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MG MGB Technical - hif carbs/ erractic idle/erratic mixture

I have come across a really oddball problem. I have converted my 1979 B to dual HIF 44's. The car runs fine in the mid range. It idles at 1500 rpm and then drops to 1100 rpm. A lower rpm seems to cause a rough idle that tends to want to die. The carbs are syncronized properly. The mixture is set properly. However, prolonged highway speed tends to enrichen the mixture to the point of black smoke and a very low idle. The tsca switch is connected. I don't know how to test it for correctness. The valves are set. The timing is at 10 degrees btdc. It has a 45D4 points dizzy. I believe the car should have originally been equipped with an electronic dizzy. Any suggestions?

Matt Harkis

Matt: The problems could be from several sources. Worn throttle shafts/bushings is a common cause if there are more than 50K miles on the carbs. To compensate for air leaks the "right" mixture at idle would be too rich at higher speed. A stuck throttle disc poppet valve would be another air leak source that would also give a misleading idle mixture setting. Others to check include faulty bi-metallic compensator on the carb jet(s) keeping the jet pulled downwards and leakage past the choke ("starter valve") "O" rings.
Andrew Blackley

Thanks Andrew. The carbs have been rebuilt by the local guru. It would seem to be more of an ignition related problem. I'll keep working.
matt

At the base of each piston there is a groove about 1/4 inch wide. The metering needle and spring goes inside a gray holder piece, and that assembly goes inside the center of the piston and is held in place by a set screw. Make sure that the assembly goes all the way up into the piston. My 74 had HIF's, and one had a casting imperfection in the piston where the needles were setting at different heights, causing an erratic idle, and erratic operation at different speeds. i'm now using a Weber 32/36 and it doesn't have the erratic behavior any more! The piston has to be matched to the dashpot, and both carbs need to be matched (sold in pairs only), so if there's a casting flaw, you're SOL. Eventually I'd like to get a new pair of SU's, but will go back to the 70-72 version which will have the traditional fuel bowl but with the plumbing for my evaporative cannister.
Good Luck!
Ken Thompson

Weird problem. I have the HIF's on my car and still have the Lucas 25D dist. Did have something similar happen, but it was all the time, ran rich, no idle, ect.. Turned out to be distributor. Ended up taking it apart and cleaning it and problem solved. I wouldn't neccesarily recommend this, but you may want to check your points, condensor, ect. Seemed like @5 degrees made a difference in it running well and not at all also. Hope this helps.
J Arthurs

Thanks Guys:
In addition, I have been told that the dist. must be carb ported for correct advance. I is currently tsca/manifold ported. Does anyone know where I can port? I cannot locate a position on either carb.
That same someone recommended a Crane ignition. Any thoughts? The car has fresh paint, a fresh chrome bumper conversion and a fresh suspension lowering. It handles and looks great, It needs to run that way!
Matt
matt

Matt, I put on Crane XR700 electronic ignition and a new Lucas coil last December, and still had that erratic idle. But now with the Weber, the Crane elec ig seems to be much better than points. At .30 plug gap it ran great. At .35 it ran too hot and stumbled. Now I've got it back to .25 and it seems to run the same as at .30.
The factory setup for the HIF carbs vacuum advance comes off the intake manifold. I don't believe there is a way to take vacuum off the HIF carbs.
I don't know much about the 45 distributor other than "get a Haynes manual".
Hope that helps.
BTW, glad to hear of a 79 with chrome bumpers and lower ride height!
Ken T
Ken Thompson

Matt: On earlier cars with the HS4 carb there is a little "barb", or vaccuum take off, located on the the rear carb, on the top of the body between the dashpot and the mounting flange.
I believe that the same should be found on the HIF, but on the underside. Fig.3.23 of the Haynes Manual shows this (item 9).
I have the 45D "euro" dizzy on my 69 B and it runs great. It uses the carb porting I described in the first paragraph.
Regards, Andy
Andrew Blackley

I have two pairs of HIF carbs and neither has the barb, perhaps they were in non North American cars.
Mine have plugged holes in that area, I think there was thread last year covering how to install a barb
in HIF carbs. You may want to search the archives. Clifton
Clifton Gordon

Matt

You say you are using HIF44 (44mm) or 1-3/4inch.

On some of these carbs in the UK there's a valve operated by a solenoid which I believe has to be activated by a 12V supply at certain times, these also have stepper motor to control mixture.

Probably your's is older than this type!!

I'm fitting a pair of HIF44's to a Rover O Series engine and finding it difficult to fit stanard K&N's, because of space.

Was wondering what filters you are using on your B Series engine.

Steve

Steve Williams

Matt,

My '74 B's carburetors are as Clifton's, dual HIF4's, and the carb port is sealed with a brass plug. I understand that's the standard for North American HIF4 sets. The good news is that beyond idle, there is little if any difference in ported vs. manifold vacuum.

Back to your original problem: Floating idle is most commonly a carb problem. I'll take your word that the rebuild was done correctly. Even so you can have a vacuum leak in the mounting via a bad gasket or loose nut. You may have a cracked manifold or phenolic spacer, a vacuum hose with a crack or one which is loose on its mounting. Your vacuum advance unit may be leaking, as may be your brake booster if you have one.

If your floating idle truly is an electrical problem, then most likely it's a case of incorrect advance. i.e. Your mechanical or vacuum advance could be hanging up. Either can be resolved by taking out the distributor and giving it a good cleaning and lubrication. See that all moving parts slide easily and smoothly. Examine the fit of the shaft through the bushing in the body. Your Haynes manual should give you the allowable play.

BTW, I've driven my MGB to Chicago. The roads all the way from NC were smooth and fine. Once in Chicago, I thought I was going to be rattled to death, and I swear I heard cries of fear from my muffler as the asphalt cracks threatened to tear it out. Best of luck with your lowered ride.

Matt K.
Matt Kulka

Last night I took my HIF4's apart to refresh my memory. The main tube through the center of the piston goes into the base, into that 1/4 inch groove that I described. The metering needle, spring and gray holder goes into the base of it and a set screw holds it in place. There is a small arrow that must point to the groove to line up the parts so the set screw will go all the way in. The base of the long tube looks like a metal ring looking at it from the 1/4 groove in the base of the piston. If that ring protrudes down into the groove, then the metering needle will be out of place and obviously not set in the jet at the same level as in the other carb. That is what caused the erratic idle in my 74 roadster. Or it's possible that it is supposed to protrude into the groove on both carbs. But when one is different, the metering needles set at different levels. And it's possible that mine was a one-time only imperfection from the factory.
Hope that helps. Good luck, Matt.
Ken T
Ken Thompson

Hi Matt,

I had a similar problem with my 74 with HIF SUs after I rebuilt the engine. The idle could not be adjusted by the idle adjustment screws lower than 1200 rpm. I had the throtle shafts rebushed and replaced by Apple Hydraulics that made the situation even worse, the idle was now 2200 rpm.

The problem was the poppet valves in the throttle discs were opening at idle due to the increased suction in the engine.

Solution was to solder the pooppet valves closed which I did and now there's no problem. Idle back down to 800 rpm and great performance.

Good luck Andy
Andy Preston

Gentlemen
I have come a long way with this car. I have succumbed to the idea that this problem is beyond my expertise. I took it to a local shop today with instructions to call when it will idle and start when warm. They are going to install electronic ign., plumb the advance to the rear carb. Their plan is to then check the entire ign. system as well as adjust and sync the carbs. The car runs through the gears fine without hesitation or miss. The shop foreman stated that I should have gone with a Weber. I don't totally agree. (yet)
matt

As Andy said check the poppet valves. I replaced mine with the earlier solid ones after some ideling problems and this solved the problem.
J Arthurs

Mine already had the poppet valves soldered shut when I got the car.
Ken Thompson

This thread was discussed between 20/06/2002 and 27/06/2002

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