"trailer hitching" at low speeds

[William F.]

At speeds just above idle when I ease along in 2nd gear, such as when going slow through a parking area, in my '67 L79 I often get severe "trailer hitching-rpm jumps up then back down and will continue until I push clutch in. Is this probably due to vacuum advance rapidly advancing then retarding? In addition, car is very sluggish in general below 2500 rpm. Later may be partly due to factory sloe centrifugal advance, but I suspect problem with vac advance for this second problem also. Runs great at higher rpm. Guess I need to know which vac can I have and how it relates to engine vacuum at low rpm. You think vac fluctuation is problem? Thoughts on problem and Advice on how to diagnose will be appreciated.
Thanks

[Mark E.]

Re: "trailer hitching" at low speeds

Did the "trail hitching" begin after something changed? Start with that.

Then see how the car runs with the VAC disconnected and its line plugged.

Could the symptoms be from too much ignition advance?

[Perry M.]

Re: "trailer hitching" at low speeds

Trailer hitching? WTH. Will you please speak in English? No wonder people are losing interest. They ( and me) don't know what your talking about.

[Frank D.]

[Rich G.]

[Bill M.]

Re: "trailer hitching" at low speeds

Ill add to that it feels like you are towing your trailer with the wrong size ball.

[Edward J.]

Re: "trailer hitching" at low speeds

Perry, another term sometimes used is surging/bucking at low speeds. Bill, you are on the right track vacuum advance or sticking mechanical advance weights. See if the vacuum can is holding vacuum.

[James W.]

Re: "trailer hitching" at low speeds

I would say your vacuum advance cannister is bad OR it has has the incorrect one installed. My '65 L79 wouldn't run worth a damn at low rpm. I asked some questions regarding this issue here in the TDB and received some good advice from many including Duke who asked what vacuum advance cannister I had installed in my car. At that time, it was a B1 cannister which Duke said was incorrect for my car. I think he used the term "boat anchor" and said get rid if it and install the correct one that that passes the 2" vacuum rule (B26). I found a correct one from Rock Auto but also know that Car Geek had them also. After installing the correct vacuum cannister, the car runs great. I did the same thing for my '64 L75 that was exhibiting the low speed "trailer hitching". I changed it and put in a correct vacuum canister and the problem was solved.

James

[William F.]

Re: "trailer hitching" at low speeds

Mark,
I've had the car 10 years and always trailer hitched.Previous owner had engine rebuilt with Comp "vintage cam" and lower compression pistons. I've just lived with it since runs great from above around 2500 rpm to redline. Thinking I need to get around to checking causes. Duke says I need to get centrifucal advance to come in quicker to help low end but don't know if this could also be related to trailer hitching.

[William F.]

Re: "trailer hitching" at low speeds

Well Perry, settle down. For you, I'll explain (although others have talked about this phenomenum in other posts.) It's like pulling a trailer and the trailer hitch is a little loose on the ball and it pushes the car a little and then pull s back, a back and forth motion.. In my case, at low rpm, just barely moving, engine speeds up abruptly then slows abruptly, causes violent jerking of the car until I push the clutch in.

[William F.]

Re: "trailer hitching" at low speeds

Thanks to those who knew what term meant and gave considerate helpful advice. Sounds like i need to check vac can against what vacuum my engine pulls and also check for sticking centifugal advance

[Timothy B.]

Re: "trailer hitching" at low speeds

Under high vacuum conditions (throttle blades near closed) with overlap in the camshaft (both valves open at the same time), vacuum can pull spent exhaust back into the combustion chamber diluting the mixture, this can cause a lean surge.

It also could be to much advance causing the point of maximum combustion to occur before or at TDC pushing the piston back. I believe to little timing can cause the same to happen at the other end.

Some carburetor idle circuit tuning may be in order if you are a skilled hand to try and correct.

[Duke W.]

Re: "trailer hitching" at low speeds

This phenomenon is not uncommon on ANY car, especially with a manual transmission as drive train lash increases, and this can be significant on Positraction axles that most Corvettes have. As the clutches wear differential gear backlash and side yoke end play increase, and the axle gets "sloppy". Transmission gear backlash can also be an issue. Sometimes a richer idle mixture can eliminate or at least mitigate this issue.

Having a functionality correct vacuum advance as previously stated is important. Also, many Corvette optional engines have medium to high overlap cam engines. Without going into all the details of the physics involved it can be sometimes hard to get these engines to operate smoothly at light load at less than 1500 to 2000 RPM.

Try letting the engine run down to idle speed in first gear while slightly dragging a brake. Hook up a vacuum gage in the cockpit. Does vacuum vary a lot and get below the Two-Inch Rule minimum as you let revs drop to idle speed on a level surfaces in first gear?

You may have a 100 point concours paint job and interior, but a lot of such cars have lots or mechanical gremlins that weren't corrected during that "restoration" a few years ago.

Let me drive a Top Flight or Duntov winner and I'll likely find operational issues that need attention.

Duike

[Perry M.]

Re: "trailer hitching" at low speeds

Sorry guys. I didn't mean to come on so strong. I must have had a bad day due to this 110* temps we've been having here. Thank you for the explanations. I don't think I have ever experienced that condition in my 63 fuelie.

[Timothy B.]

Re: "trailer hitching" at low speeds

You are here-by forgiven because of high summer heat and not enough cold beverage to lubricate the brain... :-)