jerking at 30-50 mph

[flamingofarm]

Car ran fine except hard to start, questions came up regarding plugs, wires, distributor points...all that stuff...we replaced it all, had carbs rebuilt....and car starts fine but, keep getting this mostly soft jerking between 39 and 50 mph, something can't be right. Checked vacuum hoses etc, timing had been checked and within range....we are baffled as to what is causing this, are we missing something?

[GriffinGuru]

Just to throwing out another option aside from motor related stuff... is the tranny oil good, is there oil in the diff? Play in the diff or U-joints could also cause subtle jerking and noise. If the car is new to you and you haven't checked the diff oil that would be wise to look at that stuff as well since that can sludge up or leak out.

There are also multiple things motor related that could do that as well; spark misfire, from poor dist components like cracked rotor, not fully clipped on plug wires??

[bbodie52]

Welcome to the Corvair Forum!

Could you tell us more about your Corvair? Automatic or manual transmission? Model year and engine configuration? Are there any non-stock modifications? Is the vacuum advance connected to the correct carburetor spark port? Is there a possibility of a faulty vacuum advance?



I would like to encourage you to expand on your initial post and tell us more about yourself, your Corvair, and your goals for the Corvair. If you can describe your personal assessment of your mechanical skills and abilities, that would help a lot. Members of the Corvair Forum love to be helpful in assisting other Corvair owners with technical support and advice, but it helps a lot if we have some understanding of your technical background and mechanical abilities, Corvair-related knowledge, etc. Helping us to know more about you will help us to write comments to you that are tailored to your needs and experience. Knowing your location is also useful, because knowing where you live can sometimes suggest possibilities.

[flamingofarm]

Thanks for the interest.....engine size I listed wrong, it is a 102 hp, four speed dual carbs. Car ran fine last year other than hard start. We did the common sense stuff with help from club, new plugs, points, condensor, plug wires, car then started as it should but just did not ran like it did before. Keeping it timed seemed to be a problem, feeling distributor might be original and worn, we put a new one in from Clarks. That timing issue seemed resolved....also done was both carbs rebuilt, we did replace the vacuum advance, we did replace vacuum hose to the advance and also put ties on each connection. Other than idle a little high we we prefer, everything seems in order, car goes through all the gears fine, no indication of any mis firing......but, you cruise between 30 and 50 you can feel this soft jerking, sometimes a harder jerk near 50 and none of us hear anything suggesting misfire or similar sound. When this was talked about early on, one of our members in Buffalo who also teaches repair always felt it might be a vacuum leak, maybe not from a house but from a port which can only be determined with proper vacuum check equipment which i guess none of our members have. Hope this info will help you, be happy to quickly answer any additional questions.
just so very frustrating.

[66vairguy]

There could be many causes, but it is most likely a fuel or ignition problem.

Are you running mechanical points or an electronic ignition? If points what is the dwell reading on the meter? A large dwell (too small a gap) could cause higher RPM missing.

Next something easy -- disconnect and plug the vacuum hose to the distributor and go for a drive. If that cures the issue your timing may be excessive at higher RPM. The distributor mechanical advance and vacuum advance would need to be verified.

If the timing is fine then check the fuel pressure. It should be between 4-5 PSI. Many of the newer pumps have been reported to deliver 9 PSI, too high.

BTW - AS Brads diagram indicates, each carburetor has two vacuum ports. One for the choke pull off (which can leak) and one to the distributor vacuum advance. Obviously only one carburetor port is for the distributor, so the other carburetor has a rubber cap installed on the distributor port.

If all else fails, then you need someone who is good at troubleshooting old car problems.

[bbodie52]

Just to jump off of the engine track for a moment, is there any possibility of slop in the drive line causing the jerking effect while cruising at a steady state? I was thinking along the line of a sloppy universal joint in one or both drive axles, a sloppy clutch, etc. Of course a sloppy clutch with loose rivets would usually rattle with the engine idling in neutral (foot off of the clutch), and a faulty universal joint would usually produce a clunking sound as power was applied or removed as the u-joint cross shifted back and forth within the bearing cups.

Back to the engine... are both chokes fully disengaged and wide-open with the engine at normal operating temperature? Were the carburetor base gaskets and insulators inspected for cracks or potential vacuum leaks? If the choke vacuum break vacuum diaphragms are old, you could try disconnecting them and plugging the vacuum hoses to see if the problem goes away... an indicator of a leak in one of the vacuum break diaphragms.




Surging at Cruise Speeds

http://corvaircenter.com/phorum/read.ph ... 362,442452

[64powerglide]

Check the carb float settings, if they are set to high you could be getting a bit of overflow at crusing speed. Set them to shut off the fuel sooner so the bowls don't fill to close to the top.

[flamingofarm]

Thanks to all who voiced in, very much appreciated. Going to try those vacuum test suggestions. Also i had ask a couple members about those two vacuum diaphram and hoses but was told they only have to do with the coke operation. Will check those suggestions out HAVE A GREAT 4TH.

[flamingofarm]

Is there a reasonably safe way of checking around the carbs etc to see if there is a vacuum leak......someone suggested wd40. Guess if there is one you will hear the engine increase

[flamingofarm]

Some additional information.....did use some starter fluid, checked all the hoses and around the carbs and found no fluctuation in engine; also took the vacuum hose off vacuum advance, plugged it as suggested and took it for a drive and although it seemed better, was still there. The only thing I did hear will checking for leads was a click that came from the carb vac on the drivers side, whether something go hung up (or gets hung up) that might cause the problem, not sure, just thought I would mention it. What we did try was this.....I have nuver understood why the car jerks going down the driveway in first gear, you would think once you let the clutch out it would be fine..and sometimes it will do that for a second after shifting into second gear............so, we took the car out until we could in fact feel the jerking, pushed the clutch in and motor ran as smooth as could be. We tried this a number of times just to find out if in fact one could still feel or hear some hesitation from the motor...nothing, each time we did that. As soon as you let the clutch out and if at that 30-50 mph you could feel that slight jerking again. I originally ask in the club if the clutch might be slipping or something was going on with it and got quick answers that if it was slipping you would definatelly feel it as a slip and going up a hill, never felt or heard a slipping feel, in fact car rarely if ever jerked when under such a load.
So.....all of your advice taken and checked and what additional information I am sending, any thoughts?
I do know one of the carb vacuums was replaced at one time but not the one on the left side so not sure if in fact that remains a possibility. AND THANKS AGAIN FOR YOUR PATIENCE, THIS JUST HAS BECOME SUCH A FRUSTRATION PROBLEM.

[bbodie52]

That's why I was asking about possible slop in the drive train (between the crankshaft and the drive wheels). A worn-out universal joint might cause this, if the needle bearings inside the universal joint bearing cups had rusted to powder. I have actually seen such wear in Corvair universal joints where the needle bearings have essentially disappeared — leaving a gap between the bearing cups and the bearing surface on the cross. As the cross shifts back and forth within the gap in the cups, a jerking action may be felt as the axle (drive wheel) speed mismatch with the differential is "telegraphed" to the vehicle as a jerking effect. This jerking effect might be more pronounced in lower gears, but also might be noticed in a steady state vehicle speed as engine speeds vary lightly in comparison to the steady axle speed.

[flamingofarm]

Good thought. As i think back, i have always wondered why when you take off in first gear, especially as i go down the driveway, the car would always rather jerk in first gear, sometimes in second, that is why i thought maybe this had to do with the clutch but it seems like members balk at this idea.

[bbodie52]

If you get an opportunity to observe gearbox of an automobile, observe the sizes of 1st, 2nd and 3rd stage gears. You'll observe the 1st is the biggest gear and then size reduces successively. This is because initially car needs maximum torque and bigger gear would result is larger torque multiplication.



Torque multiplication is force multiplication. In the lower gears, the forces exerted on the vehicle between the driven wheels and the engine are multiplied, so differences also cause a more-noticeable effect on the car. By the time you get to fourth gear, the multiplication factor is significantly reduced, so the stresses imposed on the vehicle are less-pronounced. If something in the drive train is loose — as in failing universal joints — the effect on the car will become increasingly noticeable and to some extent multiplied in lower gears.

The universal joints in the rear axles are often neglected on Corvairs. Some have no grease fittings (zerk fittings), so those universal joints are less-likely to be lubricated periodically. They will likely be allowed to deteriorate even more than normal since greasing them requires disassembly. Once the small amount of grease in the bearing is washed away, the needle bearings begin to wear grooves in the cross, and eventually the needles begin to break and disintegrate. If your universal joints lack grease fittings, be sure to replace them with quality units and then grease them periodically with a grease gun.

[1949chevy]

Sounds like the carbs are not right. If the carb linkage is like mine...about worn out, the two carbs may not be putting the same amount of gas in each of the engine sides...they really need to be balanced and even flo or you will get the result you are getting.

I used a turnbuckle and moved the throttle main rod about 1/4 open and looked down in the carbs...they need to have the same opening of the butterflies on each side....then I checked the butterflies at 1/2 throttle....they were different on both accounts. I adjusted the linkage at those settings and now I have a completely different running car....no surge and a good idle.

Nothing in the differential nor the transmission can cause a surge...they would have to have missed teeth and it would sound like the world is coming apart. A universal joint would clunk real loud and it would be from 1 mph but no surge if a steady gas was given...never had a bad UJ surge.

Good luck.

[GriffinGuru]

I just remembered an experience my brother and I had almost 18 years ago. My brother was trying to sell a car to a friend of his. It was a 86 VW Golf with 5 speed. The trouble was our friend did not know how to drive stick, but he really wanted the car. My brother took him out and tried to teach the guy how to drive a manual transmission on the fly, during the test drive...

Well, I don't remember exactly what he did- possibly drop the clutch while shifting into the wrong gear or something like that, but after that one drive the car all of a sudden had a rough jerking while driving it down the road. Because our friend was new at driving manual, and we at our younger age and less diagnostic experience, naturally thought that somehow the clutch was shattered or something like that. After a two weekend ordeal removing the transmission and replacing the clutch, the problem was not fixed...

After spending more money on more parts, we ended up finally replacing the motor mounts and that fixed the jerking. We were less skilled at repairs then, and I seem to remember stripped bolts and other things not going right with that whole transmission ordeal. Anyway, that particular problem was in a front wheel drive VW Golf. I do not have enough experience with Corvairs to know if bad motor mounts (and/or trans mounts) in these will cause similar issues to what we experienced in the VW, but I thought I would share that experience and throw that out there as well.

[64powerglide]

timing had been checked and within range

What is it set at? 102hp will run best when set higher than book says, 16 advance will give you better performance. Ran my 62 at 18. Did you hook up a dwell tach & see if the needle is steady at different rpm's? No wheel cylinders hanging up? Just grabbing at straws, when the problem starts does it feel like it slows then accelerates or accelerates then slows? Sounds like a sloppy distributor, but if that was bad I would think it would happen at any steady speed.