Carburetor (Linkert M21) Boiling Over

[Wilkit11]

Help!

I’ve got a 28 JD with a Linkert M21 carburetor mounted conventionally. Now that temperatures have risen into the high 90’s here in California, after a short ride the carb boils the gas and creates bubbles in the fuel line . . . causing the engine to stumble, repeatedly. I’m looking into a spacer to move the carburetor further from the cylinder fins and into the airflow. I don’t think that will be enough . . . so I’d like suggestions to deal with my dilemma. As I’m entered in the Cannonball, I’m preparing for the heat of Texas in September . . . so any suggestions would be appreciated. Thanks!

[T. Cotten]

Help!

I’ve got a 28 JD with a Linkert M21 carburetor mounted conventionally. Now that temperatures have risen into the high 90’s here in California, after a short ride the carb boils the gas and creates bubbles in the fuel line . . . causing the engine to stumble, repeatedly. I’m looking into a spacer to move the carburetor further from the cylinder fins and into the airflow. I don’t think that will be enough . . . so I’d like suggestions to deal with my dilemma. As I’m entered in the Cannonball, I’m preparing for the heat of Texas in September . . . so any suggestions would be appreciated. Thanks!

Just a 'short ride', Wilkit?

My library pic of a carb support suggests it might conduct some heat to the carb, although I cannot imagine it boiling.

J manifold support.jpg

A bowlstem nut with a stud for a bracket might make a difference. Maybe?

By any chance do you have the unique bronze Linkert bowl?

....Cotten

[knucklehead 61]

i'm running my 25JDCB in the cannonball & have an m51 on it. it hasn't ever had the issues you mention & i run the carb support bar as shown in T.Cottons picture.
you could use a phenolic type carb spacer to help insulate the carb.
i have also seen a few cannonballers reverse the intake manifold & run the linkert on the right side of the motor to help keep it cooler.
Are you running a fuel filter? I use the standard linkert filter on mine.

[Wilkit11]

Guys, thank you for the replies! Here’s what I know:

1. Yes, short ride (30 miles). Basically stumbling along at 1/2 horsepower towards the end of the ride. Ambient temperature above 97. Clear fuel line showed a steady stream of bubbles coming from the carb and pushing to the tank.
2. Yes, I have the center support as shown. I’ll have to attempt to insulate it or, if I’m understanding you, use an alternative support on the bowl bolt.
3. No, common bowl (aluminum?), not a bronze bowl.
4. Have ordered a Linkert 3/8” spacer. Though metal, will use copious amounts of gaskets to insulate.
5. Had to pull the filter off as it was obstructing the fuel flow due to the excessive bubbles. Would have to pull over as it basically was starved of gas. Could restart few minutes later after the bowl filled up.
6. Interesting idea . . . reversing the intake. May have to consider.

Thanks again guys, really appreciate you chiming in. Bike ran well when weather was in the 70 - 80’s. Find it odd that I’m running into this problem now. Would assume this would be similar weather in Texas in September. Trying to be prepared.

Bill #43

[T. Cotten]

...Clear fuel line showed a steady stream of bubbles coming from the carb and pushing to the tank...

If it were truly boiling, Wilkit,..

It would be a bomb.
Bubbles always go up; The question becomes: where is the air getting into the line?
Pressure within the bowl should shut the needle tighter, if not relieved at the vent. Air entering the fuel line must either come from the vented headspace above a leaky floatvalve, or from a leak at the line connection.

2. Yes, I have the center support as shown. I’ll have to attempt to insulate it or, if I’m understanding you, use an alternative support on the bowl bolt.

Its an option, but there is no reason the correct design should cause your problems.

3. No, common bowl (aluminum?), not a bronze bowl.

I can bring one to D-port!

4. Have ordered a Linkert 3/8” spacer. Though metal, will use copious amounts of gaskets to insulate.

The OEM spacer 1109-41 was 7/16" thick, and I seriously think even its insulation would be minimal, and gaskets only add places to leak. And some folks still think manifold icing is an issue! (I have boards of Micarta if anybody needs some....)

5. Had to pull the filter off as it was obstructing the fuel flow due to the excessive bubbles. Would have to pull over as it basically was starved of gas. Could restart few minutes later after the bowl filled up.

I gotta ask, Wilkit,... Are you using some funky fuel additives? The surface tension of gasoline should let bubbles fly through a filter.

And I know I'll make some people whine for bringing this up, Bill, but if you haven't bubble-tested your manifold assembly at a constant, regulated 15psi, you may well chase your tail forever.

...Cotten

[exeric]

Bubbles in the gas sounds like a problem that was common to Volkswagens in Florida Summers, which everyone called 'vapor lock'. The good old boy remedy for that was spring type clothespins on the fuel line, or wrapping the line with rubber bands. I was told those remedies would dampen the high frequency vibrations that caused the bubbling. I only had one VW and never had the vapor lock problem, so I'm just repeating what I heard. Tom always gives good carb advice so I would listen to him.

[tfburke3]

I have a similar hot weather problem with some SU car carbs.Hot day if car sits off for about 30 minutes it runs rough for a few minutes after.I cant see the fuel in my lines but I think its similar.
I think your bubbles are not air but the gas changing state to vapor.I also think its happening in the line,not the bowl ,as like tom mentioned the float valve would be closed and block vapor travelling back to tank.Either way the gas is to hot.Modern cars circulate the fuel and avoid these problems.
For my SU's they even make little float bowl insulating "coozies"like for your beer,that wrap around the bowls.Insulating fuel line is also reported to help.
I
Tom

[T. Cotten]

Let us please consider, Folks,,,,

There is no source of heat except the engine. If the carb or fuel line is over-heated, it can only be radiant from adjacent fins, or marginally conducted from the manifold.
Incoming air and evaporation of fuel cools this assembly significantly.

A quick search for the flash temperature or boiling point of fuel isn't exactly definitive (with the flash point anywhere from ~100F to ~600F) other than suggesting different fuels can be quite different.

Imagine that.... ... .. .. .. . ..

....Cotten