Bouncing causes throttle application

I took my '5 out to the hills to play this weekend. (Finally! After 150+ hours of labor, and and a large sum of money I haven't tried to keep track of ) Over all she's running pretty well, but I found a new and interesting problem. Sometimes when she takes a bounce, I'll get some throttle application. I've checked the linkage, and everything /looks/ normal to me. It seems to only happen when going down hill, or maybe that's the only time I notice it. Its a bit disconcerting when trying to ride the gear box down though, every once in a while it hits a bump and feels like the throttle has been punched. It got sketchy enough that I had to take her out of gear and ride the brakes. Very ineffective because I'd have to keep stopping to let them cool back down! Has anyone ever seen something like this before? Could it just be my throttle return spring is shot, and momentum of the linkage is pulling on it, it still seems fairly strong?


-Ken

EDIT: I forgot to mention, the jeep has a V6, with a Rochester 2G carb... Carb has been replaced by the PO, and the roll stamp identifies it as Jeep application for a '71 with 225, I believe this to be identical to the '68 carb that should be in place.

 

I don't know what you have done to your Jeep already or what your experiences with Jeeps are so here are the first things that pop in my mind,
I hope it is of some help.

Does it have New motor and tranny mounts ? any mods that can interfere with throttle linkage? You have checked the frame throughly for cracks?

One other possibility you are bumbing the throttle when you bounce with the jeep. I have had that happen to me. I now drive with heal on the floor board and the side of my foot on the tranny hump (2 points of contact for me makes throttle operation much smoother). or try a hand throttle.

 

I'm sure its not my foot hitting the gas, as it happens even when I'm fully clear of the peddle and on the brakes. My jeep is fairly stock, but I may have the wrong return spring. One end looks like it may have been cut from a longer spring and had a new end bent as a trail fix. Overall length is 3", spring stretches 1.5" longer with a 6-pack hung on the end. I can't for the life of me remember how to calculate spring rate from this though. Apparently I've forgotten everything I learned in school. I chose the 6-pack (4.5lbs) as a weight in the hopes that someone else can go check their spring so I have something to compare against.

I don't think I've got any issues with the motor mounts, there doesn't seem to be any excessive movement with the engine. I may have a problem with the body mounting, as I have about 2" of lean to the driver side caused by bent tub mountings when she was rolled onto the cage by the original owner. They seem 'solid' though, jacking from the tub tries to lift the frame with it; it doesn't separate.

Does anyone have a throttle return spring they could try measuring for me? If not, I'll try to pick up a stiffer spring from the hardware store after work tomorrow and see if that changes anything. Its just hard to test because the problem only occurs when bumping around on the trail.

-Ken

 

Frame flex?

 

Soft or worn, loose or flexing body mounts?

 

Yeah, that was my first assumption as well. I'll try a heavier spring to see if it helps out, I think my spring may not be correct. When yours does it, is it just a little blip, or does it feel like its really punching the gas hard? My jeep accelerates heavily when it bumps, hard enough that the brakes can't hold it back easily.

-Ken

 

I read an article somewhere about a throttle dampening system for that problem. I'll look for it again and post it.

 

Might as well tear it down to the frame and start over..... chainsawguy

 

My take is that it would take a rather huge amount of flex, either in motor mounts or frame, to cause that. A weak return spring and the stock iron pedal could, as Patrick suggests. The older, floor mounted pedals really do work better-way easier for me to keep an even foot on them. You still get some bumps in the really rough stuff but better than the swing type and easier to brace foot off something.

 

Well, I've swapped the return spring out with a much heavier replacement... Pedal feel still seems acceptable, I thought the original feel was a bit light for my taste anyway. Hopefully that will be enough to make the problem a little less hair-raising when driving down the side of a mountain.

 

My 68 did the same thing. It would bounce and surge so bad I would have to
take my foot off the pedal. It was not drivable on the trail. I took out a couple of leafs to smooth the ride a little, but what solved the problem was rebuilding the Rochester 2G. The accelerator pump was shot and the float was set wrong. Now it is perfectly smooth, no more surges.

 

Yeah, I have definitely considered this, but I've rebuilt this carb twice in the last two months. First was from 13+ years of sitting, second was from a defective fuel filter letting junk past. This is the first Rochester I've worked on, but I have done a fair bit of work/tuning with motorcycle carbs. It is possible I've missed something somewhere though, or maybe my manual has the wrong specs for the float level. Anyone know what the height and drop should be set as so I can double check it again?

 

Mine does the same thing, always has, no lift, 4" lift, full rebuild... Foot position seems to help.

 

Not for a Jeep, but should work.

http://www.duramaxforum.com/forum/enervative-design/47808-accelerator-spring-mod-99-05-a.html