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Triple Mikuni Questions


tauh

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I have triple mikuni 44's ready to go on my 240Z. They have the external float adjustment. I rebuilt them but have some questions about setup. On the bottom of the carbs are some plates that fit under the accelerator pump. One of these plates has 2 vacuum fittings (I think) on each carb. I can not find these fittings anywhere in the books or printouts that I have. Anyone know what they are for?

 

The starter discs on mine are attached to a spring that holds them all the way to the left (looking from the carbs toward the intake). In reading, it seems that all the way to the left is full choke and that under normal driving (after warmup) the discs should be all the way to the right. Why would you spring something where the normal position puts pressure on the spring? Does not make sense to me or am I mistaken about the disc position.

 

If this is covered somewhere else, please direct me to the right place.

 

Thanks

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The fittings under the carb are for running a coolant (not the engine coolant) through. some have plumbed the fuel return through them to keep the carb bodies cool. I never used them when I was running Mikunis.

 

All the way to the left is closed, i.e. inopperative. These are not chokes, but a separate circuit for starting. I found that I never needed them, just pumped the throttle to use the accel pump when cold.

 

Sam

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They indeed are for using fuel cooling of the bottom of the bodies. It helps quite a bit with fuel percolation on hotter days. The Mikuini setup states they are fuel coolers, not coolant lines. You use fuel as the coolant. The tank is your sink for heat.

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That is their design function. It keeps the fuel in the Accelerator pump below it from percolating out and making a TERRIBLE lean flat spot on quick throttle application. Heat boils the fuel in the accel pump and it discharges up therough the pump nozzles, leaving them dry when the time comes to quickly hit the throttle and inject the accel squirt. Running a volatile light solvent (gas) over the top of the accel pump (and under the float bowl) sucks out heat from both areas and keeps the bowls and accel pump full of liquid gasoline.

 

These are an OEM part for some JDM cars with hot non-crossflow engine compartments!

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  • 2 months later...

Glad this was answered, I also have those and was told to cap them off because they may leak fuel? but now knowing the feature I believe I will use them as they were intended, Thanks Tony again Im thinking to use some type of heat insulator on the fuel lines would help also along with the heat shield, do thicker heat shields work better or have the opposite effect due to time to cool thicker metal? here is what is being discussed

post-3501-081721600 1301688952_thumb.jpg

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Heat shield should be a low-transmission materal. Ideal metal would be Stainless. Really best is a ceramic/carbon aerospace composite sandwich. NO radiant heat is transmitted.

Putting asbestos blanket under the heat shield seems to help more than metal alone.

 

I have used Aluminum, with asbestos cloth underneath...I'd use stainless if I had some on hand.

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OK. I hooked up the fuel coolers to the return line and installed a heat shield but I still have the issue. It will rev through the entire range slowly. It has a HUGE bog at just off idle on a quick punch and will not recover until I let off the gas. Any ideas?

 

Pat

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I'd be tempted to say the accel pump jets where too small, but having read other posts on here from ppl using Mikunis and having the same issue as yourself, its more than likely the pilot jets which are too small.

 

You could really do with having the AFR monitored while your engine is bogging to tell you whats happening fuel wise and base your next move on the results of that test.

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Does it spit when it happens? What is your curb idle set at? What is your idle set to, and are you using vacuum advance on the 1st carb through the damper pot?

 

Mikuinis like 900 for curb idle, and usually idle jets in the 55-62 range with initial timing bumped up accordingly. A recurved distributor is not necessary, but it helps.

 

Having the idle too low, timing retarded, & heavy flywheel, will all exacerbate the 'bog' of insufficient accel pump.

 

And if it doesn't come off bog when you keep it floored likely there are venturi size and vacuum leak issues as well. They may stumble, but even 44phh's on an L20A will start revving and once they hit 1500-2000 will take off like ZOOM!

 

If they don't you have other basic mechanical issues that need to be addressed before monkeying any more with the carbs.

 

The cooling bodies and heat shield only help with a percolation issue which may or may not have these same symptoms.

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  • 3 years later...

Sorry to resurrect a thread from the dead, but Wow!! so THAT is what those are! I didn't know, so I plugged them up... so let me get this straight.  ALL I need to do is run my fuel line past the last banjo bolt, then loop it down, go in one hole, out the other repeat twice more and get back to the fuel return line? is it really so simple? And it will really improve my 44 PHH performance without any drawbacks? This seems too good to be true!! (so please tell me if it is :P )

Edited by Badler
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I read getting rid of the flat spot on quick acceleration as an improvement  ^_^ and there really isn't a drawback to running the return line through there, is there? If it can't hurt, and might help, then do it, right?

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The flat spot on quick acceleration caused by percolation will be cured by using them...

 

If you have incorrectly sized idle jets, pump squirters, pump check balls that are stuck, wrong pump stroke....

 

It won't help anything but keeping the carbs cooler.

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