Engine RPM sticking at 2000 rpm

a)Is the carb mounted solidly to the intake with just a single thin gasket, or is there some kind of spacer, factory or otherwise involved?

When I had the 600 on there I had a 3/8" thick composition gasket that probably insulated the carb from heat pretty well, so had I used it again with the 750. Thinking that the larger butterfly plates of the 750 might hang up on the gasket somehow I switched to a new flat gasket. No change though.

b) Did you try the "spray carb cleaner around looking for a vac leak" trick?

I have not - that's a really good idea.

c) Throttle plate position is critical when you remove and replace them,
since you swapped them I would look to that as well.

I agree - that was one of the first things that I chased after. I have those things lined up so perfectly now it's silly.

d) Maybe even swapping a known good carb in place if the plates seem to be aligned correctly but the idle still won't come down.

Good idea. I'll put my 600 back together and give it a try. If it works correctly with the 600 then we know it's not the distributor.

SO THE THINGS TO TRY LIST GETS BIGGER:

1) Plug the PCV line off
2) Dig down to the mech advance and see if anything is messed up there
3) Look for a vacuum leak with carb cleaner spray
4) Try a different carb
5) Mow the lawn before my wife gets mad

From what you have said it's obvious you have a
air leak somewhere.

Sticking throttles, leaking hose, or even a poorly sealed intake/carb.

Timing aint causing it.

Ok, I took a good look at the distributor. The advance mechanism doesn't have any slop in it, and it is firmly attached to the distributor shaft and that's firmly held to the gear by the roll pin. All of the slop that I'm seeing in the rotor is due to slop between the distributor gear and the cam gear.

But, I don't see anything but a normal wear pattern on the distributor gear. So either the cam gear is wearing out, or this is just the way they fit.

In reality, when the engine is running the oil pump provides resistance to the power applied via the cam gear, and the cam gear to dist gear clearance closes up. There's really never a situation where the oil pump would provide no load to the gear train, so the backlash between the two gears never opens - it's always closed. So it should be consistent.

And not the problem I'm looking for here.



Edited by Greyhounds_AMX - Jul/21/2018 at 5:52pm

Too high fuel pressure?

a) Too high fuel pressure?

I suppose that's possible, but the same stock type fuel pump has been on there for 18 years and 10,000 miles, so I'd expect too low fuel pressure out of it before anything else.

b) If you go for a drive and it act`s up can you stop and to be sure the throttle blades are closed (@ idle position)When it does it can you lower the idle with the base idle screw???If you can it`s not the primary side of the carb..If you can`t something is holding it open (Choke cam)

I did exactly that - you can see the result on the first post on page 2.

c) ...I find it hard to believe ignition timing would cause a 1,000 to 1,200 rpm rise...it would have to be really fat(rich) for a vacuum leak to cause it also...I would drive it with the air cleaner off if it still does it i would test it as i listed above...I`m leaning towards the choke cam or the vacuum secondary pod

I'm doing all the testing now without the air cleaner, but it's still doing the 2000 rpm thing, so that isn't the culprit.

I did swap both the electric choke assembly and the vacuum secondary pod and their respective levers from my 600 on to the 750 and it made no difference at all. I expected the choke to be the problem as well, but it's working fine. At this point I have it cranked clockwise far enough where it's always 100% open.

I'm going to take the ones that were on the 750 and stick them on the 600 now so I can do a complete carb swap and see what happens.