Allpar Forums banner

1 - 17 of 17 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter #1
Hello,
I would like to know the most common way to tune a 74 Thermoquad's mixture screws. The reason I ask, is because it seems strange to me that I'll start the procedure with the screws at 2 turns out from being closed, engine starts right up, I'll go to about 4 turns out (counterclockwise) to give me the best vacuum reading (18.5") and highest rpm. I can turn an additional one turn counterclockwise and nothing happens, aka: no drop in rpm or vacuum reading. Is it normal to go more than one full turn richer before a drop in rpm or vacuum is observed? I'm not sure where my optimal mixture screw setting is.
 

·
Super Moderator
Joined
·
21,398 Posts
These have fuel restrictors in the throttle plate so that idle mixture enrichment is limited. There may be no further mixture change after 2.5 screw turns out. They should however give plenty of adjustment range to give a satisfactory idle. Keep the screw turns equal.
It sounds like there is either an intake manifold or internal carburetor vacuum (air) leak or the internal fuel passages are restricted by deposits/debris.
If this carb sat for an extended time, it should be gone through with fresh carb reconditioning parts (gaskets, seals, o-rings, etc).
1974 may have used the propane enrichment method or an exhaust gas analyzer for idle mixture adjustment (covered in the service manual). You can get close by getting a best warm idle and then turning the screws in equally for a slight idle speed drop. Not to the point where lean misfire occurs.
Don't try to do this by ear if you aren't experienced. You may be able to get it 'better', but not right.
An exhaust gas analyzer is used here:
| Repair Guides | Idle Speed And Mixture Adjustments | Adjustment Procedure | AutoZone.com (at http://www.autozone.com/repairguides/Dodge-Aspen-Volare-1976-1980-Repair-Guide/Idle-Speed-And-Mixture-Adjustments/Adjustment-Procedure/_/P-0900c15280087880 )
 

·
Registered
Joined
·
1,291 Posts
10% ethanol gasoline has less energy than straight gasoline, which makes a lean carburetor even leaner. The idle jets are intentionally restricted to limit the amount of pure gasoline that goes into the engine. It's a mixture of air and gas that flows thru the idle jets.
 

·
Premium Member
Joined
·
20,094 Posts
And also noted, since the jets sit on the bottom of the carb in this model, any slight amount of garbage inside the bottom tends to "mess" with the functioning of these carbs (one reason to like Holley carbs), so a cleanout may be warranted.
 
  • Like
Reactions: pt006 and Brian T.

·
Registered
Joined
·
8,096 Posts
Check the bowl plate too. Sometimes they won't be flat. A little filing (slowly) across the plate face will take care of that. And you want to make sure you clean all passages.
 
  • Like
Reactions: Brian T.

·
Registered
Joined
·
7 Posts
Discussion Starter #6
These have fuel restrictors in the throttle plate so that idle mixture enrichment is limited. There may be no further mixture change after 2.5 screw turns out. They should however give plenty of adjustment range to give a satisfactory idle. Keep the screw turns equal.
It sounds like there is either an intake manifold or internal carburetor vacuum (air) leak or the internal fuel passages are restricted by deposits/debris.
If this carb sat for an extended time, it should be gone through with fresh carb reconditioning parts (gaskets, seals, o-rings, etc).
1974 may have used the propane enrichment method or an exhaust gas analyzer for idle mixture adjustment (covered in the service manual). You can get close by getting a best warm idle and then turning the screws in equally for a slight idle speed drop. Not to the point where lean misfire occurs.
Don't try to do this by ear if you aren't experienced. You may be able to get it 'better', but not right.
An exhaust gas analyzer is used here:
| Repair Guides | Idle Speed And Mixture Adjustments | Adjustment Procedure | AutoZone.com (at http://www.autozone.com/repairguides/Dodge-Aspen-Volare-1976-1980-Repair-Guide/Idle-Speed-And-Mixture-Adjustments/Adjustment-Procedure/_/P-0900c15280087880 )
Thanks for your reply imperial crown member. Is this fuel restrictor located in the throttle plate causing me to not be able to enrich the mixture screws to the point of dropping off the RPM and or lowering my vacuum reading? The engine idles ok at my starting point of two turns out on the mixture screws. I do not have an exhaust gas analyzer to use when adjusting the mixture screws so I'm trying to do it via the vacuum gauge and rpm readings. Here is some background information of what's been performed on the carburetor and engine so far. The Thermoquad is the factory stock carburetor on an engine with only 53,000 original miles. The carburetor was professionally rebuilt by carburetor rebuilding service out of Rockford Illinois. Every rubber hose on this engine has been replaced. All original emissions controls are still intact and working properly. (OSAC, CCEGR, TIC valve, etc) I've checked for vacuum leaks around the carburetors base and it's gasket and found no leaks. What I'm ultimately trying to do with the mixture screws is to eliminate a slight hesitation off idle. I had noticed that when goosing the throttle, that I didn't get a very strong accelerator pump discharge. I checked the accelerator pump's S link and pump stroke height and they were correct. I am not suspecting the O-rings in the carburetor because of the professional rebuild. I thought it could be a clogged fuel filter and or a weak fuel pump. I remove the old fuel filter and cut it in half only to find that it was clean inside. I also replaced the fuel pump with a new factory Carter. The Excelerator pump now seems a bit stronger.The hesitation went from mild to slight. This is why I am really trying to eliminate this hesitation with mixture screws adjustments but I'm wondering if it's just gonna be an adjust and drive trial process.
As a sidenote, I've been taking temperature readings of my exhaust manifold ports. Passenger-side bank averaging 450 to 550° and the driver side bank averaging 350 to 450° I hope to balance these out with the mixture screws. That's another reason why I really want to nail down this mixture screw setting. Is there something I'm missing or is the exhaust gas analyzer my only option at this point?

PS. Sorry for the extremely long post.
 

·
Super Moderator
Joined
·
21,398 Posts
Idle mixture adjustment may not be your hesitation. Idle mixture screw adjustment is basically for idle. As the throttle blades open, they sweep past the transfer ports (which are like slots) above the idle holes in the bore below the throttle plate. These must also be free of restriction.
Is the accelerator pump cup leather or Viton? If leather is allowed to dry out after a car sits for awhile, it may not swell back to its necessary size after being wetted with fuel again. Before Viton, the rubber cup could be damaged by modern fuel additives. You need to see 2 healthy streams of fuel from the accelerator pump while slowly opening the throttle.
The power valves must also be adjusted correctly.
Timing is important. Can you get the car to slightly ping under hot acceleration?
 

·
Registered
Joined
·
1,291 Posts
Also check that your heat riser valve on the passenger side exhaust manifold is working properly.
And that the EGR valve is closing.
And that the mechanical advance weights in the distributor are free and oiled.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #10
The timing is dead on at 5 degree BTDC. (Factory setting) The engine is a 400 cid high performance "P" code.(Dropped Magnum ?) I have read something about the slotted transfer ports in relationship to the position of the throttle plates. I believe the article was saying, that if your throttle plates were exposing too much of your transfer port slots, that this may cause the stumble upon acceleration. If this is the case, I do not know how to check this. I can only assume that on the carburetor rebuild that the setting was already checked. I'm not sure if the accelerator pump cup is leather or Viton, it's whatever would come in a newer rebuild kit. Hopefully that doesn't depend on where the rebuild kit came from! The gentleman who rebuilt the carburetor is well known in the Chicago land area for dealing with Mopar products. I've got to give him the benefit of the doubt. Now that the new fuel pump has been installed, I do see to healthy streams coming from the accelerator pump. (I'm not sold on the fact that the new fuel pump helped out with the weak acceleration pump shot....but it seem to have) I have not noticed any pinging under hard acceleration.
The EGR valve not closing completely brings up an interesting point. All I can say to that is, I took it off, cleaned it, make sure it was seating properly, installed a gasket and tested the diaphragm. If this was to still leak, how does one test it ? I have no heat riser valve on the passenger side exhaust. Is far as the mechanical weights inside the distributor, when I get on it, there's no shortage of power. The distributors vacuum advance I believe has also been tested, when I was testing the T.I.C. valve. The engines rpm went from 900 rpm (factory settings) to 1200 rpm once the radiator temperature reached 225°. This is a completely stock 74 Dodge charger automatic with the above engine combination. I'm still wondering about my mixture screws, just for the fact that one bank of the engines exhaust manifolds are 100° hotter than the other bank. Could this be partially due to an inbalance of mixture screw settings? If the temperature readings of exhaust manifolds ports can vary 100° then I'm fine with that, I just don't know personally. Is it normal to have a 350° heater hose temperature? Radiator hoses and the radiator itself temperatures are fine. I mean the heat under this hood is incredible. Hopefully that's just an air pocket in the heater hose or is it being preheated by the exhaust manifold? I know I've strayed from my original question but this is really starting to get frustrated. Thanks to all that have replied!
 

·
Super Moderator
Joined
·
21,398 Posts
If the throttle is opened too far for an acceptable idle, it will take fuel from the transfer slots which will be leaner than if the blades were more closed and drawing fuel from the mixture screw holes. The most common reason for jacking the throttle adjustment open too far in order to attain an acceptable idle would be to mask a lean idle circuit or an intake manifold vacuum leak.
Is there timing chain slack? You might try bumping the ignition timing to 10°BTDC and see if the hesitation gets better. You can always adjust it back if it pings under load.
With the EGR valve off the car and disconnected, you should not be able to blow through it. A leaky EGR valve will behave like a vacuum leak.
The bank-to-bank exhaust temperature difference may also be because of flow if the heater riser valve is closed. It will direct the exhaust gases of one side under the intake floor to help warm the incoming mixture. Looking and comparing left to right side spark plugs would also help determine mixture balance issues.
The appropriate spark plug for best burn should probably be Champion Copper Plus RJ-14YC.
These engines always seemed to run very hot. They would cook valve cover gaskets and hoses.
 

·
Premium Member
Joined
·
20,094 Posts
You mentioned exhaust temperatures being slightly different and all that. Are the two inner ports on one side and the two outer ports on the opposite side of the engine more equal in temps? As in 4 and 6 are near the same temp as 1 and 7, 2 and 8 similar to 3 and 5?
 

·
Registered
Joined
·
2,951 Posts
What timing do you get when fully advanced?
 

·
Premium Member
2017 Charger Pursuit AWD
Joined
·
592 Posts
On both the TQ and QJ, CCW is lean. 5 full turns out is full lean, with the screws barely engaged. You're using the factory-recommended method of adjusting, so my guess is you probably have a vacuum leak, typically at the base plate on a TQ.

A hesitation is generally the pump shot, if your timing is correct. On the TQ, you should have a couple of slots, maybe 3, that the pump shot arm hangs from. You can move it closer to the carb to increase the leverage and get a bigger shot of fuel.
 

·
Super Moderator
Joined
·
21,398 Posts
Ported vacuum to the distributor is important. Manifold vacuum to the distributor will have full advance at idle and can cause off-idle hesitation.
Make sure that the distributor advance can diaphragm holds vacuum and isn't leaking.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #16
Hey Imperial Crown member, i'm hoping there's not timing chain slack because it only has 53,000 original miles. I'm assuming that if I bump the timing from 5° to 10° BTDC and the hesitation is eliminated, that there was slack in the timing chain. I'm confident that the EGR valve is not leaking. There is no heat riser on either exhaust manifold for an explanation of temperature differences on the exhaust manifold ports that's why I think it has to do with the mixture screw settings. The under the hood temperatures I agree with you on, Chrysler's way of eliminating NOX gases. The ported vacuum on the base of the carburetor goes to the OSAC valve then out of the OSAC valve to the TIC valve. My distributor does not see manifold vacuum unless cooling temperatures are above 225°.
Hey Dana44, i'll check this weekend and record each exhaust ports temperatures and do the comparison that you referenced. It will be interesting to see.
 

·
Premium Member
Joined
·
20,094 Posts
The reason I question this is, the stock intake manifolds and all dual plane intakes are an H pattern that divides the engine into two 4 cylinder engines of sorts. If the set of cylinders I pointed out, two inner and two outer on the opposite side and of the same temp, it is a fuel issue inside the carb, or a vacuum leak causing those four cylinders to be running lean compared to the opposite. Heck, you can literally pull every other spark plug wire off the distributor cap and she will run just fine, just down on power. It will give us a little better idea of where to look for your stumble problem. Discovered this issue on an AFB carb in a '62 Cadillac, was lower on fuel than I thought and fuel was not completely getting to half the carb.
 
  • Like
Reactions: Bob Lincoln
1 - 17 of 17 Posts
Top