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MG MGB Technical - MGB with 2100 engine and Kent716 cam

One of our club members has recently bought a B with the 2100 engine and a Kent 716 cam. He is having great difficulty getting it to run well. The tick over is very variable and seems to wander from 1100 (lowest possible to achieve) to 2000 for no apparent reason.
Any ideas, what would be the correct procedure to correct this?
regards
Graham
G Cherry

For a start one should always be able to reduce the idle to a stall with the fast idle screw. If you can't the butterfly is either hanging up on the cable (insufficient free play), the linkages between twin SUs are incorrectly set up (one butterfly closed holding the other open), a butterfly isn't seating in the throat properly, the butterflies have poppet valves in them and they are sometime sticking open a bit, or possibly extreme wear in the throttle spindle and bushings allowing mixture to get past a fully closed butterfly. Sort that out, then you may well find that the varying has gone too.
Paul Hunt

IIRC the 716 is a pretty wild cam, may explain the high idle. Doesn't explain the fluctuation; what carbs are on there?

N
Neil

Found it: 302 degree duration... wild!

N
Neil

As of yet I have not seen the car, the comments were made to me at our last meeting, when the guy asked for help. The carbs are HIF4 and have had new needles but not new jets, are the set up proceedures similar to HS series carbs which I am familiar with, so if necessary we can start from scratch and set the carbs up correctly.

Graham
G Cherry

Don't know anything about HIFs, sorry.

But if it is really 2100cc with a 716 cam, then maybe there are other modifications eg big valve head, different distributor? I know the bigger displacement calms the cam, but maybe not enough...? Again it wouldn't explain the fluctuation, but ignition timing will have a big effect on idle speed. Also, if the carbs have the vacuum port downstream of the butterfly (which they do on some later cars) then the long duration cam may be having an effect on the vacuum signal.

N
Neil

Hi
I had the same problem on HIF carbs, if you solder up the poppet valves on the butterfly it solved the problem for me. Just remove the valve and put a dab of solder on the hole, do make sure the solder has taken because if it comes off it only goes one way.
I have heard of people using a nut and bolt then peening over the threaded part of the bolt to stop the nut coming off
Can not take credit myself an old time BMC mechanic told me what to do it was/is a common problem
Hope this helps.
Trevor
T. E. Jones

I bought plain butterflies to replace the ones with the valve in. You may then get run on though.
David Witham

Sometimes the obvious is overlooked, verify the hardware holding the carbs on is tight. I ran in circles for quite a while on my A before finding this.
Russ
Russ Carnes

This thread was discussed between 14/05/2009 and 07/06/2009

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