Popping/Coughing noise from 1 cylinder (w/ video)

Another installment in my ongoing "help a noob" series.

I'm getting a popping/coughing sound from one of the cylinders on my 1979 KZ750 twin. Its audible down by the exhaust pipes on the right side of the bike. The left side sounds fine.

You can hear it in action here . The loudest pop happens about 3 seconds in. It's sort of like a little back fire.

I've been trying to get as many things up to spec on this bike as I can since I got it a few weeks ago. Recent work includes:

carb disassembly and cleaning
set fuel (float) levels
carb bench sync
set spark plug gap
set point gap
static timing adjustment
idle and pilot adjustment

I'm still planning to run a dynamic timing test with a strobe light and perform a better carb sync with some vacuum gauges once I borrow the right tools from a friend.

At this point I'm wondering if this is likely a valve clearance issue or if I messed up on the fuel level and one carb is running too rich.

Any thoughts on this?

Almost sounds like a slight lean "pop". Does it still pop or get louder/worse if you just creep the throttle open slightly?

Are the exhaust pipes around the same temp? If you dont have a laser guage, you can mist some water onto the pipes and see if it sizzles off the same. The "poping" pipe should be hotter if it is indeed receiving more air.

You say you set pilot adjustment... via mix scres? What method did you use? Are there any mods here? Stock jetting?

Did you perform a bench sync of the carbs?

Often it can be very difficult to detect small air leaks with the "WD40" trick, even watching the exhaust for smoke during. Best preventative measure is to make certain that the carb holders are completely sealed to the engine, and everything is strapped in snug (carbs, filters, etc.) No missing vacuum caps, no open air access, except for carb bowl vents.

Yeah, now that i listen to it again at high volume, it really sounds lean, like an air leak, or imporperly tuned idle mixtures.

AFAIK all is stock though I'm the third owner and have only had the bike a couple of weeks.

I set the pilot via the mix screws by following the manual. All the way in then back out 1 and 1/2 turns and make slight adjustments from there.

The popping goes away at higher rpm but I haven't tried a slow creep on the throttle.

I'll try that and the water test tonight and look for any possible air leaks.

Worst case scenerio, bad or burnt valve. I used to get a backfire through the carb. And it was a bad valve seat.

A leakdown tester is the only way to check it,

Topper wrote:

AFAIK all is stock though I'm the third owner and have only had the bike a couple of weeks.

I set the pilot via the mix screws by following the manual. All the way in then back out 1 and 1/2 turns and make slight adjustments from there.

The popping goes away at higher rpm but I haven't tried a slow creep on the throttle.

I'll try that and the water test tonight and look for any possible air leaks.

Right on.

Use your mix screws to add fuel, and turn them slowly, until the rpm drops significantly/stumbles (1/4 turn 1 carb at a time), then back off some to re-attain a smooth idle.
Do this on both sides seperately (they will end up different number of turns). Then turn them both slightly leaner (1/4 turn) one carb after the other, and you will hear the idle increase slightly, when the increase stops, stop there. If it drops some, return to the previous position.

You will like the results... best luck.

Could try backing out the mixture screw another 1/4 turn on the right side carb, to test for effect.

A leaking intake valve is more apt to backfire through the carb.

It's generally better to assure valve clearances are within specs, and good properly timed sparks, before carb tuning.

Good Fortune!

Ok, here are the results of tonights tests.

The water test on the pipes was inconclusive but if anything showed the right pipe cooler than the left.

I tried WD-40 to look for air leaks and did not have any luck. I checked all the carb boots and everything was tightly sealed.

When I creep the throttle open the popping goes away. It comes back as the rpms go down.

Backing out the pilot screw on the right hand carb 1/4 turn had no effect. Backing out another full turn seems to have helped slightly.

If the valve shims need adjustment I'm going to have to take it to the shop. I don't have the tools for it and its cheaper to have them do it than buy the tools and shims myself.

Do you still think it's running lean? If so could it be that I set the fuel level a little low when adjusting the carbs?

Did the water sizzle off the pipes (both nice and hot)?

Has the timing been checked recently?

Single coil twin... bad spark would show on both sides.
Maybe compression/engine wear issue...

Tune the mixture screws as I described. Proper tuning of the mixture screws has nothing to do with "X number of turns" crap.

I guess this bike is a 750 Twin... What carbs are we talking about? OK, I just checked, CV type.
Your mixture screws enrichen when removing (counter-clockwise), and lean when tightening (clock-wise) - OK.

You can make that lean pop go away with the mixture screws if it is directly mixture related. If you need to go out several turns on one carb, and not the other, than no matter what - there is extra air getting in somewhere.

Good luck.

Topper wrote:

...Backing out the pilot screw on the right hand carb 1/4 turn had no effect. Backing out another full turn seems to have helped slightly.
If the valve shims need adjustment I'm going to have to take it to the shop. I don't have the tools for it and its cheaper to have them do it than buy the tools and shims myself.
Do you still think it's running lean? If so could it be that I set the fuel level a little low when adjusting the carbs?

Where turning the pilot screw has no appreciable effect on rpm, it's an indication that the pilot circuit is less than perfectly clean.

Simply checking valve clearance with a thickness gauge is fairly quick and easy. And the existing clearances may be found already within spec and no adjustment being needed.

The adjustment, when necessary, is slightly more involved.

Good Fortune!

I pulled the valve cover off and checked my valve clearances. Sure enough some of them were off.

On both cylinders the two valves in the front have almost no clearance at all.

Now I need to decide whether or not to try to replace the shims myself.

Any advice on how difficult this job is? I'm not a mechanic, but am trying to learn to do this sort of basic maintenance on my bike myself.

Will one of those valve shim removal tools really help or can I just do it with a flat head screwdriver?

NO! You cant do it with a flat head screw driver! :laugh: Sorry... just dont want you to @#%&! your bike.

Here's what ya do. The cheap and easy way.

17mm socket and wrench
Large zip tie
Feeler gauge
New shims
All plugs removed
A small pick tool, preferably metal - to pop the shims out. They are usually suctioned into place by the oil underneath.

OK, so you've measuerd the gap and know what shim you want to replace, good.

Roll the crank forward with your 17mm over the large nut behind the ignition side cover, till the lobe comes down and pushes the valve your doing right down.

Now bend the ziptie in half in the middle. Crush it flat at the bend with some plyers. Make a small bend 45 degrees about 1cm from the end of the bend.

Stick the upward bent zippy into the plug hole and up behind the valve you just pushed into the cylinder. You can feel the valve stem, just stop there and hold it in place.

Simultaneously - roll back the crank to let the valve travel back up, except the zippy will stop the valve form travelling all the way up!

Keep rolling the crank back and there will be considerably more gap under the cam lobe - just enough to pick out your shim and change.

Change the shim, then reverse the steps - roll cam forward till zippy is loose - remove zippy. Then keep rolling cam forward to the next shim change.

One more tidbit - spin the bucket so that the small opening to wedge out the shim is where you want it before you compress the valve.

Hope this helped :laugh:

You'll be a shim changing expert in no time!

Well that's not in the manual! Sounds like a great tip though.

One question. Which end of the zip tie am I inserting into the plug hole? I'm thinking the flat end, not the one with the box right?