When I freshened the engine a few years back I "tewaked" the HEI with a cheapo Mr Gasket spring and weight kit. I just put in the lightest springs, removed the module and added an 6AL box.

The car ran good, felt like it had decent power, idled nice. I knew the motor had more but decided to leave well enough alone.

A few years later the car became a dog out of the hole, turns out the mech advance was not advancing, upon inspection I found the Cheapo weights to be worn. Replaced them with stock weights/cam (106's then 139's), but ligher springs. (Med&Light)

Problem now is I can't get enough advance out of the unit. It will only give me 12 degrees at 3000. So to run at the 36-38 my engine likes my initial is at 24-26...makes for crappy hot starting, fumed out idling, and bad fuel mielage.

I searched every post with HEI in the last year...
With all of you talking about bushings and such...did I miss something with the HEI? Should I buy another cheapo kit or is there better stuff to use??

I feel like just throwing the HEI in my swap meet bin and buying an Accell HEI for 110.00 or a GMPP HEI for 177 from jegs.

Rob, Not enough advance is a common problem with GM HEI's. The aftermarket weights usually don't fit at all. The bushings and slots on these don't usually limit the mech advance like they do on a points type dist. It's all in the weights and springs. What you need to do is do some careful grinding on the inside of the weights and on the center cam plate to get the weights to sling out a little further. Sometimes flipping the center cam plate over helps. Be careful not to grind away too much or the weights can sling out past the support pads and get stuck there. GM HEI's are a very good dist but just need some curve work. I've seen aftermarket HEI's with the same problem (not enough mech advance). BTW if you haven't already, you better make reservations asap for CB '06. June 8-10, 2006 per Chuck.

OK Von,

I understand the theory behind do some creative grinding, but just can't visualize where to grind for the desired effect. Maybe you could 'alter' my diagram or explain further and I'll alter it.

Made reservations yesterday... all six this year.

If I understand this correctly, in the attached diagram, the red is the grind area for the weights and the blue is the grind area for the center plate/cam.

?????

Hmmph...berry berry neat! I'll use those tips on my weights!

Usually aftermarket weights are smaller and lighter than stock.

Therefore advance starts SOONER with stock (heavier) weights, just like if you put in lighter springs. You also said you put in still lighter springs. Now the advance comes in REALLY SOON.

First guess: Your distributor is ALREADY advancing at idle. You retarded the timing to get the initial "right". Therefore you don't have much additional centrifugal advance.

Toss some really heavy springs into it, and see if your initial timing retards. If it does, that's the problem.

I went through this a while back Rob, and you are on the right track. I found, through some email conversations with I-man, that GM stock weights #41 would a good starting point - I actually did not need to grind them at all, might have been becasue I luckily started with a compatible center cam, not sure. I went to the local junk yard and popped off about a 6 or 7 dist caps until I found the #41 weights, so they are out there. This is not a requirment, but I didn't know what I was doing and wanted to start somewhere close.

I did run into an issue: there are (at least) two types of HEIs: small and large diameter pins for the weights. My original HEI that came with my car (and had Mr Gasket weights) was small pin - my pro shop guy told me the smaller pins require the bushing and are less desirable, as they can wear over time and strongly urged me to get another distributor, which I did. Not sure of the general opinion on this, but I followed his advice. The stock #41 weights are the large pin style, I think most of the older ones are.

I found the same issues with the aftermarket Mr. Gasket weights as mentioned, too light, mine would only give me 10-12 degrees of mechanical advance. I tried flipping the center cam over etc., but hte two sets I tried would not do better than that. If I understand correctly, many stock set-up HEIs (not all) are set-up for 20° of advance - but the heavy springs don't let the full advance come in until 4500+ rpms - I think this is an emmissions thing

You may be getting all the advance you desire, it just is not "all in" by 3000 - be sure it is still not increasing at higher RPMs. I think you are likely on the right track with one medium, one light spring, good starting point, that is what I use.

After all that, I think the next thing to start with is to grind a bit of the tails of the weights (red) but just such that the tail have to move farther before they hit the center cam - the whole tail should need to be ground, but may depend on the weight/cam profile you have. Do a bit at a time so you keep them even as possible. I saw a post a while back here where someone had taken a 360° protractor and screwed it on the top of the distributor to check how far the weights would move at max travel. I think this is a great tool (but I think the rotor rotation is 1:2 vs degress of timing at the timing tab, so keep that in mind that 10° at dist = 20° at crank).

Hope that helps some...

Well, I took out some HEI's I have in my parts bin and played with several weight/cam combos. All appear to be giving me the same amount of travel. I do notice that there is more advance in the HEI than the weights will allow so that definately is the limiting factor.

Problem is I don't se how grinding the weight will allow more travel, it seems as though the 'tits' on the side of the cam is what is stopping the weights from pushing out farther.

Von is also correct that the aftermarket sets don't fit. I tried all 6 of the HEI's I have and only my original has pins that will fit the aftermarket weights.

Ron, the weights I currently have are 105/106/139/60. All seem to do the same thing.

The grinding takes a little experimentation due to the many factory combinations of weights and cams. On some I've had to grind both the weights and the cam. On some flipping the cam over helps, some not. Without a dist machine and not installed in an engine, you could measure across the furthest points of the weights in the fully "slung out' position to see if you're making progress.

Well after playing with several HEI's I have laying around in various states of dissasembly and using several weight and cam combos for several hours, I made the following observations.

The weight/cam combo is definately the limiting factor. But I also noticed that although all of these combos that I tried would not allow the HEI to 'travel' to full advance there was something more interesting. They also would not allow full 'travel' to the fully retarded position either.

The amount of advance you get in the dist is the difference between the starting point and the finishing point. It really doesn't matter if you start at full retard or finish at full advance as long as the difference will net you the desired amout of CHANGE in degress.

Looking at it that way, I held the distributor on my hand with it turned to the "full retard" position (felt quite at home there). I then installed a cam and tried to fit the weight on without changing the advance position. What I found was that only one cam I had would allow this to be done..if I flipped it over.

Now using that as a base I installed different weights until I found a set that would yeild the most advance. Of the selection I had #60 gave me the most noticible advance. However none of them would let the dist advance enough to the "full Travel" permitted.

But with more travel on the retard end, my advance will be greater than it was before, so maybe I don't need full travel after all, only testing will tell the net amount of degrees I picked up.

Now In my original problem I was looking for 8-10 degrees more. If this change doesn't give it to me then the grinding comes in. So I figured I give it a try anyway.

Using #60 weights, for the reasons above and that I have a few sets. I started some grinding. By removing only 1/16 inch I was able to get a little more advance, not full but that was about the limit before the weights were to close to slinging out too far and getting stuck on the 'pads' they ride on.

Hopefully tomorrow I'll get to test it in the car and see what the numbers really are.

The attached pics are:
1. Swing Before Grinding,
2. Swing After Grinding,
3. a comparison of the grinded weight laying on top of a stock weight.
4. The picture of the cams shows the ones I tested, the cam on the right is the only one that would allow full retard, when it was flipped over (numbers down).

Seems like you are the right track. I would suggest, and perhaps Von can confirm as he is more experienced than I am: the "full" travel available in the distributor, assuming the weight set never restricted movement, is likely more than you want.

I looked at my records and all of the HEI's I've done ended up with 22-24 crankshaft degrees of mech advance. If I remember right that's about as far out as the weights will safely go without getting stuck. Most of them had about 16 deg before mods. 24 deg plus 12 deg initial gives 36 deg total which is the ideal starting point IMO.

OK, I finally got to some real life testing. Remembering that I was starting with 12* of advance originally. With the addition of the cam plate mounted upside down allowing travel to the 'full retard' end an stock weights (#60) I picked up 4 degrees and had a 16* advance.

Then changing to the #60 weights that I had done a little grinding on I picked up another 6 degrees to a total of 22 degrees. Pretty close to where I think I may need to be.

With my initial set at 16 and the centrifigal bringing me up to 38 the car 'feels' much more responsive. No indication of pinging. I started with a two med springs, but advance came it a little at idle. Then I tried a heavy and a light and the effect was the same as the two mediums. Finally a heavy and a medium did not allow any advance at idle and brought it all in by about 2700. Note: I used the heavy supplied in the kits, the original GM heavy springs are yet even much heavier.

16 is about as low as I can go at idle. Any less requires too much idle screw and I loose the carb adjustibility. Any more timing and the car is fuming me out at idle.

Now after all this, I have a slight stumble if I mash the pedal from a start. :mad: Any Ideas??

I started with adjusting the idle mixture screws to get the proper setting. I also adjusted the acc pump to the correct setting.

It still stumbles, but if I ease into the throttle (even relatively fast) there is no stumble. It only happenes if I mash it hard from a dead stop.:confused:

I have the same situation, except I am down to just the slightest "mash the peddle" stumble. I think it is in the acc pump cam. For comparison, I am running 20 initial with 36 total. I think my motor would like more initial, but 20 seem to be the limit for hot start. THe key is to run the lightest spring possible to where you have a stable idle. Prior to the intake swap and intake gasket leak fix, I had similiar situation where the idle was very eratic and the timing was jumping up/down. Ended up having to go to heavier springs to slow down the timing at low rpm. Now that you have your total advance worked out, you need to find the limit to where you can get the timing coming in, yet maintain a stable idle.

I worked out most of the timing issues, at least as they relate to the timing curve on the mechanical advance.

Now the stumble I developed has grown into a deeper problem I encountered while attempting to get it tuned

Thread continued here (http://www.chevelles.com/forums/showthread.php?t=106827)

I have one that is marked 78 305 HO. It is the only one that has 20deg of advance from the factory that I have come across. Like has been shown a little grinding can bring any HEI to the same total advance. It looks like you can't get much more with the stock tower.

I just set up a new Summit HEI for a friend. It had the same problems most GM HEI's have. As delivered it had max 18 deg at 2400 rpm mech advance and 24 deg of vacuum advance. It took a LOT of grinding of the weights and trial on the dist machine to get it to 24 deg mech advance. Had to get the weights all the way in against the center cam at idle and all the way out at full advance to where they were just short of going off the plate. I also have trouble with the advance coming in TOO SOON on HEI's. Usually end up using stock heavy springs. The Summit HEI came with an adjustable vacuum advance but no limiter. I fabbed a limiter and got the vac advance down to 12 deg.