Spark plugs: Black as cole

[Glaerox]

Hello All,

This year I bought a 410M consoleboat with a suzuki DF15. I used it a couple of times now and never experienced any engine problems. Only a little low RPM @ WOT

I dont know the history of the engine, so I wanted to do some maintanance on the engine myself. Starting with the spark plugs. I was very suprised to see that both sparkplugs (2cyllinder 4stroke) are carbon fouled. What could be my problem?

The engine is from 1999. IT could be that those spark plugs are still the originalls!

The engine always starts good (but also without choke). and idles very smooth. The only thing is that it only does 4800-5000RPM @ Wot, where 5400-6000 is normal. Im not sure iff this is Prop pitch related or due low on power (dont got any comparisson)

Dont know if it iss possible to finetune something on the carburator?

Got some hints where to look?

What I know from the engine:

4stroke 15HP from 1999 with CDI
-looking still very nice, not worn
-Starts, run and idles very smooth. But it also does a cold start without choke (but takes a little longer)
-Very strong tale tell
-dont know the maintance history. I do know it has been stored for 4 years without running. After that de carb is cleaned.
-The engine oil is about a 1/4 inch above the MAX
-The Fuel Squeeze Pump on the tank does never become hard after squizing.

thanks! Bastiaan from The Netherlands

[9D280]

That drop in RPM could be as simple as too high a prop pitch for the boat. could you borrow a prop a couple of inch pitch smaller to test?

If it's 4- stroke a manky plug would hint at slight overfuelling. (I assume its a carb and not fuel injected?) Does it smoke?

ASSUMING it is a carb and is throwing a bit too much petrol in, I can't think of a modern carb that has "Fast jet" adjust, so the next place I'd look is the float / valve. Sometimes the thin metal lever will bend slightly over time, or the valve gets worn. Tis means the "off" valve that fills the chamber (bowl) never quite shuts, and so fuel is pumped straight through and into the engine. (no or reduced action by the valve to "regulate" the fuel pump pressure)

Other things could be ignition, buit if you say it runs OK-ish, I'd try the prop change first.

[Glaerox]

Thanks 9D280 for the reply.

I can not borrow a other prop, i dont know anybody with the same type. The one fitted now is a standard 9 1/4 x 9P. I could drop to 8 or 7P. But, i dont assume a wrong prop could lead to a black spark plug like this?

Was thinking about the float chambre to. I asume this could declare why the Fuel Squeeze Pump dont het hard. And why the engine also starts without choke.

But the engines does not use much fuel on a day... about 10 litre with 5 hours driving at low and high speed.

Saturday i replace the spark plugs, oil and oil filter. Is there a way how to check the float chambre switch? I asume i can not ajust the main jet, and it would be the right one, when fitted in the factory.

Ordered a service manual in the states. But I guess it would take some time till i get it

[slimtim]

Is this a sib or a rib? If its a rib at 4m long its ridiculously under powered which would result in not hitting full rpm at wot. You could also check the throttle linkage under the cowling. Make sure its pushing the throttle on each carb all the way to its limit. My old yamaha was out of adjustment so it didn't open the throttle all the way. I reckon I got another 2-300rpm out of it by correctly adjusting the linkage.

[Glaerox]

This is a Console boat. So not a rib or sib. it takes max 40HP



The boat weights with engine, gear and 2 persons about 950~1000LB. it does about 20mph.

It only have one carburator for 2 cyllinders. It opens Full.

[9D280]

Originally Posted by Glaerox

Thanks 9D280 for the reply.

I can not borrow a other prop, i dont know anybody with the same type. The one fitted now is a standard 9 1/4 x 9P. I could drop to 8 or 7P. But, i dont assume a wrong prop could lead to a black spark plug like this?

Try Brownspoint for a prop chart - you might be surprised how many other Suzuki engines use the same prop! (they also have Microfiches for all Suzis) My thought was if you are running it slower than the fuelling (i.e too much load) then over time it might soot up.

Originally Posted by Glaerox

Was thinking about the float chambre to. I asume this could declare why the Fuel Squeeze Pump dont het hard. And why the engine also starts without choke.

That is shouting too much fuel!

Originally Posted by Glaerox

But the engines does not use much fuel on a day... about 10 litre with 5 hours driving at low and high speed.

Slightly too much fuel it can still run, if your float or the valve it operates are not 100%, you only get a small amouht extra, which is not enough to flood it, but probably enough to soot it up.....

Originally Posted by Glaerox

Saturday i replace the spark plugs, oil and oil filter. Is there a way how to check the float chambre switch? I asume i can not ajust the main jet, and it would be the right one, when fitted in the factory.

Yeah, most engines have a fixed main jet, as it doesn't need quite as much fine tuning as the idle jets. The only way to check the float is to remove the carburettor, and take the bottom off. Once inside, there will be a small needle type valve linked to the float. If the float is full of fuel, you'll need a new one. I don't know this carb, but most carbs if you hold them upside down, the top of the float is usually in line with the joint that you opened up. Your manual will confirm the exact setting.

[Glaerox]

Thanks for al the answers.

I would look at the carburator tomorrow. When i will squize the fuel pump, and the float chambre switch does not works, i asume I would see fuel dripping somewhere?

I also could borrow a compression tester, and will perform that to.

The spark-plugs fitted not also does not look like it are the prescribed ones by suzuki. I have read that to cold plugs also turn black. I replace them tomorrow.

I forgot to mention that i can operate the engine with the warm up lever on the remote. It opens the carb about 1/3th. The engine than runs about 6000rpm, and even higher after a couple of hours sailing.

p.s. a Evinrude E-tec 25/30HP will do the job to(A)

[9D280]

For the "pump squeze test" if you open the throttle so you can see down the carb body (engine off!) and pump, you will likely see the leak through the main jet, which will be sticking up somewhere in the middle of the main air passage.

Fast idle lever - any engine will do silly high RPM under no load.

[Glaerox]

Got the compression tester. nice proffesional tool!

I know that the engine has to be on working temperature. But do i have to remove all the spark plugs before testing, or only on at the time?

[Glaerox]

Today:

I replaced the spark plugs, oil, oilfilter, gearoil and fuelfilter.

Checked the float chamber of the carborator. There did not leak any fuel out of the main jet. I think the reason why the Fuel Squeeze Pump dont get hard, is because it apears to squeze fuel to both directions (also back in the tank), so I will replace that to.

As for the compression test. I ran the engine till working temperature. removed both spark plugs. And spinned the engine with gauge about 6 seconds. But stupid as i am...I did not open the thottle, so it was fully closed. (I realised this after I took the tester back to its owner). But with the carb closed, both cillinders gave the same 87 PSI (6bar). I think it would get higher with the throttle fully open.

I would soon look if the new plugs become black again.