Modified Classic Gas engine mods for BUB

[Oldspoke]

Searched through this site a lot lately and still confused about modified classic gas....I tried the bub tech email but no response for a while.....

My motor is not an OHV , 2-stroke or flathead so here are my questions.......

In modified classic gas class can you change from stock carbs to a larger venturi size using a different carb (one that is still available for racing but can be purchased by the public)?

Also is allowable overbore in classic .050" past the maximum displacement for your class or must the displacement of your class include the maximum overbore?

OK to lighten and balance crank?

Thanks all...stay warm in the polar vortex!   

[Peter Jack]

I'm not a motorcycle guy but if I were making a request like this the first thing I would explain is what I had and what I was trying to do rather than what I didn't have. It's a lot easier for those who might help to do so if they know what they're dealing with.

This is landspeed racing and the guy who's liable to be the most help may be the guy who presently owns the record. You can save a lot of time and grief (which equals money!) by getting the advice of those who have already tried what you're about to attempt.

Good luck with your project. I hope you get the information you need.

Pete

[Old Scrambler]

I run in M-PG and MPS-PG with records in both......................Now I am building another bike to compete in the M-CG and/or MPS-CG. Based on passing tech with my current bike, the carb you speak of should be fine.  I have my own hesitations about the 50-thou over rule...........from stock or over the class limit. Right now I think its the class limit but would like others to confirm.

Good Luck

[Oldspoke]

Thanks Oldscrambler for the quick reply.  Good straight talk from a "motorcycle" guy.

What are you buildng for this year?

[Old Scrambler]

CB750 sohc with permitted mods   I will continue to develop my Tiger Cub.

A crank mod is OK

[Old Scrambler]

I thought I would pose the following so that answers would be available to all....................

So the latest rules revision for the CLASSIC category states "Allowable overbore in this class is + .050- inches over OEM standard bore to remain in displacement class".

My standard bore is 61mm x 4 cylinders. I would like to think this means that I am allowed to use 62mm bores which is slightly less than the .050-inch allowance.  BUT.......this takes my motor to 760cc............Am I still going to be legal?

Second question: Could I use the same base motor with a major overbore and compete in the 1000cc category?

Third question: If OEM motor size is actually one or more cc's larger than the class, is it automatically in the next larger category.............and if so, is it still limited to a .050-inch overbore?

[donpearsall]

The key is that after the .050" overbore, the displacement must STILL be in the original displacement class. So if you were at 749cc before the overbore and as a result of the bore you are now at 751cc, that puts your engine out of it's original class and therefore you can't run in the Production Classic.

Nor can you jump up in displacement class to the 1000cc class, as that would not be the original production displacement.

I was denied a record in the 750 class because mere wear on the cylinders put the displacement over 750cc. No overbore at all, just lots of wear on a 30 year old bike.

Don

[Oldspoke]

Thanks Don!

Appreciate the detailed reply. All the best.....

[Koncretekid]

The key is that after the .050" overbore, the displacement must STILL be in the original displacement class. So if you were at 749cc before the overbore and as a result of the bore you are now at 751cc, that puts your engine out of it's original class and therefore you can't run in the Production Classic.

Nor can you jump up in displacement class to the 1000cc class, as that would not be the original production displacement.

I was denied a record in the 750 class because mere wear on the cylinders put the displacement over 750cc. No overbore at all, just lots of wear on a 30 year old bike.

Don

Are you sure about this for Classic classes other than Production Classic? Here is the way that Classic Gas is written:
 "Allowable overbore in this class is + .050- inches over OEM standard bore to remain in displacement class. Further increased displacement beyond the class limit shall place the motorcycle in the correct class."

It seems to me that the increase in displacement beyond the stock OEM size could be run in the next displacement class, according to the above (i.e. up to 1000cc, which is all that is allowed at present.)
Tom

[Chris V]

My wife (Great Granny Sandy) ran production /altered engine (modified engine) P/AG also P/AF then went to classic with P/CA and P/CF didn't seem to matter at tech scrutinering that we had modified engine, stock bore 347cc. AMA rule #'s 12.0, 12.N address overbore. Vetter Velocity   (3 records).

[Uncle Jimbo]

Hey Chris - Was nice seeing you & Sandy in Columbus last month. 
 I would like to hear your opinion on why the Classic class has engine classifications only to 1000 cc ?  What about all the Knuckle, Shovel, and Pan Heads that are 1200cc ?  Why do you think all those years of bikes are excluded from the Classic class ? 

Also - any hints on what you & Great Granny Sandy are putting together for this year ?

Have fun - stay happy & healthy
Jimbo

[donpearsall]

Koncrete, you are correct about other classic classes. My comments were only about Production Classic.
Don

[Chris V]

Uncle Jimbo, good to see you and Lou there also. You're right with all the old Harleys out there one would think they could have an open class over 1000cc.Sandy was in the 500cc class with her 350. New rule has 350cc class. We're doing ceramic engine bearings, 6 speed transmission, a little more cylinder porting. Hoping to break 100mph
Chris

[Old Scrambler]

Tom...........that's how I THINK the rules should be applied. But their not exactly clear. Why can't they list examples?

[Old Scrambler]

So my basic question was answered by Curtis Smith, the AMA/FIM Technical Steward.

"You are correct the .05" overbore allowance is from the stock bore. So if you have a 60.00 mm stock bore you would be allowed to bore it to 61.27mm. .05" = 1.27mm, A 1mm should put you within the allowed overbore.

One thing to remember is that we take a physical measurement of the cylinder on the salt. If you have any other question don't hesitate to ask."

So tomorrow my pistons and cylinder block will arrive at MILLENIUM TECHNOLOGIES for proper sizing.