K75S 1988, engine stalls on throttle and banging sound

[xyxy]

Hey all, working on K75S I recently got and trying to find a cause of those two issues:
1. engine halts if I try to throttle fast enough but if I gentle, then it revs high
2. sometimes there's banging noise in the engine that concerns me, not sure if this is a problem or not

I made a video that demonstrates those issues if it's of any help:


Things i have changed on the bike so far, that could be related:
- fuel filter & fuel hoses including the which is submerged
- air filter
- spark plugs

Initial issue with the bike was shorted alternator - I have replaced it with 50amp version

Thank you all!

[Past-my-Prime]

Hey and welcome! Classic bike, great to ride (although I see you've got a much newer cruiser as well). 

Way smarter people than I will likely provide additional insights, and I'll follow along, always like learning things. 

From your video, it looks like it's not charging when you're in idle. Mine does the same thing, probably normal. Related to whether an alternator can make juice at low idle. If you advance the throttle (via the "choke" that isn't actually a choke) does the charge light go off?

Once you've warmed the bike up, does it throttle up better, by which I mean, without stalling?  I wonder when you quickly open the throttle, if your fuel / air mixture gets out of whack, too much of one or the other, not sure which: my K75 will bog down if I crack the throttle to quickly especially if it's cold and although mine hasn't stalled out.

The fact that it seems to be worse when it's cold leads me to consider the possibility of over-fueling in that scenario. I "baby" it a bit until I've got it up to operating temperature, and it doesn't bother me that much. So thanks for posting the question, I look foward to others' answers.

[xyxy]

Hello hello, thanks for the message!

When I gently advance throttle - charging light goes off. I've read that it's normal that on idling RPM alternator isn't charging.
In my case it's worse probably because of the higher-power alternator I've installed (50 amps) as original one was dead.

I've never got it actually warm-up so I'm not sure if the problem will go away, my feeling is that it will.
I'm concerned with banging sound so don't want to force engine to work until I'm sure it's safe :)

I think your idea about overfueling or underfueling could be right, that was my guess as well.
I'm going to try and clean injectors next - I've also ordered "upgraded" bosh injectors https://www.ebay.com/itm/112299763780
Another things to check in my list: TPS and air-mass sensor.

[frankenduck]

Your tach isn't working - but you probably knew that already.

It is not normal for the alt warning to illuminate at idle. If that were the case then the bike would kill the battery and die when you get stuck in stop-n-go traffic.

Your idle sounds a little low. You should use the "choke" when the bike is cold.

If you let the bike warm up before applying a lot of throttle it probably won't die like that.

[Past-my-Prime]

How are the valves? It's not too hard to check, and replacing the shims is easy enough.
Injectors replacement may not be needed.
I don't know with any certainty that your banging sound is due to that, but assuming oil level is good (the video was a bit too quick for me to note that) and the motor is running OK, I don't know what else would cause the banging.
Someone will surely ask what do the spark plugs look like?

[Laitch]

I've also ordered "upgraded" bosh injectors https://www.ebay.com/itm/112299763780

There is little evidence that the 4-hole injector "upgrades" performance. There are anecdotes that it causes a slight increase in fuel consumption. Getting the injectors evaluated and cleaned by a professional service with a good reputation like Mr. Injector is the way to go before anything else is purchased.

Once you know the injectors function well, the spark plugs are the correct type, the coil wires are in good shape, and the air filter isn't clogged, a throttle body balance should follow.

[The Mighty Gryphon]

Stalling and stumbling off of idle is often a sign of a lean mixture, usually caused by an air leak that lets unmeasured air into the cylinders.  The most common source of this air is the notorious "z" tube which is the crankcase breather hose.  It connects the crankcase to the air box and is located on top of the engine block between the relay box and the air box behind the rear end of the throttle bodies.

The hose frequently cracks at the clamps that hold it in place, allowing a tiny bit of air into the air box that is just enough to cause the problem you are having.  Experienced owners will replace that tube as a matter of course when they get a new bike.  It's cheap insurance on a 30+ year old bike.

It is also possible, but less likely, that there are leaks in the rubber bushings around the throttle bodies, but in my experience, it's the "Z" tube that is the culprit.

[frankenduck]

Where to find the "Z" hose:

https://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/ccb/ccb.htm

[xyxy]

Thank you all for comments.

I actually have replaced that Z hose (crank case breather) at the same time as other fuel lines - it was included in the bob's motorcycle maintenance package I got.
So I assume it's not because of the Z hose, but I'll check again if I installed it right or if there's any visible damage.

Oil level is fine, so banging is not due to that.

As non-charging on idle isn't normal, probably I have too low RPMs on idle and should tune that first and fix tacho as well. BTW speedo is also broken it seems along with tacho.

[Martin]

Check the rubber caps on the vacuum ports on the throttle bodies. The bricks engines are designed to use one hole injectors, they spray onto the back of the inlet valve atomizing the fuel and cooling the valve. There are no Dyno comparisons between the 1 hole and the 4 hole by any company selling the 4 hole showing their benefits.
Regards Martin.

Cutaway K Engine Injector angle .jpg.png (59.96 kB . 768x568 - viewed 293 times)

[stokester]

I don't see where you checked the valve clearances.  They are easy to check and the shims are readily available.  Swapping them out will require a couple of simple tools but a quick search on this site will give you instructions and sources for tools and shims.  I traded for shims at the local motorsports dealer.

My former commuter mule, the '93 K75S, was getting very "cold blooded" in that it was taking much longer to warm up where I could take off the "choke".  Searching on this site I found that tight valves may be a cause.  They had been checked and were within tolerances when I first purchased it but it now had another 30k or so on the odometer.  I found a couple of exhaust valves tight, figured out needed shim size and replaced them.  Problem solved.

[blackie1]

what banging noise? sounds pretty standard to my ol' ears
I would agree with the comments above that if there are no leaks in any rubber components allowing unmetered air into the system then i think your next move should be checking the valve clearances and remediating any descrepancies with the appropriate shims, followed by rebalancing the throttle bodies. you will need something similar to this

https://www.beemerboneyard.com/carbtune4col.html

both of these procedures are fairly straight forward and a basic maintenance requirement.
With a repair manual at your fingertips it is not too difficult
If I can manage it anyone can.