The Stovebolt Site Forums The Parking Lot Big Bolts 150 mile test drive - engine stumbles up a long grade

Hey All

I just took my 46 2 ton for a long drive.
I drove from Hayward CA to Modesto CA to see how it would do.
Its 50 miles one way. And goes over the Altamont Pass which is about 1,000 ft.
Coming back, about 3/4 of the way up, my truck started to miss, then ping, then backfire. I slowed down, pulled over, drove about 40mph for a bit, took it out of overdrive and drove the rest of the way up the hill at 3k RPM in direct drive (its a 261 engine).
Any thoughts on what caused this?
Im guessing my truck is running lean and the full throttle long run got the engine too hot. (water temp stayed low). Either that or the fuel pump cant handle the volume (I have 2 carterYF2100 carbs).

The thing that is new for my truck in this scenario is the full throttle long uphill run. The pass is probably 10 miles long.

Also, the driving was all high speed driving on freeways. I was constantly driving 55-65MPH (the truck as an overdrive). Its the farthest I have driven my truck.

Do you have a vacuum advance with an octane selector? The octane selector was used for occasional/temporary adjustments in timing due to changes in gasoline octane and significant changes in altitude.

I live at sea level, and the pass is only 1,000 feet, so I didnt change it. I also run premium gas.
The distributor was also recurved to deal with modern gas.

I had something like that happen to me one time and it turned out to be a loose vacuum line leaning it out.

Mike B

I'm planning on installing a vacuum gauge in mine. Since my 2-speed switch isn't set up for an adapter,I'll plumb it into the line at the firewall. A friend of mine had one,and he could monitor for performance,economy,and,best of all,will tell you when your engine is running "below-par". That usually happens,when it's time for a tune-up,anyway!L.O.L. Just goes with what Mike B. says. It's like an on-board,old-fashioned diagnostic tool. Works best when combined with tach.

A quick check would be to T in a fuel pressure gauge at one of the carb inlets and duplicate the climb. The pressure should remain high because the pump should be able to move more gas than the carbs could use. Maybe a marginal pump or partly clogged filter with the engine rpm down in overdrive, could not keep the supply up.

Are you running a hot spark plug? The long pull could cause the tip to over heat and cause pre-ignition.

If the timing is advanced for premium gas and is just a little too far, you might not notice anything in normal driving, but the long steady pull might make it show up.

Finally, again, if the advance is set for premium and you got some "not so premium"....

I do have a fuel pressure gauge on my fuel log between the carbs, but its only viewable with the hood up.
I will install a temporary flexible hose and run the gauge into the cab.
I will also pull the spark plugs and look for signs of running lean.
I will also recheck the timing - its pretty far advanced, but I have no problems around the hills by me, but around my house I can only get about 2-3 minutes of pure full throttle under high load.
the spark plugs Im running are rj8c
the engine is a 1958 261 with a high compression head.

Jay, the symptoms you experienced are exactly like what my truck was doing a couple of years ago and it turned out to be a failing fuel filter, most likely effected by the new ethanol gas. Middle to top of any long grade and it would stumble.
Just a thought...

thanks billy, i will make the fuel pressure gauge visible in the truck
and examine the spark plugs to get a baseline
then the first thing i will change is the fuel filter.

i think i found the issue.
1) replaced fuel filter - no change
2) reduced timing - no change
3) replaced front carb - big change

im running 2 yf2100 carbs.
yesterday it started spewing large amounts of gas out of the base. and i mean a lot. i replaced it. i think it is clogged somewhere and was running lean.

more power too. i just drove it on the freeway over hills that i normally have to downshift, didnt have to downshift with the replacement carb. less stumbling on initial throttle use as well.

lets hope i didnt burn a valve while it was running lean

Jay,

It is always good to find a definitive problem. Hope all is good with your engine.

I look forward to 150 mile road tests.

Paul