Stuck Throttles Comments
After reading your article on stuck throttles (“Stuck Throttles Discussion,” May ’14), it brought me back to some experimenting I did some years ago about this. This was brought on by a throttle sticking on start up after removing a carb. It got me to thinking about some of the reasons I have seen them get stuck over the years. Most of my experience is with the Holley 350- and 500-cfm two-barrels. But I’m sure much of this will also transfer to four-barrel applications.
The top of the list is not using any form of a throttle stop. Outside of the obvious of bending the shaft and having it hang up, it places all of the force and mechanical advantage from the linkage fully on the carb itself. Over time, this can bend the parts on the carb to a location where the plates can hang up on the manifold or a spacer or bind the linkage.
I have seen a few times where spacers or adapter has been tight enough on the hole bore that it can cause the butterflies to get stuck in the open position either from the bolts not being torque properly or the lack of a throttle stop allowing movement of the carb.
As for the throttle stop itself, I have seen many times where a guy will use a 1/4- or 3/8-inch bolt as a stop and it was OK on the shop floor, but not large enough to stop the linkage from slipping off of it and the pedal from getting caught up and capturing and holding the linkage.
Some guys have ground the exposed threads of the screws holding the butterfly plates in the shaft thinking this will be a huge benefit in airflow when all it really does is remove the staked area of the threads that is there to keep the screw from backing out. When they do back out, it can cause the plate to move and jam the carb open.
The last of the major issues I have seen is a throttle stop being located on the carb itself and had loosened up and fell into the linkage or became loose enough to trap the linkage and hold the plates open. I really feel that stops should be away from the carb and located on the pedal or firewall and only placed on the carb as a last resort.
—A tech official (name withheld)
This is all good information. We can’t pay enough attention to the linkage and carburetor functions. The more we know about the ways this can mess up, the better we can prevent a stuck throttle.
More about Stuck Throttles
I read your article, which by the way, had some very interesting ideas. I’ve not heard of a brake-for-your-life kill switch. I wished I would have last summer.
I’ve been racing going on 20-plus years now. I’ve taken a season or two off from driving, but not from racing. Early on in my career, around 1991, a veteran racing friend died as a result of a stuck throttle. There were other bad design choices made, but Super Mods are usually handbuilt and reflect the skill of the craftsman, or lack thereof. I studied at length and understand what went wrong.
I made a conscious decision at that time to make the things I had control over the safest I could, and I did. Even with an eye on the ball, I let this one get by me. I should have known to check it, the signs were there. I just didn’t.
The car that I crashed last summer had a throttle linkage very similar to my Sprint Car as I helped with the design. The root of the problem was throttle pedal hinges (heel pivot) that were allowed to wear.
This provided some side to side movement over the 3-4 years of service. The floorpan attachment bolts had just enough threads exposed above the nut for the pedal to catch on the outside of the threads locking the pedal to the floor.
I had felt what I now know to be the pedal edge hitting the top of the bolt. It would hold it off the floor by a half inch, which you could feel when it clicked loose to the inside. It just happened that the pedal was screwed to the floorpan right where it was. A half inch to the inside and it would not have happened. A pedal stop would have prevented it from getting that far down too.
A brake pressure kill switch wouldn’t have prevented walling it up, but the car may have been repairable as a result. I had just enough time to slap at the ignition switch and grab the wheel before impact. Quarter-mile tracks are close quarters at 13 second lap times.
The moral of the story is just because you check and double check doesn’t mean it won’t happen. Racing is hard and it’s harder when you go straight in by your own hand.
Good luck and God bless,
I get your message to be, “always look for the unexpected.” Imagining what can and will go wrong with the installation of all of the throttle parts and their proximity to other parts. You are correct, nothing is foolproof, but we can help to eliminate problems by being attentive.
In the event the unexplained and unexpected does happen, the brake pressure kill switch is the fastest and most logical way to deal with the stuck throttle. We’re just not fast enough to reach for the ignition switch in the time it takes, but we are already on the brakes and the overwhelming tendency is to push harder when we see we are not slowing. This activates the pressure kill switch and turns off the engine. It’s a no brainer.