R

RonBeasley

I'm New Here
I have a Marvel MA-4SPA carb on my 0320 powered 6. Recently, I have experienced a problem shutting down the engine when pulling the idle mixture to cut off. The engine continues to run at very low rpm and very rough. The only way I can kill it is to shut the fuel selector to off for a couple of minutes then pull the mixture to off...and it shuts down as expected. I replaced the float/needle and seat thinking this was the problem but it continues. I have played with the idle mixture setting too to no avail.

Does anyone with extensive carb experience know what my problem may be. Also, what should the idle mixture setting be.

Ron
 
Check and make sure the idle cut off lever is fully closing. If that is not the trouble, and the float level is OK as well and the needle valve not leaking, it could be the cut off valve is leaking.

The proper setting for the idle mixture is to set the mixture so that when you pull the idle cutoff, you should get a momentary rise of about 20-30 rpm just before the rpm drops off. I set mine to idle about 680 and see about 700 to 710 before the engine quits. This should be done on a fully warmed engine.

If you see no rise in rpm, the engine is too lean and the screw should be backed out turning counterclockwise to enrichen the mixture. If the rise is too great, the engine is set too rich and you should turn the screw in clockwise to lean it. Usually 1/4 turns to get close, then 1/8 or less to tweek it.

Hope this helps,

Roberta
 
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That I don't remember and I could not find it in my manuals. If you can not get the engine to kill using the cut off, the problem is not likely to be the mixture screw adjustment. It may be in need of ajustment, but to properly adjust that, the cut off needs to be working. All the carb functions like float level, the float needle valve, and the cut off need to be working properly in order to set the idle mixture, otherwise, you are wasting your time trying to adjust the idle mixture. Fuel enriching or leaning problems due to internal carburetor problems all affect the adjustments for idle mixture and speed, among other things.

Roberta
 
I had the same problem a while back and here's what I learned.
Your engine can only run on (Diesel) if it has fuel and air getting to it. You have already checked some of the major culprits, but do you have a primer system installed? If so, make sure that there are no leaks in it. Verify that the primer O rings are good and that the primer is closed.
In my case, nothing helped and I realized that my carb had a lot of hours on it. There are a lot of working parts there and they eventually wear out. You may not see a lot of wobble, etc in the moving parts but...
In the end, I finally exchanged my old carb for a factory reman unit and all of my problems finally went away. Past experience has taught me that overhauled units are hit and miss in their quality and for me, it has been cheaper, in the long run, to go either new or with factory remanufactured units.
Hope this helps.
 
A base setting for the mixture screw, that will work, is 1 1/2 turns out from all the way in. Fine tune as Robertta sugested from there. Sure sounds like a leaky primer! If not that, and the engine runs good otherwise, my bet would be the mixture valve.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at you own risk."
 
In the mean time.

My vote is for a worn mixture valve or the primer system leaking. In the mean time when you pull the mixture to kill the engine open the throttle and the engine will die.
 
It's an old post, but I had the same problem on my carb/engine w/ just 300hrs. Turns out my mixture cable throw isn't long enough to hit idle cutout. Hits the stop full rich, but doesnt on the lean side.
FYI
 
MA4SPA mixture

The mixture "valve" is a very simple brass piece which rotates in a bore at the bottom of the bowl. The mixture control lever clamps to the top, and the actual valve is tied to it via a braided cable similar to a speedometer cable. The entire assembly is about 2" long, and if a small piece of trash is stuck in the bottom, it won't fully close. Also, if the cable is binding or failing, it won't close. I recomend taking the bowl off, and looking at that if your control cable/travel checks good. I am replacing one now on an MA3 due to "birdcaging"of the valve cable.
 
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