Triple Mikuni thread

[gundee]

Still plugging away with this. 

Latest:

Pilot:  60 (1.5 turns)

Main air:  200

Main fuel:  140

Outer venturi:  34

Idle:  13.2 afr

3000rpm in 2nd (30mph):  13.4 afr

WOT in 2nd:  11.5 afr

Going to check my valvetrain tomorrow, and confirm a few other things while I am at it.  Surprised I am still a little rich with 140's for main fuel.  

I have a set of 37 outer venturis I could use.  What effect will switching to them have?  Presumably more air, which might help with the richness issue?  

 

I can say in my case, the 37mm venturis were undrivable on the street. Not enough velocity on low end to get the car moving.

Went with the 34mm. Took care of that problem.

[madkaw]

Need some ideas folks. I seem to be chasing my tail on my tune. My carbs will be great , steady idle , smooth thru the dreaded transition areas and then it changes, all in the same day. The car will die at idle arbitrarily. I can watch my AFRS go lean until she just seems to run out of gas.

The other day, probably the second day driving it the tune was amazing. Transition stayed right around 14 and it didn't lean out and felt powerful right thru the rpms- this is normal driving conditions. Then all the sudden I have lea out issues in transition.

New fuel pump, regulator set at 3psi.

So I guess I need to check my tank and lines for some kind of partial blockage?

[240260280z]

To verify fuel loss before carbs you can try a few things:

 

Hood open and drive while viewing the fuel flow in a clear plastic filter or temporary clear vinyl line inserted before the carbs.

 

Dead head the carbs so there is no return to tank.

 

The only other things I can think of to mildly consider:

 

ignition dropping

floats hanging

heat soak

clogged fuel strainers in carbs

pump failing/loose connection

Edited August 20, 2015 by Blue

[madkaw]

My gauge always seems to show good pressure and I run my set up dead headed.

What if I was getting a partial blockage on the supply between the tank and the regulator. Would this drive the pressure up but limit volume and my regulator would keep it at 3psi.

Thinking about just pulling the fuel line after the regulator and just watch it run for 10 minutes into a gallon container.

[Chickenman]

Is the fuel pressure gauge located where you can monitor it in real time? If not, temporarily install a second gauge and tape it to the cowl where you can see it while driving. That will take a lot of guess work away.

 

I suspect a fuel venting problem or fuel feed problem from the tank.

[madkaw]

I agree with your suspicions. The point I was trying to get confirmed was that the pressure gauge might not tell me much. If partial blockage raises pressure the regulator will just adjust, but am I getting the same volume of fuel? Is my reasoning sound or not. not venting also crossed my mind. I've tried to duplicate the venting system with my Jeep tank. Maybe my tank vent valve in the engine bay is not working.

[240260280z]

If the pump cant draw gas to push due to a build up of vacuum in the tank trying to pull the fuel back from the line,  the fuel bowls in the carbs will run empty (by the engine running) and the fuel pressure feeding them goes to 0 (rel. 1atm) as air goes back into the fuel lines from the bowls through the open needle valves.

[madkaw]

Checked fuel cap and didn't seem to vent easily, so I removed vent-no difference.

Time to check entire supply system

[gundee]

Checked fuel cap and didn't seem to vent easily, so I removed vent-no difference.

Time to check entire supply system

 

My issue turned out to be that holley regulator. It was restricting my fuel flow or pressure or both,

I don't know. But when I removed it, It solved my Mikuni problems. The car would loose power

like it was running out of fuel.

A  sledge hammer took care of that regulator. I made sure that was never going on another car.

I was a little pissed.

[madkaw]

Interesting.

I guess that will be suspect as well as everything else. It's interesting that my gauge will show 3 psi at the regulator after it stalls out from running out of gas. 

[RyanT67]

As I understand it, the Holley regulator regulates by cutting off flow, and in doing so you lose volume.  Your pump might be blasting out a healthy 60 gallons per hour, but after the regulator you might be getting only 30 GPH actually reaching the carbs.  I'm making up these numbers mind you...  But losing a good portion of flow isn't ideal all the same.  

I'm planning to switch to a bypass regulator next Spring.  Ideal route will be:
Fuel tank - Filter - Fuel pump - Filter - Carb - Carb - Carb - Fuel Pressure Gauge - Bypass Regulator - Tank

This way the flow and pressure will remain absolutely constant at the carbs, and any excess pressure beyond 3.5psi will bleed off back into the tank.  Seems like a more sensible solution.  

As to whether or not this is your issue though, couldn't say...

Are you running one of those LOUD fuel pumps?  If so, can you hear it well enough to discern whether or not it's running consistently?  If it is cutting out intermittently - overheating or wiring issues - that could explain your problem.  If you can't hear it well enough over your exhaust, you can maybe try recording it with your phone or something while driving.  

[madkaw]

Running an rx7 pump and it's quiet.

Changed whole system over to a Mr Gasket regulator (non bypass), and car ran no different. In fact , on the way home it kept acting like it was running out of gas- almost didn't make it home.

So I like your thinking Ryan. Stopping fuel flow to 3 carbs sounds like it wouldn't keep up with volume. Even today the car ran great WOT, but transition, idle was crap. So build up of pressure would happen at low demands so causing this type of regulator to shut off fuel. High demands at WOT the regulator just opens up.

Is this what your saying?

Lots to think about and research. Might need something better than a 40$ regulator.or maybe run a return