backpressure: what are the facts?

4 cycling

Thanks for the info. So the 4 cycling I am having at cruising rpm is caused by a lean situation. How do I rejet or get more fuel into the mix? As for pushing the rpm following the guideline in the motor instruction of " avoid sustained full throttle operation or motor life will be shortened" and the fact that bicycle type brakes aren't that great above 12 mph. Should keep our motor together. Also shaving the head .020-.040. Adds some. GuTs to the little beastie!
 
4 cycling

Since we are on the topic, I have 2 HT equipped bikes....both are the 80/66 cc versions. both seem to be a little different (I bought both bikes already converted with low hours on engines) but they both pretty much run the same. Both will 4 cycle until a certain RPM.....approx. 3000 by ear. This was with the exhaust unmodified, and later with modification. Thats when they start to run right, like a 2 stroke should, throughout the rest of the RPM band. Unless there is a load on the engine; like climbing a hill, then it 4 cycles again. When I got my first bike I thought something was wrong, until I watched some videos of bikes in action. When I got the second bike, thats when I assumed it probably was 'normal' for these engines. Anybody find a way around this?
 
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4 cycling

Ratty, after further thought I seem to remember model airplane motor beak-in called for a RICH mixture causing 4cycling. As the tank emptied bubbles would enter the fuel line leaning the mixture causing the motor to run normally (2cycling) I'm starting to think that racing carb for sale at some sites might be worth it iF. It allows more adjustability. Anyone tried one of these? How about the low restriction air cleaner from S.B.P.? It looks a lot cooler than the black plastic one too. As I said earlier took the head off both my motors , and using apiece of plate glass with 3m stickit sand paper on it I planed .040 off one head and a more conservative.020 off the other, this increased compression and resulted in a significant power increase. Be carefull of piston/head interferance. If you goof or don't like the change a new head is a whole 12 dollars. Party On!
 
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It would appear that way,4 stroking at low throttle would occur esp. in powerful 2 strokes like the Kawasaki 550 for instance, if you were just poking along.If it's at medium throttle you're likely to be too lean (beware of overheating),does it idle fast too ? (intake leaks!).
 
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4cycling

We can assume the 550 kaw has a much more refined exhaust intake carb etc. ThAn. Our little china motor. Intake leaks would probably also be accompanied by uneven throttle response and backfiring but the general imperfections in the making of our little mills serve us well as they will run in less than perfect conditions and states of repair, also ride the kaw everywhere in one gear 4th should do. Rich 4cycling will probably occur.
 
4 stroking cannot really be prevented,it's part&parcel of the '2 stroke condition',a lean micture would promote it.The carb may be improperly designed.Normally at higher air speeds the micture tends to get richer (Bernouilli's law),so possible the needle taper may not be right and raising the needle position one notch would improve matters.However that may cause rich running at higher throttle,this would usually not be deleterious,but would increase fuel consumption&might cause plug fouling.It is important to ALLWAYS get enough oil into the engine,even at low throttle,that's why I am opposed to higher than 30:1 mictures.The only occasion a 2 stroke began to seize up on me was when had I tanked up with premixed gas once and took off at high speed on a 4 lane road.The engine seemed to lose power all of a sudden for no apparent reason,realizing what was prob,going on I cut the engine,coasted to a stop&let it cool down,luckily it freed up,never ever did that again.
 
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The engine specs posted by cabinfever77 have mistakes in them.For the 48 cc engine the max power was quoted as 1.61 HP at 6800 rpm.However if one computes the power at 5500 rpm based on the torque of 1.77 lbs.ft ,it turns out to be 1.88 HP,this is hardly credible the output at 5.5 k skould be lower, or the max power at 6.8k higher (more likely the latter.)
It turns out that the figures for the 68cc engine can not be trusted either.The max torque of
4.57 lbs.ft is not credible,it can not possibly be 2.5 times as high as the 48 cc engine.That would correspond to a power output of 4.9HP at 5.5 k rpm,The max output quoted at 5.5 k is 2.68HP.The power ratio between the two engines at 5.5k turns out to be 1.42 ,the displacement ratio is 1.39 ,which makes sense.The torque at 5.5 k for the 68cc engine based on 2.68HP output turns out to be 2.52 lbs.ft.For anyone interested in these sort of computations,I can give a handy-dandy formula (not that hard),here goes:
P= 0.193 x (rpm/1000) x Torque , P is in HP, rpm in rev/min and Torque in lbs.ft.
For instance in the last case P= 0.193 x 5.5 x 2.52 or P=2.68 HP.
It would be appropriate to post revised numbers in the specs.
There happens to be an ballpark approximate empirical relation between engine displacement and max torque: Tmax=D/25 and Pmax=D/20to25. (50/40HP/liter).This applies to typical small engines, not the souped-up variety.
 
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4cycling

Thank you DUIVENDYK for increasing our knowledge of 2cycle motors a very great deal. This is precisely why I joined this site. I just went for a ride on my 80cc screamer and it was happy 4cycling at low speed but as you mention too much can lead to plug fouling so I get on it frequently. Performance is so hot with the head planing and reduced back pressure its unbeleivable. I weigh 290 lbs. ANd this thing just flies! Just few more power tweaks and I'll trade my pedals for pegs and a pull starter!
 
Yes, the effect of back pressure on performance is quite pronounced,did you shave the head 0.040" ? (1mm),should have raised c.r. by about 10% from 6.5 ? to 7.2 or so,may be more.I would not push the revs too much,if the con. rod bearings croak you can prob. kiss the engine good bye.I am not an advocate for high oil ratios,it has more to do with environmental concerns than promoting engine life exp.The design of these engines goes back 40/50years or so and I assume they are designed for something like 20:1 ratio,as was pretty standard in those days.Good luck with your screamer!
 
compression ratios

From 6.5 to 7.2 thanks again for the math, duivendyk I first tried .020 that was fun so I took off another .020 Do you know at what point higher compression ratios become a detriment to the performance of a 2cycle? Can it interfere with crankcase mixture pressure? Another thing I did when I put the head back on I turned it180 so the spark plug gap faces forward causing the flame front to advance toward the exhaust port I'm not certain if this hAd. Any effect on performance. I to am concerned with the environment and was pleasantly surprised to hear the synthetic oil I bought is bio-degradable.
 
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