1977 F350 not enough pulling power HELP
 

I am restoring my 6 cylinder 1956 Chevrolet truck and got myself a temporary replacement.

It is a 1977 Ford F350 that was originally a wrecker, or tow trucks as we call him here in Australia. It has a 351C and someone has recently converted from 4 speed to auto. It does not run on petrol anymore, it has a Liquid Petroleum Gas (LPG) system fitted.

My problem is that at a steep boat ramp, the truck will not pull out my 26ft powercat that weighs about 3500kg (7000 pounds?). The Chev had no problems, even with my bigger boat and even our automatic Range Rover has no problem getting it out of the water. Last weekend we had to use the boat motors to push the boat, truck and trailer out of the water. The truck just got to a point where it loaded the torque converter and wouldn't go anymore. No wheelspin, just no go.

Later it also boiled the coolant just maneuvering around the yard trying to position the boat. It doesn't overheat at other times and has a new heavy duty radiator, twin thermo fans etc. It tows great once going, it is just working way too hard trying to get off the mark.

I guess the questions are:

1/ Is this normal, do automatic F350's struggle of the mark?

2/ Any suggestions on what to do?

First off, the 351 wasn't anywhere near a powerhouse- but it should still get a boat moving up a ramp. I'd start with the basics- beginning with a compression or better yet, a leakdown test. Then I'd find someone knowledgeable with propane and have them make sure the equipment is working. If your mixer or regulator have issues, you could have flow restrictions to the engine.

Is there some reason you'd want to retain the propane?

I had an old F250 with the 351 auto and same problem. It was the tourque converter something to do with the stall speed. I traded it in on a new truck with the 300 6cyl and 4 speed, 4X4, just put it in 4 low and used the granny gear, could pull anything up the ramp.

Bowtie, I was thinking the same thing with the torque converter, maybe I should look at a higher stall speed.

Chris, to go back to petrol I need fuel tanks, lines, carbies, basically everything and with petrol prices are something like $6 a gallon, it would make this truck not usable on a day to day basis.

I am thinking that step 1 is compression/leakdown test.

Step 2 get the propane system looked at by someone who knows what is going on.

Step 3, investigate if a higher stall converter will get me moving

Step 4, see if diff ratio swap is feasible,

Step 5 get Chevy back asap.

Definately got me curious! What kindda ratio ya got in the rearend? I know propane cuts a bit a power but being a 350 the gearing(ie converter) should at least get ya boat outta the water! Does sound like a motor issue to me.

Not sure of the ratio, but turning the wheels 1 revolution, the diff yoke turns about 4 1/2 times, so I am guessing about 4.5 ratio.

No tacho on the truck, but the converter sure seems to kick in real early. Not sure if it is down on low end torque, or is a very low stall converter. I did chat to an automatic transmission specialist and he seems to think that I should be looking at the engine side.

Acceleration is not too bad, it can almost keep up to econo cars on a traffic light drag. Feels responsive too.

Are you SURE its taking off in 1st gear? Older Fords will take off in 2nd gear(AT)if your shifter is in 2nd or your linkage is inncorrectly set up

I'm kinda curious, our Toyota Forklift at the shop developed All the same symptoms as your truck over the winter this year. Its also ran on LP.

My thoughts too.

You are right, it does start in 2nd if in "2" and yes it does feel like it is starting in second but I have counted the gear changes and I get 3.

Maybe when at that particular angle at the boat ramp, it moves the linkage just enough to put it in 2nd. The shifter is something that was adapted to fit and the workshop that fitted the auto kept it there for a month until they 'got it right'.

It is Saturday midnight here, so I am going for a drive to count gears again but this time on a very steep hill. I will keep you posted. At least that is what I am telling the other half...