checking fuel pressure

[jonbills]

Hi there,
when I was at Shakespeare county for that drag round, my car was misfiring in 3rd when it got up to around 5k rpm in 3rd (fine in 1st two gears)

I guessed it was down to fuel pressure (my pump was always marginal, and I'd just upped the idle jets).

I thought I'd proved the problem by sticking a pressure gauge betweeen carb 2 and 3 (per attachment)
and seeing the fuel pressure collapse to nothing on full throttle in any gear.

I bought a new pump, claiming to be a facet red top clone 12 Volt Electric , Electronic Fuel pump , | eBay and got a petrol king fuel regulator (this is what attached photo is actually of)

I was expecting with this to see a constant 2psi all the time, but I don't - its better than the old one, but fuel pressure falls by the time I've changed gear once at full throttle.
(no misfire though)

Eventually I get to a question!

Has anyone got experience of checking fuel pressure like this with a real facet red top? how does it behave? my Des Hammil book says it is possible to get constant 2psi under all conditions.

engine is a L24 with triple dellorto 45s, about 170 fw bhp.

Thanks!

Jon

 

[Rob Gaskin]

I've only checked my fuel pressure without the engine running.

Have you checked your fuel filter for blockage - old chestnut of mine

 

[jonbills]

Thanks Rob, yes, new fuel filter. I even tried with no fuel filter for one run round the block, just in case it was the restriction but made no difference.

 

[SKiddell]

Regulator should be in a non restricted return line to the tank not on the carb inlet

 

[jonbills]

ah, thanks, so with regulator on return line, and nominal 8 psi on the pump, you set the regulator to let through 6spi effectively, leaving a 2psi on the carb inlets?
is that how its meant to work?

 

[Mr.F]

I normally advise the Filter King V8 version with gauge port in conjunction with a genuine Red Top Facet for your application.

For carburetted systems the pressure is regulated and measured before inlet to the carbs. Most triple system don't even have a return line.
For injection systems the regulator is fitted at the end of the fuel rail and before the fuel return (which will always be present on an injection system AFAIAW)

 

[zbloke]

Jon,

You really should be aiming for 3>>3.5 psi, 2 is a bit low IMO

 

[rallymanDP]

Have you checked further back along the Fuel feed line to make sure that there is a good flow of fuel actually coming out of the Tank ?

Another common early 'Z' problem - ( partially blocked ) pickup pipe in the Tank - from scale & detritus falling down to the bottom over the years and collecting at the end of the pipe.

The Red Top in our experience is a superbly reliable piece of kit and I wouldn't think that it is the problem. Have you checked the Filter in the Pump itself for any blockage ?

 

[Rob Gaskin]

I normally advise the Filter King V8 version with gauge port in conjunction with a genuine Red Top Facet for your application.

For carburetted systems the pressure is regulated and measured before inlet to the carbs.

That's me, set at 3psi.

 

[moggy240]

i presume you are using braided fuel lines aswell

 

[jonbills]

Thanks everyone -
Duncan: I should have been more explicit: its not a real facet red top, just a clone claiming similar specs, and I'm questioning whether it's really up to it.
I did have the tank out to clean it a couple of years ago, and don't think that the pick up is blocked.

 

[SKiddell]

For carburetted systems the pressure is regulated and measured before inlet to the carbs. Most triple system don't even have a return line.

Maybe why there are so many **** poor triple setups around, many will benefit from one, even if its just taking heat away, for a pressure regulator to work correctly it needs to bleed pressure off somewhere otherwise its just acting as a flow restrictor which isnt the best way of controlling system pressure, a "dead end" system (no return line) can also result in an over pressure scenario (rapid throttle closure) which can flood a carb out, also many pumps are not designed to run a dead head of pressure and facets are one of the worse for not liking it.

 

[Rob Gaskin]

I still use my return flow.

Before I went to Spa in May I moved my pump to the rear and noticed that the return pipe was 'flexible where it comes through the rubber clamp above the diff nose. It was on the verge of rusting through! Lucky find it could have 'gone' at any time.

 

[Mr.F]

For reference, Speedflow's generic diagram of a simple fuel system for carbs:

Pressure regulator can include the pre-carb filter (as per Filter King - the clue is in the name). The bleed back to tank can be anywhere after the regulator, but end of the line preferred for triple carbs on a Z, and may need a restrictor (e.g. NOS jets are popular) to tune the system.
The implication is that not only does fuel keep moving in the lines to aid cooling, but any pulsing of pressure caused by regulation will be smoothed out - win / win.
Note that some pumps need a pre-filter (i.e. between tank and pump), but the Facet Red Top has a built-in pre-filter.
Injection pumps, particularly the aftermarket roller vane types, MUST run some pre-pump filtration - tank debris can destroy an injection pump in a very short time.

 

[jonbills]

Thanks Mike & Steve. That gives me something to think about and try. I'll start with figuring where I put. My 3rd 'T' banjo union.