View Full Version : Carbs, fuel pump or poltergeist?

08-07-2005, 08:27 PM
re. : Coil Problems...
It's been two weeks since I last posted about the problems I've been having. In that time I've dismantled both Webers and thoroughly cleaned them, changed out the jets and re-set the floats. Before putting them back on the car, I checked for anything loose around the manifolds and found a stud I had forgotten to tighten. ALL of the studs are now tight. I replaced the coil with a new one. With these changes I felt confident I could finally enjoy the summer driving season. Not to be. I took the car out Friday afternoon for a test drive, went about 12 miles without a hitch. Decided I would try to take it on a weekend trip (my wife took a seperate car, smart woman that she is) and 8 miles into the trip it started bucking so hard I doubted I would make it back home, yet it would idle fine whenever I got to a stop light. I'm convinced I have a fuel delivery problem.
So I have a number of questions about the best way to deal with this (by the way, the fuel filter is being changed out tomorrow): Should I re-build the fuel pump even though it looks fine (no tears in the diaphram, screen clean, seems to have plenty of suction when I place my finger over the intake hole) or replace it with an electric pump (Redline recommends a Carter Pump; 60-70 GPH @ 4 psi and no regulator)? If I use the electric, how do I blank off the hole where the mechanical pump mounted? I remember hearing years ago, in the age of carburators, of "vapor lock". Could this be a problem and could it be caused by the heat from the headers since it seems to happen only when the car is warm? On both occasions, the car ran fine on test runs for 8-12 miles. An hour later, I get in it to go somewhere and it hesitates after 4-5 miles of driving. The Pierce manifolds have a passage way for fitting coolant hoses in order to reduce the effects of heat in the engine compartment on the fuel. I have done this, but I wonder if the headers are adding more heat to the equation? Would it be a good idea to try to fabricate a heat shield? Has anyone tried it and has it worked? The only other thing I can think of would be a poltergeist, but I never heard of one bugging a Triumph...

08-07-2005, 10:11 PM
it's got to be Joe's ghost. Have you called a priest?

08-07-2005, 10:27 PM
Hi Dave,

Yes, it sounds like a fuel starvation, vaporization or vapor lock issue. You are on the right track checking for anything that might plug up the jets in the carbs or interrupt flow from the fuel tank.

Sure, go ahead and change the fuel filter. Look for any corrosion or particlates coming out of the fuel lines, perhaps from the tank or metal lines. However, since the car recovers at idle and after it cools down, I think a blockage is unlikely.

More likely is heat related vapor lock or actual overheating of the fuel in the float bowls of the carbs.

If I recall correctly, those are downdraft Webers, aren't they? I'm more familiar with sidedraft Webers, but yes it could be a problem with overheated fuel, especially with headers.

Headers radiate a lot more heat into the engine compartment than the original cast exhaust manifold. A heat shield shoudl be installed to help protect the carbs. Fabricate a thin piece of steel, painted silver, or stainless steel if you wish, cut and shaped to provide a heat-reflective barrier between the headers and the carbs. It can be mounted by tabs under several mounting points and hose clamps can be used. Just be sure there is some air space between the shield and the carbs and betweent the shield and the headers.

Only if your headers are stainless steel, they might be wrapped with high temp cloth tape to reduces underhood temps and help gas flow within the header. Mild steel *can* be wrapped, but only partially (70% coverage max or they will have a fairly short life in the high temps). Often ceramic coating is a better choice for mild steel headers. WIth wrapping or ceramic coating a heat shield might not be necessary.

Coolant hoses to the manifold, such as you mention, are generally for the opposite purpose: to warm things up in the Winter. Have you talked with or emailed Pierce for more info about that particular manifold? Their website is www.piercemanifolds.com (https://www.piercemanifolds.com)

I can tell you that sidedraft Webers are very fussy about proper fuel pressure and a good, steady supply. They use more fuel than SUs or Strombergs. For that reason, I think an electric fuel pump is a good idea. Facet makes one that's quite small and can be hidden, if you wish. Just be sure to get the low pressure model.

For sidedrafts, a separate fuel pressure regulator *should* be installed close to the carbs to insure 3 to 3.5 psi maximum. Mount the fuel pump below the lowest point in the fuel tank. A lot of folks put it in the corner of the spare tire well in the trunk. That means running hoses in and out of the trunk. On my TR4, I instead mounted the Facet on the inboard side of the LH shock bracket (your IRS might not allow for that). Facets should be mounted on shock proof rubber mountings, too.

It's pretty easy to fabricate a blanking plate for the hole in the crankcase where the original fuel pump sits. Just get a thick & rigid piece of steel or aluminum, cut out the shape of base of the original fuel pump and drill a couple holes. You'll need a new gasket. In fact, that gasket might make a good template for the blanking plate, or mock up a template with lightweight cardboard. Alternatively, for originality some folks like to just keep the mechanical pump in place, but modify and disable it so that the electric pump just pushes fuel straight through.

I'm not sure if downdraft are, too, but sidedraft Webers are fussy about float position. You might see if you can find anything out about this, specific to your carb model. There are also various needle valve sizes, be sure you have the right ones. Too small would lead to delayed fuel starvation such as you describe.

Hope this helps!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

08-10-2005, 01:41 PM
I replaced the fuel filter last night and cut the old one open to see how dirty it was- there wasn't enough dirt in there to fill a super model's belly button! Same with the filter inside the fuel pump. I then tested fuel pump pressure and got a reading of 2.5 psi-right on spec according to the Bentley manual. I didn't get a chance to road test it, although I will tonight. I did notice while I was doing the pressure test that the car ran out of gas fairly quickly. Those fuel bowls hold a fair amount of fuel and I would have thought the car would run five, ten minutes before running out. It only took about two. Is it the fuel pressure that feeds the fuel into the carbs? And if the pressure is too low will that cause fuel starvation? Or is the fuel vacuum fed into the carbs and I only need enough pressure to get the fuel into the floats?

Geo Hahn
08-10-2005, 01:53 PM
...I then tested fuel pump pressure and got a reading of 2.5 psi-right on spec according to the Bentley manual...

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Does the Bentley manual have that spec for the Webers you are using? Or are you quoting the spec for SUs?

I carry an electric pump as a spare for fast hook-up when the mechanical poops out -- could also be used to test for weak pump or vapor lock if I ever thought I had those problems.

Guess you really need advice on fuel delivery requirements for you Webers as you may have to go to an electric with a reliable pressure regulator.

An 8 Mile drive seems unlikely to have induced vapor lock.

08-10-2005, 03:16 PM
Yes, Geo, I was quoting the specs for Z-S carbs. A website for a supplier of Weber carbs says the fuel pressure should be around 3.5-4 psi for all Webers. That's why my question on fuel pump pressure above. If it's there only to get gas to the floats and engine vacuum takes over to feed the fuel to the combustion chambers, then 2.5 psi should be fine, wouldn't you think? But, if the pump pressure also needs to feed the carbs then I could see where a higher pressure would be necessary.
As for the vapor lock, the 1st time the hesitation started it happened only a mile into the drive, after having sat for a couple of hours after an 8 mile test run. The second time, nearly 8-10 miles into a drive that was nearly one hour after a 12 mile test run at speeds approaching 80 mph.

Geo Hahn
08-10-2005, 05:02 PM
Here's a site about Webers on a 4-pot TR that may provide some useful info... certainly an extensive study.


It seems to suggest that our mechanical pumps are adequate (at least for the application described). I have always just checked fuel pressure when deciding if a pump had a problem -- not sure how flow (gph) enters into it or whether your Webers need more volume at speed than SUs would.

Dave Russell
08-10-2005, 05:28 PM
not sure how flow (gph) enters into it or whether your Webers need more volume at speed than SUs would.

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The type of carb, within reasonable air/fuel ratio limits, has no bearing on the required fuel flow.

A general rule of thumb is that an engine requires 0.6 pounds of fuel per hour per horsepower. This is called "specific fuel consumption". This number obviously will vary somewhat with engine design & efficiency.

At about 6 pounds per gallon for gasoline, the engine would require 0.1 gallons per hour per horsepower.
Therefore, a 100 hp engine at full power output would require 10 gallons per hour fuel flow, 150 hp would require 15 gph etc.