My carbs are making me angry

Ok, I've been fighting a little annoying bug with the 750 carbs...

The bike runs great in all power bands. Idle is great. It starts very easy even on cold days with about 50-75% choke, and runs without choke once its warmed up.

The carbs are clean enough to eat off (on the inside, at least). Floats are set to 17.5mm. Main jets are stock, idle jets are 41's (instead of stock 40's). Plugs look great -- a very light tan on all of them.

Here's the problem: If I let the bike warm up, and let it sit and idle for a while, and then twist the throttle, it bogs down and coughs like it's super-rich, but only for a second. Then the throttle responds normally all the way from idle to redline. Again, the sputtering only happens if I let the bike idle for at least a minute or so. It's almost like gas is pooling behind the butterflies and then gets dumped in all at once when they're open.... Another thing I noticed -- looking at the colortune, when I blip the throttle, the flame goes way into darker yellow before normalizing on 3 out of 4 cylinders. The other cylinder looks more "normal", as it goes to a bright white-blue flame.

This wouldn't bother me much except for those times when you have to take off from a stop light, and I have to "clear its throat" before it will go... Also it seems to be eating up more petrol than usual.

I'm thinking maybe the float needles are shot.. The rubber tips on them have very slight ring impressions, but they're not torn or otherwise damaged. The seats look very smooth on the inside.

Whats your opinion?

 

After the bike warms up a bit, do you make sure the choke is off? And not just at the lever, but that the starter plungers are fully "closed/seated"?


The worn-out float needle valve rubber tips really shouldn't affect this, as the worst they could do is cause the bowls to flood.........which brings up another question, have you checked the fuel LEVEL in the bowls with the "sight tube" method? It's all fine-and-dandy to set the floats at the "proper" height, but then you really must check that setting(s) via the fuel level measurement, as it is THAT measurement that really tells "the tale of the tape" (in other words, the float HEIGHT setting is just a method of manipulating the fuel LEVEL setting which is really the parameter that you are aiming to control).

 

I usually kill the choke as soon as the bike can idle on its own, which is either after a couple of miles of riding or about 5-10 minutes of warm-up.

I'll check the levels using the tube method as soon as I get a chance... Which brings up another question: when you check the level this way, do you keep the tube towards the back of the bowl or the front, or the middle...? Since the carbs are at a slight angle, those measurements are a bit different... also, does the bike have to be running, or would setting the petcock on PRI be sufficient?


thanks!

 

You need to prop up the front of the bike so the carbs ARE level, both front to back and side-to-side. And maybe try switching back to the stock idle jets?

 

Not sure but if I remember right if your bike coughs back through the carbs would'nt that be lean?. Your plugs are telling you that its running good but you could be aliitle lean at idle. I see your color tune plug says rich Dark yellow then blue white. Maybe Rick could give a better answer.

 

My XS1100 had a similar situation. Turned out to be a slight leak in one of the starter plungers. I had to make new washers to replace the old ones. When the starters were leaking it was running to rich at idle and when the throddle was opened it would almost die. I tested the old ones by checking if they would hold a vacuum, used a hand pump on the fuel pickup for the starter circuit.

 

Stereo ...

Before you do anything else ... let's try doing a Old School Short Cut and seeing what (if any) success we have.
Who knows ... we might pull a fast-one, here, and have some fun.

Take a "Old Style" Glass Barrel Fuse with Metal Ends.

Measure from an end ... Precisely: 3.55mm
The line is shy of 4.0mm -- Not 4.Omm -- 3.55

With a "Scribe Tool" get a line on the end you have no trouble seeing.
Use this Fuse as a "Depth Gauge Tool" and Set the Tops of your Pilot Mix Screws ... Up or Down ... Using the Line on the Fuse LEVEL with the Flat area surrounding the Pilot Mixture Screw Hole.

Get the Pilot Screws all at that Depth and lets see how she runs from there.

 

Fitz, that makes sense.. I'll try that.. but not until next year... I think my buddies would lynch me if I spent new years eve wrenching, instead of helping them put away a few brewskis

Rick -- I have the idle mixtures set about a degree rich from where they start to miss and idle deteriorates. I'll compare them anyway with the "Buss Depth Gauge" to see if there's a big inconsistency between the carbs.

 

rick,
You need to write a book with all these tricks. You (and others) post a huge number of them, and when I need them I won't be able to find them.

I'd pay.

Please?!

 

Fitz, that makes sense.. I'll try that.. but not until next year... I think my buddies would lynch me if I spent new years eve wrenching, instead of helping them put away a few brewskis

Why not do both?

 

well, because consuming beer takes 100% of my concentration, but so does carb work... See the math problem there? :mrgreen:

 

I have a garage full of project bike and a fridge full of Smithwick's and Killians and plan to do a bit of both, but no carburetor work.