Diagnosis?

I have an 84 virago 700, got it about 2 years ago very cheap. The bike
has aftermarket pipes on it, (which i understand will increase airflow
due to reduced backpressure, and i'm not sure if the original owner
rejetted the carbs to compensate for this, or if it would be neccesary
in this case), but other than that i think it's stock. Anyway, in the
past i've had trouble running the bike at 60+ miles per hour for an
extended period of time. Generally after running at a higher rate of
speed for a while the bike pops and fizzles it's way down to nothing
and i end up sitting on the edge of the freeway for ten minutes before
it will start again. Lately it's also been extremely hard to start, it
has to turn over about 8 times before it will finally purr to life.
Any ideas what could cause behavior like this. Any input is much
appreciated.

-Mike

 

What he said.

 

Kinda hard to diagnose without actually seeing what it is doing.. Could be
that running the motorcycle for a while produces a kind of vapor lock ..
Check the gas tank vent hole.. If that puppy gets clogged, you might be
experiencing a vacuum build up in the tank which would cause the gas to not
flow from it.. Damned near drove me nuts, one time.. Sheesh.. I was ready to
commence a carb rebuild..

Bill Walker
Irving, Tx..

 

This sounds like a plugged fuel tank vent OR a restricted fuel line (i.e.
plugged filter or screen).
OTOH, it could be running WAY lean causing it to overheat on long runs.
That could be a factor in hard starting too.
If you got it real cheap, maybe it's time to invest a little $$$ in a good
mechanic.

 

That was my first thought....fuel starvation due to blocked tank vent.

 

Do your problems happen mostly in colder weather? If so, it could be
carb icing. Cold, humid air enters the carb, condenses and freezes
(this can happen quite a bit above freezing, too, because fuel
evaporation cools down the walls of the carb to below freezing). This
buildup of ice blocks fuel from getting to your engine. When you pull
over and wait for ten minutes, engine heat eventually melts the ice and
the bike will start again.

Unfortunately there is no easy fix for this, but improperly set up
carbs can certainly make the problem worse.

-Gniewko

 

That and the fuel filters.

If you can loosen the cap and it gets better it's the vent.

You can also disconnect a fuel line, put a quart bottle under
it, turn on the tap and measure how long it takes to fill the
bottle. You ought to be able to fill a quart bottle in maybe
5 minutes or quicker.

 

Thanks to you all for your input so far, this usually happens
midsummer, or after the bike has warmed up a lot. It could be a
problem in the fuel line as you have suggested, or possibly in the
venting, i went ahead and changed the fuel filter, and we'll see what
that does, i'll take a look at the fuel cap for proper ventalation this
afternoon too. As far as the hard starting goes, that may have
improved with new plugs torqued to the correct specs. That was a new
issue just this spring, hadn't had any trouble in the past. Thanks
again. Any more advice is appreciated.

 

depending on the age of the bike, your fuel feed lines might be old
enough to collapse some when under pressure.

--
Mark Johnson, Ft. Worth, TX
http://www.bikes-n-spikes.org
"Some people will tell you that slow is good - and it may be, on some
days - but I am here to tell you that fast is better. I've always
believed this, in spite of the trouble it's caused me." - Hunter S. Thompson

 

Besides any in-lin filters, there's probably a screen inside the
tank that ought to be looked at.

 

It's an ancient Virago with a gravity fuel feed, so this can be ruled
out.

 

I'm *still* with you on this one.

 

It's an ancient Virago with a gravity fuel feed, so this can be ruled
out.[/QUOTE]

I am not so sure about that O.G. The Virago 1000 has a fuel pump and
although the tank for a 700 is different than the 1000 I thought both
of them had fuel pumps.

Of course I could be wrong.
--


Don Binns


2000 - Yamaha Venture Millennium Edition
84 - Virago 1000

You start with a bag full of luck and an empty bag of experience.
The trick is to fill the bag of experience before you empty the
bag of luck

http://www3.telus.net/public/dbinns/reeky.htm
http://www3.telus.net/public/dbinns/
http://www3.telus.net/public/dbinns/banff.htm
http://www3.telus.net/public/dbinns/kananaskis.htm
http://www3.telus.net/public/dbinns/walkercalgary.htm
http://www3.telus.net/public/dbinns/calgarybrowning.htm

 

If it's valve clearence that's causing the problem why doesn't it
happen anytime the bike is hot, why only when i'm traveling at higher
speeds? I do probably need to check them, but i'm curious why the
specific speed is causing the issue. For instance i can drive 55
forever without the bike taking an issue, but when i top 65, thats when
it starts acting funny. Thanks again for your input.

Brune

 

I agree that it sounds like a fuel flow/venting problem. Here's a
couple of things to try.

Start off with the gas tank almost full, and disconnect the fuel line
from the petcock to the carbs. Run this line into a gas can or other
suitable container, and turn the petcock to the "PRIME" position, or
the "ON" position if your petcock doesn't have a "PRIME" position.

At this point, the gas should flow freely out of the fuel line. This
is a *much* higher flow rate than your carbs could ever draw with the
engine running, so if you have a flow problem or a venting problem, it
should show up now.

If the fuel isn't flowing out as fast as you think it should, try
opening up the gas cap and seeing if the flow rate speeds up. If it
does, you definitely have a venting problem. If the flow rate stays
slow even with the gas cap opened, you might have a partial blockage
in the fuel filter, fuel hoses, or the petcock itself.

Hope this helps,

 

I had a 750 Virago - same bike, 1982 model - and that was definitely
gravity feed.