Author Topic: 73 CB750 Oil Tank breather spews oil when throttling (Read 24986 times)

scottly · « **Reply #25 on:** May 21, 2013, 07:30:08 PM »

A tight valve would not cause oil to spew from the tank, but could definitely cause the low compression on #2. A couple of squirts from a trigger type oil can, maybe a spoon-full of oil should be enough to see a difference if it is a ring/piston or valve issue...

Vincent P · « **Reply #26 on:** May 21, 2013, 07:56:28 PM »

Thanks Scotty; will give this a try tomorrow night and let you know.

Cheers;
Vince P

Duke McDukiedook · « **Reply #27 on:** May 22, 2013, 08:08:28 AM »

Quote from: Vincent P on May 21, 2013, 06:53:08 PM

Well compression test has revealed the following:

1 and 4 Cylinders have 90psi.

2 has 30psi.

3 has 125psi.

Will pull her apart soon and keep you updated on what I find, but am leaning heavily towards rings...

Thanks for all the input brothers....

Vincent

Sounds like you might have some issues, bummer.

Vincent P · « **Reply #28 on:** May 22, 2013, 05:31:11 PM »

Ok Scotty,

I squirted oil in the cylinders; the ones which had 90psi now get over 150psi. Cylinder number 2 which had less than 30psi jumped to 60psi.

I adjusted the valves which did not improve the situation.

Looks like I'm pulling the motor, and doing a set of rings.

Henning · « **Reply #29 on:** May 22, 2013, 11:43:35 PM »

Some history of the bike would be useful. Has it ever run properly while you had it, and why did you do the valve job? Milage?

Vincent P · « **Reply #30 on:** May 23, 2013, 04:07:30 AM »

Henning,

The bike has just under 36k miles. I purchased the bike from the original owner 5 years ago. The original owner told me that about 15 years ago he had done rings and a valve job on the bike putting in a set of 836 or so he claims to remember.

The bike rode fine up until 2012 were I started getting smoke out the left side exhaust, and would need to constantly change number 1 spark plug as it was getting fouled by the oil it was burning. I took the bike apart and found that I had a burnt out valve in both 1 and 2 cylinder. I had the head sent to a machine shop where new valves, oil seals, lapped, and the head planed.

While I had the head off, I inspected the cylinders for any scoring, but did not take the jugs off to inspect the rings (thinking now, I should have done this) I simply rotated the engine and looked at the walls; looked good.

Installed the head and began to setup timing (new points), synchronizing the carbs. during the first set of starts I noticed my oil was not circulating so I dropped the pan and found that the "No Smoke" oil additive I had put in the motor when it was smoking, had put a film on the pickup screen. I cleaned this checked the pump and reassembled. Oil was now running through the engine.

After the setup, I started with small runs around the block no real heavy runs. Bike was running good. I took the bike to work which is 25 Kilometers from home; when I got to work I noticed I had oil all over the exhaust and back tire. I topped up what was missing and rode home slowly. It seems to only spew oil under load at high RPM.

Now I have smoke coming from the right side again. Smoke coming from the breather at the top of the rocker cover. A constant steady stream, and it will pour out like a smoke machine when throttled. Raw oil coming from the oil tank breather under high RPM.

I will add more if needed.

Vince

Vinhead1957 · « **Reply #31 on:** May 23, 2013, 05:47:44 AM »

I think you already know your answer! Blowby. Any pressure to oil breather system.

Vincent P · « **Reply #32 on:** May 23, 2013, 08:32:27 AM »

Yep, excessive blow by = rings.
Easy calculation is it not???

Vinhead1957 · « **Reply #33 on:** May 23, 2013, 08:37:49 AM »

Or a hole in that one piston

Vincent P · « **Reply #34 on:** May 23, 2013, 08:49:33 AM »

I hope its not that bad

Vinhead1957 · « **Reply #35 on:** May 23, 2013, 10:32:01 AM »

Good luck, look at the bright side. Nice new power

Henning · « **Reply #36 on:** May 23, 2013, 12:28:15 PM »

Quote from: Vincent P on May 23, 2013, 04:07:30 AM

It seems to only spew oil under load at high RPM.

That's how it works. There's about one thread a year about this and so far no conclusive explanation. My guess is that your head gasket is defective, incorrectly installed or not torqued enough. I take it you replaced it on re-install. One of the (many) things I did to fix my issue was to crank down the head bolts some more. It's a lot of work to pull the motor just to re-torque the head, especially if it turns out to be not enough, but at least it doesn't cost anything.

Guys, the bike ran fine before the valve issue, so the pistons and rings _should_ be OK, and he has new valves. But I do wonder why it trashed two valves.

Vincent P · « **Reply #37 on:** May 23, 2013, 03:06:08 PM »

Henning; what do you suggest as a torque value when you say crank down on the head bolts?

Vincent P · « **Reply #38 on:** May 23, 2013, 03:17:33 PM »

if the head gasket was gone or placed incorrectly would it also not seep/leak where I would see it on the outside?

Henning · « **Reply #39 on:** May 24, 2013, 12:12:15 PM »

Quote from: Vincent P on May 23, 2013, 03:06:08 PM

what do you suggest as a torque value when you say crank down on the head bolts?

Between us, my fancy, expensive torque wrench was playing up. I had to finish in a rush so my son could help me re-install the motor, so I just cranked the nuts down as much as I dared, maybe 1/4 to 1/2 a turn each. I tightened down the 6 mm bolts as well, one of which was loose after I did the 12 mm head nuts.

Quote from: Vincent P on May 23, 2013, 03:17:33 PM

if the head gasket was gone or placed incorrectly would it also not seep/leak where I would see it on the outside?

I would think so, but maybe you haven't done enough mileage yet.
Bear in mind there's no guarantee that this will solve the issue; it's up to you to decide if you want to take the chance.

My story: I bought the bike off Ebay, so no history, but I can see that the PO has messed with the motor. The first time I pushed it hard on the highway it blew oil from the tank. The head gasket leaked, so both side covers were covered with oil. I lived with it. After 20 000 km I pulled the head to repair a bad spark plug thread. I didn't touch anything apart from the head. On re-install I oiled the studs and nuts and torqued the new head gasket to the maximum value. After the repair it didn't blow oil for maybe 500 km or more, but then started again. The head gasket now leaked only on the left hand side, oily left side cover. This past winter and 8 000 km later I pulled the motor again to replace the crappy leaky pucks from the cheapo gasket set I bought from CMSNL, and cranked down the head while I was at it (and did a bunch of other stuff). There is now noticeably more compression felt from the kickstarter, it seems to have stopped leaking from the left of the head, and no more spewing of oil in spite of riding harder.

Along the way I replaced the valve cover gasket that leaked so badly it made the bike unusable. That's why I bite the bullet and use only original Honda gaskets and seals; it's not worth the aggro.

74750k4 · « **Reply #40 on:** May 24, 2013, 04:21:54 PM »

Speaking of blowby from oil over fills...

Anyone heard if the Leak Stopper Valves have ever been made available again???

Someone with a 3D printer oughta be able to make a few!

Vincent P · « **Reply #41 on:** May 24, 2013, 04:27:47 PM »

Henning,

Booked out of work early this afternoon and have the motor ready for some help from buddies to pull out of the frame in the morning.
I think I will look at as much as I can rings and all and do it right. In for a penny, in for a pound if you will.

Realistically in my case I don't think cranking down on the head bolt will give me that much of an improvement considering how low the pressure is in number 2 cylinder.

I will document this experience with video and pics so stay tuned to what I find!!!

crazypj · « **Reply #42 on:** May 24, 2013, 06:03:11 PM »

Quote from: Vincent P on May 21, 2013, 06:53:08 PM

Well compression test has revealed the following:

1 and 4 Cylinders have 90psi.

2 has 30psi.

3 has 125psi.

Will pull her apart soon and keep you updated on what I find, but am leaning heavily towards rings...

Thanks for all the input brothers....

Vincent

You did have throttle wide open?
That sounds like closed throttle numbers
Doesn't really make a difference though, going to have to get it in pieces

Vincent P · « **Reply #43 on:** May 24, 2013, 07:38:56 PM »

No did not have the throttle open when I did this; did not think it would make a difference.

crazypj · « **Reply #44 on:** May 24, 2013, 08:10:02 PM »

Makes a big difference, that's why I asked
seen low numbers too many times when there was nothing wrong with engine although the super low on #2 says you have something serious wrong

Vincent P · « **Reply #45 on:** May 24, 2013, 08:15:17 PM »

Agreed Crazy,

And hopefully by tomorrow night when she is split, I'll have some answers!!!

Henning · « **Reply #46 on:** May 25, 2013, 12:36:30 AM »

Good luck, and keep us informed.

Vincent P · « **Reply #47 on:** May 25, 2013, 03:13:53 AM »

Thanks Henning and all,

Should be an interesting day....

ekpent · « **Reply #48 on:** May 25, 2013, 03:50:45 AM »

When you had it apart the first time for head work did you verify that it had a big bore kit.? Sometimes finding rings for some of the older kits can be a problem.

Vincent P · « **Reply #49 on:** May 25, 2013, 03:55:12 AM »

No just going on what the original owner told me. I have my guages ready to check the sizes of the bores when its apart.

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Author Topic: 73 CB750 Oil Tank breather spews oil when throttling (Read 24986 times)