How to do a Compression Test and 7900 BB Kit

[dean06919]

What is the proper way to do a compression test on a chainsaw? Do you do it with the saw cold or hot, with the carb open or closed?

I did a hot compression test on a Home Depot DCS 6401 last night, the results were 130 with the carb closed and 140 with it open. Are these results in the acceptable operating range? What would a new 6401 measure?

Lastly, while reading past threads, I noticed Bsnelling did a review on the Bailey's Dolmar Big Bore Kit. During installation, he did not install a base gasket and reported that the saw had 160 pounds of compression. On another thread, a representative from Bailey's reported that on the 6401, the compression numbers were about 140-150 on the Dolmar and 195 (if I remember correctly) on the Solo 681. Is this why Brad left out the base gasket on his installation, to gain more compression? How will this affect the power and reliability of the saw and why would the Solo measure higher than the Dolmar?

Thanks in advance for any help on this issue.

 

[blsnelling]

The Home Depot 6401 I did the 7900BB review with had about 170 PSI compression if I recall. I installed the BB kit without a gasket because I still had plenty of squish and wanted the extra compression.

 

[mtngun]

If you were testing compression on a car engine, I'd tell you to do the test with the throttle wide open.

In theory, the same goes for chainsaws, but its hard to hold the throttle and hold the saw and pull the cord all at once. I use baling wire to hold the throttle open during a compression test.

Compression affects power. More is better, as long as the fuel can handle it without detonating.

You will lose compression at altitude, about 3% per 1000 feet. Ditto power.

I usually test cold, but there is no right or wrong way. I suppose a hot test is more indicative of real world performance.

One guy may report 150 psi, the next guy may get 160 psi, another gets 170 psi. There are many variables -- altitude, gage calibration, testing technique, how well the rings seated, squish, etc..