I got a 250 six in a row that I just can't get the stumble off of idle out of. I have put in new points, plugs, cap, rotor, vacuum advance, rebuilt carb, plug wires are good, and compression was between 120 and 140 on all six cylinders. The only thing that I don't like is that the idle needle has a little bur on it that I have tried to polish down. It stumbles bad enough that you really have to nurse it to get it started in each gear. Once you get to 1500 rpm it runs great, very smooth. Could that needle be the problem to make it run this bad. I have played with timing all over the place. It runs awful at the factory setting of 4 btdc, runs better are more like 14-18 btdc. It could be that the dampner has slipped on the rubber ring. Please help me out guys.
Fatten it up. These engines were often lean when made. Try two sizes of jet first and see if you get an improvement, any improvement. if you do go 2 more and see. Also when you have it apart recheck float level. Don
you can verify tbc with a pos***ive stop screwed in the spark plug hole to see if h balancer has sliped. you wouldnt think a factory rochester would perform that bad though. how much vac are you pulling? does it have a vac leak?
I have to agree with Dolmetsch, those engines were set pretty lean from the factory to try to get every ounce of fuel mileage they could. Other than running lean, a vacuum leak which would cause it to run lean or something plugged in the low speed circuit of the carb the accelerator pump may be out of adjustment.
Dang, them monojet carbs. That engine needs multiple carburetors. Then it will run correctly, or still run crummy, but look traditional.
I put a new accelerator pump in it, but don't think that there is any adjustment to be made on these. I need to make sure the float is set properly. Would the float being too low cause this? Do I just adjust it to make it sit level?
Too much advance will do that, sixes don't like a lot of lead. Ignore the marks and retard it a bit at a time, the stumble might go away.
Also try and take a look to see if it is trying to flood the motorat idle.And like everyone else said check for va*** leaks.
I would start with a dwell meter on the points. The fact you have to increase your initial timing makes me think ther may be an isssue with the points.
You can fool around all you want. I ran a specialty tune up shop for a living. Worked and cured many many Monjet problems. In reality it doesnt matter if the pulley slipped since you have tried power timing it anyway which cares little about where the mark is. It didnt make a huge difference. Vacuum leak is possible but not likely since it idles and these things would die in a heartbeat with a vac*** leak. Set the float level at factory sopecs for the carb number. Rochestor carbs in general are sensitive to Float levels and need to be right on. Used to fix a couple of these per week. I am ***uming you have everything else right on such as Dwell/point gap etc. That is where to start first. For instance the wrong dwell or point gap if the engine has points would cause this but that should never be an issure because it should be set right in the frst place. If that is ok and vac*** advance its working and hooked up to the proper port then fatten the jetting up and see. Us carb guys always thought of Monojet as a carb with mononucleosis. With a bit of rework though they can be a good carb. Don
I see that vaccuum leak has been mentioned a few times. Tighten everything up, a leak at the intake manifold will cause the motor to run terribly. You can drive around like that but when you come to a stop it will be apparent. There's a line to the vaccuum modulator at the Trans that might be worth looking at. Torque the head bolts and a valve adjustment wouldn't hurt anything. good luck. what are you driving?
The 230 and 250 are bad about leaking manifold gaskets. If they are older, I'd replace them. THEN, they need to be retightened on a regular basis. The carb ideas are great as well.
The Jalopy Journal
