Major carb lean problem

Advertise with us
Shop deals on ebay
shop deals on amazon
VMAX Forum

Help Support VMAX Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
J

jcox04105q

Member
Joined
May 26, 2015
Messages
5
Reaction score
1
Location
westchester
I have a 2003 vmax with 9,000 miles (original owner). Engine developed major off throttle miss and I am stumped. Engine will idle decently, but opening throttle results in major stumble. If I apply choke, acceleration is better, but not good. If I spray ether into carbs it revs right up. This tells me problem is lean mixture. The "wild card" is when I spray ether below carbs; it will rev up. Bike is unrideable.:bang head:
I have done the following:

  • taken apart carbs, everything looks great, synched carbs, removed plugs & adjusted idle mixturenew spark plugs, fuel pump, fuel filter
  • compression check, all cylinders about 120 psi
  • intake manifolds; replaced o rings & lower boots, upper boots look good, plugged & filled manifolds with fluid, no leaks
  • the carb slides (c.v. pistons) are working
 
A Rev after spraying under the carbs usually means cracks in the boots somewhere, but it could be something different.
 
The pilot jets (the air fuel screws) control part throttle from 0-4000 rpm. The pilot jets on the Vmax ate 37.5's.....so very tiny....and easily clogged. Sounds like its time for a "Shotgun", a very easy procedure that will flush out the pilot jets. This procedure works probably 90% of the time, the other 10%....you might have to do it a couple of times.....or manually clean out the pilot jets.

http://vmax.lvlhead.com/tips/shotgun.htm
 
If I spray ether into carbs it revs right up. This tells me problem is lean mixture.
Click to expand...
Trying to understand fully what you meant by this statement.
Sounds like you're spraying ether directly into the carbs, which can only be done if your airbox is off, or you have modified air intake stacks, and filters.
Are you running stock configuration as to the airbox, and shooting ether into each carb?
If so, it will rev up, which is expected.
What the other guys are commenting on is this.
With the air box in place, everything mounted and tightened down, and your spray ether, or some other combustible gas, around each carb, to see if the engine's RPM changes. If it does change, then you'd have an air leak around 1 or more carbs.
Make sure you're using a small hose that you can direct the ether directly where your want it. if it gets sucked in from another opening it will basically give you a false reading.

Oh BTW.. I finally got my 91 running smooth by running 2 tanks of seafoam. Used 1/2 bottle for each 4 gallon fill up.

Hope this helps.
 
D-Max2012 said:
Trying to understand fully what you meant by this statement.
Sounds like you're spraying ether directly into the carbs, which can only be done if your airbox is off, or you have modified air intake stacks, and filters.
Are you running stock configuration as to the airbox, and shooting ether into each carb?
If so, it will rev up, which is expected.
What the other guys are commenting on is this.
With the air box in place, everything mounted and tightened down, and your spray ether, or some other combustible gas, around each carb, to see if the engine's RPM changes. If it does change, then you'd have an air leak around 1 or more carbs.
Make sure you're using a small hose that you can direct the ether directly where your want it. if it gets sucked in from another opening it will basically give you a false reading.

Oh BTW.. I finally got my 91 running smooth by running 2 tanks of seafoam. Used 1/2 bottle for each 4 gallon fill up.

Hope this helps.
Click to expand...

I would say that he did it with the airbox lid off.....how else would he know that the slides are working?

Its also apparent that he has some mechanical knowledge just in what he posted.....and he NEVER said that he sprayed the ether at the boots. I still say the pilot jets.....with my A/F hooked up....and atleast 2 clogged pilot jets.....I was getting A/F readings of around 18 while trying to go down the road. If you look at it this way....if the pilot jets are clogged (and lets just say the number is 2) you are getting the same amount of air.....but half the fuel.
 
Thanks for the input.

Yes the airbox is off, as I have removed and replaced carbs and manifold several times. I will go back and look for leaks and or check pilot jet circuits.
 
When checking for leaks below carbs (boots) I like to use a unlit propane bottle ..
Click to expand...
I did this method, along with all the other stuff the OP mentioned, while trying to diagnose my stumble.
 
Checked for leakage under carbs by spraying ether; motor definitely sped up when spraying carb to manifold boot. I retightened clamps which helped but still a major off throttle stumble. It does idle pretty good.

I guess the boots are leaking. They visually look good with no cracks or other obvious problems and the clamps are very tight. Time for new ones.

The bottom boots were just replaced (manifold to v-boost butterfly), no apparent leakage.

Good tip on the propane!
 
Re: Major carb lean problem. July update

I replaced carburetor boots (previously replaced manifold boots), and now have no apparent vacuum leaks. Spraying either under carbs onto boots makes no difference, previously engine speed would increase

Bike starts up ok and idles ok, but still does not want to accelerate when opening throttle. If I spray either in carbs (filter and intake rams are off, spray directed into carb inlet) it will rev up ok.

As stated previously, I have disassembled carbs where everything looks clean, float level ok, no clogged jets. Replaced fuel pump and filter. Plugs are new and with either, engine sounds good when reved up, which tells me ignition system is functioning ok.

When running, CV slides are all moving and I can see fuel entering through main jets.

I am open to suggestions
 
You must log in or register to reply here.

Similar threads

Spyglass
Carb Tuning
Replies
5
Views
697
Zeus36
Zeus36
MrMAX1985
Carb diaphragm repair
Replies
0
Views
366
MrMAX1985
MrMAX1985
Corrugated
2004 fuel / carb / running problems
Replies
3
Views
690
VeeAte
VeeAte
Corrugated
2004 out of storage fuel issue..
Replies
5
Views
781
Corrugated
Corrugated
B
engine cutting out and now it died on hwy???
Replies
18
Views
1K
brian hicks
B

Latest posts

Back
Top