Getting more power, high compression 66cc...

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So I ended up taking off my expantion chamber exhaust I bought and puting back on the chrome stock one. No more 4 cycling! Way better rpms. Runs great with the high compression. Guess the moral of the story is I know expansion chambers do work, but build one that fits your applications on your engine. Not all ones you can buy will work for you. So that is my next project!
 
So I ended up taking off my expantion chamber exhaust I bought and puting back on the chrome stock one. No more 4 cycling! Way better rpms. Runs great with the high compression. Guess the moral of the story is I know expansion chambers do work, but build one that fits your applications on your engine. Not all ones you can buy will work for you. So that is my next project!

NONE of the ones you can buy will really work as well as a properly designed and fabricated one.... ;) making a pipe CHEAP means its made cheap, not properly! no matched ports, no spring mounted headers etc, no removable silencers, no actual dimensions that are similar to what should be used... all the jazz that makes companies like FMF famous... and, noone is interested in paying for any of my pipes, that for $180 or so, means im making them for charity basically. (a local welder charges 90 an HOUR, and he would still take at least two hours, more considering he would make jigs as well)

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obviously this pic was taken before i completed it. yes, that is a watercooled head.


the guy i made this for tried haggling me to $120, finally settled at $140, plus postage...never heard a word back. making me say you can ALL go and get well and truly. at least the guys with chainsaws PAY. dont ask. i wont.



usually... a lean condition will get slightly better as the engine warms up, as if its rich, it will get slightly worse (richer again)

my only, oft repeated criticism of the mikuni vm18... why stick an 18mm throat carb onto an intake manifold and port, that are only 16mm in diameter? work on port matching, removal of daggy bits, cleaning the ports up, etc etc.


simply reading the plug can be deceptive, requiring the correct heat range to work properly, as well as other factors. i have never seen a HT engine get an insulator any lighter than a fairly dark tan without refusing to run (no danger of lean seizures, trust me)... nowhere near as clean as the plugs in my saws or trimmers etc, that all run MUCH hotter than the HT.


its always better to determine mixture simply by riding the things and seeing whats working best... at least on anything as low performance as a HT ;)

tweak all you want...i will still overtake you :giggle:
 
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Any tips on getting the clamp on carb with oring to seat better on intake manifold? I got a small leak there nothing crazy but would like to get it stopped. That oring seal is a poor design...
 
erm....get rid of the oring. if the oring isnt in a properly machined groove, its useless. just measure the depth of spigot in carb, and make sure its goes that far on the manifold (within 1/16). smear of silicone/sealant will help, but i dont even do that. you may have to file certain parts so it will slide on all the way. which parts, will be perfectly obvious if filing is needed.

there is a mikuni vm16.... quite rare, and memory tells me its a flange mount type. i may be wrong. usually am :wacko:

(and a 17 too!)
 
the Mikuni VM18 was suggested because of its price and availability.
of course if you can make a 18mm I.D. manifold that would be better.
please dont anyone comeback with the old wives tale of bigger carburetors causing worse running at low rpm. god, will that fairy never die?
 
So I ended up taking off my expantion chamber exhaust I bought and puting back on the chrome stock one. No more 4 cycling! Way better rpms.

If you make any changes to the exhaust or intake system, you will need to rejet the carburettor to suit the new configuration.
 
the Mikuni VM18 was suggested because of its price and availability.
of course if you can make a 18mm I.D. manifold that would be better.
please dont anyone comeback with the old wives tale of bigger carburetors causing worse running at low rpm. god, will that fairy never die?

nope. not saying that at all. oversized carbs DO cause bogging when opened up wide when operating at low RPM but. no old wives there, just plain, well documented truth.

im saying that the port only has a cross section of 16mm, 17 at the most. theres not enough material there to try and enlargen it, even by a whisker, and ive made properly matched manifolds before. at which point, the studs and the nuts really start protesting about clearances... then try bunging a weld in there... have to stick that tube to a flange somehow...

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make the port WIDER, and you cant get the flange to match, without some tomfoolery with the studs. oh yes, its possible, but awkward. look closely and youll see where i had to bash it to clear one nut...

make the port higher, or deeper...there simply isnt enough metal left to create a good seal... i like my gaskets to have at least 2mm of contact all around...

you can just see that that manifold is curved smoothly, not "cut and welded" through some angle, and the angle it attaches to the flange is the same angle the port itself has... i get awesome flow... but cant improve it with a dremel, at all. other than to take the seam out of the tube, maybe...

now compare my manifold flange to the best "port matched" flange you can find on the market... ha!


nothing about worse running at low rpm. everything to do with minimising flow restriction.

its like sticking a brick behind a high reynold number wing... the wing makes a lot of lift (suction on the jet in the case of a carb with a properly shaped throat, such as the mikuni, which is , really an airfoil shaped as a cylinder) that just gets broken into destructive turbulence afterwards.

if you can TAPER the manifold smoothly from the 18mm (or even a 20 if you can get the taper right...) to the intake port, then its all good.

an abrupt step... sharp edges dont have great airflow. its why we port match.

and whats that bit about sound waves being reflected by sudden changes in diameter? hmmmmm?

ill stick to my stock standard NT ;) possibly taper bored.
 
yeah but no good rider opens the throttle up more than what the engine can handle. Normally the throttle is gradually opened, not full throttle at low RPM. It is called "throttle control". I have an 18mm on my 55cc and I never have any low RPM problem with it. Also I have a custom made adaptor for the RS reed valve.
 
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