The carb on my '78 parts truck finally crapped out. I was running the engine about once a month to keep the battery charged and internals lubed, when I went to run it last time it would not stay idling. I suspected the carb as all else checked out (fuel, spark, etc) I pulled the carb off and decided to put a kit in it. I also have another monojet off the project truck a '76, but it has been off the motor over a year and the last time I ran it it was running rich.
Here is what I have:
From the 76 carb# 17056002 1MV. Per GM this a correct carb for a 76 250 with automatic and no AC.
From the 78 carb# 17058021 1ME. Per GM this a correct carb for a 78 250 with manual trans and no AC.
Both had obviously been rebuilt at least once in its life time, the screw heads had the scares.
Here is what I noticed:
The '78, with electric choke had a 103 jet and 90 rod while the 76, standard heat coil choke, had a 113 jet and a 86 rod. I measured both rods and jets to verify what I thought was stamped on them. I measured them +/- .002 of the assumed markings, allowing for wear and measurement error this seemed reasonablely correct. With these combinations the 'opening' at part throttle on the 76 is almost 200% larger then the 78. Of course this assumes that no one has re-jetted either carb.
Question 1, is it normal to see that big of a jet/rod combination difference for same size engine in roughly the same applications? I know that the 78 probably had more emission standards and less output, but that seems like a tremendous difference to me.
Question 2, is there any reference for carb jet and rod sizes used by GM on monojets?
Question 3, which combination should I use, assuming I going to rebuild the 78 model with electric choke for use on mostly stock 250 with a automatic and no AC?
Any other monojet suggestions and references would be a great help too.
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