Sputtering, bucking at 70 mph uphill

[Winslow]

New member. I have a '70 F100 (302 V8, auto. trans.) that runs fine except when going up over mountain passes, etc. where the speed limit is 70 mph. Sometimes, the slope causes me to begin to lose speed and so I give it more gas. I think at slower speeds, it would downshift from 3rd to 2nd, but according to the manual, it won't downshift over 65 mph. As I give it more gas, it starts to sputter, buck and lurch. At that point, it seems like it's flooded or something, because even slowing down doesn't solve the problem for some time. My mechanic put in a new fuel pump (which includes the filter) but the problem persists. Seems like I should be able to drive in all conditions at the speed limit, but maybe I need to hold it to 60 or 65. Preparing for a trip that takes me over the mountains and hope to avoid this problem. Any thoughts apreciated.

[Wes]

what is the elevation of this pass. Your carb has a fixed air fuel ratio. You may be experiencing to much fuel and not enough air. Not much you can do to fix this unless you live 4000 above sea level then you can jet the carb for higher elevations. It may be possible to compromise on jetting. You might want to check your secondary ignition, It's possible the spark is breaking down under the sever load.

good luck

[mrtleavitt]

check your jetting. Most carbs run rich to begin with, rises in elevation makes it worse. It would be ideal to hook up to an air/fuel mixture gauge to find a perfect carb setting for your home town elevation and then you'll just run a little rich on your passes. My guess is that you're running rich in town and super rich over the mountains. I live at 4,000 ft elevation and on my holley carb I jumped about 6 jet sizes smaller. I can't wait to move back home to Vegas at about 1,000 ft to open the jets back up and gain back some power!

[Winslow]

Thanks very much for the replies. I have had this problem on the steeper sections of highway approaching Snoqualmie Pass, WA (3,022 ft.) and also on the steep climb from Vantage, WA (663 ft.) up to George, WA (1,227ft.). I will follow up with my mechanic on your thoughts.

Winslow

[Dragon]

Not the carb jetting at that altitude. The fact the engine got hotter with all that wide open throttle then has to cool down to run right again. With wide open throttle you lost vacuum advance. All that load and heat probably hit you one place you did not expect,the coil. Most carbs will climb to 9000 or more before they get sick. I am a Colorado native by linage and flatlanders could get to 9000 or 10,000 before the carbs made them sick.

I drive at 5500 feet all the time and my holley is 2 jets down from 72 to 70 now and I am still a hair on this side of lean. North to Santa Fe is 7500 with about 8500 ft pass in the way and East is almost 9000 and West is 8000 from this valley. So I don't get too much worse treatment. My 69 f350 360 when loaded to 8000 gvw with stock coil and Accel points eliminator did that one after noon coming south. I got done with that job and put an autolite reman carb on then did it the next day, same problem. I did have a bad carb before and it did it again. I stopped and picked up a 8140 Accel coil and it went away the 3rd and final day hauling that load of tools south to home. Mornings were cold afternoons were almost 100.

Take a hot air gun and set you idle high with the truck parked and heat your coil up and see if it gets sick.

[mrtleavitt]

Ya that's not enough elevation to give you those problems due to jetting. I agree with dragon, check for electronic problems showing themselves when things get hot.

[Winslow]

Thanks so much. Will check that out.

Winslow

[Winslow]

Thought I would report back in case anyone is curious about the outcome. Dragon was right. Had a new coil put in this week. Took the truck up to the top of Snoqualmie Pass yesterday at 70 mph as a test. No problems.

Thanks again,

Winslow

[Dragon]

No problem