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MG MGB Technical - Starts from cold not when hot???

Anyone help please...


1979 MG BGT...Twin SU's

Been standing for about 6 months, All service items were replaced before laying it up..

Took it for MOT at weekend.. started quite quickly.. ran well but on the way to the MOT station revs rose to just over 1000rpm

i quickly adjusted the throttle stop to lower it slightly and it passed..said the readings were really good for a twib carb setup.

But it really struggles to start when warm or hot.
I left it for a few hours & started 1st turn..

Where do i start?

Im thinking a rich mixture, BUT the MOT readings were good? And it drives really well.


Thanks in advance


FOZZ





FOZZ

I'm thinking the same thing (rich) so you should test that first to eliminate the simple. (How do the plugs look?) If you know how to adjust the carbs, I'd do it. If you're not familiar, it goes something like this:

There are lifting pins on the sides of each carb. With the car at 800 rpm, lift each pin, about a 1/4 inch. Work on one carb at a time. The ideal is to have the rpms increase slightly and momentarily. A rich mixture is indicated by a sharp rise in rpm and a lean mixture produces a sudden drop. Adjust mixture nut one flat at a time and retest. The threads on the mixture adjusting nut are standard--clockwise to raise the jet(lean) and counterclockwise to lower the jet (enrichen). So as you look and work from the top, clockwise becomes counterclockwise, etc.

Familiarize yourself with all the adjustment pin/nuts first as you need to work somewhat quickly before your plugs start loading up. If it takes more than a minute, stop and rev the engine at 3000 rpm for thirty seconds or so and try again.

One last thought, I believe you should synchronise each carb idle first, do the above, and re synchro the carb idle. Don't forget to top up the damper oil. HTH

Paul
Paul Hanley

Fozz

You say the tester said the readings were good, did he say what they were or just he thought they were good, remember that the older cars are totally different to modern cars, what’s his experience with old cars and does he know what is normal for them? [No disrespect to the tester but it happens, my son is a MOT tester and the first time he did my MG he was surprised at the allowable results compared to the newer cars he normally tested].

The MOT test station should have given you a paper copy of the electronic test results. [It’s a legal requirement that they keep a copy with their copy of the certificate and the machine I know prints out 2 copies, one for the customer].

The printout will show the CO level [should be set at 3%] and the engine emissions in parts per million (ppm) [less than 1200 to pass]. The CO is the mixture so will tell you if, for the test only, if the engine was running rich

Brian
Brian Davis

When it won't start put a timing light on each plug lead to check you are getting a spark - the ignition can develop heat-sensitive faults - and when on No. 1 check the timing is approximately correct. A 79 will have manifold vacuum advance which will tend to advance the timing during cranking which is not ideal. See if disconnecting the vacuum pipe and plugging the carb end makes hot starting any easier, or you could crank with the throttle wide open, but remember to let go when it catches :o)

Another thing is to check the coil boost circuit is working. When the engine is idling put a voltmeter on the coil +ve with the other side to earth. You should see about 9 or 10v. Then measure it when cranking and it should be about the same. If it drops to about 7 or 8v the boost circuit isn't working, which can also make starting harder. If it keeps starting OK when doing this last test (Sod's Law) remove the plug leads.

Paul Hunt

This thread was discussed on 12/05/2004

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