The Stovebolt Site Forums The Shop Area Engine Shop Looking for More Umph......

I've been really enjoying my 59 GMC these past few summers. I've taken it on seven 400 mile round trips up in the sierra Nevada mountains to fish, each time cresting a 10,000' pass. (With all drum brakes....) There is one particular stretch of 15% grade early on that really bogs down the motor. (At fairly low elevation.) Carrying a load, the rpm's get low enough that in a few spots I have to shift into 1st and move over as other cars are doing 60 plus mph. Every time I slog through this 1 spot I wonder if there is anything I can do to this motor to give it a little extra power. Just enough to -not- have to reach into 1st to keep moving.

It's a bone stock 270 with the Zenith single downdraft. Aside from installing a dual M/C and radial tires, I've not done anything to it. Wondering what my options are. At 140K original miles, it's not ready for a rebuild yet so I'm more interested if there are any "bolt-on" options that make sense. Split exhaust? Dual Zenith carbs? Re-jet the single carb.....? I know once I have to open this motor up there are lots of options, but I'm just not there yet. Any ideas? Thanks, -Steve

Your problem is the altitude- - - - -at 10K feet the carb is running WAY too rich due to decreased air density. My 59 Burb with a 283/3 speed setup used to climb Sonora pass and then go uphill to Leavitt Lake with no problems at all. Definitely DON'T get tempted to increase the main jet size. That will make the engine lose even more power. A Zenith carb with an adjustable main jet might help, but getting the adjustment right every time you start climbing hills will be difficult. Too lean, and you'll start burning pistons and/or valves, and neither one is cheap for GMC engines. What you probably need is some more gears, like a under/direct/over setup with a 3 speed Brownie between the transmission and the rear end. That would let you keep the RPM in the best power range without losing so much road speed. A 270 running at its HP peak should flatten those hills without much of a problem. Those engines won WW II in a whole bunch of different military vehicles.
Jerry

San Jose is saying it is running bad at lower elevations, if it is rich enough to loose power to have to go into 1st at a low elevation it would cease to run at 10000 feet. I say start by checking your mechanical advance in your distributor

That's Correct...... I really don’t have much of a problem driving up at 9,000’-10,000’ as the road is narrow and speeds are only 30-40 mph. My 270 with 3 on the tree can do that all day long. It’s a rather enjoyable ride. I run into trouble at the start of the grade. I’ll do 50 mph for hours crossing the valley, but once I hit the hill it bogs down……bad. In 10 miles or so I go from ~3,000’ to 5,000’ where traffic is still moving at 65 or better. I hit the hill doing 65 and watch my speed slowly bleed off and my temperature climb as I get farther up the grade. Just as I’m thinking; “This was a bad idea” the grade eases up and all is good. I’ve not checked the vacuum advance mechanism other than to acknowledge that I have one. If the diaphragm in it is shot and not advancing the timing, would that cause the motor to bog down so badly?

Leavitt Lake BTW is a spectacular place to visit if you can handle the rough trail and several creek crossings to get there. Jerry, is your Burb 4WD? Every time I go, I think that with enough ground clearance, a 2WD vehicle could make it. Never had the guts to put that to put that to the test. I did see a guy tow a 14’ aluminum boat on a trailer up there once though. -Steve

Also check for a restricted exhaust. If you have access to a delay-flash timing light, check for correct ignition timing- - - -initial, centrifugal, and vacuum. Total advance at light-throttle cruise RPM should be in the 35-40 degree range, and it should drop to the low 30's or slightly more when pulling hard at low vacuum.
Jerry

No, the old Burb was 2WD- - - -and the last time I drove that road was in the late 1970's when it was just a set of ruts between some big rocks. Getting in there was fun! In the early spring just after ice-out those fish were so hungry they would bite anything I threw at them. I carried a 12 foot aluminum boat on a set of cartop racks- - - -no trailer needed!

The vacuum advance is only there for fuel economy at light throttle cruise. As soon as a little load goes on the engine like passing or pulling a hill, the vacuum advance goes away. What you need to check is the centrifugal advance- - - -a set of springs and weights under the breaker plate where the points are. Lift the distributor cap off and grab the rotor. Twist it clockwise. The tip of the rotor should move 1/4" or a little more, and then snap back smoothly when you release the pressure on the rotor. If it doesn't, you'll need to pull the distributor out and dismantle it to clean up and re-lube the weights and springs.

Once you're sure the advance system is working, here's a way to check for over-rich mixture. Install a ball valve in the intake manifold fitting below the carburetor where the windshield wiper hose is attached, and rig up a push/pull cable inside the cab so you can open and close it while driving. When the engine starts to bog down, pull the cable a little at a time to create a small vacuum leak. If the engine is actually running too rich, leaking a little air into the manifold will lean out the mixture and increase the performance.

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Jerry

At Higher altitudes one can also advance the ignition timing, however, in your case since you are starting near sea level in San Jose, it would be a little awkward to keep fooling with it every time. I used to live in Mammoth in the early '60's, advanced my 134" Jeep engine that way.

You could also remove the head, do a valve job, mill the head .062" for slightly increased compression(new head gaskets are available) but then is that more than you want to do?

Ed

TURBOCHARGING.

You didn't say what gears you were running other than it had a 3 speed transmission. The GM corporate rear end was a 3:90 but some GMC's of that era had a 3:07 which wouldn't be great with a load at that altitude. You may want to look at some lower gears for the mountain trips and add an overdrive for the freeway driving.
Fred