I was approached and asked what the possibility was to lower the compression on an 02 1200? I asked why and he said that this was going to be his main mode of transportation where he will be stationed. I asked where and he said he could not say but it is always warm and high test fuel is almost non existent. The guy just got the bike for a song only had 12K on the clock but it was in really rough shape cosmetically. We did some initial checks the engine ran fine, compression was with in specifications.
Now keep in mind this was a budget decision on our process and there are several methods that could have been used. Like changing heads, thicker head gasket, changing the squish and so on. I have never detuned a bike and have worked on other motors so this would be a challenge.
1. Pulled the heads and cleaned them
2. Weighed each head and found one head weighed about 2 grams more than the other.
3. Took two one foot x one foot one quarter thick section of lexan, fitted it with a soft gasket set the heads on them drilled holes and bolted them to the lexan.
4. Installed a fitting from a compression gauge in the spark plug hole put in about 30 psi, no leaks.
5. Removed the fitting and installed a clear tube about 6 inches tall.
6. Used a graduated beaker and filled each combustion chamber with equal amounts of water and marked each tube.
7. Broke everything down then took a die grinder and commenced to remove material from each head's combustion chamber, we ensured that we kept the chambers hemispherical so it does not muck up combustion.
8. We removed about 10 grams of material from each head being real careful not to nick the valves.
9. Weighed each head again confirming total removal of material, bolted them back on the lexan poured in the exact amount of water as we previously measured, marked the tubes and noticed the drop was exact in each head so this means each head has the same volume.
10. Did some calculations on volume compared to what we thought would be enough to lower the compression.
11. Reinstalled the heads and got her ready to run, checked TDC on the flywheel marked the flywheel with a scribe to a best guess where the new retarded timing mark should be.
11. Filled the tank with regular gas
12. Fired the bike up no issues cranked over a couple of times and then fired up.
13. Set the timing to where we thought it should be did a quick check of the exhaust gas with a 4 gas analyzer, readjusted the carb which has stock jets.
14. Took the bike out for a spin there was a slight ping when rolling on, we checked the oil and head temps all was well.
15. Retarded the timing just a hair test rode hard no ping.
16. Checked compression and noticed from original readings the compression was down exactly 22 psi per cylinder.
17. The bike did not feel sluggish as I thought it might he is going to have it DYNO tested for HP to see what the difference from stock is.
18. Shade tree mechanics prevail once again, guess and be damned or get lucky.
He has been running 85 octane gas and the bike is running great.
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