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MG MGB Technical - Mikunis

Due to extreme under hood temperatures, I am still considering a switch to Mikuni carburetors. The HS4s have the bowls sitting there over the exhaust manifold. With the A/C on, there just isn't enough air movement to clear the heat from the engine compartment in traffic.

The problem is a rich running condition when fuel starts to boil. The car will not hold a steady idle. HIFs are probably less susceptible to under hood heat since the bowls aren't hanging out there so exposed.

If anyone is happy with their Mikunis, I would like to hear of their experience with this type of carb.
Glenn Mallory

HIFs cope with heat better as they have thermostatically controlled jets. However the float bowls are still directly above the exhaust manifold, albeit the top is 'covered' with the main part of the carb. A good heat shield should prevent radiated heat affecting them, but they will still be surrounded by general air heated from the radiator as well as block surfaces and exhaust of course. Carbs should be cooler to the touch than surrounding parts, as the vaporisation of fuel when mixed with the air acts like a refrigerator, and HIF float bowls should benefit from that more than HS. Maybe look at getting the heat out of the engine bay more effectively, with RV8-type holes in the inner wings, or even raising the rear edge of the bonnet. As designed all the hot air has to flow downwards under the inner wings, very little gets down the tunnel. Stationary, especially in a following breeze but even in still air, a significant amount of engine-bay heat flows forwards under the front of the car and gets recycled through the rad.
paulh4

Louvred bonnet/hood?
Michael Beswick

Ahh
I've thought about this problem on and off and have come up with an idea that might or might not work
A car with AC has basically two hoses/pipes in the engine bay
The high pressure pipe, which is the smaller one from the compressor through the filter to the condensor and on to the inside of the car will be hot/warm while the return hose(larger will be cold/cool
The PLAN
With the return(suction) hose, run a flexible hose from the firewall up to near the rear of the carbies, then connect it to a normal rigid metal pipe and run that along under the carbs close to the float bowls and then off to the suction port on the compressor
Like this there should be enough cold left in the return pipe to keep the carburettor area cooler when the AC is being used
Might work-might not---just a thought
willy
William Revit

Glenn. Experienced pretty much the same issues here in Florida. Temps in the 90's F (32-C) with matching % humidity.

My 79 MGB has a Weber carburetor.

1- removed the rear rubber seal from around engine compartment, so air can escape

2- installed an 7mm insulator between the intake manifold and the base of the carburetor.

This has been a major help. Temp gauge indicates the engine is not running as hot as it was.

Cheers

Gary
79 MGB
gary hansen

I too had been thinking of how the suction side of the A/C could be harnessed to cool the carbs as William had mentioned. Maybe the line could service refrigerant coils wrapped around the HS bowls? It would at least look cool.

I agree with Paul that HIFs would be better adapted to the hot running condition and the bad placement over the exhaust manifold. There is a set available locally so I might rebuild those and give it a try. I assume that Mikunis would have equivalent exposure as the bowls are about the same size and are located where the HIF bowls are. I don't know if there are bi-metal jet thermostats on Mikunis. I doubt it since the heat situation on motorcycle engines is certainly more favorable.
Glenn Mallory

"the smaller one from the compressor through the filter to the condensor and on to the inside of the car will be hot/warm while the return hose(larger will be cold/cool "

Surely in A/C it's the pipe going into the car that is cold, and air is blown through the radiator there to cool the car. The fluid then becomes hot and is returned to the front of the car to be cooled by external airflow.

If that's the case, then wrapping the cold pipe round the carbs may well do a goo job of cooling them, but do less for the car.
paulh4

Willy's crash course on AC as I first explain it to the new fellas starting in the trade--

AC gas is magic stuuff
As it turns from a gas to liquid it gets hot
As it turns from a liquid to gas it gets cold
So---
The compressor compresses the gas which turns to a hot liquid---the liquid passes through the filter(receiver/drier)and then to the condenser mounted in front or the cars radiator where it is cooled
The cooled liquid then travels via the HP pipe to the TX valve(or orifice)near the AC unit in the car'
Here the TX valve(or fixed orifice)works like a carburettor and releases the pressurized liquid into the evaporator core in the AC box
as a gas----The process of depressurizing and reverting to a gas is where it turns cold
The cold gas does it's job in the evaporator and flows back to the compressor (still fairly cold) via the LP hose where the cycle repeats
The HP pipe is usually the smaller dia one

This is how ;most AC s are set up but there are a few that might have the orifice half way along the HP pipe and also some cars have the filter back near the firewall instead of up front--depends on room I guess
-----------------
Therefore
It would need to be the larger return hose to use for floatbowl cooling unless it was a strange system with the orifice in the HP pipe right up the front--havn't seen one like that but nothing surprises me anymore
willy
William Revit

Glenn, if you have your manifold ceramic coated inside and out you will be amazed at how much cooler your engine bay and carbs will be.

You will also get a slight increase in hp because it runs more efficiently.
Steven Rechter

Glenn,
I run an 18V in the MGA and did a couple of things to lick the hot fuel problem in New Mexico.

First: installed 3-pipe exhaust manifold that was Jet Hot-coated; Second: I took a thick phenolic carb spacer and had machinist make 2 thinner (3/8") spacers, then mounted these between the intake manifold and the heat shield. Third: I bought a small heat shield from Yearwood (in ABQ) (same size as the MGB heat shield) and pop-riveted it to the MGB heat shield; and, Fourth: installed the HIF4s. For the HIF4s, I routed the fuel line across the firewall to the left fender, then forward to the front carb, so all of the fuel line is far away from the engine (the old set-up ran the fuel line over the intake manifold to the HS4s).

You might have seen this setup on my car at a BAOA meeting.

Tyler
C.T. Irwin

I have successfully dealt with excess float bowl heat on the race cars by fabricating a small alloy sheet shield attached to a carb fixing bolt. It reflects enough of the heat from the exhaust to eliminate issues (in my case, tube headers not to far from the floats).
Bill Spohn

This thread was discussed between 07/07/2017 and 23/07/2017

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