what causes the engine to run away?

[Guest_User]

something strange happened on our last ride, on the last start up, let me explain:
Took the boat out to have a last season end ride and run the fuel down.
Boat ran great now that all the Rave assembly`s have been replaced. I usually don`t push the boat, but today I wanted to burn down the 40 gallons of fuel... so we mostly cruised at 6K or better with a few decent hits at 7k.
I haven`t had the boat at max speeds all summer , those that read the build thread no why...
so today I just wanted to see what max speed felt like, and again the boat ran great, no issues until I was on the ramp.
I usually start the boat on the ramp incline while the drain plug is out and any residual water drains, I`ll fire up one engine and push the shifter into forward so I can tap the throttle a few times to blow out some of the water in the exhaust from idling in the NWZ to get to the ramp... I don`t go nuts and rev the snot out of it, then shut it down. Since I ride in fresh/brackish waters I do not flush on the hose... dam carbon seals..lol...

anyway--- as I went to pull the lanyard while the starboard engine was running, keep in mind, it only takes a few moments to fire up, push the shifter into forward, blip the throttle then shut it down, the engine did not shut off! took me a second to realize what`s happening, I had the engine hatch open, so I quickly ran to the back and pulled the electrical plug thinking it was still having ignition, when that didn`t shut off the engine I then realized it was a lean runaway or some type of pre-ignition going on... I saw that the rpms climbed to 6500 no higher for the brief seconds it took me to push the throttle linkage open to stall the engine...whew...

after I moved the truck and boat off the ramp and parked, I restarted the engine , it idled it`s normal 3K+ out of the water for a second or 2 and shut off like it`s supposed to do... I also did this again when I got home about 45 minutes later and it did not do anything strange. started right up and shut off quickly when the lanyard was pulled,,, so what the hell happened here???

there are now 22 hours on these SBT engines. after the engines broke in, I ran JT4 synthetic at 40:1. used up my stash, and this last ride was on XPS synthetic. There are 4 hours on these spark plugs and they are def lighter than the set used last and the set before that. although not terribly lean, have yellow to brownish colors, compared to a more Rich darker brown previously. I attributed this to defective RAVE assemblies not opening all the way and preventing the starboard engine from reaching max rpm. The new Rave assy`s was my missing rpms cure. So prior to that I haven`t been really pushing the boat and running between 5000-6200 rpms...
these last 2 rides had more upper rpm time in the 6-7K range.

what has me stumped is that the boat runs good, and impressed with how well it handles the chop and overall handling and stability at speed... it really steers like a huge jet ski! love it... comes up on plane like an animal when you want to get aggressive and push it... no bogging, no hesitation, it just scoots...
Im really starting to like this boat given all the work that I put into it this year...

so, this thing that happened freaked me out, even the misses knew right away something was wrong...

can anyone tell me what had happened? what should I look for now even tho we are done for the season...
Thanks...

 

[Dr Honda]

Generally... it's a lean mix.

At idle... and unloaded... if it's lean, the heat can actually push the ignition timing high. (from a hot spot in the head) So... it's not using the sparkplugs, and that's why it doesn't shut off when you pull the lanyard.

Anyway... as the timing goes high... the RPM's go up. In turn... the vacuum gets high enough to force the fuel draw. (so it gets more fuel) And this aggravates the problem.

As you found out... a blip of the throttle will sometimes shut it down... but the best way to do it is to pull the choke. It will force a rich mix, and cool the combustion.

My solution is to open the low mix screws about 1/8th of a turn, and that should drop the RPM out of the water by a few hundred RPM. BUT... do not raise the RPM back up. You should still have the same RPM in the water.

Lastly... Don't try to set the idle out of the water. (I saw the 3000 RPM comment) Put the boat in the water, and adjust the idle for a low, stable idle. (1400 to 1600 is normally the best) A lot of times... it will be lower than 3000 out of the water, and that will help with run-away.

 

[Guest_User]

Generally... it's a lean mix.

At idle... and unloaded... if it's lean, the heat can actually push the ignition timing high. (from a hot spot in the head) So... it's not using the sparkplugs, and that's why it doesn't shut off when you pull the lanyard.

Anyway... as the timing goes high... the RPM's go up. In turn... the vacuum gets high enough to force the fuel draw. (so it gets more fuel) And this aggravates the problem.

As you found out... a blip of the throttle will sometimes shut it down... but the best way to do it is to pull the choke. It will force a rich mix, and cool the combustion.

My solution is to open the low mix screws about 1/8th of a turn, and that should drop the RPM out of the water by a few hundred RPM. BUT... do not raise the RPM back up. You should still have the same RPM in the water.

Lastly... Don't try to set the idle out of the water. (I saw the 3000 RPM comment) Put the boat in the water, and adjust the idle for a low, stable idle. (1400 to 1600 is normally the best) A lot of times... it will be lower than 3000 out of the water, and that will help with run-away.

Thanks Doc. I actually followed the specs when adjusting the idle speeds. Turning the idle screw after it made contact with the carb tang 2 turns in to start off with was perfect at 1500 rpm when the engines are warm in the water... The only time I`ll have the engines run for a few seconds is right before I`m leaving the house to ride and at the ramp so I know the boat will fire right up and all is GTG. I can understand this happening after a hard run, but after a short no wake zone idling and docking, then shut down until it was on the trailer def caught my attention when it did this.

I have the LS screws out 1.5 turns now... and will do what you suggest and turn them out an addition 1/8 turn but I won`t know anything till next season.
I will do an end of the season (today) compression test for my notes/file and my plan was to remove the drives/engines and store indoors, while I update the fuel system from the fuel pumps forward and install all new engine mounts during the off time. remember that thread about the filter debate and water separators? all have been replaced with the exception of the fuel hoses from filter to tank sender baffle.
also to fix the drooling ever so slowly oil tank and come up with something a little cleaner even tho all those hoses have been replaced as well. Just have the tank for the RV...
Thanks Doc!...

I`m sure a pressure test during the off season wouldn`t hurt?