Topic: Dead spot in throttle

[sneakypete]

Hi Everyone,

I have a 78 550 K.  It's a pretty good runner, I think, but has a dead spot in the throttle.  Under slow acceleration you'll never notice it, however, if you draw hard on the throttle, in between 4000 - 5000 RPM it bogs out and runs really rough.  Once it's through the gate it roars off to 9000 like it's new.  I enjoy turning wrenches but I'm no mechanic.  Also, after reading how many people have so many problems that stop them from even getting down the block I'm a little reluctant to do anything.  But I still want to iron it out.  I've had two suggestions that keep coming up and I was going to try them both one after the other. 

1.  Clean and sync the carb.  Maybe the issue is with the jets.  As the machine draws from the small to the large jet something might be clogged and causing a fuel imbalance.

2.  Replace the points with a solid state gizmo.  For a mere 300 clams I can have smoother running and no adjustment necessary.  This sounds too good to be true which is why it's #2 on the list.

Before I take my daily driver and turn it into an unnecessary project does anyone have any cautionary notes or better ideas about how I could proceed?

Thank you.

[Jv550]

I don't know but I'm wishing you luck. Mine does the same thing...

[TwoTired]

I can't tell yet what is wrong with your bike.  But, I might suggest you do some diagnostic tests to determine what the cause is before you start changing things and throwing money at bits that won't necessarily make you or your bike happier.

My favorite way of troubleshooting is to collect enough data and let it show where the problem lies.

1) Have there been any changes to the bike from when it left the showroom floor?  Air filter and exhaust changes can certainly effect the engine's carburetor tuning needs.

2) Tune up history; tappets adjusted, cam chain adjust, spark plug numbers, gap, point condition and gap, ignition timing, air filter life history?

3) If you're going after carb bits, know that you have mechanical slides directly linked to the twist grip.  Throttle position determines what part of the carburetor is dominating the fuel mix, not RPM.  So, temporarily mark your throttle so you know at what settings the distastefull behavior occurs.  This helps focus on the area of the carb that needs to be addressed/ scrutinized.

4) The deposits on the spark plugs are your window on the combustion conditions inside the cylinder.  To make this useful, learn to read the spark plug deposits, and how to do a "plug Chop" that preserves the deposits under the problem conditions for you to later read and understand if the mixture is too rich or too lean.
See:  http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

A "bog" is not self explanatory and subject to interpretation.  Could be a rich condition, could be a lean condition.  Wouldn't we want to know which and at what throttle position before we took the carbs apart?

A couple other diagnostic techniques are:
To temporarily run without an air filter, which should make the carbs run leaner over the entire range.  How does that effect your "bog"?
Alternately, add partial choke, which should make the carbs run richer over the entire throttle position range.  Note it's effect.  Better? Worse?

Cheers,

[sneakypete]

Thanks TwoTired.

I'll work through your suggestions and see what I end up with.  I prefer knowing what's wrong to just randomly changing things anyway.

Ciao.