Herculies 6-cylinder Chris Craft
 

Hey guys, sorry if I'm messing up the spelling on this. I'm at a family member's who have owned a 1957 Chris Craft for many years. The engine is a flathead Herculies 6-cylinder with 2 carberators. It is a 135 hp model, with a newer electronic ignition.

The engine is misfiring under load, and running rough at idle. We changed the sparkplugs, the rear 3 were considerabley fouled (carbon only - no oil) and the gap was around .46 thousands. We installed the new plugs at .35 gap and found that it ran better, but stil has a mid-range miss and is 'lopey' at idle. It's not smooth the way this engine has typically been. This boat is in perfect shape - beautiful in fact - it would be nice to get the engine running the way it typically does. Any thoughts or suggestions?

did you check the timing?

It would seem like a fuel problem to me.If you have two carbs and half of your cylinders are running bad it is not too much of a stretch to suspect that maybe the carb that feeds those three cylinders might be part of the problem.
Do the carbs feed a common intake manifold or are they separate?If ignition were the problem the three good cylinders would be bad too.I would also consider bad fuel but carburation would be on the top of my list.Can you swap the carbs with each other and see if the problem follows them ?
If I removed the carb I think I would just rebuild it and be done with it.Good luck.Ron G

I grew up with a '49 Chris Craft 20' Custom Runabout, and a '48 Chris Red and White - 25' Express Cruiser that both had the 130 hp Hercules - ours had the single carb version.

The R&W had a miss that was traced back to a bad coil. It was failing most noticeably under load. This was back in about 1970 and it was still the original 6V system; but none of this may apply since you said your friends newer boat has an electronic ignition on it.

Yeah, when running right that old Herc should be able to idle smoothly down to under 400 rpm to the point where you can almost count the teeth on the flywheel as they go by.

Thanks for the suggestions so far - I'm not there now but I'll e-mail this info to the owner. I believe, but am not certain, that it's the dual intake system. I would also think it's one of the carbs that needs a rebuild. That would likely do the trick.

The timing would be easy to check, as bulabula stated, with the engine cover open you can easily see the flywheel and the rear of the starter motor. Also the belt-driven generator is right on top. Have to love a simple engine....

Is it raw water cooled or closed system? How many hours on the block and heads? If raw water (even in fresh water) you might want to do a compression test on all cylinders and make sure there is no evidence of water in the oil. Water in the combustion chamber will foul plugs very quickly. Though the engines I worked on were in a salt water environment, even fresh water takes its toll, especially for a boat in the northern climes that has to be winterized. It is hard to get all of the water out of the lower cylinders (meaning physically the lowest as most inboards are angled). When you say the rear three plugs were fouled, did you mean on the lowest part of the engine (depending on mounting, it could be a v-drive or direct out the stern so I'm not sure which you mean by rear)? If so, you might check for a crack in the head or block (head usually goes first on v-8s but I'd suspect block on 6 cyls.). It is very difficult to diagnose. Compression test and any white milky stuff in the oil pan would be a clue. Good luck!