A week or two ago I posted about an issue where when the car was warmed up and turned off and then restarted a few minutes later the RPM’s upon starting shot up to 2500 or more. I thought I solved the problem by adjusting the return spring arms to ensure there was no more binding or pulling the linkage the wrong way, but I had the issue again today.
After taking the car in for its state inspection (passed) I drove to the DMV in the car to complete the annual registration. While the car was not yet warmed up yet, I still had a small amount of choke on (triple SU conversion done many moons before I acquired the car). If I tried removing all the choke, the car would stall at intersections as the idle would drop below 500.
After going into the DMV for literally ten minutes, I came out and started the car, only to have it rev up to 2500 + without my foot touching the accelerator. When I goosed the accelerator to try to get it to come down, it only went up higher. I allowed it to run at 2500 for less than a minute and it slowly came back down, but was touchy all the way home and wanted to rev higher several times.
The linkage has no binding. I even had it disconnected near the washer motor and the section to the accelerator is smooth as butter, as is the section from the washer motor to the three SU’s. The issue does not appear to be the linkage. The diaphragms are all new and were installed a few months ago by a pro who rebuilt them with new parts. Last week, per some advice I received here, I pulled the SU domes and checked for puddled fuel in the carbs (there was none), stuck butterflies (they weren’t), and adjusted the mixture screw to 2 1/2 turns. At speed the car runs great, but its being finicky at idle and with re-start RPM’s after the engine is warm.
Your butterflies are not closing completely or you have an air leak.
Likely suspects: 1. fast idle screw has no clearance. Pull a .005" feeler gauge through and adjust the screw.
2a. Clamps that connect the butterfly spindles are holding a butterfly open. Disconnect the two, snap each throttle closed a few times reconnect making sure all the spindles open in synchronization.
2b. Grab each spindle’s end and move up &down. If rebuilt you should feel no movement.
Try 1&2 first. With each carb in your hand, shine a light in the carb ( damper out), you shouldn’t see much light around the circumference of the butterfly.
Your rebuilder should have addressed this. Might want to take it back. If not: Loosen the butterfly screws, push the butterfly closed and carefully retighten the screws. Careful. Do not over tighten , and insure they can’t come loose. Then spread the split at the end, I used red locate too. This is a fidgety adjustment so they’re probably one time use screws so be careful. You can’t get.them perfect, but you can make them better.
You could push it up against the spring. But at that point what’s another 4 (x 3) screws?
Try the first two possibilities, they’re more likely to be the cause.
It’s usually a basic, fundamental that’s been overlooked.
D
Have you any idea what kits were used to rebuild the carbs? Do you know what car/year the carbs were donated from? It matters because there can be multiple causes for sticking throttle valves besides the most obvious one of misaligned butterfly valves, and different year cars had different potential trouble spots.
I chased this same problem on my 3.8 for months and it finally turned out to be the “upgraded” rubber side seals that expanded when hot and dragged enough to overcome the return springs. It was the one suggestion not made by anyone.
I have no clue as to when the triple SU’s were put in or where they came from, that was long before me.
I had purchased all the SU replacement parts from one of the usuals and provided them to the rebuilder. He used Delrin bushings instead of the rubber SU provided set.
The rubber parts in the SU kit aren’t bushings, but rather “seals” which were meant to replace the cork seals on 3.8 cars. The 3.8 used bronze bushes and the 4.2 uses Teflon strips.
So I’m unclear where he used Delrin. Did he machine the carb bodies and make actual spindle bushings? Or did he machine Delrin to use as packing pieces on the sides? The problem with experimenting with different materials in these is that they have different expansion coefficients. An assembly can be smooth as silk when cool, but then get bindy after being heated.
Mine were like that with the rubber seals. They functioned better than ever until it got hot and then it gave me a 2500rpm idle. I disconnected the linkage arms from the spindle and could feel how sticky they became when I moved them by hand.
This would be my suggestion. When you encounter the problem, isolate which carb is sticking by pushing against the spindle stop. Then shut it down and remove the linkage arm and try gently manually rotating the spindle. The slightest drag the means something in that carb is sticking. It could be the bushes, seals or valve. If it’s not sticky at all, then it’s in the linkage or return.
The Delrin was used as a bushing on both sides of the butterfly shaft as it protrudes from the sides of the carb. Supposedly it has good properties as far as not expanding with heat, impervious to fuel (ethanol or not), etc.
Well, it’s novel. I’ve never heard of anyone doing it so can’t comment. As I said, the only way you can tell whether it’s causing binding is to remove the linkage when hot and sticking. There must be no resistance at all. The return springs aren’t strong enough to overcome it. You could increase the spring tension to overcome the binding but then your pedal feel will get worse.
Too late tonight to go back out to the garage to check the great ideas you and David have given me to check out. Tomorrow, I’m planning on spending most of the day at Texas Motor Speedway for the Goodguys car show and vendor area.
I have plans to acquire a 1953 Chevy 1/2 ton pickup truck for restoration in the next month or so and will be going there to get some ideas and to start looking for parts. The carbs will have to wait until the weekend after I take my granddaughter homecoming dress shopping…apparently my Amex card hasn’t been exercised enough lately at Moss, Welsh, and SNG
As I’m sitting here thinking through what I will be checking on the carbs given the input from @David_Ahlers and @Erica_Moss I’m a bit confused.
When I turn off the engine after its been warmed up and running fine, the RPM’s are at idle. When I restart the car several minutes later, I only turn the key and do not touch the accelerator pedal at all, yet the RPM’s are jumping immediately up to 2500 +. Given that the cars engine when I stopped it was purring at idle, that would mean that the butterfly valves are closed, no?
How to they get opened during the period of time that the warm/hot engine is sitting and not running?
Dunno, it’s not the way mine bound up. It would be stuck until I either blipped the throttle and let them snap closed hard, or I manually pushed against the spindles.
If I cap off the vacuum port on the intake manifold, I won’t have the vacuum assist on braking, correct? I can baby in on the street in front of my house with no assisted brakes, but won’t take it around town that way. This might make sense. How would I test it at the reaction valve?
Yes the brakes will take more pressure to stop. Be careful, but I doubt you will have a problem. Just do it for a test, this disables the secondary safety brake circuit. Then if it is the problem, next find the solution. Could also be a cracked vacuum hose, check valve etc. but reaction valve may be more likely, But first see if it is in this area.
Tom
Bob, when all is running properly, there is a set amount of air/fuel entering the engine. If one wishes to increase the speed, the amount of air/mixture must increase. To do that, one must open the throttle plate or in some way bypass it but still increase air flow. So if your engine is running at 2800 rpms, there must be enough air flow entering somewhere to allow it to run at that speed. You seem to have checked and so far have not found the throttle plates to be stuck open. If that is true, the air must be coming from somewhere, so I am looking for that somewhere. It could also be an intake manifold leak, but I would expect that to be consistent. Leaky carb gasket. Somewhere air is leaking in. If there is a problem with the reaction valve, I could see it leaking air in. Chances are, as you restart the engine, you put your foot on the brake pedal. This would move the reaction valve. It should simply shift it and use a little bit of vacuum, and then stop, but maybe yours is not operating properly. Or maybe something else. Just a thought. Maybe there is something else in the carbs, bypass circuit screwed up, but you have not found it yet so I was looking for something else where.
Good luck,
Tom
Tom, that makes sense that air is coming from somewhere. Last night I went out to the garage and disconnected the vacuum line right at the intake manifold and put a short 4 inch piece back on with a bolt threaded and clamped in the other end. Later today, when I get back from the auto show I plan on starting the car up and letting it warm up in the driveway to see if it replicates the situation. I’m guessing that an air leak might also explain why the choke needs to remain on longer than I think it should to keep from stalling.
