The Conversion That Worked!
The Conversion That Worked!
JODY NICHOLAS
THOSE READERS who are familjar with the Bultaco. Matador series motorcycles would probably think I was crazy to mess about with one of the most tractable and reliable engines around. We road tested a Bultaco Mk IV Matador Six Days Trials Replica in the Jan. 1972 issue of CYCLE WORLD, and of course I participated in the test.
Having owned two Matadors previously (a 1964 200cc and a 1966 250cc) I was amazed at how many improvements had been made in a few short years, and just how ideally suited the Mk IV Matador was to my style and type of off-road riding. So, I placed an order and took delivery.
After some minor fettling with small changes to make the bike suit me better, I entered the Hi-Mountain enduro, finished the run with absolutely no problems and was ready to do it all over again after a Coke break!
But there were a couple of annoyances I just couldn’t get fixed to suit myself. One of these was the inability to get the Amal Concentric carburetor’s float level high enough without machining part of the float chamber to allow the float’s pivot to ride deeper in the chamber, which in effect raises the float level. Another was a tendency for the bike to misbehave at ultra-low rpm and throttle openings, and no matter how much I fooled with the carburetor I couldn’t get it perfect. A little more power down low wouldn’t hurt, either. Quite coincidentally, Dale Herbrandson’s first article on reed valves for two-strokes also appeared in the Jan. 1972 issue, and I had a chance to study it closely. For some reason it didn’t occur to me until Dale’s second article on reed valves (CYCLE WORLD, May 1972) that a reed valve set-up on my Matador might cure my annoyances and could promote even better low-end power.
I contacted Dale about the possibility of us doing a project together, using my Mk IV Matador as the guinea pig. Tom Patton from Bultaco Services, Inc., was technical advisor. Will Pfitzenmeier owns a dynamometer facility in Glendale, Calif., which we used as the gauge.
Although Will built his dyno, and his readings may not agree with someone else’s, the machine is useful for comparison work that we wished to do. It really makes no difference if the dynamometer’s readout is in ft.-lb. of torque, gpm of water, or in candlepower, as long as the same units of measurement are used for the following tests!
As Dale noted in his article this month, we found that the Matador would pull under full throttle operation at 3000 rpm and produced good power even that low in the rpm scale. In fact, after the first test of the stock machine, I was beginning to have reservations about continuing for fear that the beautiful low-end of my machine would be spoiled. But my fears weren’t justified at all. Dale’s assertions about improved low rpm performance in his May article were correct.
All testing was performed using a Bosch W 190 P2 platinum tip spark plug, Union 76 Super gasoline and Bardahl VBA two-stroke oil mixed at 40:1 in the gas.
Because of the rather wide bolt spacing on the DH reed valve manifold we had to use a clip-on type Bing concentric carburetor. Remembering some pretty bad experiences with the older side float Bings, I was at first apprehensive. But the fears have been unjustified.
In order to get everything we might need, 1 called Paul Hunt at Hercules Distributing Co. (distributor for Bing carburetors in the western U.S.) and he sent down a No. 1/32/101 Bing concentric carburetor and a selection of main, needle, vaporizer and idle jets. After some fettling it was found that the Matador ran best using a 145 main jet, a No. 51-592 vaporizer jet, the standard No. 45-118-265 needle jet (needle in the 2nd position) and a 30 idle jet. These settings are well nigh perfect for this area and so far everything is working well in the carburetion department.
After seeing the windows that had heen cut into the rear of the piston I thought that vibration might become worse. But no apparent difference was made. There’s still enough material between the two windows to keep from weakening the piston and they are right in the middle of an area of low wear anyway because of the inlet, auxiliary transfer and boost ports.
Judging from the dyno curves the reed valve conversion was worthwhile. And testing the machine proved us right. Riding the before Bultaco makes the after Bultaco seem as though someone had added a dose of nitro to the fuel. The actual power characteristics weren’t charged around that much because the change to port timing netted an increase in intake duration only. Everything else was left standard.
But the difference of a nominal power increase of 15 percent is just short of amazing. Certain of my favorite hills could be climbed using the next higher gear and ultra low rpm pulling power is much smoother and more predictable.
Being one of the less sound, more ground freaks, I immediately noticed a side benefit of the reed valve conversion in the reduction of carburetor intake roar. It really wasn’t that loud to begin with, but it’s almost silent now, even at full throttle when climbing hills in the lower gears.
I’m of the opinion that reed valving my Matador was a worthwhile modification and wouldn’t hesitate to recommend such a change to anyone. [Ö]
