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Triumph TR6 - Fuel overflow

Your help is greatly appreciated.

My Triumph is a 1971

I have had the carbs rebuilt and rebuilt yet again professionally. The issue is that when the car runs the carbs are totally dry. Once the car is turned off, over the next 12 hours the carbs continue to fill with gas and it leaks out of the choke assemblies on both carbs. It would seem that a syphon effect is occuring
by which the carbs are pulling fuel thru the system.


I appreciate your thoughts!
JS Shirhall

I've got a 71 also so... I'll try to help out. First, I don't understand when you say that your carbs are totally dry. Are you refering to the float bowls not having any fuel? I had my carbs adjusted professionally a few years ago and found that they were not familiar with our carbs. Therefore, they did not adjust mine correctly. This can very well be your problem also.

By dry I mean that they do not leak when the car is running. The person who works on my carbs was actually factory trained to work on Zenith product. I have no doubt about his ability. We have checked the float height on a couple of occassions now and that is not the issue.

When the car is not running the fuel level rises above the gasket line by which fuel leaks from the gasket as well the choke assembly which is open to the bowl.

To test this theory I plan to clamp off the fuel supply to the carbs and run them dry, wait for the carbs to run out of gas, release the clamp and then see if the carbs fill up again when the engine is off.

I will report back with the results.....
JS Shirhall

Right after my car hit the road in 2003, I had the mysterious 'overflow' as well. It put about a gallon of gas in the crankcase, which plays havoc with the oil pressure. Even worse, if the vapors from that 'overflow' ignite, imagine what can be "BLOWN" off the cylinder block!!! I went to an electric fuel pump, back by the tank, and switched over to HS6 SU carbs.
Since 'oldtimers disease' has set in, I can't recall whether the fuel got in through the fuel pump diaphragm or the carbs. But I haven't had the problem since. I do, however, remember being very PO'd at the time!
Is the fuel just leaking from the carbs or into the engine? Might want to check that too....See if the level is up on the dipstick.
Rod Nichols

If you determine what the problem is, let me know as I have the same issue. I am extremely frustrated. I tore the carbs apart several times with no luck. Floats don't leak, levels set correctly, 100 % clean fuel, but I still have leaking problems. I abandoned the problem untill spring.

When I look again before the car goes on the road, I will weigh the floats. TRF told me they should weigh 15 oz. Another consideration, is that the jets are not sealing. However, these are new as well.


I hope you mean 1.5 oz. as 15 oz. is almost a pound!

It's not likely to be siphoning since the carbs are likely above the fuel level in the tank. Besides, the floats should stop this anyway.

As an experiment, install a 0-30 psi pressure gauge in your fuel line near the carbs. With the engine running your fuel pressure is probably in the neighborhood of 2-3psi. When the system is shut off, pressure should maintain or drop off. If it increases and the leak is large, I suspect you have a blocked vapor system in the tank and pressure is increasing from fuel evaporation. If it is a small leak, it is probably the engine warming and expanding the fuel in the fuel pump and the close by fuel lines. Either of these may overpower the floats, causing the leak.

Tom Sotomayor

Hi JS,
I went out to my 75 and disc the fuel line in front of both carbs and placed a small container under the line...a few drops of gas then nothing..after 1HR still nothing the container was dry. Try this and see if you still have fuel coming in.
If you disc the fuel line in front of the fuel pump it will constantly leak as the tank is the highest it possible your fuel pump needs a rebuild as it is allowing fuel to slowly seep by and creep up the line to the carbs ??..your float should stop any flow in unless it's under you have grose jets ( the small ball bearing ) or the regular type with the rubber tip ?

Charlie B.

JS Shirhall,
Sorry I didn't get to you any sooner.
I just want to make it very clear that these carbs are not difficult to deal with once you know where to look for issues. So, I hope you don't get frustrated with them. There is a reason why people pay top dollars for used ZS's.
Here's a couple more questions I've got for you.
1) When was the last time your fuel tank and fuel filter been cleaned or replaced?
2) Are you buying quality gas? Or putting cheap dirty gas into you car?
I doubt you have a vacuum problem. (I don't think our cars were built that tight!! ha ha)
For your fuel pressure, it's always nice to have a couple fuel pressure gauges on your system. I've got 1 on the output of my holley fuel pump showing 3 1/2 psi, one right before the input to the carbs showing 3 psi, and one from the output of my fuel pump to the cockpit showing 3psi.

Thanks for the comments.....this is a frustrating "problem" that I look forward to having behind me.

I will disconnect the fuel line in front of the carbs and see how much fuel comes thru. The carbs have grose jets. The fuel pump is only 4 years old but it surely is possible that something has failed with it.

Sepcific to the Fuel tank, it should be worked on based on age. Not sure how it would be a factor on this though. Fuel Filter is clean and I run the expensive higher octane gas. I do have a fuel pressure guage in front of the carbs and I run it at 2.5 psi via a regulator.

Leaving town for a week and will work thru this next weekend. These are good points and correct me if I'm wrong......fuel should not pass thru the pump if the car is off? Seems as though my pump may be a problem.

JS Shirhall

Our fuel tanks are not very large. Therefore, particle built up on the bottom of your tank swishes around. Some particles pass through the fuel filter and clog up the needle valves. My experience with Gross jets are that they don't get stuck as much as needle valves but, still can get stuck open and create the overflow problem. This may not be the answer but, you should examine this area as part of your troubleshooting.
Is your fuel pump mechanical or electronic?
Over a year ago, I had the exact problem you had and my fuel filter was clean. Somehow, debris stuck the ball bearing open creating flooding when the car was off. It drove me mad because I kept taking the carbs off to re-check the adjustment of the float. When I finally added a fuel pressure gauge at the input to the carbs, (look at the picture) I was able to verify that it was not a fuel pressure issue. Today, I have no problems.


I was frustrated with my carbs leaking as well and had previously disconnected and plugged the fuel line for the winter. After reading the threads above, I released the plug to see if the fuel would continue to flow when the car was not running. As suspected, the fuel continued to drip at a steady rate.

Should the fuel pump stop the flow of fuel to the carbs? Is the pressure to great that the float and valve cannot seal?


As Tom mentioned there could be pressure building up in the fuel tank due to vent obstruction.. I don't know if a 71 has a charcoal canister but either way the tank needs to vent either into a charcoal canister or to air. An easy test would be to run the engine until the carbs are dry again then shutdown and leave the fuel cap on the tank open. There won't be any tank pressure with the cap open. It's a long shot but it's an easy try..

Good luck and by all means let us know what you find.

HP Henry Patterson

Pretty well everyone else has told you what I also think the problem is. Let me summarize though and add one point.

First, Charlie mentions probably the most important point on this thread. The bottom of the carbs are lower than an amount of fuel in the fuel tank. As John B. above found out, the fuel will flow after he reconnected his fuel line. I think it has something to do with gravity:) The fuel pump will not stop fuel flow to the is designed to do quite the opposite.
Next there is only one place that fuel comes into a float bowl AND there is only one means of shutting it off. JS you have said you have grose jets. Designed to be better at stopping fuel flow but you could have a gummed up seat. I think the jets and seats need to really examined as the cause of your problem.
Finally John B. mentions the weight of the floats. A good point....are they fuel logged?

PS Henry, the '71 also has a charcoal canister.

Rick Crawford

It seems that my fuel is simply siphoning out thru the carbs as I did not have this problem when the fuel in the tank was lower.

My fuel is clean and the floats do not contain fuel. The valves and seats are new but are not grose jets. I suppose the only thing left is to change over to the grose jets. Would you agree?

I know that there is no pressure build-up as the cap is off.


About all I can say to you is that when I had my carbs semi rebuilt I was told and did replace the needles with grose jets mainly for the purpose of preventing the carbs from overfilling.
Rick Crawford

Thanks Rick

This thread was discussed between 28/03/2008 and 08/04/2008

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