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MG MGB Technical - popping out carb

I have scoured the archives and can't get a handle on this...I have a '79 B with a HIF 44 carb that has been running great for the last 4 years.Now when I floor it it starts to misfire and pops out the carb at about 3500 rpm
if I just accelerate normally it is ok.
I have changed the dizzy to the pertronix flame thrower one and a new coil and wires,I have adjusted the valves,I have installed a new Facet fuel pump,I have adjusted the carb to what I think is correct.But I still get the problem at about 3500 when stepping on it.The compression is a bit low on 1&4 but within 10% of all cylinders and
will come up when I squirt some oil in them so I know the valves are ok

Any ideas?
Thanks Pat
patrick bailey

Popping out the carbs usually indicates ignition when an intake is open, either because the ignition is at the wrong time, or the valve is open when it should be closed. Maybe a valve is sticking. By disconnecting one plug lead at a time you should be able to determine the cylinder.
Paul Hunt 2

Pat,

there are distributers that are designed for manifold vacuum and those for ported vacuums.
Normally cars of this period were exported with dizzys designed for manifold vac and this worked well with the Stromberg carb. Most applications of the HIF44 made use of ported vac.
Check what type of dizzy is fitted and where the vac line is connected to the inlet. May be there is a 'crossover' of both systems on your engine and this would show the results you want to get rit of.

Ralph
Ralph

The distributors are identical in operation with carb vacuum and manifold vacuum, the only difference in use occurs at idle. There were many different distributors for North American engines, including when the vacuum take-off changed from carb to manifold, and it is this that leads people to assume that the two must be different and cannot be interchanged. UK cars kept the same engine and distributor for the whole of rubber bumper production, including when the take-off moved from carb to manifold. It is simply an emissions measure which gives lower emissions at idle. Stromberg engines used manifold vacuum (as did SU carbs from 1971) but do have a port suitable for distributor vacuum that is originally connected to the EGR valve. If EGR is removed this port can be used for the distributor, or indeed the port can be teed to both devices as there is no air flow through either, the vacuum simply pulls on a diaphragm. Carb vacuum gives easier slightly starting.
Paul Hunt 2

Pat,

some time ago i bought a 1975 B california version and had the same problem. Although Paul is right with the information he gave, i changed to an other distributer that came from a MG 1300 (D25) and did not have the problems you described any more.
As it seemd to be a falt on the original D45 42599 i had it checked by a Bosch service station on a distributer test bench and there was nothing wrong with it.
With the SU, ported vac. and the dizzy from the A-Series engine the engine in this car did work O.K. again and did not show these problems anymore. I did not do any further investigations upon this malfunction of the original dissy but it seems to give too much advance for this engine configuration.

Ralph
Ralph

Reading your list of replaced items, I did not see a spark plug replacement. Try changing those also.

Look at secondary coil wiring as well. Time has a way of deteriorating this one.

Let us know how it turns out

cheers
Gary :>{D
79 MGB
ghansen

Gary
The plugs were the first thing s I changed I went down below the other day and it didn't do it at 1500 ft, I live at 4000 ft so it has something to do with altitude
It must be leaning out at altitude Any other ideas?
Thanks Pat
patrick bailey

Did not realize you were in thin air. Carburation is obviously effected by altitude. A richer mixture and some ignition timing may need experimenting.

Again, let us know how this works out

cheers

Gary :>{D
79 mgb
ghansen

This thread was discussed between 22/06/2007 and 26/06/2007

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