Limiting HEI mechanical advance

[85 Cutlass Brougham]

I have a late 70's HEI on my 355 and I need to limit the total amount of mechanical advance in the distributor. I have heard of welding or epoxying the slots, but I was wondering if anyone offered limiting bushings for the stock GM piece like MSD has for it's aftermarket HEI? My engine likes 20 degrees initial advance, but this gives it way too much total advance when the mechanical advance comes in. Surprisingly, it starts fine with no kickback on the starter which makes me wonder if my Craftsman dial back light may be somewhat inaccurate. However, I do hear some spark knock sometimes, which keeps me from running the initial advance I want.Any ideas? I don't own a welder by the way ( I borrow one from a neighbor if I need it) and was thinking of just using JB Weld to narrow the slot.

 

[87MonteSS]

It sounds like there may be other issues. What is your base timing set at? What is the total mechanical advance? What is the total vacuum advance and lastly what is the total advance over all? You'll also want to see how the advance curve looks. It could be that you have sticking weights in the distributor.

There are a number of companies that make timing curve sets that come with different weights and springs for the mechanical side and an adjustable vacuum advance unit. Then you can customize the advance curve to best match your engine's needs. Crane Cams is one company that I can think of off the top of my head.

 

[85 Cutlass Brougham]

I have 20 degrees initial advance, and over 40 mechanical if memory serves. I can't go check it right now because I am in the middle of a carb swap to a really nice rebuilt Q Jet I pulled off a 1980 Chevy Van with a 350 for $30 yesterday, and I forgot to get the right Q Jet fuel line while I was at the yard. I have tried different springs, but went back to the originals because the recurve kits allowed too much advance.I did rebush to advance weights to take out the slop in them.

 

[Uncletruck]

You state that the amount of mechanical advance you have is causing some detonation, which makes sense to me with that amount of initial timing advance that you say your engine "likes." With your engine cam/manifold setup you might need to forego the stock look for a performance distributor that gives you more timing setting options- remember, the stock HEI distributor is not considered a performance item.
I think you are underestimating what your car CAN do with what you have, and are suffering some of the conciquences of some of the stock items that are somewhat lackluster performance wise.

The jetting of the Quadrajet you are putting on from that van might not be ideal for your setup either. I like the way Quadrajets work, and would never fault you for using one. But you might need to modify it a bit for best results.

I used to get caught in these traps too when looking for a good balance between performance, cost, and MPG. I found you can never really have it all, because if you were able get it all with this vintage of stuff, you would win the Nobel Prize in physics.

-Mark-

 

[85 Cutlass Brougham]

Therein lies the challenge! I want to make it look stock and haul *ss. I have a number of different rods and hangers for the Q-Jet, and swapped the secondary rods to ones from a 1968 Cadillac with a 472, but kept the deeper engagement hanger from the van carb. I plan on taking a lot of time to properly dial in the combination as this car is once again just a toy and not my daily driver. I also modified the secondary air valve door to allow full opening and 800 cfm rather than the 600 cfm most late Q-Jets have. This was done by trimming the stop where it contacts the air horn by the secondary air valve spring. The Q Jet is a very capable carb when set up properly, and there are some on 11 and 10 second cars, I figure it should be more than capable of working with my mid to high 13 second street car. I really like it because when I add cruise control, it has the provision for it as well as giving me proper TV cable geometry and better hood clearence than a Holley , Edelbrock or Demon would.Plus, it has better idle quality and fuel economy than a square bore carb would. I like having it idle smoothly at 650 RPM and 60 PSI oil pressure cold, and 30 PSI hot. Ii also takes the stock throttle kicker for the A/C.As for the distributor, it is not a stock HEI except for the housing, shaft and some of the advance mechanism. It has an MSD cap (Soon to be painted black)and rotor, as well as an MSD module with a built in rev limiter. I also used an ACCEL coil, but plan on replacing it with an MSD coil since it fits under the stock coil cover.It also runs a set of MSD plug wires run in the stock 80's Chevy wire looms. They are blue and I plan on getting a set of direct fit black MSD wires for a MCSS.I may replace it with an MSD HEI, but would rather save the $140 the local speed shop wants. I also run a chrome Holley mechanical fuel pump which I painted black so that it would not stand out. The only things stock under the hood of the car are the block, crank, rods and the brackets and pulleys. I also used a stock oil cooler off a 89 9C1 Caprice. I tried to use a stock 84 Z-28 PS cooler, but it hit the fan shroud...🙁

 

[Floydsbuick]

Have you considered an adjustable vacuum advance? Say you set your total mechanical advance to 35. But that leaves your initial timing at 10 (these are just example figures out of my head) You can hook your vacuum advance to Manifold vacuum, adjust it for 10 degress of advance, and theres your 20 initial. You can buy an adjustable advance, or make a simple, sliding plate that bolts on top of your exsisting advance.

If that doesn't sound like what you wanna do, than start looking for different weights and centers till you customize your advance to your like.

 

[87MonteSS]

I still say there's something else wrong. There's no way that 20 degrees of initial timing on a 13 second engine is correct.

Put it back to stock initial and use the mechanical and vacuum advances to get the curve correct.

 

[85 Cutlass Brougham]

Well, I will be playing with it again in a month or so. It has not even been started in almost 2 months due to a lack of time on my part, and that it is now disassembled for paint and bodywork. The hood is off and the engine covered in a thick rubber trunk mat I had lying around. I am trying to step down from my management job and be a delivery driver as it will let me concentrate more on school and my other projects.