odd colortune carb issue

I was colortuning my bike tonight and cylinders 1,3,4 all tuned great. With the colortune plug in cylinder 2, I just get spark on idle with no ignition. However, if I give it throttle or choke I get ignition. Now, in my mind this appears to be a fuel delivery problem. I checked fuel levels in the bowls and they were good. So I pulled the carbs and checked for any blockage in the #40 jet and the little mesh dome. None were seen but I blew them out with carb cleaner and ran a wire through them again.

I reinstalled the carbs but it got too late to fire it up again. Is there any other issue that might cause this?

FYI, valve clearances, bowl levels, and carb sync have all been checked.

 

If you haven't synced it you should. If you've got one butterfly (#2 in this case) completely closed it'll mess with you.

Otherwise you've either got something plugged up or an air leak.

 

Next time you have the carbs apart, lose the little mesh domes.

Beyond that, I agree with Carl. Did you vacuum sync or only bench sync?

 

The Passage that connects the Pilot FUEL Jet with the Pilot AIR Jet and SUPPLIES the Pilot MIXTURE Screw with AIR & FUEL Flow could be clogged.

TEST ~ FLUSH:

Use a Rubber Ear Syringe.


Place the Syringe's end into the Pilot AIR Jet after you have Squeezed the Air out from the Syringe. Release.
>>See if FUEL is being brought UP the Passage from the Pilot FUEL Jet.

>>Remove the Pilot MIXTURE Screw, Spring, Washer & O-Ring.
Squeeze the Air Out from the Syringe and BLOCK the Pilot AIR Jet with a Oil Soaked Q-Tip.
>>See if FUEL is making the trip to the Pilot Mix Screw Cavity.

++FILL the Syringe with Carb Cleaner.
(Shoot some in a glass or Tuna Can. Fill the Syringe.)
>>FLUSH the Pilot MIX Cavity BLOCKING the AIR Jet down through the Pilot FUEL Jet.

When you are satisfied that that Passage is clear, ... Install the Pilot MIX Screw.
Be 100% sure the O-ring SEATS Flat, ... below the tiny Flat Washer.

Bottom the Pilot MIX Screw.
Turn the Screw OUT 2-3/4 Turns.
Colortune.

If the Color Fails to change, ...

TEST:
>>Remove Manifold Vacuum Port Plug.
Insert Syringe containing small amount of Carb Cleaner or Fuel into Vacuum Port.
>>Run Engine. Look for Color as Syringe Fuel Supply is used.
Color? Y/N
Yes: Pilot FUEL Jet likely clogged or damaged.
No: Check Compression in Cylinder not firing.

 

Carl and fitz. I did both a bench sync then vacuum sync.

 

Which method for vac sync?

Or maybe Rick has something for you (I'm not Hitachified yet) with the above clogged passage theory.

 

AHA ! Thanks Rick !
Now I know how to attack (or verify) cleaning the Venture carbs Pilot circuit.

 

I used the carb tuning gauges that attach to the vacuum port for the vacuum sync.

I will need to check for any pilot passage obstruction.

Rick - I'm guessing this should be done with the carbs on the bike and the bowls filled?

 

Yep.

While you are Tuning.
But, only if the hole has Spark but isn't firing.

I've seen some guys take a Short Cut and just Pull the Pilot Mixture Screw and dribble some Fuel right into the hole to see if that makes the Cylinder fire.

If you have Spark and Gas dribbling-in from the Pilot Mix Orifice and the hole don't fire, ...
Ship the flushing and go right to a Compression Test.

 

Just out of curiosity, if I have to do a compression test, what kind of numbers would cause this behavior?

 

Not firing at idle - - about 90 , give or take.

 

You can do a Pass/Fail Test.

Hold your finger on the Spark Plug Hole and crank the Engine.
There ought to be enough Pressure to push your finger off the hole.

If you crank it and are able to hold your finger on the Spark Plug Hole; that's a pretty sure sigh that you got troubles to deal with before getting combustion back in the dead hole.

 

Ok so it was a weird afternoon...here are my findings:

1. Compression - I knew it was low across all cylinders - but cylinder 2 tested at 125psi

2. I cleaned every pilot passage in the carbs again.

3. squirting starter fluid in the vacuum port made cylinder 2 ignite.

4. I backed the pilot screw completely out until it was unseated and the cylinder started firing on idle. It was a yellowish flame but it quickly turned blue with a few turns. I backed the pilot screw out and back in a few times and the cylinder stayed ignited.

5. I put the standard plug back in and she idles nice and smooth now. I went for about an hour ride (thank you spring like temps!) then tested with colortune again and cylinder two was still firing a nice blue flame.

So my question is, what do you guys think happened here? Was something stuck in the pilot screw? I did clean that out completely again and made sure the oring, washer, and spring were seated correctly. I'm going to continue to monitor it over the next week since temps should be warm enough to ride to work!

 

Fuel began to flow within the Passage.
Once it started moving, ... it cleared-out a spot that was clogged.

Not everybody is so lucky.

Be careful that the Pilot Mix Screw isn't so loose it will fall out.

 

Makes sense Rick. I'm still going to keep an eye on it but thanks for the help!

 

I know the horse is out of the barn, ... and you have the Pilot Screw turned-out a good bit.

Did you look to see if you had the >> right << Pilot Jet (Size) in that Carb.

A Jet of the wrong size will have a dramatic effect on the Air-Fuel Mix of the Pilot Circuit.

The size is Etched into each Jet.
>>Hitachi: Right on the Face of the Collar ::: Both Main & Pilot
>>Mikuni: Face of Collar / Main :::: Barrel of Jet / Pilot

 

yep its the correct pilot air jet - #195 and its in the correct hole.

 

Look for something that might make the Pilot Mix Metering off.

Placement of the O-ring below the Flat washer.

 

It should be from bottom up o-ring --> washer -->spring-->pilot screw.

Right?

 

Exactly.