Posted by bookel on November 22, 2016 at 05:11:01 from (73.147.215.129):
HI all. New to the forum but this looks like a good place to ask. I have a 1983 GMC C600 school bus. I use it occasionally to haul. I'd noticed for a while now that when trying to get over 40mph it accelerates slow at this point. Maintaining over 35-40 is hard to do as well. Starting out in lower gears she pulls up fine. Its getting into 4th where she falls on her face. I have great vacuum according to the gauge and it hasn't changed from before. I checked for vacuum leaks and did tune up with plugs and wire. No effect. Then I checked the vacuum secondaries. Its the stock holley 4150EG. When I open the throttle by hand under the hood the secondaries move freely. I pulled the secondaries vacuum linkage from the carb. When I work the diaphragm and plug the vacuum ins and outs the rod stays up and doesn't leak down. I put my manual vacuum pump to it and this lifts the linkage no problem. I re-installed everything and tried the paper clip trick to see if the secondaries would engage but still no luck. I am sure I have no vacuum leaks so I don't know what else to try. I can get a rebuld kit for the carb but other than the secondaries not opening it runs great.The kits are $35. Or I can get a used Edelbrock performer 600cfm which is vacuum secondaries for around $100. I've heard this is a good upgrade fora carb on this engine but not sure which I should do. Rebuild and it still might not work or just slam on a different carb and hopefully be done with it.
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