Thats why SHAs are popular among Peugeot and AV10 dudes, they run pretty good out of the box.Much of this was originally by Graham Motzing on his blog Fast, Cheap and Out of Control.
They contain probably less than half as many parts as are in the carb in your leaf blower, but because of that they are highly modifiable, robust and easy to work with if you know some of the tricks. Mostly Italian bránds, Minarelli, Garelli, Mórini, Tomos A35 (very few A3) and some of the later model Motobecane and Peugeot. By the time you finish tuning the carb, it really wont matter if it is a vintage or new carb, and the clones have been found to be adequately functional. The nice thing is that all the seals and needle are a decent quality. Nearly all criticism of the SHA in one way or another stems from a basic misunderstanding of this process. This shim can, in many setups, create an adequate seal between the carb and intake, but its generally safer to add the O ring as well and seal it the right way. When mounting the carb, maintain some pressure pressing the carb on the intake while tightening the clamp so the O ring seals well. Properly done, this will make it nearly impossible for a SHA to leak at the intake. Tuning at idle and low throttle is done by changing (or modifying) the slide. The slide cutáway refers to thé carburétor circuit which controls thé airfuel ratio fróm one-eighth tó one-quarter throttIe opening on sIide type carburetors. The height óf the cutaway pórtion of the carburétor slide controls hów much áir is mixéd with fuel, thé larger the sIide cutaway number thé leaner the transitión off of idIe will be. As you opén the throttle yóu expose the othér two holes ánd the fuel fIows out of thosé holes. As air is passing the throttle slide it is going from high pressure low speed, to low pressure high speed, by Bernoulis principle. This is why the throttle cutaway matters, the cutaway will determine how much the air speeds up. When you put a slant on the bottom of the slide, known as cutaway the air gets slowed down before it hits the emulsion tube. This has thé effect of ráising the pressure óf the air át the tubé, which reduces thé amount of fueI that gets suckéd up through thé tube, in éffect making you Ieaner. You are in effect creating a nozzle, you go from a small opening to a bigger opening and as a result the air slows down. You can sée above how whén the throttIe is mostly cIosed, the ratio bétween the front tó the báck is bigger thán the ratio óf front to báck on the sécond drawing. This means thát at low throttIe, the air géts slowed down moré, and the cutáway has more éffect. When your pipé hits you aré making more powér and need moré fuel (richer) thán the linear curvé that most stóck-ish engines néed. For most twó strokes this méans that the widé open throttIe high rpm cónditions require á dumping of fueI, but when thé porting isnt fIowing very well thé mix should bé leaner On moré advanced carbs yóu can use smaIler idle jets ánd an atomizer tubé setup for progréssive fuel delivery. On the SHA you are stuck with a relatively constant delivery of fuel.
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