Topic: Newbie question -- stalls after riding hard

[Benjamin]

Hi all, very happy to have been learning from you the past few months. Many thanks for your consistent generosity, and thanks as well for whatever insight you can offer this newbie.

I have a '71 CB500 that runs quite well except for a noisy camchain and a balky, clunky clutch. But it has a couple of oddities.

1) Most alarming is that if I run it hard (on the freeway, for instance, or up a long hill), as soon as I ease off the gas it'll die before it stops moving. I never have this trouble if I run it around town below 4000 rpm. It starts back up again, almost always, if I hit the electric starter (even while still moving) and give it some gas. I wonder if it has to do with the following, second issue:

2) To start it when it's cold, I have to both give it full choke, and turn the main idle screw at least a full turn. Then it'll start, initially, at 1000 +/- 200 rpm. As it warms up, it idles higher and higher -- even up to 3000 rpm, if I don't periodically turn the screw back with my thumb. I'm just used to it now. After about 15 mins, I've turned it back down to the starting point, where it remains pretty consistently. The tach needle jumps a bit, but only +/- 100 rpm, usually.

So, I'm wondering if (1) and (2) are related -- that is, after I ride it hard, I've noticed that sometimes the engine drops back down to a much lower idle, below 1000. Why should this be so?

Thanks for reading,
- Benj

[howardstapley]

Well, just my 2 cents here but first, never hit the starter while you still moving, not so good. As for the stall/start problem... When you stall, does it sputter then stall, just die out, back fire or what? Have you tried hitting the throttle a bit when you start it rather than play with the idle screw?

[MRieck]

Vacuum leak(s).

[TwoTired]

1) check for fuel contamination.  Drain and catch the contents of each carb.  Anything larger than .016 inch can block the slow jet, which will starve the engine of fuel at idle throttle settings.  Engine vibration at higher throttle settings stirs up crud in the carbs.  When the engine stops, it also stops sucking on the jet and the debris falls back to the bottom of the bowl clearing the jet and it starts and runs again.
Oh, and if there is crud in the carbs, where did it come from?  Check the fuel tank and filter bowl, too.

2) There is no fast idle cam coupled to the choke.  You are expected to keep the engine running with the twist grip until the engine warms.  Then the idle screw is set for about 1000 RPM.    NOTB

Cheers,

[Benjamin]

Thanks, guys, here's some clarification:1

Howard - it just dies, no sputter, no backfire.
MRieck - vacuum leak(s) where? and how do I cure'm?
TwoTired - thanks, I'll start by cleaning the carbs. I've looked in the tank and it's pretty clean, but if I see any sediment when it gets low, I'll dump it. I'll also put on an in-line fuel filter. BTW, how do you feel about carb cleaners like Seafoam?

Thanks,
Scott

[TwoTired]

Before you clean the carbs, just remove the drains screws at the bottom of each carb and examine what come out.

If you already have seafoam in a pressure can with a nozzle/tube, poke the tube in the drain hole and flush each carb bowl.  Catch the drainage to see if bits come out and stop flushing when it drains clean.

When fuel evaporates, it leaves a residue that can restrict fuel passages in the carbs.  Seafoam is pretty good at dissolving this residue.  It won't dissolve rust and scale, though.  The carb bowls are vented to the atmosphere, which is how the fuel can evaporate from the carb bowls and leave deposits therein.  If the bike is run regularly and frequently, evaporative residue is seldom a problem.  Rust and scale from the tank is another matter.
Do check the fuel filter on the petcock bowl for debris/sediment.  If there is any in the tank, you should find some in the petcock bowl. An added in line fuel filter shouldn't be necessary if your tank is clean and your petcock filter is working properly.

Just to clear up a misconception.  The starter on this machine engages via a roller type slipper clutch.  Hitting the starter button will spin the starter motor, but it will not engage unless the engine is turning at a slower speed than the starter motor.  It doesn't matter if this bike is moving or not for starter operation.

Cheers,

[MRieck]

MRieck - vacuum leak(s) where? and how do I cure'm?

Thanks,
Scott

  Spray some brake cleaner around the carb boots (between the carbs and engine) while the engine is idling. If the revs pick up for a couple of seconds you have a leak(s). The boots eventually do dry out and crack causing leaks. Replacing the boots is the best cure I know of.

[Jay B]

The previous suggestions will probably get to the problem, but if they don't, try a good carb synch. My 350f did sort of the same thing, erratic idle, etc before I synched the carbs after I rebuilt them.

[Benjamin]

Thanks, one and all. I'll try your suggestions tomorrow morning, and let you know how things work out.

Best regards,
Scott