98 Mercruiser 5.7 Thunderbolt V cutting out randomly

[ksmauck]

I have a 1998 21' Reined with a 5.7 GM motor that has a 2bbl carburetor and the Thunderbolt V ignition system. Last year I had some major problems with the engine and ended up having to replace the valves and repair the cylinder heads after I discovered the valves were tuliped. In the process of this repair I did replaced the ignition coil, plugs, plug wires, distributor cap and ignition sending unit. Boat is timed per the manual. I also had the carburetor rebuilt.

The boat starts and idles great. When on the water the boat seems to randomly cut out. Generally it appears to be when cruising around 3000 rpm but it's also cut out getting up on plane. It started this last year and several attempts to diagnose/fix it were made to include disconnecting the neutral safety switch while under way, (which didn't work), and bypassing the on/off tether safety switch. When I bypassed the on/off tether safety switch I took the boat out and could not get it to do it again on that trip. Stored the boat for winter and took it out 2 weeks ago for the entire day and again the boat ran flawlessly. Not one hiccup all day. Second time I put the boat on the water I was about 5 minutes into cruising at about 3000 RPM and it cut out again. It's like it loses spark for a second and then comes right back. Occasionally if it loses spark for more a little longer it will backfire when it comes back.

It also yesterday at one point cut out and I had an audible alarm. The alarm then quit. It's the first time I've had the alarm and I'm not sure it was related. It almost seemed like maybe it was a low voltage alarm or something.

Any ideas? Seems to be a problem a guy could throw hundreds of dollars at replacing random parts and not find a fix. I have done a very good visual search for chaffed wires etc.. and so far haven't found any.

 

[Bondo]

Second time I put the boat on the water I was about 5 minutes into cruising at about 3000 RPM and it cut out again. It's like it loses spark for a second and then comes right back. Occasionally if it loses spark for more a little longer it will backfire when it comes back.

Ayuh,.... Pull a few spark plugs, 'n inspect their Colors, 'n burn patterns,.....

Did ya find the reason for the valves gettin' detonatin' into tulips,..??

I know ya rebuilt the carb, but ya gotta Know that was the problem,.....

Is it proped to be able to pull 4800 rpms at Wot,..??

 

[ksmauck]

Well last year before I had tuliped the valves I had a friend help me install spark plugs. He had cracked one of the spark plug ceramics during install. That particular plug was completely trashed. So after rebuilding the heads we put in new plugs, wires, coil, distributor cap, etc...and rebuilt the carb. I will say that my tach doesn't seem to be entirely accurate but I've hooked up another tach while on the water and it doesn't seem to get up to 4800 rpm at WOT ever. I have a 17p prop on it now, but last year had a 19p prop on it. It does seem like it's not getting great RPM. It seems to cruise fine. I'm able to reach 44 mph out in the sound on fairly calm water but the RPM seems down. I do know that the compression in every cylinder is good, when checked after rebuilding the heads, with the exception of #8 and it is lower than the others but still "good enough". Oh I should also point out that before the engine failure last year I had replaced the spark plugs with the wrong plugs. The boat was relatively new to me at the time and I pulled one of the plugs and went and bought 8 of those... Found out later that the guy who had owned it before me had the one wrong plug in the hole I pulled it out of. It now has the correct plugs in it..