Can't get rid of spark Knock!!!!!

[ekeef01]

Engine Specs:
Chevy 350 CI 275HP
Holly 650 4 Barrel Marine Carb
New Long Block with a 3/4 Roller Cam
Timing is Set to 8 Degrees
180 Degree Thermostat
I am having problems with spark knock. If I retard the timing I loose a lot of power. I do have some cheap autolite 144 plugs in now that I am about to switch to NGK BR6FS and I am going to try to put a cooler thermostat in it to keep the engine around 142 degrees. I was able to get rid of the spark knock last summer by using exxon 93 grade unleaded gas mixed with 6 gallons of 112 octane racefuel. But that gets a little rough on the bank plus its really starting to become frustrating that I can't find the problem(s). Any other ideas? Thanks...

 

[Don S]

Re: Can't get rid of spark Knock!!!!!

So, what is it? Mercruiser, Volvo, Other????? What ignition system have you got in it?

 

[ekeef01]

Re: Can't get rid of spark Knock!!!!!

Oh yea its gonna be a mercruiser and a electronic pointless distributor...

 

[Don S]

Re: Can't get rid of spark Knock!!!!!

Thunderbolt IV or V ignition?

 

[ekeef01]

Re: Can't get rid of spark Knock!!!!!

Sure sounds like a burnt valve.

I don't know the engine only has about 15-20 hours on it. Its pretty much new I thinking more along the lines of a fine tuning problem.

 

[ekeef01]

Re: Can't get rid of spark Knock!!!!!

Thunderbolt IV or V ignition?

Thunderbolt IV

 

[Don S]

Re: Can't get rid of spark Knock!!!!!

New Long Block with a 3/4 Roller Cam

That cam may be your problem.

Have you checked your total advance timing?

 

[ekeef01]

Re: Can't get rid of spark Knock!!!!!

You mean check the timing under a load instead of idle? If, thats the case than no. The cam was the same from the original block, now the block is basically from a 3/4 ton truck which is really the only difference on the engine other than the carb. I hope the block is not the problem cause I paid a boat mechanic to rebuild it.

 

[jtybt]

Re: Can't get rid of spark Knock!!!!!

The major causes for 'knock'(detonation) comes from too much compression for the octane or too much advance.

What is your compression?

Initial timing is only a starting point. Check total advance. It could be your EMC.

 

[Don S]

Re: Can't get rid of spark Knock!!!!!

You can check the total advance by rpm. The OEM manual has the advance curves in them. Not sure what year Mercruise it was, but if you go up to the adults only sticky at the top of this forum (second thread to be exact) you can download the manual for your engine. It has all the advance specs. You also can check the advance in neutral. Does not have to be under load.

 

[ekeef01]

Re: Can't get rid of spark Knock!!!!!

Downloading now, thanks for the help.

 

[ken_23434]

Re: Can't get rid of spark Knock!!!!!

A lean condition can lead to spark knock also.

That condition usually happens under load when you have something in fuel system that cannot supply the appropriate amount of gas (clogged fuel filter, weak fuel pump). At idle or under light load it might run fine, but as you increase the load you get to a point where the engine is starved for fuel and you get spark knock.

 

[ekeef01]

Re: Can't get rid of spark Knock!!!!!

A lean condition can lead to spark knock also.

That condition usually happens under load when you have something in fuel system that cannot supply the appropriate amount of gas (clogged fuel filter, weak fuel pump).

Thanks for the reminder, the fuel pump is new but I need to replace the fuel filter, that's what I forgot to buy at the boat shop today.

 

[Don S]

Re: Can't get rid of spark Knock!!!!!

It could easily be more than just the fuel filter causing a lean condition. Have a look at this thread on how to test the fuel system up to your engines fuel pump.
http://forums.iboats.com/showthread.php?t=283269
One other note, you need 3/8" ID fuel hose from the tank to the engine, nothing smaller.

 

[ekeef01]

Re: Can't get rid of spark Knock!!!!!

Turned out to be a cracked head and blown head gasket. Only found the problem by checking compression and cylinders 3 and 5 were jumping compression. $400 bucks for a set of heads and fel pro top end gasket kit and it's back in the water running great! Thanks for all the help guys!

 

[mnypitboat]

Re: Can't get rid of spark Knock!!!!!

Turned out to be a cracked head and blown head gasket. Only found the problem by checking compression and cylinders 3 and 5 were jumping compression. $400 bucks for a set of heads and fel pro top end gasket kit and it's back in the water running great! Thanks for all the help guys!

Til I read this, I was gonna say blown head gasket between 2 cylinders. Mine did exactly the same thing, and eventually broke valve. Looks like you found it though.

Where did you find heads and a top end kit for $400?