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MG MGB Technical - Dual Point Mallory w/ Petronix

After months of replacing every thing I could on my 73B roadster I am still having problems. The wabely orig dizzy was replaced with a mallory dual point but never could get it in tune. I bit the bullet and installed a Petronix and in the garage I noticed an immediate improvement however now that I can road test, things are not going well. I think my problem is the wrong combination of centrifigal springs in the dizzy so I'm wondering if someone else has installed this dizzy w/ a petronix and if so what spring combination you have used?

I have a rebuilt SU fuel pump converted to solid state. the carbs have been rebuilt. I removed the A/C compresser, Air pump and alternator. A Saturn alternator was installed and the air rail was removed.

The car performes in the following manner. It starts immediatly without touching the throttle. It idles at about 500 to 750 RPM, very steady idle. When I apply throttle she coughs a little until 1000 to 1250 RPM. As long as I am not too agressive with the throttle (what fun is that) she runs very smoothly to 2000 to 2250 RPM. than starts to break up and cough. If I keep the RPM's between 1000 to 2000 and baby her she runs exceptionally well.

I think that I have the wrong spring set in the centriffigal distributer. Both springs are the lightest. I think I should have a stiffer primary spring and a very stiff 2ndary spring. This will give me less advance on getgo and the 2nd spring will take over at higher RPM. Some one asked me not to long ago about my curve, I didn't understand what that ment but I guess this is what was ment.

Any one with similar experience.
Ken Knize

Ken, what is your timing set at at idle? How many degrees of advance does the distributor offer?
Jeff Schlemmer

Jeff, Thanks, Because of the electronic ignition I have not been able to set a static time but at low idle I have been setting with a timing light at approx 9 to 10 degrees. My engine is low compression and my manual says to use a lower advance, I didn't think to check how much it advances at a higher RPM. Your question makes me wonder if I'm setting at to low an RPM. My guess is that I'm reaching max advance to early with no additional advance at high RPM. To make make matters worse I cannot locate the other springs. We are in the middle of a move and a little disorganized.
Ken Knize

Jeff, We just went for a Ruby ride for ice cream. Prior to taking off I gave the distributer an approx 10 degree twist. This increased my idle by about 750 RPM, a bit high but tomorrow I can bring that down with the throttle adjustment. But, she ran great, I didn't try to go to to high an RPM (wife and dog were with me) but I seemed to have just about all I wanted. I'll put a timing light on it tomorrow to see where I am. I guess it makes sense that my initial timing should be higher, I have no vacumn advance.
Ken Knize

I thought you were describing an engine that had retarded timing! A good curve for your distributor would be a start point of anywhere from 1000-1500 rpms up to about 3000-3500. If your distributor offers 10 degrees of timing, you want your idle timing to be set at around 12 degrees (34 total minus 10 distributor degrees (20 crank).) You really should rev your engine with the timing light on it to see how much total advance you have, then you can set the static timing AND the curve!
Jeff Schlemmer

How did you set your fuel mixture and what weight of oil is in the carbs?

OK Guys you are way over my head but I've reread a few times. Jeff, It's been raining this morn in Binghamton, washing away some mud from the flood. So I havn't been able to tinker. Rain stopping now I check out your advice and post back later.
If I understand correctly, at idle of 1000-1500 I should be at 12 deg. My guess and I'll verify is, that is where I am now. I think you suggest it should be 12 deg up to 3500 RPM then I should see a continued advance? I don't think that will happen because my dist. springs are too light. My guess is that max dist advance is happening too early because of light springs. If I don't find set of springs today I'll order new set tomorrow.
Mark, As I recall I used Marvel Mystery Oil when I rebuilt carbs. Front carb holds rpm when piston is lifted and rear carb increases rpm when lifted. Can't seem to lean out rear carb but I have moved my effort to the timing issue as this seems to be off the most ie. most profound issue.
I cannot tell you how encouraged I am over improvements, thank you for your help.
Ken Knize

Ken. Remove the line from the vacuum advance can and plug the line. Set the timing mark on the harmonic balancer (crankshaft pulley) at the 20 deg BTDC mark and make a second mark at the top dead center pointer. The second mark is 20 degrees advanced of the first mark. (Only needed if you do not have one of the adjustable timing lights. If you do have an adjustable timing light the 20 degree advance marking is not needed.)

Start your engine and have an assistant run it up to about 3,000 rpm (3K to 3.5K works fine) while you adjust the timing to 32 degrees before top dead center. (Either dial back your adjustable timing light, or use the mark you made and align it about 12 deg BTDC which will give you the 32 deg total you are looking for.)

Drop the engine speed to 2,500 rpm and read the ignition advance. Drop the engine speed to 2,000 rpm and read the igntion advance. Drop the engine speed to 1,500 rpm and read the ignition advance. Drop the engine speed to 1,000 rpm and read the ignition advance.

Now, you have set your overall advance to 32 deg BTDC at between 3,000 and 3,500 rpm and you have determined your mechanical advance curve at your initial timing speed (1,000 rpm) up to the max advance point of 3,000 rpm. That is your base point for any future reference and will be used to set your dynamic timing and check the advance curve in the future. Any changes to springs would be documented in the same manner and would become your new baseline standard.

Les Bengtson

Les, To begin with I have no vacumn advance. The Mallory is strictly centrifigal. I do not have an adjustable timing light either. I attempted to check the arbitrary timing from yesterday and found I could not measure it, perhaps 35 deg. I was astounded to see it that far advanced and the car still ran at all. Then to start I set the low idle advance to 0 and reved up to 3500 with my mirror on the steering wheel. My advance went to 25 deg. Or to an extrapolated space beond 20 deg. But, after rereading your advice, Am I adjusting in the wrong direction???? I have allways been confused by the books, I have them ALL, It seems the drawings are upside down. My car is a 73, as I stand in front facing to the rear of the car I assumed the right arrow was tdc. If I understand correctly you and Jeff have been teling me I'm retarding the timing not advancing. I'm really confused, as I rev the engine the mark on the pully moves to the left hence I thought it was advancing. I was totaly defeated today My car looks like a million bucks, for years I have not been able to solve this timing thing I just keep buying new parts and it never improves. I have not been able to get he out of the garage (long story) and even if I did I don't know any one who could help me. You all are the best help I have had.
Ken Knize

Back to Mark's question about carbs. After rebuilding and installing carbs I found that the low and high speed screws were stripped. I ordered new ones but a mix up at Moss left them undelivered prior to my departure south for last winter. I tried to drill and tap and install nylock nuts on the screws. It was a big effort but after closer investigation the screws are still in stripped holes. So the adjustments are not working. I have the new adjusters but hated the thought of removing the carbs again. I guess it will have to be done, it is the reason I have been having problem with carb adj. I'll report back after correction, hopfully tomorrow.
Ken Knize

A 73 B should be high compression. With 87 octane gas you should be able to get the engine to ping under load. Try advancing your spark to the point it pings under full throttle, then retard it a bit. If you are running the low compression pistons then you can twist the distributor way too far and still get no pinging.

An engine with a vacuum advance distributor set correctly will not ping a cruise at 2500 rpm but will ping as you add additional throttle - with more throttle yet the vacuum advance will retard more and no more ping. That whole sequence is missing with a mechanical only advance distributor.

My experience with low compression and a hot cam was that I could rotate the distributor all over the place and there wasn't much noticable difference. If your running the smog low compression engine, you'll find timing can be plus or minus 5+ degrees and it won't make much difference.

Mallory distributors can be set up with a little or a lot of advance. The advance can be set to come on at low rpm or higher rpm or a combo - 2 stages.

You need to figure out what you spark curve is and what engine you have.

Barry Parkinson

Ken, I'll try to explain my previous post better.
Turning the distributor clockwise will advance your timing.
You should have your timing set at idle around 12 BTDC, give or take a couple degrees. From about 1000-1200 rpms, your timing will advance (further BTDC) another 22 degrees or so, ending with your ignition timing around 32-34 degrees BTDC at around 3000-3500 rpm.
You might do yourself a favor and use the timing mark on the balancer, but also add 3 more marks each 10 degrees apart, so you have a TDC mark, 10, 20, 30, and 40 degree marks on the balancer. The marks will be added counter-clockwise from the original TDC mark. That will allow you to not only read idle timing, but advanced timing at different rpms. You can then put a mark on the TDC or zero pointer and use the balancer marks, or a combination of both.

You seem to be adjusting in the right direction, but could use more advance. When you go too far, you will start to experience knock under load such as climbing a hill in too low a gear or full throttle at low rpms. At that point, retard your timing (turning the distributor counter clockwise) a couple degrees.

If you can adjust your distributor to give 20-22 crank degrees of timing (plus the 10-12 degrees idle timing), you will have the best (generic) timing curve you can get without some expensive dyno time.
Jeff Schlemmer

Barry, Thanks for the post, I have no vac advance. I'll have to look back and check but my engine # in the books is supposed to be low compression. I don't know what my cam is but I doubt any change from stock. PW did not like to change anything.

Jeff, Thanks for the patience. I think I'm OK with idle timing and distributer advance. In the midst of my frustration I found a new spark plug wire came loose from the distibuter cap.

I think my timing is very close to your recomendation which leads me to think my problem is not in timing. I guess I'll have to remove the carbs tomorrow and replace the levers and screws for idle and choke adjustment.

My car starts breaking up at about 3500 or sooner if I'm too agressive on the throttle. Any Ideas where to start after I get rich/lean and idle adjustments right?

I had fuel pump rebuilt, it's not that.
Ken Knize

Typically with a carb problem the engine will either run lean or rich. With lean it will sort of fade as you give it more throttle and if it gets too lean it will cut out or buck

With a rich carb it will usually run fine unless it is so rich you are fowling plugs and flooding the engine.

Either way with a variable venturi carb - giving it more or less throttle will momentarily change the mixture and give you a hint that something needs to be adjusted.

Starting to "buck" at a certain rpm sounds much more like an ignition problem than a carb problem. Is it load based - or rpm based? However, as indicated above, a too lean condition can cause the fuel to fail to ignite and cause "bucking" Part of the problem is trying to describe it in print.

If your fuel pump is not keeping up or you have a bad fuel filter, you can usually get a moment of power when you open the throttle, until the carb runs dry and then the car quits, fades or bucks.

Barry Parkinson

Ken, if the carbs are running ok, but your needles are too big (lean), you will probably experience surging at higher rpms even when just cruising at a constant rpm. It may feel like a "popping" or sound like a misfire. It will be more prevalant the higher the rpms - if its bad at 3500, it should seem worse at 4000. Its also not safe to drive with a lean condition!

If you can't get your carbs balanced, you'll be chasing a tuning problem forever, so its a great idea to repair those issues while you know the ignition is close to where it needs to be. Hopefully you have a carb balancer - tuning by ear is close, but usually won't find the "sweet spot."
Jeff Schlemmer

Thanks Guys, I did spend the day removing my carbs and replacing the linkage and adjusting screws. I have achieved the best operation to date but I need to spend more time trying to get the right combination of timing and carbs.
Today I was able to get smooth operation up to 3500 rpm and if I accelerated slowly 4000 but at that point it would start breaking up. I wouldn't charicterize it as bucking just starts missing and losing power. I reread my Mallory advance curve installation instructions and they indicate that the combination of 2 pink springs is popular with 4 cylinder engines I guess that is why I used that setup. I was on the road attempting to get to an MG meet in a park just out of town. I hoped I might meet someone who could help me. I gave up because part of the route was on an Interstate. When I could I stopped to tweak but such an unsystematic approach caused a more erratic effect. I guess I'll have to be more methodical tomorrow. The only question I have tonight is should I continue to leave my idle advance at 800-1000 @ 12 deg?
Ken Knize

I forgot to address the fuel pump question, Dave DuBois just returned my pump to me and he commented that he doubted that my pump prior to his rebuild could have caused my problem.
Ken Knize

Jeff. Re-read your post about additional timing marks. You got it backwards.

Ken. Re-read my post on how to make an additional timing mark that will allow you to use a non-adjustable timing light to set your timing. Then, do as I have suggested and set your timing at 32 deg BTDC at 3,000 rpm. Then, read what your timing is at 1,000 and use that as your baseline figure.

For the carbs, you need to know what needles you are running. Most of us find the AAA needles work quite well and they should be used as a baseline for further testing. Joe Curto, at Joe Curto, Inc. has needles available at a reasonable price. He has a website and his telephone number should be listed in the archives.

As to the fuel system, the pump is only one part of the system. The tank, the fuel pick up, the lines and the fuel filter are other parts of the system and a pefect pump is no good if you have a clogged pick up tube, clogged hard line, clogged fuel filter or bad rubber hoses. Dave has a tech article on fuel pumps (which gives flow pressure and volume specifications) and another on fuel system troubleshooting on the MG portion of my website. Got to and click on the MG section, followed by "articles", then Dave's articles are listed under his name.

Les Bengtson


Get a distributor from a known working car and throw it in. Set the base timing and see how it works.

Same problem ? == not distributor related.

Works okay ? == something wrong with your dist.

Keeping the trobleshooting clean and simple will help keep your moral up through this.

You'll get it !!

Good luck !

Tom M

Les, thanks for that. I don't like leading people astray! I was picturing the timing mark while using my advance timing light - going the other way. The additional timing marks should be added in a clockwise direction, and with a small artist's brush could be labeled 10, 20, 30...

Jeff Schlemmer

Thank you for posts, I did a flow test on the fuel line at the carbs. Dave DuBoise gave me the rate and volumn criteria and she passed. Fuel filter was changed twice, now have a clear one so I can see condition of fuel, crystal clear.
Could my use of Marvel Mystery Oil be too light? Could this be causing the piston to open too quickly?
Engine has been showing a slightly elevated temp, I read somewhere this could be a result of lean fuel mix? Does this give a clue?
I have been using high test fuel, is this OK? As to marking the pully I have used a silver sharpy it works very well. I recomend for this proceedure.
I don't know anyone who has an MGB so swaping distributer is not an option. The Mallory in the car was purchased new by me and doesen't have 10 hours of use. I had this same problem with the dual points so I gave up and purchased the petronix. In the garage she ran like new but on the road same old problem.
As to needles I installed K&N air filters and switched to the recomended needles, I cannot find my record of what that needle is. Are they marked in some way?
I'm taking the day off from the car, too frustrated, I'm going to write down the proceedure steps and try to follow it maticulasly tomorrow. Any suggestions would be apreciated.
Sorry for the rambling, very frustrated.
Ken Knize
Ken Knize

Barry, You sugested that I need to find out what engine I have, as I have been saying it is a low compression. This is the bases for my comment. My engine number is 18V672Z-L18920. Per the first page in Haynes In Oct 1972 (American 1973 Model) the North American model # 18v672Z. The engine prefix is followed by one letter, H for high compression ratio and L for low, and then the serial number. Hence I assume mine is a low compression.
You also suggest I need to know my spark curve. I think I understand how to do that and it is my plan for tomorrow morn. But, I have no idea what to do with it. I guess from what you and Jeff have written I am to work backwords from 3,000 rpm and hopfully end up with 12 deg at 1,000 rpm. If at that point I guess I'll call timing good enough and move on to the carbs. But what are the intermediate readings for? I have found my Mallory Single Stage, Matched Springs,Advance Curves for my pink/pink spring set.
0 deg=800 rpm, 10 deg=1,500 rpm, 23 deg=2,000 rpm, 40 deg=4,000 rpm. Max limit however (I'm reading from a chart) is indicated as 2,200 @ 32 deg. Doesn't the slope of the curve seem rapid, ie. advance is quick which makes sense because the springs are very light.
I'll have to study more befor I know what to do about carburation.
Ken Knize

Do you have some different springs to try? Might give you some more feedback to go on.

With a stock cam, compression pressure and torque is good from low rpms on up. If you advance the spark too much at low rpms the engine knocks/pings and doesn't run as well.

With a modified cam the compression pressure is reduced at low rpm and increased (relatively speaking) at higher rpms.

Because of the low power/low compression pressure with the modified cam you will typically throw in lots of advance at low rpm and not much more at hi rpm. With a modified cam full spark advance can be all in at 2000 to 2500 rpm. With a stock engine - in contrast 3000 to 3500 rpm.

The distributor compromise is one soft spring, one stiff spring. Or you can have one light weight and one heavier wieght, or equal weights, any combo. The weight retained by the soft springs advances quickly at low rpm. The stiff spring doesn't start increasing the timing until higher rpms. It could be that you are getting too much additional timing at hi rpms. But with a stock smog engine you probably want more advance above 2500 rpm. 1st question is what is your initial advance - next, what is your total advance at hi rpm, and 3rd what does the advance curve look like. With a distributor machine this is easy to do. With a timing light that can be calibrated to show advance you can do it at home with some practice.

A distributor set up for an engine with a modfied cam is not going to work very well in a stock engine without recalibration.

A distributor without a vacuum advance is not going to be mileage effecient or that responsive at part throttle. You appear to have an inappropriate distributor for your engine. If you advance the spark enough for ordinary driving, instead of setting for full power driving, the spark will be too far advanced for full throttle operations.

Barry Parkinson

It's Alive, It's Alive. OK after placing all those whitness marks on my pully I was able to try many advance settings at idle then on the road. 12.5 or there abouts was the best I could get. So I left it there and went to the carbs. First check the needles they were ABD's but looking into the jet I couldn't see any fuel, this can't be good. Pull the carbs off one more time. Everything looks OK but lets check the SU Book again. Where did .040 in. below casting come from, I was sure it was supposed to be 1/8th above. Might as well try it. Carbs back on and HOLY SHOOT hang on to your hat why are the tires making that squeeling sound? Why is that old man screaming around town with that silly grin. She has run good befor but this is rediculas no wonder these cars are loved so much. Thats all it was, I've learned a lot along the way, say baseline boys and girls. I'm sure I'll be back with other questions and I know I will have pleasent dreams to night as for tomorrow, where did I put that car wax? Thank you everyone for the patience and encouragement.
One happy MGer.
Ken Knize

Ken. Glad the car seems to be running better now. However, the AAA needles will make it run even better.

Again, what you are looking for, in terms of distributor advance, is 32 deg BTDC at 3,000 rpm. Set the engine up that way and see what happens. Idle setting is strictly an approximation of what the final setting should be.

Les Bengtson

Thanks Les I'll get those on order and check timing, my pully is marked so I can get that baseline set up. I'm unsure what curve I should be looking for. At the current spring set I'm at a single stage with a steep slope. In my euphoria I was more focused on driving the countru roads. Each day sees more improvement I'm sure there will be future frustrations but that day is not today. I'll report back. Thank you, Ken
Ken Knize

Les's recommendation for the ignition curve is a great place to start, but don't be afraid to run the rpm range a little higher - especially with no vacuum advance. Maybe leave one spring in and replace the other with a stiffer one?

I've had great luck with low ignition curves - all in by 22-2500 engine rpms, but you can typically get better throttle response by getting it up to 26-2800 rpms. You'll be able to tell the best curve by driving in 4th gear at low speeds and measuring your acceleration!
Jeff Schlemmer

This thread was discussed between 01/07/2006 and 06/07/2006

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