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MG MGA - 1600 engine resonance/vibration at 2600 rpm

engines, I
Andy Preston

Before you panic about the engine, check the U-joints in the propshaft. Also, does it vibrate in the same rpm range in 3rd gear (engine), or does it vibrate around 3600 rpm in 3rd gear (propshaft).
Barney Gaylord

Hi Barney it vibrates at the same rev range in every gear (T9 5 speed) and also when it's not in gear and not moving. I did install a new clutch and pressure plate and did not have those balanced with the flywheel. Otherwise it's very smooth and loves to drive at 3000 to 3500 on the freeway all day.

Thanks Andy
Andy Preston

Although it's unusual that it would happen under no load, I would try making the mixture a couple of flats richer to rule out pre-detonation. If the needles are incorrectly profiled, it will run lean, which then manifests itself as a vibration at the range where the needle is letting too little fuel into the engine

If the range of vibration shifts with the change in mixture (I am assuming that the carbs are correctly set up in that they are balanced and one is not running rich and the other lean) then you know it is mixture related rather than a mechanical issue.

I'd also try shifting to a higher octane fuel as a further check that it's not fuel related. 10.5:1 is very high compression, and a higher octane will also reduce any pre-detonation issues across the range. If it is pre-detonation, it will be happening even when you don't feel it, and it will wreck the engine quickly, so fix it soon.
dominic clancy

Further to Dominic, you might want to check the distributor mech advance curve. Running 8 retarded over the full range is pretty drastic, and not very good for fuel mileage or running temps, especially for exhaust valves. The curve may still be such that you have too much advance at that rpm. Max sensitivity to advance tends to be about 3000, depending on breathing etc. I always suspect advance for problems in the 2500-3500 range. The nearest dist I have numbers for is early MGB, which gives max centrifugal advance of 20 deg at 2200 rpm. You may need to reduce this, or bring it in slower/later. This is a prime case for a rolling road tune; if it is a timing issue it should be obvious. And, if this is the case, the thing should be a lot better once it is sorted.

FR Millmore

Pretty high compression for an old style engine Andy !
I guess it could exacerbate any minor internal problem that would not bother a std 8. + comp. Before jumping in the deep end
remone the belts to eliminate the water pump and fan also the generator. Ask me how I know ! Chasing a simular problem on our under warranty
Mercedes the Alternator was the culprit, after Mercedes stuffed around looking for an internal problem. On ignition timing ,a new Pertronic Electronic Distributor will give you early B advance curve. They are available on the net for around $200. Good luck Sean
S Sherry

Thanks for your advice guys. The highest pump octance gas I can get in Ca is 91, but I filled up with 96 octane at the local race track last week and it made no difference to the vibration.

I start with the easy recommendations before doing anything to drastic. I'll also send the distributor off to Advance Distributors for rebuilding and see if that helps.


Andy Preston

The change to 96 octane with no difference tends to rule out detonation from timing error, but doesn't mean there is not a chance for great improvement by getting the tuning correct. I note that you "guess" that comp is 10.5, so not a reliable figure. Still suggest RR before sending dist away.

If it is a balance error, either from something like Sean suggests, or the clutch, or error in the balancing done, then I would expect the vibration to be there at 2x your observed problem rpm. It's pretty rare that you get such vibrations in a range you can actually double, guess you are lucky!

FR Millmore

At that compression, timing and tuning are going to be critical, and if the carbs are out of balance as well you do't have a chance. if it stops vibrating when you REALLY richen it up you will know it's not a balance problem, but as Fletcher says a RR is probably the only way to find out what is actually going on. Mine has much lower compression, but hates even 94 Octane, so I run on 98. Even then it will predetonate if the mixture is too weak, even if the timing is spot on.
dominic clancy

Andy my memeroy tells me that these engines required 100 Octane leaded fuel when new ? I also understand that low Octane means low grade and less H/P & MPH.. We get what we pay for. Sean
S Sherry

Low octane does not mean low grade or less H/P. Higher octane is used to prevent pre-ignition or detonation in high compression engines. It may also help in reducing run-on after switch-off. If your engine is on the edge of requiring high octane or not, you may have the choice of more expensive fuel or retarding ignition timing a few degrees. If you think you need to retard timing more then a few degrees, there is something dramatically wrong with the engine setup, not the fuel.

The addatives needed to increase octane rating displace some of the energy producing components of the fuel, so high octane fuel may have a tad less energy than lower octane fuel. Ergo, high octane fuel may give slightly lower fuel mileage. You may get more power with high octane fuel, but only if your engine is set up to take advantage of it. Higher compression (not too high) makes for higher thermal efficiency, so it could produce more power and better fuel mileage with higher octane fuel, if it is done right (but then costs more to operate).

If you don't have any problem with pre-ignition, pinking, or detonation, then higher octane fuel is a waste of money. MG engines are noted for run-on, because the were (at the time) pushing the limits of compression ratio (wanting high octane fuel). Run-on by itself is a non-issue if you just put it in gear and let the clutch up after switch-off. Or you could get fancy and install an anti-run-on valve.
Barney Gaylord

Hi guys, I removed the fan belt and generator and there was no differrence to the vibration. It still peaks at 2600 and dies away around 2800.

I returned to racetrack and filled up with 100 octane gas at $8 a gallon. I returned the timing to factory spec and of course have no pre-detonation with the higher octane fuel. I checked the carb balance and they were perfect. I enriched the carbs by 2 flats on each and there is little difference to the vibration. It still coes in at 2600 or though it seems less or though that could be wishfull thinking.

Could the new clutch pressure plate be out of balance because the flywheel was lightened 3 lbs less than a B flywheel? There also seems to be a grumbling type noise coming from the driveshaft and rear end. The driveshaft is new and came from Hi Gear with the 5 speed so should be OK but I think it's worth taking it out and having it balanced jsut in case. The grumbling rear end is probably play in the ring and pinion. Could this have any effect?

Many thanks,


Andy Preston

Weight of flywheel has nothing to do with clutch balance.
If the flywheel was balanced correct;y, a correctly balanced clutch will not affect it regardless of the weight of either. Given today's parts quality, out of balance clutch is possible.

Under these circumstances, "grumbling" bothers me a lot. Flywheels or clutches that are coming loose of the crank cause grumbling before the explosion. Such grumblings travel up and down the driveline.

Did you try it at 5200-5600?

FR Millmore

Accelerate the engine with the clutch in and see whether the vibration is there. You have to isolate the source of the vibration, i.e. the engine or drivetrain. Mark your damper pulley and see whether it walks on you. I have seen bad damper pulleys on other cars.
Mike Parker

FWIW, I have owned a 1600 coupe for 8 or 9 years and I have a bit of vibration in my mirror in that rev range. I don't think the original mirror (which I replaced shortly after getting the car) vibrated; which made me think the replacement mirror was lighter and not as adept at damping the vibration.
M. D.

This thread was discussed between 31/10/2011 and 09/11/2011

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