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madkaw

Triple Mikuni thread

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Hey fellas!  Just stumbled upon this thread and the info is outstanding!  Just curious if anyone has continued on with their tuning projects this year?  I'd love to see some updates.

 

Some things I've been curious about recently are how people are fuelling the carbs... where are the lines ran, pressures, distribution blocks, regulators, mounting locations, return vs. no return line... etc. etc.  So I've been scouring the interwebs looking at engine bay pictures. 

 

I picked up my setup as a complete unit that the previous owner was swapping out for a RB25 turbo project.  It ran great when he pulled it out and it ran great, and still runs great, after I dropped it in.  Rebello 3.2L stroker w/ 44's.  I'd did the swap right before moving to Florida in 2013 and since then I have had very little chance to drive or tinker with it.  Between the humidity here in the FL swamps, absolutely horrible drivers and high car theft and vandalism, I only occasionally take it for short cruises... which is killing me slowly.  However, with a change in job and location this summer things should start to free up for me to get back at it :)

 

The other day I ran into my first starting issue though... she had to cool down after getting fuel... vapor lock I'm thinking?

 

Anyway, thanks for all the time you guys have put into this extremely valuable thread, and as soon as I have something useful to contribute I'll make sure to post.

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I was wondering how that swap was going. I met you at C&C in Indy.

My car started better with a rich idle mixture. My Mikuni's are dead headed at the carbs, no return line. Holley regulator. I think it's all in this thread.

Glad to hear from you. 

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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?  

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So my engines apart and I'm wanting to check the carbs over before putting them back on. Since I was having running issues with my engine I really wanted to eliminate the Mikunis as the source. Everything checked out on the carbs, but I was suspicious of throttle plate damage from slamming the throttle(maybe a few times:o)

Looking down the barrels with back lite I tried to eyeball the light making it through and see if the openings were the same. Checked for looseness in the throttle plates also. Some looked suspect, but still not sure how to measure the opening.

Then I had an idea(careful now), what I checked the air balance with the carb off the engine. If I had a consistant suction I could use my unisync to measure any differences. So I dragged out the Oreck( my wife got excited for a second), and I hooked it up to each carb and measured. If this is an accurate test, then my carbs were pretty spot on.

I plan on stteing all my idle screws using the vacuum. At least I will know they are all set equal. never did like using the feeler gauge

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So I finished tweeking the carbs with the Oreck and quick put it back in the closet before she got ideas!

I would suggest a fresh bag in the cleaner before attempting this. I noticed that some of my readings were fluxuating as if the suction was varying. I pulled the bag to see it was very, very full. So I got a new bag and proceeded. I did find one carb that seem to have a tweeked throttle shaft. A few yanks on the opposite throttle plate nuts and she dialed in.

I moved on to slimming down my vacuum log. ive been trying to come up with a way to eliminate the hoses running over my carbs. I think this set-up will work and look much better.

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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.

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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?

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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 by Blue

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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.

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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.

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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.

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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.

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

Time to check entire supply system

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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.

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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. 

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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.  

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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

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99.99% sure it's fuel. haven't pulled the plugs but I'm pretty sure what I will find.

I'm running my MS which turns the pump on and off.

Not sure why it got so bad so there is much to check, but had to walk away for while. 

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Wow, so many different ways to do one task. Been all over looking at triple fuel line routing and it's hard to compare anything. Racetep recommends Carter fuel pump and No regulator. Some run regulator to deadhead, some regulator with return . Some say you have to have a return, others say no.

Then you throw in the variable that everyone's car runs perfect on their set up (whether it does or doesnt). Other variables like what pump they are using .

I think there is something to be said that most FI use a regulator after the injectors and use a return. I think that's the route I'm going .

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I ran a fuel pressure test right at each carb with no return line and got 3.5 psi.

I then did the same test with the return line hooked up and got 3.25 psi.

I decided then to leave my return line hooked up. Continuous cool fuel supply that way.

 

This is without  a fuel pressure regulator of course.

Edited by lumens

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Well it wasn't terrible, but the pressure wasn't stable. At WOT pressure dropped to 2psi. Pressure was also shaky in transitional areas. Upon looking at the numbers on my fuel pump I realized that I had installed (and recommended-sorry) a pump that probably is slightly to small in volume. The rx7 pump was made for a 1.2 liter rotary and requires only 15 gph(2-4psi)

Maybe sufficient for a stock Z with SU'S , but not enough for my engine.

I have ordered a Carter p60504 to replace my pump. It's rated 4psi at 30 gph.

Reading Racetep ' s page they run Carters with 4psi and no regulator on all there set ups.

Waiting on the pump and leaving the pressure gauge in place to see the changes.

I will say that after adjusting my Holley regulator up right to 4 psi the car ran really well- EXCEPT it stumbles in transition sometimes and I can watch the wideband go lean. I'm thinking -hoping - the increased flow with help keep float levels at optimum level to combat my lean issue.

We will see

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