Honda Rebel 250 & 450 Forum banner
1 - 20 of 88 Posts

· Registered
1986 Rebel 450
Joined
·
181 Posts
Discussion Starter · #1 ·
Hello all, My bike has developed a knocking. I have adjusted the valves. All the valves open and close and the clearances were checked. I also don’t have any vacuum leaks in the carb boots. I was told it could be from non balanced carbs and I plan to sync them soon. I doubt that is the problem though. It’s really hard to judge where the knocking is coming from. It seems to be coming from lower case and transferring up to the valve cover. I really can’t figure out if it’s at the valve cover or the lower end. Does anyone have an idea on what could be causing this? I’ll post a YouTube video below. Thank you!

 

· Premium Member
1985, 86 CMX250C, 81 CM200T, 74 CL360, Invisible to cages, treat them accordingly. Avoids Road RAGE!
Joined
·
3,806 Posts
It almost sounds like a wrist pin to me. If this is the problem, (I hope that it's not), oil may not be getting enough pressure to get there.
 
  • Like
Reactions: B40dash8

· Registered
1986 Rebel 450
Joined
·
181 Posts
Discussion Starter · #4 ·
well it started knocking before I adjusted the valves I was told to check the valves to make sure they weren’t sticking or out of wack. I adjusted them but I set them mistakenly to the inches but used mm. So they chattered when I put it back together. I did them again and now they are quiet. But no there wasn’t a big change in adjustment from when I first checked. Although I didn’t measure it before adjusting. I’m just going by how little they moved while loose in the compression stroke at tdc
 

· Registered
1986 Rebel 450
Joined
·
181 Posts
Discussion Starter · #5 ·
could that happen from oil being to thin? I changed the oil today and it smelt like gas. It was a brownish color No metal shavings or anything in the oil though but it did indeed smell like gasoline
 

· Premium Member
1985, 86 CMX250C, 81 CM200T, 74 CL360, Invisible to cages, treat them accordingly. Avoids Road RAGE!
Joined
·
3,806 Posts
Poor lubrication was my guess. So, yes thin oil would make for a noisy engine.
How is your compression?
 

· Registered
1986 Honda CMX 450 Rebel
Joined
·
116 Posts
Could be a "loose" timing-chain. It seems to vanish at higher rpm. Could also be a loose adjuster-arm of the counterbalance shaft.
 

· Registered
1986 Rebel 450
Joined
·
181 Posts
Discussion Starter · #8 ·
I just did a compression test on the engine cold and warm.
Left cylinder= 145
Right cylinder=165 Left is lower by 20psi. I would say that’s not good :(

There is no knocking when just using the starter to move the pistons up and down.
 

· Premium Member
1985, 86 CMX250C, 81 CM200T, 74 CL360, Invisible to cages, treat them accordingly. Avoids Road RAGE!
Joined
·
3,806 Posts
Here's something you can try: with the engine running while making noise, try pulling the spark wire (possibly the plug), on one side (if the engine can still run this way), then put it back on & try the same with the other side & see if the sound lessens. If the noise lessens/disappears this will at least isolate the side of the noise to a cylinder/piston.
 

· Registered
1986 Rebel 450
Joined
·
181 Posts
Discussion Starter · #11 ·
I tried that and the engine will only stay running on either side of I keep it running with the throttle. The knocking sound is there when removing either side. I synced the carbs today but still have the knocking. The knocking does dissapear above idle speed though. You can hear it in the video. Just as a experiment… I do not have a leak down tester yet but I filled the left side cylinder with 100 psi of air on the compression Stroke at top dead center and the air escapes out of both of the exhaust pipes. In theory shouldn’t the exhaust valve be closed On the compression stroke at top dead center? Here is a video of the sinking and knocking sound that I still have. At this point about a week into it I’m going nuts trying to find out what is causing this knocking.

 

· Super Moderator
1987 CMX250C AC
Joined
·
3,762 Posts
because of the exhaust crossover you may hear air escaping from one valve in both mufflers..
check TDC of compression stroke again..
bent/warped or burnt valve is about the only way exhaust leak would occur on compression/power stroke..

Before tearing into engine I'd pull the oil pressure switch, attach manual gauge to know exactly what the pressure is.
 

· Registered
1986 Rebel 450
Joined
·
181 Posts
Discussion Starter · #13 ·
I didn’t even think of the crossover!!! Thank you for suggesting that! My next thing to do was to take the exhaust off and just go with that’s coming out of the head. Hmm I will look into the oil pressure switch. I will have to consult the manual for that. Off the top of my head I don’t even know where it is. It’s tough working on these bikes because there’s very limited info on YouTube being it’s such a rare bike. Thank you brother for the crossover info. I never even thought about that!
 

· Registered
Joined
·
3,878 Posts
Get a long screw driver or a wooden rod. Start the motor and listen. One end touching motor. The other end put your ear to it. You will hear the noise through the screwdriver or wooden rod. Start at the valve cover and work you way down the side of the motor. The loudest point will be the location area of your knock.
 
  • Helpful
Reactions: Emil

· Registered
Joined
·
3,878 Posts
An exhaust leak at the head can sound like a knock sometimes. If you have pulled the exhaust a couple of times. You need to change the copper exhaust rings.
 

· Registered
Joined
·
3,878 Posts
When you put the 100 lbs of air to it it should have pushed the piston to the bottom. Remove the exhaust pipe and listen. Make sure both rockers are loose. Then if you hear any air coming out. You have a bad valve.
 

· Super Moderator
1987 CMX250C AC
Joined
·
3,762 Posts
Font Parallel Circle Art Line art


timing chain has spring loaded mechanical tensioner that sticks up under rocker cover.. tap down on it to assure wedges are loose enough to self adjust..
Automotive tire Motor vehicle Machine tool Bicycle part Saw
 
1 - 20 of 88 Posts
Top