Triumph GT6 MKIII Fuel leaking from carbs into air box

[NDP]

Any suggesting (apart from sell it) welcome. Here is the sequence of events:

1). Fuel flooding into airbox via carbs.

2). Replaced fuel pump with a new one (manual). Standard unit as per original.

3) Problem not solved.

4) Carbs removed and sent away for complete refurb.

5) Problem not solved

6). Tank removed, cleaned and relined.

7). All fuel pipes renewed.

8). New fuel filter fitted.

9). New air filters fitted.

10). Problem not solved, but wallet considerably lighter.

I have read around the subject a bit and there is a view that the fuel pump could be too strong for the system. Does anyone know what pressure the fuel pump on a Triumph GT6 MKIII should run at? The second question is how can I tell what my pressure is running at and then if it is too strong how do I alter it?

Thank you for any help. 

 

 

[dougbgt6]

N,

Here is a recent thread it says 1.5 to 2.5psi

Are the floats in your reservoirs the right way up? Easily put back wrong and flooding the result.

 

Doug

[Paul H]

Is it both carbs leaking or just one . I had the same issue with the rear carb on my Vitesse , and rubber slivers were holding the float valve open .

I also replaced the fuel pump with a modern copy and this made it worse , too much pressure . Suggest reconditioning your old one or move to electric fuel pump. I chose the latter and now use a Huco 13301 , nice bit of kit .

Paul 

[Pete Lewis]

and dont forget with all the messing around you make those sneak rubber slivers   they float about and jam in the back of the float needle valves 

Dougs correct the later stromberg floats can be easily be fitted upside does  this does affect fuel levels 

with two needle arms they can be wrong   buckeye have good pictures of the float 

TR250_TR6+Carbs+Part+II.pdf (squarespace.com)

Pete

[dougbgt6]

1 hour ago, Pete Lewis said:

Dougs correct

Hoorah! 

See! I do know stuff. 

db

[chrishawley]

Let's just assume for a moment that the £££s recon carbs are 100% tickety boo. Assuming so, then pretty much the only cause of float chamber flooding would be excessive fuel (pressure) deilivery. I've not experienced this myself but I've heard that repro mechanical  pumps may deliver excessive pressure above the 1.5 - 2.5 psi required. And that the simple expedient of putting a couple of extra gaskets between the pump and the cylinder block reduces the action of the pump lever and resolves things.

If you want to measure pressure directly then that means using an in-line gauge. Demon Tweeks do a kit of the gauge and connecting pieces. Won't go into detail as it's pretty self explanatory (see website).

But the biggie for float chamber flooding is detritus, not least rubber bits, getting on the back of the needle valve, leaving them always-open. I've been caught out on this with repeated blocking of the valves despite assidusous attention when refitting the rubber interconnections. Ultimatley tracked down to 50 years detritus in the U-shaped pipe between the carbs. Absolutely full of crud; pieces of which were intermittently being washed off and getting on the back of the valves.

Now, even if the carbs have been rebuilt, professionally or otherwise, there's a couple of way the needle valves can leak. Firstly, I've had needle valves fresh out of the packet that didn't close properly. So 'new' is not a 100% guarantee of 'right'. Secondly there's the crucial little alloy washer under the head of the needle valve that makes the seal to the body of the carb. Ideally they should always be new but old ones can be reused subject to flatting with wet'n'dry to remove any burrs and annealing to be soft and comformable again. And tightened up well.

That doesn't cover everything but I hope it gives an idea about some common scenarios. But just to add that I've learnt to use two in line fuel filters (the see through paper type). One right below the fuel tank but also one just before the carbs. When sorting carb problems the latter is very handy as it allows direct inspection of what the fuel flow is (or isn't) doing.

 

 

[NDP]

Gents, thank you all for your input. I have read the replies and all of the associated links to previous postings etc. 

I will slowly work my way through the suggestions but some clarification on the pump pressure for the GT6 MKIII would be useful. It has been quoted above:

1.5 - 2.5 psi required

but also quoted in a link as 2.5 to 3.5 psi

this seems like a fair difference if taking the mid point of each range?

Cheers

[Ian Foster]

NPD

For reference the suction version of the Huco electric pump has a specified pressure of 2.1 PSI viz:-

Specification:-

• Pressure 2.1 PSI
• Flow 27.5 Gallons per Hour (125 Litres per Hour)
• Union fuel pipe size 5/16 inch / 8mm

This pump is a suction type and designed to be fitted in the engine bay of the vehicle.

This pump is ideal for use with Dellorto or Weber carburettors, the low pressure eliminates the need for a pressure regulator.

Manufacturers part number 133010

Ian

[Stratton Jimmer]

I have had the same problem with the HS6 SU carbs on the Sixfire. The mechanical pump was overpowering the float needles. It is a replacement pump which I believe came from a certain company with a "grimmer" reputation than some. Additional spacing between pump and block resolved the issue.

[NonMember]

9 hours ago, NDP said:

some clarification on the pump pressure for the GT6 MKIII would be useful. It has been quoted above:

1.5 - 2.5 psi required

but also quoted in a link as 2.5 to 3.5 psi

I'm pretty sure the lower range is the correct one. The carbs will probably tolerate 3psi - they will definitely leak with one of the modern 4psi+ pumps.

[Clive]

59 minutes ago, Ian Foster said:

NPD

For reference the suction version of the Huco electric pump has a specified pressure of 2.1 PSI viz:-

Specification:-

• Pressure 2.1 PSI
• Flow 27.5 Gallons per Hour (125 Litres per Hour)
• Union fuel pipe size 5/16 inch / 8mm

This pump is a suction type and designed to be fitted in the engine bay of the vehicle.

This pump is ideal for use with Dellorto or Weber carburettors, the low pressure eliminates the need for a pressure regulator.

Manufacturers part number 133010

Ian

But take care, as teh Huco pumps are available in a higher pressure too, but look identical (they are clear about teh difference, but something to watch for)

Likewise they are offered as Suv=ction (pull) for engine bay mounting, which most other pumps are NOT suitable for. Huco also offer boot loacted (pusher) pumps. All higher or low pressure.]

People often confuse pressure with flow, I ran a pair of dellorto carbs on a 160bhp engine at 1psi. The flow was plenty. 

[NDP]

Thanks one again chaps.

 

Stratton, when you said extra space between the pump and block, just how much extra space are you talking about and did you have specific gaskets in mind?

[Stratton Jimmer]

1 hour ago, NDP said:

just how much extra space are you talking about and did you have specific gaskets in mind?

Only about 3mm. Self cut gaskets from sheet.

P.S. Stratton is where I live, The name is Jim but round here, anyone named Jim is called Jimmer.

[NDP]

Cheers Jim!

[NDP]

I thought I would post a final update on this topic because of how frustrating it has been for me to find a cure and hopefully this helps someone else short cut the problem. Since my last post I did the following:

1). Get a 3mm gasket (has to have one made but cost was negligible). Didn't cure the issue but it did make a difference when I measured the fuel pressure with a gauge I bought.

2). Had a 2mm gasket made and connected together with the 3mm gasket ( For those of you asking why I didn't get a 5mm gasket made, they only make the "Unigraph Unimetal sheets in 1, 2 or 3mm that the gasket was cut from). When fit this cured the problem. Fuel pressure was reduced to 2psi (but only when up to running temperature, prior to this it hardly registered on the psi scale).

Once again for all of those that put forward suggestions, thank you. The full rebuild of my GT6 is ongoing and I have a few more questions / issues for which I will open new threads.

 

Kind regards and keep motoring!

 

[Stratton Jimmer]

Good to hear that you got a result.