bucking in 5th gear at 70mph & a wee bit of fire

[timspider]

I had a peculiar problem last night in the last hour of a 3.5 hour highway drive, mostly cruising at 70mph. Had done the same drive earlier in the day with no issues.
I think of lugging as what the car does when you are engaged in a gear to high for your circumstance, so this isn't lugging since I was at highway speed. I'll call it bucking. Just started jumping a bit and lurching while at highway speed in 5th. Shifted to 4th and slowed a bit and was still getting it a little, slowed to 60mph and it smoothed out.

Slowly gained speed and carefully shifted back into 5th and life was good for another 15 minutes. Then the same thing, had to coast on the shoulder for a while, drop down to 3rd before working my way back up to speed.

At this point, I was pleading with her to get me home. I promised to get her transmission issues addressed if she got me home in one piece. She obliged.

This morning, I hesitantly used her to go to the grocery store, warning the family that there was a slight chance that I will be very late with the groceries, and possibly on foot.

She ran famously well, although I was only doing 55-60mph on country roads, not 70 on the highway.

I went through all the gears repeatedly and everything was fine EXCEPT 5th gear occasionally felt like it had a very slight grinding feeling to it. No bucking, just the feeling that something was a little off.
Of course she did send a flaming belch through the aircleaner at the conclusion of the ride, which set my foam oval element on fire briefly. But hey, that's all part of the fun, right?
Does anybody know of a fireproof replacement for one of those oval elements?
Any thoughts on the bucking or an alternate air cleaner element to foam are appreciated.
thanks much

[redcars]

It sounds to me like you where starving for fuel. As in your fuel filter is starting to get plugged up or you have a restriction in your fuel supply some place.

[So Cal Mark]

I agree, sounds like a fuel starvation issue, and the backfire through the carb indicates a lean condition

[timspider]

thanks so much. the fuel filter looks a little grungy. that would be really great news compared with the other possibilities that were going through my head.
I'll get a new one in there and post the results probably by monday.

[timspider]

replaced the fuel filter today and the car ran immediately smoother. had an hour long errand to run, with about half on highway, everything felt much better as far as the gears. Very smooth.
I think you both got it- thanks so much

I still am having some backfiring at the carb, mostly just when starting out in 1st. I need to learn how to tune this carb next.
have a great week

[timspider]

so my final test yesterday was a drive to Chicago and back, a 7 hour trip with an hour break in the middle. It was the last time I made this trip that the bucking thing happened and I barely limped home.
No issue with that now, held 70 to 75 for most of the last 3 hours.

I'm really grateful for the help. What a relief to find out that a problem that felt so major could be cured with $5 run to napa.
On to the next issue, backfiring at the carb.
The list is slowly getting shorter.