Suzuki DT55 RPM drop

[imported_stinger]

On my Suzuki DT55 1994 with few usage hours recently bought, it occurs an RPM drop several times without any fault indication on monitor instrument. Water temperature is OK, oil reservoir is full, the oil consumption is OK, the water flow sensor is OK and the engine returns to the normal behaviour after about 10 minutes it is off. The above occurs only randomly and it is not related to the engine functioning period (e.g. I travel for 90 NM without any problem before a new occurrence of it). It was in my intention to check the oil flow sensor but its screws are sealed. Could somebody, that experienced a similar issue, give me some advices?

Thanks and regards

 

[bossee]

Re: Suzuki DT55 RPM drop

Hi,

Do You use an external portable gas tank?
Is the tank, fuel houses, connections and bulb known to be working without problem?

When You get RMP drop next time try to pump the fuel bulb and see if helps. If it helps You have probably a fuel delivery problem.

Have you new spark plugs with correct heat range and gap?

Have the carburators been overhauled? Maybe some dirt in them?

When was the impeller (waterpump) last changed?

Have you checked the thermostat is opening and closing at specified temp?

If any overheat problem the overheat alarm should get on but.. impeller and thermostat must work for the engine to work optimal. Since it works fine after 10 min at rest it could be an overheat situation (but that is only one of several possible tracks to follow).
Could be fuel pump rubber diaphgram set also that is bad but I doubt.

/Bo

 

[deejaycee_2000]

Re: Suzuki DT55 RPM drop

My geuss would be, to replace spark plugs and set correct gaps, give the carbs a good clean .... my 30hp suzi does the same thing occasionally ....

 

[imported_stinger]

Re: Suzuki DT55 RPM drop

Hi to all,
I give you more infos: I used two different portable gas tanks but the problem occurred with both; sparks have been replaced and impeller is new. I’ve drained carburettors, in the fuel line it’s installed a new Mercury water filter. The drop of RPM to about 3000 should indicate that a malfunction has been detected by the engine safety system, but no light on monitor nor buzzer sound help me to identify failure. For the above, in my opinion, it can be excluded that a poor fuel supply occurred. From the symptom it is reasonable that the malfunction occurs due to overheat, but water comes regularly out from the water pilot discharge hole and from the exhaust hole and its temperature seems to be correct. Perhaps, I presume, if the thermostat does not completely open in certain conditions, the engine cooling does not completely satisfy the cooling requirements. I’ve a little doubt about of the amount of water quantity coming out from the exhaust hole, I’ve the feeling that on my previous DT30 it was more, but it’s only a feeling. Next tests in water will be performed not before mid of September and in that occasion I hope that malfunction occurs again in order to allow me to understand the possible cause (it’s not easy to cure a random failure) , in the meantime all your consideration are welcome,
bye

 

[imported_stinger]

Re: Suzuki DT55 RPM drop

Hi to all,
with the following I inform you about the result of further tests I performed in order to try to identify the problem cause:
With the engine running in a water tank and the gear in neutral position I increase the rpm until 4000. Below is reported the list of the performed tests and relevant results with the a.m. conditions
- simulation of low oil tank level extracting the level probe: Red oil level caption lights on monitor, buzzer sounds, no rpm drop neither rev caption illumination
- simulation of high temperature through sensor wires short circuit: red temp caption lights on monitor, buzzer sounds, no rpm drop neither rev caption illumination
- simulation of insufficient oil flow through sensor wires short circuit: red oil caption lights on monitor, buzzer sounds, no rpm drop neither rev caption illumination.
From the above it appears that two conditions I expected did not occur, i.e. rpm drop and rev caption illumination. I can presume that this is caused because of the gear in neutral position. Can anybody confirm me this theory? Actually I’m totally confused because I experienced rpm drops without any failure indication. In next days I will perform test in open water
Regards to all

 

[deejaycee_2000]

Re: Suzuki DT55 RPM drop

From the above it appears that two conditions I expected did not occur, i.e. rpm drop and rev caption illumination. I can presume that this is caused because of the gear in neutral position. Can anybody confirm me this theory? Actually I’m totally confused because I experienced rpm drops without any failure indication. In next days I will perform test in open water
Regards to all

Yes you are right, rpm drop will only occur when the motor is under load and in gear ... so you would need to test it on open water under load ...... it could be something simple for eg. little bit of dirt in one of the high speed jets on the carbs or your fuel pump diaphram ... have you cjecked those properly?

 

[imported_stinger]

Re: Suzuki DT55 RPM drop

Hi to all,
with the following I inform you about the result of further tests I performed in order to try to identify the problem cause:
With the engine running in a water tank and the gear in neutral position I increase the rpm until 4000. Below is reported the list of the performed tests and relevant results with the a.m. conditions
- simulation of low oil tank level extracting the level probe: Red oil level caption lights on monitor, buzzer sounds, no rpm drop neither rev caption illumination
- simulation of high temperature through sensor wires short circuit: red temp caption lights on monitor, buzzer sounds, no rpm drop neither rev caption illumination
- simulation of insufficient oil flow through sensor wires short circuit: red oil caption lights on monitor, buzzer sounds, no rpm drop neither rev caption illumination.
From the above it appears that two conditions I expected did not occur, i.e. rpm drop and rev caption illumination. I can presume that this is caused because of the gear in neutral position. Can anybody confirm me this theory? Actually I’m totally confused because I experienced rpm drops without any failure indication. In next days I will perform test in open water
best regards to all

 

[imported_stinger]

Re: Suzuki DT55 RPM drop

Thank you very much deejaycee for your opinion, initially I was focused on an electric type problem, now I begin to think that a fuel problem may occur randomly. Due to the fact that the engine has three carbs, and during the event it does not lost any cylinder, in my opinion malfunction can easier attributed to fuel pump that effect all cylinders feeding. In any case I intend to proceed to jets cleaning, have I to take particular care during that operation? I never been experienced it before.
Many thanks again

 

[deejaycee_2000]

Re: Suzuki DT55 RPM drop

Yes you have to be carefull when rebuilding your carbs, do them one at a time so you don't mix them up, soak in carb cleaner for 24 hours and blow out with compressed air ... good luck

 

[imported_stinger]

Re: Suzuki DT55 RPM drop

Last news, after carb drain today I sailed for 43 NM without any problem. I simulated an oil flow failure shorting the sensor wires in order to check the warning system: RPM drop at 3200RPM, buzzer sound, red oil lamp on, but no rev lamp illuminated. Bye