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Wayne S.
Wayne S., Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 1255
Experience:  Experienced and Licensed Motorcycle Mechanic - all sizes and brands welcome.
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Suzuki motorcycle GS850G: Motorcycle: 1980 GS850G.I'm experienci

Customer Question

Motorcycle: 1980 GS850G.
I'm experiencing a significant loss of power that seems to be getting worse, and I have fuel pooling in the air box when the engine runs. Carbs have been rebuilt and the float height settings double checked. Also replaced all four spark plug resistor caps, which didn't help.
I would like to find a motorcycle specialist who is sharp on the vacuum-operated carb and airbox setup.
Thanks.
-Nate
Submitted: 1 year ago.
Category: Motorcycle
Expert:  Wayne S. replied 1 year ago.

Welcome,


Carbs only leak fuel when there is a problem
with the float, needle valve under the float
or plugged float bowl vents or overflow tube.
What likely occurred is that you have some bad
stuff coming down the fuel lines from the tank
or a faulty petcock or bad fuel hose.
Without a good fuel filter inline the float valves
will immediately start to stick and leak fuel.
The float valves have to be in perfect condition
and the seat has to be very clean as well.
Some needle valves have orings on them
which can also leak fuel into the bowl making
it overfull.
If any fuel runs from the carbs when the bike
is not running then the float needles are leaking.
If you have a vacuum petcock it could be faulty
and may be allowing raw fuel to be pulled into the engine
through the vacuum hose.
Even if they were just cleaned the carbs have to have
an external fuel filter or they will cause problems
immediately and have to be cleaned again, at least
the float valves.
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Customer: replied 1 year ago.
Thank you for your fast response.The carbs on the bike came from a 1981 GS850GL. Float height settings were double checked. I also did a hot water test on all four floats, but it wasn’t obvious if anything was wrong. Two of the floats did emit more bubbles than the other two, so I replaced them using floats salvaged from the original carbs. I didn’t notice any difference after that.All four needle valves were replaced with new ones in the carb rebuild. I checked all four of them while I was doing the float checks, and all appeared to still be in perfect condition. All four o-rings for the needle valve housings were also new, and they were still in place and intact. No debris of any kind was found that could have kept the needle valves from seating. No debris was found in the bowls; they were still very clean. Also, I’ve never had a problem with fuel leakage when the bike is not running.The tank was cleaned and sealed using a Caswell epoxy kit.The petcock was rebuilt. It does have a pretty fine mesh screen which should serve as an inline filter.So I still have these items to check:• Overflow tube plugged?
• Float bowl vents plugged?
• Shine flashlight in the tank. Any debris?
• Check petcock screen.
• Petcock vacuum valve.What is the best way to check the petcock vacuum mechanism, to make sure it’s not pulling in fuel?So I know I’m looking in the right place, where are the float bowl vents and overflow tube?Also, do you know if the jetting would have been different on the 1981 GS850GL that my carbs came from? Last Sunday I ran the bike at idle with the air box removed. When I placed my hand over any of the four carb inlets, fuel burbled out the inlet into my hand. That doesn’t seem normal to me. What are your thoughts on that?-Nate
Expert:  Wayne S. replied 1 year ago.
Used a #115 main jet on both years.
If the petcock vacuum diaphragm is leaking
you will have fuel in the vacuum line.
Also the petcock may not work very well.
The float levels could be a little high
depending on how they were set.
They should be measured without depressing the
spring loaded end on the float needles if it has those.
Fuel height in the bowl is much more accurate
than float level. 5mm below top of the bowl is
standard.
Covering the carbs air intake is like choking
the air supply so the engines vacuum draws in more
fuel from the main jet area. This is why fuel
comes up and out the carb inlet when you do that.
If any carbs drip fuel when stopped then the
float needles or orings are not sealing or
floats are not floating or you have some other
problem. There is often a brass tube
in the bottom of the float bowls. This is the overflow
when the carb is too full of fuel.
If these develop cracks down lower the carbs can leak.
Big no-no is depending on the petcock screen
to protect your carbs from debris.
They need a fine paper or gauze type inline filter
available at auto or cycle stores.
This will stop alot of problems with leaky carbs.
If your air box fills with fuel the fuel is likely
coming from the carbs being overfull if the
needles are all sealing solid.
I have seen aftermarket needles leak and also
floats hitting on the bowl gaskets when made
poorly.
Replace the float needle valve seat orings as that is
a major source of problems. They might look
good but any shrinkage will ruin them.
Fuel height in the bowl can sometimes be checked from outside the carbs
using a hose on the float bowl drain.
Get this manual:
http://www.mtsac.edu/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf
Let me know if the carbs leak when engine is off or just need
adjustments. Check that all cylinders (pipes) are running the same heat.
Air box is necessary for it to run well and must be sealed
as new were.
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Customer: replied 1 year ago.
Here are responses to the items you discussed, and some additional information:• The petcock does seem to be working fine. The bike actually ran very well just a few weeks ago.
• The float heights were set correctly, without pressing down the needle springs.
• No fuel dripping with the engine not running.
• #2 and #3 are not hot like #1 and #4 when idling. The 2-3 ignition coil checks OK at 11,976 ohms, and all 4 5000-ohm resistor caps have been replaced with NGK's.
• The 4 inlet boots going into the air box are kind of stiff. I think they’ve gotten worse from being in contact with fuel. One or two of these fit loosely in the air box.
• I believe the sealing inside my air box has been compromised by contact with fuel.Thanks for the link to the manual. I will take a look at it and check the remaining items.
Expert:  Wayne S. replied 1 year ago.
Is it possible to swap the coils to see if the
cylinders 2 and 3 run better then and check for
air leaks around the rubber carb inlets?
Cylinder 2 and 3 sound like they are not pulling
their weight so to speak.
Also the carb butterflies have to be synchromized
to open at the same time as outlined in the carb
manual I believe. Carbs 1 and 4 may be opening
sooner than 2 and 3.
Hard or leaking rubber carb boots can cause lean fuel
mixtures which may not burn effectively.
Good carb boots between carbs and engine are a must.
The air box boots may have less effect but still some.
The engine compression should be close to even as possible
and valve clearance could be an issue in weak cylinders.
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Customer: replied 1 year ago.
Unfortunately, the connectors for the primary coils for 2-3 are different from 1-4, so it would be possible, I think to swap them, but it will take some trickery.These carbs have been bench synchronized only so far, using a piece of wire to make the openings even when on the idle stop. I'm saving the full running synchronization until I get this fuel and air box issue sorted out.Earlier today, I started going down my list of things to check, and with the air box removed, I discovered that I had fuel running out of #3 and #4 carbs as soon as I switched the petcock to PRI. I was just trying to perform a fuel level check on #1 when this happened. I did this check before with the airbox still on, and I didn't see fuel leaking from any of the carbs. I can't explain that yet. Anyway, I ended up pulling the gang of carbs and separating #4. I removed the bowl and still didn't see anything abnormal. I then performed a fuel level check on #4 with it in a soft-jawed vice. I think the check came out normal, right at .203" (13/64"). My manual says they should be .18" to .22", so mine is pretty much dead nuts. I'm doing this with the bowl and gasket seated, and I'm measuring to the bottom of the main housing, where the main housing meets the bowl. The main housing overlaps the top of the bowl by about 3/64", so if I measure to the actual top surface of the bowl, I get .25". Which method is correct? My Suzuki factory manual also says to do this check on the bike with it idling, but the carb rebuild manual you linked me to shows a single carb in a vice, like the way I'm doing it. ShoulWith the way the fuel poured out of #3 and #4 with the carbs still on the bike, I expected to see something like that
Customer: replied 1 year ago.
My posting got messed up so I re-did it here:Unfortunately, the connectors for the primary coils for 2-3 are different from 1-4, so it would be possible, I think to swap them, but it will take some trickery.These carbs have been bench synchronized only so far, using a piece of wire to make the openings even when on the idle stop. I'm saving the full running synchronization until I get this fuel and air box issue sorted out.Earlier today, I started going down my list of things to check, and with the air box removed, I discovered that I had fuel running out of #3 and #4 carbs as soon as I switched the petcock to PRI. I was just trying to perform a fuel level check on #1 when this happened. I did this check before with the airbox still on, and I didn't see fuel leaking from any of the carbs. I can't explain that yet. Anyway, I ended up pulling the gang of carbs and separating #4. I removed the bowl and still didn't see anything abnormal. I then performed a fuel level check on #4 with it in a soft-jawed vice. I think the check came out normal, right at .203" (13/64"). My manual says they should be .18" to .22", so mine is pretty much dead nuts. I'm doing this with the bowl and gasket seated, and I'm measuring to the bottom of the main housing, where the main housing meets the bowl. The main housing overlaps the top of the bowl by about 3/64", so if I measure to the actual top surface of the bowl, I get .25". Which method is correct? My Suzuki factory manual also says to do this check on the bike with it idling, but the carb rebuild manual you linked me to shows a single carb in a vice, like the way I'm doing it. Should running or not running make a difference in the level check?With the way the fuel poured out of #3 and #4 with the carbs still on the bike, I expected to see something like that in my off-bike level check. But it just came out normal. My theory on that is that when it’s off the bike, I don’t have the higher fuel pressure exerted on the needle valve that I do when it’s being fed by the nearly full tank on the bike. So it must be able to unseat the needle when on the bike, but not when in the vice. What are your thoughts on that? Should I try adjusting the float tab to the extreme low-level (lean) end of the spectrum? Or maybe even a bit lower than that?
Expert:  Wayne S. replied 1 year ago.
If the needles leak then that is the problem.
Even with a full tank of gas or even a fuel
pump the needles should never leak.
If they leak even occasionally that is not
acceptable. You might have some bad aftermarket needles.
The originals are much better but are hard to find.
The fuel height should be just a little under
a full bowl and carbs will not flood unless the
float needle is not sealing properly.
You could try seating the needles with some fine
grinding compound. Just like resurfacing an engine valve.
Spin the needle until it seats evenly in the valve seat.
The leaks will prevent you from ever getting it tuned right.
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