DT75 revving problem

[Illusmare]

Hello,

I'm new to this forum and been trying to figure out what could cause the problems I'm having on my 87' DT75.

It starts well, and idles decent. Problem is it won't rev past about 4.000rpm.
It also struggles to get up the RPM. If I give it full gas it takes about 10s to get to those 4.000rpm, even more if there are more ppl in the boat.

It also uses alot of fuel. Don't know what is a normal amount, but ~2 hours (maybe even less) at 3k rpm and it used up 40L of fuel. Normal?

Can't remember the exact size of the propeller, but it's pitch was 17. The boat is a rescueboat and weights about 600kg without engine and ppl in it. But I still asume the DT75 should rev almost instantly to max rpm.

Thank you in advance for your answers.

[Solarman]

'87, dt75

Problem sounds like carbs might be needing cleaning, have you looked at plugs? If all dirty and wet/ black the carbs for that cylinder may be flooding it.
good luck, tell us what's happening.

[Illusmare]

Here is some more info on this. The propeller is 13 1/4" x 17.

Took few pictures of the spark plugs. Here is what they looked like. Not very good imo. Last 30 mins it ran at about 1500rpm so guess that might have some effect on what the spark plugs look like, but still I think they shouldn't be this "wet".

[Solarman]

pics of spark plugs

Well not able to see pics of plugs, just a black screen.
might send pics again with a different program. Apparently am unable to view them on this program?
If plugs are dirty, does it look like they still are in good shape, or worn out and needing to be replaced? Rounded electrodes in middle, gap of plug wider caused by worn electrode?
If only dirty, then carefully dismantling each carb, making notes on all mixture screws, and soaking in a carb cleaning solution (can purchase gallon can with parts basket inside at auto parts store) - read label for soak time, and using compressor blow through passages after soaked, then re-assembling just as it came apart.
If not mechanically inclined, may wish to find a Suzuki mechanic that can clean them for you. You do not need to remove linkages except to separate the carbs, and label each carb & the linkages to go right back in the same place (top, middle, bottom.)
Let us know how it goes. If any questions?

[Solarman]

first important thing to check

You should also check the compression on each cylinder. rule of thumb, 10% max difference between cylinders.
If running with poor compression, motor will be extremely sluggish and drink up the fuel. Basically, the worn out motor, would either need to be replaced, or rebuilt.
So before spending money, take a basic compression test.

[Illusmare]

Changed the link to the picture. Should work now.

Took a compression test few days back and it was good. So that isn't the problem.

The last plug is pretty black and not sure if those 2 other plugs are that yellow cause of the flash or if they are really yellow. Have to take a better look tomorrow.

[Solarman]

pic of plugs

From the look of the plugs (good pics), I feel the third plug is missing, not firing. Do you have a timing light that clips onto the plug wire?
Have someone drive the boat while trying to get up on plane, while you clip the light on each plug wire. I suspect the black plug is not firing, either because of flooding or lack of spark.
It might be good to clean the plugs, before putting back in.
you might be able to tell just sitting at the dock, or on the muffs if on trailer. Don't need to rev it high. If all flash the same at idle, then may need to try in water.
ONLY IF, flashes are different, swap the ecm wire from bottom coil to middle, and ecm wire from middle to bottom coil, then plug middle coil plug wire onto bottom plug and bottom coil plug wire onto middle plug. Basically, you're just changing coils to see if both coils are good. Now run motor again with light on each wire, any difference?
If no difference, then put coils and wires back like they were.
Your problem might be the ecm, or the gear counting coil - (have you made sure the distance from gear counting coil to flywheel is correct? this is important!), or under the flywheel. in the magneto area, there are 3 pulse/ timing coils. Look in manual wiring diagram, to see which coil is for bottom cylinder. Swap that coil with either of the other two. Make sure the plug connectors are same, all three should be identical? Then try the light again, has anything changed?
Let us know.

[Illusmare]

Just took out the bottom carb and took it apart. Looks clean, but will clean it with carb cleaner to be sure. One thing I noticed is that the float distance was 14mm when it should be 10-12mm on '87 DT75. Not sure if that would cause it but adjusted it to what the manual says.

[Solarman]

cleaning carbs

Have to ask, when you clean your carbs do you remove the needle valve seats? Important to unscrew those seats because it is common for some small trash to lodge on the backside of that start. This could cause your motor to bog at higher RPMs if fuel flow is restricted partially, it would act like not enough gas. And if there is something there it is important to know what it is. This might tell you if something before that valve is coming apart. If you have cleaned all carbs, you should go back through them all and check that seat. And yes, all the float levels need to be set correctly by the book.
Let us know.

[Illusmare]

Have to ask, when you clean your carbs do you remove the needle valve seats? Important to unscrew those seats because it is common for some small trash to lodge on the backside of that start. This could cause your motor to bog at higher RPMs if fuel flow is restricted partially, it would act like not enough gas. And if there is something there it is important to know what it is. This might tell you if something before that valve is coming apart. If you have cleaned all carbs, you should go back through them all and check that seat. And yes, all the float levels need to be set correctly by the book.
Let us know.

Didn't unscrew the seats. I only adjusted the floats and sprayed carb cleaner through the main nozzle and the needle. Everything inside the carbs looked like new tho, and tested the main nozzle and needle by blowing through them. Already put everything back together so guess I will go test it before unscrewing everything again. Will inform here how it worked once I have had the time to test it.

Also, seems I had adjusted the pilot air screw of the bottom carb half turn the wrong way, so 2 carbs were at 1,5 turns while the last one (with the black/wet spark) was turned 2.5 turns. How much would this affect it?

[Solarman]

yes, the adjustment matters

The idle mix adjustment is very important at low speeds, but has nothing to do with high speed. The high speed jet is in middle of float, it has to be clean, but no adjusting it.

[Illusmare]

The idle mix adjustment is very important at low speeds, but has nothing to do with high speed. The high speed jet is in middle of float, it has to be clean, but no adjusting it.

Ok so it wasn't the culprit to the problem. Let's hope those floats were just letting too much fuel in then and made it flood. Every carb had 1-2mm too much space between main nozle and float bottom when turned upside down. If I still have the same problem I will have to check the needles.