Big Cam Cummins has low power

You didn`t say what kind of terrain.not enough info about truck specs(trans,rear ratios)are air filt new?Sometimes not as clean as they look.Also fuel not near what it was in 1977.older trucks dont perform as well.fuel line could be collapsing internally(hard to diagnose)without knowing condition of your engine there are lots of things that could be wrong(same with a new one)THere are ways to boost power but you could severly damage the engine if not done correctly.My advice-find an older mechanic who specialized in Cummins big cams and let him tweak it to your needs.you will be happier with the results,and wont be as likely to screw something up.big cams are the best overall engine ever made as far as i`m concerned.I have ran them in few of my trucks and got better service than ANYTHING i`ve ran since.Good luck,be patient and set your cummins up the right way.you`ll be glad you did.

 

I can't remember the name of the part that is bad,(getting old) but I think it is the AFC valve, or something like that. It is located on the fuel pump and controls the speed in which fuel is given to the engine when the accelerator is pressed.

There is a rubber diaphragm in the AFC, and when it goes bad the truck will only run like it is getting 1/3 throttle. I had this happen to a 400BC, and it ran like a 250.

It is easy to fix, the pump does not have to come off, and your truck will run like new when this is fixed.

 

Well I do know a little bit about that engine. First of all how many miles on the engine. There are several things that can cause your problem. You first need to determine the contiditon of the engine, meaning compression. If you have low compression it won't matter how much fuel you feed it, it isn't going to run right. I have been a truck mechanic for over thirty years and I even find other mechanics that the first thing they want to mess with is the fuel pressure, and injectors without ever running a compression check and possibly a leak down test. If you have good compression then is the time to start looking at your fuel system.

 

I'd like to know what you are comparing it to. afterall it is a 300 hp motor and the big cams had much less torque then the new motors do and you are killing it with that 10 speed. the big cams like to humm along at a fairly high rpm and don't have a very wide power band. and being a 77 I don't think it should be a big cam III unless it's a transplant. My 86 had a big cam III originally and now had an 82 big cam II in it.

Old wrench I wish you were closer I'd like to bring my big cam to ya to be worked on. these young guys look at me crazy when I pull into the shop with it. if they can't plug a computer into it and have it tell them what's wrong they can't fix it.

 

Replace the diaphragm anyway. It could be stretched. They are cheap and easy to change. I have seen them do what you describe and look OK, but once changed the truck ran great.

Is your AFC the old style that connects by lines and hangs behind the pump, or is it connected directly to the pump.

The reason I ask is the Big Cam III engine did not even exist in 1977. I am not sure the Big Cam was even out in '77. I believe it was '78 or '79, but the odds of your engine being original are slim anyway.

The not pulling past 1600 keeps me going back to the AFC circuit. If the compression is low you would have a lot of blow by out of the tube, but still should run better that you say it is.

I have driven a big cam 300 quite a few miles, and they will pull good when right.

 

Unfortunately that is the way all shops, truck or automotive, are going. The days of being able to diagnois an engine by hearing it are over and those mechanics have long since gone.

 

they aren't gone but they are getting rare... (and not getting younger either)