The Stovebolt Site Forums The Shop Area Engine Shop 55/56 GMC 270 swap for '53 228

I am replacing a sick 228 with a 270 in my '53 GMC 1/2 ton. The truck has the hydramatic trans which I want to keep due to it's uniqueness. The 228 engine Zenith carb had the automatic choke which I understand were standard with the hydramatic. I swapped the 270 Zenith throttle body, venturi and jets to the 228 carburetor. The engine runs well at idle and higher rpm but when driving runs very rough but I can't be certain if the problem is fuel or timing. One mystery I haven't resolved is not getting a spark from the coil wire to ground when setting the points, only a very slight spark on the points when they open and close. I would expect a bad coil but the engine does run well as described above. So, a few questions I am seeking help/advice...were the carb modifications correct to use the 228 carb on the 270, is a bad coil the probable problem with setting the points and is the correct position for the timing pointer at 5 degrees BTC for the 270.

Carbking or another carb-knowledgeable will address the carb issue. It sounds too simple to me that a swap-over of those few parts will make the 228 carb satisfactory on the 270 (but, your logic/action might be correct).

My friend with a 270 engine in a 53 GMC 1/2 ton long-bed used a GMC Holley AA-1 carburetor with adjustable jets. It ran fine (he now has a 302).

You might want to adjust the carburetor and timing on a warm, running engine, at idle speed using a vacuum gauge (adjust to maximum vacuum reading at the wiper port on the intake manifold).

I hope Carbking or someone can comment on the modification. Beside the engine running rough we also had difficulty getting the idle speed below 500 rpm, recommended idle speed with the hydra-matic is 400 rpm.
We did set the carb with a vacuum gauge. I also resolved the coil question...I wasn't holding the coil wire close enough to ground and initially the sun was shining on engine making it difficult to see the spark. I am also going to do a compression check to make sure the valves are closing. The cam grinder specified .012"lash on both intake and exhaust. Original spec was .020" on the exhaust.

I'd stick with .020" on the exhaust.

I agree with Cletis. A GMC was meant to have .020 lash on the exhaust. Remember you are dealing with a GMC, not a Chev. What kind of plugs are you using? I always had my best luck with an AC45 in a 270. The book may call for a colder plug, but they assume that the engine will constantly be at full throttle under full load the way we drove them 60 years ago. I doubt that you drive your like that. I would also check the points for dwell. Are you running the 270 distributor or the 228 distributor? If its a 270 larger truck engine, the distributor was not designed for a vacuum control. Using a vacuum control advances them too much.