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| | | | Joined: Oct 2006 Posts: 99 Wrench Fetcher | | Wrench Fetcher Joined: Oct 2006 Posts: 99 | I have a Holley 1904 on a 1955/56 GMC 270. On the bottom of the carb flange there is a vacuum outlet/inlet port that the gasket seals. Gaskets I have seen online have four cutouts that would appear to allow engine vacumm to connect to that port. I am getting terrible gas mileage- 6.1 city and 8.1 highway, have been thru several carb rebuilds. The engine runs fairly well, seems to misfire a little, and it backfires occaisionally on acceleration, and when I shut down the engine, there is a lot of gas vapor in the carb. The float level was off, still may be. Should I connect the vacuum port via gasket cutout?
Last edited by Dave Douglas 56; 11/17/2012 11:38 PM. Reason: spelling
Well, my time went so quickly,
I went lickety-splitly
Out to my ol' 55.
As I pulled away slowly,
Feelin' so holy,
God knows, I was feelin' alive..... Tom Waits, Ol' 55
| | | | | Joined: Nov 2005 Posts: 6,100 'Bolter | | 'Bolter Joined: Nov 2005 Posts: 6,100 | Hy Dave Douglas 56, I'm not at all familiar with that carb, but I'm wondering if that port connects to the power valve system, the same way that a similar port connects to the power valve system in the Rochester carb. With that port covered in a Rochester, the power valve is always open, because the valve is not connected to engine vacuum. Hence fuel mileage will be low and up here the vehicle will not pass an emissions test, hope that helps. | | | | | Joined: May 2006 Posts: 8,351 'Bolter | | 'Bolter Joined: May 2006 Posts: 8,351 | You're right on the mark, it is for the Economizer Diaphragm, which operates the Power Valve.
Another thing that might cause the symptoms you describe, mainly the backfire on accelleration, is a lack of timing advance. Many times that is blamed on the carb, when it is actually an ignition issue. Check the vacuu advance for proper operation, as well as your base timing. You may need to advance it a few degrees.
Bill Burmeister
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