OMC Cobra Stern Drive Tech Info Part 3 Featured

Written by Stuart Hastings
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     This website and its author are not affiliated in any way with OMC. See complete disclaimer below.

Adjusting your Cobra Shift Linkage

     Dave Brown (marina owner and frequent contributor to rec.boats) once said that a properly adjusted Cobra will "shift with a finger." I urge the reader to demand as much from your own stern drive. If your stern drive requires more than one finger to shift, get it fixed soon, because your gearset depends upon it.

     If you're serious about working on your stern drive, I urge you buy all of the appropriate documentation, especially the "shop manual." I strongly recommend you get the original documentation from the people that built your equipment. I'm not impressed by the aftermarket manuals (Clymer, Seloc, &etc). The manuals should be available from your local dealer, or you can call the manufacturer directly and pay with a credit card. The price will be similar; locally, you'll pay sales tax, and if you order from the manufacturer, you'll pay shipping. Even if your shop manual is out-of-date in some respects (as mine is), it is a wealth of information, and very useful when diagnosing an obscure problem.

     I'm not listing part numbers for the documentation here because OMC may have different manuals that apply to different years of drives, and I'm too lazy to confirm or disprove this with OMC. However, here are the names of the documents I recommend you get:

OMC Cobra Owner-Operator's Manual

OMC Cobra Service Manual

OMC Cobra Parts Catalog

     The third item may surprise you; it's a booklet of exploded parts diagrams for an entire stern drive, with part numbers. This makes it very convenient to order parts from a dealer; you can order by part number, avoiding the usual colorful "thingy that connects the whatsit to the thingumajig" discussions.

Below is a link to my by-hand transcription of an OMC document that explains how to adjust the shift linkage on the Clutch-Dog Cobra. I believe this procedure supersedes the procedure outlined in the OMC Cobra Service Manual. My copy of the OMC document is dated "10/17/96", and I believe it to be the latest and final word OMC will ever utter on this topic. Fortunately, it is reasonably clear and easy to follow, and I am very satisfied with my results. I hope this information is as helpful to others.

     When you read this document, be aware that it assumes you have a serviceable Transom Bracket Shift Cable installed and adjusted correctly. If you are replacing your Transom Shift Cable, you'll need to adjust it with the ponderously-named Transom Bracket Shift Cable Adjustment procedure from the OMC Cobra Service Manual (page 11-16 in my copy). I have the two applicable pages 11-16  and 11-17 below.

 

 This procedure requires only common wrenches, a straightedge, and a ruler; note that subsequent Service Bulletins from OMC require you to use special Cobra-specific tools to make this adjustment.

     This Service Bulletin describes how to recognize a defective spring in the Cobra shift linkage, and the p/n of the correct replacement. It cost me about US $2.50 for this part, but the new spring was still too stiff. However, it's straightforward to crunch the spring by hand to get a more appropriate tension: "Wrong Shift Interrupter Spring" below

 

 

 

     This Service Bulletin just says to install the latest ESA (Electronic Shift Assist) module. When OMC converted from a two-degree clutch dog to a five-degree, the dog was more reluctant to disengage. The new ESA makes the engine idle slower and weaker to make disengagement easier.

If you have the original two-degree gearset, and your ESA works, you probably don't need the new ESA. "Hard Shifting" below

This is a catch-all of typical problems that make the Cobra hard to shift. My own unit had none of these problems, yet I found these pages very interesting; some very obscure and un-intuitive problems are explained here:

Diagnosis of Cobra shift problems, page 1

 

Diagnosis of Cobra shift problems, page 2

Diagnosis of Cobra shift problems, page 3 (197k)

Diagnosis of Cobra shift problems, page 4

 

Diagnosis of Cobra shift problems, page 5

 

    

Just for completeness, here's a letter that came with some of the service bulletins that OMC graciously sent to me: OMC Letter to Stuart below

 

Here is the letter informing Cobra owners of the gearset recall. I I was fortunate that the original owner of my boat kept this now-historic document. I believe there was a subsequent Transom Bracket Shift Cable Recall letter, but I do not have a copy; presumably it was given to the dealer when the cable was replaced. OMC Gearset Recall Letter below

 

Pertronix Ignitor

     Many Cobras were built with non-electronic ignitions, using conventional points. These ignitions work fine, but the points are subject to wear, so the boat needs re-timing or even new points every season. You can end this cycle of maintenance by installing an electronic igntion retrofit; this is a gadget that replaces the points in your distributor. You install a plastic ring with magnets over the distributor cam, and the gadget senses the magnets as the distributor rotates. The beauty of the system is there are no points to wear and go out of adjustment, and I suppose it's a little more tolerant of moisture.

     OMC installed such "point-less" ignitions in the later Cobras (I've seen a '92 Cobra with factory electronic ignition). One option is to purchase this and mate it with your boat; in fact, OMC had a set of "kits" that included an electronic ignition gadget plus a new Electronic Shift Assist (ESA), all for the price of the ESA alone. I don't know if these are still available now that OMC is gone.

Another, cheaper option is to install the Pertronix Ignitor(tm). (See http://www.pertronix.com.) This is problematic, as the OMC ESA expects to see its sense line dragged down to zero volts every time the points "close," and the Pertronix gadget doesn't do that. Your engine will run fine, but the ESA won't see the pulses it expects, and it won't work, so shifting is stiff.

     I telephoned Pertronix tech support (the phone number used to be on their website) and inquired. They said the OMC ESA was a known problem, and they mailed me a sheet with instructions for a simple circuit that would interface their gadget to the OMC ESA. Specifically, the circuit has two identical resistors, and two identical diodes, for a total of four parts.

I visited my local Radio Shack, bought the parts, plus a small plastic box, and built the circuit. I installed it with my Pertronix Ignitor(tm), and my ESA works fine with it.

     The diagram that Pertronix sent me is below

I grabbed this off their website some time back, and it subsequently disappeared. However, it had reappeared when I checked in early 2003 (look for "Ignitor," then "Support").

 

Part 4 How To Adjust Your Cobra Shift Linkage

Part 3 OMC Cobra Stern Drive Tech Info

Part 2 Problems with the Cobra

Part 1 OMC Cobra Stern Drive Tech Info

 

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Source: http://www.hastings.org/~stuart/cobra/index.html

Recreated with the permission of the Author for CBOC

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