MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fast idle with new distributor?

Hello,
I am having an issue with a fast idle speed (1500 +-)
on my '72 B. The engine is in good nick (about 2500 miles on a minor rebuild) and has no modifications.

I recently installed a new distributor from the BHive and am now having this idle issue. Mind you it idles smoothly, just too high.

I have the timing set (by timing light) at 10 BTDC, and have the carbs adjusted properly. I've found that the idle increases when I attach the vacuum advance line. I've checked my manifold pressure with a gauge and it seems ok.

The thing is, I have the carb. stop screws backed all the way and checked that the choke screws are not touching.

I know I'm missing something simple, any suggestions?

Regards, Dave
David Steverson

I suspect you have the vacuum advance hooked to the manifold. The new distributor probably has lots of vacuum advance compared to your old emission distributor. To prove my theory unplug the vacuum advance. If that is the problem you'll need to make a vacuum limiter for the advance on modify one carb for ported vacuum. I don't think you can switch the vacuum advance between the distributors, if you can that is another possible solution.

Clifton
Clifton Gordon

Yes Clifton, it is hooked to the manifold.

So I should have it hooked to one of the carbs instead?

David
David Steverson

PLease be sure to check that you do not have worn carb shafts - this will allow air to pass into the intake. I had new bushings and shafts installed on my carbs and it allowed me to slow my idle down to the 800-900 range. I had a similar prob earlier.

To check for this - just spray carb spray on the shafts and see what happens to the engine while running.

Jeff
J Delk

After poking around in the archives a LOT more, it appears my issue is ported vs. manifold vacuum, as Clifton suggested.
I just ordered the spacer with vacuum port that AptFast sells. Hopefully, that will do the trick.
Dave
David Steverson

Clifton, how de you make your vacuum limiter?
wim

David, I have never made or used one. I still use older distributors and manifold vacuum. I select a vacuum capsule with a short vacuum range that was originally used with a manifold vacuum set up. Jeff Schlemmer has mentioned that he has made some limiters when he sets up a distributor. On the older distributors I think you can install a short length of tube around the operating link to limit advance movement.
jeff@advanceddistributors.com
Sorry I don't has specific information.

Clifton
Clifton Gordon

Clifton,
Since I've already plunked down the money for that new distributor, I guess I'll have to work with it. Wim puts up a good question on making a limiter.

I've just thought of another issue on the AptFast spacers, if I'll have to change carb mounting studs to make up for that extra thickness.

Anyone know about that?

Dave
David Steverson

Doesn't matter where you connect the vacuum pipe, or what distributor you fit, if you can't turn the idle screws down to stalling speed there is a problem with the carb or linkages. With the butterflies fully closed there will be no mixture reaching the engine so it cannot run. Either some mal-adjustment of the throttle cable (not enough slack) or linkages (one butterfly fully shut holding the other open) is preventing the butterflies from fully closing. Just possibly the throttle spindles are so worn that they are allowing mixture to get past a fully closed butterfly. Having said that it is quite possible that a different distributor is allowing more efficient combustion, and hence a higher idle speed for the same carb settings. But if you can't tune that down with the carb idle screws it is, as I say, a carb problem.

The only difference manifold vacuum to carb vacuum makes *does* affect idle speed - it will be higher with manifold vacuum for the same carb settings, but I say again you should always be able to tune that out with the carb idle screws. UK cars changed from carb vacuum to manifold vacuum for the 77 models, but neither the engine, distributor or carbs changed throughout rubber bumper production. It is simply an emissions measure, increasing the advance makes for more efficient combustion, so the idle speed goes up, so you can reduce the idle screws and hence fuel consumption and emissions at idle. The down-side is that the vacuum capsule spends a lot more time at maximum advance, which may cause it to fail earlier, and it can make starting more difficult.
Paul Hunt 2

Well, I can't get the idle down on my stromberg MGB when the vacuum is connected directly to the manifold and the idle screw backed completely off. It's supposed to be connected to the 4th gear vacuum switch on the brake booster, but it's not working.
So vacuum does affect idle, and no, there are no air leaks around the carb.
when I finish my twin HS4 conversion I will be replacing the dizzy with a new 45D4 that's modded for less vacuum and more advance and it will work with manifold vacuum(my '71 HS4's use manifold vacuum)
Ron

I do know this, my car did idle at less RPM before the new distributor.
I rebuilt my carbs about 9 months ago including new throttle shafts/seals.

Mind you, it idles smoothly and runs well. But, if I disconnect that vacuum line, I get an immediate drop in idle speed.

BTW, thanks for all the input thus far!

David
David Steverson

David, you definitely need to change to carb vacuum, and that will cure your problem by decreasing advance at idle.

The vacuum advance limiters I install are not for the faint of heart. They are for those folks who are very tech savvy when it comes to engine tuning. Basically, I weld up the slot in the lower half of the points plate where the points retaining screw fits. Then, the micro-adjuster on the 25Ds can be used to both alter ignition timing AND the amount of vacuum advance.
This becomes complicated, because as you alter the amount of vacuum advance you also alter the engine timing. With this method, you always need to make 2 adjustments at a time!

I've tried other methods of altering the amount of vacuum advance, but without disassembling the vacuum module, I've found no real reliable way.
Jeff Schlemmer

Yes vacuum does affect idle. And yes manifold vacuum will result in a higher idle when connected compared to when diconnected. But however high it increases the idle the point is that the carb idle screw should always be able to shut the mixture off altogether or at the very least low enough to make the engine stall. If it doesn't there is something wrong with the carb or throttle cable setup.
Paul Hunt 2

David, Paul's reply reminded me I didn't give you a good answer. I had a too fast idle speed after replacing throttle shafts several years back and couldn't get the idle speed as low as I wanted it. I removed the carbs and backed the idle adjusting screws out so the ends weren't touching anything. I loosened the throttle plate screws and snapped the throttles open and closed a few time to make sure the plates were properly seated. Tightened the screws and installed the carburetors. That solved the problem.

Paul, HIF carbs have a bypass idle system. The only place I have seen information on the system is in the Haynes mgb manual. On page 78 section 24 # 5 Haynes reads, "An emulsion bypass passage allows unevaporated fuel drops to be drawn along at small throttle openings and mixed more completely with the faster traveling air". Do you think that system would have any effect on trying to slow the idle speed? Just curious.

Clifton
Clifton Gordon

Clifton - my Haynes shows the bypass tube and its exit but it is not exactly clear where it is in relation to the throttle plate when at idle (and I don't want to disturb mine to check). However I think the phrase "allows unevaporated fuel drops to be drawn along at small throttle openings" indicates that it only comes into play when the plate is slightly open from its idle position, i.e. is on the jet side of the butterfly at idle, not the engine side. If it *were* on the engine side then it would have an effect on the minimum idle that one could achieve. However since the number of these complaints of not being able to reduce the idle speed low enough are tiny in relation to the number of cars (many BL other than MGB, plus other manufacturers) with HIF and Strombergs it *must* be a correctable problem and not one of design only get-roundable by moving the vacuum take-off from the inlet manifold to the carb - if it has a port anyway. North American cars used manifold vacuum from 1971 (i.e. the majority of production) with HIF and Zenith carbs, the UK not until the 77 year with HIFs. Your experience with the misaligned throttle plate is exactly the point I have been trying to make.
Paul Hunt 2

I just went out to the shed and looked at an HIF. If I'm looking at the right thing (it's the only hole I saw), it's a fairly large hole -- its diameter is about the thickness of the throttle plate -- that sits right at the plate. It looks as though, with the throttle completely closed, the hole would present a very tiny opening on BOTH sides of the plate. I doubt however it would contribute much to idle speed.....
Rob Edwards

So why would Stromberg MGB's have a switch to allow vacuum only in 4th gear and very little or none at idle?
Ron

when I had my carbs rebuild and new throtle shafts installed I could not get the idle down .
Turned out to be the new shafts were a hair longer and therefore binding just a bit keeping them open slightly. I had to remove the carbs and file down the ends of the linkage shaft to create a little clearance.

Try and wiggle the connector shaft back and fourth to see if there is any end play between the carbs . If it is tight it could be the problem .

good luck Derek
Derek

Paul, Thanks for your comments. Rob, Thanks for checking. I thought it was as you describe but I wasn't sure since I last looked several years ago.

Clifton

Clifton Gordon

That woul be intersting to compare if possible the given idle advance with the actual dizzy and the previous one to check out a possible failure/discrepancy ?
Renou

The Zenith carb was introduced in December 74, but the TCSA vacuum switch was only introduced on 77 models to prevent engine surging when the clutch was operated, which was a 'feature' of all the other emissions rubbish North American cars were lumbered with and kept being added to. This only connected the vacuum in 4th gear, so there was no vacuum advance at all at idle or in the other gears. Only North American spoec cars had the TCSA and Zenith.
Paul Hunt 2

This thread was discussed between 27/07/2006 and 31/07/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.