HEI Mech advance spring help/timing adv advice

[greywurm]

I am trying to think through an issue I have with some combination of timing, mech advance and vac advance. Hoping you guys can help me out troubleshooting this... I am leaning towards a mech advance issue, but I really dont know a lot about it.

Specs: 350 sbc, edelbrock 1406, aftermarket intake, hugger headers, true dual exhaust with black widow mufflers, HEI, TH350

I have had the initial timing set around 14 degrees as this seemed to be a happy medium between initial and total (I dont remember where total timing fell). But I was still noticing a bit of a ping coming out of the exhaust when letting off the throttle going downhill on the highway (engine almost at 3000 rpm).

Being that I also felt like I am lacking some power, I decided to throw caution to the wind and time to 36 degrees total (vac can plugged) and see what it would get me. This brought initial timing at somewhere over 16 degrees.

I have a short hill close by, but its a little short and in a residential area so going 80 isnt good, so I threw it into second to get the RPMs up at a lower speed. As expected, the pinging through the exhaust is a lot more frequent and wont stop until i put the trans into 3 to drop the rpms down.

Now, I am thinking since I either have to have too little total timing or too much initial timing that there is something wrong with the mech advance, like stuck or worn springs. Problem is, I really dont know what I am doing or what I am looking for, so would need some advice from you guys.

Also, I dont know how to test the vacuum advance can to see if its functioning properly. I did a couple runs with the vac adv in ported vac and manifold vac ports on the edelbrock carb... Not really any difference in the pinging, but it always idles better on manifold.. i've also read that ported is garbage and an early emissions trick from the 70s.

[greywurm]

A couple of pics since I hear you all like pictures...

[geezer#99]

First you need to prove your timing tab is correct.
Look here.
http://www.crankshaftcoalition.com/w...op_dead_center

Here's a wiki on your distributor.
http://www.crankshaftcoalition.com/w...EI_distributor

Do you have a breather on the drivers side valve cover?

Good to see the fuel reg but bad to see the rubber fuel line.
What's your fuel pressure set at?
As for the ported or manifold vac thing. Depends on your motor combo. Some motors (mild ones) like ported.

You're not hearing pinging out the pipe. That's after fire. Incomplete combustion due to lack of timing or too much fuel.

EAsy to check if your vac advance is working. Hook it to manifold port and plug it in. Idle should go up. And drop down when you unplug it.

[greywurm]

Thanks for the info geezer. I probably should spend some time also making sure the tab is correct.

I dont have a breather, just an oil hole plug.

I have the fuel pressure between 5 and 6 if i remember correctly. I do have plans to replace the fuel hoses with either braided or the regular steel lines.

I used to have a hesitation when initially hitting the gas if I didn't ease on the pedal to get going, like the engine almost choked out. Replacing the pump in the carb (forgot what its called) where it sprays some fuel when going between idle and primary jets didnt help, but going with manifold vacuum did.

Vac can does work - idle goes up... but i was curious if its operating properly like coming on at the right time or letting off when it should.

[greywurm]

Originally Posted by geezer#99

You're not hearing pinging out the pipe. That's after fire. Incomplete combustion due to lack of timing or too much fuel.

If its after fire, then adding timing should have cleaned up the noise a little, but it actually made it worse... So should I be looking more at the fuel?

I also should note that I am pretty sure the engine is stock internally. I also have a slight exhaust leak because of the headers and challenges with the bolts.

[geezer#99]

You need a breather. You need air in to mix with the crank fumes and make the pcv work.

An exhaust leak can draw air in and make the popping noise. Especially on deceleration.

To check the vac pot, hook a longer hose to it, and use your lung power to pull a vacuum on it. When you release the vacuum you'll hear it snap back.
Or you can pop the dizzy cap off and watch the plate inside move.

Sey your fuel pressure at 5 psi.

And last but not least!!
Get a new air cleaner!!
If you get a backfire thru the carb those cleaners light on fire!!

[Wrenchbender Ret]

That "pinging" you are hearing is not the pinging noise refrerred to when setting timing. It is usually caused by lean idle mixture. Try screwing the idle jets out about 1/2 turn. The total timing without vacuum will be best at about 34 degrees. If it pings on hard acceleration when thouraly warmed up, it is set too high. If it kicks back when you start it it is also too high.

[greywurm]

Originally Posted by geezer#99

And last but not least!!
Get a new air cleaner!!
If you get a backfire thru the carb those cleaners light on fire!!

I actually dont like the looks of it, but it makes adjustments on the carb easy without having to pop it off all the time. I also have plans to replace it though. Working out one thing at a time.

[greywurm]

Originally Posted by Wrenchbender Ret

That "pinging" you are hearing is not the pinging noise refrerred to when setting timing. It is usually caused by lean idle mixture. Try screwing the idle jets out about 1/2 turn. The total timing without vacuum will be best at about 34 degrees. If it pings on hard acceleration when thouraly warmed up, it is set too high. If it kicks back when you start it it is also too high.

It sounds similiar to when I let go of the throttle linkage when working the timing and the timing is high...
But I have no kick back under acceleration or start up. Only when letting off the throttle going downhill when the engine is at high rpm's.

What has me stumped or at least thinking of the adv springs is that the difference between total time and initial is too small... I have to trade off with either high initial/normal total or normal initial/low total.

The truck does pull off from a dead stop a lot more now. I think that is due to having higher initial time.

[lil hoodlum]

Your thread/questions are very timely for me as I have been researching these issues as well.

My truck isn't "pinging" though.


While researching the edelbrock carberator, I ran across some installation and tech videos by Edelbrock.


http://www.edelbrock.com/automotive/...h-videos.shtml

[ol'blu72]

You are on the right track with advancing the timing and it sounds like it is all good except under high load right. If it is pinging (detonation) there are two factors that will play in, your timing and your mixture. Timing is easy you set it to and accepted value, the set point that is most accepted is 36 total (up for debate but is a great starting point). Then you need to do some carb tuning. If you are pinging at 36 total, and your dist is working as it should, you have to richen it up a little. In an eldelbrock you may just be able to switch your needles to go up a step or two on the power circuit (since you are pining under load).

[lil hoodlum]

OK, I have have found 2 more links that talk about "pinging".


This link is from our very own forum.

http://67-72chevytrucks.com/vboard/s...d.php?t=214486



This link is to edelbrock, troubleshooting -Erratic idle and pinging.

https://www.youtube.com/watch?v=niHQMY2tSXc

[geezer#99]

You know Greywurm, you're living at a higher elevation means your motor likes more initial. And a slightly lowered float. And slightly higher jetting.
If you run a bunch of initial, then you need to take out some mechanical and limit your vac advance.
And take what the edelbrock dude says with a grain of salt. Every motor needs a custom curve on the timing and depending on whether it's mild or wild the use of which port for vac advance changes.
And nowhere in any of Edelbrock's literature or vids is there ever a mention of limiting your fuel pressure. It's a known fact that eddy carbs and their ancestors the carter afb don't like more than 5 psi. It's in the dual float inlet at the back design.

[elkniks]

I had an issue with my 72 not taking the gas and backfiring off idle, found out that I had original resistor wire (7v I think) feeding my HEI.

[greywurm]

Originally Posted by elkniks

I had an issue with my 72 not taking the gas and backfiring off idle, found out that I had original resistor wire (7v I think) feeding my HEI.

I replaced the wire when I changed engines and put the 350 in to return the truck back to whats on its spid. A PO put a 250 with HEI and also left the resistor wire.

[greywurm]

Originally Posted by geezer#99

You know Greywurm, you're living at a higher elevation means your motor likes more initial. And a slightly lowered float. And slightly higher jetting.
If you run a bunch of initial, then you need to take out some mechanical and limit your vac advance.
And take what the edelbrock dude says with a grain of salt. Every motor needs a custom curve on the timing and depending on whether it's mild or wild the use of which port for vac advance changes.
And nowhere in any of Edelbrock's literature or vids is there ever a mention of limiting your fuel pressure. It's a known fact that eddy carbs and their ancestors the carter afb don't like more than 5 psi. It's in the dual float inlet at the back design.

Currently, after messing with it before making this post, i am running 20 degrees initial, 36 total, and vac looks to be adding 10 at full idle vacuum.

I have the regulator in because I read a lot of issues people were having with the edelbrocks, and it a lot were due to having too much pressure. I wound up getting the 4.5-7 psi regulator thinking I would be covered now and in the future if I were to change carbs. Its been a while since I had a gauge inline, but I am pretty sure I dialed it out as much as it would allow.

I have been cruising around town just in drive and havent had any issues... It did almost try to stall out when I threw it in drive leaving home depot yesterday... but it recovered. I am considering taking it back to the highway for a quick spin to see what happens at highway speeds downhill.

[greywurm]

Originally Posted by geezer#99

You're not hearing pinging out the pipe. That's after fire. Incomplete combustion due to lack of timing or too much fuel.

After much reading the last couple days, I do concur that it is after fire, and I may have been combining a couple issues... I think the timing thing could be a separate issue. It only happens on decel (let foot off throttle), downhill, about 3000 rpm or more

Anyways, for the after fire, I am wondering if my exhaust is too free flowing. Its some generic hugger headers into 2.5" exhaust all the way out. I have mufflers, but they are pretty open. When the guy at the exhaust shop showed them to me i laughed because they dont look to do much other than reshape the sound. you can see from one side to the other.

So if its its not the exhaust and lack of back pressure, i think it has to come to the carburater or spark plugs leaving too rich of an exhaust mixture.

I am still trying to think this through before I start buying parts.

[airdale94]

Some of those vac adv are adjustable, aftermarket. You pull the hose off and stick a alen wrench in the port. I had a Pontiac with the same symptoms. Also a intake leak will do that. If your running a alum intake you should use fiber gaskets instead of blue PermaTex's.