Re: M37 Carb Question


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Posted by Alan Bowes on March 18, 1998 at 20:31:02:

In Reply to: Re: M37 Carb Question posted by Joe Sacco on March 18, 1998 at 19:30:42:

As Bill mentioned, the 230 seems to start up fine with some choke when it is really cold, but if the choke isn't working, you can't statically "prime" the engine by pumping the gas pedal like you could on a carb with a mechanical accelerator pump linkage. When the choke is fully closed, it will draw enough vacuum in the carb venturi to suck fuel through the venturi nozzle itself. It will also, as Bill noted, raise the manifold vacuum, since the choke can close much more tightly than the throttle plate, which is held part way open by the idle speed screw. This means that if you have the choke closed and pump the gas pedal while cranking the engine, I would imagine that there may be enough manifold vacuum to operate the accelerator pump, at least to some degree, even if you don't get a full squirt each time. This will depend to a considerable degree on the spring tension on the small air-bleed flap on the choke plate. This flap is designed to allow a certain amount of air through the carb when the choke is fully closed, and at the same time, to guarantee enough vacuum to draw the fuel through the venturi nozzle. From looking at several different ETW1 carbs, I noticed that some bleed plates had considerably more spring tension than others. It may be (just a theory) that the stiffer spring (which would produce a higher vacuum in the carb venturi) may have been used on models that did not come with the hand-operated priming circuit, as some M37s were equipped with. This priming circuit squirted fuel directly into the intake manifold. On models without the priming circuit, the greater vacuum could suck more fuel through the venturi nozzle and/or allow the accelerator pump piston to respond better to pumping the gas pedal while cranking the engine. Again, this is just a hunch.

If the choke is not working, you still MIGHT be able to get a little action out of the accelerator pump if you pump the gas pedal while cranking the engine. The faster that your starter cranks the engine over, the better the chance of some action, since it will develop more manifold vacuum at a higher cranking speed. I've never actually checked to see what the manifold vacuum is while cranking the 230...with or without the choke closed...but at normal cranking speed and with an open choke, the vacuum should be significantly less than it would be when the engine is idling.

With an inoperative choke, the determining factors would be cranking speed, the amount of initial tension on the accelerator pump plunger spring, and how far open the throttle plate is held by the idle-speed screw. With a closed choke, the spring tension on the choke air-bleed plate would become a factor as well.

So, I don't know all the answers to this one, but I think we're close.

Alan




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