Mercruiser 5.0L Spark Knock Over 4,000 RPM

[JJBoatr]

Hey All,
What I'm working on is a 1990 Thompson Cutlass 195 powered by a 2 barrell 5.0/Alpha Gen I, and the base timing is set at 8 degrees BTDC per the spec. I just ran the boat on vacation for 11 days straight with zero problems except that I would get what seemed to be a random spark knock above 4,000RPM. Sometimes it wouldn't act up until I hit about 4200. The engine only has about 6 hours on the new long block. It will happen once, kind of a "Tink" sound, then it will do it again 2 seconds later then again 5 seconds later then 3 seconds later etc. And it is always random like that. I am suspecting the carb has something obstructing one of the mains at this point since the boat acts perfect until you really get her spun up. No hesitation getting out of the hole, good response mid range and doesn't miss a beat when you're just cruising at 3500. Which I know that if it is a fuel obstruction then the sound I'm hearing is detonation from leaning out. I can also feel the power drop a little when I hammer it from about 3900. But if you slowly move the throttle the power doesn't drop off. If the carb is good my next suspect is ignition. My engine has a Thunderbolt ignition system with the module mounted on the port exhaust riser and I'm not all that familiar with this system. The fuel/water separator filter is new and the old one had no water in it. I ran 3 bottles of dry gas through it, and I use Marine Stabil or Startron mineral treatment at every fill up. Any help and info on my ignition will be greatly appreciated.

 

[cr2k]

Re: Mercruiser 5.0L Spark Knock Over 4,000 RPM

What octane gas are you running? If you are running regular you should drop your timing a few degrees. Boat specs are for 1990. Gas being used is 2011. (not the quality it was in 1990)

Or your HS jets are in need of service.

 

[JJBoatr]

Re: Mercruiser 5.0L Spark Knock Over 4,000 RPM

Thanks for the reply. I always run 93 octane if available at the dock. The marina I was at only offered 89 octane but the "tink" didn't get any worse running lower octane fuel. At the beginning of the season I had the timing set at 4 degrees BTDC for break in and it was still knocking. I will be taking the carb off in a couple hours and I'll let you know what I find. By the way the carb is a rochester 2 barrell sorry for not supplying that info before. Ya just gotta love ethanol

 

[JJBoatr]

Re: Mercruiser 5.0L Spark Knock Over 4,000 RPM

Hey All,

Sorry for the extreme delay in posting results of my troubleshooting. The problem ended up being a poor seal between the intake and heads, discoverd by spraying carb cleaner along mating surfaces while running on boots. A good friend of mine with lots of SBC experience told me to tighten my intake bolts beyond the spec of 30ft/lbs, having some experience myself I was reserved about cranking on what I thought to be critical bolts. I re-torqued the bolts to spec and ran the boat, it once again it stumbled while spraying carb cleaner at mating surfaces. I tightened the bolts free hand (in the correct pattern) with a standard length 9/16 wrench and the stumble went away. I checked the torque that I applied and it was 45ft/lbs +/- 2ft/lbs. I evened all the bolts out to 45ft/lbs with a torque wrench and headed to the lake. The knock is gone!!!!!!! The engine accelerated to 4500RPM with no hesitation!!! I may have to prop up next :0) What a relief. Sorry again for the very late closure of this problem if any one was counting on this thread.