Air Flow

[tmc3c]

How many of ya'll and under what conditions do you take the rubber disc out under the seat on a high breather frame for better air flow? When doing this does it effect the jetting ?

Thomas Carmichael

[firstturn]

Thomas,
  We tested this back in the 70's and came up with results that proved (to us) that it made no difference.  I believe that the area you are talking about is so far from the actual air intake of the air filter/carburetor intake is the reason for no difference.  I will say that while I was working on this little project I did discover the need to not use certain filter oils due to there restrictive properties especially if too much oil is used.  Just another thing to besure your bike is breathing properly and still getting clean air to the engine which is the REAL life of the a two stroke.
  Also there are other tricks to do inside the airbox to boost horsepower.  But, I will leave that up to the experts to discuss.

Ron Carbaugh

[tomale]

Ok what tricks to the air box. I got passed going down a straight on my 400 and that blew me away. I did not think that was possible. Not that I am all that fast it is just that my bike should be.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
78' 400 MC5

[Mike Lenz]

Maybe on the smaller bikes it doesnt make a differance but it sure does on my 400. I even have to go down one size on the main jet with the plug in.

[tmc3c]

Thank You Guys for your input!! I am dealing with my 250 and as far as air box tricks Thom,I think you might want to repost to get some feedback OR we can corner BAJA Ron and get him to spill it !!:D

Thomas Carmichael

[firstturn]

Mike,
  What size carburetor?  How did you measure performance?  Truly interested.

Ron Carbaugh

[Mike Lenz]

Ron, 38mm older style Bing. However I cant remember what jet I went down to. A 180 I think, from a 185. It just wouldnt clean out on top with the main jet I had in it without the plug. Went down one and it cleaned out on top as it did with the one size bigger m jet before the plug??

[firstturn]

Mike,
  Thanks for the follow-up.  I will tell you that we were using 36mm carburetors on the 250 and 400 Pentons.  I only knew one guy in our area that could ride either with a bigger carburetor(guys name is Keith Duffy*).  Many tried, but they would have been faster with the smaller carb.  And I do agree with you that the cap, in your case being off, would help, but in your case you could have have had a brush hog tied behind your bike and you still would have been fast(in my book).
*Keith Duffy was a very talented rider out of Houston and on a good day put away the likes of Howerton and Stackable.  He did have a lot of trouble with that kid named Wise.  Keith finished his career riding a 250 & 400 Pentons for Richard Sanders and the bikes were prepared by a great tuner named Richard Herman.

Ron Carbaugh

[Mike Lenz]

Thanks Ron. Im running a 40mm carb om my 77 400, with some porting! The trick with the 400's is to figure out a way to get more air up from the crank area thru the transfers. They are limited by the case design. The 79 400 cylinders had a center finger in the V boost port and as we know Carl Cranke opened up this boost port like crazy in his 250 porting specs. We actually tried a 250 Cranke copy porting job on a 400 back in 76. It ran like fire...for two races...before the crank came out the bottom of the cases. It was fun while it lasted though! I have a 79 cylinder with the three finger boost port and am going to experiment a little more with that cyl than the one Im currently running. I think why the 76 blew up back then is because we raised the exhaust quite a bit and the crank (being the weak point of the 400 engine) couldnt handle the higher RPM's. This time I am going to work on transfer flow to the max...but leave the top of the exhaust port alone! I have to make a faster 400 than Teddy Landers has!!! JP Morgan did an unbelievable porting job for Teddy's 400. Took out so much of the liner it scares me, but it RUNS, even with the reed valve "air blocker system" JP put on it!