1980 DOHC - Idles rough / White smoke

[harishsk]

Hello, I'm not that familiar with CBs and I am hoping experts here can answer some questions I had regarding my motorcycle.

1. Engine idles rough
I hear loud rattling sounds from the cylinder block on a cold start. Let it warm up for few minutes and this rattling goes away almost completely. Also, I observed the rattling is much less when I pull the clutch in.

Had a mechanic (Marietta Motorsports) take a look at this issue. He explained clearly back then, I don't remm much now, but if I recollect correctly - he said the noise is from the primary cam chain and that the oil pump was perhaps taking a while to pump up the oil up top and lubricate its way down - I could be sorely wrong, I tried to reconstruct the sentence best using the keywords I remembered from that conversation.

Once the engine is warm, the rattling barely exists and the motorcycle performs pretty well.

2. White smoke
I've had this issue ever since I purchased this motorcycle. On a cold start, along with the rattling I mentioned - the engine also spits out some fumes, sometimes a crackle/misfire. I want to say, a puff of thin black smoke emerges on start and as the engine warms up, it emits a thin screen of white smoke. By no means, does it emit a whole ton of white smoke. But there is enough smoke to know that it isn't normal. I will try to upload a video about these soon.

Also, the white smoke more or less disappears after a good 20-30 minute ride.

Now, when I bought the motorcycle, I had a feeling the engine was all gunked up and not running smooth from the way it sounded and ran - he would struggle hard to even hit 70mph. I ran a cycle of sea foam - thru the tank and some in the crankcase - and after sea-foaming, the performance improvement was immediate. The engine ran much smoother and I could accelerate much quickly off the line and easily hit speeds up-to 100 mph (I just sprint, and get back to cruising speeds )

I also have a feeling there is some oil leak from the head (not around the valve cover but more around the exhaust fins) but I have not cleaned up the motorcycle thoroughly to see if the oil stains comes back on. When I replaced my headers a while ago, I observed thick black carbon deposits in at least 2 of the 4 exhaust ports. I have a feeling oil is making its way into the combustion chamber maybe?

What do you guys think? Any ideas? I would like to start diagnosing and fixing this issue. Spring is almost here :) I would appreciate if you guys can help me figure out a list of potential items to diagnose and fix (beginning with the most simplest to identify and fix). Help is much appreciated!

[TOOLS1]

The rattling could be two things. One is the carbs are out of sync. When the carbs are out of sync the engine will sound like it is coming apart. Two is the bolts on the starter hub are coming loose. As for the white smoke, white smoke is water. You mentioned "Marietta" so I am assuming Georgia? There is a lot of humidity there! I know because I have lived in the south for many years. You are most likely getting condensation in your exhaust, and after 20-30 minutes it burns out. That is why it quits smoking. As for carbon deposits in the exhaust, this is normal.
TOOLS

[harishsk]

Thanks Tools.

I will check if the carbs are out of sync. Also, if the carbs are indeed out of sync, how does the rattle go away after a quick ride? Just trying to understand how it works :)

Can you be a little more elaborate about the starter bolt being loose? The ratting sound occurs somewhere in the cylinder head (the rattling sounds like loose tappets on an old Triumph or an Enfield), so I am little confused that you'd mention the starter, but I'd like to explore this.

As for the white smoke, yes, I am in Georgia! The white smoke also appears after the engine is shut down for few hours after a decently long ride, which would tell me its not because of the moisture. Also, the white smoke tends to smell like gasoline, as if the engine is burning rich and spitting out a lot of unburned gas.

Any thoughts?

[sgtslag]

Tools is one of the guru's here, that's for sure.  Have you downloaded the Factory Service Manual yet?  it is linked to on the Home Page.  After you get the carbs tuned, make sure to adjust the Cam Chain Tensioner (two bolts on the cylinder block, in the book, easy-peasy to do, just follow directions).  Cheers!

[harishsk]

Alright sgt. I'm going to work on the suggestions this week. I'll post updates. Thanks!

[TOOLS1]

In your first post you said the noise was from the "cylinder block", now you say from the "head". The DOHC engines are also known for the cam chain guides to wear out. I guess I should have asked if the noise gets louder when you rev the engine, or quiets down. As for the "white" smoke, that is water, now if it was "blue" it would be oil, and "black" would be gas.
TOOLS  

[harishsk]

Thanks Tools - So, one less thing to worry about @ the smoke. The rattle goes away when I rev the engine up, rattling occurs only at idle revs. What do you think?

On Mon, Mar 17, 2014 at 9:09 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

The DOHC engines are also known for the cam chain guides to wear out. I guess I should have asked if the noise gets louder when you rev the engine, or quiets down. As for the "white" smoke, that is water, now if it was "blue" it would be oil, and "black" would be gas.

TOOLS  

Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
1976 CB 750-A X 2
1977 CB 750-A X 2
1977 CB 750-K X 2
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)

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Harish Nair

[TOOLS1]

Since it goes away when you rev the engine, it is most likely the carbs out of sync.
TOOLS

[harishsk]

Perfect! just ordered the carb rebuild kit online, figured I'll go the mile if I'm going to get my hands dirty anyways. Thanks Tools.

On Mon, Mar 17, 2014 at 9:46 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

Since it goes away when you rev the engine, it is most likely the carbs out of sync.

TOOLS

Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
1976 CB 750-A X 2
1977 CB 750-A X 2
1977 CB 750-K X 2
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)

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NAML

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[LukeM]

Relocated this thread to a more appropriate area.

Luke M

[SoonerRon]

The clutch on these old bikes is notorious for making noise at idle. They have rubber dampers that get old and hard with age. The rattle typically goes away above ~ 1500 rpm or when pulling the clutch. I've seen custom made replacement dampers for sale on another site, oem is unobtainium.

[harishsk]

Oh nice, can you point me to any link that tells me more about this part, just so I can open up and inspect the dampers you refer to. Thanks!

On Wed, Mar 19, 2014 at 2:28 PM, SoonerRon [via Honda CB750'S] <[hidden email]> wrote:

The clutch on these old bikes is notorious for making noise at idle. They have rubber dampers that get old and hard with age. The rattle typically goes away above ~ 1500 rpm or when pulling the clutch. I've seen custom made replacement dampers for sale on another site, oem is unobtainium.

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[SoonerRon]

http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=16388&highlight=dampers

[harishsk]

I see. Thanks for the link. The same mechanic had also mentioned to me about a loose clutch basket causing the rattle - this could be it. Thanks Ron!

On Wed, Mar 19, 2014 at 2:44 PM, SoonerRon [via Honda CB750'S] <[hidden email]> wrote:

http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=16388&highlight=dampers

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[harishsk]

Guys, here's a link to the video.

If you listen closely, the rattle is intermittent and continuous. On close observation, it did seem to be coming from the clutch box and not the cylinder (block or head) as I had thought earlier.

Also, I have tried to capture the smoke from the exhaust in this video towards then end. I dont know if you can tell if it's white or blue from this video.

https://www.youtube.com/watch?v=P0qxm9MKOMo

Thanks for the comments!

[LukeM]

Tried to click on the link, but it's a "private" video.  NPI.

Luke M

[harishsk]

Apologies. Give it a try now.

http://youtu.be/P0qxm9MKOMo

[SoonerRon]

Clutch for sure. Mine does the same. Up your idle to 1000-1200 to minimize the noise and ride the hell out it.

[harishsk]

Thanks, I'm going to see if I can grab the dampers you mention. If not, just tighten up the clutch basket nice and good for now.

[LukeM]

Works great.  Double check the sync on your carbs, but from what I'm hearing, it sounds like clutch rattle. After 30 years or so, there's bound to be some stuff that's gotten bent or warped or just plain worn out.

As far as the smoke occasionally coming from the exhaust, it looks like it has just a hint of blue to it.  That could be a bit of blow-by getting past the rings.  If it were upper head related, I'd expect the smoke to be darker, with some oil spotting on the insulator of the spark plug(s). You could try a compression test on all the cylinders (wet and dry) to see if it's a problem with the rings. Do a search for "compression check" on our forum, and pay heed to throttle openings and how to compare the readings. This is left as an exercise for the student.

Luke M