Yamaha XS650 backfire

Wow, haven't though of Pacers in years... there was a house on my paper
route when I was about 14 that had TWO of them, his 'n hers, in the
driveway... owners were very proud of them. In a freakish sort of way.

 

You could start here: http://en.wikipedia.org/wiki/AMC_Pacer

The UK import story is quite, err, interesting...

Mind you, with a 4L out of a 90s Jeep it might are work reasonably well.

 

Isn't it, just? What an amazing lash-up.

You mean, drink even more fuel and go even slower?

 

Na, same fuel consumption, roughly twice the power.

 

Well, there goes the neighborhood. Next we'll see Ivan Reid confessing
a predilection for Humbers in a thread about XS650 fart-bangs...

 

And what, precisely is the fuel consumption of an AMC/Jeep 4.0 liter
engine?

A local woman was offering to trade her Jeep Grand Cherokee for
*anything* that would save gas last summer, during the height of the
price gouging, because she had to haul her kids to school and back 10
times a week.

The Wikipedia article also claims that the AMC/Jeep 4.0 liter engine
will go 300,000 miles between overhauls, which is double the
expectancy of the 3.7 liter V6 used in the Jeep Liberty models from
about 2001 on...

The 3.7 Liberty gets 22 mpg on the highway and about 16~18 mph in
town.

 

Humbers? I think a neighbour had a Super Snipe when I was five or
so. Cars weren't too numerous in our neck of the woods at that time.

--
Ivan Reid, School of Engineering & Design, _____________ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

I think I'd test the compression just because it's
an easy test if you're eliminating suspects. I like
the idea of the carb swap too.

 

Compared to the AMC Gremlin, I always thought
the Pacer was a thing of beauty. (sort of).

 

I like the idea of the compression test better than the carb swap... 2
minutes vs. 1 hour. What the hell, I'll test it. Swapping the carbs looks
to be a bit of a whore, I'm not sure if they're actually identical... one's
got the idle-adjust screw on it and I don't think there's a tab for that on
the other one. I might be wrong. Other than that I think everything could
be swapped, I'd need to exchange throttle plate rods I think to get the
fittings in place for the throttle cable and throttle plate linkage.

 

The only way to really be sure that the idle circuit passages are
clean is to squirt
carb cleaner into the pilot air jet and watch it squirt *freely* out
the idle jet and the three acceleration transition ports.

 

Even if ring wear on one cylinder would be unusual,
it could be one exhaust valve on it's last legs or very
slightly bent so it's not sealing correctly.

There's definitely a point in troubleshooting
where you stop trying to figure out what's likely
and just start methodically eliminating suspects
one by one.

In this mode, you hit the easy ones first.

Best of luck with it.

 

Just wanted to update everyone that's offered advice. I removed & cleaned
the left carb again, used pretty damn close to a whole can of cleaner, it
was SPRAYING out of the three holes in the throttle body and pilot jet
orifice when squirted through the air jet passage. I also noticed when I
had them off that the carbs seemed to move just a tiny bit in relation to
each other. The machine screws which hold them onto the angle-iron bracket
were wrecked from an over-ambitious philips head driver (rather than an
impact driver) so needed replacement, so I may have eliminated a bit of an
issue there. Put it all back together and...

It idled perfectly on both cylinders. Finally. Good news.

The bad news is that I am WAAAYYYY over-jetted on the mains. Bike ran well
when cold but stumbled up near redline, say 7,500 RPM. So I had an idea I
was a bit oversize. Then started climbing hills, outside and engine temps
warmed up, and 4,500 - 6,000 RPM was unusable, the engine was just choking
on too much gas. The left plug fouled badly and by the time I got home I
was idling on one cylinder again. (Note to self: next time maybe not do the
middle-of-nowhere-through-the-mountains ride as a shakedown ride). I read a
bunch of stuff about the XS650 with BS34 carbs which said one size up on the
mains would improve stock bike. I put on 14" free-flow mufflers (yes, very
loud) and went up two sizes and it was way too much. I think I'll probably
end up back with the original jets, but will try one size up over stock
next.

Next up is front forks... this bike is SCARY on bumpy corners for someone
used to a sport bike with decent handling.

Dave

 

<snip>

Well done

Oh yes.

 

Ummmm, what do you think is meant by "one size up over stock"?

Many amateur tuners seem to be under the impression that if they have,
say,
#110 round jets in the stock carbs, going to a #120 round jet is only
one jet size larger.

It's not. It's four sizes larger. The next size larger than #110 is a
#112.5, which passes more than 2.5% more fuel.

The next step above that is a #115 round main jet, etc.

Since the area of a circle = pi times radius squared, the area of the
hole in a 120 main jet is 20% larger and will pass 20% more fuel.

No wonder it drowns the engine at high RPM...

 

Stock is 132.5. I'm running 137.5, two sizes up from stock like I said.
One size up would be 135, which is what I'll try next.

 

Well, work out the relative areas of the various sizes, and you'll see
that
the area varies *exponentially*, with the amount of fuel flow
increasing *dramatically* in the sizes above #125.

I have a GS1100 Suzuki bored out to 1168cc and I couldn't use anything
bigger than a #135 in it on the race track.

 

An 1168cc motor (4-cylinder) would have 4 x 292cc piston displacement. My
650cc twin has 2 x 325cc. So your 292cc displacement piston should draw
commensurately less fuel than my 325cc displacement piston, n'est pas?

Or is this calculation not valid? I'm sure there are other factors than
piston displacement which influence main jet size...

 

He knows what he's doing.

 

A considerable percentage of the fuel burned by your XS650 is actually
used to keep the pistons from melting...

Your calculations are a starting point for comparison, but my GS1100
has four valves per cylinder and peripheral valve area reduces the cam
duration requirement.

The Suzuki 16-valve I-4's use less camshaft duration and they take
about 1/3rd less time to burn the mixture, so less heat is transmitted
to the piston and cylinder head, and the engine requires less fuel.

One of Honda's GP I-4's actually decreased the total burn time to 1/2
that of what their older 16-valve CB750/900/1100's with pentroof
combustion chambers required.

The CB750/900/1100 designs ran hotter than hell...