OMC Timing for 30+ years Joe

[Solittle]

You have posted several times a method for checking timing at cranking speed at 4 degrees off the factory setting. It is clear and I understand the procedure. On to my question - -<br /><br />I have a pair of 1987 OMC V4 SeaDrives. The recommended setting on the carb decal says to set spark advance adjustment at 28 degrees BTDC. My OMC manual confirms this. Timing is set by an eccentric screw if within 2 degrees and by the position of the spark control rod in one of three holes of a retainer (small nylon block) if the timing is not within 2 degrees. Moving the spark control rod from one hole to another changes the timing by 4 degrees - - so says the manual.<br /><br />Before doing anything I checked the present timing timing pointer per the manual. The starboard motor was off by about 2 degrees. I then set up my timing light and checked the timing. I get a reading of 16 degrees BTDC. According to your method if my timing was OK it should be 24 degrees or the timing is now 8 degrees off. Something does not seem right. I would think a motor would not run if it were off by 8 degrees.<br /><br />I did not check the port motor but suspect it to be pretty much the same as I did a rough timing check a few weeks ago before I checked the timing pointers.<br /><br />As to why I am checking timing - both motors start but not as quickly as other OMC V4s that I have owned over the years. My gut tells me that they do not have the oomph (oomph is not a term I got from the manual and there is no OMC procedure for troubleshooting oomph) that they should in getting out of the hole. When on plane they seem to run well.<br /><br />The manual also calls for checking the cam follower pickup timing at 4 degrees BTDC by loosening a jam nut and rotating a thumb wheel after the maximum spark advance is OK. I have no idea what this does or how/if it relates to your procedure.<br /><br />I am hesitant to change anything without your input.<br /><br />Also big thanks for your help to all of us - <br /><br />Howard <br />Miami Fl

 

[Joe Reeves]

Re: OMC Timing for 30+ years Joe

SoLittle.... The "Cam Follower Pickup" 4° BTC timing pertains to when the throttle butterflies should start to open. That is the exact point where the scribe mark on the cam lines up with the center of the carb roller.<br /><br />That would simply involve having the engine running, then increase the throttle until the timing light indicates that you have the engine running at 4° BTC. Then adjust the roller and the cam as needed. (I'd suggest shutting the engine down before making those adjustments).<br /><br />The full spark advance setting can be safely set at cranking speed (engine not running) as I've indicated previously... 4° less than the actual designated setting. In your case 28° less 4° = 24°. I have no idea why you're only reading 16°. Double check to make sure that the timer base is up against the full spark advance stop screw, wire it there if necessary.<br /><br />I'm assuming that you know which cylinder is #1. Standing in back of the engine... top starboard (right) cylinder. Not trying to insult your intelligence, just avoiding a lack of communication. Let us know what you find.

 

[Solittle]

Re: OMC Timing for 30+ years Joe

Joe - Thanks for the response. I understand now the cam follower pickup but am still at a loss on the max spark advance.<br /><br />My motor does not have a full spark advance stop screw. If you have a Clymer Outboard Shop Manual 48-235 HP 1973-1990 Pn #B736, the procedure and photos are on pages 244 & 245. I also have the OMC manual & could scan the procedure & photos and email (need you email address) them to you if that would help.<br /><br />I would like to use your procedure but am a bit reluctant to do so without better info. How could someone (prior owner or mechanic) set the timing as it is now and be 8 degrees off? I would think being 8 degrees off would hurt my motors big time or they wouldn't run at all. I don't want to add to the problem & only would like my motors to run better.<br /><br />Thanks,<br /><br />Howard

 

[Solittle]

Re: OMC Timing for 30+ years Joe

Joe - Hoping that you will catch this this time around. Forgot to mention that I do know where #1 is. - - - I'm tempted to say between 0 and 2 but that would be corney.

 

[Joe Reeves]

Re: OMC Timing for 30+ years Joe

SoLittle..... It had slipped my mind that the SeaDrive your speak of does not have that rubber capped spark advance stop screw. However, a engine running at 16° at full throttle when it should be showing 24° (at cranking speed), to me simply indicates that the timing is retarded.<br /><br />However, I'm not there staring at the engine and I don't want to commit myself as there's always the possibility that something strange is lurking in the shadows.<br /><br />Check the other engine (from your post, I assume you have dual engines) and let me know what you find there.

 

[Solittle]

Re: OMC Timing for 30+ years Joe

What would the effect be by running the engine retarded? I have never understood what retarded or advanced really means. I know that it means the point at which the spark plug fires relative to TDC but I don't understand how that relates to performance or reliability. It would seem to me that for a basic design, say the V4, that all like motors would be set the same but aparently this is not so.<br /><br />Yes I do have two motors and it is to late to recheck the 2nd now. I did check it a several weeks ago and it was within two degrees of the one I posted about. I forget whether it would then be 6 degrees off or 10 degrees off rather than the eight for this motor.<br /><br />Can you think of any harm I could do if I set the timing at 24 degrees per your procedure?

 

[Joe Reeves]

Re: OMC Timing for 30+ years Joe

Running the engine in a retarded mode would result in a power loss. The figures of 16° or 24° whatever, pertain to the degree of ignition before the piston reaches Top Dead Center (BTC).<br /><br />Think of it as 16° being 1/16" and 24° being 1/4". At full throttle, the piston firing 1/4" before it reaches TDC would allow the fuel to burn thoroughly, forcing the piston downward with great force. The piston firing 1/16" before it reaches TDC would not allow the fuel to burn thoroughly and might even have the combustionable mixture going out the exhaust ports resulting in a power loss. There would be a power loss in any case. I have no doubt that someone here could explain it better.<br /><br />In thinking about what you've said about the other engine being right around the same timing degree, I think that before I'd change anything, I want to water test it, using a timing light to see what degree shows up at full throttle. Something just doesn't sound right. If by chance, a change results in actually having the engine exceed the normal 28° while underway at full throttle, that would result in pre-ignition which will melt a piston. Don't want that.

 

[sparkroost]

Re: OMC Timing for 30+ years Joe

Think of a fan blade. You want to make the blade go faster with your finger. If you get the angle just right and the speed of your finger you can hit the blade and make the fan go faster. Now, what happens when you are at the wrong angle and are going to slow? You inturrupt the motion of the blade and actually slow it down, then once the finger catches up, your force increases and the fan spins faster. Now.. this time you point your finger right at the blade but you move in too early , either stopping the fan..or if the blade is very heavy.. you break your finger.<br /><br />If the explosion in the cylinder happens too early, that force will collide with the piston and work against it.<br /><br />If it happens too late, the explosion will never interact with the piston with efficency. Anywhere in between power is lost. It must be dead on to perform efficiently.