Quote:
Originally Posted by massey478
'Back in the day', 1970's, when I worked at Meyer Motors GMC/Allis Chalmers dealership we would have complaints about these motors. We would drill out the main jets about .010" size increase and power would come up, run cooler, and mileage would actually increase because the driver did not have to have his 'foot in it' so much. When the weather conditions were right we would have motors 'backside' pistons, burning them. After repairing the damage we increased jet sizes and the problem never happened again.
For my Massey with the 478 (hence my name) I had several drilled jet sizes I tried until I got what I wanted. There is also an enrichment piston in the carb that drops dowm as engine vacuum decreases, opening an enrichment jet to give the motor more fuel. My 478 would drop more RPM than I wanted it to in a herd pull before the enrichment came in so I stretched the spring on the rod for the enrichment valve until I got it to come in at less RPM drop. When it let the RPM drop too far, when it came in the power boost would cause the tractor to spin out. I also throw away the timing light and 'power' time motors, moving the distributor and trying it until I get the proper performance. We also do that with out street/race motors then mark the distributor base so we know where to set it. I also find that the standard ignition system with a MSD 6 or MSD 6al and the associated MSD coil added will help tremendously. These units give five spark pulses per plug until they reach 3,000 RPM at which time they give one. Much better than Pertronix. The MSD 6al is different from the MSD 6 in that the 6al has chips that can be plugged into the side that limit the high end RPM the motor is allowed to reach. Made the motor run much better. I think the 5 sparks per plug help these motors light the fuel more efficiently in the large bore size they have. Believe me, you do not want to get shocked by one of these units! Big no-no! Just my experience here. Others may be different, but it has always worked for me on these motors.
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I can give that a whirl. The jets that were in the original carburetor I messed with some and opened up last year. Probably to around .057+/-, and it made a noticeable difference when I did that. Compared to those modified jets, with the new 57's I have to keep my foot into it a lot more and as you said, I'm cooking through gas. For the few flat stretches of road here in NE just touching the throttle keeps it at 45, but most of the time I'm laying into it keeping it at speed or climbing hills.
Power piston I'm not sure if it's working. I'm matted it a few times and there is no noticeable difference that the plunger has hit the valve. Might not be coming down... or heck, who knows... might not be moving up. But I know my vacuum is good since the truck is hydro-vac brakes.
Thought about the ignition upgrade just wasn't sure what to get. I'll take a look into the MSD's!
Quote:
Originally Posted by jimjaz
I see Ken has mentioned exhaust.....
Back in the early 70's, I took my 63 GMC with 305 V6 to Midas Muffler and had a dual exhaust setup installed. Don't know if your big 5000 has the same single exhaust like all the 305's did coming from the factory that way.
That being said, I went from a steady 11 mpg to over 13 with dual exhaust and nothing else- nothing crazy- just two inch pipe through some low back-pressure type "turbo" mufflers that do not make a bunch of noise like some do today, and I noticed a BIG difference in how the engine performed, and it absolutely was better on the hills. The engine just breathes so much better. It was the best $80 I ever spent (back then). In my own situation, I had the mufflers installed outside the frame rail and dumped the exhaust out the sides at a 45 degree angle in front of the rear wheels/tires. Sounded great too! With a 20 percent jump in fuel "economy", I was impressed with this change.
Something ELSE to consider......FWIW,
JimJ AZ  |
I still have the factory setup on this one (seems like factory up until the Y). 2-1/2" pipe out of the manifolds, Y's under the drivers side and then runs 3" pipe about 6ft back into a muffler. So it might not be a bad idea to put dual exhausts on it with pipes straight back. Seems like it breaths pretty well with what it has but it might not be enough.
Thanks guys, I'll start with modifying the old pair of jets in the next week or so and I'll report back.