Team Chevelle banner
1 - 20 of 86 Posts

·
Registered
Joined
·
115 Posts
Discussion Starter · #1 ·
Hey guys need a little help here, I have a 496 bbc with a msd street fire HEI ignition and a Holley 850 with mechanical secondaries. Car was running perfect before I did a recurve on the ignition timing and now it idles great and in park will rev easily through out the whole power band but under load I’d say between 2000-2500 rpm it stumbles and once I get her going above 3000 rmp she’s gone. It happens if I take off quickly from a stop or at cruising speed sitting at the 2000 rpm mark. Was thinking it could be the accelerator pump? Or the cam for the accelerator pump needs to be bigger to help the fuel keep up? Any help on where to start would be good.

- Rob
 

·
Registered
Joined
·
115 Posts
Discussion Starter · #3 ·
Please outline before and after timing curves.
Hey sorry I should have added that, so the timing curve I first started with was the original springs on the street fire HEI which apparently give a total of 22* mechanical advance at 4000 rpm and I had a initial of 12 for a total of 34*. The vacuum advance would pull another 12* revving up to about 2500rpm. Not going to lie I’m not a huge fan of this distributor and the tuning aspect of it isn’t great either. But for now I gotta make it work until I get a MSD pro-billet. The advance curve I was able to make now is:

16* initial , I used a light and medium spring on the advance curve kit which “apparently” gives me about 21-22* of mechanical advance all in by about 2200-2500 rpm. Also I limited the vacuum advance with a stop plate that pulls anywhere from 5-8*.

total timing me revving all the way up to 4000rpm for a short period was around 44-46* total.
 

·
Registered
Joined
·
177 Posts
What was your total timing at before you made the changes, and after you made them? Was total timing 34* before and now is 44*-46*? Or am I understanding your post in-correctly?
 

·
Registered
Joined
·
115 Posts
Discussion Starter · #9 ·
What was your total timing at before you made the changes, and after you made them? Was total timing 34* before and now is 44*-46*? Or am I understanding your post in-correctly?
The total all in was close to what I have now by what I remember it was about 46* and now it’s about 44-46* but what changed was the actual curve I’m making the timing come in a lot sooner 2200-2500 rpm instead of 4000 rpm. Also I have more initial timing now going from 12* originally to 16* initial now.
 

·
Premium Member
1966 Chevelle SS396
Joined
·
579 Posts
I would take that stop plate off the vacuum advance.
 

·
Registered
Joined
·
458 Posts
Your total timing numbers seem really high. Does that include vacuum advance? I had a similar experience though in that I had my timing curve ending at around 3000rpm. I put in lighter springs to bring it all in by 2400rpm (based on recommendations saying it was better) and the car did not like it. I had bad bucking/stumbling under light load in the 2000-3000 rpm range. Put it back to 3000, problem went away and I haven't messed with it since.
 

·
Registered
Joined
·
124 Posts
When you did the "recurve" on your HEI, did you just change the existing mechanical advance to lighter springs?... or did you also change out the centerplate and advance weights on your HEI?
 

·
Registered
Joined
·
115 Posts
Discussion Starter · #14 ·
When you did the "recurve" on your HEI, did you just change the existing mechanical advance to lighter springs?... or did you also change out the centerplate and advance weights on your HEI?
Yes I bought the advance curve kit for the street fire hei and changed weights, springs, and centre plate.
 

·
Registered
Joined
·
115 Posts
Discussion Starter · #15 ·
Your total timing numbers seem really high. Does that include vacuum advance? I had a similar experience though in that I had my timing curve ending at around 3000rpm. I put in lighter springs to bring it all in by 2400rpm (based on recommendations saying it was better) and the car did not like it. I had bad bucking/stumbling under light load in the 2000-3000 rpm range. Put it back to 3000, problem went away and I haven't messed with it since.
Ya that seems to be pretty much the issue I’m having. Yes the “all in” number would be with the vacuum advance. Without vacuum advance I had 36* total timing.
 

·
Registered
Joined
·
124 Posts
What advance curve kit did you install (part #)?... Did it look like either of the kits below?
715272

Did you verify the amount of total "mechanical only advance" (VA disconnected) after you installed the new weights and centerplate?... and at what rpm was total mechanical only advance seen?
 

·
Registered
Joined
·
115 Posts
Discussion Starter · #17 ·
What advance curve kit did you install (part #)?... Did it look like either of the kits below?
View attachment 715272
Did you verify the amount of total "mechanical only advance" (VA disconnected) after you installed the new weights and centerplate?... and at what rpm was total mechanical only advance seen?
Yes I bought the kit on the right, and I also did disconnect the VA when testing the mechanical advance. I need to double check what rpm it maxed out at just because I put it at 4000 to guarantee it was all in to make my total 36* without VA. The only reason why I ran it to 4000 rpm is because the kit and diagram is very vague, I find you can’t get it bang on with these springs/kit

here’s the diagram:
715273


I put one medium spring and one light spring so you kind of have to guesstimate what rpm it’ll be all in by without using a timing light to verify which I will later today.
 

·
Registered
Joined
·
124 Posts
When you set your initial timing to 16° BTDC @ idle, did you verify with a timing light that you were able to get a total of 36° of "mechanical only" advance while revving the engine with the VA disconnected (20° of mechanical advance from the kit)?
I have found in the past that this type of advance kit only provides about 16° of mechanical advance.
 

·
Registered
Joined
·
115 Posts
Discussion Starter · #19 ·
When you set your initial timing to 16° BTDC @ idle, did you verify with a timing light that you were able to get a total of 36° of "mechanical only" advance while revving the engine with the VA disconnected (20° of mechanical advance from the kit)?
I have found in the past that this type of advance kit only provides about 16° of mechanical advance.
Ya I did exactly what you mentioned, when I got her up to 4000 rpm with vacuum disconnected I had a total of 36* maybe I had a bit of luck with this kit hahaha. Like I said I can’t wait to get a pro-billet MSD but for now I gotta make this street fire work.
 

·
Registered
Joined
·
3,180 Posts
What squirters and P/V's do you have in the 850? also remember to make sure your float levels are correct as if they are low that will start the mains later. Also dual or single plane intake?
 
1 - 20 of 86 Posts
Top