Intermittent Bucking… (1 Viewer)

[faalkinaey]

I could use some help figuring out this bucking issue, I have a 87 fj60 and it has a very intermittent bucking problem under load. The truck will run completely fine on the highway for maybe about an hour then it starts to buck, it goes away as soon as you lift your foot from the throttle and GENTLY reapply throttle to get back up to speed, I have to do this every time there is “bucks” when going under load and I can not figure out what it is for the life of me.
Things done to truck within past couple of months
-Man-a-fre HEI distributor with vacuum advance (OEM broke)
-Toyota fuel filter
-Toyota fuel pump
-new spark plugs, gapped to .50
-Timing set to 13° BTDC because egr is bypassed. (I also don’t have any pinging, running 90+ octane)
-Replaced ALL vacuum hoses, no leaks. This thing Idles great at 700-750 rpm
-ICS seems to be grounded by P.O, the solenoid clicks every time.
The truck never bucks or dies when at low load / speeds only highway or going up steep hill, and even then it’s intermittent. One thing I can confirm is that it ONLY starts happening after 45-60 minutes of driving, never before. I would prefer to keep a carb rebuild as a last case because it was allegedly rebuilt before I bought it couple months back. I Appreciate all the advice I could get

 

[Skreddy]

How quickly after you let off the throttle does the bucking return? I’m assuming the bucking is like a heavy miss feeling?
How much gas is in the tank? Tank condition and fuel line condition? I’d be leaning towards fuel starvation under constant load.

 

[OSS]

My guess: It's intermittent fuel starvation that's causing the bucking.

I'd bet $100 that there's debris in the carburetor float bowl drifting around and intermittently blocking a jet.
When the vehicle is angled up a hill, the problem (bucking) can get worse because the jets are at the back of the carb float bowl and any crud drifting around inside it has an easier time migrating back against a jet.

One possible reason why the bucking stops for a while when you let your foot off the gas pedal on the freeway and then slowly apply gas, is that the jet stops sucking in fuel when you let off the gas which may allow that mythical piece of junk in the float bowl to drift away from the jet a bit since there's no more suction.

At any rate - my prime suggestion is to examine the carburetor (float bowl)

My 60 experienced the exact same symptoms you described - and there was a small piece of crud I found in my carb float bowl. Once I got rid of it - engine ran fine again.

 

[faalkinaey]

just found out today that it seems to be consistently happening at 2500 rpm

 

[toy_tek]

How old are the wires? Might have high resistance to causing internment misfire after they’re heated up.

 

[faalkinaey]

So I drained the fuel bowl and replaced both spark plug and wires as you guys suggested and no change, it bucks on the highway after being in constant load for a while..

 

[faalkinaey]

I’ve noticed the bucking seemed harsher when EGR is hooked up

 

[faalkinaey]

one more note, is there supposed to be bubbles in fuel filter? Just changed the filter out to a clear glass one non OEM, and I can see bubbles at the top of it

 

[OSS]

the EGR system will make all FJ60 2F engines hesitate a bit at light acceleration - because its injecting exhaust into the intake stream = normal.
its ok to have some air bubbles in the fuel filter.

If this was an ignition problem, you'd be able to duplicate the issue just at fast throttle with the car parked & transmission in neutral.

Bucking is a classic fuel starvation symptom

 

[faalkinaey]

So where should I go next?

the EGR system will make all FJ60 2F engines hesitate a bit at light acceleration - because its injecting exhaust into the intake stream = normal.
its ok to have some air bubbles in the fuel filter.

If this was an ignition problem, you'd be able to duplicate the issue just at fast throttle with the car parked & transmission in neutral.

Bucking is a classic fuel starvation symptom

Where should I go next? Verified filter is good, verified pump is good, draining the carb bowl is the most I’ve done there. My tank is vented to atmosphere. What else could I check

 

[OSS]

If the float bowl was drained just by unscrewing the "drain" plugs on the front of the carb - that may be insufficient to completely clean it out if something is drifting around in there, If there's a gremlin glob in there, it could be resting on the bottom of the bowl still.

The bucking may not be caused by debris in the float bowl, but you've got to absolutely rule out the possibility. Just draining the bowl doesn't ensure that. You've got to examine it visually.

Also - make sure when the engine is idling, car parked, that the fuel level is in the middle of the sight glass on the carb.

 

[faalkinaey]

Okay so out of frustration when replacing the fuel filter, I broke the hose clamps, replaced hose clamps at the filter and drove 1 hour home with ZERO issues… what the f***!!!!! Gonna drive 2 hours tmrw to see if the issue is still there.

 

[faalkinaey]

I guess 91 octane and hose clamps was my solution…..