distributor recurving - what's involved?

jpnovak · 07-13-2004, 02:12 PM

I am running carbs on my 78 3.0. I know that the advance curve is supposed to be adequate for the conversion but I assume its not optimal.

What is involved in recurving a dist? I am not looking for answers that say "send to shop X or mechanic Y."

I assume that small amounts of the plate are milled out to allow more swing on the weights. This should control the total amount of advance. I then assume that different springs are used to move the advance curve around with respect to rpm. Is this correct?

Just curious since there are no dist's I can swap in my 3.0. I know the ring gear and advance mechanism spin the wrong way.

Couldn't find any information in the archives that describe the actual process. Has anyone recurved their own dist? If its as easy as I think, I am willing to try.

dean · 07-13-2004, 03:24 PM

Quote:

Originally posted by jpnovak
I assume that small amounts of the plate are milled out to allow more swing on the weights. This should control the total amount of advance. I then assume that different springs are used to move the advance curve around with respect to rpm. Is this correct?

You are correct sir. I have done it and you can with a timing light. Noting the advance at different RPM's. Change the springs to make the curve as you like.
Dean

Fishcop · Fishcop's Garage

Jamie, from what I understand (have had two done now) is that it is a combination of weights and spring tension. Depending on what is trying to be achieved a different spring with less or more tension is swapped. I believe it is rarer to change the actual pendulum/weight.

I stand to be corrected on this

911pcars · 07-13-2004, 04:05 PM

There's a mechanical stop that limits weight travel and thus how far the top half of the dist. shaft rotates. However, it's the springs AND the configuration of the weights (shape and weight) that determine the rate at which the distributor timing advances.

The ideal curve is matched to your engine via a chassis dyno. The operator manually advances and tracks that amount at specified rpm under full load. He can then plot a curve that allows max HP and torque minus a safety factor for fuel octane, air temperature, air density, etc.

Sherwood

tsuter · 07-13-2004, 04:12 PM

To change the overall ignition mechanical advance the mechanical stops must be modified by bending at the top. This limits the travel as 911pcars states.

Both sides should be modified equally.

For example mine (1978SC) is modified to limit the two mechanical advances to 10 and 10 respectively.

Thus when static advance is set at 5 BTDC the total advance is now mechanically limited to 25 BTDC. or 10 and 10 and 5

You will NOT be able to do this with vise grips!!! You will need a distributor recurving machine to do it properly.

Good luck.

Shuie · 04:23 PM

Quote:

Originally posted by jpnovak

Just curious since there are no dist's I can swap in my 3.0. I know the ring gear and advance mechanism spin the wrong way.

What's the difference between the advance curves of the US '78-'79 distributors and the euro distributors? Would there be any advantage to switching to a euro distributor for a carb'ed 3.0 or 3.2?

jpnovak · 07-14-2004, 05:17 AM

Shuie, The difference between 78/9 is the amount of total advance available without the vacuum system attached.

Right now I only have about 19 deg total advance in my dist. spec is 22+/-2 deg. My problem is that I need to set the advance at 30 BTDC at 6K rpm. This would leave me at 11 BTDC or so at idle. I am not able to get that setting since the weights are starting to swing at that point and I get full advance at about 3.5K. It also starts to induce creep where the weights swing to advance timing, the rpm goes up, the weights swing more... Its a vicious cycle.

I was wanting to do a combination of two things. One is mill the stops in teh diamond shaped hole on the dist. advance plate. In combination with moving the stops, I can get more total advance. Goal is 26 deg total advance. Next I would add a slightly stiffer spring to prevent the weights from swinging until a higher rpm.

Now if I can just get my CDI to work again.

911pcars · 07-14-2004, 10:23 AM

Jamie,
If 11º BTDC at idle is causing the weights to swing, how about lowering the idle speed so the threshold is below that point? Beyond that, you'll probably need a bunch of springs to arrive at the desired advance curve. I haven't looked recently. Is there a spring anchor post in the distributor you can bend to vary the tension?

Looking in my spring catalog, there are a staggering number of springs and spring variables. It would help to know the factory spring specs. Anybody have that tidbit of info?

Sherwood

jpnovak · 07-14-2004, 11:16 AM

I have the idle as low as it will go. idle speed screws are all the way out and air adjusters are almost all the way in (as far as I can go and still be balanced. The shafts are tight and there is no wear on the bore of the carbs.

I assume I could try stiffer springs first. Do you know a source? Even new springs are worth a shot. I searched Pelican and did not find dist. rebuild kits or spings.

I can take the springs out and measure if needed. In fact, i could probably do it with my eyes closed. I have been inside this dist 5 or 6 times in the past two weeks.

911pcars · 07-14-2004, 01:26 PM

I suspect an air leak if you can't shut the air off normally.

Century Spring is one large outfit (in Los Angeles).

Sherwood

jpnovak · 07-14-2004, 01:40 PM

I have suspected that too but have never found one, even with the propane test. The carbs are sitting on new PMO insulators (for the CIS heads) surrounded by new gaskets complete with a little silicone grease to help them all seal. I have used this combination for a long time with no problems.

Is this the right company?

http://www.centuryspring.com/

Time to get the spring out and take some measurements.

dean · 07-14-2004, 02:10 PM

Go to any speed shop and get some dist springs. Summit racing has them on line. MSD is a supplier.

jpnovak · 07-14-2004, 02:20 PM

Dean, is this a universal fit or are they a particular spring constant? I have a good shop here in town.

911pcars · 07-14-2004, 02:25 PM

Jamie,
Yes. That's it. They have a catalog (hard copy and CD).

If you're near a company that carries springs (tons of them), it might be better to visit them where one of their experts can measure yours and get you in the right direction.

Summit has dist. springs too, but not sure if Chevy and Ford springs are close in size and tension to the ones in a 911 distributor.

I'm sure the companies that provide recurving services have the correct assortment of springs. Whether they will sell you just the springs is a question only they can answer.

Sherwood

dean · 07-14-2004, 02:25 PM

You will get a set of 3 different spings in a kit. Is that wihat you mean?

jpnovak · 07-14-2004, 02:33 PM

Sort of. I want to make sure that a spring from a 'mericun dist will fit a Bosch. NOt just in force constants but in mounting dimensions, OD, ID etc.

Sherwood, good idea on a local supplier. I guess I will all Mooresped down the street.

I noticed that a Mallory kit at Summit has 9 sets of springs. I sense some experimentation.

Now if I can just get my CDI to work again.

jpnovak · 07-14-2004, 06:58 PM

I found a few more problems with my dist tonight. First the spring for the smaller weight did not do anything. I think it may have been replaced at sometime. It was too long and just barely started to pull when the weight was at maximum. I also noticed that the large weight would hit the stop and not pull to total advance. I assume this is for low rpm range.

I know it does not matter since I have it disconnected but the vacuum advance plate was frozen. I fixed it. Just goes to show that all is not right in the dizzy world.