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MG MGB Technical - High RPM Break Up

I have been posting about a fuel pump problem, that seems to be working OK but I am back to a high RPM skip that has plagued me for a few years. My car is a 1973 Chrome Bumper in pretty good shape. Low compression, no OD. Peco exhaust modified to Peter Burges stock manifold specs. Cabs rebuilt new needles that correspond to the K&N air filters. New coil, new dual point dizzy. The Mallory Dizzy has a number of springs for the cyntrifical advance and I have tried every possibility. Vacumn advance removed. I have been maticulas about setting the points, removed the dizzy set points on the bench reinstalled. removed dizzy after unsucsessful drive and checked points, spot on. I next loosened the dizzy and tried all conceavable advance and retard settings while driving. Some settings better than others but none releaved the problem. My feeling has been a timing problem and I still think so but I'm at a lose as to what to do. This is getting real old.
Ken Knize

Your timing may be retarded at higher speeds since you disabled the vacuum advance, or did you mean you disconnected it while timing the engine?

How did you set your points? If you didn't use a dwell meter, get one. It will show a more accurate reading of where your points should be set. It will also give you a smoother idle.

Have you tried switching distributors? Do you know what your current timing curve is? My suggestion is to install the lightest springs you have for starters. Maybe put one light one in, and one slightly stronger one if that doesn't work well.
Jeff Schlemmer

Hi.

Assuming that fuel delivery is adequate (have you measured this ?) I would guess that it is a weak spark problem, Jeff's suggestion of measuring the dwell would be my next step.
As the revs increase the coil has less time to build up magnetic flux, which can result in a weak spark at high revs.
Getting the dwell right helps a lot (as does fitting electronic ignition).

Don't forget that a new component is not necessarily a good component.

Don
Don

Great, first my MG won't work now having problems with my computer, I was just able to get back on line. Weak spark was my thought when I went to the Mallory dual point, longer dwell. I also replaced the coil. The Mallory is strictly centrifigal advance. Vacumn has been eliminated. The dwell meter is one thing I havent tried, I guess because I dont know how to use one. I have been old school feeler gauge type. I'm sure the meter will have instructions so no need to get into that. The old distrib was toast actually wabbled so my only option is the one now in the car. I have however give thought to electronic ignition I don't think they are readily available for my distributer, but your advise incourages me to try.
I will also check my books on how to measure fuel flow. As far as the springs, last summer I decided I would find the right combination once and for all, I don't think I missed any option, no joy, gave up and let the car and myself think about it for a year. New energy and the 5th wheel is out of the driveway this year. I guess I don't have to play golf tomorrow, I try a dwell meter and let you know how I make out.
Ken Knize

Ken - Read the last entry on your previous thread "Replacement Fuel Pump". I tell you how you can measure fuel flow in that. Good luck - Dave
David DuBois


Ken, FWIW, I also have the problem you are experiencing with the Mallory dual point. Mine was eating up the condenser. New ones went out in a relatively short period of time. I heard this is common problem with these distributors. I used a condenser for a Ford and it seemed to work better and lasts longer. It still has a slight skip at high revs but not as noticable as before. pertronix does have a electronic replacement for these distributors which I'm going to try.
Mike

I thought about the dwell meter and decided that rather than that I'll order the electronic ignition for my Mallory distrib. Why buy something I may only use once to convince myself that I need the electronic Ignition. Well the rep I got in touch with at Moss cannot find it in their system. Would have been nice to take adventage of the current price break. I anxious to get my car on the road. I didn't intend for her to sit in the garage for 5 years. Any advice on who might carry the Petronix Electric Ignition for the Mallory dual point distrbuter?
Ken Knize

I forgot to mention that I checked out the flow from the fuel pump and Dave I found that it is a little shy of the flow you told me about but very little shy. Not enough to cause the problem I have, at least in my opinion.
Ken Knize

Hi all.

May I describe what I feel is the ultimate answer ?.

I have an old Sparkrite electronic ignition unit on my 80 GT, which uses the standard distributor points to trigger a capacitive discharge unit.
This provides healthy sparks at high revs irrespective of dwell, massively reduces the load on the points, can be quickly switched back to standard ignition in the (unlikely) event of the electronics failing, and is appropriate for the age of the car.. what more could you ask for ?...

Don
Don

Don I tried to find the unit you speak of and I came away with the impression it is no longer being built. Do you have any information to the contrary?

Mike KCK, Any ford condenser?

Thank you everyone for your help the car is in souch good shape otherwise it kills me not to be able to drive it.
Ken Knize

Hi Ken.

I don't know where (if anywhere) these units can be purchased new, they were quite a common retro-fit in the late 70's and 80's and used ones do turn up occasionally.

Some more modern cars have Transistor Assisted Contacts, which reduce the load on the points but do not offer the enhanced spark generating properties of CDI (Capacitive Discharge Ignition).

I would guess that modern optical types could be readily modified to operate from conventional points, but most of these do not appear to be CDI.

Does anyone out there know of a source of contact driven CDI units ?.

Don
Don

It is a well known problem for Mallory condensors to burn out. This is caused by too thick plating on the condensor housing which doesn't allow it to ground correctly. You must sand the condensor where the screw attaches it to the dizzy body to get a better ground. A bad condensor will cause all kinds of nasty problems.
Mike MaGee


Actually Mike I think the condenser sold with the Mallory dual point distributors are not up to the function they are suppose to resist. Not surprising since not many are sold. I was in a situation that I needed to get the car home and attached a Ford V8 condenser that had its bracket bonded to the condenser. It worked.

Ken, go to http://www.carshopinc.com/index.php/cPath/21_24_25.
They advertise the electronic replacement for Pertronix. Check the particulars since I haven't made the purchase of this part.
Mike

Thanks for the advice. I'll try the condenser trick. I did get a petronix ordered should have it next week.

My new 5th wheel trailer came in today so the MG won't get much attention next week. I wish I had a way to pull the MG behind the 5er.
Ken Knize

About 30 years ago my race B, developed a miss at 5,500 rpm. The cam had a small flat spot, just before the top of the curve, that would float that valve. I know its a long shot but it can happen.
Leland Bradley

hello
just for the record spakright2000 was sold by halfords and have long gone out of production best chance of finding one of those little red box of tricks is at a auto jumble its a small red box with gold finished heat fins that clips onto your coil i think the full name was sparkright sx2000
Richard H
Richard H

Before geting too excited about the dizzy, timing and other gizmos - have you tried changing the plugs?

I find that high speed misfires are sometimes caused by electrode insulation breakdown.
Chris at Octarine Services

Yes chris I have changed plugs and wires. Initially I couldn't find the timing marks, seems my books had them printed upside down or may it be lookin from under the car. This has been such a long process I've just been throughing mony at it. If I think it could be the fault buy a new part. I would like to detox the car but befor I make any changes I would like to get it running right. Next step wait for elect ign.
Ken Knize


Ken,
When you get the new electronics installed let us know the results.
Mike

Not sure what I have learned but this is current results.
1. I still haven't received Moss order, no elect ign yet.

2. Installed Ford condenser, condition much worse. Haven't tried a better ground on Mallory condenser guess I'll try that tomorrow. What's that Dear it's not going to rain, then golf it is.
Ken Knize

The only person who had a similar problem to you was someone also with a mallory dual point dizzy also fitted. Strikes me the problem lies there. Have you tried putting a standard dizzy in and see how that goes..? Otherwise you have changed just about everything else in the ign system and still not found the trouble.

If you know someone else with a B I'd ask to borrow their dizzy (offer them a re-tune afterwards or something) or else pick up a unit at an autojumble and try your luck

~PHIL
Phil

Thanks Phil, Old Dizzy was toast, it wabeled. Hopefully the Petronix will arrive this week. You see it in fact didn't rain today and golf it was. Although it would be nice to take my clubs for a ride in the MG.
Ken Knize

This thread was discussed between 07/08/2005 and 22/08/2005

MG MGB Technical index

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