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Old 06-09-2013, 08:55 AM
jtomasik jtomasik is offline
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Runs rough/intake popping when secondaries kick in.

Hey guys, just joined. I'm a boat owner with a 351W that has a Holley 600cfm on it. Posting here because I need info from guys with a lot of engine/carb experience, and I don't see much of that on boating forums.

So, I bought the boat late last season, and I only used it twice. Honestly, I'm not sure if it did this last year, as I might not have been getting into the secondary carb circuit, 'cause it goes pretty good down low, and we were only wake surfing with it (which doesn't require a fast start).

I'll give you all of the info below, but now I'm to the point that even after rebuilding the carb (it was horrid inside...very dirty...and it had this problem before I rebuilt it), when I the secondaries start to dump, the engine 'pops' and runs rough. It's like it won't take the fuel. I think the rpm's are about 4000 before I hit the secondaries, and it runs very smooth until then. It starts fine, it accelerates fine (note that I'm testing out of the water), but once those secondaries open up, the issue occurs.

That's the Reader's digest version....here are the details that've lead me to this point:

So, I put the boat in storage over the winter, but I didn't drain the fuel. When I brought it out of storage, the boat wouldn't start. There wasn't any spark coming from the coil. This is a points ignition system. I checked the points, and they were fine. I bought a coil, and the boat started. However, when I took it to the lake, it would start to stumble after running for a few minutes. It got to the point where I couldn't start the boat.

I got the boat home, found a kinked fuel line, it still ran like crap after fixing that. I checked the timing, and it was about 10 degrees ATDC. I adjusted it to the spec (I believe it's 6 degrees BTDC). It started and seemed to run fine, except at high revs it'd start exhibiting that popping situation. I assumed it just needed to get into the water and run some fuel through it. I did check the fuel filter, and it looked clean.

I took it to the lake, it ran ok, but when I tried to put it into full throttle, it'd run like crap. I cruised around a bit more, tried again, and it ran so bad that it stalled and I couldn't get it started. When I looked at the carb while trying to start, fuel was dumping into the carb at what seemed like a high rate, so I thought I had a hung fuel float. I assumed it was time to rebuild the carb...thinking that crud got into it.

So, I brought it back home and I rebuilt the carb. I pulled the fuel lines after the filter, blew them out, and I put the carb back on the engine. Although it starts and runs well, I noticed that as soon as the secondaries opened, the problem would occur. Before deciding to replace the carb, I checked to see if there was play in the timing chain (assuming it might have a slipped chain - I checked this by rocking the crank back and forth, but the rotor shaft seemed to move immediately), checked plug gap (they were in good shape but over spec...set to 0.034"), checked the points (0.016"), and I sprayed starting fluid around the base of the carb to see if there were any intake leaks. Everything seemed fine.

One weird thing I ran into was that I checked the timing, and it was ATDC. Maybe I missed something like not tightening the distributor lock bolt enough, or maybe I had a couple too many brews. I readjusted the timing, and it runs fine....until those secondaries open up. I'd pop the valve covers, but two bolts are covered by these huge cooled exhaust manifolds, and I might have to remove those to get at those bolts. I'm afraid that trying to remove those exhaust manifold bolts is going to result in a broken bolt...which is no fun to remove, as most of you already probably know.

Note that I haven't tried any changes to the carb. I'm in Denver (5280 ft. altitude).

So, before I drop $600 on a new carb, I thought I'd pass this along and see if someone here could help.
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