Sunday, January 31, 2010

Cleaning the carbs

So it's winter and it's been especially cold these last couple of weeks. We've hit -21 degrees Celcius before the wind so working outside on the cars is a little bit unrealistic. Burger is off designing our custom jackshaft which he will be posting CAD drawings of shortly. For the rest of us the next logical step would be to work on the snowmobile engine and get it running smoothly. Well a broken bolt lodged in one of the cylinders has put a stop to that, so all that's left to work on is the carbs.

Now, I honestly don't know when the last time the engine on the snowmobile ran or how long it sat since it was disassembled. The truth is that everything looked like it had been stashed in a barn for a few months. There were clumps of dirt, grass, and other strange and wonderful bits of nature covering the parts.

I had to set up shop inside the house since it's freezing outside and my garage is a mess. I took a shot of my setup in my basement so you could get a laugh.
I had the laptop on my desk with a "how-to" guide for cleaning these carbs. I have cleaned mikuni carbs before for my 6.5 hp honda engine on my racing kart, but I've never tackled one for a snowmobile, let alone 4.

The guide I used was here:
A big thanks goes out to the VMax4 forums for posting this article.

I first read through the carb cleaning article and it talked about resetting some tuning settings if your engine is having idling problems. Since we've never actually run our engine, I tried not to mess with these too much. I did notice that all of the fuel adjustment screws were set at a different level. All of which were significantly different than the recommended "reset" settings listed in the article. For now, I've left them as-is and if we have idle problems when we get the engine fired up, then we can play with them then.

For the most part, the entire experience was uneventful. I only had to remove the float bowl on the bottom of each carb to do the cleaning and I didn't have to mess with the reeds at all.

Cleaning a carb can be basically broken down into a few short steps:
  1. Remove the float bowl
  2. Remove and clean the pilot jet and fuel tunnel
  3. Remove and clean the fuel screw and fuel tunnel
  4. Remove and clean the main jet and fuel tunnel
  5. Remove the floats and check the status of the float pin
All of the main jets were quite clean and shouldn't give us any problems *fingers crossed*. 3 of the pilot jets were in pristine condition. One was clogged. I tried for a long time to open up the jet using a thin strand of brass wire. The guide said to use copper, but I figured it wouldn't matter. The wire entered the jet quite easily, but could not break through the dirt. I eventually went and got some copper wire from a recently disassembled computer power supply and noticed that it was a thinner wire. Of course, one poke with it and the dirt came right out. Let this be a lesson to all of you, always follow the guide ;)

The fuel screws were all set to different levels in each carb. The guide specifies that these should be reset to "1.25 turns past the point where it just gets snug". For these carbs, that was between 13 and 14 turns of the screw (from not being threaded at all). I returned the screws back to their previous depths which were anywhere from 14.5 turns to 17 turns. These values may cause problems for us, but thankfully this screw can be adjusted when the float bowl is on and the engine is running.
The last portion of cleaning came to the floats. These had some dried gas residue on them and needed to be scrubbed with a carb cleaner soaked paper towel. They came out much shinier and hopefully will not mix too much old gas with the new stuff we throw in. Here is where another tuning piece came in. As you can see in the picture, the floats are made up of 2 plastic floats and a metal "hangar". In the middle of the hangar is a small tab. This tab is what pushes down on the rubber needle in the carb. Now, the guide specified that the floats should be aligned with the machined base of the carb body. All of the floats were at significantly different angles to the machined body and I did adjust them to be closer. I'm not entirely sure what this does, but hopefully the adjustments I made will help the carbs flow smoother.

After cleaning and reassembling all the carbs, I cleaned all the rubber pieces that attach to them and installed them for visual effect. As far as I know, the carbs should be good to go as soon as we get the broken bolt out of our cylinder.

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