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Author Topic: K3 CB750 engine knocking  (Read 1093 times)

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Offline Jinsonphoto

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K3 CB750 engine knocking
« on: December 24, 2014, 10:41:58 AM »
Okay so I have a 1978 CB750K with 1973 engine in it. Not long ago I noticed a knocking sound that happens when coming to a stop with the clutch pulled in and I think it almost always happens when in first gear. Im pretty sure it never happens when in neutral. When it first started happening it was not a frequent occurrence, but now it happening every 2 out 3 times I come to a stop. And it seems like the knocking is faster and louder than it was originally. I can almost feel it when its happening. It sounds as if its coming form the lower left side of the bike.

Now Ive been slowly customizing & replacing parts on the bike for the past year, but I have yet to do any work with engine other than replacing oil & plugs.

Any help would be greatly appreciated!

All the best,
Anthony
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Offline flybox1

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Re: K3 CB750 engine knocking
« Reply #1 on: December 24, 2014, 11:07:33 AM »
Quote from: Jinsonphoto on December 24, 2014, 10:41:58 AM
Now Ive been slowly customizing & replacing parts on the bike for the past year, but I have yet to do any work with engine other than replacing oil & plugs.
:o kinda backwards....but ok.
you definitely need to complete the 3000mi recommended maintenance items.
this will alleviate the majority of the noises.
once you've done it all, and the problem persists, we can better assist.
the shop manual can be downloaded in the FAQ section of the forum, if you havent already done so....
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline krusty

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Re: K3 CB750 engine knocking
« Reply #2 on: December 24, 2014, 11:10:34 AM »
Easy stuff first. Check all your engine mounting bolts.
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Honda
1976 CB750F1
1978 CB750F2
1972 CB350F
1961 C100 Cub
1962 C100 Cub
1959 C76
1963 C92
1964 C95
Suzuki
1963 M15D 50cc
1961 250TA Colleda
1961 250TA Colleda x 2 primed ready for paint and assembly
Yamaha
1977 DT175E x 2
1978 DT125E
1979 DT125F
1976 DT250E
1978 DT250G
1984 FJ1100
1990 FJ1200

Offline Vinhead1957

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Re: K3 CB750 engine knocking
« Reply #3 on: December 24, 2014, 11:39:37 AM »
Lower left.    Get that front chain gear cover and inspect !   Not pretty when it goes thru the case
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Offline Jinsonphoto

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Re: K3 CB750 engine knocking
« Reply #4 on: December 24, 2014, 12:48:57 PM »
Yeah I think Ive done most of the maintenance stuff when I first got the bike, but Ill check the factory manual. Ill definitely check the engine mount bolts and the chain gear cover.

Thanks for all the advice so far. If anybody else has thoughts be sure to chime in. I will follow up once I check the recommendations that have been made.

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Offline flybox1

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Re: K3 CB750 engine knocking
« Reply #5 on: December 24, 2014, 01:41:32 PM »
cam chain tension...
drive chain slack and/or chain jumping on a worn drive sprocket...
chain cover vibrating


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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline ekpent

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Re: K3 CB750 engine knocking
« Reply #6 on: December 24, 2014, 02:14:57 PM »
Not where your problem is at but I thought that the top end on my CB900F was coming apart. Between 2-3500 rpms there was a very loud and weird buzzing sound coming from what I thought was the engine and was freaking out. Come to find out when I was running the engine with my helmet off carefully listening to the engine that I found it was the loose glass that was radically vibrating in the mirror housing and not the engine at all. I would swore it was the engine but was a happy camper in the end.
   Hope you find your Gremlin, look things over well. Make sure you don't have a major clutch rattle also.
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