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MG Midget and Sprite Technical - Idling too fast

I have just got my 1275 midget started after a long time.It now starts but immediately goes to approx 3000 rpm. If I blip the throttle it revs up then down to 3000 and then starts to creep up to about 3500 rpm before I shut it down. The throttle screws are backed right off and the throttle springs are returning the spindle back to the right place.I have built the carbs up from "bits" any ideas where I start to look for the problem? It has no air filters on and the vac advance isn't connected,would they be sufficient to cause that high a tick over?
m fairclough

At this stage it's not really about fuel, it's about air. Your engine is drawing in too much air, either over the throttles or by another means.

Things to check:

1) You say the spindles are returning, does that mean the throttle plates are shut? Check this by lifting the carb pistons and looking through. The throttle plates must be COMPLETELY closed when the idle screws are backed out. I suspect this is what your problem is.

2) Either connect the vac advance or plug the port at the manifold. This shouldn't make a huge difference but if air gets in through the hole the engine will rev up.

You don't need the filters on. In fact for access to the carbs you don't want them on.

It is climbing higher as it warms up and friction drops. Be very careful though. There's a narrow band (about 2000rpm) between reving to 3000rpm and reving faster than it should ever get to. You don't want to overspeed your engine else it might not survive! Definitely check that the throttle disks are closing tightly - I'm 99% sure that this isn't the case given what you've said.

Ant
Ant Allen

You sure that you have the throttle and choke hooked up right? Once I hooked the choke up to the throttle linkege and vice versa by mistake. The result was that when I pulled the choke and started it up, te revs would shoot straight for the redline. The other possibilities are that you have something wrong with the throttle cable or the linkages. try to make sure that the butterfly valves are completely closed. If they are the tipe that have the built in spring loaded pop off valves, solder those shut and try again.
S.A. Jones

Agree with all that Ant has described, would add the following:

You need to disconnect the interconnection rods between the carbs. If the rods are connected to each carb then one carb can be on its stop whilst he other will be held open and cause the engine to rev.

You must understand that for an engine to rev at 3000rpm at idle either throttle disk only needs to be open by a knats!! or as has been pointed out a small air leak will cause it to happen.

When the rods are disconnected each carb is set up on its own screw to a point where both are "sucking" by the same amount, at this point the shafts can be secured together. (if that was indeed the cause. :-))
Robert (Bob) Midget Turbo

Worth also checking that you don't have a leak in the intake manifold. usually between manifold & head or between manifold and carbs. Do you have all the gaskets in place & are they clean & in good nick? I've got a 1500 so will be slightly different & can't tell you exactly how many gskets there are.
Try spraying some carb cleaner around the manifold joints, a drop in revs will indicate that you have found your leak!
I was told to always tighten the manifold to head bolts from the indie out to spread the loadings & make sure you don't get any little gaps.

As the guys above have said, air not fuel would seem to be the problem and it should still be fine with the filters off - just put them back on before you do too many miles!

-Craig
C Robertson

Seems a bit pointless trying to find a fault that may not really be there. Wait until everything is correctly connected before deciding you have a problem.
Cheers John
JOHN HALL & JULIE ROBERTS

If its not your butterflys or some amazingly huge airleak....Do check back and let us know, I got a really "retarded" alternitive theory as to the problem.

Btw start the engine up and spray WD40 around all the intake joints and connection,,,if the engine speeeds up for a bit, then you found your air leak.


Prop
Prop

Thanks for all the advice, sorry its been so long to get back, only managed to work on the car today. The problem appears to be air getting past poor fitting butterflies. They were a second hand pair of carbs I got hold of and I should of in hindsight rebuilt them properly
m fairclough

Your not in bad shape...The bushings are a harder material then the brass shafts and flys, you should be able to just replace the shafts and small butterfly screws and be done...if you have the popit valve, on the butterfly, id certianly replace the butterflys also.

Just remember to make note which way the butterfly sits in the throat of the carb.

Prop
Prop

This thread was discussed between 15/04/2009 and 20/04/2009

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