DT140 EFI high rpm

[SeriouR]

I have a 97 Dt140 EFI with 517 hours. I have owned it since 490 hours and its on a 16 ft fiber glass cabin boat. It originally came with a 3x13x19 SS suzuki prop which managed to push the boat to over 70KM per hour at 5200-5300 rpm. After doing a hardcore de-carb and changing spark plugs and fuel filters I decided to get a lower pitch prop (3x13 1/4x17 SS suzuki prop). It has now developed a bogging issue at high rpm (5300-5600). Basically it will start and run perfectly fine up to 5200-5300 rpm but over that it will run for a short while (sometimes 2-30seconds) up to 5500-5600 rpm and then it will bog down to 4800 rpm at WOT. If I reduce the throttle slightly it will (after a couple of seconds) increase to 5000 rpm and stay there quite happily. I have done numerous things to try to correct it and have gone out a few times (because i can still easly got 50+KM per hour) to see if the high rpm bogging has been repaired but still does it.

Compression is 120-125 over all four (cold)
NEW Spark plugs, fuel, fuel tanks, fuel line, primer bulb (second one as first new one failed after 2 hours), fuel fittings from tank to motor, low pressure fuel lines, fuel pump diaphragm/o-rings/insulator, first fuel filter, VST gasket/needle valve/cone fuel filter to high pressure pump.
Cleaned VST tank, high pressure pump/lines, injector screens/rail (all looked very clean)

I haven't yet checked the fuel pressure (as i don't have a gauge handy) or the fuel pressure regulator.

Starting to run out of idea's and looking at the very costly high pressure pump, also have made sure tanks are breathing well etc

It does seem to me as if it starved for fuel at WOT. Here's the strange thing if i change back to the 19Pitch prop it does not do it. May be it is more of an electrical issue at RPMS over 5300?? Any help would be much appreciated

[Solarman]

Several things you haven't mentioned.

First, when replacing diaphram in fuel pump, did you inspect those clear disc check-valves (if broken -replace pump, also if broken, pieces may be restricting fuel on back-side of needle valve seat)? This can limit supply of fuel to vst.

A portable tank could help rule-out any built-in tank issues.

Any leaks (air leaks) in fuel lines, or connectors. Check filter housing for cracks, those plastic filters can leak?

When motor starts to bog down, have you tried to pump bulb? Does it do anything when pumping it?

Did you replace needle valve seat, and re-set float level to proper hight? This can cause vst to run out of fuel at higher rpms.

It does sound more like a fuel issue, than anything electrical? Bogging is more like a restriction of fuel flow, especially at higher rpms.

If it is an electrical issue? The only area that might act as you describe, might be the TVS? But it might act like a dead spot in throttle at WOT, like it's not opening fully?

Yes, it would be helpful to check fuel pressure at rail, however, I feel if the regulator and hp pump are working at/up to 5k rpms, I think it is a restriction of flow at WOT.

Post back when able to let us know how it's going. Good luck.

[SeriouR]

Several things you haven't mentioned.

First, when replacing diaphram in fuel pump, did you inspect those clear disc check-valves (if broken -replace pump, also if broken, pieces may be restricting fuel on back-side of needle valve seat)? This can limit supply of fuel to vst. Yes the check valves are both mint. I inspected the needle valve previously and when i changed to a new needle valve (and needle valve seal) I also setup the float (it did appear to not be opening as much as it could). The needle valve/float are working correctly and it opens fully and shut off when float is at the top.

A portable tank could help rule-out any built-in tank issues. I am running two new portable tanks (45litres each) i have only been using one at the moment and I run Mobil 98 fuel with yamaha fuel stabilizer (did run yamaha ring free previously)

Any leaks (air leaks) in fuel lines, or connectors. Check filter housing for cracks, those plastic filters can leak? The primer bulb pumps up hard and stay hard (even after a few hours only requires a pump or two at most to be fully hard) i have gone over the lines a couple of times. i also running a external filter/water sep which has the primer bulb after it. The fuel filter housing is also new and the previous one has no cracks.

When motor starts to bog down, have you tried to pump bulb? Does it do anything when pumping it? No i haven't been able to prime the bulb when the problem is there as i am by myself. If the problem starts and i back the gas off the primer bulb is still quite full and again only two pumps and its fully hard again.

Did you replace needle valve seat, and re-set float level to proper hight? This can cause vst to run out of fuel at higher rpms. Yes was setup. Slightly pulled the float down to bend the stopper to open needle valve 1mm more.

It does sound more like a fuel issue, than anything electrical? Bogging is more like a restriction of fuel flow, especially at higher rpms.

If it is an electrical issue? The only area that might act as you describe, might be the TVS? But it might act like a dead spot in throttle at WOT, like it's not opening fully? I have only just bought the throttle position test loom so will be doing this next chance i get. I have checked timing at idle and that is spot on but again when i go out with another person will check WOT timing and try prime the fuel. I assume TVS is the throttle position sensor? (what does V stand for?)

Yes, it would be helpful to check fuel pressure at rail, however, I feel if the regulator and hp pump are working at/up to 5k rpms, I think it is a restriction of flow at WOT.

Post back when able to let us know how it's going. Good luck.

Thanks for jumping in to help. I have done countless search's and see you have contributed to most DT140 issues. It seems most have to do with the needle valve and the mechanical pump check valves...

[Solarman]

TVS (throttle valve sensor - used on EFI throttle body models), the tps (throttle position sensor - used on carb models, usually mounted on bottom carb throttle butterfly). They are tested differently.

If you cleaned vst thoroughly, and blew out all pas-sages with compressed air, and have float adjustment correctly, then i would suspect low pressure pump is not 100%.

I don't think timing is off at idle or wot. If timing or spark is off, the motor will stutter. What you describe sounds like a fuel flow restriction (or, running out of fuel at wot, when fuel demand is the greatest).

Providing compression in each cylinder is good, as indicated.

One other thought, you mentioned bending float down? The only thing you should bend is the small tab at the needle valve, the float should remain straight, unless it was damaged?

Good luck, post back when able.

[SeriouR]

I will test the TVS as per the workshop manual. This is why I got the proper test lead. I have tested the resistance and it seems to have a stable gradual change as you open the throttle.

I agree I don't think its the timing and more a fuel related issue.

I will check compression again.

The float was not damaged and initially I did just bend the part the needle valve sits on but it didn't look right (the needle valve still wasn't open enough to my liking, also when float was at the top the tab was at a slight angle to the spring loaded needle part). The part I ended up bending only slightly was the tab on the float that stops the float from going lower and sits on the side of the needle valve when at its lowest point. This allows the needle valve to open further. I tested it with the old needle valve in the original position by blowing through the inlet to VST and opening and closing the float and then the same with the new needle to see how much restriction there was and to ensure it did in fact close.

Hmm looks like i might have to get a new low pressure pump... And maybe another float

[Solarman]

It is crucial to set the float by the manual, if not set correctly the motor may run out of fuel at higher rpms.
Very much like it is running out of fuel.
Same with the low pressure pump, if not pumping enough fuel.

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

[SeriouR]

I will set the float to the manual and see how it goes. I may also bypass the external fuel filter just to rule it out before getting a new pump

[SeriouR]

It has been a while, i have been working on my boat and have just got everything running again. I still have not fixed this issue but i will be looking into it over the coming months. I now have a spare complete motor and a spare block. When engine was reinstalled i had the engine temp sensor fail (engine check light 1-4).

Has anyone here done any modifications/tuning on a DT115/140 efi?

[Solarman]

What modifications are you talking about? Tuning motor is done with the electronics, the gear counting coil, TVS, and ECM. The sensors for heat and oil will throw the motor into safe mode if there is a problem. Overheat is a serious problem for two stroke motors.

Good luck, post back on what you find out.

[SeriouR]

Modifications as in ignition timing and injector pulse width changes. I see Magnum tuning in the US do some tuning boxes. I have had all my vehicles tuned and they run a lot better with better fuel economy. Don't these motor also use a IAT an MAP sensor to alter timing and fuel? Still havent seen the MAP sensor except for the exploded diagrams and what i think is the IAT sensor looks the same as the overtemp sensor but just sit under the cover by the throttle body.

I have been out for the last two days and motor is running pretty well. Managed to get to 5800 RPM with the 13x17 prop. Still have the rev dropping issue, though doesnt seem as bad, but I have to to agree with your previous comment that it is the VST valve/float setup or the low pressure pump which i will get on to this week. I don really go above 5000 RPM anyway but I want to have the motor 100% for sea missions

Had another issue on the second day. It stopped firing on number 4 cylinder. Went to the nearest landing and pulled the plugs (had new spares onboard) and 1-3 looked good but number 4 was oil fouled. We had just done a bit of lower rev running (below 2000 rpm) and stopping the engine and restarting with 10-30 min in between. If I leave the engine off for 10min+ i get a bit of 2 stroke smoke which I though well i should adjust the oil pump cable again but looking at the plugs it is just number 4 that is the problem.

I have not looked into this yet but thinking I will ensure that number four coil gives a strong spark, get the injectors cleaned and flow/spray pattern tested, another compression test. Is there anything in the oiling system which could cause over oiling of one cylinder? Is there also a way to clean the oiling system while running a premix tank (ie seafoam in the oil tank)?

I am planning to order more parts for this soon and thinking of overhauling the oiling system (replace check valves, lines and sensors)

[Solarman]

Well on 2 stroke motors there is no MAP sensor to adjust timing, or fuel flow. The ECU, computer does that as I stated earlier.

The amount of oil being more in the bottom cylinder, may indicate your piston oil rings, or crank shaft sealing rings could be wearing out, or possibly the reed valves on that cylinder could be weaker? Not sure where you should look. But the oil pump is a set flow rate, that can be adjusted to increase/decrease oil flow. I have not ever needed to change oil flow on any motors, so not experienced there. But, if you do change any settings, make sure you're premixing until you know it is working well.

Good luck, post back on what you do find.

[SeriouR]

Ok so a compression test is the first point of call, then the reed valves. They are pretty hard engines to tear down as the corrosion is everywhere. Spare motor i have i removed the heads and the water seal seat in between the two heads is corroded to hell even though everything else is not too bad.

I will probably try rebuild the spare power head i have. I have heard previously though that boring the cylinders cannot be done easily because of the type of metal that is used? Have you have any experience rebuilding these?

[SeriouR]

Compression are:

#1 - 112PSI
#2 - 110PSI
#3 - 115PSI
#4 - 98PSI

Done cold with plugs out and plug coils disconnect with throttle body 1/4 open.

I assume this would indicate it is not the reed valves but more the piston rings?

I would really like to rebuild this but so many bolts are seized and buying heads is becoming difficult.

[Solarman]

I doubt it is worth rebuilding, as most parts are NLA, and anything dealing with the crankshaft is non-serviceable. Boring cylinders can be done, but again, most parts can't be bought. Used parts are the only way to find anything, but even those parts will be worn.

Your compression in #4 is just below the allowable 10% difference between cylinders.

Best to run it as long as it will run, then replace.

Good luck, post back on what you do.

[SeriouR]

I am going to try a shock ring free with yamalube ringfree and carry spare plugs for now and see if there is any improvement. I have a full working spare engine with good compression so just need to swap all the good parts over and im away again. i will keep posting as to outcome. Gutted as it does run really well otherwise