MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Engine stops at 3700-4000 rpm's

I have a 1969 MG midget MK III with the original 1275cc engine. I recently had the engine rebuilt at .020 over with new pistons, the head was rebuilt with larger valves and unleaded seats, and I had a new APT VP266-BK cam installed. The crank, rods, and clutch/flywheel assembly were all balanced. I installed a new Petronix ignition coil along with a new Petronix electronic distributor. Also installed were new Cobalt "high performance" plug wires from Moss Motors. I have 1 1/4" twin S.U. carbs with standard needles. To have the car idle properly I did have to enrichen the carbs by lowering the jets.

Today, I set the timing to 32 degrees at 3100 rpm's. It had been set at 38 degrees previously and the car accelerated very well. (subjective, I know.) My understanding is the timing should be set at approx. 30 degrees at 3000 rpms so this is what I attempted today. I also read that 30-32 degrees was satisfactory so I left it at 32 degrees as stated above.

When I test drove the car all seemed well until I reached about 3800 rpm's in fourth gear. At this point the engine lost power and I pulled off the road. Within seconds the engine sounded normal and I proceeded home. Within two miles I again reached 3800 rpm's and the car lost power again. As the car slowed for a few seconds the power was restored and I completed the drive home. Once home I parked the car and ran the engine to 3800 rpm's and the engine again lost power. I immediately turned the ignition off and checked the carb. float bowls, both were full.

The feeling of losing power was just like the feeling I experienced in my BMW M Roadster when the rev limiter actived. I no longer have the BMW and I do not have a rev limiter in the MG.

Please give me your thoughts. Thanks, Keith
Keith Moore

Does it reach 3800 and stay there or does it die?
Art Pearse

hi Keith,
I had a similar problem in an old honda i had...
it would be fine until you floored it or went up a steep hill.
I replaced the fuel pump twice but it made no difference, eventually tracked it down to a kinked fual pipe under the car.....

Andy Phillips

It does sound like fuel related. Sounds as if the pump can't cope or the filter is blocked.
Max max-at-midgetmax-dot-com

I can't see how the standard needles could be suitable given the new engine now has a big valve head and higher lift cam. Has the distributor been recurved or just had a module fitted? The changes are significant enough to have the car set up on a rolling road,to correct the fuelling and make sure the ignition advance is optimised.
F Pollock

I thought it might be fuel related, so as I stated I checked the float bowls immediately after the last incident in my garage...the bowls were full. I think this negates a fuel problem, other than the needle issue F. Pollack mentioned. The distributor has not had anything done to it, it was just a simple electronic replacement. My Son's Midget has the same mods. except for the needles...He changed to a richer needle. Unfortunately his car is still in the final stage of restoration and he hasn't been able to run it on the road, so we don't really know how it may drive. I think I will ask him if I can try his needles for a comparison.

Does anyone see a possible electrical issue?

Thanks for your input.

Keith
Keith Moore

Try pulling the choke out a little at a time to see if it takes you through the bad spot if it is too weak. What air filters are you running?

Apart from too weak you could be looking at tappets too tight. Check the carb pistons are lifting freely when running.

Peter
Peter Burgess Tuning

If it only happens with a warmed up engine, then it could be binding valve stems. Overtight tappets that Peter mentions would cause this. When mine did that, the drop in powered was very sudden and felt just the same effect as the limiter coming in on a on a diesel engine. More pronounced than just won't rev any more, it felt like the revs being physically braked.

The other possibility is that the carb butterflies are not opening fully, if the cables are adjusted wrongly. But this would just stop the engine from increasing in revs, rather than the braking effect on the revs.
Guy W


The problem is super simple You screwed up the curve

You said....

####Today, I set the timing to 32 degrees at 3100 rpm's. It had been set at 38 degrees previously and the car accelerated very well.####

The car used to run well....then you re-adjusted the curve, and now it dosnt work to well...

put the curve back where it was and it should run well agian

My guess... your using a intermeddiate curve as your maxium curve...thus you are running out of advance long before the engine should

Try 36-38 degrees at 4500rpm for total advance

Prop
Prop and the Blackhole Midget

Just curious...

at 2000 RPMs...what is the degree reading

At 800 rpm... what is the degree reading?

Prop
Prop and the Blackhole Midget

I didn't check at 2000 rpm's, nor did I check at idle. I was double checking the max. advance timing, as when I previously set the max. advance timing I had difficulty with my digital timing light so I set the timing by ear.

Keith
Keith Moore

I agree with Prop- you changed the timing and now the engine is acting up, so what happens when you change the timing back?

Growler

####  I had difficulty with my digital timing light so I set the timing by ear.####

Yeah, Im all out of sarcastic comments at the moment and probably for the rest of the week... So I will be sitting this one out,

As often as people come on and have issues then we Find out...."oh, btw, I timed it by ear" we really need a stat pre written answer that can automatically be uploaded just to respond to that little highly valuable tid bit of info...its really gotten to the point of being so common, its not even offensive anymore.. and as far as laughable... wow, years ago maybe

Kieth...

I certianly wish you all the best... but to make any real progress from this point on you will need a functional working timing light...try harbor frieght, sears, walmart, ebay or amazon..a little patience you can find a decent one for under $20

Prop
Prop and the Blackhole Midget

This thread was discussed between 28/04/2013 and 29/04/2013

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.