Looking for a little advice on the timing numbers I just pulled from my 73’ Duster (backfire issue)

[tonyp25]

The best answer will require knowing the timing vs. rpm. For example, you set timing at 5* at 750 rpm What was timing at 900 or 1000 rpm, 1400 rpm etc.?

My guess is the throttle was too far open,, so when the vacuum advance was reconnected, it added timing and the idle rpm floated up.
(Just as Chrysler explained it.)

Further guessing the engine is not stock.
I would reduce the throttle stop position (slow idle) and increase the initial a bit more so the engine again is idling at 750 or or slower if it will do so.

Plot the mechanical timing vs rpm out on the chart. (no vacuum advance connected, plug the port)
As long as the timing is between green lines and blue lines it should be fine.

Hey Mattax,

Advancing the timing can’t cause the car to start having some smoke come out the tailpipes right? Finally felt like it was running well but now I noticed that it seems like I might be burning oil or something is clogged causing it to smoke? I’m still tinkering with getting the idle and timing just right but wanted to make sure that this is likely a totally separate issue.

Thanks

 

[Mattax]

Small changes in timing wouldn't effect it (exhaust) so directly or obviously.
Too much timing will cause a gurgling or pinging sound under load. Under heavy load (full throttle) this is real bad and need to get out of the throttle immediately. You can look at the spark plug porcelain for bits of aluminum - that would be sign of detonation under load.

Too advanced for the conditions can contribute to excess NOx which is sort of brownish. I just think that's unlikely. It would ping or at light throttle, trailer hitch (feel like the car is getting jerked ).

What color is the exhaust smoke and under what situations do you notice it? White? Brownish? blackish? bluish?

 

[tonyp25]

Small changes in timing wouldn't effect it (exhaust) so directly or obviously.
Too much timing will cause a gurgling or pinging sound under load. Under heavy load (full throttle) this is real bad and need to get out of the throttle immediately. You can look at the spark plug porcelain for bits of aluminum - that would be sign of detonation under load.

Too advanced for the conditions can contribute to excess NOx which is sort of brownish. I just think that's unlikely. It would ping or at light throttle, trailer hitch (feel like the car is getting jerked ).

What color is the exhaust smoke and under what situations do you notice it? White? Brownish? blackish? bluish?

It seemed to be very dark grey. It was a hot day yesterday and it didn’t start happening until after it was warmed up. Then it continued throughout the day. Also went to a car show and it happened on startup too but I think the engine was still warm.

My idle also was rough when this was happening. It had dropped about 200 rpm’s in drive and sounded like it was struggling.

I did run seafoam through the tank and carb last week but I thought I had got all that out and that was more white. I bought a new pcv valve and breather just to see what happens. Both those hoses seemed for the most part clean.

I ran it today and didn’t notice smoke or rough idle (it was slightly cooler). Driving me crazy.

 

[Mattax]

My guess is too rich when warmed up or oil.

Did the choke fully open?

Also look at the plugs.

 

[tonyp25]

My guess is too rich when warmed up or oil.

Did the choke fully open?

Also look at the plugs.

Thanks, looks like I’ll be pulling the plugs next weekend. I believe the choke did fully open but maybe I’ll check again to see if anything is sticking.

If it is too rich is that typically adjusted through the air/fuel mixture screws?

 

[Mattax]

Choke is fully open your first photo.

If it is too rich is that typically adjusted through the air/fuel mixture screws?

For idle mixture - yes.

For the idle mix adjustment to be effective, the primary side throttle blades need to be positioned so only a bit of the transfer slots are exposed below the plate.

Soooo, if you've adjusted idle speed up (or down) with the 'idle speed screw' what you've actually done is open or close the throttle position for idle. That's fine, but there's a limited range of adjsutment before too much or too little trasnfer slot is showing below the blade. If the transfer slot is too exposed then the idle port doesn't do much.

PS. 'Idle circuit' is a misnomer. Better to think of it as the low speed or low throttle circuit.

This illustrates how both the transfer slot and the idle port contribute fuel mixture at idle.

 

[tonyp25]

For idle mixture - yes.

Thanks, looks like I’ll need to brush up on a few YouTube tutorials on how to do it…wish I had an air/fuel ratio gauge but maybe that’s something I’ll try installing this winter.

 

[Mattax]

Thanks, looks like I’ll need to brush up on a few YouTube tutorials on how to do it…wish I had an air/fuel ratio gauge but maybe that’s something I’ll try installing this winter.

Ignore the you tubers, and you don't need an AFR gage. If you were doing idle AFR then you really need a CO meter, just like it was done when your car was new - but that was for emissions reasons.

 

[Mattax]

With a Holley.
First. Check fuel the level in both bowls.
Can't see which Holley you have but most have sight plugs on the bowl. Traditional sight plugs are located so the fuel level should be just at the bottom of the plug (primary) and a little lower on the secondary bowl. New big clear window plugs aer usually set near the middle.


Then check the primary transfer slot exposure under the throttle plates at idle are in the range of .020 - .040"
You can check with feeler gage or drill bits.
While you're at it, write down how many 1/4 turns in of the idle speed screw from just touching gets .020, 030, and .040. Then you'll always know how far open the throttles are.

The drill bit is next to the transfer slot.
The hole is the idle port.

 

[Mattax]

Now if the idle screw has to be turned in too much to get the idle speed up so it will run smoothly, add a couple degrees more timing. Then you should be able to back the screw down. Then adjust mixture. Finally try it out in gear.

 

[Mattax]

PSS. if your car has a timing delay (OSAC) bypass it. If the hose from the carb timing port goes directly to the distributor, that's fine. OSAC delayed the timing advance and was very annoying when trying to moderatley accelerate.