98 Speedster- When one motor starts the other dies

[Oljoe]

Have a 98 Speedster, had starter replaced now when you try and start the second motor the other one dies, no matter which one you start first. They run fine by themselves but can’t run them together. Any ideas??

 

[Oljoe]

Ok so I checked all the fuses, they are good, batteries are good. If you start the P side first then the S side the P side dies. If you start the S side first the P side hit and dies, hits and dies. Help please. I’m stuck.

 

[Oljoe]

Did compression check and it’s 144-145 on all four cylinders

 

[Oljoe]

If you kill the S motor the P will start and run like normal.

 

[noviceskier]

*No expert but just guessing and using the little information from the reading posts about these*

Don't both the motors run off of 1 mpem? Could it be related to that? If everything mechanical is sound next would be electrical.

 

[Oljoe]

That’s what I was thinking, but wasn’t sure if that’s what it would be since they both run fine independently.

 

[Oljoe]

Would a bad ground cause this kind of issue??

 

[noviceskier]

Would a bad ground cause this kind of issue??

yes, it could

 

[Oljoe]

Gonna trace every one and check the ground blocks. Thanks for the help

 

[jts32]

That's a head scratcher. I don't see how it would be a ground issue, as both run fine individually, and don't see how starting the other would steal ground from the running motor. Is there a ground connecting both motors together? You had zero issues before replacing the starter? What type of starter was installed? If there were no issues before replacing the starter, I would look at the starter installation, or starter it's self.

 

[Sportster-2001-951C-Stock]

If system voltage drops too much, the running engine will stop. Dual or single battery? Check battery, might be weak?

 

[Sportster-2001-951C-Stock]

If system voltage drops too much, the running engine will stop. Dual or single battery? Check battery, might be weak?

BTW, never jump start these from most other running vehicles or leave connected a charger that might go over 14.5 volts b/c, too much voltage will damage the MPEM.

 

[Oljoe]

It’s a dual battery setup but it only kills the P engine. The aftermarket starter was replaced on the S side. I had a friend install it and thought maybe he had pulled some wire lose or something while changing it. It’s the strangest thing I have seen and so far no one has a clue.

 

[Oljoe]

I greatly appreciate all the help from everyone

 

[Sportster-2001-951C-Stock]

Well, if it wasn't doing this before, then maybe he made some subtle change to the position of the grounding cables or something.

Each engine has an flywheel "alternator" (magneto) and sometimes over the years one stops working and is abandoned instead of repaired but if you have dual batteries they must be wired in parallel? Or you possibly have a battery A/B/Both select switch, in which case you might try the BOTH switch position and see what happens (assuming one isn't already flat/dead).

When a starter is changed, it's common to install the negative cable onto the painted spot surface of the new starter, causing problems of high resistance. Not sure how that could affect the 2nd engine but if the grounds cables are looped from one to the other then off to the battery then maybe.

So take those grounds lose and clean them well using a wire brush, make sure to brush ring terminals/scrape/sandpaper clean any paint/corrosion off the spot face of the starter ground as well.

 

[Oljoe]

I will definitely address the grounds and it does have a three position switch (1-2-all). Gonna replace the battery in a day or so, they show they are good but are 3 years old. Thx sportster

 

[Sportster-2001-951C-Stock]

Also, keep the OEM starter (assuming it's an OEM) b/c the quality of aftermarket starters is notoriously poor, it's better to rebuild the original starter using the rebuild kits.

 

[Oljoe]

Do you think it could be the mpem? I have a guy wanting to sell me one said they are 1700 and he knows that’s they problem. Sounds like an very expensive guess. I’m just not sure that’s the issue since they will both run just not together.

 

[Oljoe]

Maybe rectifier?

 

[Sportster-2001-951C-Stock]

Maybe rectifier?

I'm curious, does the engine shut off while cranking the other? Or does it shut off AFTER the engine fires up? If the latter, then possibly the regulator (some are integrated rectifier+regulator, other older design is just a small rectifier).

But yes, it's possibly an issue with the rectifier/regulator stator system, if the voltage is dirty or too high the CDI ignition will malfunction. I'd especially be suspect if it's the regulator/rectifier type, more to go wrong with voltage control.

To isolate an issue with the charging system, unplug the regulator/rectifier, it will have at least two or three yellow wires with a red one. This device converts the AC generated by the permanent magnet magneto into DC and if it includes the integral regulator it conditions the voltage to around 13.8V DC or so.

So temporarily disconnect the rectifier/regulator and see if the problem goes away. While disconnected, that particular charging circuit will be disabled thus won't charge the battery.

 

[Sportster-2001-951C-Stock]

Usually when an MPEM dies, it's dead but you know some of the twin engine boats used a special dual MPEM that's hard as hens teeth to find a replacement for. So IF you ever do see one on the wild and can snag it cheap then do so.

It should be possible to convert over to jet ski MPEMS if you get into a pinch but a lot of work wiring it all up. Do your leg work there, b/c some later engines were different, they used a differently timed ignition system with twin ignition coils (one coil for each cylinder) and a lot of parts associated with ignition timing will need changing (flywheel?), I'm not sure on all the minute details.

FWIW, if you keep the test runs down to no more than a minute in length, you don't need cooling water. Could probably stretch that a little if need be but remember heat will build up quickly.

I usually fire mine off before backing down the ramp somewhere off to the side just to make sure it'll be ready, I don't use water b/c I figure if it starts it's ready to test in the water. But I practically live on the ramp, not 20~30 miles away.

 

[Oljoe]

Yeah it shuts off after the other starts. It has two connectors on the rectifier. Do I unplug the red and black connector and the yellow? Or just the red and black?

 

[Oljoe]

The second the other starts it just shuts off

 

[Sportster-2001-951C-Stock]

Okay, that's interesting.... So it doesn't seem to be a low voltage condition that occurs while cranking up the 2nd engine.

As far as the RR connector goes, the yellow wires carry current from the AC magneto coil to the RR, the red wire is output current from the regulator (or rectifier, it's not clear but probably it's a RR) which eventually makes it's way to the battery charge current and the black is ground.

Personally, I prefer to disconnect the yellow wires from the RR b/c it's IMO less chance of damaging the RR. You can disconnect them all and accomplish the same goal, but the idea is to disable the charging circuit to isolate it as a potential cause. Either disconnecting the RR completely, the RR input AC(yellow wires) or the output would disable the charging circuit.

Ordinarily, both P and S engines have magneto stators with RR's, so you have to do this for both.

 

[Oljoe]

Ok I’m lost unless it it’s the MPEM. I have unhooked the rectifiers, same deal, checked all fuses and grounds cleaned all grounds. Switched ignition switches, changed DESS post from my jet ski, still no luck.Run diagnostics and got the all clear and engines started but still can’t run them together. Everything works as it should except the S motor will shut of the P and if you try and restart the P it hits and dies.