High speed issue - "bucking" at 65mph (1 Viewer)

[Sparky_Mark]

Strange... Any have an idea of the root cause?...

'79 FJ40
2F engine
Original manual trans and transfer
IGN wiring all new
Compression is good
Rochester carb (I know)
No emissions control


I was driving down the highway a few days ago and experienced the vehicle "bucking" when I got up to around 65mph (3,000 on the tach). Being very concerned, I dropped speed and got off at the next exit. I had no further issues during the rest of my trip (about 30 more minutes) on surface streets - 50 mph max.

I wouldn't characterize the problem as stalling as it runs fine at lower speeds.

I'm thinking of 2 possibilities (but certainly open to other causes)...
1. Fuel filter. I have recently switched gas tanks (cleaned the "newsed" one well before installing it). I used a glass filter with a replaceable cartrige so I could monitor it regularly. I have had a issue on 2 occasions with stalling - but the problem went away after replacement. Also, I have a rotary fuel pump but don't suspect it because I've had it for about 5 years. Could it be a fuel flow restriction from the filter?

2. Clutch. I'm not sure how much life is left in my clutch, but it does slip briefly on occasion (twice in the last 6 months) between 1st and 2nd while under high load. Not having done a clutch job before on my vehicle, is there a way to inspect the thickness of the friction material without removing the major parts of the assembly?

Any other suggestions?
Comments appreciated.

 

[toddslater]

Don't think it's the clutch and it definitely sounds like fuel starvation at high constant load. Give your filter a change out and see what happens.

 

[CruisinGA]

In addition to what todd said, i would give the inside and outside of your carb a hefty spray down with carb cleaner.
I had a dirtbike carb that didn't work at all, 10 min and a half can of carb spray and it worked like a champ.
I've had a similar symptom twice, once it was a loose sparkplug wire, and another time it was a worn out distributor bushing. I doubt it is the latter in your case, as it only affected the truck at 70+ mph.

 

[Hot Rats]

Can you describe in more detail what you mean by "bucking"? That word (and the apparent dependency on speed) reminds me of "death wobble" (search this forum for that quoted phrase and you will get plenty of descriptions and tips). I know death wobble doesn't quite jive with the rest of your description, so, given that this is my second post (and with a penny for my thoughts) my suggestion can easily be worth less than $0.02

 

[Sparky_Mark]

"Can you describe in more detail what you mean by "bucking"?"

hard to put into words, but here goes...
The issue seems to be speed dependent (I don't experience this when in lower gears, although I don't spend much time near 3k rpm). The condition feels like the engine is stalling, then restarting. This results in an abrupt surging of the vehicle.

Another way to describe it is: If you were driving on the highway and "goosed" the clutch. It would result in the same sort of surging.

I had an EZ locker installed previously and experienced the same thing, only it seemed more severe. The locker kept racheting when this happened. I had originally though that the locker was worn to the point that it had slipped frequently. I removed the locker and, unfortunately have the same issue.

Hope that helps, thanks for the interest. All comments are worth much more than $0.02 to me as it at least generates dialogue. Time is money and 2 cents worth of advice can save a lot of head scratching.

I'm going to change out the filter and spray the carb clean. Results to follow...

 

[65swb45]

My vote goes to fuel starvation as well. That Rochester flushes gas thru the motor like a toilet bowl; it needs a LOT at the high rpm.

 

[Jetboy]

I had a problem that was very similar in my last mini truck. It would "buck" so violently that I thought it was going to make the rear end jump. I though for sure it was a rear end problelm, but it turned out that it was a fuel filter. the extra slop in lockers realy exagerates the problem as the ring gear slows down and then gets some fuel and slams foreward. I had a detroit softlocker in that truck and it would go from normal driving to so rough so fast that I couldn't hear the sputtering of the engine to know it was starving for fuel.
I finnaly figured out what it was and it never occured again.

also be glad the filter isn't on top of the fuel tank where the trucks have theirs, I had a custom flat bed and the filter was a PITA to change.

 

[Advent]

It might also be the fuel pump. I had the same problem at one point, and I replaced the fuel pump, gas tank, fuel filter, AND all the lines. Didn't solve a thing.

Took it in to Cruisers Only, where Mark decided to test the fuel pump. Turned out it was a bad replacement fuel pump, in addition to whatever it was before.

 

[Sparky_Mark]

Issue resolved...

It was in deed the fuel filter. I replaced the glass filter with one that is an aftermarket replacement (FJ60 style - inline instead of a "y" type because of the new position near the tank). I also spent half a can of carb cleaner on it as well.

I got down the highway for a few miles without any problems at near 70mph.

Looks like my plan to monitor the glass filter was not such a good idea. For future reference, it was a Puralator and I've seen these in every chain auto parts store I've been in.

Thanks all for the help. I'm REALLY glad it wasn't something major like the clutch.