MKL’s ’98 1200C – Yost Power Tube Installation & Impressions

The following was my experience installing and testing the Yost Power Tube CV-40 Master Kit, as originally posted to the XL List 12/7/98. [NOTE: This was my first self-completed engine project.]

“Last night, my newbie hands ventured into my carb for the first time, as I attempted (successfully) to install the Yost Power Tube (YPT). To review, I chose this jet kit over the others due to the feedback I got from some of you listers, most notably Rickko and Maurice. Installation was relatively painless and a great learning experience. To install, I relied 90% on Rickko’s instructions, 5% on TFM, 4% on Yost’s instructions, and 1% on the “101 Performance Harley-Davidson Projects for Evolution Big Twins and Sportsters” book by Kenna Love / Kip Woodring (more a “why-to” than “how-to” book). Keep in mind that I already had the CV carb breathing through an S/E cleaner and properly jetted (175 main / 45 slow), as well as S/E slip-ons and all the other usual Stage I stuff. Here’s my impression after my first test ride today:

1) Throttle response is improved. The engine seems more sensitive to any manipulation of the throttle.

2) The mid-range (2,000-4,000 rpm) flat spot isn’t as prominent as before, but still remains. The bike feels the same as always from 2,000 to about 2,500rpm, after which the improvement is more easily felt. Overall, the change is about what I expected from this mod – noticeable, but not wildly significant.

One more interesting note: Yost’s instructions included an extra step not mentioned anywhere else – I had to drill the BOTH the needle hole and the vacuum port on the bottom of the throttle slide with a 1/8″ drill bit. According to Yost, this must be done on all bikes 1996-up. I did so, with no negative side effects detected in today’s test ride. I suspect this throttle slide mod is responsible for the quicker throttle response …

On the whole, I would say that this project was positive in that I finally wrenched on the bike myself (a truly great sense of accomplishment for a technical newbie) and the bike runs a bit better. But is this mod itself worth $90? It’s your call – But if you’ve already got your CV tuned properly (rejetted, S/E air cleaner, adjusted low-speed mixture, etc.), I would consider saving up for something that will make a more significant difference, like cams.”

NOTE: Since this was posted, I’ve noticed that the bike runs a bit richer than spot-on perfect (which it was prior to YPT), but not enough to worry about IMO.