DT100 rough idle and stalls under load

[staffan]

I have a 1992 DT100, which idles very rough and stalls under load. Earlier this summer it ran good even though it was most likely lacking some power (could not get RPMs higher than appr. 4500).

We have checked fuel related aspects (but of course we could be missing something), a lot of electrical/sensors and there is spark on all cylinders.

Compression is a bit low and not completely even:
Port-top 115
Port-bottom 120
Starboard-top 100
Starboard-bottom 105

I also checked compression after pouring some oil in the two top cylinders and got the following readings:
Port-top 130
Starboard-top 125
So a considerable improvement.

When starting the engine there is white smoke coming out of the exhaust and after running the motor on idle for some time the plugs are oily and covered with carbon.

I then removed the cylinder heads and they look like this:



and saw a lot of carbon build up on the pistons. The cylinder walls looked ok to me but I am not an expert.

So here are my questions:
- Could the carbon build-up be the source of my problems?
- What is the best method to get rid of it?
- Could there be issues with the piston rings? And in that case is it possible to replace/clean those by only removing the cylinder (what the cylinder heads are attached to) and not by taking the whole engine apart?
- Is there any chance that the head gaskets (probably from -92) can be reused just for testing purposes so that I don't get new ones (they are more than a 100$ a piece in Sweden) if it turns out there are more serious problems with the engine?

Thanks!
// Staffan

[Solarman]

reuse head gaskets?

Staffan, compression could be the problem, rings worn, could cause problems, but before pulling heads I would have gone through the fuel system inside and out. It sounds like a partial blockage of the needle valves, restricting full flow of fuel.

You can clean the carbon of the pistons if you rotate each to its outermost position, covering up the other cylinders first.

There are many electrical components, but you were reaching 4.5k rpms. That does suggest a fuel problem, or slight loss of compression.

Never reuse head gaskets. Once compressed there is no guarantee you will seal the compression, or water from leaking by them. Always use new. If water or compression leak by them, you have waisted your efforts, even for test purposes.

Using a better quality, or grade of fuel, or even a fuel additive might help with carbon buildup.

Am in America, and can't even find a parts listing for your motor. The dt100's i've found here online, are 3 cylinder engines. Yours must be specific for European engines. Maybe you have a European forum that could help more.
Good luck.