SLORYDER

Yeah Jeff and i think it was Kevin Herr that told me the same thing. It just doesnt make sense that 2 venturis of the same size (33mm at 10% and 28mm at 18%) could give you different results if they were passing the same amount of air. People talk about velocity ect which is pertinent in a four stroke application because the flow goes straight from carb to cylinder but in this instance the transfer phase is a separate ordeal from the intake. A smaller carb just creates more pumping losses at higher rpm's

4 mill long rod is the way to go IMO. If your cylinders already have a clean up port it's basically drop in and go. Port work is relatively simple for the do it yourselfers too. That's Gonna be my next build fo sho

Ps sorry for the caps lock ok i SEE WHAT YOU ARE SAYING. yOU ARE SAYING THAT TOO SMALL AN ENGINE DOESN'T PUT ENOUGH SUCTION ON A BIG CARB TO ALLOW IT TO PERFORM AS IT IS INTENDED. bUT EVEN IN THIS INSTANCE, IT SEEMS LIKE YOU COULD JET DOWN ON THE SLOW JET/ DROP THE NEEDLE TO COMPENSATE FOR THIS... tHE REASON i BRING THIS UP IS THAT i HAVE SPOKEN TO MORE THAN ONE BUILDER (SPONSER)THAT SAID THE BOTTOM END LOSS IS BOGUS.

Make sure your jetting is right before you jump to any conclusions..

Say, for instance that rpm is 1,000; it may take say 100 cc's of air per minute. Now to me that can be achieved by having a 26mm carb running at 10% OR a 35mm carb running at 4%. Either way, the same amount of air is being fed into the engine, and if the engine is drawing the same amount of air from each carb, and the orifice passing the mixture is the same size (which in my mind it would have to be), then the velocity would be the same, which leads me to have doubts about the velocity theory. Anothe reason I don't but the velocity theory is that the final velocity of the stream is determined by the size of the transfer ports and the pressure pushing the stream. IN MY OPINION

This is how I view it. I may be way off. A banshee engine needs a certain volume of a/f to run at a certain rpm. Say, for instance that rpm is 1,000; it may take say 100 cc's of air per minute. Now to me that can be achieved by having a 26mm carb running at 10% OR a 35mm carb running at 4%. Either way, the same amount of air is being fed into the engine, and if the engine is drawing the same amount of air from each carb, and the orifice passing the mixture is the same size (which in my mind it would have to be), then the velocity would be the same, which leads me to have doubts about the velocity theory. Anothe reason I don't but the velocity theory is that the final velocity of the stream is determined by the size of the transfer ports and the pressure pushing the stream. IN MY OPINION Thoughts?

HAHA I just bought the boy one of these. I paid 250 and it was in need of a new top end. While it was apart I opened the exhaust port (made the sleeve flush with the cylinder) and let me tell you this thing is more fun than my wife's blaster. Thing moves for an 80. Needs about another 2mm off the exhaust port roof about 40 thou off the head and a little clutch work though. I'll try to get some vids...

You sure it isn't the jetting?

You say that going from 26mm carbs to 33mm carbs will cause you to lose bottom end performance. Please explain why, so I will know Thank you.

So if one side was constantly under more stress you don't think that would accelerate the problem?

I've heard of this. I'if it's a big kink chances are it'll take more than just that. PM AKheathen about his air compresser method. Seems like it would work good for a bad kink

Naw, just spent 3 hrs out in the hot sun with a pair of dark coveralls on. Feels like my brain is on simmer lol It just seemed like maybe having one cylinder putting more stress on one side of the crank than the other constantly could lead to something going out of wack eventually..?

i wonder if your carbs aren't synced properly if that is a cause of the crank going out of true..?

Depending on whether or not the transfer ports have been raised you may not be able to use those cylinders. Take a measurement in millimeters how far from the deck the transfer and exhaust port roofs are. It is really easy to port to a 4mill crank. Mostly involves lowering the port floors and raising the exhaust a little. Take those measurements, and pics if you can and pm me or just reply to this post.

still

If this were the mentality of everyone, no one would learn anything. Unless we asked you. You're just a bad MoFo

Well doesn't it feel better to get things like that out in the open? You know I wouldn't judge you and I'll always be here if you need help. As for your little problem let me suggest cutting back on the masturbating and possibly doing some yoga -Marcus

YGPM

delete

OK. I was thinking there was only a single reed cage for the two cylinders. Now it makes sense. So the only factor here would be the overlap between the two carbs, which IMO would be minimal. I would like to hear sodapopinski's opinion on why the setup does not work and the associated dyno sheets.

After thinking more about this, it appears that there may be minimal overlap of the intake cycle of the two cylinders, however it shouldn't be enough to make a significant differende, miiiiight make it seem like a pair of 33's instead of 35's. IMO. The thing that bothers me is the fact that when one cylinder is in the transfer phase and pressurizing the reed cage area, instead of building pressure to push the transfer stream through the flow follows the path of least resistance and goes into the other cylinder. Unless of course there is a check valve in each intake runner separating the two sides...Is there?

How do 99% of us learn 99% of what we know? FROM OTHER PEOPLE. That said, I've heard guys that have run every pipe under the sun say that they like the t-6 best for low end out of all the pipes they've used. I don't have that kind of experience with pipes, and this is ONLY their opinion, so take it for what it is.

OK, after much thought it is clear to me. This statement that I said holds some truth, but is really irrelevent to compression vs. octane relationship. I now realize why it is UCCR that determines octane. Cranking compression, like I mentioned before is affected by port timing, UCCR is not; this is due to the fact that when you are kicking over the engine, at that low of an RPM there is not pressure being built up inside of the cylinder prior to the exhaust port closing; all the pressure goes out of the port. This is why the height of the port is important when measuring compression w/ a guage. Now, the UCCR (un corrected compression ratio) is calculated by determining the entire volume above the piston at BDC, compared to the entire volume above it at TDC. You may wonder why we use the entire volume above the piston at BDC, since the piston doesn't seal the cylinder until over half way through it's upward stroke.. The reason is that at the RPM when the exhaust pipe is performing its duties most efficiently (peak hp rpm) the pipe is returning a pressure wave that is compressing the cylinder before the port seals, then the piston further compresses the mixture until it reaches TDC. So basically the piston draws in mixture from the carb to the extent of the displacement of the cylinder (175cc) then it transfers this mixture to fill the cylinder above the piston. At this point the pipe is actually sucking even more mixture from the carburetor, which ends up being even more cc's of mixture than the 175cc that the piston draws. Where does this excess mixture go? Out the exhaust port into the pipe. So you have mixture filling the cylinder (175cc) PLUS a good deal in the pipe. Well as the piston travels upward, unlike at the low rpm you get from kicking the engine over, your tuned pipe is not allowing that mixture to go back out of the exhaust port, rather it is shoving the contents of the pipe back into the cylinder with the pressure wave that is returned from the reverse cone of the pipe, giving you compression BEFORE the piston seals off the exhaust port. Sooooo bottom line. I stand corrected. Even if you go higher w/ your exhaust port, it is not safe to assume that you can go lower on your octane. I believe the rule of thumb stated in my previous post should hold true though. If anyone sees a flaw in this please feel free to correct me.

Upon further thought, what Daj is saying makes sense. Although you lift the E port 2mm say, its relation to the entire trapped volume is likely small in relation to the .010" you take off the head and its associated reduction of the compressed volume at TDC. So, to be safe .025"=Pump gas .030"=50/50 race fuel, pump gas .035"(squish allowing) race fuel Methanol=Re-chambering This should be a safe medium timing and other things considered.

So is the trapped volume a factor or the entire volume above BDC? Also at higher rpms when the pipe is returning the pressure wave its amplitude will affect the final compression inside the combustion chamber, correct?