Triumphrat

Jamo · 05-25-2009, 09:17 PM

My '79 Bonneville runs beautifully so long as the gas tank is almost completely full. Once it burns through just .8 gallons of fuel it starts to stumble, as though fuel weren't getting into both cylinders any longer. It won't maintain idle and will stall. It sputters when running, as though only one cylinder were getting proper fuel. Then I fill it up and it runs normally again. The engine seems to have access to only the first .8 gallons of gas.

Any clue as to what is going on with this or why the tank has to be full for it to run?

GrandPaulZ · 05-25-2009, 10:43 PM

Make sure your gas cap vent isn't plugged.

Jamo · 05-26-2009, 01:19 PM

THat's a good thought. I'll look at that.

I'm not sure though. I had checked the gas in the tank by opening the gas cap just a few miles before it started last time. THat should have let air in.

jimballard · 05-26-2009, 06:05 PM

You may have let in just enough air to get you .8 miles. The cap vent can be plugged by simply waxing said cap. Try a small needle to clear it and see if you can blow through it.

Been there, done that: Jim

Jamo · 05-27-2009, 10:41 PM

I don't think that's it. I could see daylight through the pin hole and I blew through it easily. Any other ideas?

t100rc · 05-28-2009, 12:13 AM

I keep hearing these horror stories about the clogged vent in the gas cap ... my cap doesn't even have the little vent hole ... what gives??

jrector · 05-28-2009, 12:56 AM

Is it possible that the problem isn't really manifesting in the time it takes to burn .8 gallons? If you run it until it starts acting up, then top off the tank without shutting it down, does the problem go away?

If that is the case, I would drain the fuel tank on the off chance that you have some sort of contamination in the remaining gas that is lighter than the new fuel, only getting sucked in once the good gas is gone, but that seems like a stretch. I suppose the filter screens on the fuel tappets could be plugged enough to start failing once the static pressure in the tank drops, but again, seems like a stretch if the bike is running fine when topped off.

t100rc · 05-28-2009, 01:33 AM

Does this problem happen when running either petcock? How about running both? If you let the bike cool completely and then re-start it, does the problem continue? If the bike was running lean, the carbs may get hot enough to vapor lock. If you disconnect your fuel lines and open the petcock(s), how is the fuel flow? Have you removed the petcocks and looked at the screens? How about the filters in the bottom of the carbs?

Seems there are a lot of troubleshooting steps to take ...

Jamo · 05-28-2009, 11:54 AM

Thanks for all the thoughts. I guess I'll have to take it one step at a time. Maybe turn the problem over to my wrench.

Snakeoil · 05-28-2009, 06:05 PM

Before you turn it over to your wrench, do a few troubleshooting tests to determine if you can pinpoint the problem. The parts involved in the system are:
Fuel Cap
Tank
Petcocks
fuel line
Check valves
Floats

At 0.8 gal of use each time the bike acts up, I would have bet on a plugged tank vent. If you pulled the cap, but did not wait for the bowls to fill up again, the problem would persist for a few seconds, not unlike switching to reserve while riding. First thing to try is riding until it acts up and when it does, take the cap off or at least crack it and then retighten. If that does not work, switch to reserve and give it time to kick in. If this does not work then let's toss out a few other ideas.

You could have a sticking check valve in one of the float bowls or a float that is not properly set. The bike starts to stumble when one of the float bowls runs dry and/or the check valve does not open. Here's what I suggest.

Run the bike until it exibits the problem. Then push down on the ticklers on one of the carbs and see it it clears up. If not, try the other carb, if it clears up, you found your problem.

I have another question that might shed light on this. When the bike runs fine, is it also cold? Or does engine temp not matter? If it happens that the engine is cold and then after it runs a bit it starts to stumble, it would lead me to believe that one of the chokes is partially closed. This would show up once the bike warmed up a bit. I also understand that late model Bonnies had a choke linkage that did not fully open the choke on one carb. An old friend who was a dealer told me about this problem.

The suggestion to open the reserve petcock is a good one. If opening the reserve does not clear up the problem, then either your tank is full of crap and is plugging both screens or the problem is not in the tank or petcocks. If the problem clears with the reserve open and then returns, I would say that your tank is full of crap and the flow thru the petcock is pulling the crap to the screen and plugging it.

You do not have to run the bike to check the tank and petcocks. Remove your fuel lines by unscrewing the compression fitting and attached a clear hose to the threaded end. Open the petcock and let it run into a gas can. If you can run a gallon or so thru the petcock without any reduction in flow, it's not your tank, cap or petcocks.

Assuming it is not a choke problem, the symptom you report of one cylinder seeming to run out of fuel leads me to believe it is a float or check valve problem. Could also be dirt in the floatbowl that eventually gets sucked up and blocks the fuel port in the bowl. Put a clean white rag under your float bowls and remove the bungs to drain each one separately. If you see dirt or debris come out, that could be your cause.

It's hard to truly troubleshoot this type of issue without having the bike there to work with. How you are describing the problem could be leading us down the wrong path. For example, it could have nothing to do with fuel. It could be that your electronic ignition begins to die or "get confused" as it warms up and causes the bike to run poorly. I had a Harley that did this, and it started to do it right after I did some major mods to the carburetor. Needless to say, I played with that carb for almost a week before I decided it was not the carb and bought a new brain. The new brain solved the issue.

Hope this helps,
Rob