Author Topic: 1977 cb750f Smoking - Finally, more details (Read 1889 times)

robdrobd · « **on:** March 14, 2010, 06:47:45 PM »

My friend called me tonight and said his 77 cb750f was smoking badly and losing power. Apparently, it smoked a dark gray smoke, and was leaking oil from the top head breather area/hose as well. He said by the time he got home , the top end was pretty well covered with oil. What do you think the diagnosis might be? Is it fixable, or should we swap out the engine? I have a 76 cb750f engine that runs. Would that work?

BlindJoe · « **Reply #1 on:** March 14, 2010, 07:13:24 PM »

some details would be nice

jaymz · « **Reply #2 on:** March 14, 2010, 07:24:49 PM »

Has he checked the oil level? my brother just pulled his bike out from the winter and ended up over filling the oil. was the bike setting for the winter? if so was it properlly stored and prepped? fresh gas? I would start with pulling the plugs and checking them to see if they are oily. I am no master mechanic by any means but with the info you gave that is where i would start then I would check my valves and timing.

robdrobd · « **Reply #3 on:** March 14, 2010, 07:28:31 PM »

The losing power I told him was the big piece. I was thinking rings?

w1sa · « **Reply #4 on:** March 14, 2010, 07:47:40 PM »

Get more details of the symptoms, when they occur etc., and what he's done to it recently.

It might be as simple as soot fouling (black smoke) 'cause the ignition or carbs/ mixtures have developed a problem etc.

jaymz · « **Reply #5 on:** March 14, 2010, 07:54:50 PM »

It could be but it is really hard to tell with out details what color was the smoke? when is it losing power hot cold both? it could be a range of things rings, gasket, incosistent spark, timing, bad fuel but without some more details I am no where near able to give you a real answer

robdrobd · « **Reply #6 on:** March 14, 2010, 08:29:30 PM »

Stay tuned, I'll take a look at it this week and come back with the "details"

BeSeeingYou · « **Reply #7 on:** March 15, 2010, 06:02:14 PM »

Keep in mind that the 77/8 F had the issue with premature valve/valve guide wear. Though this would probably not show up all of a sudden it will cause smoking.

TwoTired · « **Reply #8 on:** March 15, 2010, 06:08:11 PM »

Given the year/model and known issues with valve guides for that model, I'd go with that diagnosis, given no other details.

The guides are just the beginning, allows the valve to wobble in the seats. Seats and valves wear, loses compression, valve guide seals fail, and it blows oil and loses power.

robdrobd · « **Reply #9 on:** March 15, 2010, 07:13:43 PM »

Wow, you guys really are amazing. Once I get a chance to see it, I'll let you know if you were right.

bryanj · « **Reply #10 on:** March 15, 2010, 09:18:19 PM »

Yup 350 miles per pint and 105 max with just guide wear, 150MPP and 90 max when stem seals go as well

Freaky1 · « **Reply #11 on:** March 16, 2010, 03:30:00 AM »

I read that if you change the exhaust valve clearance to .004 instead of .003 you can solve the problem, provided you do it before it starts. It said that heat was the culprit and this relieved that, I hold some doubt that .001 will make that much difference, any thoughts anyone?

MRieck · « **Reply #12 on:** March 16, 2010, 05:52:21 AM »

Quote from: TwoTired on March 15, 2010, 06:08:11 PM

Given the year/model and known issues with valve guides for that model, I'd go with that diagnosis, given no other details.

The guides are just the beginning, allows the valve to wobble in the seats. Seats and valves wear, loses compression, valve guide seals fail, and it blows oil and loses power.

Exactly.

robdrobd · « **Reply #13 on:** March 16, 2010, 08:32:17 AM »

Details are now in the original post.

robdrobd · « **Reply #14 on:** March 16, 2010, 11:32:10 AM »

Is it worth replacing rings and pistons, or is it easier to drop another engine in it?

BeSeeingYou · « **Reply #15 on:** March 16, 2010, 11:58:53 AM »

I would drop the 75F engine in and be riding in a few days. I get the impression that this happened suddenly so there may be other issues than just the head. It seems like the cheaper and faster alternative. This way you can rebuild at your leisure or not. The 75/6 does not have the valve issue.

Grnrngr · « **Reply #16 on:** March 16, 2010, 12:02:41 PM »

You have to pull the motor to do rings and valves anyway... pull the dead motor and put in the runner. Then rebuild the dead one so you'll have a spare when the runner dies. Minimizes downtime. Since the "dead" one isn't really dead yet, it'd be an easy rebuild, and you could even "pump it up" a little...

robdrobd · « **Reply #17 on:** March 16, 2010, 07:46:02 PM »

Will a 76f fit in the frame without any problems?

Jerry Rxman Griffin aka MuthaF'er · « **Reply #18 on:** March 16, 2010, 08:43:28 PM »

TwoTired · « **Reply #19 on:** March 16, 2010, 09:13:48 PM »

There is the 530 to 630 chain difference.

robdrobd · « **Reply #20 on:** March 17, 2010, 05:16:54 AM »

Would that involve a sprocket change as well, or can you use a 530 chain on a 630 sprocket?

w1sa · « **Reply #21 on:** March 17, 2010, 05:34:39 AM »

Pitch of chain used must be the same as sprockets.

eurban · « **Reply #22 on:** March 17, 2010, 06:55:04 AM »

Taking a step back here for a moment . . . . If one overfills the oil tank significantly the motor will tend to puke oil from the breather and could potentially smoke. Perhaps your buddy put in too much oil? It's pretty easy to do on the 750. Obviously the problem could be with the engine mechanicals and the 77/78Fs do have wear prone top ends, but it might be something simpler . . .. .

robdrobd · « **Reply #23 on:** March 17, 2010, 09:18:29 AM »

I will definitely have him check that first!

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Author Topic: 1977 cb750f Smoking - Finally, more details (Read 1889 times)