Surging

My bike is a 79 KZ650 and I've recently noticed some surging that occurs intermittently. It's most severe after I've had it wide open in top gear for several seconds and then back off to 80 mph.

It doesn't seem to be starving for fuel - I've checked the filter - and I also run it wide open for several minutes at a time, without loss of power.

It just starts surging afterwards. Weird! Any clues what I should be looking for? Thanks in advance.

This is just a wild guess, but when was the last time you checked the valve clearances?

Usually the first symptom of valves needing adjustment is the bike will stall when you come to a stop after a hot highway run.

I haven't checked the valve clearances yet. I purchased the bike late last summer and it had a broken odometer... so I've no idea how many miles are even on it.

I'll say this, it backfires a helluva lot as I decelerate. Is this another symptom of the same problem?

Looks like I'll have to take it apart for some work :dry:

I've never noticed backfiring as a symptom of lash that was just starting to go out of tolerance. Backfiring on deceleration can have several causes, but start by making sure you don't have any vacuum leaks on the intake.

Idaho_Spud wrote:

...I'll say this, it backfires a helluva lot as I decelerate. Is this another symptom of the same problem?...

Backfiring on deceleration is a symptom of leanness. Check your float level! ..and vacuum leaks, rubber cracks, etc...

Until you have adjusted the valves and performed ALL other tune up and maintenance procedures, trying to adjust the carburetors is essentially a waste of time. Backfiring can result from the exhaust valves hanging open a bit... partial and poor combustion can occur and the gases can escape when the valves are not supposed to be open. Please have a look at ANY manual and note the tune up steps. Carbs are always last... get the other stuff right FIRST and narrow your problems to the carbs... Check compression, valve clearances, ignition timing, replace wires/caps and plugs and if you have points/condensors, those as well. Check the voltage going to your coils and the primary and secondary impedence at your coils, adjust the cam chain and remove/lube the mechanical advance. See if something in that list doesn't make your problem go away. Also, on an older bike, sediment collects in the bottom of tank where you petcock is installed. Have you removed the petcock to see if there is sediment or even clogging of the petcock filter screen?

Alrighty then! Sounds like I have my work cut out. It'll take a while to get all that accomplished, since I commute on it daily.

I have had the tank off, cleaned it out and rebuilt the petcock though.

The rest doesn't sound too difficult, except for the valves, of course. It smells like it runs really rich, which is a cooler flame, and yet the pipes are quite blue-colored, which would indicate a lean condition - so I'm a bit confused about that too... like wiredgeorge says though a good tune up should come first. I'll follow that with the other suggested remedies.

I'll update this post when I make some progress. Meanwhile thank you very much to everyone who has contributed.

If a potential customer says he needs carbs or carb work, I generally like to find out if he has tuned his bike up before proceeding because carb work won't fix problems with the electrical system, valve clearances, or timing, etc. Get a good set of feeler gauges. Sears sells a metric gauge with the smallest gauge at .051 I think. Very thin gauges are needed to check the valves. You can approach the valve issue two ways. First, you can line up the correct tools and shims OR you can buy the correct tools and figure out which shims are needed. I suggest, if you plan on keeping the bike, to buy a "shim kit" from z1enterprises. You will need ONE SHIM that is a 200 (2.0mm) in the event you can't measure clearance on a valve at a minimum. You will need a torque wrench. The cam cap bolts can generally be torqued with a 1/4" torque wrench which is also useful for many other engine cover bolts. A 3/8" torque wrench will work but is at the limit of its range when measuring under 100 in/lbs and its scale will be in ft/lbs. A 1/2" torque wrench will not work. I mentioned the feeler gauge. If you buy one at an autoparts store, NOTE THE SMALLEST GAUGE SIZE. You need to have a guage that can measure about .051mm at a minimum. You need to buy a shop manual. A Kaw manual OR Clymer will do the trick. If you premeasure the clearances and estimate which shims are needed you will save a couple bucks BUT you will probably not buy EVERY shim you absolutely need. You will surely end up needing one more shim size you don't have. Buying a shim kit will fix this problem for a few bucks more. Once you have done the valve clearance job once, it will only take a few hours in the future as you grow familiar with what is needed. You might also want to pick up a new valve cover gasket as well.

Last tip... to turn the crank shaft, use a 17mm socket and not a wrench.

wiredgeorge,

Thanks for those handy tips, especially about the torque wrench. I purchased a valve cover gasket a while back, intending to do this very job, but never completed the chore during the winter off-season.

I'll get the shim kit. I like this bike quite a bit - most of the ones you see on the road nowadays come in two flavors: Harley and Sport bike. It's kinda cool to ride around on a retro machine that nobody recognizes anymore

I don't think adjusting the valves are beyond my abilities, but I prefer to do these sort of things *observing* someone else do them correctly. I've read how do measure and shim the valves in the Articles section, so I've got a pretty good idea how to do it... just need to take the bull by the horns and get this bike running nice, right? Again, thanks for your sharing your knowledge.