87 Yamaha SRX 250

[user removed]

I felt the transition at close to 65 MPH yesterday. Just as you described. At first it kind of startled me and I was worried about it progressing to something worse, 4 miles from home. Now after reading your post I realize it is a good thing, because it gives me fair warning that my fuel mileage will drop significantly if I go beyond that point. I guess the bike was designed for either 55 MPH or 100 KPH speeds. Above that and it starts to drink a lot more fuel.

regards
Mech

[renault_megane_dci]

A lot more fuel being not this much since my four valver 250 single with very poor aero never got me over 3,5 L to the 100 kms including some fair trashing ...

[user removed]

After almost 150 miles some observations;

The replacement chain was not long enough. I now suspect the front sprocket was changed for one with more teeth. I replaced the rear sprcoket with one that was susposed to be the stock number of teeth, can't remember if it was 43 or 41.

This bike does 65 MPH (indicated) in 4th gear at redline of 9k RPM. In 6th gear at the same speed its under 6k RPM. The speedometer was fluctuating so I lubed the cable which seemed to help. Based on the fact thjat I had to enlarge the slots in the trailing arm to get the chain to fit, I think the front sprocket is one or two teeth larger than stock.

The engine is remarkably smooth, even down to close to 2500 RPM in 6th gear in my neighborhood. I have had it up to 75 MPH, max speed is listed at 84, which I think this bike might do in 5th gear. Maybe one day I will find out on the interstate. It pulls good from low revs, and the secondary works well if you have the revs up. Accelerates away from lights well enough for me and easily better than the average traffic.

You can really feel the difference when you tuck behind the windshield in the amount of throttle you need to maintain speed.

Even the gas gauge works well, considering the tank only holds two gallons of fuel!

It would even be OK on the Interstate and I think it will do nice as a replacement for the Vulcan 500. Maybe when I quit dogging it my fuel mileage should approach 75-80 MPG if I keeep the speeds below 50 MPH.

regards
Mech

[renault_megane_dci]

As a comparison point, my GN is +1 front, -2 rear.
It used to get an indicated 83 mph tops with stock sprockets, then it fells to 79 or so with taller gearing.
Although with the cardboard kamm back, 86 was seen once.
Anyway, the GN got a lame front end as far as aero goes so you should be much better there.

And my average speed is more 60 to 70 on the highway and I get 75 mpg so I assume you should get much better.
Even with the taller gearing I use 5th from 30 mph on up.

[user removed]

Big update.

Found a barn find SRX 250 on craigslist. Had not run since 1990, seller was asking $1000.
After a while he dropped the price to $700, no title. I contacted him and offered $450 for a bunch of parts and he accepted.

Gas tank, front fairing, all park-signal lamps, exhaust (OE), speedometer, gauge fascia, light fascia, as well as all the attaching parts and brackets, and to top it off the very rare unobtanium lower front fairing (needs a little repair). Gas cap assembly which I swapped out my original lock cylinder to and does not leak like the old one when I fill the tank completely to the ring in the filler neck with the bike vertical.

Today I pulled the spark plug. That sucker must have been in there forever and it wasn't tight enough to compress the crush washer. Seems to run better and should get better mileage with the new plug torqued properly.

We'll see if the mileage increases with the new plug.

regards
Mech

[user removed]

My 6th gear final ratio works out to 100 MPH at 9k RPM which is redline. The bike was supposed to do 84 MPH tops (was probably close to redline in 6th originally). I have had it up to 70 and I can tuck down where I feel the wind flowing over my back, indicating fairly smooth airflow. I still need to get the secondary emulsion tube out and clean it better, as well as rebonding the rubber to the aluminum intake manifold for the secondary carb. I'll probably wait until winter to do that. I had the primary one rebonded when I had the carburetor off the first time.

Got a Rebel 450 project to work on now.

regards
Mech

[renault_megane_dci]

Air leaks on intake sounds like potential for hole in the piston to me ...

[user removed]

I agree, vacuum leaks in a manifold can be the source of real problems. These manifolds are aluminum bases, bolted to the engine, with a rubber portion that was glued or vulcanized to the aluminum, that completely enveloped the aluminum base and attached to the carburetor with a clamp. The other end of the carburetor is clamped to a molded hose that fits to the air cleaner housing. Even with the bonding partially separated the mechanical connections still hold the carb in place and the design means any vacuum works like chinese handcuffs to prevent external air from entering the engine. The spark plug looks fine and I feel no indication of lean stumble. I had the other side rebonded after glass bead blasting the aluminum base and cleaning the rubber with lacquer thinner, then using urethane structural windshield adhesive.

If it holes the piston, then I will just have to get another engine and I know where one is sitiing right now with less thna 6 k miles. If I thought it was a serious issue I would fix it now, but I feel it will be fine until cold weather comes abck this fall. Until then I am going to work of the 87 Rebel and sell the Kawasaki Vulcan. Who knows what other project I might pick up.

regards
Mech

[renault_megane_dci]

To be honest, I got an Helix with a big crack in the manifold.
The plug was guilty of being white (couldn't resist) but it ran fine for 2K miles (before I repaired and sold it)