Dual Quad Problem

[ERA Chas]

I have twin vac. sec. 654's on a Tunnel Wedge. They work and meter perfectly.
This is a combination that has run 15 years with minor maintenence and no changes. There are no bogs, hesitations, leaks or any other imperfections.
EXCEPT-over the last two seasons they will not return to idle while running on the road and coming to a stop. The throttle will come down to maybe 1800-1500 rpm. If I just poke the pedal it will come off and drop to my normal 1000 rpm idle-and idle perfectly.
I will eliminate all the easy answers-no vac leaks, no binds in the linkage (heim joints), all springs and geometry perfect -nothing I've checked or tried has altered the problem. I've been very methodical but I'm no genious.
I'm thinking of swapping carbs front to rear to see effect. The best suspicion I have is the throttle cross link is naturally sloppy on these (no bushings or bearings on these) and may be the source of the hang-up. If the replaced carb does it too, then that's all I can pinpoint. If I'm right, does anyone know where I can get cross-linkage which is bushed? Remember, the shaft which goes through the plate is brazed to the levers on both sides.
Any experiences similar to this and the fix if found would be greatly appreciated.

[Jac Mac]

Are the secondaries closing completely, next time it does it , open the hood & manually check that they are both closed- return link may need a slight tweak if this is the case.

[Rick Parker]

The throttle shafts in the base plate or the bores they run in (more likely) may be worn not allowing the throttle plates to fully seal in the bores. Some carb shops can rebush the baseplate and or replace the shafts. If you want to check it, remove the base plate and check it against a light source (or wiggle the shaft) while it's installed on the car. If you can feel movement you have excess clearance and a source for a vacuum leak which will raise the idle speed.

[ERA Chas]

Jac,
Thank you, excellent thought, I will check that.

Rick,
Thank you too. Wear can definitely be a factor here given the age.
But if the shafts or bores have wear, would the carbs still work perfectly? They are razor-sharp in operation and you can't feel them open. The motor just goes into warp drive.
Wouldn't they vacuum leak?
In the recent past I did check them for movement and couldn't detect any.
Really appreciate the difficult answers.

[wtm442]

Make sure that the linkage is not binding. Spray some penetrating oil on every joint and see what happens. You may also need a stiffer return spring.

[undy]

Originally Posted by wtm442

Make sure that the linkage is not binding. Spray some penetrating oil on every joint and see what happens. You may also need a stiffer return spring.

I'm thinking linkage/gas pedal problem too.. If it's cable operated, check for frayed cable (could be hanging up internally in the housing). It's happened, with like symptoms, to me too.

Dave

[RICK LAKE]

ERA Chas Does this happen when the motor is hot or cold? If cold you should be able to seperate and check all the pivot points one by one. Make sure the pivot center piece is free and moving easy, If you feel any binding replace that joint. What I used for linkage lube is chain lube for a motorcycle. It's waterproof and stays where you spray it. 15 years out of this setup is a long time and you got your mileage out of it. I would next check the hiem joints on the bars for the car linkage to make sure they are free too. It could be the gas pedal pivot heim joints. After this you are looking at the carbs, the first thing to check is the choke high ilde plate and that it moves free and doesn't stick. I have had a couple of carbs with chokes,( 600, 780 ) that this plate wears on the shaft and hangs up, some times hot other times cold. If you get the car to act up, turn off the motor and pull over to the side of the road and look at what is not in the right postion on the carbs. DON'T touch anything right off, just LOOK. If you don't see anything then start to move the linkage and see if anything changes. Next would be the carbs themselves, Check shaft end play and wobble. If you are running strong return springs to the main linkage arm you will wear out the throttle shafts and base plate unless they are roller bearing. I don't think you would have this done unless racing. You would have to check to see if there is even bronze bushings the shafts ride on. If not Send out both base plates and have thing done. You will not always feel a shaft hanging up unless you move it to the stop and feel a small jump. If it is a vacuum leak coming from the motor, I have found that using a small bottle of propane with a little vacuum hose and needle point end under LOW pressure will help you fine and external leaks.If there is an external leak the motor will pick up rpm's. The intake gasket could also be split and leaken after 15 years of running. I have seen a few #1020 felpro gaskets split and leak. If you want to talk about this some more , call me 732-254-3536 in the evening est time Rick L.

[jhv48]

Before you get too deep into this, check one very simple thing:

With the engine fully warm and parked, disconnect the linkage and throttle cable and manually rev the engine (at the carb) to about 2500 RPM and then VERY SLOWLY let the carb linkage return to idle.

I'm betting the carb will not return to idle but will hang up somewhere.

If it does, BY HAND, grasp the throttle linkage at the carb and see if you can manually close the throttle. AT THE CARB!

If you can close the throttle so the car returns to normal idle, you have narrowed down the problem to the throttle shaft or maybe some deposits are preventing the butterflies from completely closing, thus binding slightly.

If this happens, take the carb off and turn it uside down. Now look through the bore and shine a flashlight in the bore from the opposite side. Manually open and close the butterflies and look for even light around the entire throttle plate. If a dark spot appears, it means it is hitting the bore and could be slightly binding at that point. Sometimes a simple adjustment is all that is needed. Possibly needs cleaning too. Either are very easy to do.

Good luck.

[tboneheller]

Check the distributor for a binding mechanical advance. Mine dose the same thing. For some reason the MSD distributor"s advance likes to hang up. I may need to switch over to heaier springs.

[ERA Chas]

You guys are great.
I have already checked/done many of the things you all suggest. I'm in the process of doing the few I haven't.
Tbone, I have already changed springs in my Unilite to no improvement but it's really appreciated.
Undy, the linkage is all mechanical, no cable and I've spent many hours making it friction-free.
Dirt/debris is never an issue on my stuff so that eliminates the easy stuff. BTW, ther are no chokes here.
The throttle shaft/bores will be scrutinized again and Rick and JHV, you make good points to check.
Sincere thanks to all and I'm working my way through all the applicable suggestions.
Thanks all.

[SuperHart]

I had exactly the same problem with the dual quad setup on my 351W. Although my engine is different I still use the 427FE medium riser carb linkage and run my carbs reversed. I found that the throttle plates on the REAR (secondary) carb were not completely closing unless I "blipped" the throttle. I had a return spring only on the linkage itself. Nothing I did corrected that problem UNTIL I added a return spring to the secondary carb throttle arm. Yes, doing that made the accelerator a little firmer but more than made up for it by eliminating the fast idle problem caused by the throttle shaft sticking at slightly open position. Try it, I think you will find that it works especially since you claim that your carbs seem to run just fine otherwise. One last thing, if the nuts retaining the carb to the intake are not snugged down evenly or if one of the nuts is too tight that might contribute to such a problem.

[Tony Aprile]

Charlie

You probably found the problem already, and the guy's gave excellent suggestions. I have run across a multi carb with a similar problem, the main linkage spring would close the front carb throttle plate. The rear carb after the linkage released tention was dependent on the holley incorporated throttle spring. The spring was not strong enough (for what ever reason) and would not close the throttle plate to the fully closed position...

Tony

[ERA Chas]

Tony is absolutely correct-everybody gave excellent advice including him.
I have tested and rechecked all the major suggestions-I even changed all the
heims to brand new ones and checked geometry. Checked secondary links, springs, gas pedal, the works. Yes, I can wiggle a little, the shafts in the throttle bores but they're all the same.
If I haven't fixed it, I'm on the right track. Now you're all gonna groan when you read this. Because it's so basic that I missed it.
The idle speed screw on the second carb. I took out 1 1/2 turns and added about 3/4 turn on the primary carb to maintain my 1000 rpm idle. The screw was holding the plate open to a fast idle. Now this doesn't explain how poking it made it come down but for sure it had an effect.The linkage to the second carb is progressive which means it would barely open when "poked".
Drove like a wildman on the back roads and stood on the Wilwoods.
1000 rpm each time, all temps normal.
I want to put more time on it but it's substantially better. Will get around to the bearings in the baseplates after I drive it a while.
Sincere thanks to all for well though-out replies. Now you guys know I'm a Bozo...