Inconsistent Fuel - tripple carb setup

[Hakosuka07]

Hello all,

 

Question for all you knowledge carb ppl. I bought a car with a triple mikuni setup and Im having some inconsistent fuel pressure issues. Shortly after I bought the car my Electric fuel pump went out so I replaced it with a NEW electric (Facet) fuel pump w/rating between 4-7psi. While I was replacing the fuel pump I decided to have my fuel tank boiled & sealed. I replaced all my rubber fuel lines blew out all my hard lines with compressed air & installed a fuel pressure gauge.

 

Well once all that was done I ran the car for a few hundred miles. I later realized the carb closest to the firewall was leaking. So I went out & purchased a MR gasket fuel pressure regulator. Before I installed it I read a few threads (on this site) where ppl stated to place the FPR on the return line not the supply line to the carbs. Well the way my carbs were setup they don’t have a return line that’s connected. So I installed it on my supply line to carbs.

 

Now Im new to the triple carb setup so please be patient with my ignorance. However Im a bit confused with a few different answers that I’ve read here on this site.

 

My main issue there are times when I have 4psi Fuel pressure & there are times when my gauge reads zero. If anyone could help understand the difference between FPR on the RTN line vs. supply directly to the carbs. Also possibly advise why Im getting inconsistent fuel pressure readings. I forgot to mention before I installed the FPR. The fuel pressure gauge would sometimes read 4psi & sometimes read zero. So this issue was present before the FRP was installed.

 

 

Any help, guidance is GREATLY appriciated.

Edited October 19, 2010 by Hakosuka07

[madkaw]

Not a Mikuni expert -BY FAR, but you didn't mention if this fuel pressure problem is affecting the engine performance? Could you just have a guage problem?

[Tony D]

The difference is how the regulator is supposed to regulate.

Some are merely restrictions, bascially a check valve that opens at a preset PSI to hold pressure there downstream. These are not really very accurate, and as they are actually a restriction to flow, could cause an issue if they fail by totally shutting off fuel to the engine.

 

Some are 'backpressure regulators' meant to restrict the fuel returning to the tank---this keeps the fuel pressure AT THE CARBURETTOR consistent regardless of drops between the pump and the carbs. This is preferred. If your fuel pump starts clogging, this type of regulator senses the pressure drop and closes off the return allowing more fuel to the carbs to keep fuel pressure constant.

 

As long as the fuel pump has sufficient volume and pressure capability to flow through the restriction, you will have consistent fuel pressure at the carbs. I used an OEM EFI pump for my fueling for years, and at times my nasty tank plugged the fuel filter pretty good, but I could still get proper fuel pressure at the carbs.

 

I'd get a GOOD GAUGE (not one from a mass marketer, something like a fuel pressure testing gauge---many vacuum gauges for idle vacuum will test mechanical fuel pressure to 7psi as well) and compare THAT to what you see on the gauge.

 

Like someone said, is it causing performance problems?

 

Many times you will see pressure go up after shutdown then drop to zero as the bowls boil out and the floats drop relieving fuel pressure in the line.

 

Dont' konw which facet you hvae ,but it's not uncommon to have to run three of them on a serious engine to supply fuel. For a stocker with carbs it should be O.K., but at higher rpms under load it may fall short especially if you have a restrictive regulator and not one on the return line.

 

You may want to consider regulating the return line from full size to some smaller diameter by installing a jet in it---say 0.050" or whatever and see how that affects your fuel pressure. If you can put the jet in the return line so that it's not over 4psi at idle (with your fpr set to 3.5psi) then that is all the return line area you will need. This will help on high end fuel pressure loss under load as unrestricted return lines can rob fuel from the carbs when the floats fully drop.

 

Good Luck.