Engine racing on full throttle

I need help looking for a place to start to find my odd engine racing issue. I drove it to a car show today with out any troubles - about 2 hours each way. I was a few miles from home, we live on a 90 km/hr road, and though why not check the engine for smooth acceleration so I floored it - no trouble, floored it a second time and the car took off and would not slow down. I pumped the brakes a few times and then just turned off the ignition so I coasted into my driveway. I started it up again and idled it into the shop. The throttle linkages are not binding, I have replaced the engine mounts about 6 years ago so no binding noticeable on the front frame under the carbs. I’m running dual Strombergs that were overhauled 18 months ago. Auto trans 68 2+2. I added a stiff return spring on the throttle linkage as an experiment and it still did the rev to eternity. So, gentleman where should I begin looking?

How do you go about determining the throttle linkage is not binding? I‘m not sure the Stromberg setup is that prone to binding I‘ld have to check but engine mounts have been known to sag a lot faster than 6 yrs. How would you check that standing still, without having the engine under full throttle/load? Also how is the gearbox mount condition.

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All good questions. I moved the linkage by hand without the car running. I am afraid to try it again running since it revs way up within a second. I might need a second person with me to figure it out. When I bought the car 8 years ago I could see the throttle binding on the frame and replaced the engine mounts. I recheck closer once the car cools down.

Mine did that once in Houston as I nailed it while on the entrance ramp to a busy freeway; scared me silly. Turned out the throttle linkage for the triple SUs was in some situations slightly interfering with the engine frame. I checked for binding as you did and found nothing, but the linkage as very close to the frame and I could see a tiny mark where it had contacted the frame. A slight repositioning of the engine alleviated the immediate problem, but new, uprated engine mounts were the long term solution.

This is by far the most likely cause.

It’s not necessarily that the linkages are physically jamming against something. Check that none of the pivot points take their rod past the 12 o’clock position; if they do the rod can get locked in that position.

With a RHD car you can hook your foot under the accelerator pedal and pull it back but this is not so easy with the LHD organ pedal design.

thanks John and Andrew. I have to agree that it must be binding under load, but not while I’m fiddling with it. Too exhausted after 4 hours driving the girl without AC. I’ll get into it tomorrow and let you know what I find. Just thinking so fortunate to have it happen close to home…I could have been on the news “Jaguar driver reaches ludicrous speed!”

I found the trouble. Just a small amount of paint was missing on the engine frame, so the linkage must have caught the frame under load. I shimmed that side engine mount for a temporary fix and will order new engine mounts. Next surprise is the engine is running terribly now. Miss-firing, coughing and won’t even idle. I dread the thought that I may have pooched the engine with the high revs, but will resume the exploration after a few cold beers.

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Get the so-called uprated mounts from anywhere, they have a reputation of holding up longer. Regarding the rough running, I doubt the engine is broken - these engines can take serious beating. Check the vitals like ignition, sparks, carbs etc. you‘ll probably sort it out pretty soon.

Thanks Martin. I just noticed that the 123 distributor rattled loose, so that has messed up the timing. I’ll re-set that tomorrow and hope for the best.

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I found when I installed mine I had to fettle a bit to get both the 123 distributor and the clamp seated correctly. Once done, there shouldn’t be any movement.

Check out the diaphragms in the Stroms.

Yes , Larry that was the issue. I guess with the high reving, the dizy got pushed out enough to not be clamped by that collar. May not have been fully seated for that last year, and just needed a strong force to dislodge it. I got it tightened up again fully seated in the collar and after resetting the timing, all is well.
thanks for everyone’s input and support. I have been finding it a challenge to wrench on the girl since shoulder surgery in the spring. Have to do a bit, stop for an hour, do a bit more… takes forever.

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