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MG MGB Technical - Variable idle speed

As the engine gets hotter in traffic (and I mean after 15 or 20 minutes after the gauge has reached its normal temp) the idle seems to drop. I normally set it at around 850 rpm (runs smooth) but it drops to around 700 rpm and does not run so smooth.
The carbies are well tuned and balanced, the throttle bushings are new and the little valves in the butterflys are soldered up.
The dashpots have SAE 20 wt oil in. I run K&N filtres and have #6 rich needles.
Any ideas
SW Tompson

Normal for HS (?) carbs. Some fuel pools in the throat of the carb, upsetting the mixture, which is why tuning instructions say to run the engine at 2000rpm or so for a few seconds or so every 30 secs. HIFs have a port from just in front of the jet to by the butterfly to stop this. When mine was running-on I had to use the fast-idle to stop it stalling in a long hold, as increasing the idle speed made the running-on much worse. Now I have fixed that I have upped the 'normal' idle so that it is still adequate even in a long hold. The standard idle speed was increased from 500 on the earliest cars up to 850 on the latest as emissions controls bit. My UK 73 should be 750, which is fine, but any lower and it really shudders.
Paul Hunt

It's a 71' B with HS4 Carbies.
IS there nothing I can do?
Even when I blip the throttle it still drops to the lower rpm.
Cheers
Thommo
Thommo

This might seem a bit out there but check that your exhaust manifold nuts haven't slackened off at all. If they have, check the manifold gasket isn't damaged (replace it if it is) and tighten up the nuts. If the manifold is slack, this could be one cause of your changeable idle.
Tim Highman

I share the same problem on my '69B. It gets worse during hot weather, and no I don't think there is anything you can do.

The problem has been explained to me as follows...
When the under bonnet temps get very high as when stopped in traffic (no airflow through engine), the fuel in the fuel bowls starts to vaporise more and more until it starts to boil. With so much fuel vaporising due to heat (rather than needle/seat venturi effect), the mixture gets very rich and the idle speed starts to suffer as the motor splutters with rich fuel.

There are two solutions to the problem, rev the engine regularly to cool the under bonnet temps and get the water pump going, and secondly, pull on your choke cable a little bit (< 0.25") to increase the fast idle (no extra mixture) by opening the butterflies a bit. You can drive safely with the choke pulled on a small bit - just remember to release the choke when you turn off the engine, otherwise she'll run on.

James
James

This thread was discussed between 02/08/2003 and 07/08/2003

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