MGB & GT Forum

My 1971 B has been running rough for the past couple months and I've run out of ideas. The current symptoms are a rough idle and a misfire at 3k and above (with the alternator light flashing). At idle, the timing light is jumping around +/- 5 degrees, sometimes jumping as much as 10 degrees.

The jumpy timing is clearly a problem and I've replaced everything that I could think of that would cause the problem. I figured that the problem had to lie with the distributor, the distributor drive gear or the timing chain - all have been replaced or checked and the timing is still jumping around.

I rebuilt the distributor with an Aldon Amethyst electronic ignition system with a Pertronix trigger. The distributor shaft didn't have much play and I welded the rotating assembly in the distributor into a single piece to make sure that the feed into the electronic ignition was rock solid (the Aldon Amethyst requires the mechanical and vacuum advance to be locked out as the unit replaces those functions). I checked the end play in the distributor drive gear and found no measurable play before reinstalling the distributor.

After the distributor install, the timing was still jumpy so I replaced the timing chain, sprockets and tensioner. The old chain was worn and replacing it helped, but it didn't solve the jumpy timing.

I'm getting ready to throw in the towel on this problem so any ideas for a next step would be greatly appreciated.

Dan

Could it be just the timing light itself too sensitive the adjacent cable

Try with another timing light from a friend, MG club, or old mechanic around the corner.

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1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.



Edited 1 time(s). Last edit at 2015-10-06 08:00 PM by joron.

Did you replace the dizzy springs? If there's any clockwise play, they're shot. This will cause the timing to jump around.

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

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73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

I had jumping timing when I bought my '71. It turned out to be the person who last replaced the timing chain forget to release the chain tensioner.

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Dan, Which rotating plate did you weld? The points carrier or the mechanical advance assembly? How did you lock out the mechanical advance (if not already answered) and how did you lock out the vacuum advance?

Beverly eh?

Hmmmmm.

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Michael J. Caputo
'79, '77, '76, '74.5 (rubber dual SU), and '73 owner. Extensive experience in 12v Audio System design and installation. Vendor of Regalia and Promotional Products since 1993. Supplier of Accessories to MOSS. Forum Member since 2009; with a warped sense of humor since birth. Publisher of the annual MGB & GT Calendar, mailed worldwide.

Jeff Schemmer started a thread on this subject a while back.
The issue is the clearance between the distributor drive gear and the plate the distributor mounts to.
I believe that John Twist has done a video on the subject.

Hi Rick, I have searched but could not find either the tread or video you mentioned. Could you give some more clues as this is something I would like to investigate more too.

Thanks
Marlow

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1975 MGB

You've already checked it.

You've all latched on to the jumpy timing but skipped over the lumpy running, misfire and flashing alternator light. Are we looking at a crappy earth connection somewhere?

Thanks to everyone for taking a look at my post.

To lock out the mechanical and vacuum advance, I removed the mechanical advance weights and springs, then welded the advance in the fully advanced position. I also disconnected the vacuum advance from the points carrier then welded the two halves of the points carrier together so it wouldn't rotate. The end position of the advance assemblies isn't particularly important, but rather that everything is locked out from moving inside the distributor (aside from the rotation of the shaft/rotor to trigger the ignition, of course).

I saw the posts about how end play in the distributor drive gear can cause the timing to jump as the gear walks up and down below the distributor - I checked the end play during reassembly and didn't find any wear that would cause this problem. I actually removed the drive gear and the gear looked to be in good condition as well as the upper drive dog engagement surface.

I released the tensioner when I installed the timing chain so I don't think that there is any issue there. The jumpy timing actually improved slightly after installing the new timing chain so there was probably some play caused by the old chain - it just wasn't the only cause of the problem.

Bruce - the rough idle, misfire and flashing alternator light are definitely things that need to be addressed as well, but I'm just hoping to knock out the jumpy timing in the hope that it will at least smooth out the idle. In troubleshooting the misfire and flashing alternator light, I ended up finding the jumpy timing, but that certainly doesn't necessarily mean that its the only cause of my running problems. One of the first things that I did was replace the coil, which fixed the misfire and flashing alternator light above 3k for a few hundred miles. I also test ran the car with the alternator unplugged to see if the alternator was causing a power surge above idle, but the case still misfired above 3k. I don't think that I mentioned it before, but the tachometer was also jumping up and down when the alternator light flashed -- do you think that a bad ground somewhere could cause these symptoms?

Also, I wanted to add that I checked the gaps on the rocker arms to make sure that all the valves were closing and I ran a compression test - all cylinders were between 150-155 psi.

The alternator should never be taken loose from + battery or it will go to very high voltage and probably internal failure as a consequence.
Always leave its thick +battery wire connected, however the line to the warning lamp may be taken off temporarily if you so desire. But it does no harm...

My gut feeling tells me that somehow the battery feeds either on + or minus/ground are no longer OK or something is shorting this circuit from time to time without blowing a fuse.
That situation would explain intermittent flashing of the alternator warning light AND ignition failures showing themselves via the tach ( I guess your 1972 tach is the RVI kind so this figures )

It's a pity you do not have a normal dizzy with points left now, in this case feeding the coil temporarily directly from the bottom fuse position and taking the one but bottom fuse out would soon tell you more about the feeds condition without checking all battery wiring going to and fro inside your car and its relation to the ignition cicrcuits.

PS You do know you can always start the car any time by letting the whitebrown wire on the starter relay touch this bottom fuse location, don't you?

I'm not certain about this but, since you locked the distributor in an unconventional manner, you may have taken the rotor out of phase with the distributor terminals. That would cause the spark to jump excessively and would worsen at higher speeds, straining the coil.
Easy to check. Mark on the distributor casting the center of terminal 1.(The 25D4 has a raised mark on the rim which can be used to align the back edge of the rotor.). Remove the cap and position the rotor in firing position and note if rotor is in position to fire the terminal. Normally, the vacuum advance holds the contacts in phase when adjusted properly and Jeff has written about major adjustments taking the distributor out of phase.
Oddly, what you describe could be caused by a failed condenser in a points ignition.

This is a cut and paste from a thread from Jeff at Advanced distributor from 5 years ago.


This is a situation I've seen a lot recently. Here's the skinny:

Timing marks jump around both at idle AND at high rpms, even with a rebuilt distributor. I've hashed this concept out with several "bigwigs" in the field, and the general consensus is this: The drive spindle in the block that acts as an intermediate shaft between the cam and the distributor probably has wear. If that shaft has end float and is allowed to climb up and down the cam gear (both have helical cut gears), the end effect is a change in timing. With no load on the engine, this change in timing is displayed clearly as jumpy timing marks.

As a secondary effect, the total amount of timing your distributor gives you will grow by the amount of travel the spindle can rotate as it climbs the cam gear. I had one customer (and MGE member) find 14 extra degrees of advance in his engine due to this problem.

The repair: well, its not a simple one.
The cast iron sleeve (distributor housing) that holds the gear in the block needs to be trimmed down to reduce the end play in the drive spindle. I'm talking about the face of the sleeve that sits against the engine block. This is a step that gets missed in most engine rebuilds, and since the spindle isn't available new, is a common issue. That's where low mileage engines benefit in a HUGE way - less wear!!! The back side of the sleeve can be milled or filed down to reduce the end play in the drive spindle. By hand, filing could take a couple hours and a lot of patience. The good news is that you can easily check your own progress by simply reinstalling it and checking end float in the gear. You're looking for just a few thousandths of an inch - barely perceptible movement end-to-end and virtually no rotation in the gear. Use a long bolt (air cleaner bolt?) as a handle in the drive spindle for good leverage.

If this repair still leaves your timing marks jumping, address the timing chain tensioner or the distributor - the distributor is the number one cause of jumpy timing marks (worn out advance springs).
Jeff

Is there a way to defeat the Amethyst's advance algorithm?
Maybe it is confused about the engine environment (rpm and/or vacuum), and is trying to adjust the timing prematurely?
You could try an experiment at cranking speed with plugs removed and see if the timing is stable there.