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Author Topic: Rebuilt HIF4 Carburetor Problems  (Read 463 times)

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Rebuilt HIF4 Carburetor Problems
« on: August 09, 2020, 01:35:01 PM »
I have a 1973 MGB with original, recently fully rebuilt, HIF4 carburetors.

Since the carb rebuild I am having a very difficult time starting the engine when it is cold. 

When the engine is at normal operating temperature it is very easy to start and it runs fine with a smooth idle around 950 rpm.

I am unable to get in touch with the technician who rebuilt the carbs and am wondering if you have any suggestions as to what I could try to rectify this problem. 

I'm wondering how far the fast idle screw needs advance along the choke cam at full choke to ensure full enrichment?  I checked the mixture looking for the 50 rpm blip that you mention in your video.  Not sure if I noticed that working alone without a tach but the engine neither raced or stalled out when I move the pistons with a small screwdriver.  If the carbs are set too lean would that be part of the issue with cold start problems even at full choke?

The service that included the carb rebuild included setting the timing, tappet clearances, new Champion plugs.  The car has Pertronix electronic ignition.

Thank you for any suggestions/guidance that you can provide.

Joe Prior
Toronto, Ontario, Canada


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    • University Motors
Re: Rebuilt HIF4 Carburetor Problems
« Reply #1 on: August 09, 2020, 01:43:46 PM »
When you pull the T handle choke cable from the dash, it should move as much as two inches.  Ensure that when itís pulled to itís fullest that the interconnecting link between the carbs has rotated the choke cams as far as theyíll move.  That alone makes the mixture so rich that the MGB should start in even arctic conditions!

When setting up the fast idle, pull the choke out until the engine rpm reaches itís fastest.  Usually, if you pull beyond that point the rpm begins to drop.  Then, balance the carbs at an idle speed of about 1600-1800.  Thatís all there is to setting the choke idle.

John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids MI 49506-2410
Cell  616 307 6737
John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids, MI  49506


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  • chfwrench
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  • MG information: '73 red B roadster
Re: Rebuilt HIF4 Carburetor Problems
« Reply #2 on: September 15, 2020, 09:10:05 AM »
Hi Joe,

The HIF4 carbs have a feature that is often overlooked.  They have internal Thermal 'Temperature Compensators' (Moss Part No. 373-590; about $40 each).  These are bi-metal 'L' shaped pieces inside the float chamber that hold the jet and, if working and adjusted properly, regulate the mixture when cold.

They work like a thermal choke on an American car, but not as dramatically.  The thermal action raises and lowers the jets slightly for a richer mixture when cold/leaner when warm. 

There is a screw adjustment on the side body of the carb that would set the mixture when cold, but these wear and can fail with age.  If they have not been replaced in the years you own the car, chances are they need to be.

Might want to check their adjustment and/or replace them if your problem continues.

Safety Fast!


Art Isaacs
AMGBA Tech Staff


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