MisFire above 4000RPM after 20 min

[Beech2000]

Now 21 hours on fresh 350 vortec. Boat runs great. Planes out nicely and runs 50MPH with my wife and I but not for long.

Aftter sustained operation above 4000 RPM for more than 20 minuets I have what I call a mis-fire that will not go away until throttle back to idle. I can then throttle right back up and go again but not as long.

Its driving me nuts as all is new. (Wires, Cap, Rotor, Plugs)

The misfire I descride sounds like some one is using a small air hammer on valve covers.

Very strange...

Help and Thanks Kevin

[StianM]

What octane rating do you runn, 92,95 or 98?

[TerryKing]

Originally Posted by Beech2000

(snip)..The misfire I describe sounds like some one is using a small air hammer on valve covers.

This definitely sounds like "ping" or "detonation". Google that...

Do NOT run with that noise! You can break engine parts...

It is typical for it to happen at high temperature / heavy load after some running.

Possible cures:

• Higher Octane Fuel (easy to try!)
• 'Colder' spark plugs
• Reset ignition timing: not so much advance

Questions:

• What do the plugs look like right after a run like this? Stop, and pull one.
• What is the compression ratio?
• How was the ignition timing set?
• Does the distributor have a "Marine Advance Curve" set up?

[Beech2000]

Questions:

• What do the plugs look like right after a run like this? Stop, and pull one.
• What is the compression ratio?
• How was the ignition timing set?
• Does the distributor have a "Marine Advance Curve" set up?

[/quote]

*Haven't tried pulling spark plug after high power run. Can I tell if detonation has occured by pulling cold?

*I think compression ration is 10.0:1 (Flat top pistons, Vortec heads)

Timing is 10 BTDC using timing light with engine at 700RPM

Ignition is factory Mercruiser Thunderbolt not sure if sytem has advance curve feature

Thanks

[Beech2000]

Originally Posted by StianM

What octane rating do you runn, 92,95 or 98?

89 octane is all we can get in local marinas here in the states

[Beech2000]

Could the engine also be running lean as another contributor to TerryKing list?

Carb is 600 CFM weber AFB but largest marine cam Comp Cams makes. (Extreme marine 270)

[TerryKing]

*Haven't tried pulling spark plug after high power run. Can I tell if detonation has occurred by pulling cold?
*** All you can tell is the approximate plug temperature from the color. Should be Tan. If it is white or very light they were too hot, which can cause detonation.

*I think compression ration is 10.0:1 (Flat top pistons, Vortec heads)
***Ouch! That seems way too high for 89 octane.

Timing is 10 BTDC using timing light with engine at 700RPM
***The problem is what is the advance at 4000+ RPM ??

*Ignition is factory Mercruiser Thunderbolt not sure if system has advance curve feature.

This all sounds like several marginal things have come together:

• Low Octane fuel
• High Compression
• 20 hours and a little carbon build-up compared to new
• Ignition advance too far, at high RPMs
• Full Throttle Operation for minutes at a time

I think that what is happening is that at that temperature, and compression, the fuel mixture in the combustion chamber is exploding fully before top dead center, possibly firing even before the spark, on a hot plug or carbon particle.

I would try:

• High Octane fuel (Carry from station)
• Set ignition advance back by 5 degrees. Less power, but Ping gone?
• Colder plugs if they were white
• Less Lead-Arm

You are running that engine harder than a race car!!

Let us know what you find...

[Beech2000]

Thanks

any concern with mixture?

[Frosty]

If its fuel octane or cold plugs or advance ---why only one cylinder,?????????

[Beech2000]

Sounds like I need to figure our which one is not fireing.

This might be hard to do unless I remove sun deck cover and run.

[Beech2000]

Frost do you think its detonation?
As you can guess I am concerned of damage if yes.

[TerryKing]

Originally you described it like this:

The misfire I describe sounds like some one is using a small air hammer on valve covers.

That sound is variously referred to as "Spark Knock", "combustion rattle", "ping", or "Detonation".

I expect the sound is not perfectly even like a valve click, but a little random, more like a rattle. Is that right?? If so, I think you are hearing multiple cylinders doing this, not just one.. This is common, in my experience, with 'ping' at high speeds. so, the first issue: Is It Just One Cylinder, or not ??

Mixture: Sure, this type problem (if that's what it is) is made worse by lean mixture. But why is it OK when cold(er)??

Newer Computer-Controlled engines, trying to make the last bit of fuel economy, have a piezo vibration sensor on the head, which listens for 'ping' and backs off the ignition timing advance.

If either trying high-octane fuel or retarding the timing a bit makes this go away, you've proved it, without taking anything apart...

Take a look at: http://www.larrysengine.com/techarticles.asp for some discussion of these problems.

[Beech2000]

I used to remember detonation sounding like falling marbles under the hood of cars I used to drive. This falling marbles sound was mainly on acceleration and never caused the engine to misfire that I could tell. Not sure if this scenario is called pre ignition or detonation but never the less it wasn't consistent and new to back off throttle.

My current problem marine sounds like extreme case of same but at high RPM high load.

There is no question it must be addressed ASAP.

Question:
Does the thunderbolt 4 ignition change ignition timing at higher engines speeds?
*It seems as if distributor speed is the only input to the control box so how could it change other than with RPM?


I have tried retarding timing and have been running it as such with better results but still there.

I haven't tried higher octane gas yet. One company here in the states does offer 93 octane. Maybe I will try that as well.

I will also try colder spark plugs. I am currently running NGK BPR6EFS.

Any recommendations on alternate to try?

Kevin

[StianM]

I think your compresion ratio is too high for the octane rating.

There is a few cures for this:
*Runn a fater mix would cool the combustion down.
*Install a nozle to spray water into the manifoils(bether suited on turbos).
*Mix some tecnical alcohol in the fuel will also increase the octane rating but require a litle more fuel to get the same hp.
*Some boy racer shops probartly sell octane booster you can add yourself so you don't have to bring high octane gasoline to the marina.
*Less advance ignition.

If you install a nozle to spray wather into the manifoil mixing some alcohol in the wather will increase it's efficensy since the alcohol in the water would add some octane to the fuel/air mix.

I think playing with the ignition timing and maybe use a octane booster would be the best option and the water thing has only 1 big advantage and that is that low octane fuel are cheaper

Originally Posted by Beech2000

I will also try colder spark plugs. I am currently running NGK BPR6EFS.

Any recommendations on alternate to try?

Kevin

Switch the 6 with a 7.
I have played around with colder and hoter sparkplugs aswell, but for pre detonation I newer had luck with it, the only time I had was when my engine stoped and once i take out the plug I discover the tip off it was burned away. Newer had the same problem after changing to a colder one.

[TerryKing]

Originally Posted by StianM

I think your compression ratio is too high for the octane rating

One long-term possibility: lower the compression ratio with:

• Different Heads (Ouch!)
• Special thick head gaskets (some 'performance' shops have them)

Also, another possibility is that the heads are getting too hot, maybe locally, after hard running. Do you have separate temperature gauges for each head?? It's easy to put in an aftermarket temperature gauge. I have mine on a switch so I can compare the two heads..