Engine Surging - 1967 435

[Mark K.]

I have a 1967 435 that is mostly original. The original carbs on the car were leaking which drove me to take them off to be rebuilt (though I haven't sent them out yet). I bought a set of new Holley carbs to replace the old ones so I can drive the car this year. The new carbs are on the car and the car has no problems starting.

The problem I have is when I head out on the road. Upon pressing the accelerator, the engine RPM just keeps heading higher and higher. To get the RPMs down, I have to tap the accelerator pedal (sometimes a few times) to get the RPMs down. There seems to be no RPM (above idle) where the engine will obey my commands (rather than the other way around). It just wants to race.

How do I eliminate this surging problem? Remember the carbs are new. The float levels have been set and the linkage seems fine. My Holley books seem to make suggestions that incompatable with new carbs. Any thoughts here?

[Richard R.]

Re: Engine Surging - 1967 435

Sounds like you have a serious vacuum leak.

[Joe C.]

Re: Engine Surging - 1967 435

I have a 1967 435 that is mostly original. The original carbs on the car were leaking which drove me to take them off to be rebuilt (though I haven't sent them out yet). I bought a set of new Holley carbs to replace the old ones so I can drive the car this year. The new carbs are on the car and the car has no problems starting.

The problem I have is when I head out on the road. Upon pressing the accelerator, the engine RPM just keeps heading higher and higher. To get the RPMs down, I have to tap the accelerator pedal (sometimes a few times) to get the RPMs down. There seems to be no RPM (above idle) where the engine will obey my commands (rather than the other way around). It just wants to race.

How do I eliminate this surging problem? Remember the carbs are new. The float levels have been set and the linkage seems fine. My Holley books seem to make suggestions that incompatable with new carbs. Any thoughts here?

The problem which you describe is not surging: a rhythmic pattern of increase/decrease of torque while cruising. That condition is attributable to a vacuum leak and/or lean condition.

What you describe sounds downright dangerous, and more suggestive of a linkage/return spring/throttle plate issue. I would start by disconnecting the outboard carburetors, and be sure that their throttle plates are "locked out" so that they cannot open on vacuum signal. Next, disconnect the linkage from the center carb, and be sure that the linkage is not binding. Check the return spring, that it's free, and not getting hung anywhere. With linkage disconnected, work throttle plates open/closed. Reconnect center carb, and get engine fully to op temp. Remove linkage and recheck throttle plates on center carb............the heat might cause them to bind...........adjust as needed. Reconnect center carb and road test car..........again, being certain that the disconnected outboard carbs have their throttle plates "locked out". Reconnect outboard carbs and road test.

Joe

[Mark K.]

Re: Engine Surging - 1967 435

The problem which you describe is not surging: a rhythmic pattern of increase/decrease of torque while cruising. That condition is attributable to a vacuum leak and/or lean condition.

What you describe sounds downright dangerous, and more suggestive of a linkage/return spring/throttle plate issue. I would start by disconnecting the outboard carburetors, and be sure that their throttle plates are "locked out" so that they cannot open on vacuum signal. Next, disconnect the linkage from the center carb, and be sure that the linkage is not binding. Check the return spring, that it's free, and not getting hung anywhere. With linkage disconnected, work throttle plates open/closed. Reconnect center carb, and get engine fully to op temp. Remove linkage and recheck throttle plates on center carb............the heat might cause them to bind...........adjust as needed. Reconnect center carb and road test car..........again, being certain that the disconnected outboard carbs have their throttle plates "locked out". Reconnect outboard carbs and road test.

Joe

Joe, Thanks for the advice. To lock out the end carbs, should I both disconnect the linkage and disconnect the vacuum lines? How do I adjust the center carb throttle plates if I find them binding? Is this something I should better have a professional shop look at? Thanks.

[Joe C.]

Re: Engine Surging - 1967 435

Joe, Thanks for the advice. To lock out the end carbs, should I both disconnect the linkage and disconnect the vacuum lines? How do I adjust the center carb throttle plates if I find them binding? Is this something I should better have a professional shop look at? Thanks.

Disconnect and plug the hose to each outboard carb's vacuum actuator. Insure that the two outboard throttle shafts are POSITIVELY immobile, by either tying or wiring them closed.
You state that the carbs are new, so I'm assuming that all of the shaft bushings have no "slop" in them.
If you find a problem with any of the carburetors, then why mess with them. Exchange under warrantee. There is no reason why you should have to adjust any of the throttle plates. Furthermore, they are fastened to the shafts with self-aligning fasteners which leave little/no room for adjustment
I suspect that you might have a bind somewhere in your linkage. AGAIN, double check all return springs, and make sure that you have them installed properly.

Joe