Another Boggie One

[Ricks70Duster340]

Bought a '70 Duster 340 back in late November and just starting to work on tuning. The engine is a '72 360, and heads/block looks stock. I doubt there is a special cam. Has 340 intake and 340 exhaust manifolds. Original type distributor has a Pertronix I. I replaced the original AVS with a Holley Avenger 670 because the AVS had a bog. Same bog in the new carb as well. I've made no changes to jets. Vacuum advance set to use ported spark. No vacuum seen at that port at idle, so I know the primary butterflies are not too far open. Auto transmission (probably stock converter) with 3.23 posi & G60/14 tires in back.

Car has a huge bog when starting out from a dead stop, or when pressing the gas after it coasts down a long hill. If I very gently open the throttle, the engine will not bog. Mid range and above seems fine. I can see good flow from acc pump squirters when I barely move the throttle. Here is what I've done:

Tried initial timing from 14-22*. Idles at 650 at 14* initial in gear. Idles at 850 in neutral.
With initial at 14*, the total timing is 40* around 1800 RPM
Set idle screw down low and adjusted mixture on all 4 corners for best RPM
Made sure no clearance between accelerator pump arm and screw
Made sure fuel level is right at the bottom of the sight glass on both bowls

 

[forphorty]

670 street avengers seem to be crap OOTB . Holley 670 Street Avenger Holley 670 VS Street Avenger Hesitation Fix Holley 670 accelerator pump problem Holley Street Avenger Carbs: Not recommended - Chevy Message Forum - Restoration and Repair Help 670 Holley Street avenger?

 

[CRUZE 418]

Put the AVS back on. Tighten the air valve spring to 2 1/2 turns. That's release the spring lock, tighten till the secondary blade closes and then 2 1/2 turns. Usually a bog is too much air too soon and not enough fuel. So tightening the spring slows it's opening. That's what I did back in the day.

 

[Bewy]

What post #3 said. Check float levels FIRST.

 

[George Jets]

Put a 1406 Edelbrock on it. They run excellent on a range of stock engines, 318, 360, 383s.
Bolt on and go, no re-jetting necessary, run great right out of the box. No Bog . . .

 

[413]

Is the heat crossover under the center of the intake manifold blocked off or open? It should get hot pretty quickly from a cold start.

deos it have old gas in It? This can cause unusual running problems. Have you run fuel through it and put some miles on it since you’ve owned it?

 

[Ricks70Duster340]

Is the heat crossover under the center of the intake manifold blocked off or open? It should get hot pretty quickly from a cold start.

deos it have old gas in It? This can cause unusual running problems. Have you run fuel through it and put some miles on it since you’ve owned it?

Could not tell you if the crossover is open or not, but I do wait until the car is at full operating temp before getting on it. Have run quite a number of tanks through it at this point, so everything is fresh. Have worked on other areas of the car (starting with the rear and working forward) up to this point so have about 1500 miles on it now. The car runs, but not nearly the way it should. Others posted about going back to the AVS, or an Edelbrock. I had the same bog with the AVS, so putting it back does not seem like the right way to go.

 

[Mattax]

I had the same bog with the AVS, so putting it back does not seem like the right way to go.

The advantage would be that most AVS don't have weird issues as the Avengers do. However some of the smog era AVS carbs are unique to just a year or two and need to be understood to do this sort of tuning.

That said lets see if we can help with the current situation.

Tried initial timing from 14-22*. Idles at 650 at 14* initial in gear. Idles at 850 in neutral.
With initial at 14*, the total timing is 40* around 1800 RPM

Since it idles smooth at 14° 650 rpm your assumption that it has a fairly stock cam seems reasonable.
40° @ 1800 rpm is way too quick. Since its an unknown factory distributor, do you disconnect and plug the vacuum advance hose when you measured this?
Remeasure the timing, vacuum advance disconnected (plug the hose). measure from as slow as the engine will tolerate then every 200 or 250 rpm. This is esspecially important because you are observing a difference in rpm between in gear and out.

The difference between the idle in neutral and the idle in gear is because the load on the engine is high. Its high relative to the power and torque the engine is making at idle speed. That leads us to..

Set idle screw down low and adjusted mixture on all 4 corners for best RPM
Made sure no clearance between accelerator pump arm and screw
Made sure fuel level is right at the bottom of the sight glass on both bowls

If these are the big sight glass bowls, fuel level should be in the middle.
If these are the small sight plug holes with a clear plug, then bottom as you have done is a good starting point.

With four corner idle and a stockish cam it is very easy to provide excessively rich idle mixture. I'd start with the rear mix screws just cracked open and then adjust the front two for best rpm. Then see if adjusting the secondaries helps at all.
After getting best rpm turn any one screw in just a 1/16 or 1/8. RPM should drop. Turn it back to where it was. Then add 1/8 turn to the screws. The engine needs a richer mixture in gear than in neutral. Its just a PIA to tune with the car in Drive. But the goal is to tune it for maximum efficiency and power in Drive.

As far as using the timing port as an indicator of whether the throttle blades are open too much at idle rpm, that sounds reasonable. However I would not count on that. The carb could have a timing port that is well above .035 or .040" exposure.

Primaries shold look something like this.

Secondaries generally less. Often not exposed at all. Leave them at factory setting is often best other than to crack them a little more to see if/when more air is needed.

 

[Mattax]

As far as using the timing port as an indicator of whether the throttle blades are open too much at idle rpm, that sounds reasonable. However I would not count on that. The carb could have a long t-slot on the primary side or a timing port that is well above .035 or .040" exposure.

This from the Urich and Fisher Holley Carburetor book illustrating the importance of correct throttle relation to t-slot .
(Link in this post: Carb knowledge help needed)

The conditions where too much or too little t-slot is exposed are circled.
I crossed out the idling characteristic that may not apply especially with a four corner. With 4 corner idle, you've roughly doubled the potential fuel delivery at idle.

Adding holes or filing the throttle blades are traditional correctives, but should only be done if needed after other tuning (timing and air adjustment).

 

[Ricks70Duster340]

The advantage would be that most AVS don't have weird issues as the Avengers do. However some of the smog era AVS carbs are unique to just a year or two and need to be understood to do this sort of tuning.

That said lets see if we can help with the current situation.

Since it idles smooth at 14° 650 rpm your assumption that it has a fairly stock cam seems reasonable.
40° @ 1800 rpm is way too quick. Since its an unknown factory distributor, do you disconnect and plug the vacuum advance hose when you measured this?
Remeasure the timing, vacuum advance disconnected (plug the hose). measure from as slow as the engine will tolerate then every 200 or 250 rpm. This is esspecially important because you are observing a difference in rpm between in gear and out.

The difference between the idle in neutral and the idle in gear is because the load on the engine is high. Its high relative to the power and torque the engine is making at idle speed. That leads us to..


If these are the big sight glass bowls, fuel level should be in the middle.
If these are the small sight plug holes with a clear plug, then bottom as you have done is a good starting point.

With four corner idle and a stockish cam it is very easy to provide excessively rich idle mixture. I'd start with the rear mix screws just cracked open and then adjust the front two for best rpm. Then see if adjusting the secondaries helps at all.
After getting best rpm turn any one screw in just a 1/16 or 1/8. RPM should drop. Turn it back to where it was. Then add 1/8 turn to the screws. The engine needs a richer mixture in gear than in neutral. Its just a PIA to tune with the car in Drive. But the goal is to tune it for maximum efficiency and power in Drive.

As far as using the timing port as an indicator of whether the throttle blades are open too much at idle rpm, that sounds reasonable. However I would not count on that. The carb could have a timing port that is well above .035 or .040" exposure.

View attachment 1715722885

Primaries shold look something like this.
View attachment 1715722887

Secondaries generally less. Often not exposed at all. Leave them at factory setting is often best other than to crack them a little more to see if/when more air is needed.

Thanks for that info. I reset the float level to mid glass (was at the bottom) on both primary & secondary bowls. That required me to lean the idle mixture screws on both front and back. The front is about 1/2 turn from seated now and the back is about 1/4. The mixture screws are very sensitive, and turning them just a little really affects the RPM. In my initial post, I had the timing set without the vacuum canister connected. After doing so, the car seemed lazy. I reset the timing to the MP instruction sheet you provided in your "how to" article How To Limit and Adjust Chrysler Vacuum Advance Cans. With vacuum canister unplugged, I set mechanical timing to 35*, then plugged canister in and got 48*. Close to the max of 50* that MP said I should go for. After doing that, I reset the carb mixture & idle. Still has a hesitation coming off of idle, but the bog is not as bad. I've had cars that were very crisp, so know it can be done.

 

[George Jets]

How To Cure Carburetor Stumble

 

[Mattax]

That all makes sense.
Keep at it and you'll keep improving it.
Sometimes there may be steps back as well as forward. It does take some time and fuel but its worth it.

With vacuum canister unplugged, I set mechanical timing to 35*, then plugged canister in and got 48*. Close to the max of 50* that MP said I should go for.

That's a reasonable starting point.
Keep this in mind. They wrote that knowing the mechanical advance in the distributor would result in an initial timing around 10 to 14*.
End result was a good baseline for mid and top end performance and probably OK idle performance depending on the cam and engine.
Idle performance being the lower priority, and also the hardest to predict as there are so many different cams and compression ratios.

Here's a plot of the timing for the MP distributor when I set the timing to be 30* at 2700 rpm.

I didn't get the timing below 900 rpm that day, so I don't know the actual initial. (We sometimes learn from our mistakes! )
Anyway the point here is that your distributor probably has a different advance curve.
Perhaps the closest factory ones to the one above are in 273 4 bbl distributors (non-CAP).

Any factory CAP/CAS distributors from the late 60s, early 70s will have a super quick and long initial advance.
Here's an example of that (from shop manual specs).

Notice how fast timing jumps up from 750 to 950 rpm. This was to make up for the fact it was purposely retarded at idle (compared with a non-CAP engine).
Using less initial timing made the cylinders hotter at idle so the combustion more completely consumed all of the hydrocarbons. The made less power, so higher idle speed was needed (750 rpm). The compromise here was entirely at idle. A little less efficiency in terms of power was traded for a lot less CO and HC out the exhaust. Above idle everything is pretty much back to normal.

 

[Ricks70Duster340]

That all makes sense.
Keep at it and you'll keep improving it.
Sometimes there may be steps back as well as forward. It does take some time and fuel but its worth it.



That's a reasonable starting point.
Keep this in mind. They wrote that knowing the mechanical advance in the distributor would result in an initial timing around 10 to 14*.
End result was a good baseline for mid and top end performance and probably OK idle performance depending on the cam and engine.
Idle performance being the lower priority, and also the hardest to predict as there are so many different cams and compression ratios.

Here's a plot of the timing for the MP distributor when I set the timing to be 30* at 2700 rpm.
View attachment 1715722926
I didn't get the timing below 900 rpm that day, so I don't know the actual initial. (We sometimes learn from our mistakes! )
Anyway the point here is that your distributor probably has a different advance curve.
Perhaps the closest factory ones to the one above are in 273 4 bbl distributors (non-CAP).

Any factory CAP/CAS distributors from the late 60s, early 70s will have a super quick and long initial advance.
Here's an example of that (from shop manual specs).
View attachment 1715722927
Notice how fast timing jumps up from 750 to 950 rpm. This was to make up for the fact it was purposely retarded at idle (compared with a non-CAP engine).
Using less initial timing made the cylinders hotter at idle so the combustion more completely consumed all of the hydrocarbons. The made less power, so higher idle speed was needed (750 rpm). The compromise here was entirely at idle. A little less efficiency in terms of power was traded for a lot less CO and HC out the exhaust. Above idle everything is pretty much back to normal.

Wow, that curve sure looks like what I've seen from mine. Very quick advance. As a side note: After I bumped up the timing from my original post, the engine seems to run cooler (on the gauge) than it did. Never overheated, but did go up one notch on the temp gauge. Now it seems to be happy at just below mid-range. Think we're moving in the right direction.

 

[Ricks70Duster340]

Thank you, Mattax for all the help. I advanced the timing another 2* and all the bog disappeared. Now the engine starts & runs like I expect from a Mopar!

 

[Mattax]

Once you get the timing curve measured, we can see what adjustments to the advance may be needed for the mid to high rpms.