confused

[jafa]

I've been slowly working through a plug fouling problem on my 91 DT40. When I first got it, it was really badly wet fouling the plugs after an hour or so. <br />I have... Changed the impeller and themostat, that helped, stripped and cleaned the carb , that improved it a bit more , lowered the float level to factory spec minimum to lean the mix out a bit , that stopped the piston crown appearing wet but the plugs are still black and wet , no sign of tan yet. Decarbed and upped the heat range of plug from BR6 to BR7 and increased the gap to 1mm from 0.7 and changed oil to yamalube. <br />Both plugs are now still wet around the edge and core and a hard black "enammeled" look to the earth electrode but there is the first signs of detonation on the piston crown. All the carbon under the plug has been cleaned off and a couple of pits have appeared. They might have been under the crud all along but I'm being cautious.<br /><br />Compression is 100 psi when cranked for a second and 120psi when cranked for 4 seconds on both cylinders. Replaced the portable tank connector and motor fuel tank connector. Timing is Ok 1/3" spark , fuel fresh & revs to 6k if pushed. Starts first time. I'm outa ideas.

 

[bigbrownbuku]

Re: confused

your fuel pump diaphragm may be torn. suggest giving it a service.

 

[jafa]

Re: confused

I had thought about that but as its affecting both plugs equally i discounted it as the fuel pump is driven off the lower cylinder.

 

[rodbolt]

Re: confused

if you suspect detonation check ign timing at WOT and make sure the prop will allow the engine RPM's to run at the high side of the operating range and make sure your fuel meets the recommeneded octane requirements. then do an oil consumption or delivery test. on a two stroke, especially on a flusher "tandry" plugs are all but impossible. sounds good in the book but rarely happens in life. at low no load speeds wet plugs are normal for that motor. the black enamel is usually caused by cool combustion temps and/or excess oil. check the plugs imediatly after a 10min high speed run. also carefully inspect for signes of water on the plug. in fresh water you may see whitish to brownish spots on the electrodes. in saltwater the spots will be tiny green flecks on the electrode.

 

[jafa]

Re: confused

Ok heres an update. Cleaned out the portable tanks and fresh gas. Finally got my hands on the correct Suzuki manual, given up on the clymer, Timing was a couple of degrees out but now set as per book at 2deg atdc and 28 deg btdc at 5000 using the method in the manual ,set rpm to 1000 and rotate the stator to its end stop and set at 18.5deg. <br />Put in some new B8 plugs and fired it all up. Pulls to 6000 rpm and oil consuption is around 1 litre to 65 litres of gas , about 65 to 1 on mixed running. <br />Started up great but still wet sodden plugs. I checked the thermostat again and here's a thing. When I replaced the thermostat I didn't realise they had given me a 50deg one , book calls for a 60. I can put my hand on the top of the head for over 20 seconds before it gets uncomftable. I'm pretty certain I need to raise the cylinder temp but is a 10 deg difference in thermostat going to make a diference to clyinder temp other than 10 degs. ?

 

[rodbolt]

Re: confused

test or replace the fuel pump diaphram as well. if its leaking it can cause both cyl to run rich, you will never see dry plugs like a 4 stroke motor has. its common for large bore two cyl two cycle motors to have somewhat wet plugs. especially if its idle a min or so before checking them. i did not catch it at first but the b7 is a colder plug than the b6. that motor also idles rough. dont care what ya do its rough. will it idle ok at its proper idle rpm? there is a fuel recirc system i believe but out of memory i cannot remember how it works. if the pump diaphram is leaking it may start fuel washing the carbon from the piston.