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The empty tank vapor lock issue...
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The empty tank vapor lock issue...
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rufusswan



Joined: 29 Sep 2007
Posts: 1786
Location: Taneyville, Missouri

Post from Paul Smith on the Yahoo board Reply with quote
I thought that this was a good description of the "fuel gremlin" issue.

Paul Smith on Yahoo wrote:
I bought a used Chetak a couple of years ago that ran like a dream
with 600 miles on it. It did seem to have an electrical problem
though and I promptly took it to University Scooters in Dallas. They
initially misdiagnosed the problem as something major, but figured
out it was just a shorted wire causing the problems. however, they
recommended the first major tuneup as is recommended and of course I
went for it. When I picked the bike up the mechanic said it was the
fastest production Chetak he had ridden and in fact at the time I was
able to get it over 60 quite easily. However, after the first tune up
I began to experience major fuel problems. The main symptom was that
as I began to speed up it would stall out. This increased to the
point that I had to hold the gas a little to keep it running at the
lights and if I could get it running pretty well, then it would
backfire if I ran about 55 or so for any time. I found split hoses
the day after the tune up causing vaccuum leaks and fixed that, but
the problem seemed to come back and neither the shop nor myself could
find any more leaks. They made adjustments and I made adjustments
and eventually with carb adjustments and using premium gas I was able
to get pretty good performance, but still couldn't get up to the old
speeds or run for long at 55 and it was still prone to stalling. The
next time it was time to fork over the dough for a tune up I had it
running pretty decent and they recommended brakes, a new clutch, and
the full tune up. They did all of this and when I picked it up I
couldn't even ride a mile without stalling dangerously at lights. i
took it right back and they couldn't really figure out the problem.
The problem subsided a little over time, but it still wasn't ust
right. Of course I began to ride it less and now want to get back
out on the bike more so I started messing with it myself because at
this point it really isn't even safe to run. The problem is that if
it starts and idles okay, when I would take off on the streets it was
constantly cutting out as if it were out of fuel. I figured out it
seemed to worsen after right turns and I looked into it. After using
seafoam directly in the carb, installing an inline goodflowing after
market fuel filter, and running additives in the gas it seemed to be
doing better, but then quit running alltogether one day. I looked
and looked and realized that the fuel line was s shaped up and down
and by shoving it down lower than some things they had routed it
over, I got better flow so it seemed to be a gravity problem. I
assumed that if it was so dependant on gravity I may have a vacuum
problem somewhere else, but could not find one. It then ran like a
dream for one day before getting back up to it's old tricks today. I
now realize that it seems to be only starving for fuel when I really
start trying to take off on city streets and if I baby the throttle I
expereience the problem much less. Is this just because of the fuel
filter I put on, should it be so dependant on the gravity feed for
the fuel, are the lines too small, What what what? This is an
otherwise, good running, well maintained scooter, that rarely even
needs a choke to start if the weather is above 40. I just want to
enjoy it. I'm not mechanically deficient, but this really has me
stumped. It seems to have a gremlin.


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Tue Jun 10, 2008 2:41 pm View user's profile Send private message Send e-mail MSN Messenger
donarntz
Site Admin


Joined: 01 Jul 2007
Posts: 235
Location: Austin, TX

Post fuel pump Reply with quote
Monza, I saw you posted about adding a fuel pump. How is that working? I'm dying to know.
Mon Jul 14, 2008 2:02 pm View user's profile Send private message AIM Address
matty_x



Joined: 06 Apr 2007
Posts: 1065

Post Reply with quote
I'm locked out of the damn house with a laptop and a wireless connection. So what else to do than research fuel pumps?

I found that you can purchase a Mikuni pulse fuel pump for a single carburetor for anywhere between 20 and 30 bucks. And you don't have to go through SPD or ASC. You can get them lots of places. I checked the googles on the internets: http://www.google.com/search?q=mikuni+fuel+pump&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

I also learned the following:

Quote:
A pulse fuel pump uses the pressure differential produced by the engine to move a diaphragm inside the pump body. This pressure differential is generally transferred via a pulse tube to one side of a flexible diaphragm in the fuel pump. On the opposite side of the diaphragm, check valves are positioned in the fuel channels to only allow the fuel to be drawn from the fuel tank and delivered to the carburetor.

In a two-stroke engine the pulse tube of the fuel pump is connected to the engine crankcase. As the piston ascends and descends the pressure in the engine crankcase transitions between positive and negative. The pressure differential can be greater than 8 PSI. As this pressure differential is transferred directly to the diaphragm, fuel pressures are nearly the same as the pressure differential of the crankcase. Fuel flow is also directly related to this pressure differential.

In a four-stroke engine, the engine crankcase contains lubricating oil. Therefore, the pulse tube of the fuel pump is connected to the intake manifold instead. As the piston ascends and descends, the pressure in the intake manifold transitions between approximately atmospheric pressure and negative. This pressure differential is usually less than 2 PSI. Because of this low pressure differential, a spring is added to move the diaphragm back when the negative pressure returns to approximately atmospheric pressure. A combination of the pressure differential and the spring force is transferred through the diaphragm and fuel pressures are nearly the same as the pressure differential of the intake manifold. Fuel flow is also directly related to this pressure differential.

So, for a given pulse fuel pump, a two-stroke engine will provide a greater pressure differential and correspondingly greater fuel flow and pressure than a four-stroke engine.


The part number for the particular fuel pump that appears to be ideal for our bikes is DF44-211.

There is some excellent information on mounting the fuel pump here.

Here's a list of distributors: http://www.mikuni.com/fs-carburetor.html

I plan to look into this more later....

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Sat Sep 13, 2008 12:17 am View user's profile Send private message
donarntz
Site Admin


Joined: 01 Jul 2007
Posts: 235
Location: Austin, TX

Post Reply with quote
I wish I wasn't lazy, I would like to know how well one would work, it doesn't look hard to do. I'm getting fuel starvation much too often considering how fast my scoot will go.
Wed Sep 17, 2008 10:53 pm View user's profile Send private message AIM Address
rufuskeet



Joined: 10 Aug 2008
Posts: 66
Location: Texas Panhandle

Post Reply with quote
Does any one have problems with gas coming out of the grey vent tube once it is unhooked? Every time I look there are remnants of a small pool, roughly size of a quarter or two.
Tue Sep 30, 2008 1:33 am View user's profile Send private message Send e-mail Visit poster's website
dirkhunt
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Post Reply with quote
This sounds so familiar to what I went through. There seemed to be a flow restriction in my fuel line. Because they (Bajaj) put a fuel line on the scoot that was too short when I pulled the carb to clean et c. it also pulled the fuel line just a little and put a kink in it just as it left the petcock.

The flow from the kinked line was a small stream about 2mm across. With the new fuel line it was a full stream, not a dribble.

I don't think a fuel pump will help if you are running out of gas. Start using the "on" position, when it starts to sputter switch it to the reserve and find a gas station in the next 15 miles or so.

Maybe I misread the original post but it sounded like ... my tank is empty, why won't my scooter stay running....

Reading other posts on this site, are you sure that it is a fuel starvation issue and not overheating from a lean condition. When they tuned you up the said that it was faster, you can get more power from running a lean mix but it will overheat your scoot quicker, when you throttle down it will allow your scooter to cool enough to run, say at 40 instead of 55.

I also noticed that as I started to starve out the RPM at idle would increase and I used that as an indicator that I needed to switch to the reserve part of my tank.

One last thought.

The carb itself has 2 90° bends before it reaches the float valve. Has anyone drilled out the horizontal and resealed it to see if that would increase the flow. Because quite honestly, I have a hard time believing that the 3/16 inch fuel line is the fuel restriction. you can drain a tank in 15 min through that thing... anyone getting less than 25 MPG?

Thinking a little more (always dangerous) the tightest restriction in the entire system is the opening for the float valve that allows fuel to flow into the bowl. Anyone thought about opening that up a couple of hundredths?

Just random thoughts
Tue Sep 30, 2008 5:24 pm
monza



Joined: 26 Mar 2007
Posts: 343
Location: Kansas City

Post Reply with quote
dirkhunt wrote:

I don't think a fuel pump will help if you are running out of gas. Start using the "on" position, when it starts to sputter switch it to the reserve and find a gas station in the next 15 miles or so.



The problem is that the fuel tap combined with the line size cant flow enough fuel to keep the bowl full at full throttle for miles straight. The fuel system is barely adequate when the tank is full, but as the tank goes down flow goes down since there is not as much static head at the tap inlet. This is when it starts to starve out because you just cant flow, its not because its actually out of gas. This is where the pump comes in. It pulls fuel to the carb, and returns what is not needed through the bypass.

Quote:
The part number for the particular fuel pump that appears to be ideal for our bikes is DF44-211.
There is some excellent information on mounting the fuel pump here.
Here's a list of distributors: http://www.mikuni.com/fs-carburetor.html
I plan to look into this more later....


I finally got mine through my local scooter shop who ordered it from Sudco. It cost $30. SPD Sux BTW.

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-Paul O.

75 Vespa Primavera "Parmakit 130", P200E,
Tue Sep 30, 2008 6:22 pm View user's profile Send private message Visit poster's website
ScooterCommuter



Joined: 21 Apr 2007
Posts: 118
Location: Hyattsville, MD

Post Reply with quote
Monza, great description!!! You've saved me hours of tinkering and wondering. My fix will probably to just back off the throttle for a bit and let the bowl fill, then turn it wide open again. I live in DC and my mechanic used to be above Baltimore, so I'd get on the Parkway (295) to 95 and get off the other side of Balt. -- basically, I'd burn a whole tank of gas at wide open throttle. Always toward the end of the ride it started to cut out, and I thought that I was overheating it or something. So comforting to know what the real cause is! How the heck did you figure that one out?

SC

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'02 Chetak (90mpg - poor man's hybrid?)
Tue Sep 30, 2008 6:49 pm View user's profile Send private message
monza



Joined: 26 Mar 2007
Posts: 343
Location: Kansas City

Post Reply with quote
From observing what my Chetak would do, and what Angry_J's Chetak would do. Never cut out when the tank was full, but if you were down to the reserve level it would cut out after a couple miles of WOT. Angry_J's is worse than mine was.

On top of that, everyone I know that seriously tunes Vespas has told me at some point or another that the Vespas have always had a low flow fuel tap, so if doing lots of performance mods you have to run a fuel pump. The Chetak's fuel tap is very similar to the Vespa one.

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-Paul O.

75 Vespa Primavera "Parmakit 130", P200E,
Wed Oct 01, 2008 4:04 pm View user's profile Send private message Visit poster's website
ScooterCommuter



Joined: 21 Apr 2007
Posts: 118
Location: Hyattsville, MD

Post Reply with quote
Your wisdom is much appreciated!

SC

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'02 Chetak (90mpg - poor man's hybrid?)
Wed Oct 01, 2008 7:49 pm View user's profile Send private message
Angry J



Joined: 25 Mar 2007
Posts: 496
Location: Kansas City

Post Reply with quote
He's a misunderstood genius. Cool Too bad he sold his Chetak!

My chetak is horrid at fuel starving. Which sucks ass because it's fast. Nothing blows more than hauling ass with your friends down the road at 62 or so and having to sputter down to 40 to let your bowl fill back up. Or worse - have to pull over altogether - which happens to me all too often. I plan on trying the fuel pump on the Chetak as soon as I pay off this rally I just went to. Hell, $30 and an afternoon of work is less than a bottle of whiskey and the headache on Saturday morning.

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Mon Oct 06, 2008 2:09 am View user's profile Send private message Visit poster's website
donarntz
Site Admin


Joined: 01 Jul 2007
Posts: 235
Location: Austin, TX

Post fuel pump Reply with quote
I'm thinking of trying the fuel pump on mine. Nobody has taken the plunge yet?
Thu Feb 26, 2009 7:14 pm View user's profile Send private message AIM Address
Aggroton



Joined: 21 Nov 2008
Posts: 438
Location: Mechanicsburg PA

Post Reply with quote
Anyone get this done yet?

Its driving me bonkers.
Where would one attach the air hoses to?
I have completely removed my secondary air emissions bull stuff.

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Tue Nov 30, 2010 11:13 pm View user's profile Send private message
rufusswan



Joined: 29 Sep 2007
Posts: 1786
Location: Taneyville, Missouri

Post Reply with quote
If a fuel pump requires access to outside air, you would not need to attach that line to anything, just open air. If the pump would require a vacuum line, then you would attach it to the nipple on the carb intake nipple that you are no longer using.
Wed Dec 01, 2010 1:02 am View user's profile Send private message Send e-mail MSN Messenger
stepthrupanam



Joined: 21 Mar 2011
Posts: 82
Location: Wellington, New Zealand

Post Reply with quote
Did anyone ever get a fuel pump installed? I did a little bit of energy management at university and if I remember correctly the water pressure that determines how quickly a water turbine will spin is based on the "head" of water which I think is something to do with volume and height, and a couple of less important variables. If you have half a tank of gas, you have half as much fuel weight to force the gas out of the tube.
Fri Mar 02, 2012 5:30 pm View user's profile Send private message Send e-mail Yahoo Messenger
Aggroton



Joined: 21 Nov 2008
Posts: 438
Location: Mechanicsburg PA

Post Reply with quote
Hooked up a mechanical fuel pump.
Seems to flow even less fuel then the Vac operated petcock.

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Thats a sweet bike.

I have MotoStrada's left over stock.
If you need anything please let me know.
Email is best
Tue Jul 07, 2015 10:57 pm View user's profile Send private message
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