M74B Carb rich at mid range.

[partshunt]

Hi All: - On a 54 FL with an M74B carb, I am running rich above idle when just cruising, no power on. Idle mixture screw is about 4-6 nothes backed out from misfire lean. The hi speed needle is closed shut. Acceleration is excellent. Power on it runs smooth all the way down to idle. The problem is at around 1500 to 2500 coasting it blubbers and burbbles, seems to load up richer. Float is set at 5/16ths. Float needle holds good. Would it possibly be something worn or wrong with the spray nozzle? Fuel mileage is about 26-30 MPG with brass mesh OEM air cleaner..Thank you for any ideas...Joe

[knuckleheadtim]

Remove the bowl, put a cork in the bottom and hook up a fuel ine to it; the wet fuel level should be 5/8" below the top of the bowl. Too high will cause a mid-range burble.

Also, I've had much better luck with the main jet at 1/16". You can solder it and redill it. Make sure the venturi isn't loose in the body, and definitely make sure you have the 5 hole main nozzle.

[flat-happy]

if you are running anything but cotten's float in your carb it is probably the float sinking too low in which case you can't set it with a measurement. the floats are all soaking up the fuel and becoming heavier because of the ethanol blended into the fuel at the pump. i have to replace 2-3 a week in other peoples bikes

[T. Cotten]

Joe!

If you can run with a shut HS needle, somethings wrong.
(Please inspect the fixed jet to be certain that it has NOT been attacked with a drillbit!)

Why did you feel you needed to set your float so deep?
Even my competitor's floats of similar buoyancy should be set at the Service Manual's spec of 1/4".
If you have brass, or one of the new India black boatanchors, you will be lucky if is works at all at 3/8".

The only wear issue on the main nozzle is when the venturi has shrank so loose that it rattles and wears upon the nozzle spigot, thus defeating its accellerator pump effect.

As always with ANY performance issues, the manifold should be pressuretested with soapy water upon it to make certain that the carb is not masking other problems.
And of course, timing and valve adjustment must be on spec.
Only then can carb issues be addressed sensibly, one at a time.

....Cotten

[partshunt]

Cotten:- Thank you for your input. I will check out your sugestions. Timing and valves are correct. My float I believe is still the cork original, just coated with por 15 to keep it durable. I can park it all day with the fuel shut off open, no flooding over, engine stopped or running. The float needle and seat is new. What is the main jet number most common of these M74B carbs, # 19? (Not shown in my 48-57 shop manual)??....Thnx...Joe

[partshunt]

Thanks also to Flathappy and knuckleheadTim. All your input is well taken and will add to my list of checks. Been collecting to build another M74B. All your points are important to check when building up another carb. Possibly, Cotton can supply his float, main jet and spray nozzle. The rest I have for a second carby. Thanks again to all...Joe

[T. Cotten]

Joe!

Floats are cut to order by my own hand, but please support your local independent dealer or favorite mailorder vendor for odd hardware.

Liberty only does service and only sells what goes with those services,... as I really do not want to conflict with my good friends who only retail!

I just tried POR-15 again on an etched glass plate in this year's Shell Premium and it didn't last the night. Never did.
Please do not be surprised if your float eventually looks like the attachment.
Any sealer upon a float weighs it down anyway, leaving you chasing your tail for a proper setting even if it does survive.

Reproduction nozzles are decent (unless obviously undrilled or otherwise unfinished!) But the loose venturi and the egging of its nozzel hole is a gamble, as most repops are already "shrunken".

Good luck!

....Cotten