Help me diagnosis engine noise/symptoms

I have a 1976 KZ900 with original electronics (points).

The bike runs pretty darn good, however, I get a strange sound under certain conditions that I am going to do my best to describe... but I'm sure I won't be able to do a very good job.

When you are at lower RPMS and especially under a slight load (maybe around 4k rpm) and you are not really accelerating hard, just maintaining .. I get this sucking/popping sound. Let me try to describe it.

It almost sounds like a click, but there is air involved somehow.. It might be coming from the carbs. If I let off the throttle or give it a bunch more throttle it quits right away.

I can't decide if its coming from the points or the carbs.. It might even be a lean condition on the carbs at that spot in the throttle. But I am no expert by any means.

I was thinking of taking the carbs off first and seeing what the jets, needles and settings are and start there, but other than that.. it runs ok. The only other spot is when you are really getting on the throttle.. around 6-7k rpm, it bobbles a bit and then after that, it screams... So I assume thats something to do with the needle or position of the needles.

Your thoughts are welcomed!

think u might have a faulty sparkplug there.. I had roughly the same problem and turned out to be a failing sparkplug.. so check ur sparks first before u attempt to take the carbs apart..

yeah sounds terminal to me,better sell it to me now before it gets worse.it will be harder to do later,im just trying to save you some suffering down the road!dont thank me,we're all in this together!:evil:

yggy wrote:

think u might have a faulty sparkplug there.. I had roughly the same problem and turned out to be a failing sparkplug.. so check ur sparks first before u attempt to take the carbs apart..

The guy who initially synced the carbs and put it all back together said or did put new spark plugs in it. But it makes sense given the symptoms... I guess there is no sure way to know if a spark plug is acting up other than replacing it.. So I will go buy my 4 new ones. Whats the proper plug for this thing? I will go try to search for that answer..

Save yourself the trouble..... I can dispose of the bike, and the ptoblem for you. Just deliver it her, and all your troubles will be gone!

Sounds like that are made when a valve isnt sealed.(comp. check) A vac. leak. Check the boots and clamps. Plugged pilot jet. Clean carbs. Idle mixture screw closed too much. Weak spark from loose kill switche's internal wires, points arcing, gap too small, late timing on one set. If it idles at 900 nicely, its not a pilot jet. Sounds like a bad plug or lean condition. If it makes a funny noise when you twist the throttle all the way at 6 or 7 grand its likely a pilot jet, check the idle at 900 and check the idle mix screws. Low compression on one cylinder will create a lean condition at at idle and low rpm's because it doesnt create enough suction to bring fuel into the chamber, same for a plugged pilot jet. Check the rubber plugs on the carb boots for looseness. Sometimes high resistance through one plug wire boot can cause weak spark and the misfire sounds the same as when it isnt getting enough fuel. When u hear that sound out of the carbs it usually means no fuel.

Post edited by: nads.com, at: 2006/08/17 00:51

If none of the things i mentioned help its time to take off the valve cover. Valve clearance should be checked. Make sure the cam caps are tight. If compression is down and clearances are good, Use oil to test for a bent, burned or carboned valve that isnt sealing. If you take the carbs off to clean them, hold your hand over the boots one at a time and turn over the motor. At no time during the full cycle should there be air blown back out the opening. If it pops through the carbs, an intake valve is not sealing due to tight shim clearance, or the valve is bent,burnt,carboned. Compression can be checked as can vac. by the method i described, if it has good suction and no reversion its probly ok but should be confirmed by a comp. test.

Yet another case of, "I bought this bike and..." and "The previous owner said..."

Buy a Clymers and perform a full tune up in the order given. Replace your carb holders (intake manifolds) and clean/lube the mechanical advance mechanism. I suspect the noise will go away or you will become confident the noise is a normal one.

When you buy a 30 year old bike, TUNE IT UP, change the fluids and check the wearable items like brake pads and cables. Certainly, the previous owner did all this in preparation for selling the bike but you never know these days bwhahha

These are some really good suggestions. I bought the bike from someone that spent time making it 'look' good, but didn't do lots of engine maintenance. I took it when I got it to a mechanic and he adjusted the timing, valves, cleaned and synced the carbs. It ran a million times better, but not perfect.

However.. Ive noticed that the idle has creeped up on me since Ive been riding it the last month and now I idle when warmed up at 2800k - I must have a vacuum leak somewhere. Is the best way to find it ... is to use carb cleaner spray and spray around the boots when its running?

I am going to look at that tonight and then see if I can adjust the idle adjustment screw between the 2 and 3 carbs.

I have NGK Plugs and Plug wires with caps on it. I am going to look at those too.

If the idle was not adjusted when the bike was fully warmed, it will tend to be higher than it should. You are right to adjust the knob. Turn it counter clockwise and ONLY adjust it after the bike is warmed up. Check the slack on your throttle cables; you should have some visible slack near the carbs. If you don't, loosen the barrel connectors at the switchgear (right hand lever) and turn them towards you to gain some slack. Tight cables = high idle. Last, if you have more carb related questions, post them in the carb section. Keep us updated on your progress and good luck!

Well, I found that my carb holders are shot. They are dryrotted pretty bad and if I spray them with carb cleaner while running, they kill the motor everytime. I assume this could be the root of many of my problems. They look to be fairly old, probably 30!

Anyway - new holders coming so we will see.

Oh ya, I was able to get the bike to idle around 1000 by turning in the air screws a hair. The slack was in the cables and the idle adjustment wasnt in affect.

Ive noticed as the bike gets hotter, the idle changes a few thousand rpms, must be these rubber boots.

If the carb holders don't seal, they will draw in cold air which concentrates on the piston crowns and could easily hole a piston if the bike is run. I would not operate the bike till the carb holders are changed. Try www.z1enterprises.com