jerking at 30-50 mph

[flamingofarm]

Again, thanks to all that have shared their knowledge.........did find out something this morning....always hard sometimes to listen to an engine and try to concentrate but, noticed two things.....after driving it about three miles to the village, I checked the carb vac on the left side, the one that I thought I heard a click from the other day while looking at the engine, the arm had only moved about half way thru that opening it moves in, when I got home it had moved all the way to the left or what might be considered as fully open or fully closed, don't know. I also did hear and feel a pop, not a misfire but like an air pop in the engine and there were a couple minor ones as well as I went throught third gear and fourth and this was at the same time the car jerked. As mentioned, never heard it backfire or anything like that, this pop just felt like a slight misfire or....not sure what the terms are....more like pop of air. Make any sense? Maybe what is going on becomes more evident at certain speeds like between 30-50 if timing was off or a bad condensor or.............something. Make any sense?

[bbodie52]

It sounds like you were describing a partial opening of the choke mechanism. Both sides should open at approximately the same rate. They are each controlled by a bi-metallic coil that responds to engine heat. If each bank of cylinders is receiving approximately the same fuel/air mixture from each carburetor and all cylinders are being treated equally by the ignition system. temperature rise and operating heat should be comparable on both cylinder banks. If one side is slow to open the choke mechanism, it could be an indication of one side receiving less fuel mixture, an ignition problem, or poor compression in one or more cylinders indicating that the one cylinder bank is running colder and perhaps producing less power for some reason. Alternately, a faulty or incorrectly adjusted choke mechanism might be indicated.

The images below depict the automatic choke mechanism found on normally aspirated 1962-1969 Corvairs. The attached publication, DELCO ROCHESTER - Models H, HV Carburetor Service Manual, contains a comprehensive description of the operation of each carburetor subsystem. Reading it my clarify for you how the carburetors function in response to engine operating changes. Understanding what "normal" looks like and why the carburetors function the way they do can help you in detecting, interpreting and correcting abnormal or faulty operation.

DELCO ROCHESTER - Models H, HV Carburetor Service Manual.pdf

DELCO ROCHESTER - Models H, HV Carburetor Service Manual

(1.79 MiB) Downloaded 31 times

[flamingofarm]

WANT TO THANK ALL YOU RESPONDED, SEEMS WE HAVE CHECKED EVERYTHING WE CAN WITHOUT THE PROPER EQUIPMENT. WITH REBUILT CARBS, NEW VACUUM HOSES, NEW PLUGS, NEW WIRES, NEW POINTS, CONDENSER......IT HAS BAFFLED MANY, TOOK IT OUT TODAY AND THREE TIMES IT SOUNDED MORE LIKE A MIS-FIRE BUT NO REAL NOISE, JUST VOID SOUND (HARD TO DESCRIBE). ONLY SHOWS UP BETWEEN 30 AND 50 AND VERY PRONOUNCED AROUND AND PRETTY MUCH DISAPPEARS AT 60+.
CHOKE VACUUMS ONLY THING WE DO NOT KNOW MIGHT PLAY A ROLE, EVERYONE SEEMS TO TELL ME THEY AFFECT THE CHOKE ONLY, NOTHING ELSE. GUESS I NEED TO FIND OUT THROUGH THE CLUB IF THERE ARE GARAGES THAT DO SERVICE ON OLDER CARS AND HAVE THE EQUIPMENT TO CHECK FOR A MISFIRE OR BAD PLUG (ALTHOUGHT ALL WERE NEW), A VACUUM LEAK IN THE CASTING AREA OF THE CARBS.....EQUIPMENT MOST MEMBERS DO NOT HAVE IN THEIR GARAGE. SAD, CAR STARTS FINE, HANDLES GREAT, FUN TO DRIVE, BURNS NO OIL.......JUST DO NOT KNOW WHAT TO LOOK FOR BUT THANK EVERYONE ONE OF YOU FOR YOUR IDEALS AND INPUT.

[bbodie52]

The choke vacuum break mechanisms only have an effect when the engine is cold and the chokes are applied. They have an effect of opening the choke butterfly mechanism somewhat as you drive the car with the engine still cold and not quite up to normal operating temperatures. This lets in more needed air with the engine operating at higher rpm speeds. Once the engine has warmed up the chokes are fully disengaged by the automatic choke mechanism and the vacuum break mechanisms no longer have any effect. HOWEVER, IF THERE ARE ANY VACUUM LEAKS INSIDE THE PLASTIC CASES, THOSE VACUUM LEAKS COULD STILL IMPACT THE ENGINE OPERATION BY LETTING ADDITIONAL AIR INTO THE INTAKE MANIFOLD AS IT LEAKS PAST THE INTERNAL RUBBER DIAPHRAGM. To check this, the rubber hoses could be temporarily disconnected from the vacuum break units and plugged. This would have no impact on an engine that has warmed up and is not using the chokes, but it would also eliminate any possibility of vacuum leaks through these diaphragms. Then the car can be driven to test to see if the operating performance has changed.

[Ragtop Man]

There are a lot of great suggestions here that I'm almost afraid to chime in but here goes . It seems you never answered Brad's questions about whether or not both chokes are opening fully . Did I overlook your answer ? I also was thinking what 1949chevy said about the throttle linkage. I bought the set up from Roger Parent and love it . Have you observed by moving the throttle slowly by hand with the air breather off what is happening on each side ? Does each throttle lever start to move simultaneously and do they both go to WOT (wide open throttle) at the same time too? You also said you have rebuilt carbs. I bought two rebuilt carbs ( not from Clarks!) and they look nice but I found one that had a leaking float . Now this did not give me an issue like yours but you could have more than one minor issue compounding and ganging up to put a whooping on you . Cars can seem possessed when you have two issues even when they'r'e minor. I would also , just out of curiosity , take the advance plate out of the distributor and see if they springs are on the weights and perhaps what size the weights are . I doubt Clarks would have made a mistake but it can happen . If you have the wrong weights (or the wrong distributor) I'm thinking this could throw off the advance curve. Did the car do this before you swapped the distributor ? An issue that occurred with my car was that one of the local "experts" in the club here told me that after putting on the Crane ignition system that I could open up the gap on my plugs all the way up to 60 thousandths if I wanted to ! I decided to try it but only up to 45 instead . The car would run great for a while until the coil would start to heat up . It would run ok at low speeds but would just start to run like hell during acceleration and higher speeds . I was thinking about it one night while going to sleep and just realized the coil was overheating . The next day I pulled the plugs and regapped them back to .035 and that was the end of it . I found the leaking float a couple of days later . It's running pretty well now that I don't have two things wrong . I hope this helps you out because it seems like you're about to give up !! It sounds like you have a great car to me . When you find the problem yourself you will end up being happier as well as a better troubleshooter . Lord knows I'm still learning

[flamingofarm]

BRAD, SO.....I DISCONNECT THE RUBBER TUBES FROM THE CARB VACUUM BREAKERS AFTER THE CAR HAS FULLY WARMED UP?

[bbodie52]

The functional operation of the choke mechanism is described on page 8 of the attached manual: DELCO ROCHESTER - Models H, HV Carburetor Service Manual (shown below). Once the engine has been warmed up to normal operating temperature, the vacuum breaks no longer impact the carburetor choke operation (since the choke valves have been fully opened by the choke coils). If you suspect engine performance has been degraded by a vacuum leak — possibly from an internal vacuum leak in one of the vacuum break diaphragms —each hose can be temporarily disconnected from the vacuum break and an appropriately sized screw can be inserted in each hose to plug the carburetor vacuum connection. A test drive can then be performed to determine if engine operation has been improved.

[flamingofarm]

THANKS, WILL TRY IT AND WILL KEEP MY FINGERS CROSSED; APPRECIATE YOUR CONCERN AND PATIENCE WITH ME.

[64powerglide]

Check that distributor, make sure the weights are not worn. Possible they could hang up a bit then when you get past 50 they unstick & everything seems fine. Get some dry-slide lubricant & spray them, that's the stuff used on gun parts that you want lubricated without oil. I really don't think you have a carb problem. I thought I made a mistake once but I was wrong. That's my 02: for what it's worth but it surely can't hurt to check it out.

[flamingofarm]

BRAD: FOLLOWED YOUR INSTRUCTIONS REGARDING THE CARB VACUUM BREAKERS, LET IT WARM UP, THEN DISCONNECTED TUBING TO EACH (BOY, YOU SURE CAN FEEL THE VACUUM) AND....SAME THING, JERKING BETWEEN 30 AND 50. WAS WORTH A TRY THOUGH, THANKS

[oldschoolteacher]

My Corsa had a similar problem. I did all of the things listed above, but could not solve the problem. I just found out that I have a weak valve spring, which caused the mild jerking. If you haven't checked that, it is a suggestion that might help you.

oldschoolteacher