Mikuni Triples Fuel pressure??????

[justaZcarguy]

On the '74 I have triple mikunis...

Those of you that are running this or any triple setup, what are you running for fuel pressure? I assume that since these are not a draft style carb, the pressure is important as far as performance goes, not just keeping the bowls full....

At a sustained rev (3k RPM) my pressure is dropping to 2 LBS or so and at an idle it's at 5 LBS. I think that the motor is low on pressure fuel wise which is contributing to performance issues as far as demand goes, I have a great pull in the first few seconds then the engine seems to "drop off" a bit.

Any suggestions as far as fuel set up goes? I am currently running an electric pump that has a rating of around 3-5 LBS.

I think that I need more but I don't want to blow past the seats...

Ideas or suggestions?

Thanks and have a great day!!!

~Brian

[LanceM]

Your problem is flow not pressure. Your pump can not flow enough in GPH to keep the pressure up. 4 PSI should be plenty, 5 the upper limit otherwise you will force fuel past the needle valves when the bowls are full.

What you are seeing at 3K and 2 PSI is that the pump can not handle the flow necessary to keep the pressure up to 4 PSI. I would recommend getting a higher volume pump and regulate it at 4 PSI.

[2-4-T-Z-MAN]

I have the same setup on my 240.

Lance is correct.

The pump is not able to keep up with the demand.

[justaZcarguy]

So if I go with a rotary style Carter max of 8 LBS part number p4594 and regulate it at 4 or 5 lbs I should be good?

I hate that max of 8 lbs, it doesn't state an average flow...the GPH is 72 by the way....

Thanks for the info...

By the way 2-4-t, what set up are you using?

[HS30-H]

Another possible scenario:

Your hard fuel line ( running from the tank to the front of the car ) could be compacted with years of crud. I've seen this on many cars that I have taken spares from in the past. My current project car was a great example; when I took off the hard fuel line and snipped it into sections I found enough striated deposits to satisfy a geologist!

This would allow some fuel to get to the front of the car, and about enough to build a bit of pressure, but it would not allow the volume that you need.

I've seen this any number of times. Even blowing down the line with compressed air didn't really show up the problem ( the crud was compacted and would not blow out - but air was still coming through the line ). You might even want to consider replacing the hard line if you consider that its particularly bad, and cleaning out the fuel tank itself - which is where most of the crud comes from in the first place.

Good luck!

Alan T.

[2-4-T-Z-MAN]

My 240 is equiped very similar to yours.

I have 40mm mikunis, rejetted for my L28 with K& N filters.

An msd ignition, with 6-2-1 headers.

When I built the engine I left it pretty stock, only boring it 20over to clean the cylinder walls.

Prior to everything just mentioned other than the rebuild I was using a little solid state fuel pump that was fine until I installed K&N's on my SU's. At that point I bought a new fuel pump and pressure regulator.

When I installed the Mikunis I had to adjust the flow because I was having the same problem you described.

[justaZcarguy]

What are you running for pounds setting?

[justaZcarguy]

And thanks Alan! I am hoping that is not the problem....:sick:

[Zvoiture]

Mikuni's like 3 lbs. But you gotta keep them fed. This is the cheapest pump I have found for them http://store.summitracing.com/partdetail.asp?part=CRT%2DP4594 . And that is with a clean (preferably 1/4) system as Alan suggests. Other than that, find yourself a Red or Blue.

steve

[2-4-T-Z-MAN]

Justazcarguy,

I have a carter race pump, just because I got a good deal on it.

The regulator is set at 3.5 lbs.

If you are running a mild cam you might need more.

Good luck.