78 YZ250 gearing ?

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10/22/2018 2:09 PM

Hi guys, I'm old and slow but racing a 78 YZ 250 in vintage motocross. Just wondering what gearing people are using ? Right now I have 13-50 on the bike. Any better choice ?

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10/23/2018 6:00 PM

the gearing depends in the track and your riding ability. i am racing a 1980 yz250 and the internal gears are similar. i am using a 49 rear. i will look at the front tomorrow but i thought it was a 13. the jumps come up quickly after the turns here so the numerically higher rear ratio helps me exit the turn faster and get more speed up before the jump.

you also need to be careful about detonation on that bike.

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10/23/2018 6:13 PM

Thanks, I am running Race tech springs and emulators up front and a Fox air in the rear. I am also running a Eric Gorr ported 79 cylinder on the bike.

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10/23/2018 7:39 PM
Edited Date/Time: 10/23/2018 7:44 PM

Jon_Haynes wrote:

Thanks, I am running Race tech springs and emulators up front and a Fox air in the rear. I am also running a Eric Gorr ported ...more

xlnt, and if you have a stock pipe and want a little more low end at the expense of some rpm on top, you can add 30 mm to the head pipe.

also, if eric did not put boysen ports into the crankcase, you could ask him about doing that.

you can also try lowering the front if you want it to turn better but make sure you don't lower it so much the tire hits the fender.

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10/24/2018 9:57 AM

I do have the PFR pipe. I also have the 43mm forks. You are right about raising the forks in the clamps, I'm still working on the final height. I can almost turn now, hahaha. I would gladly trade some top end for more bottom, like I said I'm old and slow hardly ever wound out. Thanks for the input.

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10/24/2018 10:48 AM
Edited Date/Time: 10/24/2018 11:01 AM

Jon_Haynes wrote:

I do have the PFR pipe. I also have the 43mm forks. You are right about raising the forks in the clamps, I'm still working on ...more

ok, you can remove the fork springs then lift the front wheel up until the forks bottom out.

raise the forks in the clamps until the front tire is almost touching the bottom of the fender then tighten the clamps.

then sit where the seat meets the gas tank in the turns and lean over the bars. if you don't sit here it will be harder to turn, plus the front tire will try to push towards the outside of the turns (basically loose traction).

in your case, you can add around 25 - 30mm to the head pipe where the pipe comes across the front of the engine, but i have never used a pfr pipe on my bike so i don't know how the pfr pipe changes the performance from a stock pipe but this mod should work on any pipe. i use a stock pipe with an answer silencer with the head pipe mod.

you can also have the boysen ports added that i previously mentioned if they are not added already. this will give it more mid range, but again, ask eric about it first.

also, if it hits too abruptly for you and looses traction, you can make a weight for the flywheel or use an ignition that has an external flywheel but this is a little complicated.

your cranking compression should be around 180. if it is much less than that you will get more power by increasing it which might mean just new rings or a cylinder head mod etc.

my bike is capable of hole shotting and winning the vintage expert class so i know the mods i have work for me at least, but a box stock 1980 yz250 could hole shot the local pro class back in 80 anyway.

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10/24/2018 11:14 AM

Are you starting in 2nd gear?

I will look at my countershaft sprocket in a couple hours and let you know what it is.

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10/24/2018 1:34 PM

I raised the forks. I got 6 mm more with a little clearance between tire and fender, that should help. I do start in 2nd gear. I will see where the compression is tomorrow. It should be pretty good. the pipe mod has my interest up although our season just ended and testing may be hard. Getting up on the tank and simple standing on the straights has been a issue. I see a back surgeon in 10 days but if talks fusion riding will be on hold for while. Thanks for the tips.

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10/24/2018 3:25 PM
Edited Date/Time: 10/24/2018 3:28 PM

Jon_Haynes wrote:

I raised the forks. I got 6 mm more with a little clearance between tire and fender, that should help. I do start in 2nd gear. ...more

i have reptured disc so my back is screwed as well and sitting way forward on the seat is the most neutral place and is where you will feel the bumps the least.

my countershaft sprocket is 13 so i would leave your gearing exactly where it is unless you are tapping out in 6th gear half way down a straight. in fact, i may try a 50 on the rear also.
.

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10/24/2018 4:02 PM

as far as fusing your back goes, if you need it fused, look into the process where they access it from your side. this is what tiger woods just had done and it seems to be far safer with much faster healing time.

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10/25/2018 3:39 PM

Thanks for the tip on the fusion, I will ask about that, My compression tester broke today so I don't have a number yet on that. I said the pipe was a PFR but it may be Jemco ? I will post a pic soon. I find it hard to sit at the tank seat junction, I seem to be on the rear of the seat in most pics.

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10/25/2018 5:52 PM
Edited Date/Time: 10/25/2018 5:55 PM

Jon_Haynes wrote:

Thanks for the tip on the fusion, I will ask about that, My compression tester broke today so I don't have a number yet on ...more

No prob, on the back info but i just hope you don't need it. I saw a video on it after I read about woods operation.

As far as sitting forward more, you can only do what you can do but it really is the only way that bike will turn half decent and not push the front much. Also, if sat back on the seat over bumps I would need an operation as well because the bumps just kill my back now that I am old and weak and brittle, lol.

Here's a story on woods newest back surgeon that did that operation.

https://sportsday.dallasnews.com/other-sports/golf/2018/03/10/exactly-richard-guyer-plano-back-surgeon-responsible-tiger-woods-return-competitive-golf

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10/26/2018 5:01 PM

I checked the compression ( with a cheap gauge) it read 150psi and I did have the throttle open. not sure what factory specs are for compression.

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10/26/2018 5:16 PM

Jon_Haynes wrote:

I checked the compression ( with a cheap gauge) it read 150psi and I did have the throttle open. not sure what factory specs ...more

I don't know the factory compression on your model but ir definitely should be over 150, irregardless, if it were me, i would get it up to 170 - 180, but unless you use a good gauge with a short small diameter hose, it might not be accurate.

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10/26/2018 5:23 PM

I could re ring it and check again but it is a fairly fresh build. Starts right up. I will buy a good gauge in a week or so, I need that anyway.

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