MisFire above 4000RPM after 20 min

Hopeing for no major changes until end of season at which time I will perform long term repairs.

Short term workscope
1) I'll replace plugs with colder
2) Use at least 91 octane fuel and maybe a can of booster each tank
3) Richen up the primary jets in AFB
4) Throttle back and baby it until close of season

Thanks all

 

PLEASE let us know how she runs with your changes!

We have to keep learning from the Real World!

 

Changing spark plug heat range will do nothing for your problem.

It is very unlikely to be pre detonation at that RPM and not at low.

Pre- detonation does not affect just one cylinder,unless you had some wierd individual ingnition sytems.

pre detonation does not come from the rocker boxes and could not be desrcribed as a knocking, more of a tinkling.

The noise "like an air hammer on the rocker boxes " could be a cam follower failure. This would cause the noise and misfire. A sticking valve would also fit your description.


Plugs ,fuel and timing affect ALL cylinders.

First locate the missfiring cylinder.

 

Wow I am so confused.



The valves are factory steel intake and inconel exhaust.

the spring set and keepers are what comp cams recommended for
the marine extreme 270. (Dual coil 10degree)

several valve guides and all seals were replaced during overhaul.

any ideas on locating faulty cylinder / valve?

 

try this ,,if it is a sticky valve,,with engine running ,,and warm ,,pour some transmission fluid into the carb,you will get alott of smoke ,but it will clear your valves,,,,,,,if that is your problem ,also while doing this have a friend either listen or pour ,so you can here a differance,,listen with a stethiscope.longliner

 

It runs perfect and strong!. just not for long!

Not sure how to simulate problem in driveway with muffs to identify culprit. I am also unable to run boat at 4000 rpm planed out with sun deck open.. That is if I want to keep cover on boat.

More info:

problem will only cease with complete off plane idle for a minuet or so and then won't resurface until I run it hard again. Water temp stays around 170 to maybe 175

Misfire and noise starts only with hard running above 4000RPM and when backing down to even 3000rpm the mis fire is still there but not as bad.


Thanks

 

also after last post ,,carb can be loading up

 

Carb loading up?

What is carb loading up?

 

well I am assuming this is a gasoline engine ,with a carbourator ? or is it fuel injected,,

 

if you have a carb,,loading up is when too much fuel is in motion,,when you close the trottel,,this can be O rings or an adjustment needs made,,have you ever heard a motorcycle rev up and then come to a stop ,,only to back fire ,,that is loading up,if you have injectors its probably the valves,sticking if thats the case ,,pull the head and have it die checked ,,look for cracks in the valve seats,,good luck longliner,

 

what also makes me think it is a crack in the head is that after heating up ,,the crack expands,longliner

 

How to troubleshoot??

Boy, Jack, I dunno....

I have heard of a similar sound/problem in high performance boats...

Friends who ran circle track race cars could get something similar halfway thru the backstretch with too much advance.

In a car you will get knock/tinkle at low speeds with heavy throttle, but in a boat you cant get that torque loading..

A valve train failure would be an even periodic noise.

So, how to troubleshoot??

Question: Is the noise VERY regular like a valve tick, or irregular??

Try the EASY ones (Octane, Advance) first...

And let us know what you find.. the suspense is killing us

 

The noise is irregular as it only happens at high power after 20 min of lake runs.

I have access to video borescope with 3.9 mm flexible insertion tube. I will bring home this weekend and have a gander through spark plug holes.

I will capture images and post. Hopefully the problem is not valve train but rather something simple that I have over looked.

Be advised that I once had a similar problem at high loads on my four winns 470 engine which turned out to be faulty ignition wires.

This one is different only because of the rapid ticking accompanied by loss of power and mis-fire.

 

Beech, does your engine have a knock sensor? It is a small accelerometer mounted on the the starboard side of the engine. On my 99 350, its mounted on a Y-connect that shares the blue drain plug. My understanding of the accel is that if it detects high frequency vibration (e.g. engine knock) it will retard the timing until the knock subsides.

 

Not that I know of. Is your ignition thundrbolt?
Mine is and due to the fact that I swapped engine out myself, I feel sure I would have seen such a device.

That doesn't mean there is some relation to the new engine and timing though. Thanks for the input.

Kevin