TR4/4A TR4 Fuel Pressure

[TR4]

I am getting a little overflow at the carbs on my TR4. A mechanical pump was installed about 6 years ago and it just started this problem. Is there a pressure range that I can go by if I check that? I know it should not be more than 3 lbs but wonder how low it could go and still be within a good pressure range. My guess would be no lower than 2-1/2 lbs ?

 

[Lou Metelko]

If this overflow just started then there is nothing wrong with the amount of pressure. Your problem is that a piece of something is not allowing the float needle to close. Take both float lids off and lung blow out the needle. To be sure it does not happen again just replace all of the rubber fuel lines. Although the rubber lines look OK, they are deteriorating on the inside and flaking off.

Lou Metelko
Auburn, Indiana

 

[Geo Hahn]

...Take both float lids off and lung blow out the needle...

Or you may find you have Grose Jets (little brass balls instead of the needle). Same idea, clean 'em and replace lines (though some may suggest removing the Grose Jets and seeing how far you can throw them).

 

[TR4]

I should have added, both carbs were just rebuilt by Paltech and I replaced all the rubber hoses and the fuel filter. I cleaned out the inside of the rubber hoses before I installed them. I talked to Jeff and he feels I have a fuel pressure issue. The rear carb was leaking before I sent it to Jeff so I am in agreement that it appears I have a pressure issue. He thinks the fuel is coming out the choke spindle since that is not sealed. I will get a pressure guage to see what the pump is putting out and let all here know.

 

[Darrell_Walker]

I recall the recommended pressure range for SUs to be 1.5 to 3.5 lbs. I don't, however, remember where I saw that.

 

[Geo Hahn]

Reading from the Book of Haynes, Chapter 3... 1.25 to 2.5#.

 

[TR3driver]

My TR4 Bentley also gives it as 1.25 to 2.5. My TR3A with SU H6 seemed to run fine with only 1 psi, measured at the front carb inlet at full load (it was higher at idle).

Although not likely, one possibility if you have plastic floats is that the floats have become porous and absorbed fuel (making them heavy). I had a lot of trouble with that on an old motorhome, back when CA started putting MTBE in our fuel. And I see where Burlen is bragging about their new ethanol-proof floats ...
https://www.sucarb.co.uk/ProductDetail.aspx?Id=33997

 

[mtlman8]

did you replace the throttle shafts? It seems more likley to leak their than the choke shaft.

 

[TR4]

Yes, Paltech installed new throttle shaft bushings as well as new throttle shafts. I had rebuit these carbs myself about 8 years ago and they worked OK but when the rear carb started leaking, I decided to have Paltech redo them because the carb bodies needed bushings and I did not want to take on that job. I realize now the leaking might have been a fuel pressure issue but I am still very happy to have treated my car to the rebuild. Jeff at Paltech does an incredible job.

 

[TexasKnucklehead]

Reading from the Book of Haynes, Chapter 3... 1.25 to 2.5#.

Did someone say Monty Python?

...and the poundage presented to the carbs will be from one and a quarter to two and a half, no more, no lees. Thy pressure shall be inclusive from one and a quarter to two and a half. Two and three quarters is right out! Less than one and a quarter (even if Randall claims will work) is outside the printed limitations and four thou shalt not attempt...

 

[tdskip]

Reading from the Book of Haynes, Chapter 3... 1.25 to 2.5#.

LOL

 

[tdskip]

Reading from the Book of Haynes, Chapter 3... 1.25 to 2.5#.

Did someone say Monty Python?

...and the poundage presented to the carbs will be from one and a quarter to two and a half, no more, no lees. Thy pressure shall be inclusive from one and a quarter to two and a half. Two and three quarters is right out! Less than one and a quarter (even if Randall claims will work) is outside the printed limitations and four thou shalt not attempt...

Except if it floats, then it is a witch...

 

[HerronScott]

Reading from the Book of Haynes, Chapter 3... 1.25 to 2.5#.

Did someone say Monty Python?

...and the poundage presented to the carbs will be from one and a quarter to two and a half, no more, no lees. Thy pressure shall be inclusive from one and a quarter to two and a half. Two and three quarters is right out! Less than one and a quarter (even if Randall claims will work) is outside the printed limitations and four thou shalt not attempt...

:lol:

Sounds like we need the Holy Fuel Pump of Antioch?

And Saint Attila raised the fuel pump up on high, saying, "O Lord, bless this thy fuel pump, that with it thou mayst blow thine enemies away at stop lights, in thy mercy." And the Lord did grin. And the people did feast upon the lambs and sloths, and carp and anchovies, and orangutans and breakfast cereals, and fruit-bats and large chu...

Scott

 

[KVH]

I'm constantly learning. Excessive pressure from a mechanical pump would be something I've never heard of on a TR4.

When my carbs kept leaking at the jets, it was due to old deteriorating fuel hoses. You say you added new ones and cleaned yours. Mine must have been crumbling because tiny specs of rubber, so small I couldn't at first imagine them being the cause, were clogging the jets.

I'd spent hours adjusting floats switching back and forth to grose jets, etc. before I just replaced the hoses, added a filter, and went on.

If your filter is near the carbs, and you have new hoses, I guess that wasn't your problem.

Let us know how it works out.

 

[KVH]

By the way, can someone here explain the principle behind the floats? What is going on there? Why a float, and why an air vent at the top of the chambers?

 

[Roger]

There's a float for the same reason you have a float in your toilet cistern - it works on the identical principle.
The air vent is to let the air out when the fuel comes in, and to let the air in when the fuel goes out. It keeps the inside of the float chamber at atmosheric pressure.

 

[TR3driver]

To amplify just a bit, the carburetor pulls fuel through the metering jet with a relatively small amount of suction (formed by the air passing through the venturi). Thus the level of fuel relative to the jet and the air pressure above the fuel have to be held within close limits to get the proper fuel/air mixture. The float valve controls the level, the vent provides the right pressure.

 

[TR3driver]

I'm constantly learning. Excessive pressure from a mechanical pump would be something I've never heard of on a TR4.

It does happen from time to time. Possible causes include having the wrong spring in the (rebuilt) fuel pump, or possibly the pump not assembled properly.

And sometimes the problem doesn't show up for a long time, since the amount of pressure that the carb can actually tolerate depends on several factors, including fuel density (and the fuel supplied in most of the US today is less dense today than what these carbs were designed for).
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Mine must have been crumbling because tiny specs of rubber, so small I couldn't at first imagine them being the cause, were clogging the jets.[/QUOTE]
Also a very common problem. Ethanol (or MTBE) will attack the original hoses, causing them to shed those tiny bits into the fuel. And pretty much anything that gets caught in the float valve will block it open and lead to overflow.

I regard the soft fuel lines as replaceable maintenance items anyway; IMO they should be replaced at least every 20 years or so. I do mine every decade, whether they need it or not.

 

[Geo Hahn]

...Although not likely, one possibility if you have plastic floats is that the floats have become porous and absorbed fuel (making them heavy)... And I see where Burlen is bragging about their new ethanol-proof floats ...https://www.sucarb.co.uk/ProductDetail.aspx?Id=33997

Looking at the OPs profile I see this is a 64 TR4 which will probably have Strombergs with that unique float (used only briefly & only on some TR4s) which was NLA for many years. They are now available but it is quite possible they did not get replaced during the rebuild as they were made of unbtainium until recently.

 

[TR4]

The floats in my carbs are metal if my memory serves me right. I shook them to see if there was any fuel in them when I rebuilt the carbs in 2003.