Sorry if I'm beating a dead horse here, but I've got a couple of questions about recurving an HEI distributor . . . I read Lars's paper regarding distributor recurving, but was unsure specifically what to do for an HEI. The proceedure that I tried to use was to do this:

1 - Disconnect vacuum advance.

2 - Start engine and set idle speed to 750 RPM (or there abouts).

3 - Set initial timing advance to 12 degrees BTDC (seemed like an okay number and is what I've been using to tool around town with, so I didn't have to change it in reality).

4 - Run engine up until maximum timing is seen (within reason) and record the number for future reference (just so I know where I started from).

5 - Turn off engine, install weakest set of springs that came with my recurve kit, tossing the weights into a spare parts bin and sticking with the ones that came with the distributor.

6 - Start engine again and set advance timing light to 36 degrees and then rev engine until maximum advance is seen, and retard (or advance if that ever happens) until mark lines up with 0 mark (36 degrees total advance).

7 - Flick the throttle past 3000 a few times and retard timing a bit more until we are only seeing 36 degrees. (I may have skimped here a little bit, it was hard to tell by myself.)

8 - Take a guess about which springs you may need based on what you saw with the weak springs. (For me this was one of the weakest on one side and one original on the other (can you mix and match?).)

9 - Start engine and see what the total advance is and then swap springs based on what is observed (put in heavier springs if necessary). Repeat process until you can get a total of 36 degrees advance in by 3000 RPM, but preferably in at exactly 2500RPM.

So here are where my questions start . . . The weakest springs seemed to be too weak (I was getting additional advance at idle). I swapped in one of the weakest and then the next weakest (my kit came with 3 strengths). . . Well, this one doesn't give me 36 degrees at 2500 RPM (or even by 3000+ RPM). I only get about 31 degrees total as far as I can tell (without revving more than about 4000 rpm)... The car now seems weaker than when I started. Arrrrgh! By the way, my new settings showed about 9 degrees at 750 RPM and about 30-31 degrees at 3000 RPM.

10 - Reconnect vacuum advance.

11 - Drive car and while lugging the engine (I put it in 2nd on my TH400) punch the throttle and listen for knocking/pinging. If there is any pinging, retard the initial timing in 2 degree increments until it goes away.

 

Sounds like you have the procedure figured out nicely. As for springs I don't see why you can't mix and match.

I took the weights and centre piece out of an old Buick distributor. Nice heavy weights and a thinner centre piece.

I found that if I pulled the weights out by hand the centre piece would not advance all the way. I had to grind some metal off the weights and centre piece in order to free up full mech advance. Maybe you could try that, I did not put the grinder to my stock weights of course.

 

Thanks. Sounds like I should look for a set of heavier weights(or different springs) based on what I was seeing then. Lars's paper gave me the impression that an HEI would have more total advance than the car could ever possibly handle and this is why you have to install the weak springs when starting it up to check the total advance. Sounds like something was strange with your unit if this was not true? I was definitely able to get 36 degrees with the lighter springs, it's just that it was advancing the timing a fair bit at idle with the weak springs. . .

 

Maybe heavier springs are the answer for you so you don't get your mech starting until 1000RPM or so. Then you may need heavier weights to compensate for your heavier springs so your mech advance comes in at the RPMs you want.

I agree with that... my friend's camaro's HEI gives him over 30 degrees mechanical so add 12 initial and he is getting too much. Him and I are going to have a "lets tune are engines and drink beers" night in the near future.


Yup there could be something wrong... maybe my springs are too heavy? I want to verify my timing mark is at TDC and get some timing tape on my harmonic balancer too. I'm still trying to figure it all out, thats the fun part right!

 

Yeah, that could be. . . Maybe I should also try the weights that came with my recurve kit as they are much smaller (most likely lighter) and may work okay with the weakest springs. Otherwise I can try to find heavier weights, but the ones in my recurve kit are *WAY* smaller than the ones that came with the distributor, so I may have a difficult hunt. . . This process is fun, but would be much less frustrating if I just had an unlimited selection of springs so I could just stick to swapping in heavier/lighter springs until it is perfect. . .

Too bad you're not local or I'd invite myself along Oh well, hopefully I'll get it all tuned in this weekend. It is amazing how much more power my new engine is making than my old one (or is it the new torque converter that I'm noticing, uh oh), but still, I know there is a lot more if I can get everything tuned in.

Timing tape 'eh? I hadn't thought about that. . . I'm just using an advance timing light which is pretty nice, but seems slightly figety when I'm alone trying to rev the engine, look at the tach, and adjust the light all at once . . .

Definitely this is the fun part. I'm having the most fun I've had in a long time and now that I can work on my car again, I can hang around the forum without the depression setting in I was starting to feel bad for not visiting, but I just couldn't stand hearing about everyone else's car troubles when my biggest trouble was not having an engine. I actually left my car, tools, and everything exactly where it was when I had last worked on it 6+ months before. . . But I'm digressing now

 

Don't worry about centrifugal advance starting below idle speed. What's important is getting the right amount of total. "I only get about 31 degrees total as far as I can tell (without reving more than about 4000 rpm)". Do you get more advance if you rev it past 4,000? If so, your total isn't all in until then and checking it at a lower rpm will only give you a useless reading. Put the lightest springs back in and you'll probably be able to get an honest 36º total all in where you want it.

 

This weekend I got it all working really nice. I found that the new weights were taller and were therefore rubbing against the bottom of my rotor, so I just ground some plastic off the bottom of the rotor to free the weights up. Now I have 12 inital and a nice 34 degrees at around 3000RPM!

 

I bought a re-curve kit from Lars and the weights were Accel but he opened up the holes to make them oval and no plastic bushings. I first did what you did and did NOT use the new weights, just new springs and vac. can. The best I got was 31*. I thought Lars weights looked bad because he opened them up and used no bushings. So I tried his whole kit (springs, weights, & can)...36* on the money, 52* with vac.

 

I didn't actually get to playing with this anymore over the weekend . . . I ended up rejetting my carb and putting in colder plugs instead. Was a bit hot around here (not that I'm complaining), so I didn't get to do anything on Saturday as my dad didn't feel like working outside in 100+ temps...

I didn't see movement much past 3000 RPM, but took it up to 4000 just to see. I could rev it more I suppose, it just scares me sticking my head down there a little bit after I heard a story from someone on the forum about a balancer exploding . . .

I'll try the weaker springs again and see what I can do, but it would seem odd to have so much initial advance (15+ degrees) to me.

Boofers: Sounds like you did some good tuning. You could probably get another couple of degrees out of that and maybe try for 2500 still, no?

mvftw: Maybe I'll try a Lars kit if I can't get it myself. I like the learning process here, so I want to play with it a little more still... I think I will need to get a different (adjustable probably) vacuum can. . . Are you using ported vacuum, or manivold vacuum? I seem to get too much advance with the vacuum port that is on the primary metering block on my Holley 750 double pumper...

 

Ported vac.

 

MINOR RANT:

If I owned a vintage car I would buy a GM shop manual!

That 1974 manual shows an L-48 has 18* of centrifugal advance @4200 RPM, while an L-82 has 20* centrifugal advance @5000 RPM. So, using the stock weights on an L-48 and 17* initial timing will give you 35* total advance. An L-82 with stock weights and 15* initial advance will give you the same total advance. However, 4200 and 5000 RPM is not where you want to have your total advance, hence the use of lighter springs to get you advance all in by 2500-3000 RPM.

Get yourself a dial back timing light, put in lighter springs and set the light for 35*. See what RPM you have and adjust the springs, you can mix and match, accordingly.

I am running a stock '76 distributor with lighter springs and am getting 35* @2500 RPM with no problem.

I assume you are/will be checking with the vacuum line disconnected. You will probably need to buy an adjustable vacuum can or check Lars specs for a non-adjustable can so you get a total advance of about 50-52* total mechanical+initial+vacuum.

Don't worry about initial advance, just total, unless you have a starting problem. Make sure the weights are free and not binding.

BTW, manual shows no mechanical advance for the L-48 @1100 RPM and no advance @1000 RPM for the L-82.

 

Glen: Not to rant on a rant, but did you read my original post? My engine is nothing close to an L-48 or an L-82. Also, I am using an HEI distributor, which was not on a car in 1974. Also, the GM shop manual isn't designed with the performance enthusiast in mind, but rather for the repair technician who is trying to put the car back to factory specs. . .

I *DID* try ligher springs and do have a dial back timing light. My question was basically what should I do if the lightest springs give me too much initial advance and the heavier springs give me too little total advance? Also, how much is too much initial advance? Obviously there is some number or we would not have variable advance in the first place. I'm not looking for OEM specifications here, I'm looking for hotrodding advice


-dath

 

Springs can be mixed; one light one heavy. This is what I am using. What year is the HEI distributor out of? If you know what the factory weights are capable of, then you can come up with a plan on what to do with the springs. Have you tried on light and one heavy spring? As I mentioned, I don't remember what my initial advance is, (just a guess of about 18*)I just go by total mechanical, all in by 2500 RPM. If you set up this way, and you have trouble starting, then you will need to change the weights because the weights are limiting your advance.

You do need to know what the distributor is capable of, in its present form, before you start changing weights, etc. Maybe your best solution is to get in touch with Lars. He should have the answers.

BTW, re-read your initial post - no mention of what distributor, other than HEI, that you are using. Being a dumb A**, I never thought to verify that a '74 does not have a HEI as stock.

When you say the mixture of springs, lightest and next to lightest, do not give you 36*, it seems you are working from an initial timing point rather than trying to set your total timing, then set (advance your distributor) until you get 36*.. If you have trouble starting using this setting, then it is time to change weights. However, until you know what the present weights are capable of, ....

I have the GM specs on some of their distributors and could possibly help if you have the distributor number/application.

 

Guys,
The centrifugal curve should not be advanced at all while idling. You want the centrifugal curve to begin typicaly around 1100 - 1200 RPM or so depending on your idle speed. Only the initial advance should be there at idle.

If you have any centrifugal advance in effect while idling you will have an unstable idle. The springs are too light if this is happening.

If you want to run more initial advance than what you had with the distributor in the original specification, you have to remove some of the centrifugal advance. To do this will require a thicker bushing or brazing of the slot in the autocam plate.

What you are trying to do is keep the sum of initial + all the centrifugal to be equal to 36° and have it all there at 2600-3000 RPM.

For example, if your distributor tune up initial timing setting was originaly 4° before TDC then your centrifugal advance mechanisim was providing 32° from the factory originaly. This is a total of 36°.

You want to recurve it to have maybe 12° initial so you would reduce the amount of centrifugal to 22° so the total of 12° initial plus 22° centrifugal equals 36°. If adding the bushing does not reduce the centrifugal advance enough, you need to braze the slot closed a bit and file it down so it moves freeley to get to 22° in the centrifugal. You can then run the 12° of initial and keep the total 36°.

Once you have that part done, you need to mess with the springs. Start light and move a step heavier until the cetrifugal advance does not start too early before 1100 - 1200 RPM or so. If you get ping - step up on one spring at a time till it goes away.

This is a general performance curve that should work on most HP cars.

If you get this done, you can start to mess with the vacuum advance! OH BOY

-Mark.

 

I just replaced the stock distributor with an MSD unit on a 77. Talk about a breeze to set the advance. I had about 20 degrees at 3,500 - 4,000 rpm with the stock distributor and the timing set at 8 degrees BTDC.

I followed the instructions in the MSD book for 36 degrees of advance at 3,000 rpm, put on the recommended bushings and springs and on first try it was dead on what they said it would be.

It's nice when something goes together the first time like it's supposed to.