Another Carb quandry ...

I have gone over this set of carbs top to bottom. Cleaned and polished every orifice that I could get something into, down to probing with the e-string from a light guage electric guitar set - not much bigger than a hair. The B-string worked well at cleaning the enrichment tube that goes into the well.

I then blew air through everything to make sure it was all clean and dry - compressor set at 90 psi. Bores for the diaphragm part polished to mirror bright, plunger itself wet sanded with 1500 grit. They move quite nicely.

Carefully measured float levels, adjusted and reset - all +/- 1mm of 2mm.

Bench synch, mount, add gas, start her up. Revs scream up to 5K.
Turn it off, verify free movement of throttle cable, minor adjustment of throttle cable to have the throttle totally at rest with off throttle, also adjusted idle screw to make certain it wasn't open too far.

Turned it back on, revved to 2K, then after about 10 sec, leaped to 4K.

Diaphragms were all checked and tested.

The only thing that makes any sense to me might be if the floats are off, but again, I measured pretty carefully and used a level to verify that carbs were level and used calipers to measure fuel level. All were at the 2.x level, except one that got to 3.2mm but I wasn't going to open them up again for .2 mm ...

What am I missing or where do you think I slipped up despite my best efforts?

 

Mike

I might be barking up the wrong tree here but what did you use to hold the gap during the bench synch - when I use even business cards, I *always* find she will scream when first started and I have to be ready and quick to bring the revs down with the idle adjustment screw.

It's sounds like you have done a good job of the cleaning and I would doubt your float heights are the culprit now (seeing as you have them set so precisely).

 

I did back off the idle screw, that was when it dropped to 2K but then screamed up to 4K. I guess I forgot to say that.

For the "business card" I used a strip of a slick mailer card that Sprint so kindly provided. About the same thickness with a polished coating on both sides.

 

It "screams" of a vacuum leak.Check yer boots with a torch.Unlit would be a good idea.Sounds like you have covered yer bases with the carbs.

 

Vacuum leak is a good possibility. I didn't realize that would cause it to rev up, I thought that would just cause it to run lean. Thanks.

 

Okay, I checked for vacuum leaking, none at this point. I held the propan torch at a downward angle so that I could actually see the With idle screw backed off to nothing, it revs at a bit over 3500 rpm.

I did find that one of the washers on the end of the throttle shaft had broken off, so I replaced that with another washer to no effect.

I am at a loss - any suggestions appreciated and needed.

 

Aer you sure the throttle cable loose enough to let the carbs close all the way? Have you checked the fuel enrichment circuit to make sure it closes off all the way, I use a small piece of tubing placed over the pickup to see if I can blow any air through it. I have had these give me trouble before, it is one of the first things I check when I am having carb problems.

 

I have checked the throttle cable and I have checked the enrichment circuit as far as movement. What do you mean regarding the "pickup"?

 

Did you check the float levels with the clear tube?

 

yep. Used clear tubes and windshield washer fluid.
Looks like I have to pull them again.

 

I've had this happen on a couple of Bikes.

IF ... the Vacuum Sync is OK, you're leaking AIR in.
You might not find the leak with Propane.

Sooner of later we are ALL going to have problems like this.
Here's what I think you should try.
This is what I do.

I am NOT going to put myself in a position where I have to "UN-Fugg" something.
So, I am NOT going to touch the Manifold Mounting Cap Screws.
Sorry.
That's for somebody else to do. The RISK is too great!
So I RE-SEAL the Manifold to the Head by: "Smooching-in" High-Temp RTV Sealant --> IN ... and, --> COMPLETELY AROUND ... the Seam where the Manifold mates to the Head.

You can make time on the middle ones.
Can't see in there, anyway.

Mask-off the Outboard ones.
Clean the Head with Carb Cleaner and a Toothbrush
Wipe it down with an Athletic Sock soaked in Lighter Fluid.
Allow to dry ... DRY.

Scissors and Gorilla Tape and Masking Tape.
Mask-off the Seam and Head.
Go nuts covering the Head.
Run Tape following a "Pleasing to the Eye" line on the Engine at the Intake Port.
Once you get the situation to where you can Lay-on the RTV and not make a mess of things ... SEAL that Outside Manifold.

Let the stuff get to the Point where you can UNMASK the Head and leave a NEAT LOOKING LINE of Sealant when you peel-off the Tape.

Seal ALL-4 Manifolds with RTV.

(Alternatively, you can remove the Carbs and remove the Manifolds, replacing Gaskets and O-rings.
FAIR WARNING
IMMINENT DANGER
The Manifold Fasteners are as SOFT as a Stick of Butter left-out on the dinner table overnight.
IF THEY ARE SEIZED YOU WILL BREAK THEM OFF RUINING SUMMER
IF THEY ARE SEIZED YOU WILL BREAK THEM OFF RUINING SUMMER!)

 

at least give a running sync a try, it don't take much to rev like that with no load
maybe the revs jumped like that because another cylinder kicked in

 

Just a thought.
Others here have had problems with reving do to the fact that the idle screw can not be turned in anymore for adjustment.
This occured when putting the carbs back together the screw was already to far in not allowing enough adjustment later.
Damn I had a hard time explaining that hope it made sense.

 

Thanks folks.

Hound, I have the idle screw backed all the way out at this point. Having done a bench synch first, the idle screw wasn't too far in in the first place.

Polluck, with it running that high, I was kind of afraid to leave it running for any length of time, given the heat, even with a box fan blowing on the engine.

Rick, I'll take a look and see what I can find. Why would it pull that much air at that point of the bike and not be affected by the propane?