Corvette Forum : DigitalCorvettes.com Corvette Forums banner

1 - 10 of 10 Posts

·
Registered
Joined
·
893 Posts
Discussion Starter #1
468 big block, solid mechanical cam 264,265 @ .050 with 630, 630 at 108 degrees. My buddy says 26 at idle but he has a VERY radical cam and it's his race car. Other than that I'm lost. I've read on other posts to start at 15 advanced at idle with mechanical advance plugged. Is that the same as a big block, I'm not thinking so. Where should I start? The reason for this post is that my car just started to not idle, it idled down till it died and it loves to backfire out of the carb from a start, and I need to start somewhere to trouble shoot.
 

·
Registered
Joined
·
1,845 Posts
468 big block, solid mechanical cam 264,265 @ .050 with 630, 630 at 108 degrees. My buddy says 26 at idle but he has a VERY radical cam and it's his race car. Other than that I'm lost. I've read on other posts to start at 15 advanced at idle with mechanical advance plugged. Is that the same as a big block, I'm not thinking so. Where should I start? The reason for this post is that my car just started to not idle, it idled down till it died and it loves to backfire out of the carb from a start, and I need to start somewhere to trouble shoot.
You didn't say what compression you have, but with that cam you ought to be able to tolerate 15 at idle easily. Look at your Barry Grant Mighty Demon manual. I think my Speed Demon one says 16-18 for recommended timing, but I don't have it handy. Do you have the MSD 8572 Distributor? You want 36 degrees at 2500rpm with the vacuum advance plugged. If your distributor has 28 degrees of mechanical advance, when you set the timing for 36 at 2500rpm, you will have 8 at idle (36-28). If it's an MSD distributor, you can change the mechanical advance amount by using the different bushings that come with the distributor. For 18 degrees initial, use the 18 degree bushing, which is the physically largest one, and the lighter 2 blue springs, and set the timing at 36 degree at 2500. You'll have 18 degrees at idle and that spring set lets the mechanical advance be at it's max ("all in") at 2500rpm.

I started out with the 28 degree bushing, then dropped down to the 25 degree, now I'm on the 21 bushing so I'm at 15 initial. It starts fine and has great throttle response. I'm working my way up slowly. There's a "sticky" above the header when you're in the C-3 Forum section..."Setting your timing, by Lars" or not sure of the exact title. Read that.

Have you checked your timing at idle now? Once you get past idle does the engine run like it did before? If you're cruising at 2500 and punch it what happens?
 

·
Registered
Joined
·
5,916 Posts
You didn't say what compression you have, but with that cam you ought to be able to tolerate 15 at idle easily. Look at your Barry Grant Mighty Demon manual. I think my Speed Demon one says 16-18 for recommended timing, but I don't have it handy. Do you have the MSD 8572 Distributor? You want 36 degrees at 2500rpm with the vacuum advance plugged. If your distributor has 28 degrees of mechanical advance, when you set the timing for 36 at 2500rpm, you will have 8 at idle (36-28). If it's an MSD distributor, you can change the mechanical advance amount by using the different bushings that come with the distributor. For 18 degrees initial, use the 18 degree bushing, which is the physically largest one, and the lighter 2 blue springs, and set the timing at 36 degree at 2500. You'll have 18 degrees at idle and that spring set lets the mechanical advance be at it's max ("all in") at 2500rpm.

I started out with the 28 degree bushing, then dropped down to the 25 degree, now I'm on the 21 bushing so I'm at 15 initial. It starts fine and has great throttle response. I'm working my way up slowly. There's a "sticky" above the header when you're in the C-3 Forum section..."Setting your timing, by Lars" or not sure of the exact title. Read that.

Have you checked your timing at idle now? Once you get past idle does the engine run like it did before? If you're cruising at 2500 and punch it what happens?
:agree: Follow the stickey. All gen 1 Chevy engines work with this formula. Initial timing is irrelevant. It was set up that way (initial) for smog emmisions, not performance.
 

·
Registered
Joined
·
1,845 Posts
:agree: Follow the stickey. All gen 1 Chevy engines work with this formula. Initial timing is irrelevant. It was set up that way (initial) for smog emmisions, not performance.
"Initial timing is irrelevant." Not sure if this really true 100% of the time.

If you set the timing to come all in at 2500, and the distributor has a certain amount of mechanical advance built in, the initial will fall where it will. And you don't need to mess with setting it. If I were using the stock 69 427/435 distributor, which has 30 degrees of mechanical advance, and set the timing at 36 degrees at 2500 rpm (with vac plugged), then I'd only have 6 degrees intial at idle. That would make for very lazy off the line performance versus 15 or 18 degrees. Stock distributors range from 26 to as high as 38 degrees of mechanical advance. These figures are from the 69 Chassis Service Manual. The 38 is for a non-corvette application. L36 and L68 are 26. Strangely, there is no L46 (350/350) listing.

Anywho, if your distributor has an adjustable mechanical advance arrangement, I'd use it to keep the initial up high.
 

·
Registered
Joined
·
893 Posts
Discussion Starter #5
I'll start at 15 and try to get 36 by 2500RPM. I'm pretty sure that I'm using the blue springs and that my MSD Distributor is set up correctly, it might have moved a bit while getting on it which I do quite too often. To answer Fred Mann's question about getting on it when it's up around 2500ish, no it doesn't do any of the other stuff, but it feels like it's not 100% though. I'll start with timing then start playing with the carb a bit. When I say a bit I mean that it was correct not too long ago but it did come loose from the intake a touch. And yes, I did check for vacuum leaks and there were none.
 

·
Registered
Joined
·
893 Posts
Discussion Starter #6
HOLY SH!T Timing is everything! My car is back to being the beast it used to be!!!! Now all I need is to get the damn thing to idle. I have a leaky thermostat around the water neck and I'm pretty sure my engine is running hot and it's not idling after it's been running a bit cause it's hot.
 

·
Registered
Joined
·
5,916 Posts
HOLY SH!T Timing is everything! My car is back to being the beast it used to be!!!! Now all I need is to get the damn thing to idle. I have a leaky thermostat around the water neck and I'm pretty sure my engine is running hot and it's not idling after it's been running a bit cause it's hot.
Timing IS 90% of it. Now, if your running hot, check the usual items. I would check for vacuum leaks, and I bet you are lean. Also, you mentioned back firing thru the carb. I believe that can take out the power valve.
I would also consider sending your carb to Lars for a refresh. These two items can bring back the dead.
 

·
Registered
Joined
·
893 Posts
Discussion Starter #8
I need to seal up the thermostat and get the damn thing to pop open to cool the damn engine first!!! After that then I can get the damn thing to idle. I do appreciate ALL the info!! Again, you guys are VERY reliable and instill a sense of confidence I can depend on. I would like to say thanks to all of you!
 

·
Registered
Joined
·
5,916 Posts
I need to seal up the thermostat and get the damn thing to pop open to cool the damn engine first!!! After that then I can get the damn thing to idle. I do appreciate ALL the info!! Again, you guys are VERY reliable and instill a sense of confidence I can depend on. I would like to say thanks to all of you!
When you seal it up, take out the stat and drill 2 1/8" holes in the flat. This helps eliminate air locks, and burps the system. Consider a Mr Gasket stat.(Robert Shaw) They have the advantage of failing open.:thumbsup:
 

·
Registered
Joined
·
1,845 Posts
I'm not sure how new your engine is, but you mentioned the carb being loose and the thermostat housing leaking. It just makes me think it might not be a bad idea to re-torque the intake.

Do you have an o-ring type thermostat housing? Mr. Gasket also makes them. No gaskets. It has a re-usable o-ring instead.

The Mr. Gasket thermostats flow more water when open than a standard thermostat. Didn't know that they failed open, that's a good feature too.

Your Mighty Demon power valve is designed to handle backfiring. There's a check valve or some other anti-back-fire protection device.

Glad you're getting back to normal.
 
1 - 10 of 10 Posts
Top