Tuning Question

[Palf70Step]

My 70 starts and seems to run Good, except when I let off the gas while driving it seems to backfire once in while. I have timing set at 6 Deg BTDC and the carb was adjusted for max vacuum when I did the idle mixture screws. Do you think I need to adjust timing or maybe carb slightly out of wack? II added an EDelbrock perfomer manifold, and new HEI., new plug wires. MY old brain is thinking timing is slightly off, but also know it could be carb. Just looking for some helpful hints to smooth it out.

[72SB]

are you using vacuum advance?

was the truck originally a points dizzy?

[sixty8HDC30]

popping on deceleration is usually a lean condition. Could be jetting or gummed up carb or fuel starvation from a weak pump or clogged filter/line.

[Palf70Step]

It's been an HEI since about 1980. Yes I am running vacuum advance. I regasketed and cleaned carb before I put it back on engine, but being lean maybe something to look into. Only differnece from before is it had factory quadrajet manifold (with adapter) to the edelbrock manifold with 1" spacer. When I tore into it it was having issue with acceleration stumble. When I did carb i replace accelerator pump as other was dried out and cracking.

[Dashman]

Are you using the original coil power source? Because that wire has a resistor built into it near the bulkhead and does not supply 12 volts to your HEI distributor. Just a suggestion because you didn't say how you supplied power to your HEI distributor.

Also check the vacuum advance unit that it is moving freely, or moving at all, and it goes to timed port vacuum, not manifold. Manifold vacuum would be advancing it when the RPM is decreasing, no bueno.

You could be slightly rich in the idle to mid range RPM.

Research key words "backfire, carburetor, and tuning". I have come across some great articles that helped me when I had carburetor issues. It's been so long since I've had issues that I can't remember what were the solutions.

One last thing, what elevation are you at? The jets could be rich throughout the RPM range.

[Burnsy01]

Originally Posted by sixty8HDC30

popping on deceleration is usually a lean condition. Could be jetting or gummed up carb or fuel starvation from a weak pump or clogged filter/line.

I second this

[72SB]

Originally Posted by Dashman

Also check the vacuum advance unit that it is moving freely, or moving at all, and it goes to timed port vacuum, not manifold. Manifold vacuum would be advancing it when the RPM is decreasing, no bueno.
.

You want manifold vacuum, not timed for VA. Otherwise you have no VA at idle or when pedal is released. with base timing 8-12 degrees, VA at idle using manifold vacuum adding 10-14 degrees gives you 20-24 degrees of advance at idle and low part throttle (read most cruising time) which is ideal.

Timed vacuum was a failed emission reduction trainwreck.

Controlling how much and when VA comes in is a secondary conversation but also very important to solid tuning. Adjustable VA can and a detent plate installed to limit how much VA can be added is all part of it. Be it home made detent or the Crane one no longer available is the way to go relative to setting up VA along with an adjustable VA can..

[Wrenchbender Ret]

Try turning the idle jets 1/2 turn richer. (Counter clockwise).

George

[Sheepdip]

Originally Posted by Palf70Step

My 70 starts and seems to run Good, except when I let off the gas while driving it seems to backfire once in while. I have timing set at 6 Deg BTDC and the carb was adjusted for max vacuum when I did the idle mixture screws. Do you think I need to adjust timing or maybe carb slightly out of wack? II added an EDelbrock perfomer manifold, and new HEI., new plug wires. MY old brain is thinking timing is slightly off, but also know it could be carb. Just looking for some helpful hints to smooth it out.

Backfires out the exhaust pipe or the carb? Makes a difference.

[Palf70Step]

Originally Posted by Dashman

Are you using the original coil power source? Because that wire has a resistor built into it near the bulkhead and does not supply 12 volts to your HEI distributor. Just a suggestion because you didn't say how you supplied power to your HEI distributor.

Also check the vacuum advance unit that it is moving freely, or moving at all, and it goes to timed port vacuum, not manifold. Manifold vacuum would be advancing it when the RPM is decreasing, no bueno.

You could be slightly rich in the idle to mid range RPM.

Research key words "backfire, carburetor, and tuning". I have come across some great articles that helped me when I had carburetor issues. It's been so long since I've had issues that I can't remember what were the solutions.

One last thing, what elevation are you at? The jets could be rich throughout the RPM range.

At sea levrl,about 15to 30 feet

[Palf70Step]

Originally Posted by Sheepdip

Backfires out the exhaust pipe or the carb? Makes a difference.

Tail pipefar as iknow. It doesnt do it in the shop, no mattet how i rev it, just under load while driving.

[68 P.O.S.]

Which carb are you running? If it has transition slots in the throttle bores, you need to pull the carb and set the slots to square. Then reinstall and setup your vac advance to provide no more than 10-15 degrees of advance. Honestly, you're best off with a Crane adjustable vac advance. Connect the vac advance to manifold vac, not ported/timed. Why? Because lean mixtures take longer to burn and need the extra advance. The lean mixtures are at idle and cruise. Connected to ported/timed vaccum, it has the opposite effect....no added advance at idle where it's needed, and increased timing during acceleration where it's not needed.

For timing, 10 isn't bad, but I'd bump it up to 12-14 degrees. Don't forget to unplug the vac advance when you set timing.

[72SB]

^^^this

unfortunately the Crane adjustable VA and detent plate are no longer available. There may be another brand of adjustable VA but you would need to go the home made route on detent to limit the amount of advance the VA provides

This is the Crane VA with the detent plate installed. You need to set, test, repeat as needed to get it to limit the amount it needs based on your timing light/damper readings. Each notch makes a difference in the amount of travel the arm can move

[Mike C]

Usually a lean backfire comes out the intake while a rich backfire is in the exhaust. I was surprised to see that Crane limiter is no longer available. That’s a bummer.

[Palf70Step]

Thanks for all the ideas. Vacuum advance is working properly. I'll just run thru it all again. Carb is an Edelbrock 1403 (600 CFM, electric choke). Choke is working, and the throttle blades were set right when I went to put it back on. I wish no I had put an AFS meter setup on it. That may happen in near future if it messes with me. JUst a bit out of practice. Haven't had to do much carb playing in a little while. Thanks again. I'll be looking at everything again.

[jeffahart]

Check for an exhaust leak. When letting off the throttle, and temps and pressures are right, you can get a pop or a backfire with a small exhaust leak. Remember the old days when low riders had what they called rappers? Used to crack me up!

j

[68 C10 Driver]

Originally Posted by Wrenchbender Ret

Try turning the idle jets 1/2 turn richer. (Counter clockwise).

George

This was my thought as well, sounds like it is running too lean.

[geezer#99]

Originally Posted by Wrenchbender Ret

Try turning the idle jets 1/2 turn richer. (Counter clockwise).

George

Before you go richer, go leaner.
Turn one screw in slowly until the idle changes. If you can turn it all the way in with no change then you’re idling on the power circuit and the transfer slot is fully exposed.
No amount of mix screw fiddling will help.

[72SB]

OP, the Edelbrock carb with electric choke and mechanical secondaries is the 1403 but 500 CFM. You can download the owners manual which shows all the adjustments. Its basically like a Carter AFB carb. Not as fussy to tune as a Holly

[weq92f]

Originally Posted by 72SB

^^^this

unfortunately the Crane adjustable VA and detent plate are no longer available. There may be another brand of adjustable VA but you would need to go the home made route on detent to limit the amount of advance the VA provides

This is the Crane VA with the detent plate installed. You need to set, test, repeat as needed to get it to limit the amount it needs based on your timing light/damper readings. Each notch makes a difference in the amount of travel the arm can move

.

FWIW, currently there are a few of these on eBay...this one here appears to be very similar to the one you've shown. Pricey though.

https://www.ebay.com/itm/22593562522...Bk9SR_S-oZrDYw

-klb

[68 P.O.S.]

Originally Posted by weq92f

.

FWIW, currently there are a few of these on eBay...this one here appears to be very similar to the one you've shown.

He needs one for HEI, not points.

[68 P.O.S.]

Originally Posted by geezer#99

Before you go richer, go leaner.
Turn one screw in slowly until the idle changes. If you can turn it all the way in with no change then you’re idling on the power circuit and the transfer slot is fully exposed.
No amount of mix screw fiddling will help.

This!

This is what happens when you don't square the transition slots.

[72SB]

The Crane kit dd make one for points and another for HEI. You can buy a VA can that makes less advance than others. The early 327 high HP ones. You could slide a small piece of vacuum hose on the small end of the "L" rod to limit its travel some to decrease the amount of advance it provides

Bottom line is controlling how much VA is provided at at what RPM it goes out is the way to really fine tune a motor relative to timing. Especially on modified motors.

People who still run without VA get plugs loaded up quick at part throttle and less throttle response. that timed (ported) vacuum thing was a BS stab at emissions. Use manifold vacuum for VA

[RichardJ]

They sell others at similar prices that adjust rate, but do not really limit total advance. Sleeving the pin with tubing would reduce the total some, but not by a lot. You would have to get more ingenious to reduce it down to, say 10*.

They sell Accel and Moroso units at around $40 that claim to adjust the total advance with the same rate adjustment, but the reviews seem to indicate they do not work as claimed.

You have to fabricate an independent limiter, the same as with the $14 HEI advance units.

[72SB]

you can buy the $14 adjustable one then just drill/tap the flat bar above the slot for a machine thread screw and use a small rectangle flat piece with a slot in it lengthwise to position it wherever you need. I have seen that type of homemade version.

There is a guy who did frequent some forums named David Ray who has a whole write up on doing it. Timing 101 I believe is how its titled.

Here is a chart showing all the VA can part #'s and what amount of advance they provide for both points and HEI dizzys