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Author Topic: Carburetor Adjustment  (Read 1245 times)

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  • American MGB Association
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  • American MGB Association
    • American MGB Association
Carburetor Adjustment
« on: May 12, 2019, 11:23:12 AM »
I have been messing with a pair of HIF4's that we put on my '79 B that recently was put back on the road.  My mentors with my car recommended that I go with dual HIF4's, I have a slightly upgraded cam in the engine, all of the pollution control stuff is pretty much off. 

The issue is that the car gets around 3500-4000 rpm under load and starts sputtering, and will continue to run, but no power.  I have watched your HIF4 videos multiple times, and everything prior to the carbs has been checked and is in order. The needles in the carbs were changed to AAM's, and the original needles were AAU's. 

It appears that they are running very lean, because if you slightly move the piston up manually with the screwdriver as you show - the engine will stop.  Also when I turn the adjustment screw clockwise to make things more rich the idle seems to drop.  I ran out of time on Sunday night, and needed to walk away from the car.

So - should I leave the AAM's or go back to the AAU's, and is that correct that the idle should drop as I make things more rich?  Also - how important is fuel pump pressure - I am running a aftermarket pump - I know that we checked the pressure and flow a week or so ago, and it was ok - per my friend JAy - I can not remember exact numbers however.

Sorry for rambling on - I hope that I gave you enough info about the adventure.

Joe Stout
Washington, PA.
American MGB Association
phone/text: 773-769-7084

North America's oldest and largest club for all MGBs, MGB-GTs & Midgets.  A nonprofit organization founded in 1975 and offers color magazines, emagazines, technical advice, member recommendations on service shops, registration of your MG, a tradition of service & FREE member classified ads that appear in the magazine & on the website.


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    • University Motors
Re: Carburetor Adjustment
« Reply #1 on: May 12, 2019, 11:42:40 AM »
How are you venting the float bowls?  They should be connected to the charcoal canisters but there must NOT be a low point in the hose.  If there is, then fuel can collect there and prohibit atmospheric pressure from pushing on the gasoline in the float bowls.  Sometimes I see those lines plugged Ė sometimes I see them connected to each other.  WRONG on both counts.  They must be open to air pressure.  If they are connected to the canisters, disconnect them to see if thereís a difference.

Fuel pressure should be low Ė maybe 2-3#.  If itís too high then the carbs overflow and thatís not your problem with this miss.
Float height is critical. Use 90 weight in the dashpots.

After all this, try changing the needles.  You can make a determination about the mixture by examining the plugs after a run at a certain rpm.  Just donít let the car idle after the run.  Pull off the road and check the plugs right there.  Black is too rich; tan is just right; white is too lean.

John Twist
University Motors Online
John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids, MI  49506


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