‘91 DT 115 Idling Issues

[Busterbrown_0627]

Hello everyone,

I’ve got a 91 DT115 that is giving me some issues. Been doing a lot of reading on the forum but would like to get some more input.

The issue I’m having is that the engine will not stay idling and just die in neutral or low rpms in forward or reverse. If I can manage to get the engine up to higher RPM’s it seems to run very well. I thought it was a fueling issue so I recently cleaned all carbs(Full disassembly, ultra sonic cleaner, compressed air through all passages) and put in new float needle valve assemblies and confirmed correct float heights. I also replaced the fuel filter and changed the diaphragms on the fuel pump. The clear white plastic pieces in the fuel pump looked to be in good shape.

After doing all of that I went out onto the lake to test it. I had a hard time starting it at first but after 5 minutes or so it eventually got going and seemed to idle okay. Once out of the marina I brought it up to speed /higher rpms and it felt very smooth and ran nicely. Took it for a good 10 minute run at higher rpms and then went back down to idle where it idled for about a minute or so. When it was idling I would hear an occasional miss here and there and it eventually died on me. Would start it up and get it to idle and it would only run for 20 seconds to a minute then die.

I tried to adjust the pilot air screws to see if they had any affect on the idle at all. I would turn all 4 of the screws up to a half a turn at the same time and the weird thing is it didn’t seem to have an effect on how the engine was idling. I even turned one of the screws all the way in to the seat while it was idling and it seemed to have no effect? That didn’t seem right to me. Is the pilot air screw just for mid to high range jets? I will reset all pilot air screws to the nominal 1 + 1/4 turn out since their position didn’t seem to have large effect.

So I’ve got a list of things I want to check out but was hoping I could be steered in the right direction hopefully so that I’m not chasing my tail to fix this thing.

1. Confirm Sync the throttle linkages by backing the idle adjustment screw all the way out on the #4 carb, loose each individual idle adjustment link closing all butterflies completely, tighten adjustment screws then tighten idle adjustment screw until butterflies open 1/16” run and check idle rpms (if it stays idling)

2. Do these older engines have the white wire issue? I’ll have to confirm but I’m fairly certain my tilt up and tilt down switches on the throttle/remote control only work when the ignition is in the on position. They do not work when in the off position.

3. Compression Check. Hopefully i can get it warmed up enough with some idle/dying cycles to do a proper check. As long as all the readings are close that should be enough information. What do I do with the spark plugs/leads while they are out of the engine while performing the check? I don’t want to damage any ignition related components while they are out while I’m cranking the engine.

4. Test electrical components. Gear counting coil, pulse coils under the flywheel. I tested the ignition coils with the spark plug boots removed and they are within spec.

Thanks for your help!

[Solarman]

Linkages are probably fine, wouldn't mess them up trying to change them.
if you cleaned all carbs as stated and blew out all the small pas-sages with all jets removed, then that's probably fine. Set all of the idle mixture screws to factory/ service manual settings, then don't change them. The main idle stop screw should be adjusted by the manual.
The only other possible test / replacement would be the TPS (throttle position sensor), should be on one of the carbs attached to the linkage, this reads throttle positon.
check or replace as needed.

Good luck. Post back on what you find out.

[Busterbrown_0627]

Thank you for the response!

How do you test the throttle position sensor? The one on my engine is a 2 wired version on the plug and seems to act more like an on/off switch. My manual only says how to test the 3 wire version.

1. I checked the linkages and they seemed linked up so that was good.

2. Checked out the wiring diagram for my engine and yes the PTT switch on the throttle lever only works when the key is in the on position. I have another weird problem though. The up button on the PTT switch on the engine itself is very intermittent as to when it works. I tested the switch itself and even tested for power at the relay when pressing that switch and it gets power. The relay couldn’t have failed either because the PTT switch on the throttle uses the same relay. It’s almost as if when I press the up button on the PTT on the engine itself it is finding ground through somewhere else and is not going through the relay.

3. Compression check seemed like it was good. This is a low hour engine. All cylinders showed 95psi. Which seemed low in my opinion but at least they were all consistent. Could be something wrong with my compression gauge setup.

4. I tested the resistance of the gear counting coil and the pulse coils and they checked out fine. I checked the resistance of the spark plug boots and there was a bit of a discrepancy there. 1 4.4K ohms 2. 3.8k ohms 3. 4.4K ohms 4. 4.8k ohms. Boots on #1 and #2 are also cracked. I ordered 4 boots to replace those.

[Solarman]

I actually traced my ptt wires all the way back to the motor, using a continuity tester, unplugging one wire at a time, till I found a small 3" jumper wire (looked new) with no continuity between the 2 ends? It was crimped well, not loose, both ends looked perfectly clean, but totally open? Replaced and all worked as it should. So no idea at this point.

As far as idling issues, testing that TPS sensor has to be by the book. It seems to be a step-up type sensor, that adjusts timing per opening of throttle? The book gives ohm values for each step I believe?

Mine didn't seem to test right, and I replaced, then problem went away.

Good luck, post back on what you find out.

[Busterbrown_0627]

Thanks for the reply. I found a way to test the throttle valve sensor. It wants .45-.55v at the idle stop and > 2.75v at max throttle. When I first check it my idle voltage was at .33v.. a little low. I adjusted it to .55v but my full throttle voltage only goes to 2.4V. After the adjustment the boat seems to be idling okay on the muffs. I want to take the boat on the water though and see how it behaves there. When I rev up the engine it seems to have a miss though. I’m guessing the throttle valve sensor could be causing the miss if it’s not outputting the proper voltage to the CDI. I have another Throttle Valve sensor ordered.

I also replaced the Spark Plug Boots and Thermostat.

[Murray]

Spark plugs?

[Busterbrown_0627]

Update! Thanks for the Response Murray. Your comment did get me thinking a bit. I’ve been using NGK BR8HCS-10s and noticed they looked a little fouled up. I decided to try one heat range higher and went with some BR7HS gapped to .035”. A few days ago I took the boat out again with the pilot air screws on the carbs set to nominal 1 + 1/4 turn out. Start up was very bad. It would start, die, start, die, start, die, eventually it would stay idling but had the odd cough now and then. Took it out on the lake and it ran good at higher rpms as per usual. Stopped the boat in the water to see how the idle was and it died on me a couple times. Then as the engine cooled off it would idle again, not great but it wouldn’t die.

On the lake I tried to adjust the idle up with the idle screw on the #4 carb that turns the butterfly valves. Openining it slightly caused the engine to die. Also tried adjusting the pilot valves on the carbs and turning out caused it to die. At this time I did not know that turning it out caused it to lean out. As you can probably guess by now the idle mixture was too lean. I put everything back to nominal and at this point I was just happy that it was better than before but knew it still needed more fine tuning. The engine should not be having any un-commanded shut downs.

I did some more research and read that turning the pilot screw in riches the idle mixture and pilot screw out leans out the idle mixture. So instead of setting carbs to 1 + 1/4 turn out. I set them to 1 + 1/32 out. Went to the lake today and it started great. Didn’t even die once. Idled pretty decent too with no coughing just seemed a bit low. Adjusted the butterfly valve idle screw to increase the rpm a bit and it seems to be excellent now! Ran it tonight in many different situations and it didn’t even think of dying on me. I think it’s safe to say that it is running how it’s supposed to now.

To sum up what I think my issues were. I believe throttle valve sensor was set too low at idle .35v (should be between .45v-.55v). Engine was running too lean at idle (Adjusted pilot needles to 1 1/32 turn out).

Not sure if the spark plugs helped but might as well mention it. Used BR7HS spark plugs. (Previous spark plugs looked fouled up, decided to try one heat range higher, could be fouled from bad starts when it was too lean).

Anyways figured I’d post this update as sometimes people post issues and we never find out if they got it fixed or not!

Cheers and thank you for your help Murray and Solarman