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Spark knock at high rpm's


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i updated the new boat (3.0 OMC) to electronic ignition/new plugs/wires/coil and now am getting a little spark knock over 3000 rpms.

thoughts so far

run premium instead of midgrade-didn't help

re-gap new plugs down to 35 from 40 (stock is 35 but I've had really good results running higher with hotter ignition in the past)

Timing could be off do to switch from points

any input on things I may be missing or should look at first?

thanks

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i would say the timing..but spark gap could also be issue..i would regap them to reccomended..i would much rather be stock than hot..then take it out and loosen the distributer and take a little of the advance out of it and see how it goes

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Hi Rob

I have been looking into the same thing and would love to hear how it works out for you. I was told that the timing needed to be adjusted after changing out (pertronics) but that it was a once and done thing. Any info and or advice when your done would be greatly appreciated.

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Electronics ignition requires a different shift module, so unplug it for now to eliminate it from the equation while searching for the problem. I don't think it could cause a spark knock but its always easier to divide and conquer on problems like this.

If you look in all of the marine catalogs for electronic ignition upgrades all that i know of say not to use on shift assisted motors which basically means OMC cobras. OMC made a special kit for upgrading those motors which was expensive because it came with another shift assist module and the distributor upgrade and is what is needed to do it right. This kit used to be $400 or $500 from OMC and is no longer available. Ive tried looking online for one for a 4.3L V6 recently and found them advertised in the $200-$300 range but every time i call to make sure they had it in stock no luck.

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the Pertronix setup uses adaptive dwell so the first thing you have to do is set the maximum advance because the adaptive dwell will play with it verses the old points. I have seen the conversion add 2 or 3 deg to the timing at high speed. so if you have a old style timing light you have to set your timing at max advance. Usually around 4000 rpm otherwise if you have a timing light with advance built in you need to use the advance to set the timing at idle. On points other than floating and misfiring once they are set all the advance comes from the advance plate and springs and weights. The adaptive dwell will retard the timing at idle making you over advanced at full throttle.

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As to the shift module if you called Pertronix for the correct setup they will tell you to add a diode and resister to the return to the coil from the shift module to let it work.

http://www.pertronix.com/support/tips/pdf/diodefix.pdf

The OMC 165 straight 6 I did did not require this and worked great without the resister and diode but the instructions did warn me that the OMC may require this. But I did have to do the timing as stated earlier.

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Thanks for all of the help guys. yes it was just the timing, although I could not get it set by just manually adjusting on the water (could have just been me) but I timed it with a light and it runs like a champ now.

I soldered in the 2 resistors and diodes per the instructions, works fine.

Corallee: I did the exact conversion that you are talking about on your 235 in my 194ss (both merc 302's) that is easier than on the OMC as there isn't an electronic shift interrupter. find the model number on your distributor and send it my way, I can find which pertronix model you need. when I did the upgrade on my merc i didn't even have to adjust timing.

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