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MG MGB Technical - rear carb too rich - needle valve/fuel press.?

Hi,

after starting up perfectly fine and good running during warm up while the choke is pulled, my 70 BGT (twin HS4, fixed needles No.6, K&Ns) starts to run uneven and even to stall when it is running with little load and having reached operating temp.

I suspect the needle valve, as the rear carb is a bit wet in the intake and plugs indicate it runs way too rich. (was balanced before)
However, it is a nearly new groose-jet and it passed visual testing and when I removed the fuel chamber lid and blew in "the connection where the fuel goes in", there were no signs of leaking by.
Float arm is bent to correct distance and float has no leak.

Could it be just too much fuel pressure? There is an aftermarket pump installed. (however, that was ok for over 15T km...)

Any other ideas?

Jochen
Jochen Beyer

Jochen. Is the jet coming up properly after you release the choke? I had a similar problem traced to the jets not rising back to the original position after release of the choke. Les
Les Bengtson

Grose Jets seem to be *at least* as prone to failure as standard float valves, probably more so given the likely difference in populations. Normally a leaking float valve (either faulty or too much pressure) will cause fuel to drip or pour out of its overflow, but if that overflow is blocked it will prevent the fuel level and hence the float from rising and shutting off the fuel supply, as its primary purpose is as a vent and not an overflow. In this case fuel *will* come up out of the jet and directly into the intake. Both of these will happen just with the ignition on and not the engine running. An SU fuel pump should not click more than once every 30 secs once the float chambers are charged. If it does so then either the carbs (or any of the fuel lines) are leaking or the non-return valves in the pump are. The fuel pump should only be delivering 2-3psi, if you go above this then yes it is possible that one carb will start to leak before the other.
Paul Hunt

As far as I remember the vent was not blocked. The jets seem to lower and rise as they should, however, that was all I could check right now, since I'll be away until friday.
Ordered a fuel pressure regulator though, maybe it does the job, if not, I have a neat "filter king" with a glass bowl and a pressure gauge in my engine compartment to look at while trying to fix the problem.
Jochen
Jochen Beyer

A pressure regulator will help but are you sure you fitted the right sized jet ?
RMW

Also possible that the needle is a touch too far up in the piston.

Also, check the top of the jet as on rare occasion the little bush pressed in there can come loose (making a .09" jet into a .1+" jet in a hurry).

Try swapping out the grose jet for a "standard one" to see if problem disappears. Although I've had terrific luck with them, I understand that some people have nothing but headaches.

Mike!
mike!

Mike may have a good point about the main jet needle being too far up in the air piston. This can happen if the needle is loose enough to move and was not clamped down tight enough. Usually, if loose, it will simply fall out, but its movement is a possibility that is fairly simple to check and correct. Just take the air piston assembly off and see if it's shoulder is flush with the bottom of the air piston and screwed down tight.

Bob Muenchausen

This thread was discussed between 12/12/2004 and 15/12/2004

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