After adjusting my throttle linkage to get full opening of the throttles, I noticed an issue where the idle speed doesn’t drop down to where it should be when the throttle is released. I think it was doing this before I adjusted the linkage, but I was driving the car so little, I didn’t really pay attention.
On startup, idle is around 600 rpm indicated. Upon blipping the throttle or running the car, it sticks at ~1,000 rpm and doesn’t drop to the startup idle speed.
Here’s where my issue seems to differ from those of the other threads I’ve read about this.
If I shut the car off and restart it immediately, the idle speed is back to the original ~600 rpm figure.
While the idle is high, if I press hard on the throttle stop of one carburetor, the idle drops to about 7-800 rpm, which is actually what I would like it to be. There is no discernable movement though - there is no feeling of slack being taken out. I’m just applying pressure.
3 No amount of blipping or throttle manipulation makes the idle drop. Unless I shut off the engine or press on the throttle stop, it will stick at ~1,000 rpm.
Because of this and the fact that shutting the engine off brings it to low idle with no touching of the throttle parts, I don’t suspect a too-taut throttle linkage.
Return springs are fine, but I put two new ones on since I had them. I followed the procedures for loosening the throttle spindles and pressing the throttle stop to full close before tightening them down. I also tried messing with the angle of the return spring arm to see if I could increase the leverage to pull it tight like I was pressing on it with my thumb, but no effect.
Could it be worn bushings on the throttle shaft? I can’t think of a test to verify that. I’m guessing a vacuum leak would not have this behavior.
Oil in dash pots, lube the entire linkage , no binding,butterflies all even, carbs tight to manifold, no vacc leaks
It goes on and on
Put the uni vac on it see if they are balanced😀
Next time it sticks, pop the bonnet and manually push back against the brass fittings on the ends of the spindles. If there is slack in the linkage wobble joints, and manually pushing against the spindle causes the speed to drop, it tells you that the problem is inside the carb. Something is preventing the butterfly from closing all the way. My experience is that they can and do behave differently when hot vs cold. Possible problems:
insufficient tension on the spindle return springs
binding valves, either poorly centered or a ring of carbon has built up
binding side seals
binding spindle because it’s bent
springs pulling the spindle sideways instead of straight. Make sure they’re lined up straight.
If instead you find that there isn’t any slack on the wobble joints, then correct that. It doesn’t need a lot of slack but it needs just a bit. The throttle rods should be able to move just a bit before the spindles start to rotate.
I had a very similar problem recently. Think I read it somewhere on here, but I sprayed the throttle shaft seals on the ‘sticky’ SU with CRC 556, problem solved. Has yet to recur. Mine also doesn’t get much driving, and I wonder if some slight corrosion product or fuel residue forms on the shaft and causes it to hang up very slightly. May also have ‘worked’ the shaft from side to side with the free play at the same time.
I tried as many of your suggestions as I had time for today.
oil checked, one carb low (no gurgling sound withdrawing the damper nor resistance from the oil replacing it)
lubed every point of articulation and rotation I could find
checked return spring angles - the carb nearest the firewall (I’ll call it #1) has an “improvised” anchor location on a starter motor bolt. I’ll have to check more closely to see if the triangular spring mount is there. I always thought that looked odd, with the newish looking nut securing the spring.
wiggled, pressed, and jiggled the rods and linkages connected to the throttle spindles - no change. Only pressing the throttle stop with my thumb lowers the idle, and it stays there when I release the pressure.
Things have improved a bit. Doing a sharp throttle blip now often (though not always) drops the idle to around 900 RPM vs. 1000, sometimes dipping to around 800. If it did that consistently I’d call it quits. Prior to this, blipping the throtlle had no effect.
But if I slowly raise the revs to 2500+, then release the throttle, it always sticks at 1,000 RPM. This implies some extra inertia from the sharp acceleration action is supplying enough energy to drop the throttle down some, though not far enough.
Also, I noticed that the revs do not fall in a linear fashion when the throttle is released. If I go to 2,500+ and release, it will drop to ~1,800, then hesitate for a split second, then at a much lower rate, descend to 1,000 (or 800 if I gave it a sharp burst.)
So that seems to suggest remaining friction or binding perhaps, but I can’t understand why turning the engine off resets everything and on restart, idle is exactly where it should be. Almost like a vacuum issue was keeping the butterflies very slightly open, then turning off the motor releases it.
I’ll keep tinkering with the linkage and see about getting the #1 carb return spring anchor sorted. Then follow with the whole uni-Syn/carb balancing routine. After all this they probably aren’t very much in sync anymore, if they were before this.
Maybe squirt carb cleaner into the butterflies? If there’s varnish that could take care of it, no?
I once had a problem somewhat like this. It turned out to be one of the throttle plates not seating properly. The only way to verify was to remove the carbs and sight down the bore against a bright light. The good plates blocked most of the light. The bad plate showed a corona of light all around the edge.
But let’s come back to this:
I’m not sure what you’re saying here. The usual adjustment is to make sure all three plates CLOSE in sync. This is done by undoing the linkage at each carb, holding the throttle fully closed, and retightening. Is this what you did?
Hopefully it won’t come to that. I’ll be removing the air cleaners and taking a look, at least to spray cleaner and attempt to balance the carbs. If the butterflies are fully closed, do they seal completely, and the air is drawn through a different opening? If so, I could use a smoke machine to see if a bad seal was the culprit without removing the carbs.
The original adjustments were because my throttle spindles only opened about 60% of the way when the pedal or linkage lever was fully depressed. So I replaced the sponge-soft pivot bushing on the end of the slave shaft and shortened the horizontal control rod to get full opening of the throttles.
When I noticed this problem (which may have been occurring before I adjusted the linkage) I did hold the throttles closed and loosen/tighten the spindle link arm nuts.
Did you verify that the wobble joint in the linkage has slack? If it does, then it wasn’t pulling the valve open again. This would suggest to me that either there is carbon buildup on the valve edge or bore, or as Mike suggests, an off center valve. You may have some luck carefully sticking a solvent soaked rag in from the top and wiping around the bore and edge of the valve. Just don’t let it get snagged. Adjusting the valve itself will require removing the carb body.
This is normal. Look at the asymmetric shape of the cam in the linkage in front of the driver position. It works kind of like a nautilus exercise machine. It’s so you can launch more gradually and not snap your neck back. Then once under way the acceleration rate increases.
I was trying to tune my carbs today, looking at the spark plugs i could tell i was running too lean. adjusting to a richer mix put the idle speed up to 1200 rpms, with the idle screws all the way shut. The problem was the check valve for the brake booster was leaking. So vacuum leaks anywhere seem to be a major factor.
The throttles are completely closed when the engine is slow idling. There’s an idle bypass in HD carbs, controlled by the “slow running volume” screw, that routes air flow around the plate. The plates probably seal pretty well all around, except near the spindles. You could try the smoke machine, not sure if it will work.
One thing you want to check is that the slow running volume screw (let’s call it the SRVS) is what’s controlling your idle. A common error is to use the fast idle screw to adjust hot idle. If you’re set up right, the fast idle screw should only engage when the choke is on. The mixture screw adjusts the fuel/air ratio, and can be set by the FSM method or with an O2 meter. If mixture is set correctly, don’t use this to adjust idle speed, either. If you can’t get the idle speed you want with the SRVS, it usually indicates a vacuum leak, worn spindle bushing, or warped throttle plate.
Based on what I’ve done so far and the suggestions given here, I should restate what seems like the uncommon issue here.
The idle always remains where I set it, if I don’t touch the throttle linkage. And if I shut off the car and immediately restart it, it goes to the set level.
I can move the choke freely and not make the idle stick. (as of course I have to do when starting it from cold.)
It is only when I advance the throttle that the idle sticks at a higher level than I set, at any temperature once I don’t need any choke. And after all the jiggery, it gets closer to the startup idle if I sharply blip the throttle. A gradual increase in throttle followed by a full release usually settles at a higher idle RPM than an abrupt increase.
This suggests that something is sticking and a sharper throttle action maybe overcomes the stickiness more. But if it’s purely mechanical, then why would shutting off the car and restarting it restore the idle? Could it be some vacuum or related element holding it slightly open, which dissipates when the engine is shut off? I’m just reaching for ideas here.
I don’t think you’ve confirmed yet whether the wobble joints are slack or not. Without knowing that there’s no way to narrow it down to something internal sticking vs something external sticking. The fact that pushing against the stop lowers idle but then it comes back up when you release is a clue. But you need to know what’s causing it to come back up.
Sorry, I tend to get wordy and this may have been lost.
When I press on the throttle stops, the idle goes to my target and stays there after I release the pressure (just as though I had shut the engine off then restarted it.)
There is slack in the linkages. They wiggle a bit from side to side of course, and when I depress the levers, there is a small amount of movement (a light click) without disturbing the slave shaft. The throttle stop does move in unison with the small movement of the lever before it begins to act on the slave shaft below.
Dave, I believe it may be bothersome to some here that you seemed to create this problem when you decided to adjust the throttle linkage to get full throttle. It is as if you moved the range that the throttle linkage operates the throttle from 0% to 60 % to a range of 10% to 70%- my numbers are only for illustration, not what you actually stated. I believe this is why everyone is trying to get you back to ensuring that the throttle plates are totally closed at rest. I believe your last post may have indicated that if the idle is hung up, then you press them closed, it does drop to where it should. If so, this is all pointing to what everyone says. I believe you may be hung up on why it can be low sometimes, takes a blip others, etc. Do not be. Try setting them as everyone says, see what happens.
Not that it matters, but I believe sometimes when a throttle is set close, but not right it, “may” hang up due to the air rushing through the carbs placing just a little pressure on the plates to hold them open a little. When you shut the engine off, they close completely. Some times people but stronger springs on, etc. to pull them closed. IMO, not a good idea, as that pressure wears the throttle shafts more.
Go back and adjust the linkage correctly, and you may be okay. My 2 cents.
Tom
Dave. As with most vehicle problems a quick fix is sometimes doable and your problem is solved. If you do not get lucky then it is time to go back to basics and take a step by step approach to set the system up as it was designed. Ignore any preconceived ideas and start from the beginning following the steps in the shop manual. I would be surprised it that approach did not solve the problem for you.
I don’t like it when we make comments about other series Jaguars or other cars… but… When I was at the Chevy dealer, circa 1975, the two barrel V8 carbs had a chronic problem with the throttle plates sticking ONLY under vacuum load. The fix was a new throttle body or a hard centering of the plates. This was so common that parts stocked the throttle bodies - an odd part to have on hand in normal circumstance. Of course a strong enough return spring should overcome this problem, but the double springs on the Chevies would not do the job, as the bind was as though the parts would notch together rather than simply bind a bit. The double springs were added, I think I recall, at the time of the motor mount failures causing engine lift, jump out of Park, and consequent bad results. Once had a recall on a 1958 (!) Chevy! By the time I was there most were already done with restraint cables and/or mounts.
The combination of these two things tells me that problem likely exists interior to the carbs, relating to either the valves, or the spindles. The entire linkage is basically removed from the equation if those wobble joints are hanging free.
Is your car 3.8 or 4.2? The former has a greater possibility of binding between the spindle and side seals because they have side seals. They both have an equal possibility of binding between the valve and bore due to carbon or centering issues.
Did you try some cleaning? You could try spray but getting in there with something to wipe with is more effective, a toothbrush maybe, or a rag.
After that, I’d give it a try and if it still has issues, disconnect the springs and throttle rods and just try manually rotating the spindles to see if you feel any slight bind as it closes. Even a slight rub can slow down the movement to the point where the spring can’t return it all the way. This is more likely when it’s hot because things expand.
BTW, it’s not always possible to get 100% throttle range. I can’t on my car unless I sacrifice the slack on the low end. Mine will open to a max of about 80% before it stops. That’s fine for me as it’s like a built in rev limiter. One of the benefits of a cable system is being able to get 100% range.
