bb boost port and race piston question

ok first off i boost ported my bbk and swithed from walbro 603 to walbro hdb-18 19mm bore carb and this thing absolutly kicks some serious ***! easly over 40mph no problem! my question is this i cut down a factory piston to match pbu race piston! when i put it in the head i noticed that at top dead center the bottom of the piston now comes past exhaust port! this must f up the port timing right? would the air fuel mixture escape in to the exhaust at tdc? hey to anyone that has serious mods u have to get this 19mm carb it is only 37$ and out performs any carb i have seen on these bikes! also anyone that thought that boost porting a bbk does not work is crazy! get out the dremmel again and start GRINDING!

how deep did you make the boost port, I thought the main problem with the bb head was busting thru the wall when you make a boost port due to the thinness at the back of the head

can someone explain what a boost port is? When I search i just get boost bottle info.

PM me where u got that carb i wanna get one!

to answer your guestion..at top dead center your saying that the holes in the pistion skirt goes up above the exhuast floor..the air and fuel mixture will obviously excape through the hole..but when theres a hole in the piston skirt i think that it would lower crank case pressure and also engine will runn lean due to less fuel to the case..cause with the situation you have is that when the piston comes up and the reeds are opened and fuel comes in. but that hole on the piston skirt reaches ups up to the ehuast port the reeds will close almost shut and air will leave out the hole in the piston leaving you with less fuel to compress in the case. meaning less crank case pressure..and with that i think you would get the affect of having reed spacers...having more bottom end. i could be wrong

WHERE DID YOU PURCHASE THIS WALBRO CARB , I DID A GOOGLE SEARCH AND NO LUCK PLEASE HELP US OUT.

Actually you will blow fuel/air out the exhaust at TDC but all the 2 stroke Cags do this anyways b/c you piston rings are above the exhaust port at TDC anyways! When the piston is on it's down stroke it sucks the air/fuel out back out of the pipe and into the chamber, that's how a 2 stroke works.


Oh yeah.. If you didn't balance your piston by lightening the other side, you're asking for trouble!

Where did you find this carb ... I am the 3rd person who asked?

ok sorry guys for taking so long to reply! lota pm questions first of all you are going to have to bore out hp carb manifold to 19mm (to match carb) walbro hdb-18 @ jackssmallengines.com 19mm bore 16.67 mm venturi start at 1 1/2 turn out low speed and about 2 turns high speed! WARNING do not buy this carb unless you have bbk and ip2 pipe! this thing puts out alot of fuel! it is used on factory poulin 60cc chain saws! any questions pm me thanks

How do you balance a piston and when is it necessary?
Thanks for the info.

The ideal situation would be to remove the same amount of material from both sides of the piston. If not the piston will be unbalanced and it will **** sideways in the cylinder sticking the rings and destroying the head and the piston and the crank and the rod. It's messy! I've seen people add material under the piston skirt to counterbalance on larger engines but there is probably not enough room for rod clearance due to the small CC's of the piston no the Cags.

I would be leery of the piston hanging up on the bottom of the port now that it's exposed. Usually it's a good idea to leave about 2mm piston skirt overlap over the port. Also, on the compression stroke is where you are pulling fresh mix into the crankcase so now it's getting diluted with exhaust possibly. The piston skirt hanging up on the port worries me the most though. As for lightening the piston, I would only remove some material from under the piston pin and slightly on each side. Leave 3mm or so under the pin so that it doesn't crack under it. You might be able to remove some material inside the piston near the skirt where there is a casting lip, but try to remove the same on both sides. Don't make the skirt paper thin either.

Another thing that you can do is to lighten the connecting rod a little bit. Grind some material off of the beams to knife edge the rod. If you haven't seen this done, do a little research. After you knife edge the rod, polish the surfaces that you ground to remove stress risers, small nicks and abrasions where cracks will start to form under load.

The best thing anybody who wants to dive this far into an engine is to get a good 2stroke tuning book. I recommend Two-stroke performance tuning by A. Graham Bell. It is a Haynes book and is still in print.

Thanks to you both Ritchey79 and John.
Good info....

well how does pbu race piston work it has a hole only on 1 side of it where boost port is!( i doubt with a hole on 1 side that piston is balanced)and there piston is about 3/8" shorter than stock! hmm does someone here have 1 they can review and see if at tdc you can see past piston?

You should not be able to see past the piston through the exhaust side at TDC. If you can the piston is in wrong or too short.

Nate

that is what i thought!

I am running the PBU headstrong kit with the boost port on the piston but at TDC the piston skirt does not go above the exhaust port. Good luck and I hope you figure it out. Oh ya, I'm gonna try out that Walbro carb you mentioned earlier and I just ordered the billet manifold to modify also, right now i'm only running the 15mm walbro w/wyk billet manifold. thanx for sharing that with us.

You dont knife edge rods, you knife edge cranks. You do this so it cuts through the oil thats flying around the bottom end of the engine more easily.

You are correct that it is common practice to knife-edge cranks in wet-sump engines to help reduce frictional losses from the crank passing through oil or splash back from the sump. However, on an engine such as this that doesn't rev especially high, with a decent sized rod and a light piston, you can afford to take some material off of the rod with little chance of stress failure. The lightened and polished rod will take less power to reciprocate and the bottom end of the rod that has the most fore and aft movement will benefit from any reduction in drag due to moving through the mixture in the crankcase. Only a very small amount of material could be removed as the rods are not of a heavy I-beam construction.