For those of you that haven't seen my post on the Honda GX25, check it out by clicking [link=http://www.rcuniverse.com/forum/Honda_GX25__OHC%2C___NEW_%28WITH_PICS_%29/m_1727643/tm.htm]HERE[/link]. I did plenty more experimenting today and I think it's a keeper! I tried many different carbs since I am so against the fixed jet type. The peak RPM I recorded was 6600 on a dynathrust 18x6, that was with no muffler, the stock muffler caused a 300-400 RPM loss. For those that have messed with 4-strokes, you know that the pulse holes in the carb are not in the same place, therefore a typical 2-stroke carb won't work on the 4-stroke carb mount. I contemplated making a new carb mount, with a relocated pulse hole, but I decided instead to install pressure fittings, and use a piece of tubing for the pulse. I plugged the pulse hole in the carb, made a new gasket, with no pulse holes, then drilled and tapped the side cover to accept a pressure fitting. On the engine, I just drilled and tapped into the intake manifold. The 4-strokes use the pulses from within the intake manifold, since there is no crankcase pressure, this provided a very easy place to tap, But I was concerned that the pulses might be weaker than from a 2-stroke crankcase.
With all the carbs I tried, I was able to get it super rich, and slobbering, so I assume it's pumping plenty of fuel, and the pulses must be strong enough. Now here are my questions, I don't have much experience with these carbs. With all the ones I tried, if you richened the low needle enough for a clean transition without stumbling, the idle was very rich. If you set the low for a good idle, it wouldn't take the throttle, you had to accelerate it very slow. Does this mean the carb is too big for the engine? Maybe such a large bore carb requires a lot more volume of air in the venturi to draw more fuel (for the transition) and by richening the low end, you improve the transition, but then idle suffers. I tried 7 different carbs, they all acted similar on the transition, and provided nearly identical top end numbers, So the peak HP must be limited by the size of the intake port, not a restrictive carb. Based on this, I think all the carbs I tried were too big, But I have nothing to compare it with. After running the stock carb again, it didn't do too bad, the idle and transition is much leaner, the exhaust note is even different (open exhaust) The stock carb has an accelerator pump which helps for a second, but once it burns that fuel charge, it stumbles a bit before finally settling at max RPM. With the stock carb, when you close the throttle, it pops out the exhaust on decel, with the other carbs (running richer) It never did. So now I am wondering exactly what the differences are on all these Walbro carbs. Here are the ones I tried:
WYB 6B 320 (stock on GX25)
WYL 97 835 (stock on GX31)
ZAMA ?? (stock on 30cc Homelite)
I think we have some Walbro experts here somewhere, The WT 37 and the WT 160 have an extra "jet" that is removable near the fuel inlet, it looks like a large brass plug, protruding out of the housing at an angle, but it's some type of metering device. What is this for? And why do most of them NOT
have it? Are there differences between carbs for Saws and for weedeaters/blowers? I'm assuming to get more top end, you just need a larger carb, capable of flowing more air and providing more fuel (within reason of course) But do different carbs even of the same bore size, run differently, (on purpose) some idling better, or transitioning better, etc? Are there any general rules that apply, such as "chainsaws typically have better carbs" Or "don't use the WA series" or "the WT carbs make good power, but won't idle" I noticed the WYB and WYL (both of which have a rotary barrel) seem to run a lot cleaner than the butterfly type. Even on my ZDZ and Brison, they seem to dribble a little fuel at idle or low power settings. All the carbs I tried have a much larger bore than the stock carb, yet the max RPM was the same. So it looks like I don't necessarily need a bigger carb, I just want one with two needles, so It can be adjusted. Where would I find a smaller carb than what I tried? I'm thinking maybe off a little 18cc featherlite or something. Someone enlarged the jets in a WYB or WYL carb, what happens when you go too far, how do you go back once you've drilled it? Or how do you adjust the mixture when you change props? I guess back to my original idea, a new carb with two needles.