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MGB Puling air

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Carlbanan56

Carlbanan56

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Hi, for about 1 year now I have been working on my granddad's 1975 MGB that hasn't run since 1994. Yesterday I tuned the HIF4 carburetors and was finally able to take the car for a spin around the block. The car is not yet ready for daily use I still have to go through the breaks and a bunch of other stuff.

Although I tuned the carbs well enough to take the car around the block I think I may have run into a problem. When toning the carbs I think I have screwd the mixture screws all the way in and the car is still running lean. I think it is running lean because when I lift the piston the RPM instantly drops. To me, this sounds like the engine draws air not only through the carbs. Could this be true? If so how do I navigate to a fix?

Young and inexperienced, so every answer is greatly appreciated.
 
many years ago when we used to tune them we used a small screwdriver to lift the piston just a fraction, when they were right there should be no difference in engine revs,
if you unscrew the nuts on the bottom and listen to the engine revs, if the engine revs drop. then turn them the other way until the engine revs stop increasing. if the revs start to dip again you have gone too far.
and keep adjusting the tick over down to the correct level,
 
This is what I have done, when lifting the piston the revs dropped. So I turned the screw clockwise to make the mixture richer but I bottom out the screw.
 
mezy said:
many years ago when we used to tune them we used a small screwdriver to lift the piston just a fraction, when they were right there should be no difference in engine revs,
if you unscrew the nuts on the bottom and listen to the engine revs, if the engine revs drop. then turn them the other way until the engine revs stop increasing. if the revs start to dip again you have gone too far.
and keep adjusting the tick over down to the correct level,
Click to expand...
That is for the HS series carburetors, the HIF (Horizontal Integral Float) series is very different from the HS and earlier H series carburetors with their side mounted float bowls and means of fuel flow to and through the jet and then up into the variable venturi. So now on to the OP's car.

The 1975 cars were fitted with a single Zenith Stromberg CD-175 fitted with a water actuated automatic choke. There were effectively three series for the 1974 model year. The earliest was a continuation of the 1973 cars. Then came the mid-range cars which were very similar but were fitted with large rubber blocks as overriders on the bumpers in lieu of the earlier small chrome with rubber face overiders that had been used since 1970. Then there were what was referred to as the 1974 1/2 cars. These cars had an increased ride height and were fitted with the large rubber bumpers like those on the 1975 and up cars. So the question becomes is this possibly a 1974 1/2 car or a 1975 car? If it is indeed a 1974 1/2 car it would be fitted with twin HIF4 caburetors. If it is a 1975 car and fitted with twin carburetors, then the exhaust manifold, intake manifold and the carburetors from an earlier car have been fitted. If that is the case, time to determine if they are HIF4 units or HS4 units.

Here are a couple of links to help make that determination:
mossmotors.com

HS4 Carburetor

HS4 Carburetor
mossmotors.com mossmotors.com
mossmotors.com

HIF4 Carburetor

HIF4 Carburetor
mossmotors.com mossmotors.com

I'm not sure when they incorporated the information on the HIF4 units into the basic factory service manual but at least until sometime in the 1974 model year, the HIF4 was only addressed in the Emission Control Workshop Manual Supplement, part number AKD 4957. If for some reason your manual isn't late enough to have the HIF4 covered, here is a link from the Oklahoma British Car Breakfast (couldn't help but wonder if that wasn't a play on words of Safety Fast) Club. The HIF4 stuff starts in Section D.6 on page D.14.

https://britishcarbreakfastclubcom.files.wordpress.com/2015/11/mgb_workshop_manual.pdf
 
Last edited:
Carlbanan56. I somehow or another missed the small Swedish flag. That means most of the stuff (the three sub ranges of 1974 cars and the single CD-175) above that is applicable to cars imported into this market would not be applicable to your car. Hopefully I did not send you off on a meaningless chase and if so, I apologize.

What is still applicable is that you should have twin HIF4 carburetors instead of the twin HS4 carburetors fitted to earlier cars. If you do not have a manual that addresses the HIF4s, you can still pull up that manual from the Oklahoma club and use it as a guide. If your car is fitted with a brake servo, make sure you check the vacuum hose to it as well as the check valve and the check valve seal.
 
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