Looks like I have a dead cylinder. I thought that changing the plugs (and makign sure that the plug wires were staying connected) took care of the problem. I didn't have a chance to start it after I changed them out. I fired it up last night when I got home from the gym and it sounded like crap. Here's my symptoms... hope someone's got some advice.

Engine seems to start OK though the idle is a bit rough (making sure that the plug wire was attached on number two did add to the "grunt" at idle that had been missing on my last trip out). Rev'ing it causes a shudder and shakes the entire boat. You can definitely hear the miss. Could hear that hissing noise still. Took off the flame arrestor and the sound was definitely coming from there. As you try and rev the engine the sound seems to get worse/quicker. I have not had the valve covers off to see what's going on in there (and to time the interval of the hiss to a valvetrain event) but a first guess would be that the sound is being caused by a change in vacuum, probably a valve that is not closing; ie broken valve spring (on the exhaust side) or whatever have you. The engine is quiet though and exhibits no ticking or clanging. There is also no burning oil (a holed piston was my first worry). Just a dead hole and a distinctive noise coming from the carb. In question of the valves themselves, is it possible to burn a valve in a short period of time (less than 15 hours, if that). I worried, can't you tell?

I'm going to run a compression test on it tonight if my wife will let me out of the house. I'll also yank the valve covers to see what I can in there. Any ideas right off hand? Does it sound screwed or what?

One more thing... when I changed the plugs, I found that five out of the eight were loose... some even finger tight. When I pulled them, they all looked good except for one. The one (number one) had a slightly burned ground strap. The damage though was so, so slight that at first it was barely noticeable. The very tip of the strap was pitted. The electrode also had some debris on it but very, very minor. I've heard that if a plug is loose it could cause the ground strap to burn. All of the other plugs came out perfect with a nice tan coloring and no evidence of burning. The reasoning behind the loose plugs is probably my own fault having not tightened them properly. I had a problem before with trying to pull plugs that were too tight... they would break coming out. Can't have that happening on the water so that's potentially an issue. I have heard no audible detonation or pre-ignition since getting rid of the old gas on the first trip. Just thought I'd add that info.

Thanks!

This may seem overly simple, and I'm not some life long engine guy, but make sure you are not arching a plug wire while under load. Chekc this by starting the motor at night and reving until missing, while looking for an arch. Or just change the wires out. Also, firing order correct, cap and rotor okay? Maybe just a simple ignition glitch. Also, check for contaminated ignition wire from harness to box (factory uses gray wire for ign), or power supply to box, etc. Not sure whether this is remotely helpful, but just my .02, later.

Oh, as far as breaking a valve after 15 hours, I would think anything can break at any time.

do a leak down test, if you do not have that equipment, check compression, hook up a vacuum gauge on a vac. source, it should be steady ,if it is bouncing around you have something going on in valvetrain, like burned valve, broke spring, flat cam, good luck.
Mark

If it is missing at idle, why not isolate the cylinder that is missing? At least you will have a place to start looking rather than at the other 7 cylinders. With it idling, just pull off 1 ignition wire at a time off the distributor cap and listen for the r.p.m. to drop. Which ever cylinder doesn't give you a drop, is not firing. Listen carefully, sometimes it can be tricky to hear.

Is the that cylinder firing at all. A quick way to tell is attach your timing light to that cylinder. If its getting any spark the timing light will fire. If not well nothing.

Check the distributor. Arent you using the stock Thunderbolt IV? What kind of shape is the rotor, cap, and also the magnetic pickup in. If the pickup is rusted it can cause problems.

You said earlier that you had bad gas and you were getting detonation. Have you changed your plugs since you burned off the rest of the old stuff? Did you paint the heads? I would assume yes. If there is paint built up on the threads of the plug holes you may not be able to get the plugs to seat well initally and they just worked them selfs out.

But first, check the cap and rotor, and pickup. Then see if that cylinder is getting any spark or if its missing. Then if all is well then pull your valve covers. Start easy and were the spark starts and work forward. Also do a quick compression check on that cylinder.

A cylinder balance test is definitely in order... I got zapped last night trying to check it (I guess the insulation on my pliers just ain't what it used to be) so I left it alone. Anyway... I'll check out the little things first. I just changed the cap and rotor before going out the last time. I'll look at the distributor as well.

Any ideas on what would cause the funny vaccuum noise from the carb? It's strange... almost like a noise I've heard before at some point and time (not with any of my motors). Very strange.

Tom Z, Read electrode gone from spark plug post!

Yeah, been there, did not like what I read.

However, after looking at the damage and comparing it to my one ever so slightly "nicked" plug, I don't think (hope I'm right here) that the problem lies there. If something we damaged enough to burn a valve or hole a piston, then I think I'd see the damage seen his plugs. My plugs are all still serviceable, none of the porcelan is damaged, and there are no deposits on the porcelan... just on top of the one electrode under the burned ground strap.

Guess I'm hoping that it's something simple. I'll find out soon enough.

Is your motor stock or what did you do to it? Did it blow up before? Was it modified? Compression check is easiest to do, if that's OK you have it licked. Good luck. JB

Engine is a 383, 10:1, Vortec heads, Comp Cam, etc.

First engine detroyed itself (wrong bearings, bad align hone, destroyed the block, not my work). This one is the replacement.

Yeah, dreading the compression test. Hope it's something ignition related, but I really doubt it. We'll see. If it's not raining tonight, I'll probably mess with it.

Later,