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MG MGB Technical - SU HIF4 CARBS

Any SU experts know what the problem is here?

I have just installed a set of very shiny recon HIF4 carbs on my MGB, only to find that fuel pours out of the overflow vent pipes when the ignition is on. At least the fuel pumps are working OK!

I have connected everything exactly as it was when i removed the old carbs (even labeling the various pipes to make 100% sure!). I suspect sticking floats on both carbs, but surely this would not be the case with recon carbs? Is it possible for the fuel pump to over pressure, supplying fuel even when the float chambers are full?Any suggestions gratefully received.

Many thanks
David Evans

Stuck float is possible. I had a stuck float one time. With the carb off the engine and in my hand I could not get it to stick. However when the carb was in place with the bottom of the fuel chamber removed the float would stick.
Too much fuel punp pressure is possible also. Many aftermarket pumps put out too much fuel. I believe 1-2 lbs is sufficient.
Make sure that whoever rebuilt the carbs didn't replace the float valve with grose jets. People either swear by or at them. I had nothing but trouble with them.
Bill Bresler
Bill Bresler

David,
I have a couple of questions first.
What year B?
Where did you get the carbs reconditioned?
Fuel pumps plural?

Now for the fix, did you try to give the underside of your carbs a sharp rap with a hammer HANDLE or otherwise try to loosen the stuck floats? It could be the float was jarred into a stuck position while the carbs were in transport.
It could be a poor quality rebuild that neglected to replace the float and float pivot pin.
Give that a shot and let us know.
I don't think overpressurization is a problem with an SU pump.
Tom A.

Ok, I'm not the HIF4 expert (yet, as I am about to convert to a modified set of HIF4s), but the first thing that comes to mind is that perhaps you're connecting the fuel lines to the vent tubes, rather than the fuel inlet connections. I know on later models of the ZS carb, they switched the two around, so if you "even marked everything so you'd connect it up right", you'd be connecting things backwards. If you were to do this, then it would shoot fuel back out through the inlet connections after dropping in from the vent connections. I'm probably off here, but it wouldn't hurt to take a look (maybe they switched things around on the newer model). It does seem odd that you'd be seeing this for both carbs otherwise...BTW, I wouldn't beat up my new carbs just yet. And it is unlikely in the extreme that your fuel pump pressure is causing this.
Barry Kindig

Hey Barry, please give us a full report on any difference the HIF's give over your modified ZS. And thanks for pointing the way to Mike & Sean Brown- I'm sure there are many of us grateful for that!
Terry

Terry, it should be interesting indeed. Mike's and Sean's excellent work speaks for itself, so I'm sure someone would've eventually spread the word...they've created components I could not have otherwise obtained for my B and thus increased my fun factor several fold.

Have fun,

Barry

David E., any luck? I hoped you managed to clear the log jam. If not (and you're sure everything's routed correctly), it might simply be a good idea to return the carbs for another reconditioned set.
Barry Kindig

I have been told by people who have been servicing MG cars for at least a generation that one downer for the H1F model is this tendency to have the floats hang up when the carbs are R and R'd, and that some garages created jigs to hold the set, on removal from the car, in the same position as when installed. I once installed these carbs and had the same problem from one carb, and I part unscrewed the machine screws fixing the base in an effort to get the hung up float free. However, it is amazing what some light banging on the carb body may accomplish- works like a dream with the pumps- sometimes.
Ken R
Ken Rich

David,
I guess I need to clarify what I posted earlier. Please don't beat the crap out of your new carbs! Just a rap with a wooden or rubber or plastic handled tool will do.
When I purchased my 73 B eighteen years ago I had the carbs rebuilt by a local foriegn car repair place. I soon after found myself with the stuck float problem. Anytime I did any 'spirited driving' I would have gas pouring out of the charcoal canister and anti-run-on valve all over the ground.For two years I lived with this problem, every time returning the car to service by the above method.
When I had a winter to work on the B, I pulled the carbs and tried to figure out this annoying and potentially dangerous condition. It turned out that the clearance between the float and the internal float bowl on these carbs is very small. After years of use, the float and the brass pin it rides on get worn to the point that the float can snag on the inner walls of the carb.In the same way an old door sticks when its hinges are worn. In my case they were hanging up on a bit of casting flash from when the carbs were made.
Frustating problem, easy permanent solution. Replace floats and pins and check for any rough spots on the inner walls of the bowl that could snag the floats.
Never had the problem again in the 16 years since!
Sorry this got so long, but hope it helps
My 3 cents...
Tom A.

I had the same problem, fuel pouring all over the place. The float or the needle were not stuck, but in closer inspection, the float was half full of fuel, and would not float anymore (hence the float became a sink...).
Maybe the reason was the US unleaded with 10% alcohol, because after carving a little hole to the float, draining it and filling the hole with superglue, and using alcohol-free fuel, the problem has not returned (The car has been sitting still for 12 months awaiting restoration, but started every now and then).
Kari Sillanpaa

David,
Did you check your float level adjustments? I've also had jets stick open due to teflon sealing tape fragments finding their way into the jets.
Gary Cash

This thread was discussed between 05/03/2002 and 07/03/2002

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