Timing problem on 7.5 Merc...

[Woodeye]

This 7.5 is really old. Out of the 70s I think.<br />The Mag for the thunderbolt ignition failed and is worth more than the motor. I threw away the mag and coils, and installed automotive coils and condensers, and plan to use it with a dead loss battery system.<br />I set the points very accurately with a dial indicator on the pistons, but the timing is obviously off because it fires so soon, that it stops me in mid pull. It used to start with about a 6" pull.<br />My question is; Should there a timing difference between a Thunderbolt ignition, and a condenser ignition? <br />Is 23.5 degrees too much for a points and condenser ignition?<br />Thanks

 

[Chinewalker]

Re: Timing problem on 7.5 Merc...

Hi Woodeye,<br /> I'd do away with the battery ignition and just upgrade it to either a later style electronic ignition, or an earlier style magneto ignition. I'm guessing you've got the "Phase Maker" low tension ignition - not Mercury's best effort. Moving back to a regular points/coils/condensers style igntion will probably result in easier maintenance down the road...<br /> As for the timing - the piston doesn't care what makes the spark - just so long as it fires when it's supposed to...<br />- Scott

 

[Woodeye]

Re: Timing problem on 7.5 Merc...

Hey Scott:<br />Since the points on my old Merc, interrupt the grounding of the negative post on the coil, I have attached the condenser bracket directly to the negative post on the coil, and the condenser lead to the positive post on the coil.<br />Make sense?

 

[Chinewalker]

Re: Timing problem on 7.5 Merc...

Hi Woodeye,<br /> The reason you're getting the kickback is the points were designed to be "maker" points, not "breaker" points. The low tension magneto fired when the points closed, not when they opened... It really was kind of a screwed up system and Mercury got away from it after only a couple years. If you can figure out how to make your idea work, then go for it, but I think there's an easier way... <br /> There's lots of these motors out there. Mercury used a carbon steel driveshaft on them through most of the 1970s and as such, there's lots missing the lower units. Find one with a good powerhead, swap it out and get back on the water! You could probably find a parts motor missing the lower unit for a lot less than you'll spend getting a new system to work properly. I bought one off eBay awhile back for $40.00, shipping INCLUDED! Try your local marinas first - offer the mech a six-pack to let you paw through the graveyard and you might just turn up what you need...<br />- Scott

 

[Woodeye]

Re: Timing problem on 7.5 Merc...

Thanx for the info Scott. It was exactly the info I needed.<br />I could not bring the breaker timing on because the cam was completely wrong.<br />I reworked the cam by giving it a shorter ramp, switched the points to the opposite cylinders, and it timed up perfectly.<br />It now runs...for a second or two, but that's another problem. One thing at at time.

 

[Chinewalker]

Re: Timing problem on 7.5 Merc...

Hi Woodeye,<br /> Sounds like you're having fun with it. Every now and then it can be kinda fun to re-invent the wheel, so to speak. I'm sure I've done it a few times myself. Start looking into your fuel system for your stalling issue...<br />- Scott