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TR2/3/3A Is 5.5 lbs too much fuel pressure?

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TuffTR250

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My TR3A has a TR4A engine but with SU H4 carbs. Today I measured the fuel pressure using my Harbor Freight Vacuum/Fuel pressure gauge. It measured a constant steady 5.5 lbs. The fuel pump is from TRF and about 2 years old. I connected the gauge to the fuel line via the 1/4 inch hose I use to connect the fuel line to the front float bowl. In a BCF forum entry from quite a while back I saw several comments that Bentley and Haynes both say it should be 1.25 to 2.5. Will the 5.5 lbs of pressure cause a problem? And if yes, what kinds of problems would it cause? Thanks!
Regards,
Bob
 
Since you aren't having problems, and especially considering the source of the gauge, I would look for independent confirmation before doing anything else.

Too much pressure can cause the float valve to overpower the floats, and let too much fuel into the chamber. The usual symptom is fuel pouring out of the bowl vents, but it can also have more subtle effects like fuel running down the carb throat.

Since both the pressure and the floats' ability to resist it can vary somewhat, this may happen while you are driving down the road. When it happened to me (in someone else's TR3A), there was a storm blowing over and apparently the change in barometric pressure triggered the problem. Anyway, both carbs started dumping fuel out the overflow, the smell of raw fuel along with the engine running just a bit rough was what prompted me to pull over.
 
Thanks Randall! Out of curiosity I'll check to see if I'm getting any fuel running down the throats of the carbs. I have not had a problem with the float bowls dumping fuel.
Regards,
Bob
 
5.5 psi is quite a bit over the factory spec., but the accuracy of a gauge from HF might be suspect. You might compare it with a gauge from another source. A person on another forum suggests using thicker gaskets between the pump and block to reduce pressure. I can't confirm it, but might be worth a try if the pressure is excessive.
Berry
 
IF you have slightly too much pressure you will have a rough idle and will be unable to lean it out with the adjustment nut, since you have a higher fuel level in the jet. If you suspect this merely pinch the fuel supply hose off as the engine idles , it should smooth out as it starts to run low on fuel. Correct if necessary.
 
Thanks for the advice. It does seem to be running a bit rough. I'll try pinching the fuel line when it's well warmed up and see if that has any effect.
Regards,
Bob
 
I once had an inexpensive electric pump (which according to the specs had a flow rating of about 12 gallons per hour, so more than plenty) with a Holley fuel pressure regulator set at 3.5psi. This combination ran rich at idle and low revs, but very lean at higher revs and wider throttle openings. (I checked air/fuel ratio on a dyno.) I had richer needles in the carbs already, and changed to a larger internal diameter fuel line without solving the problem. It was so lean it would misfire above 4,500rpm.
I now use an SU electric pump with internal pressure regulation, and that has solved the problem.
Hope this helps, though your situation may be different.
 
Simon TR4a said:
I once had an inexpensive electric pump (which according to the specs had a flow rating of about 12 gallons per hour, so more than plenty) with a Holley fuel pressure regulator set at 3.5psi. This combination ran rich at idle and low revs, but very lean at higher revs and wider throttle openings.
Click to expand...
May have been a problem with the flow rate through the regulator. They usually have fairly small openings in the valve, which translates to needing a fairly high pressure differential to deliver a good flow rate. Unfortunately, they often don't tell you that part.
 
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