F7 Compression

"Dear Ralph" December 15, 2005 0
Dear Ralph: My father and I have a pair of 2004 Arctic Cat F7s. When the dealer prepped our sleds they filled the oil...

Dear Ralph:
My father and I have a pair of 2004 Arctic Cat F7s. When the dealer prepped our sleds they filled the oil reservoir with synthetic oil. I know from experience that synthetics are not good for the break-in process, causing the rings not to seat properly. When we checked the compression in both of the sleds, one had 120 psi in both cylinders and the other had 130 psi in both cylinders. Do you have any advice on a repair? Do I need to have the cylinders resurfaced to replace the rings? Any information would be helpful. Also, do you have any personal suggestion on how to improve the overall performance of this set-up?
Nathan LaMothe

We need to clarify some of your views here. It is merely a generalization, not an absolute, in that an engine will typically seat the rings quicker if not using a synthetic oil during the break-in period. On these new power valve motors, most of them do use synthetic oil, even during break-in, to help keep the valves cleaner. Use of synthetic oil during break-in shouldn’t cause the rings to not seat properly; it should simply take longer to do so. Like 200 miles instead of 100 miles, for example.

As for your compression numbers, we consider 120-125 psi to be the norm for F7s. As long as each cylinder is within 10% of the other, you’re good to go. These numbers depend on the exact gauge being used, and the procedure. A cold engine with plenty of oil in the rings can easily show 10-15 pounds higher than it should. We generally will do a compression check (ideally) with a warm engine, ignition off, throttle wide open, pulling the rope until the gauge quits rising. This is usually something like 5-7 pulls.

This being said, there were a number of F7s where the rings didn’t seat properly, and many suspect it was caused by the type of oil used. This condition should show up in terms of compression, but your numbers seem to be good. In these cases, shops were lightly deglazing the cylinders and installing a new set of rings, then using a different oil than previously. If your F7s are noticeably slower than most others, this is a possibility.

Suggestions on improving the overall performance? There is no black and white answer that works for everyone, and you can find many informed opinions on the subject. There are several options, but one of the best bang-for-the-buck set-ups we’ve found is the following combination; first, install an airbox snorkel from a 440 racer (Cat #1670-315 for $7.29). This fits between the accordion dryer hose and the airbox, and replaces the 2.5” inlet snorkel with a larger diameter 4” inlet to flow more air at this restrictive location. You’ll need to enlarge the airbox opening, and secure the snorkel to the airbox with something like self-tapping screws. Next, install a 2-degree timing key. Finally, install an Injector Perfector with a 290 main jet. Do all of these together as this set-up works in this specific combination. You’ll really notice an increase on the bottom end, but overall this combo provides great performance value. Clutch as needed. This set-up should be good for 15 miles of wide-open with no heat issues.

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