'89 DT75 dropping a cyl at speed

[Flatsboy]

'89 DT75 ran fine last fall, now dropping 1 cyl above 3,000 after 1 minute running, let it idle and it will run up again then drop a cyl again. Will run at 3,000 all day on all 3. Neither oil or temp alarm sounds off when cyl drops. Drained carbs last fall, new gas and oil this spring.

Since this happened I did some maintenance - high numbers on compression, all within 2 lbs of each other, replaced oil tank filter/flow sensor, replaced impeller. Took old sensor apart and the screen was pretty clogged with gunga, drained and cleaned oil tank, bled pump, rebuilt fuel pump.

Today I ran it thinking maybe the clogged oil filter/sensor was it but no go. When the cyl dropped I ran it on two back to the dock and shut it down at speed (barely on plane). Top spark plug was hot but noticeably cooler than the bottom 2, still looks like it's firing but dirtier than bottom 2.

Leaning towards a coil. Have service manual coming but was hoping someone here can clue me in on how to test the coils or have another idea. Was a dlr for Merc and Yamaha so Suzuki is new to me. Seems like a great engine, just finding my way around it.

Thanks in advance!

[Solarman]

If the plug wires are same length, could try swapping coils between two cylinders, and see if still the same?
Before swapping, could use timing light to see if losing spark on any plug when this happens, then swap coils and repeat the test.

There may be a flow switch combined in the temp sensor that may cause motor to limit at 3k rpms on your model, but because motor isn't overtemping, it might not set off alarm?

Also the gear counting coil could be acting up at higher rpms?

It might be better to wait for the suzuki manual before jumping the gun. Then you can test those items by the manual.

Good luck, post back when able to let us know how it's going. And if you find anything.

[Flatsboy]

Thanks for the reply, manual still not here but good advice.

[2014DF25ARS]

You don't say that you replaced the plugs... I have a 65HP Evinrude 3-cyl that was doing the same thing - randomly dropping one cylinder above half throttle. I could back off throttle and it would accelerate. After looking at most stuff you're looking at, I was about to replace a coil pack, but they're kind of expensive. So I decided to replace the plugs first, though all three looked similar and fine to me. No more problem. Not saying it's a sure thing, but maybe worth doing.

[Flatsboy]

New plugs were in before I 1st posted. Got 3 coils & manual, coils ohm out correctly, old ones too. That wasn't the issue, still won't go over 3,000 for more than 15 seconds. Any ideas? Thanks in advance.

[Solarman]

That vintage (89) i believe has a 3-wire sensor, this should include a flow switch. It may be the flow switch is shutting down one cylinder if switch is dirty/stuck/not working, even if temp light doesn't light up. Also, check if your model has a neutral safety switch on the motor (not sure?), should be around linkages, if yes, this will also drop a cylinder if not working.

Good luck, post back when able to let us know what you find out.

[Flatsboy]

Thanks! If you are referring to the oil flow sensor that was one of the 1st things I replaced. According to my factory service manual the only neutral safety switch is inside the shift/control box and that circuit is operating as it should.

According to the service manual, anything that limits RPM to 3,000 should activate the alarm:
Overheat
Oil Flow
Low oil
Over revving

The Warning system circuit buzzer activates at ign on and then goes out after a few seconds as it should. The engine is not overheating, there is a full oil tank with a clean screen, a new oil flow sensor, new coils that test correctly and no over-revving. Carbs are clean, fuel pump rebuilt, all with OEM parts.

I'm running out of reasonable options.

[Solarman]

No, actually talking about the heat sensor. If yours has 3 wires then it may have the flow switch.

Post back and let us know. Good luck.

[Flatsboy]

No, actually talking about the heat sensor. If yours has 3 wires then it may have the flow switch.

Post back and let us know. Good luck.

Mine is a 2 wire. What I've done so far:
Drops cylinder above 3,000 rpm. No alarm.
Rebuilt fuel pump
New fuel line, engine coupler and bulb
Changed plugs
New oil flow sensor
Replaced coils, tested new & old ok
Removed oil tank, cleaned screen, bled system

Tested today on lake, now it'll run to 3,700 before it cuts out. Back to idle and it'll run back up to 3,700 and not stall unless it goes higher. New impeller 2 yrs ago, good telltale stream. Haven't tested heat sensor yet.

Sorry to overcook this but trying to get a grip on the system, photos of difference between Service Manual 99500-95520-01E and reality (hope this helps someone else), the wiring colors for the Cyl Temp Sensor and Overheat Sensor are backwards in the manual. The last pic shows a low resistance reading on the Cyl Temp Sensor (was wondering if this could cause my issue?). If images don't open, right click on each and "open in new tab"





Questions:
1. if I disconnect the Oil Flow Sensor it seems the switch will stay open and I can see if that's the culprit (although a new switch)
2. same as above for the Overheat sensor?
3. same as above for Cyl Temp Sensor? Don't think so cause if CDI senses incorrect timing it engages the RPM limiter

Any thoughts? Thanks

[Flatsboy]

Stumped, reaching out for help

Well almost. The issue: last year it ran fine but when running hard for a while (pulling a tube) it would cut out on rare occasions but would go back to running fine after a few minutes idling. This summer it runs great to full throttle or anything above 3,000 rpm for up to 10 seconds then stutters and drops to below 3,000, if I let it idle a bit in neutral it'll run back up fine for the same 10 seconds then cuts out again. It'll run all day at 2,900 rpm. The alarm never sounds when it goes into protection mode (if that's what's happening) and drops rpm but it does sound off at startup for a few seconds so its working. I do not have the OEM gauge.

Here's what I've done:
Compression test 135 in each cyl
New impeller (good stream)
New fuel line, bulb and engine connector
Rebuilt fuel pump
Changed plugs (new NGK)
New oil flow sensor and oil lines
Replaced coils, tested new & old ones tested ok too
Removed oil tank, cleaned screen, bled system
Tested overheat and Cyl temp sensors per manual, both fine
Tested oil level sender (float works fine)
Disconnected oil level sender, oil flow, overheat and Cyl temp sensors separately and tested on the lake, same result.
Ran it on 6 gal tank with separate hose bypassing the built in tank with same result.
Just put a new thermostat in, apparently the CDI will detect the engine running too cold as well as too hot and engage protection mode. Haven't run the boat yet, will advise.

Aside from replacing the CDI I can't think of what else may be causing it. The fact the alarm isn't going off isn't helping, according to the OEM service manual only the rev limiter will engage the protection mode w/o an alarm but if the protection circuit fails supposedly everything goes south and the alarm should sound thinking all sensors are reading over-limit. The local Suzi dlr says there is no separate rpm pickup or sensor, the CDI reads rpm directly.

Any help would be greatly appreciated. Once get this sorted out I'll post the culprit. I don't mind replacing these items as the engine, while like new and healthy will have a new baseline of serviceable parts installed and common pitfalls sorted through.

[Solarman]

ok, should check 3 things. easiest to most difficult. check the gear count coil, then the output of the magneto to rectifier - ac & dc sides, then last the tps (throttle position sensor). if any one of these are reading wrong, they will not send an alarm signal. measure them by the manual tests.

Good luck. post back what you find.

[Flatsboy]

Thanks, will do. Question though, if they are off will they trigger protection mode?

[Solarman]

if not enough voltage from magneto/ rectifier/ regulator system, ECU can go into safe mode.

if gear counting coil goes out of sync, they're still debating if ECU will trigger safe mode, i guess it depends on how it goes haywire.

If TPS goes out this could also trigger safe mode, but how would you know if safe mode? it could go bad at exactly the rpms that safe mode goes to?

however, none of these failures will normally set off the alarm.

Good luck. post back what you find out when able.

[Flatsboy]

Good info, I'll be testing those in the next few days. I ran it with the new thermostat yesterday but nothing has changed. Even my local dlr says that vintage CDI/Protection system has a little voodoo in it

[Flatsboy]

Update

Grumman Suzuki 1989 DT75 07501913239

The issue: last year it ran fine but when running hard for a while (pulling a tube) it would cut out on rare occasions but would go back to running fine after a few minutes idling. This summer it runs great to full throttle or anything above 3,000 rpm for up to 10 seconds then stutters and drops to below 3,000, if I let it idle a bit it'll run back up fine for the same 10 seconds then cuts out again. It'll run all day at 2,900-3,200 rpm depending on load. The alarm never sounds when it goes into protection mode and drops rpm but alarm does sound off at startup for a few seconds so its working. I do not have the OEM gauge.

what I've done:
Compression test 135 +- 1% in each cyl
New impeller (good stream) 7/29/2017
New fuel line, bulb and engine connector
Changed plugs (new NGK)
Rebuilt fuel pump, OEM kit
New OEM oil flow sensor
New OEM thermostat
Replaced coils, tested new & old ones tested ok too
Removed oil tank, cleaned screen, bled system, checked OHMs, OK
Tested overheat and Cyl temp sensors per manual, both fine, replaced anyway
Swapped CDI from friends same yr that runs fine, no change
Swapped complete magneto from above engine, no change
Disconnected oil level sender, oil flow, overheat and Cyl temp sensors, TPS separately and tested on the lake, no change
Ran it on 6 gal tank with separate hose bypassing the built in tank with same result.
Checked gear counting coil, TPS, mag components and rectifier per manuals (both OEM and Seloc), all within spec.

Running out of options, was a Merc mechanic for 20+ years but this thing is beating me. Again, the alarm does not sound when going into limp mode.