru the vacuum hose causing a rich or outright flooding condition. And, it may not open or stay open. If there's any hint of fuel in the vacuum hose that connects it, very suspect. They won't always leak constantly - sometimes only hot or only cold.
If the voes light checks out as I described above, do a compression test as suggested. Otherwise, the petcock is where I'd go next.
That year has a vacuum operated petcock unless it's been changed. They can leak fuel back thru the vacuum hose causing a rich or outright flooding condition. And, it may not open or stay open. If there's any hint of fuel in the vacuum hose that connects it, very suspect. They won't always leak constantly - sometimes only hot or only cold.
If the voes light checks out as I described above, do a compression test as suggested. Otherwise, the petcock is where I'd go next.
Thread Starter
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Tourer
Joined: Sep 2009
Posts: 309
Likes: 2
From: Tegucigalpa
Intake leaks cannot always be found by the common accepted methods of checking but bad as yours sounds, it should have shown up if that were the culprit. Try this - watch the voes light on the Ultima ignition module at idle. At approx. 750 or more rpm, the red light should be steady. If it's flickering at all, you have an intake leak.
That year has a vacuum operated petcock unless it's been changed. They can leak fuel back thru the vacuum hose causing a rich or outright flooding condition. And, it may not open or stay open. If there's any hint of fuel in the vacuum hose that connects it, very suspect. They won't always leak constantly - sometimes only hot or only cold.
If the voes light checks out as I described above, do a compression test as suggested. Otherwise, the petcock is where I'd go next.
That year has a vacuum operated petcock unless it's been changed. They can leak fuel back thru the vacuum hose causing a rich or outright flooding condition. And, it may not open or stay open. If there's any hint of fuel in the vacuum hose that connects it, very suspect. They won't always leak constantly - sometimes only hot or only cold.
If the voes light checks out as I described above, do a compression test as suggested. Otherwise, the petcock is where I'd go next.
I'll see if I can get a compression test done on monday. Looks like I'll have to tear it down in the end.
HDF Community Team
Joined: Dec 2005
Posts: 5,394
Likes: 2,596
From: NC USA
Intake leaks cannot always be found by the common accepted methods of checking but bad as yours sounds, it should have shown up if that were the culprit. Try this - watch the voes light on the Ultima ignition module at idle. At approx. 750 or more rpm, the red green light (voes) should be steady. If it's flickering at all, you have an intake leak..............
----------------------------------------------------
Looks steady, with glare off the status lite. Don't need an answer but, the voes light does go off with engine stopped (switch on) right?
It should only be on with vacuum present, engine running. Just asking in case that signal wire is grounded.
Grand HDF Member
Joined: May 2008
Posts: 4,325
Likes: 3,594
From: Picayune, Ms.
The "All is new" may actually be a clue. Trigger wheel may not be indexed correctly, attachment bolt maybe loose. Next coil and coil wires, spark plug wires, and of course spark plugs.
Personally I don't see how fuel, carb, intake can affect only one cylinder. But, that is just me.
Personally I don't see how fuel, carb, intake can affect only one cylinder. But, that is just me.
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Seasoned HDF Member
Joined: Oct 2019
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From: OH
The OP covered that in post # 1
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Tourer
Joined: May 2011
Posts: 469
Likes: 158
From: Alberta
Of all the stuff you changed it sounds like form your description, that nothing changed. It runs the same now as it did before. So if that is truly the case then it is something else, maybe something like the ignition switch, just a guess as some in those years did have some problems..
Coils and stuff like that tend to cause more issues when they are hot, on a running bike than when they are cold. Same kind of goes with plugs and plug wires, causing more issue at lower revs as the spark will overcome some issues as the rpm goes up.
When things out of the blue start to go bad on a carbed bike, I always suspect the carb first. There are a few small passages that can block off etc. Also I would pull the petcock and see what the screen looks like. The vacuum petcock on a close to 30 yr old bike is going to cause some issues. Also any kind of moisture in the fuel system can cause weird stuff to happen.
Did you do anything to the bike, immediately before it started acting up, gasing up the bike for example or anything else? If so retrace your steps.
Coils and stuff like that tend to cause more issues when they are hot, on a running bike than when they are cold. Same kind of goes with plugs and plug wires, causing more issue at lower revs as the spark will overcome some issues as the rpm goes up.
When things out of the blue start to go bad on a carbed bike, I always suspect the carb first. There are a few small passages that can block off etc. Also I would pull the petcock and see what the screen looks like. The vacuum petcock on a close to 30 yr old bike is going to cause some issues. Also any kind of moisture in the fuel system can cause weird stuff to happen.
Did you do anything to the bike, immediately before it started acting up, gasing up the bike for example or anything else? If so retrace your steps.
