Author Topic: HELP - 1974 Honda CB750 Engine Smoking (Read 4374 times)

JonathanHValencia · « **on:** January 16, 2018, 03:00:00 PM »

I have a 1974 CB750
Motor is smoking once it warms up. I used gunk degreaser but a couple of weeks ago. I do not drive this daily, More tinkering than anything.

JonathanHValencia · « **Reply #1 on:** January 17, 2018, 12:51:16 PM »

My apologies. Here is a video of what’s going on.

markreimer · « **Reply #2 on:** January 17, 2018, 01:01:11 PM »

Mine did that when I had an oil leak and it was burning off. Earlier it did as well when I idled too long in traffic and it started to overheat. Any signs of leaks? Also the degreaser stuff can smoke off too in my experience.

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JonathanHValencia · « **Reply #3 on:** January 17, 2018, 01:05:56 PM »

There is a leak on the front left side of the motor. Had the valve cover gasket replaced, and the head gasket looks like it may be the culprit but it isn’t leaking much. It hardly leaks on the floor if at all. I sprayed gunk degreaser to clean it, but over a week ago (although I don’t drive this bike often)

Mr. Mike · « **Reply #4 on:** January 17, 2018, 08:06:19 PM »

Those star joint clamps where the header meets the head are on backwards aren’t they?

JonathanHValencia · « **Reply #5 on:** January 17, 2018, 09:33:09 PM »

Oh yeah, didn’t notice. That clamp is backwards. That wouldn’t cause an oil leak or burning, would it?

Don R · « **Reply #6 on:** January 17, 2018, 09:41:29 PM »

No, a backwards clamp won't make it leak. It's just burning off the gunk. They are air cooled so if it idles a long time try to ride it or put a fan in front of it.

JonathanHValencia · « **Reply #7 on:** January 17, 2018, 11:13:49 PM »

Oh darn, I’ve let it idle for some time before. Didn’t know I couldn’t let it sit. Thanks for the heads up! I’m new to this (clearly)

Stev-o · « **Reply #8 on:** January 18, 2018, 07:02:27 AM »

+1...dont let it idle for long periods. Idle sounds a little high too.

jlh3rd · « **Reply #9 on:** January 18, 2018, 07:36:59 AM »

I think you need to get back to square one. Take off and put on the exhaust pipe joints correctly. I don’t even see how you can get a seal like that. Also, you have the exhaust pipe collars and the gasket for your bike , right?....make sure they are installed correctly. Then I would clean your cylinder block again and make sure you rinse off the cleaner . You did rinse it off last time, right?
Now you can troubleshoot. Oil can come from the tappet caps, valve cover gasket, cylinder head gasket mainly. That should become pretty evident. It was hard to see in the video, but I wonder if your No1 cylinder is burning oil and was leaking smoke from the exhaust connection because of the improper installation of your joint. At least now you would see smoke out of the exhaust pipe if the cylinder is having issues. Regardless, with the exhaust assembled correctly you should get a better idea of what’s happening...

JonathanHValencia · « **Reply #10 on:** January 18, 2018, 09:11:09 AM »

I purchased the bike like this. Didn’t realize the exhaust was improperly installed. I have a new exhaust from Carpy coming in this weekend, so I’ll just be sure to install it properly. Valve cover gasket was just replaced so that’s good. Small oil stains by head but it doesn’t leak on the floor. When I took this video, I had my bike at idle for a few minutes. Probably 10-15

JonathanHValencia · « **Reply #11 on:** January 18, 2018, 09:48:57 AM »

Definitely. Thank you calj737.

JonathanHValencia · « **Reply #12 on:** January 18, 2018, 09:50:17 AM »

Also, my idle is only about 1200 rpm

setdog · « **Reply #13 on:** January 18, 2018, 08:28:40 PM »

id look at the exhaust gaskets first, if those flanges are on bwards, might be missing gaskets altogether

id also check the tappet cover orings, simple cheap and easy
maybe youll get a break and just need new orings
in there to prevent oil seeping onto the top fin

Mr. Mike · « **Reply #14 on:** January 18, 2018, 08:41:58 PM »

“Only 1200”?
I have mine set for 1050/1100 and I thinks that’s high enough (at least it calms the clutch clatter). I wouldn’t take it any higher though. Others may differ on what’s too high, but to each his own.
Back to your smoke issue, my thought was escaping exhaust at the clamp, but certainly the gunk spray could be yet standing in the head casting valleys, plug ports etc. Maybe squirt the front of the head off with water, The if you have a compressor, blow that stuff out, then rinse and blow again.

“Idling for 10-15”
For one, you’ll kill your battery
+1 on using the fan though

JonathanHValencia · « **Reply #15 on:** January 18, 2018, 09:03:58 PM »

The video was taken before I altered my pilot air screw, it idles better. I also took it after I’d let it sat at idle for far too long. Took her for a spin and she wasn’t smoking yesterday. Seems to have happened when I’d turn her on and let her sit at idle. Before I knew that wasn’t a good idea

jlh3rd · « **Reply #16 on:** January 19, 2018, 03:37:37 AM »

Your misunderstanding. Letting an air cooled bike idle for too,long without airflow is bad for the bike but doesn’t make it smoke like your bike was. But because it was overheated it made the smoking a lot worse....The only reason your bike was smoking was for the previously mentioned reasons , either tappet caps, valve cover , head gasket, unrinsed gunk, or internal cylinder issues escaping through the wrongly assembled exhaust components. If i overheat my 550, it will not smoke like yours because I have no leaks or cleaner laying on the engine, or mis assembled exhaust parts....
It’s not smoking now so that’s good, you must not have rinsed the gunk off good enough.
Your manual will tell you the proper idle speed. You say your new at this so all four carbs need to be balanced.

JonathanHValencia · « **Reply #17 on:** January 19, 2018, 06:53:42 AM »

I see. I’ll check it out, since I have to remove the old exhaust for my new one anyway. Valve cover was replaced. It isn’t a constant smoke, so I guess that’s a good thing. Thank you, I’ll troubleshoot it.

Stev-o · « **Reply #18 on:** January 19, 2018, 07:00:58 AM »

Quote from: JonathanHValencia on January 19, 2018, 06:53:42 AM

Valve cover was replaced.

By the PO? Some guys replace the head cover gasket but fail to replace the "pucks" and the leak continues.

JonathanHValencia · « **Reply #19 on:** January 19, 2018, 10:30:08 AM »

No, not the PO. By cycle one in Orlando. Motorcycle mechanic

PeWe · « **Reply #20 on:** January 19, 2018, 11:50:42 AM »

Idle at 1200 rpm ensure oil pressure which is positive. I prefer higher idle since it make city riding easier, not that abrupt stop when releasing throttle in lower speeds.
Remove all 4 spark plugs, check all spark plug cavities in the head for oil leaks from their upper inside where oil from the rubber pucks and cam studs will seep out if leaking.

It can also be oil from tacho connection in valve cover that leak, oil flows along the head when riding and standing on side stand. The seal might be bad. Be careful when replacing seal, too hard use of a screwdriver from outside will break the thin alu seat in valve cover that seal has.
Don't ask me how I know. I see that cover now and then thinking about possible fix.

TwoTired · « **Reply #21 on:** January 19, 2018, 11:53:56 AM »

I like lower idle speeds as it provides better engine braking effects.

Just sayin'...

Cheers,

markreimer · « **Reply #22 on:** January 19, 2018, 01:18:48 PM »

Glad the smoking issue is getting resolved.

I have mine idling around 1,100. Any lower and my clutch rattle is too much to bear haha. I’ve synced, clutch mod, everything short of replacing the chains and later basket style.

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PeWe · « **Reply #23 on:** January 19, 2018, 01:34:46 PM »

My bike could get flickering oil pressure warning lit at idling around 1000 rpm on sidestand, 1100-1200 better despite rather hot cam that idles well. Might been the oil too, do not remember which oil I used.
Another cam did not like idle under 1300-1400 rpm

markreimer · « **Reply #24 on:** January 19, 2018, 03:36:08 PM »

Quote from: PeWe on January 19, 2018, 01:34:46 PM

My bike could get flickering oil pressure warning lit at idling around 1000 rpm on sidestand, 1100-1200 better despite rather hot cam that idles well. Might been the oil too, do not remember which oil I used.
Another cam did not like idle under 1300-1400 rpm

Was the pump rebuilt? Mine gets around 15psi at hot idle with 15w40

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Author Topic: HELP - 1974 Honda CB750 Engine Smoking (Read 4374 times)