Hello all, awhile back my neighbor recruited me to help work on a 48 ton and a half with a late 50s 261. After giving it a tune up, carb swapping and fuel pump it starts and idles fine. Anything above 1/4 throttle and it starts missing whether or not it's under load almost like you flip a switch. We're both scratching our heads on it. Exhaust is unrestricted, can't find any vacuum leaks, and timing is set correctly. The vacuum advance port is capped at the carb because of a bad diaphragm. Any ideas on what to check? Thanks- Jordan
Either way it could be flooding. Stuck float, debris in needle and seat, etc.
1950 Chevrolet 3100 (Ol' Roy) 1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1956 Cadillac Sixty Special Fleetwood (The Godfather) | 1966 Oldsmobile Toronado (The Purple Knif) | 1966 Ford Mustang (Little Red) | 1964 Ford Galaxie 500 coupe | 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
There is a difference between 261 Rochester carburetors and 235s, so if it is a carburetor issue, it's probably under powered. For a 1961 chevy 235, the main jet is .055. For a 1961 chevy 261 the main jet is .063. I put a 235 carburetor on mine to get it going, it worked ok, but accelerating off idle it would stumble and have a lean condition, would backfire. When I had it rebuilt the mechanic drilled the main jet out for me. Pic is from 1961 chevy truck shop manual supplement. yours is a bit older but should be the same idea.
Check the points to make sure they have enough spring tension. Some brands of points have a copper conductor strip, and a steel spring that can get misplaced. Without the steel spring, the engine will idle, but the points start bouncing and messing up the ignition timing at higher speeds. A bad condenser can also cause the same kind of problems, and they're notorious for being bad right out of the box. Jerry
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I'm with Jerry on this one. I saw it happen 7+ years ago and again recently on this forum. If you have a dwell meter you can watch the dwell get less and less as the RPM increases. It should stay constant.
Last edited by Bill Hanlon; 11/13/20225:48 PM. Reason: spelling
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
