Throttle Pedal Issue, Help Please :)

[yoshi_w]

Hi, I was hoping someone could assist me with a drivability issue I've been having. When the car is off, the gas pedal is easy to depress, and entirely normal, however when the car is on, the pedal becomes difficult to press, and if i press too much it will gun it. The car in question is a 73 z, l28, flattop pistons, mild cam, roundtop su's, throttle cable conversion. Thanks for any help, I'm hoping its something dumb and simple. Also, the car is running a little rich as of the moment if that matters (needa tune the carbs)

[FastWoman]

Just guessing, but I bet there's a lot of wear/play in the throttle shafts.  When the engine is running, the vacuum pulls on one of the throttle plates, jamming it somehow against the throat of the carb.  To reproduce the problem without the engine running, try applying pressure against the throttle plates with your fingers and manually opening the throttles.  Vacuum at idle would be about 9 psi.

[yoshi_w]

Alright I'll go take a look today, should the throttle shafts be that worn am I pretty much SOL? I have some extra round tops but regardless is there anyway to remedy the problem short of replacement?

[FastWoman]

I'm not a carb person, so I don't know your options very well.  It's maybe a better question for the carb forum folks.  A company called Z Therapy rebuilds the old SU's.  They do a very good job, but they're expensive.  If shaft/bushing wear is the only problem, it might be possible to have a local machine shop fix it.

[Mark Maras]

 ZTherapy does have the answer. The carb. bodies are fitted with sealed throttle shaft bearings. However there may be other things that would make a carb. a throw away.

 FastWoman's suggestion is a good one. I would look at the carb. linkage as well. Start by cleaning & lubing all linkage from the carbs. to the pedal. Also wondering if bad m. mounts could be allowing the engine to rock, changing the angle at the bell crank on the fire wall. It seems that you should be able to see, with another person inside the car, the difference that occurs when the engine is running.

[yoshi_w]

Its been converted to a throttle cable so the only linkage left is between the carbs, I didn't feel like much play when I messed with the remaining linkage (the one attached to the throttle shafts) is there any particular way to check this? And I am slightly confused how with play in the throttle shafts the problem would only occur with the engine running? Unless I'm misunderstanding something to do with the vacuum?

[Mark Maras]

 Linkage or cable it doesn't matter-clean & lube everything from the pedal to the carbs.

 Try opening up the throttle manually on each carb, one at a time, with the engine off then do it again with the engine running. From closed throttle to wide open, they should operate ultra-smooth. No hard spots just spring pressure. & you might try loosening the springs for the engine off test. Be sure to hook them up before starting it up. VOE   You should be able see or feel a difference & maybe isolate it to one carb or another. Luck

Mark

[George_Zed]

I had a similar problem but my gas pedal was rough to press either when the car was running or not...But yes, it was even worse while I was on the road...

 

I was advised to change the wire and put a steel wire... so, I found that the piano (yes piano ) wire is ideal and I used this and problem is solved There are different sizes though, don't buy too thick cause it will not bend and it will be more difficult than it is now... I am sorry but I can't remember how thick is the wire I used   but just choose a thin one...

 

anyway, this is the solution I found to my problem, maybe there is something else causing you problem

[Captain Obvious]

I've not seen that phenomenon on the round top carbs, but I've seen it on the 280 throttle body as well as other throttle bodies from other unrelated cars as well. Perfectly smooth when off, but sticky with vacuum pulling against the butterfly plate with the engine running. I won't claim to fully understand exactly what's going on, but in my experience it's been caused by either worn throttle butterfly shafts as suggested above, or by dirty crud on the butterfly and/or throat walls where the butterfly meets the walls.

 

So first suggestion would be to hold the carbs butterflies completely open and thoroughly clean the the interior walls of the carbs and the butterflies really well with a carb cleaner wetted rag. Might be tough with the carbs on the car, but you could remove the domes and suction pistons and that might allow enough room to get in there without taking the carbs completely off.

 

If that doesn't help, then I suspect the chrome plating has worn off your throttle shafts exposing the brass underneath (very likely). You could put some lube on the shafts and work them around. That may help temporarily, but to really fix it, you'll need more dramatic measures.

[yoshi_w]

Thanks for all the suggestions! I have some free time today to give those a whirl. Even a temp fix would be good while i decide what to do carburetor-wise, (maybe triples? )