Yamaha XS650 backfire

Well I don't know as I'd go THAT far, but so far so good. I am no longer
scared of carbeurettors, and will likely pull the carb rack off my CB750 and
clean it next as I've been having problems with that bike too.
Unfortunately as I recall it's a helluva lot more work to get the four carbs
off of the Honda... more friction and less room.

 

I've often wondered what the benefits of multiple valves per cylinder is.
Is it just that it gives a larger orifice to suck fuel through? Is there a
particular benefit to shorter duration cam lifts?

 

Two smaller valves that fit in the same sized combustion chamber have
more peripheral area than one larger valve. The air starts flowing at
lower lift off the seat.

Camshaft designers start figuring the cam duration from when the valve
first lifts 1 mm off the seat until it returns to 1 mm off the seat.

Yamaha tried 4, 5, 7, and even 9 valves per cylinder, before settling
on 5 valves per cylinder on their Genesis motor designed around 1985.

My FZR1000 has 5 valves per cylinder, but Yamaha is giving up on the 5-
valve design because it doesn't deliver power the way they want it to.
If they lower the compression so it will rev, it loses mid-range
power. If they increase compression to get mid range, it won't rev.

4 valves per cylinder seem to be the best compromise. Honda has revved
4-valve engines up to 25,000 RPM.

What the heck? I very seldom use even 8000 RPM. Maybe once or twice in
a long day's ride through the back roads...

When the Japanese companies started producing 4 valve per cylinder
engines, the aftermarket cam grinders figured they could just use the
same long durations
on their offerings to the motorcycle hot rodders, but they were wrong.

Their cams didn't work well at lower RPM with the amount of
compression ratio the factories were using.

The new 4-valve engines did better with less duration and a little
more lift...

Most shade tree tuners don't understand what compression ratio does
for them anyway. You need just enough compression ratio to allow your
engine to get over the first hump in the torque curve and through the
mid-range flat spot into the second hump in the curve.

That's where the real power band starts. Too much compression keeps
the engine from taking advantage of the inertia effects of air in the
intake and exhaust systems...

 

Oh yes. And four hardened carb rubbers as well. Huge fun.

 

Where to start?

OK, greater valve area is the first thing. So more gas in and out,
basically.

With better breathing comes the ability to run leaner mixture and a
higher CR, but there are a lot of variables there. As a rough rule of
thumb, four-valve heads run hotter than two-valve. In times of yore,
some early four-valve heads used to run the risk of cracking round the
valve seats, but that just isn't a problem these days.

Very important: higher revs. Small valves have less reciprocating weight
than big 'uns, so you can run the bike at higher revs before valve float
sets in (that's inertia taking over).

Ignore Krusty's rabbiting on about his ShiteOldSuzuki and old Honda
CB900s - the world has moved on a long, long way in the three decades
since they were built.

Depends on the way the rest of the head is built. A longer duration
allows more time to fill and exhaust the cylinder.

However, this sort of breathing tends to work better at high revs than
low to medium revs, when the gas velocity is greater, and decent gas
veocity allows 'scavenging' of the cylinders (outgoing exhaust gas
helping to such in new mixture), which is why you get a situation where
both inlet and exhaust valves are open at the same time... "overlap".

And four-valve heads aid high revs...

There are whole books written on this sort of thing, if you want chapter
& verse.

 

And nothing to do with the desire to keep down costs? The 5-valve engine
was/is superb. Even Ferrari used the tech.

 

You only need to strip the bodywork off an old Genesis and look at the
mix and
match castings to realize that the thing was designed by feuding
samurai warlords.

After marvelling at the faggots and peas rendered in aluminum, read
the shop manual to realize how difficult one of those would be to
repair if it had starter clutch problems or alternator drive chain
problems.

 

Mix about 5 ounces of xylene (a paint thinner) with 5 ounces of
acetone, about an ounce of transmission fluid and an ounce of WD40.

Save this mixture in a sealable container, you won't use it all on one
carburetor removal job.

Put some plastic sheeting under the carbs.

Paint the rubber softening mixture onto the rubber boots with a glue
brush and let it work to make the rubber more pliable.

 

You don'ty get it, do you? The idea of a modern bike (and your old FZR
is anything but) is not to make the thing easy to mend when it brakes,
but to ensure it doesn't break *in the first place*.

Which is what the Japs tend to do.

I'd far rather (say) have a sod of a job replacing (or paying someone to
replace) some obscure component after 40k miles than have to replace
some more easily locatable component every 5k.

 

Or do what everyone else does, and just soak them in very hot water to
make them pliable.

 

And why, pray tell, have the folks at Yamaha re-designed the starter
clutch system to put it on the end of the crankshaft, where it can be
accessed more easily?

 

Mix about 5 ounces of xylene (a paint thinner) with 5 ounces of
acetone, about an ounce of transmission fluid and an ounce of WD40.

Save this mixture in a sealable container, you won't use it all on one
carburetor removal job.

Put some plastic sheeting under the carbs.

Paint the rubber softening mixture onto the rubber boots with a glue
brush and let it work to make the rubber more pliable.

Are there any downsides to using this? I mean, will it shorten the life of
the rubber boots? Not that I'd care... last time I had them off I was about
ready to haul the bike to the scrap yard trying to get those damn carbs back
on.

 

On which FZR1000 have they done this, then?

 

Yes, there is an impact on the lifespan of the rubber boots when
solvents that de-vulcanize the rubber are applied.

If the rubber is so hard you cannot remove the carbs, you need to do
*something* to soften them, or you cannot make necessary repairs.

Running a hose from your hot water heater to route very hot water over
the rubber boots is also going to devulcanize the rubber if it's hot
enough.

 

They haven't done it to the FZR1000, and never will, but they did
redesign the Seca II to move the starter clutch onto the end of the
crankshaft where it belongs.

The folks at Triumph also learned their lesson about burying the
starter clutch and later designs relocated it to the end of the
crankshaft.

 

Ae you saying that early Seca IIs had the starter clutch in a
different place?

So what you're actually talking about is completely different designs.
Not modifications of existing designs. It's not really relevant. Some
are different from t'others. And if the component is solid, it doesn't
matter *where* you locate it, really.

And yes, I'm aware that some early Triumphs had starter clutch
problems. So that's a combination of a duff component and awkward
placement. The fact rests: make it reliable to start with and who
cares?

 

Go back and read for comprehension.

*I* care, if I'm the one person in a thousand that has to tear my
motor to flinders to replace a component that is supposedly
"reliable".

I'd rather have a less reliable component that is easily accessible
and have the
replacement parts available at Wal*Mart at a bargain price.

 

I have. Because if they redesigned the Seca II midway through its
production run, that's quite interesting.

As we never called Yamahas Secas here it's hard to tell, but the Seca
Two is what we called the Diversion. The previous XJ600 was a *totally*
different bike.

Which rather proves my point.