Topic: '78 550k - rpm changes when leaning, left decrease/right increase

[toomanybikes]

Resurrecting a '78 that sat for many years. Carbs are clean (ultra sonic, compressed air, etc.) and bench sync'd, to get at least close. The float valves are all new and the floats set to 14-15mm, although one might be a little off. I have not yet checked/set the valve clearances. Carbs, jets, airbox are all stock.

While working towards a decent idle, I noticed the following weird behavior. If the idle is set at ~1500-2000 when the bike is perfectly vertical, leaning the bike to the left will cause the revs to drop and leaning to the right will cause them to raise.

I understand that leaning the bike may cause the floats to react differently and influence the revs, but I thought the reaction would be the same in both directions (i.e. leaning either left or right would cause the revs to either increase or decrease). Any thoughts?

[calj737]

My suspicion is the throttle cables are routed incorrectly. Idle after warm should be ~1,200

[toomanybikes]

Yes, the idle still needs adjustment (i'm working on that), but at this point I'm mainly trying to understand why leaning the bike would cause rpm changes.

I believe the throttle cable is routed correctly, but even if it weren't, how could that cause rev changes when leaning? The bike is stationary and I'm inducing the lean by just straddling the bike and moving my legs. I'm not touching the throttle/handlebars.

Thx

[bryanj]

Have you physicaly removed the press in pilot jets and cleaned those, ultrasonic dont do it there

[Robbo]

With the bike idling, you can test the impact of throttle cable routing/cable length by simply keeping the bike upright and turning the handlebars left and right to full steering lock.


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[calj737]

Yes, the idle still needs adjustment (i'm working on that), but at this point I'm mainly trying to understand why leaning the bike would cause rpm changes.

I believe the throttle cable is routed correctly, but even if it weren't, how could that cause rev changes when leaning? The bike is stationary and I'm inducing the lean by just straddling the bike and moving my legs. I'm not touching the throttle/handlebars.

Yes, that sort of discredits the cable theory. It's possible then that the carbs are not fully seated into the boots, so when you lean, the weight of the carb bank transfers from side-to-side causing a vacuum leak (higher RPMs). A possibility...

[DaveBarbier]

I would move the cables with your hands and see if there’s any change.

And maybe something stupid, but is it possible you’re inducing the change in RPM as you move the bike left/right with your hand on the throttle? Try moving it without touching the throttle. Throttle housing tightened correctly on the bar? Are the bars tight on the triple clamp? How about your steering head bearings? If you have the front end suspended is there play or looseness?

Is the tank tight in it’s rubber seats? It could be shifting and moving the cables.

[TwoTired]

If the floats aren't parallel to each other, the fuel level can be different when leaning left or right.  The effect would have a delayed effect on fuel atomization, though.

Does it still do it if the throttle cables are disconnected at the carbs?

The 78 also has a fast idle cam connected to the choke.  Knob at the bars.  Tension on that *could* effect idle speed.  Particularly if not adjusted properly.

Cheers,

[toomanybikes]

Have you physicaly removed the press in pilot jets and cleaned those, ultrasonic dont do it there

Unfortunately, I was not able to get the pilot jets out, they were too stuck and I didn't want to crush them.

I know the various idle passages are at least partially clear through guitar strings and compressed air flow, i.e.
- A guitar string through the air/fuel mixture screw hole will exit to the main venturi.
- If you run a guitar string thru the pilot jet, it exits through the main venturi
- If you also shoot air into the air/fuel mixture screw hole while blocking it's outlet, you then get compressed air coming out the pilot jet.

I don't know about any small side holes in the pilot jets themselves. I'll add a heavy dose of BG44k to the gas to try and clear any remaining varnish.

[TwoTired]

The pilot jets have incorporated emulsion tubes which are cross drilled,  You can't be confident they are cleaned without removing them, unless you have a very tiny borescope.

No good reason not to yank them out of their pressed in holes.  Use a parallel jaw pliers, they won't get crushed.  I use soft jaw models.  But, a leather pad should also work without marking the jet.

Cheers,

[toomanybikes]

The pilot jets have incorporated emulsion tubes which are cross drilled,  You can't be confident they are cleaned without removing them, unless you have a very tiny borescope.

No good reason not to yank them out of their pressed in holes.  Use a parallel jaw pliers, they won't get crushed.  I use soft jaw models.  But, a leather pad should also work without marking the jet.

Cheers,

I gave the pilot/slow jets a pretty good pull last time with no success, but will give it another try next time I have the carb bank out. I got spooked by a few posts about jets that snap in place or press in holes getting ruined and didn't want to face either of those disasters.

Still have other aspects to this project to tackle first (valves, check the points, front brake, leaky float bowl, etc.) Probably needs a carb sync as well, but I know that will need to be re-done once the idle circuits are clear.