Loud knock at low rpm


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Winger1957
Posts: 24
Joined: Tue Oct 13, 2020 3:47 pm
Location: San Antonio TX
Motorcycle: 1978 GL1000

Loud knock at low rpm

Post by Winger1957 »



I have been working on mine (1978 GL1000, bought October 2019 in pieces) for two days. Running about 90 MPH on a test run, the power seemed to fade. I could maintain 90, but not any faster. I finished my ride about 1/2 hour later with no issues, parked it and closed the garage. Next morning it started with a nasty sound and I figured time to replace the points, which I had on my bench. Replacement was easy but no sight window for timing, so I did it by ear. The bike starts OK, although the starter does seem to suddenly stop midway through a rotation for some reason, like there is too much pressure in the cylinders or the starter is weak. Dunno. Then it resumes and the engine starts.

I checked the valve gaps and they are all perfect. I did find a kink in the air check valve tube and rerouted the hose. I attempted to sync my carbs but #3 barely registers, as does #1. #2 and #4 were matching each other and at very good levels until I unkinked the air check valve hose, and now all four are barely registering any movement on the needles. But they are in sync :)

I checked my timing belts because I have had a few monster backfires and the right timing gear was off about two teeth, maybe a bit less. Corrected that and cranked the bike for a few seconds (no coolant in the radiator yet - cleaning it tomorrow) and wow the nasty clanking, sounds like it is from the left side of the engine.

I did the finger-compression test and #1 does feel weaker but not by a lot. #4 seems to have some oil seepage at the top of the threads, and all of the plugs are dark and slightly fouled. Sooty, and sooty pipes. I'm 2 1/2 or 3 turns out on each of the four lean/rich screw valves, and am stumped.

I really want this engine to perform as perfectly as I can get it, and I do not want to blow it up. I will order the air check valve kit tonight or tomorrow, but would love some ideas I can work on while I await delivery. [edit] - RPM's above 1500 eliminate the hard knock, and I have tested it up to 4500 RPM with no problems. It is not chain slap, I do not think. I am used to that. This is more sinister.


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GL1000K1dan
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Motorcycle: 1976 GL1000 K1

Re: Loud knock at low rpm

Post by GL1000K1dan »

Mechanical knocking from an engine is worrying. There are some real knowledgeable folk on this site who could give you far better advice than me but, for what it's worth, I'd say it's time to get your spanners out and roll your sleeves up.
Winger1957
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Re: Loud knock at low rpm

Post by Winger1957 »

I'm going to order some intake gaskets and an air-check rebuild kit today or tomorrow and see if that fixes the problem. The knock goes away around 2k rpm and up, and I do have a nicked gasket on the #1 intake manifiold. But the knock is so scary to hear that I haven't cranked it in days. The knock started after installing new points and tightening the air-check hose where it attaches to #4. Its probably a vacuum or air leak somewhere, but I am no mechanic and have to feel my way slowly through all this.
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AZgl1800
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Re: Loud knock at low rpm

Post by AZgl1800 »

Winger1957 wrote: Fri Jan 22, 2021 12:25 pm I'm going to order some intake gaskets and an air-check rebuild kit today or tomorrow and see if that fixes the problem. The knock goes away around 2k rpm and up, and I do have a nicked gasket on the #1 intake manifiold. But the knock is so scary to hear that I haven't cranked it in days. The knock started after installing new points and tightening the air-check hose where it attaches to #4. Its probably a vacuum or air leak somewhere, but I am no mechanic and have to feel my way slowly through all this.
slow down a bit....

the symptom to me, sounds more like the carbs are NOT sync'd up correctly.

on the 4 cylinder bikes, that is very critical.
if the carbs are out of sync, the carb with the most fuel will be richer and that cylinder will produce more power, thus tightening up the timing belts with a "Clang".... => the piston hits hard and produces that noise.
~John

'02 GL1800
2009 Piaggio MP3 250cc
Winger1957
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Location: San Antonio TX
Motorcycle: 1978 GL1000

Re: Loud knock at low rpm

Post by Winger1957 »

And syncing is bad on this bike. The needles on 1 and 3 move about 1/4 inch left, while 2 and 4 move maybe one inch left, with about three inches being ideal. It does seem to run rich. Plugs, all four, come out dark and funky now. Syncing to 3 is done, but 3 shows very low vacuum. Syncing 4 to 3 matches 3, but is still very bad. I'm guessing the air check valve is bad. Any other ideas?
Winger1957
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Re: Loud knock at low rpm

Post by Winger1957 »

AZgl1800 wrote: Fri Jan 22, 2021 4:22 pm
Winger1957 wrote: Fri Jan 22, 2021 12:25 pm I'm going to order some intake gaskets and an air-check rebuild kit today or tomorrow and see if that fixes the problem. The knock goes away around 2k rpm and up, and I do have a nicked gasket on the #1 intake manifiold. But the knock is so scary to hear that I haven't cranked it in days. The knock started after installing new points and tightening the air-check hose where it attaches to #4. Its probably a vacuum or air leak somewhere, but I am no mechanic and have to feel my way slowly through all this.
slow down a bit....

the symptom to me, sounds more like the carbs are NOT sync'd up correctly.

on the 4 cylinder bikes, that is very critical.
if the carbs are out of sync, the carb with the most fuel will be richer and that cylinder will produce more power, thus tightening up the timing belts with a "Clang".... => the piston hits hard and produces that noise.
And syncing is bad on this bike. The needles on 1 and 3 move about 1/4 inch left, while 2 and 4 move maybe one inch left, with about three inches being ideal. It does seem to run rich. Plugs, all four, come out dark and funky now. Syncing to 3 is done, but 3 shows very low vacuum. Syncing 4 to 3 matches 3, but is still very bad. I'm guessing the air check valve is bad. Any other ideas?
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Maz
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Re: Loud knock at low rpm

Post by Maz »

Winger1957 wrote: Fri Jan 22, 2021 12:25 pm I'm going to order some intake gaskets and an air-check rebuild kit today or tomorrow and see if that fixes the problem. The knock goes away around 2k rpm and up, and I do have a nicked gasket on the #1 intake manifiold. But the knock is so scary to hear that I haven't cranked it in days. The knock started after installing new points and tightening the air-check hose where it attaches to #4. Its probably a vacuum or air leak somewhere, but I am no mechanic and have to feel my way slowly through all this.
As has been mentioned above, if these engines are not firing on all 4 cylinders, they sound like they are about to knock themselves apart. Check for spark, verify ignition timing and measure the temperature of each exhaust header. The coldest one (or 2) is (are) the non-firing cylinder (s). One distinct fuel related possibility is a blocked idle jet. Happened to me quite randomly once.
Maz
Ironically, Common Sense is the LEAST common of all senses!
Winger1957
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Re: Loud knock at low rpm

Post by Winger1957 »

Maz wrote: Sat Jan 23, 2021 2:49 am
Winger1957 wrote: Fri Jan 22, 2021 12:25 pm I'm going to order some intake gaskets and an air-check rebuild kit today or tomorrow and see if that fixes the problem. The knock goes away around 2k rpm and up, and I do have a nicked gasket on the #1 intake manifiold. But the knock is so scary to hear that I haven't cranked it in days. The knock started after installing new points and tightening the air-check hose where it attaches to #4. Its probably a vacuum or air leak somewhere, but I am no mechanic and have to feel my way slowly through all this.
As has been mentioned above, if these engines are not firing on all 4 cylinders, they sound like they are about to knock themselves apart. Check for spark, verify ignition timing and measure the temperature of each exhaust header. The coldest one (or 2) is (are) the non-firing cylinder (s). One distinct fuel related possibility is a blocked idle jet. Happened to me quite randomly once.
Maz
Yes, all good info and thank you. I verified spark on all plugs, but I do wonder if a plug can become weak, giving a low-voltage spark. Still, this does all seem related to some singular event at 90 mph when the "oomph" went away. Maybe its a valve. I don't have a compression gauge so maybe I need that. I did do the digital compression test, by inserting a digit into the spark plug hole and cranking on the off switch. Less digital force on #1, which also has that nicked O-ring where it bolts to the head. <sigh>. Probably the best tool for this job will be a bottle of Prozac.
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AZgl1800
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Re: Loud knock at low rpm

Post by AZgl1800 »

Winger1957 wrote: Sat Jan 23, 2021 11:58 am
Yes, all good info and thank you. I verified spark on all plugs, but I do wonder if a plug can become weak, giving a low-voltage spark. Still, this does all seem related to some singular event at 90 mph when the "oomph" went away. Maybe its a valve. I don't have a compression gauge so maybe I need that. I did do the digital compression test, by inserting a digit into the spark plug hole and cranking on the off switch. Less digital force on #1, which also has that nicked O-ring where it bolts to the head. <sigh>. Probably the best tool for this job will be a bottle of Prozac.
it is not the spark plugs.... it is out of sync, or the carbs are not perfectly clean.
~John

'02 GL1800
2009 Piaggio MP3 250cc
Winger1957
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Re: Loud knock at low rpm

Post by Winger1957 »

I replaced the broken #1 intake manifold O-ring and the air-check valve internals (removed valve was indeed pinched) and still have the problem. My Partzilla timing window should land in my mailbox today. For any person reading this for timing tips, you will need a special clear-plastic window mounted to a threaded aluminum base that screws into the port through which you see your timing marks. Then, if I can get it cranked, maybe I can time it with my timing light. The static timing produced the proper visual lamp illumination (the probe) results, but the engine still runs so rough that I don't have the heart to crank it. I did try to crank it while ago in my cold garage, and the starter just whumps a time or two and gives up, then will spin her pretty good and quit. Battery is fully charged and I leave it on the charger. Its as if there is compression in a chamber that the starter fights against. But I have checked and verified the valves three times, and re-installed the timing belt on the right side (bad knock is from left side). I just do not see how the new points can be so far off that the timing is blown up. The engine was running fine until it went soft at 90mph. Rode it for another 1/2 hour with no issues, parked it. Slept. Changed the points with correct gap at TDC/T1 and T2, cranked it and presto - sounds broken. When I parked it, I did fill the tank and added about four ounces of Seafoam and about six ounces of Marvel Mystery Oil. Maybe that is why my plugs are so nasty (carbon)? But dear god the backfiring is epic. I'm talking about both internal and in the exhaust. Maybe I need to add some Royal Crown to my additives. And a straw. Hmmm, maybe I broke a ring? No smoke, though. So nah. Gotta be timing. Has to be. Or coil? No, not with a knock like that. Maybe too much fuel in the chamber, detonates early or late or in two chambers. Exhaust backfires indicate excess fuel. Carboned plugs - too much fuel. 1.5 turns out on the four air valves, changed from 2.5 turns out. Tried at 3 turns out. Still carboned plugs and scary bad timing. This did not exist until I installed new points, although the engine did get a bit soft at 90mph. Like she ran out of whatever was necessary to move her faster. Held 90, no problem, but the engine sounded softer, a bit quieter, it was sudden - that is combustion, and that points to points and/or coil? Old points were shot and I expected to blow them up on this run. Had new ones on my bench waiting to be installed. Royal Crown. Straw.
Winger1957
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Re: Loud knock at low rpm

Post by Winger1957 »

AZgl1800 wrote: Sat Jan 23, 2021 2:00 pm
Winger1957 wrote: Sat Jan 23, 2021 11:58 am
Yes, all good info and thank you. I verified spark on all plugs, but I do wonder if a plug can become weak, giving a low-voltage spark. Still, this does all seem related to some singular event at 90 mph when the "oomph" went away. Maybe its a valve. I don't have a compression gauge so maybe I need that. I did do the digital compression test, by inserting a digit into the spark plug hole and cranking on the off switch. Less digital force on #1, which also has that nicked O-ring where it bolts to the head. <sigh>. Probably the best tool for this job will be a bottle of Prozac.
it is not the spark plugs.... it is out of sync, or the carbs are not perfectly clean.
Tried sync a week or so ago. 1 & 3 are synced at about 15; 2 & 4 at about 40. Getting 4 closer to 3 worsens the problem. I'll post an update this evening after work and after I try out this fancy glass sighting/timing window.
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Re: Loud knock at low rpm

Post by winguyjo »

" ...1 & 3 are synced at about 15; 2 & 4 at about 40. Getting 4 closer to 3 worsens the problem ... "

this is nasty. a proper compression test can sometimes avoid a lot of tail-chasing. i would start there to confirm healthy engine internals.
Winger1957
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Re: Loud knock at low rpm

Post by Winger1957 »

She flies! She flies! She flies!
Winger1957
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Re: Loud knock at low rpm

Post by Winger1957 »

I'm celebrating, drinking a cold Modelo Negro in the big can and smoking a good cigar. My bought-in-pieces 1978 Goldwing runs like a deer. She flies. And I still need to sync the carbs :) What happened? I installed the glass sight window for the timing light, and my borrowed 2009 Sears timinglight did not work :) So I "winged" it. So simple. It was the eccentric cam for the points. First, I rotated the flywheel to F at T1, then I loosened the 10mm bolt that holds that point cam in place, rotated the cam 100% counter-clockwise until it hit its stop, bolted it down tight again and fired her up. It did! I didn't even try to fine tune it by rotating the base plate. I just had it positioned more or less centered and when she fired up, I rode it down the block and back and tightened the base plate up. Rode it to work, and after work took a nice 1 and 1/2 hour ride. Thank GOD. I still need to sync and I still want to put a timing light on it. A friend from work is going to loan me his, then I will sync the carbs.

Compression - I checked compression yesterday and #1 is weak, about 90 psi. Bad, I know, and i suspect a valve. 2-4 are all about 145 psi. But this GL runs like a deer, man. She flies. I am delighted. Idle is too high, but that seems to be the fault of the connecting rod from the throttle cable where it connects to the carb assembly. I can move it (with some pressure) with my right index finger where it connects to #3, and the idle drops to normal, but then it sort of sucks.

I have renewed hope. Thanks for letting me rant. I dearly love Gen 1 GL's, and bought a used Sulfur yellow '76 when I was 19 years old back in 1977. Very special bike. I went in to buy a dirt bike and spied that yellow art piece in the window. Ended up riding it to the bank. In a few weeks, I rode my awesome Goldwing 800 miles straight through to my parents house, and one hour later flipped it three times and broke my back. I bought this one because that one was so magic to ride. The yellow bike endured the wreck well enough to ride it when I got out of the hospital. The spoked front wheel didn't even lose its shape from the collision, so the wheels are bomb proof.

Gen 1 Goldwings are the most iconic, legendary motorcycles on earth. When they were introduced, they were the second fastest motorcycles on earth (Kawasaki #1). Hondas top F1 engine designer designed this engine, which is why it is so automotive in its DNA. These bikes were born to run. Thank all of you for your suggestions.


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