Double Firing?

[sschefer]

This is a long read but it you get to the end I 'd greatly appreciate your thoughts. :cheers:

I have a 91 Mariner 2+2 100hp inline 4. It runs great for about the first 5 min at WOT then the rattles start. My first thought was fuel. Fuel pump was rebuilt but prob remained. I still thought fuel so I ran it on a separate 6 gal portable. Prob remained. I pulled the pulse pump off sealed the block up, pulled the oil injection and installed a Holley Red fuel pump and ran it off the portable tank on 50:1 mix. The problem remains.

O.K. so it's not fuel and doesn't start until the engine is warmed up. These things hold enough water to fill a small bathtub so they are naturally cold blooded and only have about 2-3lbs of water pressure. They have a high volume water pump. The thermostat is a 143 and is new, it did it with the old one also. All the cooling system is working great. The exhaust cover gaskets are new and are correct for the baffle p/n. I have no indication of water in any cylinder.

The stator and trigger are both CDI and CDI verified that they were correct. They both static test good. The magnet for the trigger is clean with no cracks. The switchbox is a new Mallory. All the wiring is new, grounds are clean and tight and all connections are coated with silicon dielectric and covered with either shrink tube or boots.

I can't seem to make it do this on the hose without a load and if I drop the throttle back to around 3200 rpm the rattle subsides (that's why I was chasing it as a fuel problem).

Does this sound like double firing to you?

If you were to start replacing things would you go with the trigger or the switchbox first? I'm inclined to think switchbox.

[David Borg]

I would start by going through the carbs first , might have dirt . also check all fuel lines , I have seen many fuel lines which start to go bad on the inside with todays fuel and fill the carbs with dirt.

Good luck

[sschefer]

Thanks, the carbs are good pressure tested them on the bench and they hold up to 10lbs without leaks and flow great. The fuel lines are all new and I'm running a Holley Red with 6-7 lbs on the fuel gauge at WOT.

[David Borg]

When the engine starts acting up , use a timing light and check if you have spark on all four cylinders and at the same time look for inconsistent spark. might have a bad coil breaking down

[sschefer]

When the engine starts acting up , use a timing light and check if you have spark on all four cylinders and at the same time look for inconsistent spark. might have a bad coil breaking down

Therein lies the problem, maybe I wasn't clear in my original post but this engine will not do this on the hose. It only does it on the water at WOT after about 5 minutes. I haven't quite figured out how to check for double fire at 40mph while I'm driving and I don't want to run it at WOT when its double firing because of the damage it can cause.

Quite a dilema huh. If it is heat related component failure as I suspect then I might be able to reproduce it with a hair dryer.

[Tripps]

I had a flywheel do that,sparked like the 4th of july!!! but did it all the time:eek:,mota sunk about 4 years ago ,but i had another situation like yours and cost me a mota :( tripps

[Chaz]

Are the CDM's ( coils ) round with three wires or rectangle with four wires . If they are the round one's they need to be upgraded to the newer rectange ones .
One of the problems with the older ones was that durring the rise time while they were charging , they would sometimes release early ( premature e...err ..you know ) causing a primary spark 40 -90 degree's early . More than enough to start the death rattle ....
I just went through this with mine ... and I took an old POS aluminum prop and cut the blades down to roughly three inch's long . I can run the motor up to 6000 rpm under a load while the boat is still on the trailer ... neat tool for replicating running at speed ...

Charlie.....II

[sschefer]

Are the CDM's ( coils ) round with three wires or rectangle with four wires . If they are the round one's they need to be upgraded to the newer rectange ones .
One of the problems with the older ones was that durring the rise time while they were charging , they would sometimes release early ( premature e...err ..you know ) causing a primary spark 40 -90 degree's early . More than enough to start the death rattle ....
I just went through this with mine ... and I took an old POS aluminum prop and cut the blades down to roughly three inch's long . I can run the motor up to 6000 rpm under a load while the boat is still on the trailer ... neat tool for replicating running at speed ...

Charlie.....II

Not sure what your mean charlie. The trigger is a new CDI with red/red white and blue/blue white (two trigger coils) each fires two cylinders.

The actual ignition coils are two wire (hot and ground) same as on a early 90's 2.0L. These are the blue ones. I get what you're saying. A cracked or otherwise damaged coil could easily do just what you're saying and start firing way too early if the core was cracked but didn't open up until it reached a certain temp. If there was only one coil doing it, it would mimick my symptoms exactly. If it were a trigger, I'd be double firing on two cylinders.

I'm going to change out the coils, I have plenty of spares that all look and test good.

Cool idea for the prop.. I have an old aluminum dinger that I can cut down.

Thanks

[Chaz]

Glad you liked the prop idea ... I was wrong on the coil type all together... I was in the outboard boneyard the other day , and saw a Force V-4 that I got some rectangle CDM's off of ... that makes about a bakers dozen spares I have now ... ;)

[Jon.Bucy]

Could it be that accelerator pump that is supposed to kick in the fourth cyl over a certain RPM. Maybe that is failing.
Good luck.

[sschefer]

Could it be that accelerator pump that is supposed to kick in the fourth cyl over a certain RPM. Maybe that is failing.
Good luck.

Good likely suspect. The accelerator pump seems to be fine. It shoots fuel to both three and four through a pair of check valve style injectors. I had trouble with those initially. I cleaned and tested them and they seem to be good. The accelerator pump feels like it's doing it's job when I stab the throttle there is a sudden burst as 3 and 4 kick in. With the electric fuel pump running 6-7 lbs at WOT and the accelerator pump fully engaged there is a possibility that if it isn't working correctly it could be leaning out the cylinder. I'm open to trying anything at this point just not so willing to keep throwing money at it.

I changed out the coils last night with a set of known good coils. I think I'll order a new accelerator pump and a set of check valves today just to remove them from doubt. That's fairly cheap and worth the money and time. That would make them the last items that have not been replaced on the engine.

---Edit Ordered 2 new check valve/injectors and a new accelerator pump. 84 bucks and some change.. should be here in a couple of weeks. The coils will probably fix the problem now that I've ordered parts.

Thanks

Steve

p.s. Gotta love the money pit:)

[sschefer]

Just got back from the lake. The problem was a bad coil. I had 3 Sierra's and 1 Merc, (all new). The Merc had a crack in it. I replaced them all with a set of known good Mercs that I had. I'll give the injectors and new accelerator pump to the next owner. This engine runs awsome now. Well awsome for a 100hp 2+2 anyway.