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MG MGB Technical - Idle readings and what the book has

Doing a tune up on my 67 B GT. My car runs nicely around 900 rpm idle. I note in my factory service manual that the idle should be more like 500. I can't get the engine to even run at much below 700 rpm.

I do not know if the engine had a cam change that might affect the idle.

500 seems a very low idle.
BEC Cunha

I have had the same experience with my '64 roadster. It has the original engine which has not been messed with. I can get a pretty smooth idle down to about 700 rpm, but nothing lower than that. It seems to idle best at around 900 rpm, so I just set the timing accordingly and don't worry about it.
don scott

Bruce,

I just went through this with the 70 MGB I care for. The owner said the idle had gone to heck, unstable, dropping down to 500 and the alternator light would come on.

Since we had rebuilt the carbs a couple of years ago, when she first got it, I surmised it was time to replace plugs, rotar, cap and wires. She has a hall effect device instead of points in her distributer. We also reset the timing. Besides removing the plugs and cleaning them, and reseting the timing, this was the first attention we had given the ignition circuit. She has put about 10k miles on the car since she has owned it.

When we were done, the only thing I had to do was lower the idle back down from about 1000 to about 800 with the car in neutral, and the clutch out. Pushing the clutch in drops the idle another hundred RPM, which is a bit low.

It seems to be somewhat temperature dependent, so it does wander around a bit. I would live with whatever idle you have for now, note the conditions you are experiencing, and go for the nicer 900 rpm if it works better on your engine.

Quite honestly, I don't think any B series engine idles well at 500, and if it does, the owners Tach is probably off.

warmly,
dave
Dave Braun

A smooth idle at 500 rpm is possible on the early B series engines that had heavy flywheel / clutch assemblies - e.g. the MGA 1500.
Chris at Octarine Services

From the Leyland Workshop manual idle speed was 500 on non-emissions 18G engines, 900 on emissions 18G engines, 850 on emissions 18V engines, and 750 to 850 on non-emissions 18V engines. But that was as new and on high-octane leaded petrol for the most part. 30 years on, and with very different fuels, you have to pick your own. My 73 UK roadster always idled roughly at 750, was much better a bit higher, but then started falling foul of running-on even on leaded. In a long idle it also tended to stall as the idle gradually dropped, a function of the HSs With unleaded the running-on was so much worse I had to fit an anti-runon valve, which allows me to idle at 900, which has also overcome the tendency to stall in a long idle.
Paul Hunt 2

Is there a link between the octain level and the idle?

I was having a few problems with my idle which was about 1300 or so when fully warmed up. I was able to get the idle down to under 1000 and decided to let it run for a few days and see what happens. I have noticed that the idle has creeped up to about 1100 after the car has been running for a while.

GG Ginty

For otherwise identical fuel, maybe yes, maybe no. But there is such a big difference between the original fuel and fuels today, octane is just one of those differences, and it certainly does give different engine running characteristics, and not just idle.

If high idle is a problem i.e. you can't get it down to stalling speed with the idle screws then there is something wrong with the setup of the linkages, cables, butterflies or there are very worn throttle spindle/bushes.

It's normal for the idle to be higher when fully up to temperature than when half warm, for example, but in my experience after that, if left idling, and with HS carbs, the idle will drop. I've not noticed this on HIF carbs, and I put that down to the thermally compensated jets in those carbs.
Paul Hunt 2

This is a good thread. I have never been able to get my '64 B to idle at lower RPMs. About a year or so after I bought the car (probably about 12 years ago), I had a tune-up done on it by a guy who used to race them, and he had it idling really low (probably at 500 RPM). He took me on a test drive (he drove), told me you "point" other cars, but "drive" MGs, and fiddled with the choke the whole time he was driving. He pulled it out at stop lights, feathered it in and out as needed, etc. The car was really hard to drive without it stalling out, so a few months later, I up the idle and its been there ever since. I thought it was just me and my car.
Bill Barge

I can set my car to idle at 750 indicated, which an external calibrated tach read as 700 actual. It's Ok at this but I usually keep it at 900 indicated where it is a lot smoother and seems to keep it's RPM if forced to idle for long periods. I put this down to the fan pushing a lot more cooling air through the engine bay at the higher RPM. Before I did all the go faster stuff it would idle at the 500 RPM specced by Abingdon and with the stock manifold and exhaust in good condition was almost inaudible, and where's the fun in that?
Stan Best

my experiance altho limited because i havent had the car very long is that 500 is far too low
mine wont idle at 500 it stalls and wont take up very well
its currently set to aroung 750 when cold and nearly 1000 when warmed up
at idle its very smooth (about the smoothest of all the running of the car)
i guess its just a matter of getting it set to what suits you and your car
john
J LEARMONTH

Idle between 700 and 1000 rpm seems to be O.K. on most MGB 1800 engines. The recommandation of 500 rpm for idel was given when there wher no changes to the combustion chamber done and large, heavy tapperts were used, meaning it only was mentioned for early cars.
For making things easy i allways would set idele at 800+ rpm if the engine is fitted with the small tapperts.

Ralph
Ralph

Not sure if you read the watch new condensors posting I put up, anyway after we got back from the natter last night I re-set the idle. The car had gone up to 1200 rpm indicated from 900 as result of fitting the new cap measuring 0,25uF and with good contacts on its connecting loom. I put it back to 900 indicated which is about 800 true. It runs nicely, the ign light just flickers sometimes it keeps cool-ish and just sounds nice! This does seem to show that the spark is lot better as well.
Stan Best

This thread was discussed between 29/05/2007 and 07/06/2007

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