Pulling my hair out trying to get this DT75 to idle decent.

[Atimm693]

I bought this motor a year ago, and despite my best efforts, have not gotten it to idle decent.

It will run like a champ all day on plane, however at idle up to 1/4 throttle, it runs very rough and choppy, stalls occasionally. The bottom spark plug is always wet, black, and cleaner than the rest, the middle is a little better, and the top is a nice chocolate brown. Pulling the lower plug results in no change in RPM, spark will jump a 1/2" gap easily. I believe it to be partially misfiring on the middle cylinder, and not firing at all on the bottom at idle.

Playing with the idle air screws makes a difference on the top two cylinders, seemingly does nothing on the bottom.

Compression test yields 120# on all 3.

Here is what I have done.

1. Various sets of spark plugs, B8HS and most recently B7HS.
2. New head gasket.
3. Used TVS, adjusted to spec in the manual.
4. Gone through the carbs numerous times. Swapped middle carb with the bottom because of a broken idle speed screw boss. No change, no problems found in carbs.
5. Synced carbs twice, all butterflies appear to be fully closed.
6. New spark plug boots with the correct resistor.
7. Oil injection is currently disabled and fed premix, tank and stuff still installed to avoid alarm, but the oil pump throttle rod is unhooked, it should be injecting very little oil.
8. Fuel pump diaphragms replaced. Cannot get new check valves, currently running a low pressure electric pump for testing purposes to be sure, although I think the pump is working fine.
9. Ignition system and sensors tested according to the Clymer manual procedure with a DVA, all okay.

The motor does seem to put out some steam, but I have never seen water droplets on the lower plug, however I supposed it could be ingesting water.

At this point I am thinking it may have an air leak from the lower crankshaft seal. Any other ideas?

[CentralTexas]

Have you done a leak-down test?

[Atimm693]

I'm not sure how you would, with no way to block off the exhaust ports.

[CentralTexas]

I’ve never had to do a leak-down test on the 2 strokes I’ve owned but I think your air leak diagnosis may be right. Since changing the crank seal is a pain I thought checking first may be a good idea. Here is what I found, it’s a post from a forum. May or may not help. You can also do a leak-down test with the piston at TDC but I’m not sure what that would add to the compression test you’ve already done:

Good post skydad. I can tell you're a technical mechanic. You're right, a leak down test is a slang term for cylinder differential test. Same thing, but different from a static compression test.<br /><br />Some things folks here might find interesting....<br /><br />Leak down tests for 2-stroke outboards and 4-stroke outboards are performed different using different tools for each. <br /><br />2-stroke leak down tests are done at lower pressures, say 10 psi max, where 4-stroke leak down tests are higher, say 100 psi max. You don't want to blow out seals and gaskets so check the book for what the manufacture of the engine suggests. Thus the 4-stroke tools require compressed air, while the 2-stroke tools can be as simple as a homemade blood pressure pump bulb (that's the type I use).<br /><br />In doing a 2-stroke leak down test, you need to seal off the intake and exhaust. You will be pressurizing the engine crankcase and don't want air leaking out the exhaust or the carb. They make rubber plugs for this, but I use pieces of innertube rubber between the exhaust manifold plate and the engine, and a tapered rubber bung with a fitting (for the bulb hose) that I stuff and clamp in the carb boot. If there is no carb boot sometimes I make a flat plate with a gasket that bolts up. The 4-stroke doesn't require blocking these openings off, because they have their own exhaust and intake valves. You just have to time the engine (TDC) so that these valves are closed prior to pressurizing.<br /><br />For a 2-stroke I like to pump my bulb up to 8-10 psi. A tight motor should hold this pressure for about 10 minutes. If the pressure starts to drop I spray a soapy solution of water all around the motor to find a leak. Sometimes it's the pto or mag seal or the head or base gasket. As skydad posted, with the 4-stroke you're looking for a comparison in the pressure going into the cylinder vs. what the cylinder is holding. This is why the 4-stroke tool requires two gauges and the 2-stroke tool only one. On a 4-stroke you can actually hear air leakage out the exhaust pipe, out the intake, or out the oil filler. Hearing air out the exhaust means a bad exhaust valve. Air out the intake means a bad intake valve. Hearing air out the oil filler means you're losing compression through the rings. Bubbles in the coolant means possible cracked cylinder or block.<br /><br />Oh, yea....I always to a warm compression check first, before the leak down test. Kinda fun stuff to do. <br /><br />Skydad, Lycoming aircraft requires the cylinder differential test to be done with a hot engine. "Operate engine until normal cylinder temperature and oil temperature is obtained. Proceed with test as soon as possible." But I'm not sure what you're working on.<br /><br />I think their theory (and mine) is that things do expand when hot, and in a good engine compression should rise significantly under hotter conditions. But a cracked piston, broken ring, or cracked cylinder wall will not expand as readily (because of the fracture). Thus the compression will not come up proportionately to the other good cylinder when warmed. Cold, they may all look good. But heated, things happen. I almost think of it in opposite....what isn't expanding and sealing, and why isnt' the compression rising?<br /><br />

[mphelle8vld]

"The bottom spark plug is always wet, black, and cleaner than the rest," How is wet and black cleaner than the top plug? If it's wet with an excess of raw fuel/oil, that's quenching the idle spark. If it's wet with soot but looks like it's been steam cleaned at high rpm then water is finding it's way into the cylinder from the head gasket or exhaust cover gasket.

[Atimm693]

"The bottom spark plug is always wet, black, and cleaner than the rest," How is wet and black cleaner than the top plug? If it's wet with an excess of raw fuel/oil, that's quenching the idle spark. If it's wet with soot but looks like it's been steam cleaned at high rpm then water is finding it's way into the cylinder from the head gasket or exhaust cover gasket.

It is wet, and black around the electrode, but there is still shiny clean metal around the plug, where the top plug is brownish around the ground strap and electrode, but the entirety of the plug is covered in soot, like you would expect from a normally working cylinder.

I wouldn't say it appears to have been steam cleaned, I'm not sure if the cleanness is from the misfire or from water, I have never seen any droplets of water on the plug or other signs of water, but I suppose it is a possibility.

I did replace the head gasket, it was not leaking before.

I would absolutely do a pressure test if it was possible, like I have done on chainsaws before, however I don't believe there is a good way to block off the exhaust ports.

I have not been into the exhaust cover, but I will pull it and see what things look like.

Seems to me that if it was fouling a plug, it would stay fouled and have trouble getting up on plane, it doesn't, once you get past 1/4 throttle it takes off without a hitch, every time. But I could be wrong.

[mphelle8vld]

Just saw your edit, I'm just guessing that with higher rpms and exhaust flow, it keeps water from entering the exhaust port.

[Solarman]

I'm thinking, with your previous info, you might check the reed valves on the lower cylinder. Is there similar vacuum at carb throat, any blow-back? What's the compression between cylinders (max allowed difference is 10% between cylinders?)?

Good luck, post back on what you have.

[Atimm693]

I'm thinking, with your previous info, you might check the reed valves on the lower cylinder. Is there similar vacuum at carb throat, any blow-back? What's the compression between cylinders (max allowed difference is 10% between cylinders?)?

Good luck, post back on what you have.

I considered that as well, but am not getting any spitback on the lower carb.

When I tested the compression at the time of purchase, the bottom cylinder was 10lbs lower than the rest. Since running it, they are all within a few PSI of each other.

[mphelle8vld]

If the motor sat for awhile with water laying in the bottom cylinder, there may have been a bit of rust that has since worn off with running it. Did you notice anything unusual with that cylinder when you had the head off? Also did the bottom combustion chamber and piston crown appear cleaner than the other two?

[Atimm693]

If the motor sat for awhile with water laying in the bottom cylinder, there may have been a bit of rust that has since worn off with running it. Did you notice anything unusual with that cylinder when you had the head off? Also did the bottom combustion chamber and piston crown appear cleaner than the other two?

It has been a few months so I don't really remember the details, but there was nothing really remarkable, there was some very minor scoring, like it had been overheated once or twice, but nothing major.

I'm starting to wonder if it isn't indeed a reed valve, while turning it over by the flywheel, the top carb has a very nice "honk" from the reeds on the compression stroke, while the middle and bottom are more of a pfft sound.

I will pull them and have a look, but want to fish on Friday, so I'll wait until the weekend. Despite the crankiness, it still runs well enough to hit the water.

[Atimm693]

I pulled the reeds and didn't find a smoking gun.

They're all there and seem to seal okay. You can see light through them along the sides with a flashlight, but it seems to be far less than the gap that the service manual deems acceptable.

I'm really not sure what to do. I've ordered some Boyesen reeds, might as well put them in I guess and see what it does.

[Atimm693]

Since I had access to the lower crank bearing with the reed housing off, I filled it with diesel fuel and ate lunch. When I came back, it was empty.

I pulled the powerhead and tore it down. Found the lower crank seal to be pretty worn, and the lower crank bearing trashed. Not sure what to do now.

[Solarman]

Most parts of the DT motors are no longer available (NLA), consequently if the lower bearings are trashed, generally a total overhaul would be the answer, but not being able to get parts means the attempt is futile. Selling your motor for parts becomes your only option, unless you have a way to make all the parts you need. Sorry for that, I have 4 - DT 140's, and they all need crank and rod bearings, NLA. So, I'm only able to part them out.

Good luck.

[Atimm693]

Since it is only the lower main bearing, I think I can build a puller and get it off. The bearing is available. It is not totally trashed, just rough, probably from water.

I'm going to give it a shot, the parts are not too expensive, compares to a replacement.