Timing issue

[951mat]

I'm stumped on the mechanical advance of my MSD on my 427. Very simply my initial timing right now is +/- 12 degrees. I have the 2 light silver springs and the red bushing in my MSD distributor. The red bushing should deliver 28 degrees of additional advance for a total of 40ish.

My timing all in is only 25 total at 2300+ rpms. What gives????? By the way, if I set my initial timing much higher, starting is much more difficult and the engine "runs on" after shutting down, I assume because of the pump gas its on. I don't think any of that really matters with the main problem of my total timing not adding up. Any way to trouble shoot this?

Thanks for any advise.

Mat

[Jac Mac]

Mat,

You have me stumped too apart from two unlikely faults;
1 .Something bent or caught in the weight mechanism.
2 .If I remember correctly your MSD Dist has an inverted chevy type weight system which 'might' be installed for the wrong rotation, a long shot I know, but I struck it once before where someone fitted a marine dizzy to a car engine.-- lost a bit of hair over that one!

Another one is if the wires in the multi plug to the dizzy have been swapped this can cause the ignition to trigger off the trailing edge of the pickup and play havoc with the timing, think there is a page on the MSD website about that one, used to happen in racing where the teams were making up new weatherproof connectors.

Plus 40° total might be a bit on the high side.

Jac Mac

[ERA Tom]

Mat, I would start by calling Sumitt Racing and buying a new spring kit for the distributor. Sometimes the springs are miscolored or simply don't have the correct tension. I had the same problem a few years back, couldn't get the correct total per the instructions, purchased the replacement spring kit, used the same color springs & bushing , and it cured the problem. Good luck, Tom

[chopper]

What is your idle rpm? That is, at what rpm are you setting the initial timing? If your idle rpm is too high, the distributor will already be feeding advance into the timing, so your initial setting will appear correct but you will not have full advance available. If I recall correctly, the springs should hold the advance weights at full retard up to about 1000 rpm; past that, the weights will start to come off the posts. If your idle rpm is set up to 1200-1300 rpm, you most likely already have some advance being fed into the system.

With the light springs, total advance will come in pretty quickly. 2300 rpm seems a little fast, however; you might try the next higher set of springs to slow down the advance rate a little. As you state, the total amount of advance is controlled by the bushing; the springs just control how fast the advance comes in. Opinions will vary amongst the folks here on the site, but I found that 36 degrees total advance at 2800-3000 rpm worked best for my engine.

Some folks set the total advance at 3000 rpm to 36 degrees rather than shooting for 40. IMHO, 40 is too much and will lead to higher than necessary engine temperatures.

[Chaplin]

Originally Posted by chopper

What is your idle rpm? That is, at what rpm are you setting the initial timing? If your idle rpm is too high, the distributor will already be feeding advance into the timing, so your initial setting will appear correct but you will not have full advance available. If I recall correctly, the springs should hold the advance weights at full retard up to about 1000 rpm; past that, the weights will start to come off the posts. If your idle rpm is set up to 1200-1300 rpm, you most likely already have some advance being fed into the system.

With the light springs, total advance will come in pretty quickly. 2300 rpm seems a little fast, however; you might try the next higher set of springs to slow down the advance rate a little. As you state, the total amount of advance is controlled by the bushing; the springs just control how fast the advance comes in. Opinions will vary amongst the folks here on the site, but I found that 36 degrees total advance at 2800-3000 rpm worked best for my engine.

Some folks set the total advance at 3000 rpm to 36 degrees rather than shooting for 40. IMHO, 40 is too much and will lead to higher than necessary engine temperatures.

Yup, agree with it all.

[dlampe]

At this point I think matt is more concerned with why he is not getting 28 deg. in the advance curve than his total timing. The problem is he has the dizzy set up for 28 deg. mechanical advance and he is only seeing maybe 14. The timing light is good and his car idles great. I don't understand why his dizzy is not advancing. It did not feel like it was binding.

[Rick Parker]

Chopper is dead on. The bushing only limits the "total amount of timing" in this case (28) degreees) that the distributor will add to an any inital setting. IE:Before the centrifical advance begins to feed in more timing. The small diameter wire silver springs being the weakest, have very little control over the flyweights, and allow movement of the flyweights to begin at about 800 engine RPM. If the initial idle is too high (above 1000 rpm) it will have already begun to advance the initial setting at a very fast rate. View your max advance with the timing light, at about 2800-3000 then see where the initial timing falls while at idle. If it is idling at about 1000-1100 and you want the initial to be less, (assuming the cam, and inlet stem will allow it) you will need to go to a stiffer set of springs and a different bushing. Don't over do it, the weaker the springs the more irratic the timing will be at low RPMs and the timing will "Hunt" causing your idle RPM to rise and lower for no apparent reason. If you can lower the idle by adjusting the idle speed screw on the carb you will then begin to "Back out" some of the "mechanical timing" the weaker springs have allowed and you will see the initial reduced timing visable with the light at idle.

Clear as mud?

[Jac Mac]

Check that the 'Round' & 'Square' location pegs of the rotor are not catching on the weights of the advance mechanism, I just checked a new MSD I have here for an FE and the mech advance would not function for this reason, a quick trim with a craft knife solved the problem ( only required the tiniest amount removed ), In your case it may only be one weight caught which would explain the 14° instead of 28°.
Rotor fitted 1/2 turn out of phase ;ie square peg in round hole could cause same problem.

Jac Mac

[chopper]

Originally Posted by dlampe

At this point I think matt is more concerned with why he is not getting 28 deg. in the advance curve than his total timing. The problem is he has the dizzy set up for 28 deg. mechanical advance and he is only seeing maybe 14. The timing light is good and his car idles great. I don't understand why his dizzy is not advancing. It did not feel like it was binding.

OK, let's try this again. By using the light springs, it's likely that the rpm for zero mechanical advance is down somewhere around 700 rpm (based on the MSD installation drawings on their website: http://www.msdignition.com/pdf/8594_frm23872.pdf). Assume for the sake of the initial argument the following items: (1) the curve of the mechanical advance is linear between zero advance and full (28 degrees in this case based on his selection of bushing); (2) Mr 951mat's engine is idling at 12 degrees advance and is achieving full advance at 2300 rpm; and (3) he has his idle set at 1200 rpm (this is a guess on my part). Based on a linear advance curve, the distributor will feed in about 2 degrees of advance per 100 rpm increase above 700 rpm. In that case, the distributor is already feeding in close to 10 degrees of mechanical advance at his idle rpm. This means he can only get an additional advance of 18 degrees out of the distributor using the bushing he selected, because he is already using 10 degrees at idle of his 28 available. Assuming (again) that everything is functioning perfectly inside the distributor and he has his idle timing at 12 degrees at 1200 rpm, he can only get another 18 out of the distributor for a total of 30 at full advance.

In the real world, the advance curve of the distributor is not linear because the springs do not exhibit a linear force-per-displacement curve. The advance will be quicker at the lower rpm range, probably closer to 3 degrees per 100 rpm increase, and will slow down at the higher engine speed since the spring constant (force per displacement) increases as the spring is stretched. This may be observed by again referring to the MSD advance curve drawings. Using these numbers, Mr 951mat is probably using up closer to 15 degrees of his 28 available, and so has only 13 left. He observes a delta between idle and full of 13 degrees, pretty close to what the numbers predict. Also, as Mr Parker says, the repeatability of the spring setting at idle is not 100%, and you have enough error to account for the differences which Mr 951mat is experiencing.

Having said all of that, it is always possible that there is something wrong with the centrifugal advance in the distributor. I had problems with my MSD which turned out to be corrosion between the advance plate and weights. I purchased the MSD unit to replace a Mallory Unilite and found that MSD had tried a new anti-corrosion coating that didn't anti-corrode very well. I returned the unit to the fine folks at MSD and they rebuilt it at no cost with new parts; problem resolved.

[dlampe]

951mat is no "Mr." !

I think you are right. The night we were working on the car the idle might have been that high and he is definately using the light silver springs. I think Matt should change to slightly heavier springs and lower his idle. That would at least tell us if this idea would work.

[Jac Mac]

Originally Posted by 951mat

I'm stumped on the mechanical advance of my MSD on my 427. Very simply my initial timing right now is +/- 12 degrees. I have the 2 light silver springs and the red bushing in my MSD distributor. The red bushing should deliver 28 degrees of additional advance for a total of 40ish.

My timing all in is only 25 total at 2300+ rpms. What gives????? By the way, if I set my initial timing much higher, starting is much more difficult and the engine "runs on" after shutting down, I assume because of the pump gas its on. I don't think any of that really matters with the main problem of my total timing not adding up. Any way to trouble shoot this?

Thanks for any advise.

Mat

If you are 'only' getting an indicated 25° total advance @ 2300+ RPM and higher ( I would check @ 4000 RPM ) then it is obvious that the mech advance is sticking or being prevented from functioning. You have already confirmed that by stating that the engine becomes hard to start when you advance the distributor manually to obtain that 36°/40° Total and the over advanced state which raised the cyl head temp to provoke run on after shutdown. The talk in other posts about rates of advance dont make a lot of sense if the advance mechanism was free as the weights would be returning to the 0° position for startup cranking speed which is clearly not the case.

Jac Mac