Runaway engine on BB1390 - likely suspects?

[smackwave]

My BB1390 GPR developed a runaway condition while riding yesterday. Weirdest thing, ran fine for an hour, then noticed the revs were staying up when I feathered the throttle while running through the chop. Headed back to the ramp and sure enough the engine ran away as soon as I blipped the throttle out of the water. I pulled the lanyard and squeezed the throttle to kill the engine pretty quickly, but the is the first time I've had a ski develop an air leak mid-ride. Just to be sure I drained and refilled the fuel, checked the fuel filter and fuel lines to make sure nothing was loose/blocked, and swapped the spark plugs - but I was able to get the engine to run away again while running on the hose. Idle was uneven and surging as well, which to me all point to the classic symptoms of an air leak. If there's something else that could cause this let me know!

The last engine work I did was replacing the #3 jug and slug and rejetting (up a size on the high speed jet) towards the end of last season. At the time there were no leaks in the case or jugs, and the ski has run fine on several trips (including a couple of abusive surf jumping sessions) since then. I may have done a lazy man's cylinder swap, though, and not removed the exhaust manifold nor replace the exhaust gasket. Looks like that's my first suspect, but let me know if you can think of some likely culprits. Thanks!

[DaveC]

Had a similar deal years ago, it ended up being a bad intake manifold gasket.

[smackwave]

Yeah, I have the V-Force III reeds which leaked like a sieve when I first put them on, even after sanding everything level. I ended up switching to modified 1300 gaskets and basically cementing everything from the manifold to the cases together with Yamibond 4 to get a good seal. They've held together well for two years, and I'm keeping my fingers crossed that they're still fine. If that's where the problem is it looks like I'm out for surf jumping this weekend.

Hmm... it could be the gasket between the carb rack and manifold, though if that's what you mean. That's a good enough suspect that I'll take off the rack, reseal, and re-torque everything. Thanks!

[DaveC]

Now that you mention that, it was the gasket between the carbs and manifold. Had a piece about 1" long missing, the end result was not pretty. Sounds like you managed to shut yours off fairly quick.

[OsideBill]

I had a lot of issues sealing VF-III reeds I ended up making my own gaskets and sealing with 1211. Lowell listened to our concerns and develoed a new gasket it works great and if you install with a light coat of grease it is reusable. My biggest issue was that damn aluminum reed plate cover bends and leaks on the carbed skis. here was my solution http://www.greenhulk.net/forums/showthread.php?t=49609

[smackwave]

Great idea Bill, I missed that while I was taking my winter hiatus. I always liked how much thicker the Protec stuffer manifold was than the stock manifold, and now I have a good reason to dig it out (hope I didn't sell it) and try it again. I'll also get in touch with Lowell about the new gaskets. Are they a significant improvement over using the modified 1300 gaskets?

[Tyrael]

One of the guys on ozpwc had the same problem. His engine kept running without the lanyard and then without the spark-plug lead. Apparently turned out to be one of the powervalve cover. Do you have PV's on yours?

[smackwave]

PVs are there and fixed. I'll double-check torque on the valve covers, but they looked good when I checked them.

I did find that the center intake manifold bolt (#7 on the tightening pattern) had a sheared off head. I'm not sure that happened unless we'd been drinking a few too many beverages while putting things together. Also, it's hard to tell with such low torque specs (12N-M), but several of the bolts needed a quarter to half turn of tightening. The carb gasket looked good, and since I managed to lose several of my spare gaskets in the last move (including the new carb gasket I had on hand) I vaselined up the current gasket and put it back on. I also bought a helicoil set for repairing the broken manifold bolt, but there was no way for me to get in there to remove or drill out the old bolt without pulling the engine. That's a project that will have to wait for next week - stupid, I know. With a bolt missing and me not sure that I've found the problem yet I took the high speed adjusters out to 1.5 turns for good luck.