vapor lock, and more

art.robinson art.robinson at
Wed Aug 12 05:46:51 PDT 2009

I agree with the simplicity of adding a manual petcock and reading all
the problems with flooded cylinders, locked engines diluted oil and bent
con rods, I become nervous just thinking about it.

On the other hand when just a lad with my brand new '62 Triumph
Bonneville, I forgot to turn on the petcocks (there were 2, each feeding
a separate carb) and I managed to make it from my mother's house down
the street to the main road. 

Just about to make a right turn in front of a f*#!king big speeding
double-decker bus and the bike sputtered and my guardian angle tapped me
on the shoulder and said, "Arsehole (remember we're in England) you
didn't turn on the petrol!". I grabbed the front brake and the nose dove
right down as the bus passed inches from my front wheel.

Two more drops of gas available in the carbs and I would have made a
great impression on the front of that bus and would never had the joy of
riding a GPZ or anything else for that matter!

My heart sped up just recalling the incident!

Now, 47 years later with my memory in tatters, I'm just too scared to
fit a manual petcock, so I'll just keep fretting and sniffing around the
bike and hope for the best.

Art in TO

-----Original Message-----
From: gpzlist-bounces at [mailto:gpzlist-bounces at]
On Behalf Of chris marquis
Sent: 11 August 2009 08:27
To: gpz
Subject: Re: vapor lock, and more

This kinda stuff is the reason i ALWAYS spend 3-4 dollars for a manual
shut off and put in the line (with a filter ) cheapest insurance i know
against a whole lot of problems!!!!!!!!!!!   Chris

--- On Tue, 8/11/09, Drew Blanchard <drewblanchard at> wrote:

From: Drew Blanchard <drewblanchard at>
Subject: Re: vapor lock, and more
To: "Bob Nelson" <majordad at>
Cc: "GPZ LIST" <gpzlist at>
Date: Tuesday, August 11, 2009, 7:54 AM

Ah, well, this is not what I expected to hear.  And I DID just have the
carbs cleaned and adjusted by my local Kawi dealer (picked up some
before I installed the fuel filter, and also needed the pilot jets
adjusted).  So, this points to a bigger issue?  The petcock has NOT been
"prime" position.  I've noticed a whiff of gas around the bike for a
but it was faint, so I put it on the back burner.

When I pulled the tank, the petcock continued to dribble.  Ultimately, I
to prop it on its side and lean it againt the wall (the tank is less
1/2 full) before I could walk away and work on the bike.  This lends
credence to hydrostatic lock; the cylinders, especially 2 and 3, has
fuel in them.  At the time, I was focused on vapor lock,  and worrying
geting home before my dogs exploded from a loooong day.  It never
to me there shouldn't be so much gas in the cylinders.  Now I need to
out how to get the back to the dealer's.

Grrrr.... (again)

Thanks for the input, Bob!

Drew in VT

On Tue, Aug 11, 2009 at 4:05 AM, Bob Nelson
<majordad at>wrote:

> Drew, first things first, if the motor wouldn't turn over and you had
> squirt out of one or more cylinders after you pulled the plugs, it was
> hydrostatic lock, not vapor lock.  Hopefully you only had one flooded
> cylinder.  There are two requirements for this to happen.  First, you
had to
> have gas flow out of your petcock without vacuum applied to the
petcock  (or
> the petcock is in the Prime position, if that is the case switch the
> to the normal or run position).  The second requirement is a
> float valve in the cylinder involved allowing gas to over fill the
carb and
> drain straight into the cylinder.
> If you are lucky the flooded cylinder was one of the outside carbs.  I
> lucky because I am pretty sure you can pull the float bowls without
> the carb rack out of the engine.  If so, you can make sure that the
float is
> free to move and not stuck open.  If it is free to move, then after
> the float bowl back on, I would tap the float bowl repeatedly as I put
> petcock in prime to try to help the float overcome any stiction and
> hopefully float and thereby close the float valve.  If the float is
> open, you will need to disassemble, clean and carefully watch the
> couple of times you start the bike to make sure your cylinder doesn't
> again.  If you are unlucky, bad things can happen after hydrostatic
>  You could end up with bent valves.
> Anyway, if this occurred out of the blue and not immediately following
> carb cleaning and reassembly, you might have fallen victim to what I
> suspected happened to me.  This March I rode from Charlotte to Florida
> some friends after not having any problems for a very long time.  The
> morning I had a cylinder locked.  I attributed it to contamination
> running way down into the reserve on my fuel tank and running some
> through that partially blocked the float valve open.  After I cleared
> cylinder and rode that day, the problem didn't recur.
> As far as follow on leaking not associated with the locked cylinder,
> would have to come from a leaking hose or the fuel tubes that connect
> carbs.  Hopefully you will just find that your fuel line is cracked
> that would be the easiest repair.  But that also points to a bad
> since you should not have fuel flow through the petcock when there is
> vacuum present to the petcock unless it is in the Prime position.
> Hopefully this rambling makes sense to you and helps.  Good luck!
> Bob
> Drew Blanchard wrote:
>> After a chilly start to the day, temps went into the upper 80s. 
>> forward to my ride home, I thumbed the starter and was rewarded with
>> very
>> brief "thump" and some starter whine, but that was it.  Motor didn't
>> over, and the starter seemed to be trying to move an immovable
>> Fearing the worst but knowing everything had been fine on the ride
>> work, I pulled the tank and plugs to see if the motor would move
>> compression.  Yup.
>> Note to self, remember to stuff rages very tightly over the spark
>> openings.  Messy.  Very messy.
>> When reassembled (and what a bitch it was reconnecting the fuel line)
>> motor initially made the same noise, then turned over with ease.
>>  Obviously
>> it was vapor locked.  I never had this issue on my '95, but that was
>> a
>> stock set up.  Anyone else have this issue?  Is there a preventative
>> measure
>> I can take?
>> Didn't get to ride the bike home, because somewhere, somehow I have a
>> nasty
>> fuel leak.  If I didn't know better, I'd say the leak was coming from
>> bottom of the motor.  And landing directly on the exhaust pipe.  I
>> very
>> fortunate the motor never actually started.  Every time I turn the
>> over I get a nice, steady drip, and I can't see where it's coming
>>  I'd
>> worked on the bike for several hours in the heat, and quit for the
>> Fortunately, our maintenance guys at school let me put the bike in
>> garage for the night.  Once on the center stand, no leak.  Moving the
>> or leaving on the side stand = leak.
>> Grrr.....
>> Drew in VT

Drew Blanchard
Technology Teacher
Winooski City Schools
Normand St.
Winooski, VT 05404
(802) 655 - 3530 x6073

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