DT75 Help.. Engine misfires/sputters at high RPM

[ericgormley713]

I have a DT75 1989. I have replaced the following:

Fuel pump, spark plugs, boots, throttle sensor, fuel filter, starter, and we have cleaned and rebuilt the carbs. Engine idles great, accelerates fine, but once it gets up to full speed it begins to feel like it's not firing right. When i bring the throttle down, it runs normal. Any help would do. The mechanic has tested the voltage of the throttle sensor, along with synced the carbs. Help if you can, Thanks!

 

[mphelle8vld]

Do you have a tach? You may be under-propped and hitting the rev limiter, on my DT65 it kicks in at 5900.

 

[ericgormley713]

I don't have a tach but I doubt i'm hitting a limiter because my boat is only moving 9 mph at that point. I'm not sure though I can look into it.

 

[mphelle8vld]

Not sure what you mean when you say full speed but depending on the boat it's pushing, if you're only going 9mph it's more likely that the motor is in limp mode which should be around 3000rpm. A tach would be nice, do you have the Suzuki monitor gauge hooked up?

 

[ericgormley713]

No i don't. It's pushing a 20 foot pontoon. It accelerates fine. When I say full speed I mean full throttle. I will invest in a tach but I don't believe it's going into limp mode because when i decelerate it doesn't misfire. The prop is the proper size.

 

[mphelle8vld]

This would be a cheap way to troubleshoot: http://www.ebay.com/itm/New-Digital-...ZTadfo&vxp=mtr

The motor will run fine under 3k in limp mode, you may have low oil in the tank, or a low water flow/overheat problem, or an issue with the warning system itself. Hard to say without the monitor gauge hooked up. When was the last time it ran correctly?

If you find you're running at 6k you may have a spun prop and are hitting the rev limiter. There has to be a reason you're only getting 9mph.

 

[ericgormley713]

Is there a way to deactivate the warning system? I don't have oil in the tank and the sensor unhooked because I use a mix

 

[ericgormley713]

I just acquired the engine. It has never ran properly.

 

[mphelle8vld]

Does the oil system, (tank,pump,filter,lines) appear to be intact with only the connector to the tank sensor disconnected?

 

[ericgormley713]

I will have to double check that tomorrow. I can post a picture. Why, what are you thinking? I know that the sensor is disconnected for sure. I'm about to get new carbs for it because i'm running out of options

 

[mphelle8vld]

I have a DT75 1989. I have replaced the following:

Fuel pump, spark plugs, boots, throttle sensor, fuel filter, starter, and we have cleaned and rebuilt the carbs. Engine idles great, accelerates fine, but once it gets up to full speed it begins to feel like it's not firing right. When i bring the throttle down, it runs normal. Any help would do. The mechanic has tested the voltage of the throttle sensor, along with synced the carbs. Help if you can, Thanks!

Seems like the carbs are fine. A photo of the oil pump area will be helpful but I think the next step should be determining the rpm at which the motor begins running ragged.

 

[ericgormley713]

No Title

 

[tommarvin]

This sounds like another dirty carb post. Remove, tear apart,clean carb with carb cleaner and compressed air, install new carb rebuild kit, with all NEW gaskets, and the gasket between the carb and carb adaptor.
I know you said you already did this, but for some unknown reason cleaning and rebuilding carbs is a lot harder than it looks. Don't forget to adjust the float.
Why did you disconnect the oil injection, the motor was made for it, It's better less smoky, . You just have to maintain it, clean,it, and change the filter.

 

[ericgormley713]

I unplugged it because the wires were melted. I can try to repair it, but I definitely want to get the sputtering done first.

 

[ericgormley713]

Had someone clean the carbs. It's still sputtering. I ordered the TACH to find out how many RPMS I'm getting. One other thing I can think of that the mechanic told me but he doesn't believe is the issue, is that the top carb is from a 92, and the bottom two are from an 89

 

[99yam40]

with an unknown carb on the top cylinder who knows what it came off of and how it is jetted.
On my C40 Yamaha someone drilled out the main jets ( trying to make it a 50 I guess) and I have a high speed miss. Found that if I ran it with cowling and silencer off into the wind it ran well, but when turned around and run with the wind the miss came back as with the cowling and silencer on.

Finally bought new jets and compared the holes in jets to find out the old were different that the new ones.

Not saying this is your problem, but there is a chance

 

[ericgormley713]

Hooked up the rpm gauge. Its fluctuating from 1400 to 2300 but won't go any higher

 

[99yam40]

fix the alarm system and check to see what is calling for it to be activated.

I believe Suzuki has low oil flow and low water level along with over heat
probably low oil level too, but not sure

 

[mphelle8vld]

Sounds like the tach is not set correctly. You posted that the motor idles fine and accelerates that 20ft pontoon to 9mph so you must be getting at least 3k which would suggest limp mode. It will be difficult to troubleshoot with your melted wiring harness.

 

[ericgormley713]

I used that tach that you had me get off ebay. I'm going to try to reinstall the oil system and see what happens