Need help trouble shooting 1925 4 cylinder

[Leif in Calif]

Hello! I am responsible for a 1925 Chevy truck which belongs to the City of Petaluma CA (see general discussion forum "Restoration vs. Preservation" for a picture). Anyway...its developed what I would call a "High Speed Miss' (high speed is relative in this case). It idles OK and if you drive a low RPMs is seems OK but at higher RPMs there is a definite miss. Am I correct in assuming it's probably an ignition issue? I did take a quick look at the points and they seem to be opening correctly and aren't pitted. What should be the next step in tracking  down  the cause? 

[a sell]

So a miss could be caused by many things.  Poor wires, Poor plugs, Lean fuel, low valve clearance, vacuum leak, not enough or too much advance, low float level, Low lift on one cam lobe, ignition points, distributor wear, low compression on one cylinder.  The list goes on quite a ways.  Or a combination of any of the above.  Start at the top, start in the middle, or start at the bottom but pay attention and mark them off so you don't get frustrated.  You could start by shorting one plug at a time with a long screw driver.  If you find a particular one that does not slow the engine, then switch it with a plug that you found as good.  If it follows the plug then change that plug.  If it does not then do the same function with the wires.  Make sure the wires are real wire cored and not the carbon fiber type.  The four cylinder plugs from 28 and earlier should be long nosed because of their location in the combustion chamber.  The six cylinder plugs for 29 and later will generally be different.  The Filling Station in Lebanon OR carries everything you might need, or Gary Wallace at 20schevyparts.com  Good luck, keep us posted.  Whatever you find and report may help someone else. 

[Leif in Calif]

Thanks for that...it doesn't consistently miss at idle so I can't short out cylinders individually. This vehicle only sees about 20 miles a year ( just parades) and was rebuilt about 10 years ago so I'd like to think I can rule out mechanical issues, which leaves us with fuel and spark. Is there a way to test fuel delivery? It has the vacuum type "pump" on the firewall. 

 

Leif

[Guest]

It probably just wants to be taken out and driven.

[a sell]

Bear in mind that when the vacuum tank is NOT drawing gas from the tank you will have a slight vacuum leak that may make it seem to be running a little rough.  Especially at idle.