MisFire above 4000RPM after 20 min

[Beech2000]

Originally Posted by stonebreaker

http://store.summitracing.com/egnsea...115+4294920710

Before you start replacing jets, how much do you know about carbs?

http://store.summitracing.com/partde...5&autoview=sku

I'm more of a fuel injection guy, myself - the only vehicle I've ever owned with a carb was a 1980 camaro I had in high school. But if you don't have a buddy that can teach you how to tune a carb, these books can do a pretty good job - I learned most of my hotrodding from gearhead books.

On the plugs, it looks like the threads are a little long - the first couple of threads look like they're sticking into the chamber. That may just be because of the extreme closeup pics, though.

Hi Stone,

I see we share a vast difference in age. (Yes I am just before wheel chair.) I learned my hot rodding from moms Rocket 350 Cutlass which I raced on weekends. (That is until Dad had to have tranny replaced. Busted!!! LOL)

My hot rod days are a thing of the past. Now last gasp of V8 performance before wheel chair. Still trying to figure out why I wanted performance in my boat. I think I’m in final stages of mid life crises.

Any way, carbs are no big deal to me. I agree that it’s likely to be running lean and do wish to rejet and adjust needles accordingly. Since I am the one who installed the rebuild kit in the vessel's AFB, I should be able to manage the jet swap out. I do ask how carter marks their jets and where to purchase for which you answered and thank you.

I understand Edlebrock now manufactures and supports the AFB. Wow what happened to Carter? In my days, Edlebrock was great source for low budget intake manifolds and that’s it. Edlebrock was a very small manufacturing company in the late 70's.

Summit was also a small company with great pricing on engine parts. Now they have grown and even sell their own brand of intake manifolds and I own one of them.
In fact, one of their units is currently installed on this engine. I think I paid more for the one I installed in my 8 year old 71 nova I drove in high school. Oh how times have changed.

Anyway, any spark plug recommendations? i.e. part number, heat range etc…

Thanks

[Frosty]

I have to ecko stonebreakers comments about the reach of the plug bieng incorrect. They do look a tad weak though.

but any way discoularation on the first few (3) threads confirms heat range but carboned suggests they are basically sticking out too far into the combustion chamber.

I would put a small 4 mill lip on a peice of welding wire and measure the hole depth.

I dont feel that the plug reach is you problem though. I think you have some serious Ignition advance coming in for some reason.

[stonebreaker]

Jack,

If you look at the piston pictures he posted, you can see the areas where the suspected detonation is occurring - some of them are around one of the valves (I would assume the exhaust valve), and some of them are on the edge of the piston - away from the spark plug, and probably on the exhaust valve side of things. That boroscope is the cat's ass - I've always had to diagnose pinging from just looking at the plugs, I've never been able to look directly inside the engine before.

Beech, I'd say the oil in the cylinders in those pics is either from the PCV or maybe some bad valve stem seals. Nothing to worry about if you can live with a puff of blue smoke on startup. Now if the oil was coming up from below, THEN I'd be worried!

[Beech2000]

Originally Posted by jack frost

I have to ecko stonebreakers comments about the reach of the plug bieng incorrect. They do look a tad weak though.

but any way discoularation on the first few (3) threads confirms heat range but carboned suggests they are basically sticking out too far into the combustion chamber.

I would put a small 4 mill lip on a peice of welding wire and measure the hole depth.

I dont feel that the plug reach is you problem though. I think you have some serious Ignition advance coming in for some reason.

The ignition timing (Both initial and upper RPM advance) was controlled by the thunderbolt 4 ECM module. I was told it added 26 degrees of advance to initial for a total of 34 degrees at 4400RPM.

Thunderbolt ignition is no longer due to high cost of ECM. Replaced with MSD distributor and coil. Total timing with MSD currently at 28 degrees.

Hopefully with richened mixture and new ignition system; the detonation will cease.

Kevin

[Beech2000]

Originally Posted by stonebreaker

Jack,

If you look at the piston pictures he posted, you can see the areas where the suspected detonation is occurring - some of them are around one of the valves (I would assume the exhaust valve), and some of them are on the edge of the piston - away from the spark plug, and probably on the exhaust valve side of things.

Installed edlebrock (AFB) 1410 marine 750CFM Carb. I was told for 340 to 350 HP this carb should be ready with no re jetting necessary. Thoughts?

[stonebreaker]

Originally Posted by Beech2000

Installed edlebrock (AFB) 1410 marine 750CFM Carb. I was told for 340 to 350 HP this carb should be ready with no re jetting necessary. Thoughts?

http://store.summitracing.com/partde...5&autoview=sku

[Beech2000]

Looks like an O2 sensor.

Where would something like this mount?

Thanks

[stonebreaker]

That's correct. You'd have to drill a hole in the exhaust, somewhere before it mixes with water, and weld a bung in place. Then connect the O2 sensor to the gauge, and you'd be able to tell if you were rich or lean. The wide band O2 kit that will tell you HOW rich or lean you are, is about $600.

[Beech2000]

Originally Posted by stonebreaker

That's correct. You'd have to drill a hole in the exhaust, somewhere before it mixes with water, and weld a bung in place. Then connect the O2 sensor to the gauge, and you'd be able to tell if you were rich or lean. The wide band O2 kit that will tell you HOW rich or lean you are, is about $600.

Any other method to determine if mixture is correct? No way to install an o2 sensor in mercruiser exhaust manifolds as water cools manifolds at ports.

Thanks

[Beech2000]

Thought I would take the time to let all know that the engine is working perfect.

My little 19ft screamer sure turns heads. I have since rechecked compression, Changed oil and filter, Borescoped cylinders and all is perfect.

Amazing what desire for horsepower and stupidity can cause but I would do it again.

Our friends own a 23ft outlaw baja and last Sunday we swapped boats for the river run back to docks. He made it back first and loved it. Our friends boat is heavy and rides rough. Now he also desires more ponies. See what I started?

Anyway she's running fine and thank you all.

Happy floating..

Kevin