87 Yamaha SRX 250

[user removed]

Picked one up today. Decent shape for 25 years old. 12-86 production date so it will be an antique in 3 months. 250 CC single cylinder. I think they are fairly rare, supposed to be a competitor for the Ninja 250, very low serial number. Was running until this year, fuel tank is cruddy and petcock and carb will have to be cleaned or replaced. It has a gas gauge and it still works but the bottom of the tank looks like it has mud in it. Good battery and nice Avon tires made in June of 04. Was garage kept until the last two years then outside under a cover. Cleaned up fairly nice. Needs turn signals, removed by previous owner.

regards
Mech

[Grant-53]

Give it the treatment. The latest round of Vetter fuel economy competitions was won on an '89 Honda 250cc with a H-D racing fairing getting 157 mpg highway.

[BHarvey]

Very cool.

[user removed]

Got her running using the fuel tank this morning. Tank needed to be cooked to get the old failed sealer out and then acid treated for the rust and recoated. I stalled on the project when I bought other bikes last year, now I am thinning the herd and this one should be roadworthy after I get the park lights and turn signals working.

New chain, rear sprocket and tires set me back some cash, and I also bought a new petcock and the fuel strainer-filter for the tank. Cleaned the carb and had a glass shop reglue the rubber part of the intake manifold back to the aluminum base that bolts to the cylinder head.

Took her for a short test hop in the neighborhood and everything seems to be fine, with no obvious fuel leaks. I do need to get at least one mirror, probably both.
The bore on this single cylinder is 73 MM stroke is 59.5 so it should be a good torque motor for possibly high mileage. The carburetor is a two stage with the secondary opening mechanically at something like 60% throttle position. Since there are two separate intake and exhaust ports, at low speeds only one intake port is recieving any air and fuel. This may provide some swirl, comparable to the Civic VX, which combined with the long stroke might be a great mileage setup, especially with the 6 speed transmission. The engine also has a balance shaft so it runs very smooth and has a 9K RPM redline, with a speedo, tach and fuel gauge. Compression is 10-1.

regards
Mech

[renault_megane_dci]

Nice bike !

I didn't know about the SRX 250, we only got 600 over here.

The carb setup is that of the bigger SRX and allows for better response through higher velocity
The 6 speed gearbox is a nice advantage too.
BUT, given you had to change the sprockets and you have that 6 speed gearbox, I hope you changed the sprockets for something very radical like +2 on the front and -6 on the rear.
That is the best economy mod on a motorbike and the only way to make aero improvements worthwhile.

So, what about it ?

Ho and pics please !

[user removed]

I changed the rear sprocket to standard, I think 43 or 41 teeth (chain was 3 teeth too short before). Not sure what standard was on the front but the new chain would barely fit so it might be 1 tooth more than standard. Anyway, at 30 MPH the RPM reads right at 2700 so it looks like 900 RPM per 10 MPH speed which is pretty tall compared to some small displacement bikes I have ridden in the past.
It does seem like the power is fairly low if you stay out of the mechanical secondary.
Trying to decide whether to put regular plates or antique plates on the bike. It still needs signals and front parking lights.
If it seems to be reliable enough I might sell the Vulcan 500 and just ride the SRX around locally.
I read a neat post on another forum about using hard paper to clean the fork seals by sliding the paper between the tube and the seal. The paper picks up the gunk that has accumulated on the fork seal and is supposed to clean up some of them enough so they either stop leaking or will not leak for a longer time. I tried it on this bike today to see how it seems to work, because the fork seals were leaking but they were not hard or cracked.
regards
Mech

[user removed]

Here is a link to the fork seal cleaning tool. Probably could make one yourself.

http://www.youtube.com/watch?v=8wlzfoVroSY&feature=pyv

regards
Mech

[woodsrat]

Sometimes you can slide a piece of 35mm film between the fork seals and tubes (with liberal amounts of WD-40 as a lubricant/cleaner) and make an otherwise good seal stop leaking.

[user removed]

Got the tags and insurance today. Antique tags, never need renewal, no taxes, insurance is $79 a year. I rode it 26 miles today, had it up to 65 MPH and everything seems to be fine.

5K RPM at 55 MPH is about 10 % lower than my 2011 CBR 250 R was. The single cylinder twin cam 4 valve motor was also used in ATVs so it has plenty of cooling fins that are normally on ATVs. Seems to run very cool compared to other air cooled bikes I have owned. Even the exhaust pipe doesn't get as hot as most of my previous air cooled bikes, even coming off a 55 MPH highway.

The carburetor is a two stage, with the normal SU type feeding the left intake valve, and a staged secondary carb that is totally mechanical depending on throttle position. This is similar to the Honda Civic VX in that only the left intake valve recieves any air-fuel mix at lower engine speeds. This would provide a great swirl to the intake charge at lower RPM, while still being capable of decent power at high RPM when the secondary is opened.

This bike is very narrow, with minimal frontal area and a decent front fairing from the factory. I will wring it out this weekend and get some mileage figures. Even has a gas gauge.

regards
Mech

[mechman600]

Very cool find.
SRX400s were sold up here. I have never heard of a 250. I had an XT350 enduro bike with the same basic engine and keihin carb setup. It always ran great at partial throttle, but the transition to the second "barrel" was always jerky. Att WOT it was fine. Then when you let off a bit it would pop and bang for a second. I messed around with it for a long time - adjusting the float height, needle position, soaking in carb cleaner and all sort of other things (that I forget right now...it's been a while). I was told on Thumpertalk.com that this was a normal trait for the "emission choked" XTs. The dual carb thing was kind of a neat idea, almost like a poor man's V-TECH, but I think the later CV carbs like on my DRZ are much better. Definitely easier to tune and way more responsive, anyway.