'92 DT 85 Miss/stall

[reelfishin]

I just picked up a really clean, freshwater run Dt 85 but it developed a miss after about 3 hours of running. The motor runs cool, has 137 psi on all three cylinders but drops a cylinder at cruise
It stalls at idle and progresses into a backfire/stall after a few minutes of the problem surfacing. When got the boat, the tank was full of old gas, I changed the fuel, flushed the tank, cleaned the carbs, and changed the plugs. It made no difference. It seems to run better at higher RPM than lower RPMs.
It at first felt like a carb issue, sort of bog with some loading up at lower speed, but it now missing always, the middle plug looks new and dry, the upper and lower come out wet and black. The motor has only about 4 or 5 hours of light use on it since I got it. Compression hasn't changed.
At this point I really am not sure if it's a carb issue or a coil or something dropping out. After it stalls it fires right back up, but it had big issues accelerating again, once on plane the missing cylinder cuts in and out at will, dropping to an idle will almost always mean a stall out.
I ordered a manual but have little to no experience with these motors.
Is there any thing that their known for? I thought about maybe a bad headgasket, but the compression it good and I don't see any signs of water in that cylinder. All coils put out a good blue spark on each plug.

The bogging sounds like carbs but the sudden stall and backfire has me a bit puzzled. The odd part is that this motor will start with no choke in less than one crank of the engine, always.

I have no real history on the motor but it looks pretty well cared for overall.

Any ideas?

 

[stylesabu]

Re: '92 DT 85 Miss/stall

sounds like fuel issue. i would rebuild carbs. another way is to enrich fuel mixture, using a propane tank with a hose running to air intake. if it smooths out and quits missing then its a carb issue. or switch coils around to see if dry spake plugs follow the coil or stay with a cylinder.

 

[reelfishin]

Re: '92 DT 85 Miss/stall

I tried starting it this morning on the ears but couldn't even keep it running.
The big problem is that I'm not at home and have little to no tools here.
I borrowed a compression tester to check the compression, just to make sure it wasn't something more serious. Right now it will only run for a few seconds and it stalls abruptly when I give it some throttle. When it starts it sounds fine till I try to accelerate then I get a bog and it shuts down sharply.
The boat was bought by a buddy here with little to no history, it ran fine for about 4 hours or about 40 or so miles or running. The problem began with an intermittent flutter or bog and occasional stall on deceleration after coming off plane. It idled a bit low so all I did at that point was to bump the idle screw about an 1/8th turn. I changed the filter, drained the tank and found some minor water and the fuel smelled old, it was separated into two parts, half of the fuel was fine, the rest was pretty dark looking and cloudy. (I let it all sit and separate and the good part ran fine in my truck. I let the tank dry out, flushed it with 4-5 gallons of fresh gas with some water remover from the dealer and refilled with new fuel. A current fuel sample at the motor looked perfect. The motor has both an water separator/filter and the OEM fuel bowl. Both are clean.

Are there any other filters or screens that could clog? I pulled and checked the middle carb, the one with the cleanest looking plug and it looked good inside, I blew it out the best I counld with carb cleaner but no change.
I called a few dealers in the area and got nothing but warnings about how these motors blow up if they run lean, two told me it was probably toast and not worth fixing. The way I see it, if its got good compression, isn't making any metal and I have no signs of water in the cylinders, then most likely its just a carb or fuel issue, or possibly a bad coil or something. The part that gets me is how fast it progressed from a minor issue to nearly not running at all. The last time out on the water, when it got bad enough to really notice or realize it as a problem, the miss would come a go, I could cruise along at about 2/3rds throttle on plane, and it would drop and pick up a cylinder as if I was trowing a switch. I got the impression that it would clear out a run wide open just fine but each time it came back to life, I had to cut the throttle back due to the area I was in. (When it runs right, the boat flies, 85HP on a 16' skiff does pretty well).
I've not been able to find a manual locally and won't be here much longer, does anyone know if the DT 85 has any internal filters that may be the issue? Is there any electrical tests I can run to varify the coils or control module?

 

[reelfishin]

Re: '92 DT 85 Miss/stall

Just an update, a god carb overhaul and a new f/p kit did the trick. All is well with the motor now. Parts were surprisingly affordable at under $45 for all including two cans of carb cleaner.

 

[99yam40]

Re: '92 DT 85 Miss/stall

Good to here it is fixed, wish more people would post back that they got theirs fixed and what it took.

 

[reelfishin]

Re: '92 DT 85 Miss/stall

I agree.

On this motor, it was mostly the floats sticking open and dumping gas, which was better than it going lean. It was the most severe carb symptoms I've seen but one of the easiest to work on. The thing runs like new now. I almost thought it might be a bad coil since the middle plug kept coming out super clean and the others jet black. It would drop a cylinder and it felt just like a bad plug wire or coil. Those looked new and I am guessing the last owner had gone there first and gave up.

 

[99yam40]

Re: '92 DT 85 Miss/stall

Some of these things do cause good prices for others , You just never know pf it will work out as good as this one.