How do you determine the fuel pressure setting?
 

Hi all,
With a 13B Extend Port N/A 48IDA carb, I am guessing I need to have pressure set at 3.5 PSI.

What are the symptoms if this is:
A. Too low
B. Too high?

Thanks

Too high:
- bad idle (rough or high)
- floods easily
- stinks
- flames out the exhaust on decel

Too low
- runs lean or hot
- plugs might be white after a high rpm run with shutoff
- will idle fine and run fine up to certain rpms/loads and will then stumble

Also, if the level is too high, it can cause dripping from the venturis. It may also affect the progression. If it's too low, it may have an increased tendency to stumble around corners.

Thanks guys,
My issue is it "hesitates" for a split second when I put the boot down - usually 2nd or 3rd gear. It is fumey / smelly on start up and idles rough when cold. So, as I have no fuel pressure regulator (soon to be fitted), I will see if this changes anything.

Do you think 4 PSI is too high as a starting point?
(I ask as my intended reg is the Mallory 4207 with a range is 4 - 12 PSI.)

Sounds like it may be running with too much pressure. It messes up the progression circuit when its high.

4 psi. might be on the edge but I don't run a weber, so maybe someone with
clue can port up.

I run 4.25 PSI base pressure with my DCOE, but IDAs may want something different.

Racing Beat recommends 4.5psi on an IDA, so going a little high probably won't hurt anything.
Going from memory I'm running somewhere between 4 and 4.5 on my 51IDA on a full bridge. Can't remember the jetting for the life of me however.
Sounds like 4 is a pretty safe starting point.

I run 4.5 on my DCOE base but it drops to 3.5 under accel. It still stumbles a bit off the line if you just crack the throttle, I found revving it enough to pump the accelerator pump works great.

Thanks guys.
Great info provided.

one thing to keep in mind is the accuracy of the gauge you use. i learned a long time ago (i think it was on Paul Yaw's site) that many popular gauges are notoriously inaccurate. years ago, when i was tuning my Dell'Orto, i paid much closer attention to the engine and what it was telling me, as opposed to the gauge. if i recall, the engine pretty much liked to be about 1 PSI (indicated) lower than what was recommended by RB.

buy this one....http://mrcmfg.com/catalog/product_in..._id=163...your best bet for accuracy

+1 on the guage.

Factoid: Most guages are only accurate in the center 30% of their range.

I have a 0-15 psi guage from Summit, liquid filled, nice guage but its notoriously
inaccurate for measuring 2.5 psi. I set it using the engine running and let it
out until I hear the rpms pickup and then back it down a bit. It indicates
around 2 to 3 at this point but I can't depend on it being the same from
one time to the next. The 0-5 psi guage from ReSpeed is the best, I may get
one when I go down that way for the Mitty.

The plot thickens...
I have just installed the Mallory 4207 Fuel pressure regulator with matching Mallory gauge. This regulator has a range of 4 - 12 PSI. It was installed exactly as per instructions - super easy!

How come I can set it for 2 PSI?

I have set the Reg at 4 PSI which is half way in the Reg adjustment. (I expected it would be at the minimum setting!)
Whilst it serves the purpose set at 4 PSI, I am now wondering if the fuel pump (which I was told is 7 PSI) is actually running alot less PSI????
Car ran well before hand and the reg was filled as a precaution.
Am I missing anything?
Is the reg working?

My Mallory 4309 is rated for 3-12 PSI base pressure, but I can get it down to ~1 PSI. And that's with a pump designed for 40 PSI.

+3 for the gauge from Respeed. If you don't really know what the pressure is, then you can't make an accurate adjustment.

Good luck...




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