!!KNOCKING!!

[Guest]

I HAVE A 1979 GS750L. TODAY, I HAD MY BIKE RUNNING. ALL OF A SUDDEN I HEAR A KNOCKING NOISE. THE BIKE STILL RUNS PERFECTLY FINE. IT HAS PLENTY OF OIL. ITS COMING SOMEWHERE FROM THE MIDDLE OF THE MOTOR IT SOUNDS LIKE, BUT IM NOT PERFECTLY SURE. LIKE NEAR THE TIMING CHAIN, OR MAYBE TRANSMISSION. THE FASTER IT REVS, THE FASTER THE KNOCK. I HAVE NO CLUE ON WHAT IT COULD BE. MAYBE OIL PUMP OR SOMETHING? ANY IDEAS?

[cloudbreakmd]

Possibly the cam chain tensioner...

[cloudbreakmd]

Don't ride the bike obviously, and run it as little as possible until you figure it out... Just a thought.

[earlfor]

I would remove the spark plugs and the oil filler cap, the spin the engine over with the starter while listening through the oil filler opening. You will be able to hear what is going on inside the engine much more clearly and possibly be able to get an idea of what is wrong or where the sound is coming from.

Earl

[bjrehill]

You can also use a screw driver as a stethescope. Take the blade and put it on the engine and place the handle up to your ear. You should be able to pin point the sound. It works great.

I ahd a similar problem. Turned out it was a carb problem. Too rich.

[Tarbash 27]

!!KNOCKING!!

WELL, RIGHT AFTER I ADJUSTED THE CARBS IT STARTED MAKING THE SOUND. THE BIKE WAS RUNNING TO LEAN. I ADJUSTED IT AND THE BIKE RUNS ALMOST PERFECT NOW EXCEPT IT STARTED KNOCKING. I TRIED ADJUSTED THE CARBS BACK TO WERE THEY WERE SET BEFORE THE SOUND, BUT THE SOUND WAS STILL THERE. ITS HARD TO HEAR. YOU ONLY REALLY HEAR IT AROUND 1000 RPM'S.

[flyingace]

I hear a knocking or maybe more of a slapping sound coming from my engine too, but only when running - most noticable when at low speed with the engine under load. However, I did as Earl suggested, and the engine made no such noise when turned over by the starter.

I've been told not to worry about this noise, but it is worrisome and hard to ignore. My bike (GS850) has 36K miles and low compression - is piston slap a possible source of the sound?

Don't mean to hijack this thread, but I think the noise we're hearing is similar.

Ace.

[Cajun Cycler]

Mine is doing that and I had a host of GS'ers listen to it last year at the fall Hill run. None of them could positively identify it. I am pretty convinced it is piston slap. The previous owner had a habit of letting the engine fill with gas while it sat because he was too lazy to fix the petcock. For me I think it is time for new rings and pistons, which I have. Just need the nerve to disassemble the engine one more time. Anybody in the mood to help?? hmm thought so.... :roll:

[flyingace]

Cajun,
I think so too. I'm even more suspicious since it sounds to be coming from cylinder #1 - the lowest compression of the four.

I am probably going to tackle a top-end rebuild over the winter as well. This will be a first for me on an engine of this size/complexity. Fortunaltely, there are a lot of experts on this site who are very familiar with rebuilds.

Ace.

[earlfor]

Re: !!KNOCKING!!

I'm inclined to think possibly rattling clutch basket, or maybe loose countershaft sprocket. A rod bearing gets louder with rpm increases.
If the bores were worn enough that you could actually hear piston slap, I dont think the engine would run at all.

Earl

WELL, RIGHT AFTER I ADJUSTED THE CARBS IT STARTED MAKING THE SOUND. THE BIKE WAS RUNNING TO LEAN. I ADJUSTED IT AND THE BIKE RUNS ALMOST PERFECT NOW EXCEPT IT STARTED KNOCKING. I TRIED ADJUSTED THE CARBS BACK TO WERE THEY WERE SET BEFORE THE SOUND, BUT THE SOUND WAS STILL THERE. ITS HARD TO HEAR. YOU ONLY REALLY HEAR IT AROUND 1000 RPM'S.

[flyingace]

Earl,
Thanks for your responses. Always appreciated.

Of those mentioned, I think a rod-bearing is the most likely. However, the sound volume doesn't increase with RPM, only the frequency. In fact, it actually sounds quiter as the engine increases RPM. It is loudest acclerating from lower RPMs.

I'm going to try to run a mic out into the garage and record this as a .wav file. I'll start a new thread... I've hijacked this one enough already.

Ace.

[earlfor]

A wav file would be great. I'll wait. :-)

Earl

Earl,
Thanks for your responses. Always appreciated.

Of those mentioned, I think a rod-bearing is the most likely. However, the sound volume doesn't increase with RPM, only the frequency. In fact, it actually sounds quiter as the engine increases RPM. It is loudest acclerating from lower RPMs.

I'm going to try to run a mic out into the garage and record this as a .wav file. I'll start a new thread... I've hijacked this one enough already.

Ace.

[Anonymous]

My 450 seems to knock a bit when I run low on gas.

[Chris Daniel]

Mine does this as well.

When I get within about a half gallon from empty it starts making a knocking that comes from the tank (the only way I can hear it is sitting on the bike).

When I have more than a half gallon of gas in the tank, it doesnt make the sound......

The only other sound it makes is the clutch rattle common to these bikes.

[Anonymous]

Is there a good test for rod knock (with out taking the engine apart)?

I have an 1100 engine that I pulled because it was knocking. It seemed to be getting worse every time I rode the bike. The last time I rode it, it was so loud that people walking on sidewalks would turn to see what the noise was. It was pretty loud at lower RPM ranges but not so at higher RPMs. It also changed frequency as the RPMs changed.

I decided it would be best to not use the engine to the point where it got real bad. I found another engine and am putting it into the bike. I haven't seen it run, but it has good compression. It had sat unused for 10 years till someone ravaged the bike for parts and I bought the carcass. I do have a good GS1100GLZ frame with a clear title though. I'm also wondering if it would be worth the expense of rebuilding the old engine. Except for the knock, it ran very well.