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MG MGA - Engine Vibration

This is a follow on from my earlier thread - Vibration at Start up.

1960 MGA 1600 (1600 block)

Motor now has about 1,000 miles since complete rebuild. Bored 0.20, crankshaft 0.10, Flywheel weight reduced by about 7lbs, modified cam, (lift both ex & in = 0.371 duration @0.050 ex 216 & in 227) described as "Sporty" power at 2200 - 2500. MGB headers and big valve head Inlet 1.625 and exhaust 1.343. CR 8.5. Flywheel, crank, con rods, pressure plate all balanced.

Idle set at 1000rpm, using 95RON fuel (without ethanol) spark plugs each a light grey colour after a 230 mile run yesterday (mostly driving +2500RPM). distributor standard Lucas, vacuum advance operational, however replaced with a new unit no noticeable change in performance. Advance 20 degrees before TDC at 1000rpm. engine does not ping.

Cylinder head re-torqued at 500 miles, valve lash rechecked replaced engine oil 20/50 and new Oil filter. Cleaned carburettor dashpot(s) verified float level, and re-checked airflow / jet setting. Standard No 6 needle. Given spark plug colour these settings should be ok.

Cylinder head was not refurbished, it was unbolted from the previous motor which had 50,000 miles since rebuild. Checked compression today on cold engine (stationary for 24 hours) winding over the motor to give 3 pulses with Wide Open Throttle gave the following; 1@125, 2@120, 3@110, 4@110. (5 pulses gave higher figures of 145, 140, 120 & 120)

Problem:

Engine is rough at idle (the whole car vibrates) then smoother from 1500 to just below 2500 where vibration returns. Once about 2700 is reached engine is very smooth when compared to the previous motor (1622). Vibration is present both when stationary or driving. While the vibration remains as above it is less noticeable after complete warm up.

Possible solution;

(1) Given the Larger valve head, MGB headers, modified cam, do I need to use a different carby needle, maybe richer up to say 2000 rpm.

(2) and or Modfy distributor advance curve.

(3) Another possibility, maybe I did not match the flywheel / pressure plate correctly after balance and have orientated incorrectly 180 degrees. Before removing the motor to check would prefer to exhaust other possibilities.

(4) Varience in compression is causing the "roughness" at lower RPM, disappearing at + 2700. The fix being a refurbishment of the cylinder head.

Any thoughts please.

Russell







R J Goebel

One thing to check is for a vacuum leak...The rough idle, and smoother at higher rpm's can be a symptom of of a leak.
Take a close look at the vacuum connection at the rear carb...then at the advance....
I had a problem with the one at the carb, and didn't know it...It caused a similar problem to yours.
Check the carb stand-offs for cracks, and finally, all the manifold bolts for tightness.
You might try the "propane trick"...by waving a propane torch (not lighted, of course), around the manifold, while running...If rpms increase, you have a leak.
It is unlikely that your balanced rotating assembly is the culprit, as you said that the engine developed the problem , after driving it for a while....(at least that's the impression you gave).
Edward
Edward Wesson 60MGA

Thanks Edward,

First up apologies the vibration has been present since the rebuild, it has improved since the first drive, the best improvement being resetting the valve backlash at 500 miles. I have gone over everything distributor, carboy, checked all fasteners. All of these items resulted in a small positive change, however the vibration remains as detailed in my first post.

I am leaning toward a vacuum leak. When originally installed the "vacuum bulb" output was at the bottom (it had been installed this way by a previous owner at least 13 years ago) so during the rebuild I used the opportunity to install correctly......perhaps all of the re-bending of the vacuum line has resulted in a fracture. In addition the line had been cut (about 2 inches down stream from the bulb, the resulting two pieces of tubing being rejoined with a bit of rubber tubing. This modification was by a previous owner makes R&R of the carbs very easy.

Could I test by removing the vacuum line from the distributor and plugging? If it runs smoother then that should prove a problem in the line.

Back to the vibration:

If it is the flywheel / pressure plate is possible that the vibration would disappear above certain revs?

Russell
R J Goebel

I still don't think it's your rotating assembly....Most engines do not get a balanced rotating assembly, and they do not vibrate, at least not enough to notice....If it were a racing engine, and running normally at revs above 5000 rpm, then I would agree, but this is a road car.
Plugging the vacuum line might help, but it has to be plugged at both the dizzy and the carb...
Cracked insulator can also be the problem.
Edward
Edward Wesson 60MGA

"Plugging the vacuum line might help, but it has to be plugged at both the dizzy and the carb..."
Why?
Art Pearse

I plugged the vacuum line and went for a test drive, no difference to the vibration. Also checked that the engine was pulling a vacuum on the line and this checked out ok.

Next step, will fit another distributor previously used on the old 1622 motor and known to be ok.

Will also track down the John Twist video about compression testing and run another test.

Russell
R J Goebel

Art
To check for a vacuum leak at either end...the manifold or the carb. Even a small vacuum leak can lead to a lot of vibration at idle speeds.

Russell
Do you hear anything unusual along with the vibration?
Also, are you using the red or black rotor in the dizzy?
Edward

Edward Wesson 60MGA

OK Ed, you plug each end one at a time to check the line?
Not both at once!
Art Pearse

We don't know if the line itself is cracked, or the leak is on the carb end, or on the dizzy end...(If there is even a vacuum leak)....
I suppose you could just block the nipple at the carb, and see if there was any improvement...If there was, then you would have to figure out if there was either a crack in the line, or a leak at the advance....
If there wasn't, then you would have to look for a leak elsewhere in the manifold (like in the insulators, or gaskets), before you could move on to other possible causes.
Edward
Edward Wesson 60MGA

Edward,

Swapped out the distributor with an earlier one that had been on the old 1622 motor. Not a direct swap disassembled both and changed over the vacuum plate and Vacuum advance mechanism from the 1600 distributor to the previously used 1622 distributor.

Result, vibration has gone.

Possible diagnosis / observations;

I noticed after setting the timing on the 1600 motor that the position of the Vacuum advance can was pointing at about 11:00, this seemed odd as it had always pointed at about 12:00 on the old 1622 engine. At this time I checked the mechanical advance mechanism and it seemed ok (sprung back to its stop- just). Both springs were in place and the "counter weights" seemed free to move.

Today, when installing the "old" 1622 distributor I noticed the vacuum advance can now points close to 12:00 after the timing was set.

Given that the newly installed distributor has the mechanical advance mechanism from the old 1622 distributor and the vacuum plate / vacuum can from the distributor previously installed on the new 1600 motor I conclude the mechanical advance mechanism must be faulty.....will have a good look tomorrow.

Thanks for all the help.

Russell
R J Goebel

May be you had an incorrect primary and/or secondary spring causing an incorrect ignition curve until you reached the limit of the mechanical curve, after which the vacuum advance worked as advertised.

Perhaps worth a browse through this document: http://www.jcna.com/library/tech/tech0015.html (tuning the Lucas distributor). I used it to realise that my newly refurbished original distributor had the incorrect advance cam plate fitted.

Steve
Steve Gyles

I have to admit to being confused here. Russell initially speaks of an engine vibration and now we are talking about distributors. Do we have vibration which means something in the engine is mechanically out of balance or is it just a misfire? To me the latter isn't vibration. Is this just a misfiring, badly tuned engine Russell?
Pete
P N Tipping

Pete,

Apologies for any confusion. The problem seems to have been too much advance in the lower range RPM. I believe the spark was occurring at the end of each compression stroke, however maximum advance was reached somewhat lower than 3500 RPM. This early advance seems to have caused the vibration, rather than a misfire. There was no pinging, which would have alerted me earlier.

Steve,

I remember reading this article many years ago, got to have a refresher.

Russell
R J Goebel

Russell

Certainly sounds like incorrect mechanical advance springs. It would account for your symptoms that you have just described. Did you check to see what cam plate you had fitted. You need one that is labelled about 13 degrees. This would give you 26 degrees mechanical advance, plus the 7 degrees static advance, giving around 33 degrees total mechanical advance at 3500 rpm with vacuum disconnected. That is how I basically set up my engine.

Steve
Steve Gyles

Steve,

Currently have a 40761A model Lucas distributor fitted, these were originally fitted to the Mk2 1622 motors. Interestingly these distributors have a maximum advance of 14 degrees at 2500 RPM (28 degrees @ 5000 engine RPM). As opposed to the original 1600 distributor (40510D) which has 12 degrees at 1500 RPM (24 degrees @ 3000 engine RPM)

I plotted each of the points for the 40761A distributor and assuming 10 degrees of static timing then dynamic timing will be 32 degrees at 3000 engine RPM. Ultimately maximum advance would be 38degrees at 5000 engine RPM.

The cylinder head spec on the engine is closer to 1622 than 1600 so the 40761A advance curve might be more suitable - I have since refilled with 91 octane and still no ping or run on at shut off.

Of course this assumes the 50 year old distributor has retained its original advance curve.

Russell
R J Goebel

This thread was discussed between 17/04/2014 and 22/04/2014

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