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1981 yz 250 ?

 
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matthesjason



Joined: 06 Jun 2009
Posts: 22

PostPosted: Tue Dec 14, 2010 7:29 pm    Post subject: 1981 yz 250 ? Reply with quote

Had the Yamaha Team tips porting done to my bike in the fall. Didn't seem to do a whole lot, still really a lightswitch powerband. Generally or if you have experience, should the jetting be leaned out after this minimal porting and head work? Always seem sooo rich on bottom, and im down to a 50 pilot. Ive added 2 teeth to the rear, and am wondering if the thicker intake spacer would help to bring on the low end.
Also, maybe sacrilegiously, I'm thinking about junking the stock old mikuni, and moving up two generations of carburetors, and putting on a new keihin.
Any thoughts? Thanks guys!!
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rdnutz



Joined: 31 Dec 2008
Posts: 605

PostPosted: Tue Dec 14, 2010 9:20 pm    Post subject: Reply with quote

are you running the boost bottle?
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matthesjason



Joined: 06 Jun 2009
Posts: 22

PostPosted: Wed Dec 15, 2010 6:33 am    Post subject: Reply with quote

Boooost bottle is on.
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rdnutz



Joined: 31 Dec 2008
Posts: 605

PostPosted: Wed Dec 15, 2010 8:56 am    Post subject: Reply with quote

ok. couple general comments here 1st, then specifics: none of my 3 81 250H I've owned ever had a lightswitch powerband, but in general you will be disappointed if you expect any YZ250 from 79-81 to have low end like an open class bike, where you can just cruise into a turn at idle then roll on the throttle and rocket out of the turn without dropping a gear or fanning the clutch- or both. My mostly stock 79 has the best low end like that, but in my experience you'll never get that from the 81 even in stock form or with changed gearing.

having said that, my current 81 runs GOOD through the whole powerband but it takes some work and most people have problems tuning with the boost bottle still on there. My setup:

stock carb
boost bottle
DG pipe
Boyeson fiber reeds
transfer ports cleaned up
Uni filter in airbox (holes drilled in airbox for more flow)
32:1 pump gas with KLOTZ synthetic

Don't remember where I ended up with jetting, but I think I went up 2 on Main jet and down 2 on Pilot and my elevation is +1500 above sea level. I went through hell with the running rich down low thing too and here's a couple things to keep in mind:
1. ALWAYS use fresh gas with highest octane you can get and good oil. I also add 1 oz Octane booster per gallon. If the gas is more than 1-2 weeks old- you'll run rich down low and smoke. I learned this the hard way and spent a lot of extra time chasing my tail on jetting.
2. don't scrimp on cheap plugs- use the best 1 you can get in right heat range.
3. keep air filter clean and not over oiled.
4. make SURE your IGN timing is right on.
5. make sure your crank seals are good.

if you got all that covered, start playing with air srew and needle height and make small changes to jetting 1 bump at a time after starting from stock settings to get main figured out and then pilot.

good luck Cool
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markm



Joined: 19 May 2008
Posts: 169

PostPosted: Wed Dec 15, 2010 12:00 pm    Post subject: Reply with quote

Regarding your thought on putting on a wider intake spacer...

I tried that on my 81 and I personally could not tell any difference in the way the bike ran. In my opinion, the HUGE downside to the spacer is that it makes it VERY difficult to put your carb back on. When I did have the spacer installed and I was messing with my carb I finally gave up and took the airbox off to get carb back on. Anyone's who's taken the airbox off knows what a pain that is (remove tank and shock).

What's kinda odd about the spacer is that I've talked to a few guys who thought it really helped out the low end.


Last edited by markm on Wed Dec 15, 2010 12:12 pm; edited 2 times in total
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markm



Joined: 19 May 2008
Posts: 169

PostPosted: Wed Dec 15, 2010 12:10 pm    Post subject: Reply with quote

rdnutz, thanks for posting what's worked out well for you on your 81.

I've messed with my jetting a bit and pretty much came to the same conclusion you have. You mentioned that you have a DG pipe, did you mention that because that pipe worked well for you? There've been several different 81 owners who've told me the bottom end pull was worse with the DG when compared to the stock pipe. I've not tried anything other than the stocker so I don't have anything to compare it with.
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markm



Joined: 19 May 2008
Posts: 169

PostPosted: Wed Dec 15, 2010 12:47 pm    Post subject: Reply with quote

Has anyone had their 81 (or any other older bike) ported by Eric Gorr?

His prices are pretty reasonable and based on his experience it would seem that he would know his stuff. I was thinking about sending him my 81 cylinder.
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matthesjason



Joined: 06 Jun 2009
Posts: 22

PostPosted: Wed Dec 15, 2010 9:09 pm    Post subject: Reply with quote

The porting was minimal, but im sure eric gorr had his dremel in a lot of these cylinders back then.

Any one tried lengthening the head pipe for more bottom?
How about a ProForm pipe? They sure look trick.
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capt264



Joined: 30 Apr 2008
Posts: 179

PostPosted: Wed Dec 15, 2010 10:20 pm    Post subject: Reply with quote

I don't know where I found this but I kept it anyway for the mention of the pipe mod in it. Maybe it would work for an 81 YZ. FWIW, I would go with a ProForm set-up, race gas and jet appropriately.
Steve

1980 YZ250 mods
First of all you must accept that you now as I do also, will never be able to out ride the stock perfection that is your low hour YZ250G. With that being said why would you wreck such a bike by changing a bunch of stuff? The 1981 YZ250 was very heavy, much slower, and the big new forks sucked bad as delivered from the factory. If you want to dial your 1980 YZ250G to perfection, simply use silklene pro RSV 5wt. fork oil @ 6 1/5 inches from the top of the tubes, have the shock serviced by Sage racing 303-425-6712, mill .015 off of the head, twin air filter, Boysen reeds, add one tooth to the front sprocket and two teeth to the rear from stock, and weld a 15mm spacer into the head pipe at the lower cradle seam from the factory, Then install modern tires and Renthal handlebars and go racing. That's what the 1980 LOP Nat'l team did then, and I just copied their bikes after pitting next to them at a couple of Nat'ls that year. Merry Christmas
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rdnutz



Joined: 31 Dec 2008
Posts: 605

PostPosted: Thu Dec 16, 2010 6:45 am    Post subject: Reply with quote

capt264 wrote:
I don't know where I found this but I kept it anyway for the mention of the pipe mod in it. Maybe it would work for an 81 YZ. FWIW, I would go with a ProForm set-up, race gas and jet appropriately.
Steve

1980 YZ250 mods
First of all you must accept that you now as I do also, will never be able to out ride the stock perfection that is your low hour YZ250G. With that being said why would you wreck such a bike by changing a bunch of stuff? The 1981 YZ250 was very heavy, much slower, and the big new forks sucked bad as delivered from the factory. If you want to dial your 1980 YZ250G to perfection, simply use silklene pro RSV 5wt. fork oil @ 6 1/5 inches from the top of the tubes, have the shock serviced by Sage racing 303-425-6712, mill .015 off of the head, twin air filter, Boysen reeds, add one tooth to the front sprocket and two teeth to the rear from stock, and weld a 15mm spacer into the head pipe at the lower cradle seam from the factory, Then install modern tires and Renthal handlebars and go racing. That's what the 1980 LOP Nat'l team did then, and I just copied their bikes after pitting next to them at a couple of Nat'ls that year. Merry Christmas


that comment about the 1981 being slower and heavy is funny. The frame on the 1981 is 14 pounds lighter than the 1980 and the forks are the 43mm that everybody always changes to from the old 38s Laughing

everyone I know that ever had the 1980 said THAT model had the "lightswitch" powerband, and the 1980 also had the 6 speed tranny vs. the 5 speed in the 1981.

regarding pipes: my 1st 1981 was a thrashed ex-desert racer I rebuilt back in around 1993 and had the stock pipe on it and I never tried any others. It seemed to be a wheelie machine (especially on hard pack), but also had very light front end with 38mm Fox Factory Forks. My last 81 came with a DG pipe and when I built up the current 1 I bought a DG for it as well since it worked well on the previous 1 and I couldn't find a decent used pipe or any other new pipe that was priced right. I have no complaints about the DG and no reason to mess around trying others as I can't ride that bike to its full potential even in stock form Cool
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dct270



Joined: 31 Dec 2008
Posts: 272

PostPosted: Thu Dec 16, 2010 7:52 pm    Post subject: Reply with quote

There is no way the 80 G frame is
14 pounds heavier than the 81 H!

The 81 weighs more , i have the Dirt
bike comparison shootout. The 80
is also faster
the 81 forks,rear hub, wheels boost
bottle system,airbox weighed more


Both great bikes, but those are the
facts
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fartman47



Joined: 12 Dec 2010
Posts: 26

PostPosted: Thu Dec 16, 2010 11:16 pm    Post subject: Reply with quote

If you are looking for more low end and less light switch throttle try a smaller carb, a little bit of flywheel weight and go back to the other gearing. lowering the ration will exasterbate the problem
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matthesjason



Joined: 06 Jun 2009
Posts: 22

PostPosted: Fri Dec 17, 2010 5:29 pm    Post subject: Reply with quote

Smaller carb is a good idea. I haven't been expert speed for 20 years so keeping it on the pipe is a bit harder now! But lets think outside the box, and what a carb thats 3 generations newer, like a keihin airstriker would do for overall crispness. comments?
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