Suzuki DT100 -92 problems

[staffan]

Hi everyone,

Getting a bit frustrated over our Suzuki DT100 -92 that we bought this year. We have had various problems with rough running, dying on acceleration, poor idleing, etc. So far we have done the following:
- changed carburetors (from a spare part engine we have)
- replaced two broken reed valves
- fixed the choke
- changed diaphragm in the fuel pump
- Installed water separator filter and removed water from tank
- replaced all spark plugs

and a lot of fault tracing...

It now runs better than ever but still some problems exists:
- little rough idleing (we haven't yet tuned the idleing screws so that could cause the problem)
- when accelerating the engine stops at around 3000 RPM for a second or two and then starts accelerating again like it should. As long as the engine is kept above 3000 it runs good but if dropped below 3000 the same phenomena will occur again. Earlier we had the same problem but the one-two seconds were more like 10s of seconds or minutes.
- we recently installed a Suzuki monitor and the monitor shows oil level correctly and with no oil the alarm sounds. However, as I understand the service manual there should be a sound when ignition is turned on, which we don't have. Also, when disconnecting the oil filter flow sensor or the cooling water temperature sensor the monitor will not make any sound or light up. Shouldn't it?

Anybody has any clue what could be the problem or how to proceed with the fault tracing? We have a replacement part engine so replacing parts to test is not a problem.

// Staffan

[mphelle8vld]

When steel reeds are ingested and then spit out the exhaust, the piston is almost always damaged, have you done a compression test?

[staffan]

The reeds did not go through the engine, all pieces were found in the inlet between the carburetor and the pistons. And if we had bad compression wouldn't it allways run bad and not have intermittent problems?

[mphelle8vld]

I would have checked compression before buying, since you're having problems it's a good place to start and it only takes 5 minutes. I would swap the
CDI next.

[staffan]

Ok, thanks for the suggestion. I don't have a compression meter but could of course buy/borrow one. But would really bad compression mean that it would run good at 3000 RPM but not accelerating further (despite full throttle) for some time and then like a switch was turned it accelerates perfectly to around 4500 RPMs?

[mphelle8vld]

The compression will tell you the general health of the motor and could affect some of your problems. The 3k issue could be related to the motors limp mode which protects it from lack of oil, water, or over-rev but seems to be malfunctioning, that's why I suggested swapping the CDI.

[staffan]

Back from the boat and some improvement. Today we have changed the CDI-box, checked Suzuki monitor cabling, changed the oil flow sensor and tuned the carburetors. We also ran the engine without the carburetor cover and checked that we had good petrol flow.

There seems to be some improvements on idleing and at low speeds. However, the hesitation is still there at 3K RPM. The monitor does not show anything but maybe the pause is too short for the light to come on. The funny thing about the hesitation is that if the boat is on a plane and then I quickly lower the throttle so that the RPM goes down to around 2K and then back up to full throttle again without the boat coming off the plane the hesitation is not there. But if we start from slow speed the hesitation is there again. Very strange. Any ideas?

[mphelle8vld]

I believe you have a water sensor (float switch)on both heads, try disconnecting both and see if you still have the hesitation. Have you changed the impeller?

[staffan]

Impeller was changed in spring. There is a strong stream from the pee hole and engine does not feel hot so impeller should be ok I think?

I only have one cooling water sensor and it is not the flow switch type. It is just a temperature sensor with variable resistance. Anybody knows what resistance should be so I can check?

[mphelle8vld]

The DT100 I worked on was a 90 so they must have made some changes. Is 4500 your max rpm?

[staffan]

Yes, max RPM is around 4500. I guess that should be higher but maybe there is a problem with growth on the bottom of the boat (I could see some yesterday) or the propeller not being quite right for the boat.

[mphelle8vld]

I would think that it would be around 5500, at the risk of becoming a broken record, I would check the compression.

[staffan]

The boat is now out of the water and we have checked the compression and it was very even (7.6, 7.7, 7.7 and 7.8). The readings was I guess all a bit low but it was a bit hard to read it exact. Also the engine doesn't really get the temperature up (after running the engine for a long time the cylinders are barely warm on the outside) so that could also be one reason for the low readings. Does that indicate a broken thermostat?

[cowboycadi]

you need to check your throttle valve sensor. if the sensor is reading incorrectly it will throw your timing way off and will have all of the symptoms you are showing. chances are that your cdi is good.