Strange knock at idle at 1/8 speed of engine


Information and questions on GL1100 Goldwings (1980-1983)
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David2953
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Joined: Wed May 19, 2010 10:36 pm
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Motorcycle: 82 Honda

Strange knock at idle at 1/8 speed of engine

Postby David2953 » Sun Aug 14, 2016 3:34 pm



I just bought this GL1100 that did not run and I rebuilt the carbs and got it running fine except for a strange knocking sound at idle. It's not real loud, but it is annoying. It sounds like something mechanical in the rear compartment. If I speed the idle to 1500 it goes away completely and it never happens at higher speeds. The beat of it is like a clock or a heartbeat. It's slower than the crankshaft and slower than the camshafts. I've adjusted the timing belts and set the tappets. It's not in the valve train and its not the fuel pump (I took it off and it still did it). At 1000 rpm idle, the frequency of the knock is about 2 per second, which is about 1 knock per 8 rotations of the crankshaft. Does anyone know what this could be?



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spiralout
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Motorcycle: 1975 GL1000 (gone)
1980 GL1100I (with '77 1000 engine)
1996 GL1500 SE

Re: Strange knock at idle at 1/8 speed of engine

Postby spiralout » Sun Aug 14, 2016 4:20 pm

Sounds like primary chain slap. The best you can do to lessen it is to synch the carbs. I see people who are coming from V twins or inline 4s to these 4 cyl boxers question the noises they make pretty often.

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ekvh
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Re: Strange knock at idle at 1/8 speed of engine

Postby ekvh » Sun Aug 14, 2016 4:33 pm

A fairly good test of this is to have the bike in gear, brake applied, ease the clutch out enough to put a load on the motor. If the knock disappears, it's probably primary chain slap and nothing to be alarmed about. A good carb synch usually cures it.

David2953
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Re: Strange knock at idle at 1/8 speed of engine

Postby David2953 » Sun Aug 14, 2016 7:11 pm

Thanks for the responses. I have synced the throttles very carefully and it did not help. The primary chain was my first thought, so I tried loading the engine in gear and it does not go away. It does not seem to follow anything I do with the clutch or the transmission. I looked at a parts diagram and the primary chain is too short to make this this frequency anyway. Only the timing belts are long enough so it may take 8 revolutions of the crankshaft for the belt to go around once. So, I checked over the belts and they looked perfect all the way around, but they were set too tight and the right one was much tighter than the left. That's a very tricky process, tensioning the belts, but I think I got it right and it did not solve the problem. So, I was thinking in terms of complete engine teardown and crankcase splitting to see which gear was to blame, or sell the bike maybe.

Then, after I posted the question here I thought of a way to test the rhythm of the timing belts. I put a big yellow area on one of them with a paint marker and let it dry. I started the engine and watched the belts with a bright flashlight. Sure enough, the frequency was perfectly in tune with the knocking sounds. The belts look perfect, but there has to be asymmetry in one or both. They are genuine Honda and I don't know whether they could be 34 years old and I know the bike was stored for a long time. Maybe the belts have taken-on a distorted shape that affects the way they spin. Anyway, I've ordered new belts and I expect that will solve the issue. Are the Gates T274s any good? They're a lot cheaper than Honda or K & L.

David2953
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Re: Strange knock at idle at 1/8 speed of engine

Postby David2953 » Sun Aug 14, 2016 7:56 pm

I think the primary chain is what actually makes the sound, but it is caused by a fluctuation that is caused by the "set" in the belts. From looking at the shapes of used belts on eBay, I suspect my belts have a sharper bend where they were stretched over each of the three pulleys for many years without moving. I won't be able to see it until I remove the belts, but it makes perfect sense that this is the case. All 6 bends in the belts are on the pulleys at the same time in the rotation, causing an abrupt enough load variation and rpm hop to make the chain noise.

David2953
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Re: Strange knock at idle at 1/8 speed of engine

Postby David2953 » Sat Aug 20, 2016 11:33 pm

Well, I put the new belts on and the noise is still the same. I don't know what is causing it.

David2953
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Re: Strange knock at idle at 1/8 speed of engine

Postby David2953 » Sun Aug 21, 2016 2:48 pm

I put it in gear on the center stand with the engine running and let out the clutch and applied the rear brake and the knocking gets rougher like the primary drive has a bad spot. It does it in every gear on the center stand, but it is not noticeable when accelerating through the gears on the road. The gears all work fine, the engine is very smooth, all 4 plugs burn clean, it does not leak any oil, and it has plenty of power. I think the original owner took good care of it.

The carburetors were a real mess when I got it. The owner had been told by a mechanic that worked on it that all it needed was a throttle sync and he would have done it but he didn't have vacuum gauges. I found that the carbs were some of the most-screwed-up I have ever seen and they needed a complete rebuild. So, there was a neglegent mechanic working on it and I may not have been told some things about it's recent history.

I suppose it could have been caused by something relatively innocent like a small screw being dropped into the engine somehow and it got caught in the chain, causing damage. Has anyone ever heard of this happening?

I have not had the honor of splitting one of these crankcases, but it looks like that's where I'm headed. It's going to have to wait a few weeks.

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maintainer
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1982 GL 1100 Interstate (Sold)

Re: Strange knock at idle at 1/8 speed of engine

Postby maintainer » Mon Aug 22, 2016 5:15 pm

Could someone have used the stator/generator bolt to turn the engine over for some service? And went too far, then maybe tried to reverse it and loosened up it?
Some warn not to turn it counterclockwise but only clockwise as it can loosen it then back out and cause a rattle.
1982 GL 1100 Interstate SOLD
1977 GL 1000 Standard (naked can be good, who knew?)

David2953
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Re: Strange knock at idle at 1/8 speed of engine

Postby David2953 » Mon Aug 22, 2016 10:16 pm

Thanks, I know that can happen, but this one is tight. I checked it.
The noise and the little jerk go away at 1500 whether there is a load applied or not. I can feel it on the road if I let the engine lug under 1500 rpm. It's a slight jerk in the drive train every 1/2 second or so at 1000. At 1500 it goes away and the drive feels smooth and sounds smooth. I normally stay above it and I only have the annoyance at idle. I just can't figure out what this is or what is causing it and I wonder if it could lead to some kind of catastrophic failure.

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Maz
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Motorcycle: 1975 GL1000K1
1976 CB500T
1979 CB750L
1990 Yamaha FJ1200

Re: Strange knock at idle at 1/8 speed of engine

Postby Maz » Tue Aug 23, 2016 2:11 am

David2953 wrote:Thanks, I know that can happen, but this one is tight. I checked it.
The noise and the little jerk go away at 1500 whether there is a load applied or not. I can feel it on the road if I let the engine lug under 1500 rpm. It's a slight jerk in the drive train every 1/2 second or so at 1000. At 1500 it goes away and the drive feels smooth and sounds smooth. I normally stay above it and I only have the annoyance at idle. I just can't figure out what this is or what is causing it and I wonder if it could lead to some kind of catastrophic failure.


I'm wondering if this is something slightly less sinister. Partially/totally blocked idle jet on one carb? Had a very similar thing on my 1000 a few years back. It idled ok, around 1000 rpm, as I had adjusted the idle screw to do so, but it had a regular 'miss' which caused the primary chain to 'buck' slightly every now and then. Open throttle slightly and, of course, the idle circuits become redundant and it ran beautifully. Try removing each plug cap at idle, one at a time, to see which cylinder (if any) is not firing properly.
I could, of course, be totally wrong and you may already have done this, but worth a try if you haven't.

Maz
Nostalgia is not what it used to be!

David2953
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Re: Strange knock at idle at 1/8 speed of engine

Postby David2953 » Tue Aug 23, 2016 6:20 pm

I would not recommend pulling plug wires because it can damage ignition components, I have a safer way of doing this. I connect my spark testers with the air gaps set to 0 and I use a ground wire to short each cylinder out temporarily. I did this and the engine slowed down the same amount with each cylinder I shorted. This is a healthy engine with healthy carbs and the idle circuits are working fine. I have tried different idle mixture settings and sync strategies and that did not help. Interestingly, when I shorted #2, left front, the knock went away, the miss that I created was somehow counteracting the knock. I still don't know what is causing it.

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Maz
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1976 CB500T
1979 CB750L
1990 Yamaha FJ1200

Re: Strange knock at idle at 1/8 speed of engine

Postby Maz » Wed Aug 24, 2016 2:12 am

David2953 wrote:I would not recommend pulling plug wires because it can damage ignition components, I have a safer way of doing this. I connect my spark testers with the air gaps set to 0 and I use a ground wire to short each cylinder out temporarily. I did this and the engine slowed down the same amount with each cylinder I shorted. This is a healthy engine with healthy carbs and the idle circuits are working fine. I have tried different idle mixture settings and sync strategies and that did not help. Interestingly, when I shorted #2, left front, the knock went away, the miss that I created was somehow counteracting the knock. I still don't know what is causing it.


If you have electronic ignition then, yes, you could damage ignition components. I don't have that luxury on my GL1000 as I reverted to points and condensers :D .
Strange that the knock disappears when killing #2. Almost sounds as if that cylinder is firing too early, which, of course is not possible unless #1 is too and it would affect all engine speeds.
As for the engine being healthy, well yes and no. Something ain't right.

Maz
Nostalgia is not what it used to be!

David2953
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Re: Strange knock at idle at 1/8 speed of engine

Postby David2953 » Wed Sep 28, 2016 7:05 pm

I found the solution to this mysterious problem. It sounded like a mechanical knock at idle, but it was really a strange ignition malfunction. Only at slow idle and only once in 8 revolutions, the front cylinders were getting a spark about 38 degrees BTDC. I could see it with a timing light. I disconnected the alternator three yellow wires and the knocking stopped. The alternator wires were causing a pulse in one of the pulse coil wires in the harness where they were bundled together. I proved this with a jumper wire. I tore apart the harness, expecting to find damaged wires, but they were all perfect. The pulse had to be by induction. I separated the pulse coil wires from the alternator wires and re-taped it all and the problem was gone.

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spiralout
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1980 GL1100I (with '77 1000 engine)
1996 GL1500 SE

Re: Strange knock at idle at 1/8 speed of engine

Postby spiralout » Wed Sep 28, 2016 7:29 pm

Thanks for the update that it was a spark knock.




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