Moss Motors, Ltd

I could use a little advice. The engine in my 77B rocks side to side and I'm not sure why. When the weather is cold, the engine is relatively smooth until it warms up. Once it is warmed up, the side to side rocking gets harder. I've tried to think of why it would do this, but other than the fuel lines and carburetor possibly getting too hot, I'm at a loss. Any ideas?

zach

zachmg77,

 I suppose the obvious question is: what age/condition are your engine & gearbox mountings? They do go bad over time.. subjected to heat & vibration, the rubber eventually gives up the ghost.

Ed

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I want my MGB

I would check the motor and trans mounts.  It sounds like the rocking is too violent to be a carb issue, but you might want to check the carb just to be sure, in addition to the mounts.  I was having rocking issues after installing my weber (my rocking issues were only apparent at lower RPM's), and it was a combination of transmission mounts and tuning the carb a bit.  Had to tweak my timing, the idle mixture screw, and install new trans mounts.  Motor doesn't rock at all anymore.  You may have SU's or a Stromberg but they're all carbs.

 

So that should at least give you an idea of where to start! 

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1979 MGB, Pageant Blue.
-Weber Conversion w/ header
-Back-Dated to points ignition, Lucas 45D Dizzy, Lucas Sport coil

Hello, Ed

Good question, and I anticipated it and meant to note in my original message that both the engine and transmission mounts were replaced about a year ago.

zach

rlich8,

Actually, its a Weber carb.

zach

Hi zachmg77,

frustrating - I just typed this, then the site told me I was logged out and lost my ramblings

Have you got a solid ignition system? Vacuum advance connected or not? Air leaks in the carb/manifold? Emissions equipment working properly?

The engine & rubber mounting system is just like the old mass on a spring, and as such will have a natural frequency of vibration. This tends to match the firing rate at lowest rpms, and vibration is much exaggerated by uneven firing. Turning this issue on it's head, I once read of an owner who, unable to fix a rough idle by adjustment, noticed a bad engine mount - presumably it was allowing the engine to recoil so much that it was not allowing smooth crank rotation. This is what I first though of when you mentioned your problem.

My B's engine idle speed & consistency certainly varies with temperature, slow when cold and in really warm/hot conditions. The system is in good condition, though the engine was fitted with a light alloy flywheel and this will compromise smooth running at slower idle speeds in comparison to a stock iron flywheel.

Of course just after I post, motorbill will zoom in, like a Lotus to my Morris Minor, with his most insightful suggestion ever. Just regard my comments as bait

Cheers,

Ed

 

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I want my MGB

Hi

   the harmonic balancer could be worth a look as well. Mine needs replacing as the engine does vibrate on idle. These have a rubber ring between the two metal parts that perishes with the oil and heat.

Regards

    Mark

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1958 MGA coupe ( under restoration
1972 MGB GT
1995 Ford Bronco
2004 Pontiac GTO

Thanks for your thoughts, Ed.

I installed an electronic ignition when I purchased the car about a year ago and replaced the engine and transmission mounts. I've checked for vacuum leaks and checked the carb to manifold mount for leaks, to no avail. The emissions gear was removed by the previous owner. The saga continues.

Hello, Mark.

I'll take a look into the harmonic balancer. Thanks for the suggestion.

zach

Hi Zach

   I have a question, does the engine smooth out at higher revs or still rock and roll???.  At the risk of being offensive, have you got the ignition leads on the right plugs. It is really easy to swap around no 2 and no 3. This will cause the engine to rock as well. Other possibilities are an out of balance or warped flywheel (does happen) , the timing chain stretched beyond spec so your cam is way off the mark, and a bent crank (dont laugh, I have actually seen this in a B series engine, that still ran.). These are extreme cases but still have happened. Now that I have you really worried I do hope it is something simple and cheap to fix.

 

Regards

    Mark

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1958 MGA coupe ( under restoration
1972 MGB GT
1995 Ford Bronco
2004 Pontiac GTO

Mark,

It does smooth out at the higher revs and the odd thing is that when the engine is cold, it runs the smoothest. Still rough, but smoother than when warmed up.

I'll check out some of those other items you listed.

zach

 

Just some added information, if I increase the rpms to 1200, she smooths out. Still some minor vibration, but much smoother so that my buns don't shake.

zach

Still no fix?  :(

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1979 MGB, Pageant Blue.
-Weber Conversion w/ header
-Back-Dated to points ignition, Lucas 45D Dizzy, Lucas Sport coil

Zach - is your tachometer accurate?

The reason I ask is that they tend to age, and start reading high. One Ebay bargain I bought read about 2x the actual revs, but they're easily reconditioned! It could be that your indicated 1200rpm is somewhat less than this, and you are just trying to get too low an idle, hence the percieved roughness.

It's just a thought,

 Ed

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I want my MGB

The problem is still there, but seems to have moderated somewhat, which has me happy/worried. I spent the day yesterday working on my daughter's car and trying to fix the ignition switch on the B. When I get the time, I'm going to give it a thorough going over to see if I can figure this out.

zach