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MG MGB Technical - HIF4 Carb Idle Question

Ever since I rebuilt my HIF4 carbs on my 73B roadster this summer I can't get the front carb to idle below about 1200 rpm.

With the linkage between the carbs separated, the rear carb idle screw will unscrew as far as needed to significantly lower the idle, but the front carb idle screw leaves the stop shelf with the car idling too high.

I've already looked at the butterfly disk and it seems to be properly seating all around, but I will take another look at this over the weekend.

Is there anything else that could be effecting a high idle here. The throttle shaft bushing were right to spec and I put in new shafts and seals (the carbs had been rebuilt recently prior to my getting the car).

Thanks,
BH
BH Davis

BH,

is there enough oil in the damper of the front carb and are the jets set to eaqual hights?

Ralph
Ralph

Ralph,

Yes, all is set and adjusted correctly. Oil level is correct in both carbs.

Thanks,
BH
BH Davis

If you can't get the idle down to stalling point with the idle screw then something is holding the butterfly open, pure and simple. The only possible exception to this is with extreme wear in the throttle spindle and bushings which is allowing mixture to bypass the fully-closed butterfly. There is no other way the engine can continue to run, and it's nothing to do with damper oil level or jet heights. Another thing it can't be although it is often mentioned is a vacuum leak. An engine can't run on a vacuum leak alone, it must be getting some fuel past the butterfly as well.

If the interconnecting spindle clamps are slack both ends, then it can't be a too-tight throttle cable or unbalanced butterflys (i.e. one fully shut holding the other partially open) which can also cause it. The fast-idle screw can hold the butterfly open, other than that it must either be the poppet valves (if still fitted) not fully closed, or the butterfly not seating properly in the throat.
Paul Hunt 2

Paul,

When I rebuilt the carbs I foolishly took apart the choke mechanisim on one of them (don't remember which). Is it possible that I got that back together wrong and it's somehow causing this problem? Chock linkage is connected properly.

Thanks,
BH
BH Davis

The poppet valves are devils...and gave me idle issues......until I soldered them over...
pete

BH - Besides the poppet valves and worn throttle bushing, the most common problem is either a throttle disk in upside down or not centered properly in the bore. This situation will keep the disk from seating correctly and keep the idle speed high. Cheers - Dave
David DuBois

My throttle plates are the style without the poppet valves.

Also, I was very careful to try to make sure the throttle plates were seated properly all the way around. Given what I've read here though my guess is I didn't get it quite as good as I thought on that carb.

Thanks,
BH
BH Davis

BH - the choke on its own won't cause a high idle as the bottom line is that the butterfly should stop any mixture getting through, or at least reduce it enough so that the engine can't possibly idle. However the HIF does have a bypass port, which should be closed off with the butterfly shut, so if for any reason that is partially uncovered on the manifold side you will get some mixture through there. But I think you will have to get that carb off and shine a bright light on each side in turn and see if you can see any light coming past the butterfly from the other.
Paul Hunt 2

I had a similar problem on my 73 B and it turned out to be one disc was marginally off centre. Recentering solved the problem.

Barry
BJ Quartermaine

Thanks everyone for the input. Problem solved.

I pulled both carbs and spent some time examining all possible areas that could impact the closing of the throttle plates. They "looked" closed but were ever so slightly misaligned.

The one thing that made me realize there was a misalignment was that one of the brass tubular air bypass ports that come into the intake chamber from the bottom of the carb had more of the hole exposed on the engine side of the throttle plate on one carb then the other.

Closer examination showed that the throttle linkage was slightly bent on one carb. Also, I may not have had all idle screws adequately withdrawn when I did the original rebuild. I thought they were out of the way, but now I don't think that was the case.

Anyway, I can now take the idle down to a stall, and instead of having it set at 1200 I can now see how the car does with it at 900 to 1000.

Thanks again,
BH
BH Davis

This thread was discussed between 12/10/2007 and 14/10/2007

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