I'm running a 350 with a 6-71, Dart 2 iron eagle heads 215cc runners 64cc, dual Edelbrock 600 CFM carbs. I recently had to have the car towed and after looking into the MSD distributor 85551 I noticed that the inside was completely corroded. As I went to rebuild the distributor the advance springs fell apart from rust, I could not tell what size was on the advance. The combo I put on was the middle size springs but the car just doesn't seem to run the same. Before my idol was around 1100 RPM and now it ranges from 1100-1400 while sitting. Any ideas on what would be the best spring settings and also timing settings for my set-up or if it's something else. The car ran great before, but now I'm afraid to go very far. HELP spring is just around the corner!!! :confused:
http://www.nastyz28.com/forum/gallery/data/502/Camaro_Parts_002com.jpg

I run a pro billet as well on my car and I bought the kit and did them according to the graphs they show on the instructions. They arent that expensive and give you everything you need,I would go that route and be back on track in no time!

But back to your story,thats kinda wierd b/c I was at the track one day (local 1/8 mi) and the car was running good and on about the 4th pass it went a tenth or so slower and then all of a sudden WOULD NOT idle,period. Pedeled it back to the trailer,took the cap off and on of the springs was BROKEN. Not sure if it was from rust, I couldnt see anything,it just freakin broke. Kinda pissed me off b/c I wanted to shut this LS1 guy up but couldnt b/c of a 5 cent spring :whine: .

Silver and blue, the light ones

gotta love MSD

I install 2 Blue springs as my starting point and go from there. If you get too light a spring in it the timing will be kicking in at your 1100 RPM idle and cause it to idle up and down and generally be unstable. You might even have to go with a Blue and a heavy silver to keep the timing from kicking in at that idle speed, but once you start using those heavy springs the timing is pretty sluggish in the low RPMs and you lose some power down low- I don't recommend using the heavy silver springs if you can possibly avoid it.

Did you mess around with the timing limiter bushing at all? If not then don't worry about it- they don't wear out. If you did then post back and we'll try to determine which one you need to put in there.

Vacuum advance on this distributor? If so, did you mess with it or attach it to a different vacuum port on the carb than you used to?

I thought blown motors liked LOTS of advance at idle. We put a big blower on a buddy's bbc and had all sorts of headaches with idle and timing. Called Weiand and they said lock it out, set it at around 28-32 and forget about it. The guy got greedy and started swapping pulleys around and trashed the motor before things really got sorted out. But I'm interested in hearing how you guys that are running roots blowers on the street do the timing.

I will try to change the springs and see how that does. Any suggestions on what spacer color to use, I believe it currently has the black one installed with the medium springs. Tomorrow I plan on removing the carbs and spacer, clean and reinstall them with new gaskets. Should I go light or heavy on the springs for my first attempt? As it sits now, when I come to a stop the idle goes any where from 800 to 1400 RPM and then settles to about 1200 RPM. I have the car pictured in the 71 Camaro folder, it's the one that's red with black stripes and of course the blower. My Hot Wheels Car!! Thanks you to everyone that's helping me!

Damon, thanks for the post. I'm running the MSD pro billet with a MSD 6A module, no vacuum line.

Marv- yes, due I think, to the low compression when you're not under boost they can take quite a bit of timing. One way to get that is to lock out the timing as stated above, but that can make starting difficult. I went the other way and used a typical vacuum advance distributor with the vacuum advance plougged into a full-time vacuum source on the carb so it's kicking in fully even at idle. This is possible on my engine becuase it's very mild and still makes plenty of vacuum at idle and part throttle to work the vacuum advance canister. They key was to make sure that there was NO vacuum advance as the motor gets close to making boost. In the case of my engine, there is about 5" of vacuum under the carb when I'm just about to start making boost in the manifold under the blower.

So my curve looks like this:

8* initial + 20* centrifugal, all in by about 2800 RPMs
12* of vacuum advance starting at about 6" of vacuum and topping out at about 12" of vacuum.

So at idle I have 8* initial + 12* vacuum advance for a total of about 20*. Idles almost like a stock engine, no lope, no chop, no wavering idle. It turns over on the starter very easily, too, since I'm only working against 8* of initial advance under those conditions. Around town driving is also pretty snappy becuase of the vacuum advance as well. When I lay through on it, however, it goes to a much more boost-friendly advance curve, topping out at only 28* maximum.

Alternatively, I could have plugged the vacuum advance line into a MANIFOLD vacuum source and not had to worry about the vacuum advance kicking in at all under low vacuum/boost conditions since it would be directly reading the vacuum/boost the motor itself was seeing. However, I decided against that since I wasn't sure if a typical vacuum canister was ever designed to hold boost- I was worried I might rupture the diaphragm or do other damage to it over the long term. That's why I plugged it in at the carb, although plugging it into the manifold itself would probably still be worth a try, if anyone wants to give it a shot. This method would probably work better on an engine with a lumpier cam that doesn't draw much vacuum at the carb- but lumpy cams are certainly not the way you would want to go with one of these little blowers anyway.

If you're not going to do it the "odd" way I have with a vacuum advance distributor then, yes, you want as much initial advance as you can get, short of having the motor kick back against the starter when you're cranking it over. That's probably why you see so many guys running locked out timing.

On a an MSD Pro Billet the bushing that gives the LEAST centrifugal advance (and therefor the highest possible initial timing setting) is the Black bushing, I beleive. It will be the one that's the largest diameter. I beleive it limits you to about 18* of maximum centrifugal advance. That would allow you to run 10* initial to hit 28* of maximum advance, or 12* initial to hit 30* total, etc, etc. Still, probably not optimum for a street driven blower motor (which is one reason I tried the vacuum advance thing in the first place).

So are you saying that the black spacer is OK or should I switch? If not then should I change and what springs should I try first (blue)? Sorry for all the questions, the distributor is not my strenght. I've been lucky and not had any problems until now, I've driven the car to shows 200 miles away and it ran like a champ, the only problem was that I had to stop several times for gas! Lou

If you're not running vacuum advance then yes, the black spacer is the one you want. Two blue springs should still be about right to bring in the advance curve quickly enough but without having it kick in at idle and start doing funky things to your idle speed. You DON'T want the centrifugal advance kicking in at idle or you will never get a stable idle.

Yes, I can beleive you are sucking a lot of gas. Making it run good at WOT is relatively easy (still plenty challenging!). Making an engine run efficiently at part throttle AND still make max power wide open is just that much more difficult.

That's why you see all the gyrations I went through with my distributor advance curve (I didn't even mention what I did in the carb, which got even more attention). I wanted it to run well under all conditions to the maximum extent possible but only spending pennies on parts. In short, I wanted it ALL. I still don't think I've got it, but it's pretty close. I knock down about 15 MPGs in mixed driving. Nothing to write home about when you consider what's possible with modern EFI engines, but remember I'm not running an OD transmission and there's not really any part on my car that didn't exist 25 years ago.

Thanks again, I will give it a try tomorrow and keep you posted. Looks like rain here in St. Louis, but a good time to work on the car!

I can better offer recommendations if I knew your boost level and static compression.. assuming you are 8-10 lbs boost and 8:1 compression.. absolutely put the black busing in there (18 degrees mechanical) and set the total timing to 36 degrees which will give you 18 degrees initial timing. If you have very low static compression like me (7.6:1), she will want even more initial timing.. mine seems to want 20-22 degrees initial at the least, but will require a custom made bushing that MSD will refuse to sell you (%$#@%).

For a big roots blown street SBC, I think most would prefer initial timing in the 18-22 degree range and total timing in the 28-36 degree range depending on your set up.

Regardless, you will want that black bushing. I use the advance springs that will get all my timing in by 2200-2500.

There is a lot of benefit to getting your initial timing optimized on a blown SBC.. better idle, more low end power and response, and cooler running at idle and cruise. All this basically for free by just setting her up right.

By the way, I hope you are not running a lot of boost with those 600 edelbrocks and I hope they are not using out of the box settings. Those carbs are too small for all but the most mild and meak of blown SBC's. I would run at least 750/800 edelbrocks or 600/650 cfm holleys for a mild to moderately built blown SBC.

Well, Saturday was a very wet day in the Midwest. I removed the carbs and carb plate, cleaned and reinstalled everything with new gaskets. I made some adjustments to the linkage, reset the carb idle and changed the advance springs to the blue ones with the black spacer. On Sunday I fired the car up, after a few small adjustments the cars idle is now between 1000-1100 RPM and vacuum is at 12.8 at the carb at idle. I'm not noticing the surging that I did before, I hope to be able to take the car out this week and test it under street conditions. I also haven't checked the timing, I just timed it by ear. Looks like I'm heading in the right direction!:cool:

if i might jump in and say that damon revealed something that most don't consider when setting up a vacuum advance with a roots setup. the fact that the vacuum between the carb and blower vs the vacuum/boost between the blower and the intake valves can be drasticly different when approaching boost. damon,, that's some great info for folks. good job man.

Vacuum advance is not recommened with a big roots blower, according to all the shops I have talked to. Thus my distrubutor does no have vacuum advance.

Well Damon, It looks like the adjustment to the springs took care of the problem of my idle going up and down. I just need to make a couple of carb adjustments with the idle. The car ran good on the highway and when I came to a stop the idle was level. Looks like rain again here in STL but at least I'm heading in the right direction! THANKS!!!! :bowtie: