1965 GTO Tri-Power "Bucking" or "Surging" Off Idle to 1500 RPM
 

Greetings everyone. I hope all is well. Just last night, my 1965 GTO Tri-Power car began bucking or surging. The idle is good, but from about 1000-1600 RPM it goes down the road and kind of acts up. It is not terribly bad, but it is bad enough that I noticed it, as did my wife from the passenger seat.

A little history: stock engine completely rebuilt five years ago (low miles still), carburetors rebuilt at the same time by me with proper parts and DAG sealant, and distributor completely rebuilt as well. Up until last night, the car was as smooth as a brand new car regardless of RPM. I never run ethanol fuel.

The only thing that has been touched since my last drive (one month ago) were the slotted airhorn screws and washers. When I rebuilt the carbs five years ago, I re-used the old screws and washers as the vendor was out of stock on them. I finally got around to ordering new ones, and I installed them. Now, I did remove the front carb airhorn to take a look inside the carburetor for dirt (none), but did not touch anything else on any other carburetor, save for changing the airhorn screws slowly and one by one.

So, does anybody care to take a stab at what this new issue may be? It seems like it could be lean in that RPM range, but why all of a sudden? I can't see anything having changed with the carburetors, but I did "touch" them recently so naturally this is what I am suspecting.

Again, idle is good and wide open throttle is good. It is just cranky from about 1000-1600 RPM. Thanks!

Did you actually compare the Mixture Screws next to each other to make sure the point taper was identical as well as the length of the taper?

Sometimes the "New Ones" just screw in and do not function correctly vs the Rochester designed mixture screws.

Tom V.

Hi Tom. I did not touch the mixture screws; they have been in there for more than five years. I simply changed the airhorn screws, for sake of appearance.

I can't see how changing airhorn screws would affect how the car performs between 1000-1500 RPM, but it is the only thing I have touched.

The car ran so smooth for so many years, and is now a bit funky in that RPM range.

So has the problem been there since you changed the screws? I would open up the first carb again and check the float level and make sure the plunger and everything still seat right. Were there any vacuum lines that may have come off?

Before you start tearing into the carbs, I'd check the dwell on the distributor. Still have points? My experience dictates all of a sudden the points and condenser can go south. Aftermarket replacement pick up's can get funny too when they get warm. It may be sheer coincidence that you changed out the screws on the carb,

Tim

Hi there. I changed the airhorn screws about a month ago and have not driven the car since last night. No vacuum lines were removed. I did take the airhorn off of the front carb, but to the best of my knowledge the front carb isn't a player until wider throttle openings anyway. I did not change anything is the front carb, only the gasket and the airhorn screws. I didn't touch the float at all.

Hi Tim. I had the distributor rebuilt completely five years ago when I did everything else. I had one of the M&H breakerless modules put in at that time:
http://www.breakerless.com/

I know a lot of people hate these "Pertronix" style modules, but I have had zero problems on both GTOs for many years. I am not saying this module ISN'T the problem, but I have always had good luck with them.

EDIT: I called M&H just now to discuss the issue. They mentioned that when these modules fail, they usually fail completely. Or, they can fail throughout the RPM range.

My problem seems to be only between about 1000 and 1500 RPM.

There's no way the airhorn screws would cause the problem you describe or change anything about the way the Tri-Power functions. With your symptoms, the first thing to do is turn both idle mixture screws in 1/2 turn at a time, checking for the "bucking" with each change in adjustment.

Access to those adj. screws is difficult, but a piece of 1/4" ID rubber hose makes a good tool for making the adjustment. Put a mark of some type on the hose so you know how much you're turning the screw. On the driver's side, if you have power brakes, access to the idle adj. screw is impossible unless you turn the brass fitting in the rear carb clockwise to provide clearance. You can easily turn it back after the problem is fixed.

Hi Dick. My apologies; I have been way from the forum for awhile. I have made up a sort-of-custom tool to set up the mixture screws. I have played around with them somewhat (using a vacuum gauge and tach to "guide" me), yet I still continue to get this annoying surging.

Any chance there could be moisture in the centre carb?

The car continues to idle great, and runs like crazy under acceleration....

The bucking problem can be a power valve spring that is too strong, causing the power valve to open intermittently when there is not enough vacuum to keep the rod up off the valve.

Another cause of the bucking off idle can be ignition timing. Ideally, with vacuum advance operative, you need between 6 and 10 degrees initial timing and about 34 degrees total timing (with the vacuum disconnected). After confirming this, connect the vacuum advance and check total timing at about 2500-3000 rpm in neutral.needs to be less than 50 degrees. Some vacuum advance diaphragms allow much more than that, causing problems with surging and detonation under light load.

Thanks Dick. I don't think I have a power valve spring that is too strong, as that spring has been in there since I did the carbs five years ago and nothing but the airhorn screws have been touched.

However, I do like your vacuum advance idea, and I need to check that. I just recently timed the car to 34 degrees total at 3000 with the vacuum line off. But, when I put the vacuum line back on I did not check for 50ish degrees. So, that is what I will do next.

Thanks again. I will report back soon with the result.

Excessive advance at road speeds with the vacuum fairly high is a dangerous situation. You may not hear the detonation at light loads, which can do some serious damaged down below.

I've heard of some cases of 55 degrees plus total advance.

Pertronix.. Put a good set of points in it and use them to trigger a Vertex Z-6 CD box...

The vacuum advance is the first thing I remove, then I set the initial to what the engine likes, not what some generic manual that has to work for millions of cars says.. The 427 in my shop truck (69 el Camino) likes 22 initial 14 mechanical all in by 2400...
I don't like my idle controlled by the engine vacuum, I like it to be steady and stable..

Any luck fixing the "bucking?"

Reading the new post in the street section... it's too late.

https://forums.maxperformanceinc.com...d.php?t=843395

Don't think he understood that the damage was rod bearings getting hammered out.

Clay

I will keep everyone posted on this. We have to get it apart.