Random knock noise at high wot

[ready4lake]

I just completed swapping my 3.0 out and putting a 4.3 in its place. I took it out last week and got it timed and running good. I was happy with the results until I really opened it up. I would hear a random knocking noise at WOT. It only happens in fwd which makes me believe it is the outdrive. I had the boat on the water for about 3 hours, then it wouldn't shift into fwd. Even if I pushed the lower shift cable by hand. Would the noise be from the clutch dog? Usually clutch dogs make a loud consistant clicking noise when the don't want to engage. I have been too busy to mess with it this week, but I just wanted to see if anyone else has had this issue.

 

[Howard Sterndrive]

Re: Random knock noise at high wot

pretty hard to open it up in reverse, but that description of the sound made me think pre-ignition (detonation) due to improper ignition timing.
Are you using a 4.3 outdrive or the original 3.0 drive? If the latter, is it a 13 spline propshaft or 15 spline lower?

 

[ready4lake]

Re: Random knock noise at high wot

I am using the 4.3 outdrive. I have a donor boat that I pulled everything for the 4.3. I thought it could be pre-ignition also, it just confused me that it didn't happen it reverse. The sound did seem to be coming from the engine. I am almost positive that my lower shift cable is bad and not the clutch dog. I noticed that once the boat would plane out that it would not happen as often, so it is just when there is alot of load on the the engine. I was not able to get the timing exact with my timing light because the ring slipped on the harmonic balancer. I have a new one on order. I was able to turn the distributor til the engine responded the best, so I guess I will wait until my new harmonic balancer comes in and try again.

 

[Howard Sterndrive]

Re: Random knock noise at high wot

I was able to turn the distributor til the engine responded the best,

that rarely works well... the outdrive may have saved you some pistons

you cannot load the engine in reverse- propellers don't hook up well enough pulling backwards, and if it ever did pull the engine to full load in reverse, you'd probably sink the boat

 

[ready4lake]

Re: Random knock noise at high wot

Yes I hope there is no damage. I did not run wot after I first heard the noise, just long enough to figure out about where it was coming from. The rest of the day I anchored out and drank a few beers and thought about how much more work I have to do on the boat. I definitely don't want to pull that motor out again. It has been in and out of the boat way too many times during the swap.
Thanks for the info. I will not go wot in reverse anymore. I don't want to sink the boat. The water is still a little cold here.

 

[Bondo]

Re: Random knock noise at high wot

I was not able to get the timing exact with my timing light because the ring slipped on the harmonic balancer.

Ayuh,.... Holy Moly,... Replace the danm thing, 'n Properly time yer motor, before ya hole a piston....

If that ring slips off, it'll either go through the hull, or maybe knock ya up-side the head....:facepalm:

 

[ready4lake]

Re: Random knock noise at high wot

I have one on order. It should be here by this weekend.