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MG MGB Technical - test for bimetallic piece on HIF 4 carbs?

This is on a 74 mgb roadster with HIF4 carbs. Something is causing the front carb to go lean only at operating temperature, intermittent--sometimes it runs fine and other times it runs on 2 cylinders. I've checked vacuum lines, put on Crane XR700 electronic ignition--the problem seems to be in the carb. yesterday while I had it idling in the garage and it was cutting out, I pulled the #1 plug wire off, and no change in the engine speed. Then I pulled the #2 plug wire off and still no change. On #3 and #4 the engine almost died before I could get each plug wire back on. The front plugs are white like too lean but the back plugs are the correct tan color. I've tried turning the mixture screw up but the engine will still cut out in the same way. It acts like it's starved for fuel on the #1 & #2 plugs. Even if the mixture screw was too lean, it wouldn't come and go depending on engine temp, but just run lean all the time. Usually it only does it under a load but just once in awhile it will do it while idling.
The question is, how can I check to make sure the bimetallic lever on the mixture screw is working? If I put it in a pot of boiling water will it show a movement? I could put both of them in the pot to compare the movement.
Is there any better way to check it?
thanks for your advice,
Ken T
Ken Thompson

I forgot to mention, I realize that gas and water will have totally different heating and cooling characteristics so putting the bimetallic piece in water won't really tell much. But that's the kind of test I'm looking for. If I put it in a coffee can of gas and let it set in the hot sun for a couple of minutes, would that give the desired effect?
Ken Thompson

Ken. I would move the front bi-metal lever to the rear carb and rear to the front carb. If the problem
moves to the rear carb then you know the bi-metal lever is the problem.

If you are using Grose jet needle valves the following information may help. A few months ago I had problems with my rear carb running dry after starting from cold, it was an intermittent problem. I had Grose jet metering valves in the carbs and found the ball in the Grose jet was sticking in the closed position. I had four Grose jet valves for HIF carbs and only one would not stick in the closed position.
I had a fuel pressure regulator set to 1.5 psi and it wasn't enough pressure to unstick the balls in the affected Gross jets. I threw the Grose jets in the trash and installed normal SU needle valves, no problems since. Good luck. Clifton



Clifton Gordon

Gentleman;
I agree with Gordon (I usually do) Needle and seat problem. Also many moons ago at the dealer we had problems with the front float bending (we thought from heat) and causing usually a flooding problem. A float change may be in order? Bob/w0mgb
See ya Friday Gordon!
Bob Thompson

Following installation of a new engine in my 73 B, it initially ran like as though on two cylinders. Pulling the choke out a bit corrected the problem. Since the HIF carbs have a separate fuel supply via the choke rotary valve, I surmised that one of the carbs may have the main jet blocked. Both were rebuilt in a couple of hours with a basic kit and the problem went away.
B.J. Quartermaine

I switched those bimetallic pieces this morning. Instead of just re-creating the same problem, it gave it a very erratic idle and the same symptoms while driving. I'm sure the main jet is not blocked, but don't know about all those little emissions circuits. It does have gross needles and seats, and the ball on the end moves freely. All the jets, needles, seats, everything were replaced a few years ago. I still have all the old parts and will try switching back to them piece by piece to see what happens. And I think I'll try a new bimetallic piece. If I can't find the problem within a few weeks, I'll try to find a set of traditional SU's to retrofit!
Thanks,
Ken T
Ken Thompson

Ken. You said, "and the ball on the end moves freely". It sounds like you have the Grose-jets
for the HS carbs in you HIF's. The Grose-jet for the HIF carbs, Moss #386-340 have a plunger that
contacts the float tab and the ball is inside the valve body under the plunger. On the other hand
the Grose-jet for the HS carbs has a large exposed ball on the end of the body, Moss part #
386-390. Using the HS Grose-jets in HIF carbs will cause the floats to to have stiff movement
or may cause them to bind. How do I know? I have tried them and for me they didn't work.
The HIF Grose-jets worked well for about two years and then started having intermittent sticking
balls. Please let keep us posted on what you find. Clifton
Clifton Gordon

Thanks, I'll check on that after work tonight. The needle and seat definitely has a small ball on the end, opposite end from the rubber point. Those were in it when I got the car, and they looked to be in good condition so I kept them. I got new ones that didn't have the ball, but from what I've seen, everybody prefers the ones with the ball, so I used the old ones. Maybe that was the problem all along.
Thanks,
Ken T
Ken Thompson

I do have the correct Grose jets after all. They have a groove around the end just below the rounded surface that contacts the tang of the float. The PO had the other Grose jets with the ball end, that I took out. This is getting complicated!
I switched the bimetallic pieces back and it has the same idle and performance as before so that's one thing I should replace. This weekend I'm taking it to our local MG guru, Bill Wardlow. Check out his website:
www.motorwayltd.com

Thanks,
Ken T
Ken Thompson

Ken, Good site, looks like a nice place to spend time. Wish there was a place like
that near me. I like the cylinder head gravity racer. Clifton
Clifton Gordon

I took the carbs to Bill Wardlow this morning. Just as soon as he pulled the pistons out of the carb body, he found an imperfection that made the needle set at the wrong height. It's awfully nice having somebody around who is that good! He's originally from New Jersey and was a Rover mechanic back in the 70's, and actually attended a few MG training sessions in Leonia, so we're really fortunate to have him here.
He had a wrecked GT out in the back that had a Weber DGV 32/36 mmm set up on it, and sent the system home with me, with instructions to set it up on my MGB and drive it and see if that solves the problem. And it did! But now I've got to learn how to adjust a Weber because it feels a bit lean. The plugs look good, after about 10 miles of driving, but it didn't have the usual oompf that a good MGB will have. I'll have to check the jets. I would assume that if it's from around here, that it's jetted correctly. (We're at 4500 ft above sea level and one hour west in Rocky Mountain National Park the main road gets to just under 12,000 ft.) So now there's several hours of tinkering ahead of me!
Thanks,
Ken T
Ken Thompson

This thread was discussed between 27/05/2002 and 02/06/2002

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