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Mikuni PHH fuel surge problem


RLY240
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I've recently replaced the original twin SU carbs for a nice triple Mikuni 40PHH setup (thanks Mick) and while the power and the sound has certainly improved I'm experiencing a significant misfire that is making the car undrivable on the track.

 

I believe it's a fuel surge issue inside the carb floats as the problem is specific to tight right hand corners only and clears immediately when the corner straightens out again.  My thoughts are that the float level is too high and the fuel is flooding into the intake under corner load.

My setup includes an electric pump and a pressure regulator set to about 4psi.

Problem occurs consistently from stone cold on a hillclimb through to a 35deg day at Mallala (all right hand turns of course....)

 

What I have tried so far:

 

First, filled the fuel tank until it came out the filler cap just to prove that it wasn't a pickup issue inside the tank.

Then blocked the return line to the tank (as at this stage I though it was under fuelled). No help.

Removed all the floats and set the float height as per the Mukuni manual (12mm when the needle is just seated without compressing the needle spring).

No change at this stage so started raising the float level - problem got worse.

So then started lowering the level - seemed to get better but not fixed

Kept lowering the level until the car would misfire even in a straight line - bummer.

 

So I think it's fuel surge related but can't find the sweet spot for the float level. Another Zed running 44PHH setup on the same day (and much faster than me) has never experienced this problem but can't tell me his float level.

 

So my question is, does this sound familiar to anyone?  Is there something that can be done such as baffling or modifications inside the carb float chamber that might help? Anyone got a different opinion to the Mukuni manual about appropriate float levels or aftermarket floats that might help? Anyone think I'm barking up the wrong tree?

 

Any thoughts please let me know.

Thanks,

Roger

 

 

 

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A bunch more googling has dredged up some info from the sports car guys in the states, looks like it's a common problem with the PHH carbs and there are some internal mods that can be done to change the vents to stop fuel splash getting onto the air jets.

Will see if I can find some decent pics or instructions.

Roger

 

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I don't have mikunis but it does sound interesting. The weber book I have talks about some mods to maintain float level when rallying (extreme conditions) I'll post them up if you like. Do you think it's occuring to all 3 carbs? Have you been able to confirm which cylinders are misfiring?

Mick

 

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Thanks for the responses guys  to answer your questions, it's hard to know where the misfire is but on the worst corner (160kph brake to a 2nd gear hairpin) the engine almost dies completely for a few seconds until you wind off some lock on the exit and then it comes back to life in an instant.  My best guess is that it's across all three carbs to some degree depending on the g load.

On removing the float cover it's obvious that there is fuel in the float breather chambers where the only possible way it can get into there is by splashing up to the jet block cover and then down into the breather spaces. If fuel is getting into the jet block it will be drawn straight down the air jets and significantly richening the mixture.

 

The car has been Dyno tuned in this configuration and jetted to give an even 12.5:1 mixture right through the rev range under load so I'm happy that the configuration and tune is adequate. I don't believe the issue is related to tune or jetting but something else such as surge.

 

There is some info around ( from wolf creek and others in the US) that shows a process of blocking up some breather holes that directly link the jet block housing to the float chamber.  In my carbs there is nothing to stop fuel from splashing through these holes onto the air jets.

All the examples of this mod I can see also add an external breather for the float chamber but my carbs have a different layout and I don't think it is necessary but I guess will have to try it to find out.

Roger

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the fuel cant spill over into the main jet block as the top plate gasket seals it off unlike webers, mikunis have5 low speed progression ports and block up easily causing fuel starvation , set the floats as per height take out all the idle mixture screws spray some carby cleaner in them with the thin long spray nozzle tube and blow them out with compressed air with the throttle open, due each one a few times , im assuming that the carburettors are second hand and not had a complete overhaul that is.

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Isolators not soft mounts.

I agree the jet block is sealed from the float chamber but it also shares the vent arrangement.  If you remove the jet block cover you can see all the way down into the fuel bowl.

I have been able to replicate the g forces on the bench and you can see the fuel coming up the breather holes into the jet block breather.

I have found some examples of breather mods on type S carbs but not for my type R style although Todd from Wolf Creek confirms that they can also mod these, I'll have to work it out for myself.

Going to try a few things this weekend and hope to get on to Mallala Friday for a practice day to confirm.  Need to sort this before Targa High Country in a couple of weeks or the SU's are going back on.

 

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Mixed results on this one unfortunately and still some work to do.

Blocked off the float vents and fitted a separate external vent on the float cover and bingo, the right hand corner misfire is completely fixed.

But.

Now have the same misfire on left hand corners that didn't exist before.

Bugger.

Moved the float level up and down to the point of flooding or starving and while changing the severity I just can't get it to clear. There is absolutely no fuel getting into the jet block housing or flowing out the breather into the intake so I'm clueless as to why this is happening.

So the SU's are back on and all trace of the miss is gone along with some horsepower but I need the reliability for now, will have to dig further in the off season.

Again, thanks for everyone's feedback, hope to have better news in the coming weeks.

Roger

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  • 7 years later...

Epic thread resurrection.....

If anyone cares I ultimately resolved this by purchasing a set of modified Type S carbs from Wolf Creek and the problem went away completely.  Fast forward 8 years and I have just bolted the original type R's to my road car and colour me surprised the same surging issue is back on tight left handers, even on the road.

The intervening years haven't made me any smarter or wiser so I'm still searching for a suitable fix and am hoping that in the meantime someone else has stumbled across this too. 

Anyone?

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  • 1 month later...

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