[toomanytoys2]

I’ve been doing a lot of research and gathering parts for my CFmoto 250 engine in my new to me 2007 Joyner SV. The few times I have driven it, it seemed to be really down on power. It also seemed to be rather high geared. The first thing I noticed was that the engine wouldn’t top out in high range. I figured that the engine was on its last legs, so I did a compression test. I got 120psi for the pressure, which would normally seem ok. However, I had located two different manuals on the CF, and one said the pressure should be 1,300Kpa (185 psi) and the other stated 1,400Kpa (213psi). So that would seem to back up my worn out theory, unless the specs are wrong.

I was able to locate the CFmoto distributer in China, so I ordered a complete genuine CFmoto head. Since on any engine, the most power gains are in the head, I took the head to my favorite engine guru to get some pointers on where I should concentrate the porting. He builds engines anywhere from alcohol burning Briggs & Stratton to world of outlaw sprints. Before looking at the head, he wanted to test some new loads for his 243 Ackley Improved, so we went to his private range in an orchard, I helped him setup his chronograph, and we fired off some rounds. Afterwards I took him to lunch and picked his brain. He then said “let’s go port your head”, I wasn’t expecting that. We went back to his shop, and for the next couple of hours he worked on the head, while I watched and asked questions. For giggles, he wanted to flow it, but I didn’t bring all the parts that he needed to do it though.

During lunch he asked me what they of carb, cam, and exhaust I was planning on using. I am using a pumper / slide carb off of a Honda XR250L. The throat measures 31.6mm, but the venturi is a taper bore, which acts like a small carb at small openings, but flows like a big carbs at WO. The down side to this carb was that it was produced during the “smog” era. That means that the pilot jet was a fixed, pressed in design. So I had to take some extra pilot jets that I had and turn them in the lathe, so that they would match the dimensions of the stock jet. This means tuning will be a little time consuming, since I will have to pull the carb, to swap the jets.

As far as the exhaust, I felt that the 1-1/2” aftermarket exhausts are too big to achieve good scavenging. I told him that I was thinking of going with a 1-1/4”. After doing the head, he recommended that I go 1-1/4” for the first 10”, and then go 1-3/8” after that. The head pipe that I am using is from a Honda CRF250X. It starts at the head at 1-1/4”, then steps up to 1-3/8”, 12” later. However, the “bushing” that goes into the head’s exhaust port was too large, so I cut the pipe before the first bend, chucked it in the lathe, and turned it down.