Topic: heat related fuel flow problem? maybe?

[grumpy]

My K3 is starting to falter.
Only happens when it's hot (as in run for 30mins or more).
I'll be at 4-5000 RPM, about 1/4 throttle. If I give it bit of a twist to go up a hill or pass it sometimes stumbles. Feels like it's out of gas.
It won't do it if I'm going up through the gears hard - only happens if I'm cruzing in the mid range and give it a bit more.
No visible crud in the tank.
I'll check for crap in the bowls and blow out the fuel line.
Otherwise I guess I should be looking at the mains, right?
The heat factor has me confused, though...

Stock airbox, K&N filter, points, Hondaman ignition, new plugs & full tune up about a month ago.

[TwoTired]

Look at the plugs yet?
Time to clean the K&N?
Check all the standard tune up items?

[tlbranth]

If it goes OK when you're gunning it but bumbles on a slow roll-up, if it's fuel related, you're probably running too rich. Like 2T says, check the plugs - see if they're sooty. A plugged air cleaner will do it (ax me how I know that)

[grumpy]

Plugs look OK but it's not a precise reading as I didn't do mid range plug chop.
Other tune up related stuff looks good although I haven't checked the lash.

I'll clean the air filter & see if that does anything.
I haven't tried the bowls & fuel line yet, either (although those would be moot if you think it's a rich condition)

Any insight on the heat issue? Or is that a red herring?

[tlbranth]

Well if you are running rich, the problem will worsen as the engine warms up because then it needs a leaner mixture.

[gane]

Grumpy, Just for grins, have a look at your' fuel line (from petcock to carbs) Especially if running an in-line filter....I've experianced situations where heat has caused fuel lines to fold/kink  at bends restricting flow when lines warm/soften. luck G

[grumpy]

Well if you are running rich, the problem will worsen as the engine warms up because then it needs a leaner mixture.

Ah, OK... warm motor, fuel is more volatile... got it.

heat has caused fuel lines to fold/kink  at bends restricting flow when lines warm/soften. luck G

Never wouldda thought of that. Thanks.

[HondaMan]

On the (above) topic of fuel lines: how are they routed? They should be almost horizontal from the petcock to the tees, passing through the holes in the carb's mount bracket enroute. If they are running in other directions (like down below the float bowls along their path) then when the trapped air bubble(s) in the line(s) get hotter, they tend to restrict the incoming drainage.

Also, try this: adjust your air screws a tiny bit, either in or out, and try the ride again. They should be in the range of 7/8 to 1-1/8 turn. It often does not take but 1/16 of a turn to make a noticeable difference at the RPM and speeds you're talking about.  This has become especially true with the alcohol (ethanol) enhanced fuels, which burn a trifle slower (and cooler) than the gasolines of the past.

During the last 6 years here in Colorado, for instance, we have gone from ethanol (10%) in the winter to 10% plus hydrogen, all year around. It made a very noticeable flat spot appear in my own 750 at 4500 RPM (when pulling hills, of which we have more than a few...) until the engine gets fully warm (10 minutes minimum). 

[grumpy]

On the (above) topic of fuel lines: how are they routed? They should be almost horizontal from the petcock to the tees, passing through the holes in the carb's mount bracket enroute.

Yep

air screws are at 7/8th - that's been my preferred setting for about a year.
I'll try changing it.

[grumpy]

OK, I cleaned the filter twisted the air screws a bit.
Seems a tiny bit better - but the weather also changed. It's a lot cooler. So who knows...

[HondaMan]

OK, I cleaned the filter twisted the air screws a bit.
Seems a tiny bit better - but the weather also changed. It's a lot cooler. So who knows...

Although this test is tough to do in heavy traffic or at speeds below 60 MPH (4000 RPM-ish), it helps with the diagnosis: run at a good speed of 4000+ RPM in the upper gears, then slowly sneak the choke closed until it is interfering with the air in front of the slide. If it starts to get better (i.e., bike takes off, then further closing slows it back down), then it's leaned too far in the needle or/and mainjet. Raising the needle will help in that range.

Repeating the process at 6500+ RPM will tell you for sure if it is the mainjet.

This test works poorly in the idle circuit, though, because the physics of the needle jet and the pilot jet work differently, and the choke is too hard to adjust that precisely when bumping down the road at small throttle openings. Here. just turning the screws a bit and trying again is easier. 

[grumpy]

cool - I'll try that.