720 randomly runs rough

[H€PHÆ$TU$]

I bought a rough 98 Sportster 1800 with dual 720's. I've been sorting through all the things that the PO modified / screwed up but this one big issue has been plaguing me since the beginning. The right engine will not sustain past 6500 rpm. (Left engine runs fine.) The problem is random though. Sometimes it will run about 6500-6800 rpm for a while but it always stumbles eventually and drops back down. It will never reach the rev limiter. If I don't bring the throttle back it will eventually die and be difficult to re-start. But if I pull the throttle back as soon as it starts doing it, it will stay running and continue stumbling at around 300-400rpm. Once it starts stumbling, it won't rev very much at all. 3000 - 4000 rpm is all you can get out of it and you can audibly hear that is is stumbling, like the timing is off. But you can listen to it and hear when the stumble clears out and it will rev to 6800 again at that point for a random amount of time until it does it again. Plugs look great, cardboard brown on the tips, no matter when I pull them. This engine is a new SBT engine with less than 10 hours and 150+ psi on both cylinders and this same issue existed with the previous engine that had a completely different stator and magneto in it. (Had to replace it because PO ran the RV cavity dry and I got the last of the RV gear's life.) To date I have tried multiple sets of new plugs, which don't help at all, I have replaced the entire ignition box and both plug wires with a new-looking donor from a parts boat that I have. Today I took it out and it was still doing it but I brought a BRAND NEW (not rebuilt - NEW from Mikuni) carb with me on the off chance that it was fuel-related. (Even though I've rebuilt my original carbs 3x in the past 3 months.) It happened again, I replaced the carb out on the water, and it happened immediately again. I also brought a new voltage regulator with me and swapped it out, which did seem to change the rpms that the engine would sustain, but still wouldn't let it hit redline.

Because the problem is random it's hard to tell if anything I do affects it. If I change something and it runs good it's hard to immediately tell if it was because of the change or just one of the random times that things work. But, as a last-ditch try today I swapped all the ignition wires from the left side to the right side engine. (Pulled the connector out of the left ignition box, connected it to the right ignition box. Pulled stator connector from left engine, moved to right engine. Pulled temp sensor wire from left engine, moved to right engine.) This way the same MPEM circuitry that WAS running the left engine (which runs fine) was now running the right engine. For the first time since I got this boat the right engine hit it's redline and bounced off the limiter. I wasn't able to run it very long that way because I didn't want to flood the left engine out as it wasn't running due to the right side wires not being long enough to reach the left engine.

In summary, all ignition components between MPEM and right engine have been replaced with no improvement. Carb has been rebuilt 3 times and now replaced with new, no improvement. All fuel lines are new automotive fuel line. The only improvement appears to have been when I run the right engine with the MPEM wiring from the left engine.

My understanding is that the MPEM does not control timing, but the rev limiter is integrated into it. Could this possibly be a symptom of a bad right-side rev limiter?

I have the MPEM and key from the parts boat. Would my next logical step be to try it? Can anyone think of anything else?

Something else I've noticed is that the right voltage regulator gets warm while running but the left stays cool. I don't know which condition is normal. I know that the regulator isn't supposed to have anything to do with ignition on the 720's and that the ignition system is separate from the charging system. But temporarily plugging the new regulator in did seem to increase the rpm's it would sustain from about 3000-4000 to about 4000-5000. I also tried with both voltage regulators completely unplugged but there was no change.

 

[chipskittles]

Since you have a spare MPEM I would swap it. The MPEM does contain the rev limiter functionality. Your experiment to swap port with starboard seems to confirm a bad MPEM. Could not tell if you swapped coils between the two but the coil could be at fault as well, since these coils have the CDI built in. The coils don't fail near as often though from what i've read it's always the MPEM.

The voltage regulator is supposed to get warm. It's in an aluminum housing that is bolted to a metal sheet(heat sink) that's how you know. You could swap that with the other side as well to see if there is a change. With previous owner in the mix you can't even know if everything in the box is connected correctly unless you read and understand the wiring diagram. It helped me to print out the diagram and label everything.

 

[H€PHÆ$TU$]

Since you have a spare MPEM I would swap it. The MPEM does contain the rev limiter functionality. Your experiment to swap port with starboard seems to confirm a bad MPEM. Could not tell if you swapped coils between the two but the coil could be at fault as well, since these coils have the CDI built in. The coils don't fail near as often though from what i've read it's always the MPEM.

The voltage regulator is supposed to get warm. It's in an aluminum housing that is bolted to a metal sheet(heat sink) that's how you know. You could swap that with the other side as well to see if there is a change. With previous owner in the mix you can't even know if everything in the box is connected correctly unless you read and understand the wiring diagram. It helped me to print out the diagram and label everything.

Have not swapped coils between the engines but have replaced the right coil / ignition box with another used one from the parts boat with no change.

I'm going to get my other mpem and give it a try.

 

[chipskittles]

So I agree, the coil is probably fine. Pretty sure there is a posting out there somewhere of someone experiencing almost the same thing, and it was the MPEM in their case. When they go it seems to be either A) the diode blows from jump starting/power surge(you can fix this issue) or B) the rev limiter circuitry goes wonky.
For anyone else reading, the service manual or owners manual i forget which, specifically says to disconnect the multi-pin connector on the MPEM box any time you disconnect the battery. They can apparently be just that delicate.

 

[H€PHÆ$TU$]

The PO of the boat has battery disconnects installed as well as an on board battery tender.

 

[H€PHÆ$TU$]

A) the diode blows from jump starting/power surge(you can fix this issue) or B) the rev limiter circuitry goes wonky.

Can you please direct me to any threads you may know of explaining more about this? The MPEM in my parts boat had a service tag on it that said, "blows 5 amp fuse." It also has a small area in the front of it that is cut out and re-filled with a gray silicone-like substance. As soon as I saw it I knew it had been repaired in the past. It looks like a professional job, like whoever did it knew exactly where to cut and what to do. So between that and you mentioning it I knew I should google it and I found tons of results about the diode being replaced in ski MPEM's, many saying that the 5 amp fuse was blowing, but nothing on the boat / 2-in-1 MPEM's. I'm curious to learn more about it ...

 

[chipskittles]

This thread follows repair of a 95 speedster MPEM, not the same of course but the layout of the power input circuit will not change appreciably.
'95 Speedster MPEM Repair 278-000-601 - 5A Fuse

 

[H€PHÆ$TU$]

This thread follows repair of a 95 speedster MPEM, not the same of course but the layout of the power input circuit will not change appreciably.
'95 Speedster MPEM Repair 278-000-601 - 5A Fuse

Thanks. based on that it appears the diode problem leads to blowing the 5 amp fuse almost exclusively, which mine is not doing. I suspect my issues are with the rev limiter if they truly are in the MPEM, which at this point seems like the most logical suspect.

It will be Fri. before I can get my spare MPEM in and test it but I'll report back.

 

[H€PHÆ$TU$]

I ran by the shop today to grab my spare MPEM. If anyone is ever wondering where the diodes are on a dual-engine module, I'm betting they're right under where that silicone is. Makes sense since it's right next to the 5-amp fuse that blows.



Praying this thing resolves my issue.

 

[H€PHÆ$TU$]

I pulled the battery disconnect switches and tender out of the boat last night and restored all the battery positive and ground wires back to as close to stock configuration as possible. I cleaned all the terminals with a grinder and flap disc to get back to shinny copper. Unfortunately, it didn't seem to change anything that I could tell. If this MPEM doesn't fix it I think I'm going to start pulling apart all the ground splice connectors and investigating those.

I have a random black wire coming out of the loom that isn't connected to anything. I don't know what it goes to but it doesn't seem to change anything when I ground it. It actually is grounded without being connected to anything so I assume it comes from one of those ground splice blocks and I should be able to trace it once I remove the splice bar. The PO deleted the factory oil tank as part of his pre-mix conversion so at first I figured it went to that sensor, but looking at the wiring diagrams, that sensor doesn't have a black wire, just blue & blue / black. Wonder where those wires are! LOL

Got to love forensic mechanics!

 

[H€PHÆ$TU$]

I guess my parts boat MPEM is bad? I plugged it in but can't get any power to the 5-amp fuse. It has a repair spot on it that was cut out to replace the diodes. I tried digging all that silicone out but it's impossible to get it clean enough to diagnose anything.

I checked all the grounding blocks I could get at in the storage compartment. I didn't see anything wrong there.

I also used a 12-volt pump to suck some fuel out of the bottom of the tank to make sure I didn't have any water.

Checked compression, still over 150.

I guess all I can do at this point is either make a test harness to lengthen the right side ignition wires to run the left engine or just cut and lengthen the wires themselves.

I'm seriously getting to the point where I'm ready to part this POS out and give up though. I've never been defeated by anything mechanical this bad before.

 

[chipskittles]

It's tough dealing with 20+ year old boat stuff. Somewhat related anecdote about my 97 speedster; we got it with a seized blower fan which is not uncommon. The parts boat had one so I tested it, it works, I swapped it. It worked in the boat for a couple days but then i tried it later that week and it did not work.

After ruling out the switch I tested volts at the blower's connector and found 12v with the switch on so blower failed right? Ordered and replaced another blower fan but the new one didn't work either.
It was then that I looked at the connections at the fuse block that's under the hood storage bin on the 97 speedster.

It seems that the multi-pin connector was getting bumped by the storage bin being installed and it had some corrosion starting due to its location in the hull. I rerouted the wires outside of where the storage bin sat, which fixed it partly, but moisture from inside the hull made it stop working again a few days later. At this point I sprayed down the connector with marine lube and I have not had problems again. Basically volts are not all there is to it, apparently the connection was good enough to pass volts but not good enough to pass enough amps to run the blower. I'm not new to electrical work on a lot of vehicles and even rewired atvs and such but this one got me. I did not tell my wife the new blower was uneeded

Your parts MPEM -what is it doing for the boat? You say it's not getting power at the 5amp fuse? So it's totally dead?

 

[H€PHÆ$TU$]

It's tough dealing with 20+ year old boat stuff. Somewhat related anecdote about my 97 speedster; we got it with a seized blower fan which is not uncommon. The parts boat had one so I tested it, it works, I swapped it. It worked in the boat for a couple days but then i tried it later that week and it did not work.

After ruling out the switch I tested volts at the blower's connector and found 12v with the switch on so blower failed right? Ordered and replaced another blower fan but the new one didn't work either.
It was then that I looked at the connections at the fuse block that's under the hood storage bin on the 97 speedster.

It seems that the multi-pin connector was getting bumped by the storage bin being installed and it had some corrosion starting due to its location in the hull. I rerouted the wires outside of where the storage bin sat, which fixed it partly, but moisture from inside the hull made it stop working again a few days later. At this point I sprayed down the connector with marine lube and I have not had problems again. Basically volts are not all there is to it, apparently the connection was good enough to pass volts but not good enough to pass enough amps to run the blower. I'm not new to electrical work on a lot of vehicles and even rewired atvs and such but this one got me. I did not tell my wife the new blower was uneeded

Your parts MPEM -what is it doing for the boat? You say it's not getting power at the 5amp fuse? So it's totally dead?

Correct. Dead. Plug my mpem back in and all is fine.

 

[chipskittles]

That sucks. I also have a spare MPEM for my boat, my luck it's probably dead too.

 

[chipskittles]

I am not familiar with the 98 speedster but it's got dual 720s, so if I were in your predicament i would absolutely wire in two PWC 720 MPEMS. It's very doable from what i've seen. You will lose your neutral safety, and DESS(BFD), and will have to figure out how to integrate the two to use one start/stop switch.

 

[H€PHÆ$TU$]

I've made an extension harness that runs the left engine off the electronics that were running the right engine. (Left engine wires reached right engine in factory form, so no additional harness needed for that.) If the issue switches engines now I'll know the problem is either wiring or MPEM. If it doesn't switch engines then I'll know the issue is something on the engine. It's a little trippy starting the right engine with the left button and then watching the left tach go up when I move the right throttle but it'll be OK for temporary testing purposes.

Rain today and tomorrow so I'm hoping Sun. clears up so I can take it out for another test run.

 

[mikidymac]

Nothing to do with your issue but I hope you are still supplying the rotary bath with oil now that you removed the tank and are premixing.

 

[H€PHÆ$TU$]

Nothing to do with your issue but I hope you are still supplying the rotary bath with oil now that you removed the tank and are premixing.

Absolutely!

I didn't remove the tank. The PO did and he didn't do a good job of keeping his lines full and both RV gears were almost gone when I got it. That's why both engines had to be replaced. The engines in my boat now are less than 10 hours old. But this issue with the right one has existed with both of them exactly the same which is why I'll be very surprised if it turns out to be anything to do with the engine itself.

Anyway, I've built a new tank that functions like the original to ensure that my new RV gears stay bathed.

 

[H€PHÆ$TU$]

Just updating. My problem appears to be resolved. I think the right engine being the main problem engine was a red herring. Eventually the left engine started doing it and the right engine stopped. So I was forced to start chasing the problem on the left engine instead of the right. I decided that replacing plug wires and boots was a worthwhile endeavor and while I was working in the left side ignition box I found this:

Nothing else in the box looked great so I replaced the entire box with one from my parts boat, which looked new, and all seems to be fine now. Odd that the left engine was even running. Even weirder was the fact that it was the right engine acting up 99% of the time. Had the left engine not started acting weird I likely would never have thought to check this fuse. I had checked the right side many times. It is such a simple and illogical fix that I doubted it was even the real issue but the boat instantly acted great the first trip out after replacing the box and has been fine for 2 test runs, much longer than I had ever had it running in the past.

42mph for a few hours straight the past 2 days. This has not happened since I bought the boat: