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MG MGB Technical - Engine vibration

I have a 77 BGT and finally fitted a new exhaust.At last it sounds like a proper MG. I'm currently trying to sync the carbs and make it run smoothly. The head has been skimmed with a new Payen black gasket and tappet clearances set. The problem is with the engine itself. It rocks quite a lot during idling albeit it is running rough. Is it possible to change the engine mounts without a hoist or lift, i.e jack under engine ? Also any tips on balancing the carbs and adjustment would be appreciated. Seen the J Twist vid online but the close ups are not good enough to determine which screws are being adjusted.

Regards Paul L
Paul Liles

buy a copy of the Driver's Handbook and it will tell you about balancing the carbs and lots, lots more an owner needs to know

you could use the Handbook in conjunction with the JT vids

Ref: 0053 -

set up the engine - (tappets), CB points, plugs, timing and carb mixture in that order - if you have to adjust any item in that chain then you’ll also need to check, and adjust if required, all the items that follow it in the chain

bear in mind with the set up figures that they were for a new car 37 years ago so use them only as an approximate starting point
Nigel Atkins

What idle speed are you using? At what point does it smooth out - if at all?

I had to set mine a bit higher than book on leaded petrol, it has to be quite a bit more now with unleaded, even 98 octane, or it rocks and shudders badly. Also with the HSs on mine the idle soon starts to drop if stuck in traffic, a higher idle overcomes that as well.

Unless you know an engine mount is bad it's not worth the effort of changing them.

Nigel's order is correct, but the carbs have to be synchronised for air flow at idle and just off idle as well as each set to the correct mixture. I've always used the lifting pins for mixture, although in HIFs (which I presume you have) it needs care and practice to detect the ideal point. While learning it's easier to set it so it is just perceptibly weak, then note how far you turn the screw until it is just perceptibly rich, and set it between the two points. However with a 77 you have to meed emissions limits. If you find the mixture exceeds that then you will have to weaken the carbs, but only ever do both by the same amount or you will unbalance them again.
Paul Hunt

Your engine shouldn't be shaking that much if it's only a bit out of tune. I'd check the compression, in each of the cylinders, and compare them to see if one of your cylinders is considerably lower than the others. RAY
rjm RAY

Not all handbooks have it. The earlier ones did, but between them and the rubber bumper version this helpful info was replaced with "take it to your dealer".
Paul Walbran

thanks for that Paul I'll try to remember that, been a long time since I've seen a B handbook and that was for a '73

the handbook is still worth having for hundreds of other bits of useful information
Nigel Atkins

I hope you got rid of the opus ignition and used a distributor that works with dual carbs, plus the coil should be looked at for ohmeric value for the ignition system youve chosen. These simple components are crucial to obtaining a well tuned engine. Goodluck! Man I hope your motor mounts are ok! Late model mounts are the hardest to replace by far...not impossible but hard!
Steven Devine

"and used a distributor that works with dual carbs"

Eh? Since when did the choice of distributor depend on whether you had dual or single carbs?

And which Paul? The original poster is in the the UK, so presumably a UK car, which never had the Opus system and always had dual carbs.
Paul Hunt

Are you saying only the US got those acursed single units with catalytic bombs attached! You are probably right...I just figured you got the bastardized units like we did those last few years!

Steven Devine

Paul, to makes things simple check the colour of the plugs, if they are all a light brown, I would leave the mixture alone. You can check the balance by taking off the air filters and listen to each carb with a length of garden hose. If they sound different, undo the the clamp nuts on the throttle linkage between the carbs, so you can adjust each carb separately untill the tickover is about 900rpm and they sound the same.
Engine mounts on rubber bumper are fiddly, mine are not in the best of condition but the engine does not rock.
c cummins

The MGBs, destined for sale in the UK, had dual HIF4 carburetors until the end of production in '80. They never had to deal with the single Zenith Stromberg carburetor and catalytic converter that was standard equipment on US bound rubber bumper cars. RAY
rjm RAY

Nor with 99% of the emissions stuff either. The sum total of UK changes was to move the distributor vacuum take-off from the carb to the inlet manifold, and that only in September 76. Even then the distributor remained the same, which gives the lie to the oft-made statement that there are 'carb' distributors and 'inlet manifold' distributors.
Paul Hunt

This thread was discussed between 24/04/2014 and 27/04/2014

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