987 260 Bog at higher speeds

"After about an hour of runnin

"After about an hour of running, my '87 260 starts to surge and bog. I have access to a scope, and during the surges the kv jumps to over 30 and sometimes extra voltage lines appear between the normal patterns. It is a Thunderbolt ignition. Fuel pressure is around 6 psi. New filters and seperators. Newer wires plugs cap and rotor. Any ideas. Thanks. PeterD"

""Newer" means what, e

""Newer" means what, exactly? Are they good wires? What about the plugs- they can be bad at higher RPM and not show any signs of trouble at idle. Remember, electricity takes the path of least resistance and if the plugs resist more at high frequencies, you'll have crossfire or the spark will jump to ground. Take a fuel sample and if there's a filter at the inlet on the carb, check/change that, too."

"Newer means they were done at

"Newer means they were done at the beginning of the season. (late April) When the kv's go up, they all go up. If I had a bad wire or plug the kv's would spike for that cylinder. The scope was on the engine when the problem occured and we got close to 30 kv's on all cylinders. The problem is intermittent. The fuel looks and smells good and all filters were changed including the carb inlet. I also had good fuel volume about 12 oz in 15 seconds. Do you happen to know the symtoms of a bad module? I saw on another posting that an anti siphon valve may cause some problems. I could not leave my fuel pressure gauge on during the test ride because it leaked. Thanks alot for your help. PeterD"

Was the 30KV output when it ra

Was the 30KV output when it ran good or when it had the problem? What about the coil? I don't remember the Thunderbolt system very well and didn't work on many of them. I know there is a specific IC module but never really had to troubleshoot much on one. Did you check all of the motor grounds?

"I did not check the motor gro

"I did not check the motor grounds, but that's a good idea. I got about 30 kv when the problem happened. Otherwise the kv were around 13-15. If the coil is putting out 30 kv I gotta believe its ok. The exterior is in good shape with no appearant arc lines."

30KV does sound like a normal

30KV does sound like a normal output for a good ignition system. The 13-15KV sounds too low. The GM distributors on injected marine motors put out close to 50KV and the Northstar system is close to 80KV.

Have you looked at the pickup coil in the distributor to make sure it's tight?

"How about a plugged fuel vent

"How about a plugged fuel vent hose? Water in fuel? Dry gas the heck out of that fuel tank. Water seperators only trap a couple of oz. of water. The Thunderbolt module delivers the advance. Check the timing with an advance light and verify the timing at rpm increments. 20@2000, 25@3000, 32@4000, don't forget to subtract the intitial timing."

"Peter- do you use gas with et

"Peter- do you use gas with ethanol in it? How long has it been in the tank, and have you taken a fuel sample yet?"

"Thanks all for the suggestion

"Thanks all for the suggestions. Any ignition system is only going to put out what is neede to jump across the spark plug gap. If you have a bad wire or cracked plug, the kv for that cylinder will go up because the spark is getting more resistence. With a .035 gap, the normal kv can be anywhere between 8-16. I am wondering why the kv goes up so much when the engine starts stumbling. Someone suggested that it might be a lean condition, because it takes more voltage to push a spark across a lean cylinder than a rich one. I also found out that the scope I was using had a bad wire for the pick-up. That could explain my extra voltage lines. I am going back up to my boat on Saturday to check out the ideas everybody gave me. I'll keep you all posted. Thanks again. PeterD"

I did take a fuel sample and i

I did take a fuel sample and it looked and smelled ok and I had good volume at an idle at the pier. Around here it is 10% ethanol.

I also checked the advance. It

I also checked the advance. It is right on the specs that Harves Marine gave. Next time I'll read all the postings first and answer them all at once.

There was a filter/screen righ

There was a filter/screen right off the pick-up tube on the fuel tank that was clogged. I finally made it out there to check it out. Thanks for all the suggestions.