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MG MGB Technical - su carbs starved?

Hi,

My 79B seems to run fine in daily driving, but when I am doing a slalom race where you throw it in a tight corner fast that's where problems happen. I have dual SU carbs put on. On the way out of the corner the car seems to starve on fuel and almost cuts out most of the time, and once in a while it stops totally (and then won't start for a few minutes). I think that it's starving , but I suppose it could be floading.
thanks,
Gordon
Gordon Thompson

Do you have HS or HIF carbs? HS carbs have a separate float bowl which can cause the symptoms you describe. HIF carbs have the float bowl integrated into the carb body and are supposed to be better in this respect.

Tim.
Tim Jenner

If it cuts out and won't restart for a few minutes that is something more fundamental than just fuel being thrown away from the jets under high G, and even fuel pump failure wouldn't cause that unless the float chambers had emptied. When it won't restart I'd check I was getting a spark to all four plugs and at the right time before thinking any more about the carbs. What does the tach do when it cuts out? Drop instantly or keep registering revs?
Paul Hunt

The float bowls are a separate unit, HS SU carbs. And the rev's dropped down to nothing when the problem starts, sometimes if I step on the gas heavy I can keep the engine running. I had checked the timing and sparks before (tuned it up before the slalom), but I'll check it over again. the fuel pump does seem to be working well.
Gordon
Gordon Thompson

Gordon:
Check for the leaky straw syndrome. Ever suck on a broken straw for your drink? More air than fluid, right, even though your suction was as good as ever. Look for broken/cracked rubber hoses, loose fittings, and other ways for air to get into the suction side of either the fuel pump or the carb jets. This may not be your problem, but I have seen it overlooked more than once.
Bob Muenchausen

While i agree that what Bob says is a must, I'd also suggest that if to your knowledge the needles and seats haven't been changed in the last 30- 50,000 miles, do so. You might be very surprised after you do.
Ken R
Ken Rich

Thanks Guys, I'll give the air/straw/hose thing a try , then new needles and seats (as I don't know what has happened before the last 10,000km).
Regards,
Gordon
Gordon Thompson

Hi Gordon
I had this problem on the parade laps at Silverstone on 22nd June. Entering the corners on a trailing throttle then flooring the pedal caused the engine to miss and stall - very embarassing - tho it restarted easily enough. Everything seemed fine after we got off the track except that the oil level in the SU dashpots was low. [I was well within the mileage interval but had not actually looked at them for some weeks and there was no problem in 'normal' driving.] As my wife was due out on the next session in a few minutes I dripped in some engine oil from the dipstick. When she drove it the problem had gone and as she sweetly pointed out it was possible to go much quicker without squealing the tyres!
Regards
Vic
V Todman

Also, Gordon, don't discount that you may have crud in the tank or jet hoses that acts as a "flapper valve" whenever suction is applied to the fuel line. It will allow delivery of just enough fuel to get you going, and maybe even keep you going at low demand, but once demand becomes high enough, the crud gathers and closes off the feed tube in the tank or pulls the crud into place to block the flow in the little flexible hose between the float bowls of HS-4 carbs and the jet tube, starving the carb. A clean tank and fuel filters ahead of the fuel pump and the carbs can help keep this sort of thing to a minimum.

One other thing often overlooked is condesate water which has accumulated in the fuel tank. The pump will suck it up just like fuel, but it doesn't burn like fuel and often gives similar symptoms to fuel starvation - just enough fuel mixed in with the water to fire (well, sort of) but not enough to really allow the engine to run properly. FWIW
Bob Muenchausen

This thread was discussed between 29/06/2003 and 04/07/2003

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