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poor running after carb rebiuld

  • Thread starter Thread starter greenbraes
  • Start date Start date
G

greenbraes

Guest
Hi Guys
I need any help available, I have a GS1000EN 1979, I have just built a set of carbs from a box of bits, utrasonic cleaned, all holes cleared, needles in good condition. I have a set of bellmouths on them and a 4-1 performance pipe. I fired the engine up and it was only running on choke on the outside cylinders, occational firing in the middle ones. If I take the choke off it doesn't run....unless I put my hand over the air intakes and it fires up OK on all 4 with some throttle..only for a short time. I did put 125 jets in with the circlip on the middle slot on the needle..I know the jets may be to big (VM28's)...any help welcome...not a problem taking them off again but will need guidance on set forward, I have some smaller jets on the way.

cheers

Guys
 
all new and tight, I have just taken a good set of carbs off, these ones were built for a bit of a change

RB
 
And the bell mouths would be velocity stacks? I'm lost after that if that's the case. Never tried to tune with stacks, the airbox works better.
 
yea bellmouths are same as velocity stacks, thanks for reading and your input, all help welcome :) , I can always put the other ones back on, these ones are for a bit of fun and be good to get them working too

Cheers

RB :)
 
This is what I had to do after installing APE pods to get her to run good, before I had her jetted on a dyno:

image.jpg

(Pic is upside down.)
 
that makes sence to what I had to do to get her to fire on all 4. To much air, does that mean the jets are to big or to small????
 
What are you air and fuel screw settings? :-k

With pods, I would suggest 1 full turn or just a bit more on the fuel screws (the ones on the bottoms). With your stacks, maybe 1 1/8 to 1 1/4 turns out. Start with the air screws (the ones on the sides) about double that, or 2 1/2 turns, then adjust them for best running when the bike is warm.

As a 'rule of thumb', I have heard that you add 3 to 5 sizes on the mains for a pipe and another 4 to 6 for pods. Your 125s are 12 sizes bigger, which might be OK for the stacks, but that won't really affect how it runs at idle.

.
 
right, my settings are miles away, I started with 3/4 turn on fuel and 1 1/2 turns on air srews, needles have circlip on middle slot (2 up from bottom). When you talk about jet sizes is that 2 1/2 per size. To be honest I would not know what the original jet size was when bike was stock.
thanks for this, I will try this start point tomorrow :)

RB
 
set the carbs up as mention below and engine would not run, I think I have to take stock and see what I have done wrong in the build. Took them off and put the old ones on (also rebuilt but with 122.5 jets) and engine fired up perfect. I think I will strip them down and see what is a miss. Can anyone help me find the part number for the pilot fuel screw (VM28 GS1000EN 1979), the one under the carb that is a nightmare to adjust in situ. I know it must be on the fiche diagram but I can't find it.

cheers

RB
 
greenbraes said:
right, my settings are miles away, I started with 3/4 turn on fuel and 1 1/2 turns on air srews, needles have circlip on middle slot (2 up from bottom). When you talk about jet sizes is that 2 1/2 per size. To be honest I would not know what the original jet size was when bike was stock.
thanks for this, I will try this start point tomorrow :)

RB
Click to expand...
3/4 turn on the fuel screw is even leaner than the factory setting. :oops:

Yes, Mikuni jet sizes go up by 2.5. Other manufacturers use different numbering systems, they do NOT cross-reference directly.

Stock main jets in your bike were 95.

You mentioned you "set up the carbs as mention[ed] below". That tells me that you still have the default display settings. That makes it VERY hard to read, with newer posts on top. It is much better if you go into the "User CP" section and change your preferences to change your display format to "Linear, Oldest first". While you are in that area, change it to show 40 posts per page, it will cut down on page-flipping when reading longer threads. :encouragement:

EDIT: Well, it used to be called "User CP". Now, you have to click the "Settings" button in the upper-right corner, then scroll down the left side, look for "My Accout" and "General Settings". In the General Settings section, scroll down to the "Thread Display Options" area and do your thing.

.
 
Last edited:
hampshirehog said:
Have you got the vacuum nipple plugged off?
Click to expand...

The vaccum line to the petcock was on, I don't know if I have the right fuel pilot screws, they came off another set of bits I have, the orginal ones were broken, and yes I did clear the broken tips. The only difference I can think of are the jets are 125's unlike the set that work, they have 122.5's. Gojng to have a look later, I must have missed something daft. Cheers for the help

RB
 
ok so your carbs have 3 circuits to them
each one controlling a different throttle range (not to be mistaken with RPM range or speed) the range i am talking about is how far open the throttle slide is

circuit 1
the pilot (idle) circuit it controls from 0 to 1/4 throttle opening
there are 3 adjustments to this circuit

1) the pilot jet this small jet meters the fuel taken from the float bowl into the idle circuit and is marked with a number like 15 or 17.5

2)the pilot fuel screw (the small screw on the bottom of the carb along the front edge of the float bowl (front edge being the edge closest to the cylinder bank)
(this screw is usually 3/4 to 1 turn out from lightly seated) in your case more likely you will need 1 turn out (the more you turn this screw out the richer the fuel mixture will get)

3) the air mixture screw (this screw is on the side of the carb body) this screw adjusts the amount of air that gets added to the idle circuit usually in the region of 1 1/2 to 2/1/2 turns out from lightly seated
(the more you turn this screw out the leaner the mixture gets)

that is the pilot (idle) circuit and its adjustments the pilot (idle) circuit is probably the most difficult circuit to adjust

Circuit 2
the needle jet and jet needle controls from 1/4 to 3/4 throttle opening
this circuit is fairly simple to adjust
the needle jet is the long tube with tiny holes in it that the the main jet screws into it has a letter and number designation ( O-6 )
and the jet needle is the thin needle that comes out of the throttle slide and usually has 5 adjustment slots in it that you can put a clip in the number on it is usually similar to this 5DL31
(the farther away the clip is from the tapered end of the needle jet the leaner the mixture will be)
reminder( the main jet size affects this circuits operation )

Circuit 3

the main jet circuit controls from 3/4 to fully open throttle opening
this circuit is fairly simple to adjust
it is adjusted by changing the main jet it self to a different size ( it is marked with a number like 125 or 122.5
the higher the number the more fuel it will allow into the carb )

now also be aware that each of these circuits overlap to allow for smooth operation so getting the carbs set up right can be a painstaking experience to say the least

i hope this helps explain how the VM carbs circuits and adjustments work and perhaps points you in the right direction that you need to go

going by what you have posted thus far the main jets are more than likely not the cause of your problems
i would venture a guess that you have a problem with the pilot (idle) circuit
 
Last edited:
Brillaint to say the least...and thank you for that, I am going to print it for reference

RB :)
 
you are more than welcome
to add to this once you get it running you need to ride it because it will tell you what you need to do by doing plug chops

a plug chop is riding at a specified throttle range for a period of time and then stopping the engine using the kill switch while pulling in the clutch to allow it to stop
then you take out each plug look at it too see what colour it is on the center electrode your looking for a light tan to dark tan colour then you know the carbs are set up correctly
if the electrode is white then your too lean
if the electrode is black then your too rich

reading the plug

at the tip of the electrode to 1/4 down is the pilot circuit
from 1/4 to 3/4 down is the needle jet circuit
from 3/4 to the bottom is the main jet circuit
 
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