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Stromberg AAV carburetor tuning


Matt Harwood
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So the trials and tribulations of this stupid 1941 Limited are continuing to irritate me. Car ran beautifully with the broken manifolds, no stuttering, no hesitation, nothing. New manifolds installed and now it runs like crap. The biggest thing is that it seems to be backfiring through the carburetors, especially when cold, but even warm it still happens. It doesn't happen at idle, but whenever I'm on the throttle and particularly when accelerating, it's popping and stuttering. I've tried adjusting it according to the manual and it seems to have no effect. I suspect it's running lean, but I've fattened it up by opening up the mixture screws and it didn't help much. It's not running rich, because there's no black smoke or gas smell--it actually runs quite clean with no smoke or smells of any kind.

 

I'm pretty fed up with this car. Every time I fix one thing, something else breaks that previously worked properly. I'm hoping to do a lot of driving in it this summer, but at the moment, it doesn't have enough power to get up the driveway.

 

Thanks for any thoughts you might have.

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When you say "new" manifold do you mean an original that is new to the car or a new reproduction?  Years ago somebody reproduced the edmunds dual carb intake for the super 8 Packard.  What they didn't realize was there was some finesse going on inside the manifold that didn't quite get reproduced with the new ones.  You cannot get the engine to run correctly with one of those reproductions.

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No, not repro/aftermarket, used original parts. I got the car with broken exhaust manifolds and bought used replacements that were in mostly good condition. Car ran great with broken manifolds but sounded like a dump truck. With the [mostly] uncracked replacement manifolds it now runs like crap. No other changes, and I would assume that the exhaust manifolds are identical enough to not cause problems with carb metering. Hell, it ran great for the first few days after the manifolds were installed, then started acting up. The popping through the carburetors suggests a lean condition, but I haven't ruled out timing or valve adjustment, but both of those were fine prior to the manifold swap, why would they suddenly not be fine with the manifold change. That's what leads me to suspect carburetors, since they were removed and reinstalled.

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You say it ran great for a few days after you installed the manifolds so I would say that the problem might not be manifold related at all. The exhaust manifold won't make a difference in the way it runs, just the noise it makes. Your intake manifold may be leaking and that will make it run lean, and also pop. Squirt starting fluid around the joints while it is idling and see if it changes, if it does there is a leak there. If nothing changes then I would rule out the manifolds. 

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Matt,

When you transposed the carb to the new intake, are you sure there is a good seal at the bottom of the carb ?

Did you get the vacuum line from the carb to the distributor tight ?

Could be "lack of vacuum advance".

To check, pull a vacuum at the carb fitting and see if the dist plate moves.

 

Also, how far from bottom are the 2 mixture screws ?

Mine are only out 1.5 turns for a good idle.

 

Just some ideas.

 

Mike in Colorado

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Matt - carburetors go bad over years, even decades; the fact that it ran well for a few days tends to take away the carburetors as culprits.

 

Also, you mention that it happens when you are on the throttle. Adjusting the idle mixture control screws only alters the adjustment up to maybe 800 RPM. Above that, you could turn the mixture screws all the way in, OR, take them out and put them in your pocket.....no difference.

 

Since it happens primarily when accelerating, the first place I would check would be the vacuum advance unit; assuming it has one. But next would be a compression test, and then a full ignition test.

 

If it is in the carburetor(s), it will be stuff in the main circuit loosened from the fuel lines when they were removed. This stuff normally finds its way into the idle circuit, not the main.

 

Jon.

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Thanks for all the advice, guys. I spent a few hours going over the basics today. I checked the plug wires to be sure they were all connected--with that cover over the plugs, I never trust that everything is staying put or not shorting out. It all checked out and connections were solid. Pulled the distributor cap and noted that points and condenser are almost new. Did not pull vacuum to see if the advance was working, but that's a good suggestion.

 

Next, I fired it up and let it warm up to operating temperature so it was off the choke. I noticed that the choke was a little lazy, so I lubed everything up. Not that it helped with warm running, but it at least got better, quicker.

 

Once it was warm, I took Dave's suggestion to look for vacuum leaks and found none that were obvious. Pulling the vacuum line off the base of the front carb caused a noticeable (worse) change in running, so I ruled out a major vacuum leak somewhere else.

 

Taking it for a drive, there was still some popping on acceleration. It got better as the engine got hotter, but it never went away. So I started on the carbs again and went through the procedure described in the manual (I have a factory training manual that's FAR more in-depth than the service manual). I reset the carbs to the baseline in that book and noticed that they were recommending that the idle screws be opened 1.75 turns, where the service manual only recommends 1.5 turns. Not a big difference, but hmmm... Setting them to 1.75 turns did nothing, and maybe made it worse. Turning it in to 1.5 turns, still popping but maybe better. Turn in to 1.0 turn and it got better still. Tweak it a bit to around .75 turns and the popping vanished. That's FAR less than either manual specifies, which is why I didn't bother turning it in that far the first time. But Mike's suggestion that his were turned in more than he expected got me thinking. A fairly aggressive drive with coolant temps around 180-190 revealed no popping at all, just strong, clean acceleration. At idle, there's still a stutter in the exhaust, it's a little snuffly-sounding, but I guess I can live with that. That says to me it's still a little rich, but the way it's running, I'm content not to mess with it any more. I think the idle's still a little high, but meh, I'm not going to worry about that, either.

 

So it appears that my attempts to make the carburetors better by returning them to factory settings actually made things worse. It wanted a lot less fuel. Why? Who knows? But it's back to it's strong-running self again.

 

Now if I could only adjust that shift linkage so it's not so hard to get into 1st...

 

Thank you guys!

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I turn each screw in until it starts running rough, then I turn it out until it starts running rough, then I turn it half way between those extremes. After I do each one I then turn the idle down to about 600 to 650 rpm's and repeat the process. Once the idle stays at 600 to 650 rpm's after the screws have been adjusted I leave it alone. The screws should be half way between the rough running extremes regardless of how many turns out they are.

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Did a little more tweaking, mostly because that's how I am and I'm never satisfied. Got it running pretty well. While I was at it, I checked timing and noticed that it was pretty flat, so I advanced the distributor a few degrees and that really woke the engine up (it was backed off all the way). Big difference there. Idle is still smooth, I backed the idle down a few RPM and it pulls cleanly at any speed. Still a little unhappy when it's cold and the popping is there if I really lean on it, but I suspect that the choke is enriching too much, so perhaps I'll play with the adjustment on the choke mechanism. Or not, because it really runs well now and I don't want to mess it up (which is what I usually do and why I started this thread in the first place).

 

Running great, smooth, quiet, and powerful. I'm happy and looking forward to the 300 mile drive to Allentown in it. Going to rack up some miles in the coming weeks so I know I can trust it.

 

Thanks for the help, guys!

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Matt,

 

As I recall, your having turn the adjusting screws "IN" that far actually enriched the mixture from where it was. Frequently, a slightly richer mixture will make up for aged spark plugs and/or plug wires with too much resistance. I wonder if, during your initial manifold swap and tuning debacle, the plugs became fouled or even slightly carboned-up, and the richer mixture is compensating for that condition - or that the plug wires may be a modern "resistor" type, instead of actual wire. This had been the case on one of our Citroen DS-21 Pallas models when we lived in Ft. Wayne, Indiana back in the '70s. Correct plug wires and new plugs solved that one, and in that winter environment it did take some extra time to warm up that Citroen 2.2L hemi.

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1 hour ago, LAS VEGAS DAVE said:

Marty, turning the screws "IN" makes the engine run leaner not richer.

Dave,

 

Yup, I stated it backward, but turning the screws in was what may have made up for the issue I was describing with possible reference to the plugs and wires.

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