vm carb adjustment - running rich?

[Guest]

Hi.

Just thought I'd get this thread started even though I don't have all the info I want to put up here yet.

GS 850GN

I think my bike is running rich. I think that because the exhaust smells like gasoline, and if I touch the end of the pipe my finger comes away black, so there's some soot there. No black smoke, no obvious performance issues, bucking or flat spots or anything like that.

I haven't looked at the plugs yet. I'll do that tonight. I have an idea of what to look for from the manual, but i don't really know because I've just never looked at plugs much before.

The air filter is less than a year old. The airbox has been sealed (weather stripping tape).

I did a valve adjustment maybe 3 months ago, did a carb sync last week. On these old VM carbs though, since there's no master carb to sync to, could I have messed up my ratio by doing the sync? Meaning, great, they're in sync now, but could I have synced them all high? Or am I totally making this up, and the screws I turned don't affect the mixture, just balancing one to another?

I've never done anything to move the needles or change jetting. I don't know for sure but would be very surprised if there's anything but stock on there right now.

Just curious what the right approach here would be in terms of tuning if I look at the plugs and they are dark - how should I go about changing the mixture?

Let me know if there's other stuff I should be looking at other than the plugs to help me diagnose, and whether there's more information I can provide here that will assist people in providing advice.

Thanks for reading!

Aaron

[shwaz]

pasting this here for my reference




GS850 (77-79) the specs are for the VM's

idle r/min........... 950-1150
carb................... mik vm26ss
id no................... 45060
bore size ............ 26 (1.0)
float height ......... 23-25. (0.91" - 0.98")
fuel level ............ 3.0 - 5.0 (0.12" +/-0.20")
pilot air screw ..... 1.25 turns out
pilot screw........... 5/8 turns out
pilot air jet........... 1.2
pilot jet............... #15
cut away............. 1.5
jet needle........... 5DL36-2 (the -2 2nd notch)
needle jet .......... 0 - 6
pilot outlet........... 0.6
by pass ............. 0.8
main jet............. #102.5

[MisterCinders]

Synching the carbs won't affect the mixture. If you are running rich, there are a few things you can adjust to tune the mixture.

First, when do you smell the gas, etc.? Idle? Mid throttle? Full throttle?

Since you're touching pipes, etc., my guess is that it's either rich overall, or in the pilot circuit, at idle after the ride or when you start it up, etc. No matter, you tune VMs from pilot up, so you can start with that.

Have you cleaned the carbs recently? Ever? Adjusting the jets for dirty carbs is foolish, so if the carbs haven't been stripped and dipped, you'll want to start there. If you synched the carbs because you just cleaned them, check your pilot fuel and air screws.

Start with the spec settings, then use the highest idle method to tune the air screws. BTW - I totally suck at the highest idle thing, since I can never seem to hear idle changes. YMMV. Even if you cannot hear the idle change, but the carbs are clean, you might try backing the air screws out slightly to see if that takes away some richness.

[shwaz]

Synching the carbs won't affect the mixture. If you are running rich, there are a few things you can adjust to tune the mixture.

Thanks! good start there, i think i figured that out from some other threads, it just synchs the vaccuum across all carbs

First, when do you smell the gas, etc.? Idle? Mid throttle? Full throttle?

definitely at idle. i haven't tried it in any other state yet, it's hard to stick my nose in the exhaust while i'm riding the bike but the smell isn't really strong, i don't notice it ever while i'm sitting on the bike, just when i was looking at the rear tire while it was on the center stand idling.

Have you cleaned the carbs recently? Ever? Adjusting the jets for dirty carbs is foolish, so if the carbs haven't been stripped and dipped, you'll want to start there. If you synched the carbs because you just cleaned them, check your pilot fuel and air screws.

I never did.
*edit - just called the shop i brought it to and they never cleaned the carbs either.
PO stated that they were cleaned prior to sale. I have no documentation.

Start with the spec settings, then use the highest idle method to tune the air screws.

I'm not familiar with that method, but I'll start off by making sure the pilot and air screws are in spec.

thanks for the helpful post

aaron

[rustybronco]

I haven't looked at the plugs yet. I'll do that tonight. I have an idea of what to look for from the manual, but i don't really know because I've just never looked at plugs much before.


Just curious what the right approach here would be in terms of tuning if I look at the plugs and they are dark - how should I go about changing the mixture?

Learn to read your plugs.

http://www.dragstuff.com/techarticles/reading-spark-plugs.html

http://www.3cyl.com/mraxl/manuals/carbtune/carbtune.htm#home

Plug Chops

Plug Chops are a good way to test the jetting accuracy of your carb setup. Simply pulling a plug and looking at the color after running through a variety of throttle positions can be misleading and possibly lead to disaster if jetting is too lean.

The procedure outlined below is intended to test main jet sizing at WOT. It is strongly suggested to begin with overly rich jetting and work your way down in two size increments to avoid seizure. The same procedure can be used at marked mid throttle position to test needle jet position. The distance covered should be at least 1/2 mi, the farther, the better. It may be best to perform the test in a lower gear to avoid excessive speed. Throttle position is the critical item. Variation of throttle position should be avoided and can taint results.

It is possible to examine plugs by using a good light and magnifying glass rather than cutting the threads away. A single cylinder can be used to hone in on the right jetting to save the expense of new plug consumption, assuming all carbs are jetted and setup the same.

Be aware that changes in fuel type/grade and timing can change results.



1. Get new spark plug(s), but don't put them in yet.
2. Warm up motor to operating temp on old plugs.
3. Install new plugs.
4. Accelerate through all gears to top gear with throttle pinned.
5. When you hit the top of the top gear, hit the kill switch and
pull in the clutch.
6. Remove the spark plug(s) and ride home on the old plug(s).
7. Hold the new plug in a vise and use a hacksaw to cut the threads
away from the center electrode.
8. You should see a brown 2mm "smoke ring" at the base of the
white center electrode if the main jets are dialed in perfect.
9. if the smoke ring is darker than chocolate brown or taller
than 2mm, you're rich on the mains, but if it revs clean to full
throttle, you can leave it there and be on the rich side.
10. If the smoke ring is smaller than 2mm or lighter than chocolate
brown, you're lean on the mains, go up a size and try again.


Chopped plug Illustration:

Below is a series of plug chops ranging from extremely rich to slightly lean.

http://www.dragstuff.com/techarticles/reading-spark-plugs.html

[sammy123]

Hi rustybronco. That carb tuning link was very informative. Thank you.

[rustybronco]

And you are welcome!

[shwaz]

Thank you.

Been reading a ton all day. Seems like although it will be helpful to try reading the plugs, if they are dark I should check my timing before trying to adjust the mixture. Concur?


this was a nice little article http://www.strappe.com/plugs.html


*edit - just called the shop i brought it to and they never cleaned the carbs either.
PO stated that they were cleaned prior to sale. I have no documentation to that effect, though.

[Big T]

Thank you.

Been reading a ton all day. Seems like although it will be helpful to try reading the plugs, if they are dark I should check my timing before trying to adjust the mixture. Concur?


this was a nice little article http://www.strappe.com/plugs.html


*edit - just called the shop i brought it to and they never cleaned the carbs either.
PO stated that they were cleaned prior to sale. I have no documentation to that effect, though.

Well, the degree carbs get cleaned varies by the parameters involved in what one thinks a "cleaning" is. Read the VM carb tutorial, order up some O rings and go to town on them. Then you'll know they're clean

And yes, setting your timing is essential before tuning your carbs. As is setting the valves

Also, I'm working under the assumption that you have stock airbox and exhaust. Is that correct?

Also, confirm that your petcock is not leaking gas into the oil. Check your oil level and smell the oil and see if it smells gassy

[shwaz]

Well, the degree carbs get cleaned varies by the parameters involved in what one thinks a "cleaning" is. Read the VM carb tutorial, order up some O rings and go to town on them. Then you'll know they're clean

And yes, setting your timing is essential before tuning your carbs. As is setting the valves

Also, I'm working under the assumption that you have stock airbox and exhaust. Is that correct?

Also, confirm that your petcock is not leaking gas into the oil. Check your oil level and smell the oil and see if it smells gassy

Correct on airbox. Put weather stripping around the edges seal it.
Valves are set.

I'll check the oil as best I can. Anything more scientific than just opening the filler cap and smelling?

I found a link to a carb tutorial on Nessism list of top ten n noob mistakes, but it's for cv carbs. Know of one for vm?

[Big T]

Opening the cap and smelling will tell you what you need

The VM carb tutorial is found both on BassCliffs site and In The Garage here (on the main oage)

[shwaz]

Thanks all. Found the carb guides.

putting everything on hold till i can do points and timing.

appreciate all the info! this has been really informative and i'll look forward to getting into messing with the carbs and setting the mixture once timing is confirmed good.

aaron

[shwaz]

from http://www.thegsresources.com/garage...rb_rebuild.pdf

"37. Using a manometer, sync the carbs.
a. Remove the fuel tank and the tops of the carbs
b. Use the idle adjuster to set the engine speed to 2,500 to 3,500 rpm. Be sure to use
several fans to keep good airflow going straight onto the engine from the front. The
engine will overheat easily at that speed during the time it takes to do the sync.
c. All four carbs should pull the same vacuum. If your manual sync was done carefully,
they should be close.
d. If they are different, loosen the locknut at the top of the slide of the carb with the
highest vacuum and adjust the screw to match the lowest vacuum. Repeat until they
are even.
e. Replace the carb tops and fuel tank. "



When i recently did my vacuum synch, i did the opposite of this and synched all the carbs to the highest vacuum. How bad/important is it to synch to low?

[rustybronco]

Not important.

The only thing that is important, is that they all are equal.

[MisterCinders]

from http://www.thegsresources.com/garage...rb_rebuild.pdf

"37. Using a manometer, sync the carbs.
a. Remove the fuel tank and the tops of the carbs
b. Use the idle adjuster to set the engine speed to 2,500 to 3,500 rpm. Be sure to use
several fans to keep good airflow going straight onto the engine from the front. The
engine will overheat easily at that speed during the time it takes to do the sync.
c. All four carbs should pull the same vacuum. If your manual sync was done carefully,
they should be close.
d. If they are different, loosen the locknut at the top of the slide of the carb with the
highest vacuum and adjust the screw to match the lowest vacuum. Repeat until they
are even.
e. Replace the carb tops and fuel tank. "



When i recently did my vacuum synch, i did the opposite of this and synched all the carbs to the highest vacuum. How bad/important is it to synch to low?

At the bench synch stage, I usually check the adjustment screws to see which ones have room to move and in what direction. Then I use that to to pick a "master" slide for the bench synch. So far, when I go to the vacuum synch, they slides are pretty close and only one or two of them need any adjustment.