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MG MGB Technical - su hif adjustments


I need some assistance with my twin SU HIF carbs on my '76 MGB (no emission equipment fitted.
I have been taking apart my carbs for some maintenance, used the Haynes book for the instructions.
After re-assembling everything and doing the final setting adjustments on a hot engine, it seemed alright.
Nice idle speed, no mis firing.
Took it for a ride, when going easy its fine but as soon as i go for the extreme ride, kick down to the floor so to say, it shoots from the exhaust pipe and the engine dies and comes to life again after some seconds,shoots again and dies again and so on, this goes on for say more than half a minute and then becomes sort of normal again.

I am not that experienced with the carbs stuff, so i can use some help here.
As far I can figure out I put all parts back in the right way, cheked it more then one time. Still got the same result.
Maybe there is some useful instruction on the net somewhere i can use.

Jacob
Jacob Halma

Jacob
On my previous set of HIFs, I had a problem with a misfire at 3-3.5k rpm. I recently replaced the HIFs after checking them over and the only difference between then and now is that the new set have butterflies from the HS (ie. no emission poppet valve on the butterflies). I suspect the older set had a sticking piston. If this is the case, do NOT try and clean the piston with emery paper, but clean the piston again in a carb cleaner soloution (some suggest using a metal polish). Check the fall of the pistons in situ, both should fall evenly and cleanly with a thunk. Have you got damper oil (engine oil) in the dashpot?

Firstly the engine needs to be properly warmed up, so go for at least a 10km drive. I set the carbs by getting the jet level with the bridge and then using a gunson's carbalancer, I set the idle speed and then played with the mixture screws until the speed rose, then reset the idle speed, and then leaned the mixture until the speed dropped, and then richened it slightly to bring the idle speed back up. All the time I kept checking the balance on both carbs. You should check the plugs to see if your mixture is too rich or too lean, and you can also check by lifting the piston using the pin, the speed should just rise and settle again.

However, it is essential to ensure that the timing, valve clearances, points and spark plug gap are all correctly set first before setting the carbs.

If you are getting backfiring from the exhaust, this is normally unburnt fuel, which would indicate an ignition problem, possibly.

Try these sites for more info:
http://www.teglerizer.com/sucarbs/techtip6.htm
http://www.sonic.net/~kyle/su.html

Martin
Martin ZT

Jacob,

I just went through the same symptoms after putting on rebuilt HIFs on my 1980 MGB. The car ran poorly with a Weber also, same symptoms as you mentioned, but I thought the HIFs would cure the problem. They didn't. Turned out it was a bad distributor cap and rotor. Hence the old adage "95% of all carburetor problems are on the ignition side". I also changed the plug wires to complete the job.
Chuck Hassler

The B's fitted with a 45D distributor have a habit of breaking the wire to the points inside the distributor as it bends with the operation of the vacuum advance. So, when you accelerate the vacuum drops off & the wire breakes connection. Disconnect from the points & try stretching it - preferably when warm so the plastic is soft.
Garth.
Garth Bagnall

Jacob, I feel your pain. I replaced my ZS with HIF4s about 2 years ago. I used the Moos video to rebuild the carbs and balance them I thought that I had set them up correctly but the car was not performing the way that I thought it should. I seeked out professional help in a mechanic that knew more than I would ever know and he balanced the carbs and tuned the engine to perfection. I have been driving it for two years with no trouble. ( I think that I should take it in for a tune up now.)
Don't be ashamed to seek professional help. It solved my problems and has proved to be well worth the money. I prefer to drive my '77 B Roadster than work on it and Scott at Chequered Flag has been my saviour.

Bob
Bob Anderson

Martin,
misfire at 3-3.5k rpm is also happening on my car. No sticking pistons, damper oil inside (not realy sure about the oil to be used and the level)
When I disassembled the carbs. I noticed a diiferent setting from the needles, for the reason of balancing the two carbs I made them the same amount sticking out. But what is the right setting of needles sticking out of the pistons?

Garth,
Checked the wire you mentioned, its OK

Chuck,
Maybe my carbs problem is a ignition problem as you mentioned.

I will try once more and use the info I got from guys, see what happens.
I will let you know the results

Jacob
Jacob Halma

Jacob
I was always taught 3-in-One oil was the correct damper oil, but I gather that on the Bs, it is advisable to use 20W50 engine oil (not that the oil would ever get warm enough to get to the hot SAE50 grade). This thicker oil helps richens the mixture slightly. As to level, it should find it's own level , but usually just up to the shoulder about 10mm down from the top of the dashpot.

Last night, I retarded the timing by about a degree or so as it was starting to pink below 3000rpm, and I also fitted AAA needles as I have K&N filters and a Stage 2 head. I could not believe how smooth and responsive she ran today! Whereas she was a bit lean above 35000rpm, she's now pulling well without any spluttering up to 5500rpm.


Martin
Martin Williamson

As Martin says engine oil, of what ever grade is correct for your engine and gearbox for your local climatic conditions. Different, incorrect and confusing info in the books as to level, one would have you topping-up after every trip. If you can unscrew the plastic damper cap, lift it up, press it back down, and feel the resistance of the oil before the threads on the cap meet those on the piston cover, you have enough. How much before tells you how much of a 'reserve' you have. You shouldn't have to top up in several thousand miles. But the reservoir in HIFs, unlike HSs, is drilled right through and then plugged, and I have known the plug get lost or dislodged letting it leak out very quickly. Both needles *should* be set to the same depth in the pistons of course, and for swinging needles the bottom of the needle guide cylinder (that the clamping screw locks on to) should be flush with the bottom of the recess in the bottom of the piston. Bottom of the shoulder on fixed needles in HSs, although I don't think there is a recess on the piston.
Paul Hunt

This weekend i will have some time to play with the MGB again, I am wondering how to see if I have the right floatlevel in my HIF4's when they are on the car. I made the settings as mentioned in the Haynes workshop manual. But still i have my doubts about the carb. for cyl. 3 & 4.
My fuelpump looks OK, tested it with the hose pulled from the carb and put into a jerrycan. Lots of petrol in an regular flow, nice ticking of the fuelpump.

When having a better look at the ignition side, i see that the PO must have had problems when shifting the ignition because its all damaged (hammer screwdriver).
I tried also to make some movement there but without any result. Any suggestions? I sprayed a few times some WD40 on, maybe it will go this weekend.

Jacob
Jacob Halma

This thread was discussed between 28/05/2005 and 09/06/2005

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