Trailerhitching After WOT
Thread Starter
Pro
Joined: Jul 2006
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From: Buxton Maine
Trailerhitching After WOT
Trying to dial in my engine. Point Distributor.
I've used Lar's Papers (Thank You) to set timing. Car runs very well now under moderate driving conditions. And goes really good at WOT. But after giving it WOT for a couple seconds it will trailerhitch at light cruise speed for about 3-5 minutes. Then it will settle back to normal. What do you think might be happening to cause this? Thanks, Rick
I've used Lar's Papers (Thank You) to set timing. Car runs very well now under moderate driving conditions. And goes really good at WOT. But after giving it WOT for a couple seconds it will trailerhitch at light cruise speed for about 3-5 minutes. Then it will settle back to normal. What do you think might be happening to cause this? Thanks, Rick
Race Director
Joined: May 2006
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when is the last time you checked the rear suspension including trailing arms, strut rod bushings, differential side yoke and wheel bearing play?
when is the last time you had the rear aligned assuming all of the above are good?
when is the last time you had the rear aligned assuming all of the above are good?
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Pro
Joined: Jul 2006
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From: Buxton Maine
Everything is new and tight. Alignment was performed last week. I'm trying to solve a distributor problem. My suspension is very good.
Team Owner
Joined: Jun 2001
Posts: 23,367
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From: Virginia USA
Stop towing your boat with your Vette. Must be wild doing a WOT burnout with a trailer attached. Seriously I don't know what trailer-hitching means, never heard the term. If you mean jerking back and forth line when you have a boat or something attached could be after WOT your float bowl is almost empty for some reason and it takes a little to refill??? Check your fuel pressure
When I was in the AF a buddy of mine towed his ski boat behind a 1980 Corvette and when he launched it the rear tires would be totally in the water. Hate to know what it did to the trailing arm bearings.
When I was in the AF a buddy of mine towed his ski boat behind a 1980 Corvette and when he launched it the rear tires would be totally in the water. Hate to know what it did to the trailing arm bearings.
Racer
Joined: Oct 2009
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I was wondering what the term "trailer hitching" meant also. If the car is "surging", that is a lean condition and could be low fuel pressure or restricted fuel filter. As noted above, low fuel level in the float bowls of the carb until the consumption lessens and fuel level comes back up.
The answer is not the distributor, but EFI lol!
The answer is not the distributor, but EFI lol!
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
Trying to dial in my engine. Point Distributor.
I've used Lar's Papers (Thank You) to set timing. Car runs very well now under moderate driving conditions. And goes really good at WOT. But after giving it WOT for a couple seconds it will trailerhitch at light cruise speed for about 3-5 minutes. Then it will settle back to normal. What do you think might be happening to cause this? Thanks, Rick
I've used Lar's Papers (Thank You) to set timing. Car runs very well now under moderate driving conditions. And goes really good at WOT. But after giving it WOT for a couple seconds it will trailerhitch at light cruise speed for about 3-5 minutes. Then it will settle back to normal. What do you think might be happening to cause this? Thanks, Rick
I hope this helps Henry @ oles carb
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Thread Starter
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Joined: Jul 2006
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From: Buxton Maine
I originally thought the weights might be sticking in the advanced state. I didn't want to shut it off but probably should have to see if it corrected. When it starts running bad my first instinct is to head straight home. Tomorrow I'll loosen the gas cap and test drive. If it acts the same I'm back to the distributor.
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Try to find (Google) Lars papers on Vacuum Advance. Very interesting. Most of us run too much with the old theory that more is better.
If you can find a Vac canister that is adjustable with an allen wrench to fit your dizzy, then life is good. You only need about 10 - 12 degrees. Any more than that, you have hooked up the boat trailer again.
If you can find a Vac canister that is adjustable with an allen wrench to fit your dizzy, then life is good. You only need about 10 - 12 degrees. Any more than that, you have hooked up the boat trailer again.
Last edited by HeadsU.P.; Jul 30, 2018 at 07:03 PM.
Team Owner
Joined: Jun 2001
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From: Virginia USA
Although I agree it could be the vacuum advance but I also think if it was it would be doing it all the time a part throttle cruise. In his description it happens after a WOT romp then clears out and starts to run normally. I don't think it's the vac advance but as my wife points out all the time I could be wrong. It could be sticking weights if it's anything in the ignition. I guess it wouldn't hurt to go through the timing as Lars says "Most carb problems start with ignition, get the ignition right first then start troubleshooting the carb".
EDIT - Don't google for Lar's paper. E-mail him and ask him to send it to you. v8fastcars@msn.com
Reason being he's revised the papers over the years and if you download it from some site that is not Lars then you might get an outdated version
EDIT - Don't google for Lar's paper. E-mail him and ask him to send it to you. v8fastcars@msn.com
Reason being he's revised the papers over the years and if you download it from some site that is not Lars then you might get an outdated version
Last edited by theandies; Jul 30, 2018 at 07:40 PM.
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Joined: Jul 2006
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From: Buxton Maine
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
That should work. How do you know it maxes-out at 11 though? I have a plate inside the dizzy that offers four choices of max vac adv degrees. It stops the movement of the pin / arm going to the advance. I believe its made by MSD.
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From: Buxton Maine
"Henry Olsen of Ole’s Carburetor & Electric and Lars Grimsrud of Tuned by Lars Corvettes have found that vacuum units that over-advance timing cause a variety of problems, including what they describe as “trailer hitching” or bucking and stuttering at cruising speeds, engine misfires, reduced mileage, and detonation under light acceleration."
My can starts pulling at 6 in.hg and is mechanically limited with a bushing when the pull reaches 11 in.hg. Motor idles at 15 in.hg. I haven't checked how many distributor degrees the can advances, but a B20 will usually advance about 8 distributor degrees without an added limiter.
Do you think the centrifugal weights could be hanging up for a while after high rpm WOT? Not retracting at lower rpm for a few minutes?
Melting Slicks
Joined: Mar 2005
Posts: 2,093
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From: Midlothian VA
Here is a brief description of the symptoms from a Hot Rod article:
"Henry Olsen of Ole’s Carburetor & Electric and Lars Grimsrud of Tuned by Lars Corvettes have found that vacuum units that over-advance timing cause a variety of problems, including what they describe as “trailer hitching” or bucking and stuttering at cruising speeds, engine misfires, reduced mileage, and detonation under light acceleration."
My can starts pulling at 6 in.hg and is mechanically limited with a bushing when the pull reaches 11 in.hg. Motor idles at 15 in.hg. I haven't checked how many distributor degrees the can advances, but a B20 will usually advance about 8 distributor degrees without an added limiter.
Do you think the centrifugal weights could be hanging up for a while after high rpm WOT? Not retracting at lower rpm for a few minutes?
"Henry Olsen of Ole’s Carburetor & Electric and Lars Grimsrud of Tuned by Lars Corvettes have found that vacuum units that over-advance timing cause a variety of problems, including what they describe as “trailer hitching” or bucking and stuttering at cruising speeds, engine misfires, reduced mileage, and detonation under light acceleration."
My can starts pulling at 6 in.hg and is mechanically limited with a bushing when the pull reaches 11 in.hg. Motor idles at 15 in.hg. I haven't checked how many distributor degrees the can advances, but a B20 will usually advance about 8 distributor degrees without an added limiter.
Do you think the centrifugal weights could be hanging up for a while after high rpm WOT? Not retracting at lower rpm for a few minutes?
Reviewed all of Lars' papers. Turned out to be excessive timing advance (you can test for this by simply backing off the advance with the distributor to see if it corrects the symptom).
In my case backing off the distributor advance eliminated the symptom - the root cause though was my balancer had slipped just a bit which, of course, had me setting the timing incorrectly.
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Thanks for clarifying the problem.
Have you recurved the distributor for a total mechanical advance of 36 degree at about 3K rpm assuming you are running cast iron heads?
What is the initial timing with the vacuum disconnected?
Are you using ported or manifold vacuum?
Have you recurved the distributor for a total mechanical advance of 36 degree at about 3K rpm assuming you are running cast iron heads?
What is the initial timing with the vacuum disconnected?
Are you using ported or manifold vacuum?
Last edited by MelWff; Jul 31, 2018 at 02:49 PM.
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Joined: Jul 2006
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From: Buxton Maine
Weird that it only happens after WOT and lasts for 5 minutes, then reverts back to running great again. Motor behaves perfectly even at WOT . Just gets jerky afterwards and stays that way for a few minutes.
edit: I did check TDC with a piston stop.
Last edited by 69L46vert; Jul 31, 2018 at 03:07 PM. Reason: Added Piston Stop Note