Ran for about 5 minutes at wide open then slowed to 3/4 for about a minute before shuting it off (or trying to anyway). Engine continued to run and clatter for a bit. Checked plug wires the next day and found two just a bit loose. Hasnt done it since. Any thoughts?

My first thought would have been carbon build up in the cylinder but if you say it hasnt done it after resetting the loose plug wires......hell i have no idea!


I know that on my old two-stroke jetshi engine (highly moded) you can get a compression runon from having a vacume leak. Happened to me once and scared the crap out of me!!! Thank god there was someone right next to me that knew what to do and flooded the carb with gas (for lubrication) to kill it. It would have spun to WFO and seized the engines up!

Your engine might have been "dieseling," by how you described the clatter.

The engine was probably heat-soaked after a hard run and it didn't cool off sufficiently before trying to shut it off. This can occurr sometimes with low-octane fuel(or bad fuel) and a hot engine. It's actually compression detonating(like a diesel) and is often accompanied by a clatter.

The "dieseling" is due to something in the combustion chamber being hot enough to act as a "glow plug" to light off the intake charge. Your carb has no idea whether the ignition is turned on or not and continues to add fuel as long as the motor sucks it in and there is fuel in the float bowls.

A sparkplug electrode can be the culprit if the plug is too hot for the application. The edge of an exhaust valve can be the culprit. Usually, though a chunk of carbon on the surface of the combustion chamber or piston crown will be the cause. The carbon chunk will get red hot after a hard run.

A good way to decarbonize a motor is to get a garden hose and a nozzle with a "mist" setting. Warm the motor to operating temp. Remove the flame arrestor and while holding a steady 1800-2000 rpm, begin misting water into the carb throat (you only need to do this on the primary venturis since the secondaries will remain closed). Mist the water as heavily as the motor will allow before chiking on the water.

What this does is it rapidly cools the hot carbon. The carbon chunks will crack and break free of the surface of the piston and/or combustion chamber. The particles will be carried away in the exhaust. If you have thru hull exhaust you will be able to see the black crap come out.

If you have ever pulled a head off a motor with a blown headgasket you will notice the piston and head are extremely clean. Same principle.

Hopfully the only deiseling from now on will be my truck! Ive done the water trick on bikes but always used a spray bottle. I guess a 454 might need more water. I pulled the plugs today and aside from some oil up the threads, all looked fine. I think this motor is going to need valve seals as it doesnt consume any oil but my plugs are oily if I let it set. Time for new heads if thats the case. Run the piss out of it this year and leak it down in the fall...thats the plan. Thanx for the replies.

PS, Ill prolly lose all my compression if I take out the carbon...maaybe Ill just leave it be.

Probable cause. Very hot combustion chamber from the hard run. You should let it cool down more before shutting it down. The dieseling is real hard on the engine. Makes pressures in the combustion chambers that are out of this world.

Just take a minute or so and let it cool down before shut off.

If you watch your water temp after a hard run you will notice that it usually climbs a little. My boat will run about 130-140 at idle and climbs to about 150 after a hard run. I ALWAYS let my boat idle after a hard run for 1-2 minutes until the temp comes down a little just to bhe sure.