fuel pump me thinks ???

Lushman · Post by **Lushman** » Sun Apr 15, 2007 8:34 pm

I have recently replaced the tired old 3.5 V8 in my capri with a 3.9 V8 which been fettled with.

see picture and Spec.

3.9l v8
Compression ratio 9.35:1
Heads Skimmed 1mm and Ported
Valve Guides Shorteneted
Valve Backs Machined for narrow Seats
Piper Vernier Timing set
MGBV8 Oil Pump Spring
New Cam bearings
New Mains and Big end bearings
ARP ROd Bolts
Oil Gallery Restrictors for rockers in block
TVR Griffith Cam, followers and Clutch
Double Valve Springs + crane retainers
Offenhauser Manifold
Holley 390 Vac Secondary

I am having a problem with the motor not wanting to rev much past 4k

I think the main cause of this is the fuel pump which is facet 574a and looking at the details it's to about 4psi. prior to the engine replace I had no problems with it. but then it was feeding twin SUs on a 3.5 V8.

I have been informed to go get a red top holley pump to solve this issue. but wondered what other recommendations anyone suggests. I was told a red top should not need a regulator either (where as blue top does).

Any suggestions and advice welcome

Thanks

Lushman

Alley Kat · Post by **Alley Kat** » Sun Apr 15, 2007 8:52 pm

Red top is I think 7psi, mine read 6psi at the carb so didn't bother with a reg. It was noisy despite being rubber mounted but some guys seem to have quiet ones (recent thread about that).
Motor looks very nice btw....

Lushman · Post by **Lushman** » Sun Apr 15, 2007 9:04 pm

Cheers Alley.

My facet is fitted on rubbers in the boot of the capri, dunno why it was there when I bought the car.

My friend runs 2 of these pumps on his GT40 replica and his are very noisy.

Sondar · Post by **Sondar** » Mon Apr 16, 2007 4:35 am

4 psi is probably a bit low, but these carbs need volume flow more than pressure so I guess it depends if the pump can maintain sufficient volume flow. Can you put a pressure guage on it, or get someone else to check it for you?

I have a Holley Red on my 4.6. It's supposedly factory pre-set at 7 psi - I don't know how sensitive the Holley carb is to pressure but I've got an Edelbrock Performer which specifically states that pressure must not exceed 6.5 psi & should ideally be set at 5.5, so I'm fitting a regulator to do that.

I'm a bit surprised that your new new motor runs out of steam shortly after 4k if the 3.5 showed no problems with the same pump. That's only 2/3 of the rev range for a relatively small increase in capacity (albeit with the other goodies too). Are you sure that the ignition side of things is all good?

Does it pull strongly to 4k & then basically stop pulling or does it rev to 5k+ but slower. What's max rpm under load (say 4th gear - yeah, I know, it's illegal...)?

Eliot · Post by **Eliot** » Mon Apr 16, 2007 6:28 am

i had a similar problem. When you get to 4k, just back of a little or coast - to allow the float chambers to refill, then floor it. if it goes for a bit - then you know that the pump cant keep up.

Lushman · Post by **Lushman** » Mon Apr 16, 2007 8:14 pm

Thanks guys.

The ignition timing has been setup at 26 degrees before TDC at approx 4k (without vacumm advance).
The car is also running a little rich as well and has only just been rebuilt.
the engine seems to revs fine to just over 4k about 4200, but stutters any higher.

JP. · Post by **JP.** » Tue Apr 17, 2007 7:15 pm

Are you shure the vacuum secondairies of the Holley go open.
The vacuum secondairies of the Holley are controled by the engines vacuum so to speek. If you change cam you also change engine vacuum and so the vacuum reference of the secondairies. Changing the vacuum secondairies spring for example a lighter one may do the trick.

Lushman · Post by **Lushman** » Sun Apr 22, 2007 8:07 pm

JP. wrote:Are you shure the vacuum secondairies of the Holley go open.
The vacuum secondairies of the Holley are controled by the engines vacuum so to speek. If you change cam you also change engine vacuum and so the vacuum reference of the secondairies. Changing the vacuum secondairies spring for example a lighter one may do the trick.

Thanks Jp, is there an easy way to check this. More recently I ahve found that if I let off the throttle it will rev to 6k but will back fire as soon you press the throttle to the floor over 4.5k.

Looking at the carb, the vacuum pipe fits to the front left of the carb so it may be using the primary don't know where it should attach if not where it is.

Thanks

Tahnks