Author Topic: My 73 barn find with 15k original miles! (Read 18563 times)

seanbarney41 · « **Reply #75 on:** January 04, 2015, 12:37:18 PM »

+1 on the 3k tune up stuff! With the bike off, open the throttle to full with the twistgrip. Let go. If it does not snap back with a slap, your throttle cables are incorrectly routed or adjusted. Adjustment needs to be as per the factory service manual with the specified amount of free movement at the twist grip. The Clymer manual actually covers this adjustment adequately. If your cables are adjusted too tight the carbs often will not snap closed properly. Don't try to adjust the idle speed with the idle air screws, you are correct in using the single thumb screw. Adjustment of the idle air screws is seldom needed when the air box is used, just start with the manual's setting. It takes a long time to fully warm up a cb750 without riding it. Final tuning such as idle air screws, vacuum sync and idle speed cannot be done until the motor is fully warm. Don't use the idle speed screw to keep the bike running on warm up, use varying amounts of choke and throttle. Once the bike is fully warmed (manual calls for 15 minutes of stop and go riding and I would estimate that to equate to 25 minutes of stationary run time depending on ambient temperature)you can set the idle speed.

sulphurdave · « **Reply #76 on:** January 04, 2015, 02:29:28 PM »

Good info. The twist on the throttle feels good and snaps back pretty good. I'll report back once I perform the 3k mile service.

sulphurdave · « **Reply #77 on:** January 06, 2015, 06:58:25 AM »

Ok guys got another problem that I knew would show its ugly head sooner or later because there's evidence of burnt oil on the exhaust pipes that's been on there a long while. Where is this oil leak coming from?

calj737 · « **Reply #78 on:** January 06, 2015, 07:08:46 AM »

Oil leaks can come from LOTS of places, and simply looking at areas where oil has collected will not usually provide an explanation of their source.

Something I noticed that I would devote time and attention to is the oil pan seal. It appears the PO used RTV sealant (the light blue goo around the oil pan) and you want to drop the pan, and remove that NOW. It is infamous for chipping off and finding its way into the sump and oil passages. You should not need sealant outside of the stock oil seals. If you do, you've over-tightened the pan. Stock oil seals work perfectly. Aftermarket are hit and miss.

Take some photos of the engine at the front, side, under the shifter side cover, etc. skip the videos. Places that could be contributing to your oil leak are the oil under the shifter side cover, a head gasket, a leaky valve cover, etc. Turbulence from the wind while riding will drive the oil a long way and confuse you as to its source.

sulphurdave · « **Reply #79 on:** January 06, 2015, 12:47:44 PM »

Here's a few pics that I took last night. The oil is dripping right in front of the bike stand pivot area. The front of the motor looks dry in comparison. I knew this was coming from the look of those crusty black exhaust pipes. I'll change out the oil pan gasket but I don't think the oil is dripping from there. Could be wrong. Any other gaskets down there besides the oil pan? What about where the chain goes?

sulphurdave · « **Reply #80 on:** January 06, 2015, 12:48:48 PM »

Oh and disregard the lighter dry spots of oil that was from the oil change last week. It's the fresh wet stuff that's new.

calj737 · « **Reply #81 on:** January 06, 2015, 12:51:27 PM »

If I am not mistaken, your model has a chain oiler under the left side cover, where the front sprocket is. Remove that cover (shifter arm first) and degrease the area well. Then run the bike for a couple of minutes and watch for oil seeping. You can use a little baby powder in the aa once clean to aide in idientifying any weeping or seeping.

seanbarney41 · « **Reply #82 on:** January 06, 2015, 01:01:28 PM »

Pull the sprocket cover. Clean the whole area very thoroughly. It will be filthy. Be prepared for a cracked case in the front sprocket area.

sulphurdave · « **Reply #83 on:** January 06, 2015, 01:15:16 PM »

When you say "cracked case" is that what I think you mean? A cracked block? That's obviously worst case scenario right lol

sulphurdave · « **Reply #84 on:** January 06, 2015, 01:24:57 PM »

I just googled "CB750 cracked case" and I sure hope that's not the "case" pun intended.

seanbarney41 · « **Reply #85 on:** January 06, 2015, 01:36:50 PM »

Quote from: sulphurdave on January 06, 2015, 01:15:16 PM

When you say "cracked case" is that what I think you mean? A cracked block? That's obviously worst case scenario right lol

yup...found on probably 25% of cb750s

iron_worker · « **Reply #86 on:** January 06, 2015, 01:43:22 PM »

Poorly placed battery vent lines often allowed some acid to get onto the chains which weakened them and thus caused them to break. They would then pile up in front of the sprocket and punch through the case.

To fix it properly you need to split the cases, clean very thoroughly, have it TIG welded, and somehow ensure that the new mating surfaces are still flat (machining, careful hand filing, etc).

Some guys just clean it up as best they can with the engine still in the frame and JB weld a patch over it. It works fine for a lot of guys.

IW

BPellerine · « **Reply #87 on:** January 06, 2015, 04:26:51 PM »

did it leak before you changed the oil?bill

sulphurdave · « **Reply #88 on:** January 06, 2015, 05:53:27 PM »

I think I'm fixing to give away the answer right here. No it wasn't leaking oil as long as it's been sitting there for months. Not until I got it running and put it in 1st and 2nd gear on the center stand to see the chain move the rear tire which it obviously did. I guess that means the cat's out of the bag and should expect the worse.

seanbarney41 · « **Reply #89 on:** January 06, 2015, 06:39:37 PM »

Don't panic or cry until you get that cover off and clean it up. Even then, JB Weld can be a quick, easy, and totally effective solution, at least until it makes sense to split the cases and weld it up.

calj737 · « **Reply #90 on:** January 06, 2015, 06:58:18 PM »

...and, just because it wasn't leaking sitting there, doesn't mean it wasn't leaking before. Since you've run it, any seal under that cover could easily have dried up then sprung a leak with the recent running of the motor. Could be a couple of possible issues under that cover.

Breathe, Grasshopper, breathe.

Btw, is the chain still "taut" and in place? Did you hear any unusal clattering while running or when stopping? If not, doubt it's the case, but could be. Gotta get the cover off, clean it up and snap some photos for our perverse entertainment.

sulphurdave · « **Reply #91 on:** January 06, 2015, 07:34:21 PM »

I think I'm feeling a little better now.

Cal that makes perfect sense about that gasket drying up and springing a leak since I've ran it. There's still evidence of burnt oil on the pipes though leaving me to believe something was going on back with the PO.

After I removed these covers I didn't see anything unusual even when it was gunky. After I cleaned the area up some I still don't see a crack or anything. Plus it was mainly dry inside there where the sprocket is along with the old gunky grease. I know I still need to run it though. Can't run it now until I get a new gasket for that shifter cover. If that gasket wasn't bad before it's bad now because it tore when I took it off. But I'm praying that's what was leaking in the first place. Because that means I'm just a gasket away from being fixed.

sulphurdave · « **Reply #92 on:** January 06, 2015, 07:39:02 PM »

Oh and I'm not sure how tight the chain should be but I feel it's a little loose. It does have a little slack. How much should the chain move up and down if I were to grab the bottom side of the chain in the center? Or how should I measure chain tightness?

seanbarney41 · « **Reply #93 on:** January 06, 2015, 07:50:05 PM »

Everything looks good in your picks. The spot in the case just forward of the sprocket is the usual place for damage. You can see how tight clearance is there, but yours looks good. Sorry about being the voice of doom. Actually, that little screw in the center of your counter shaft is the chain oiler adjustment. I would tighten that up all the way in, at least until you get your leak figured out. Also, there is a seal behind your sprocket that may be the culprit, but it is uncommon for it to be a major leaker. 3/4-1" maximum movement on the chain, as you described...

sulphurdave · « **Reply #94 on:** January 06, 2015, 08:05:18 PM »

I've heard that from a few other people too on a Facebook group. I'll do that and change that shifter cover gasket.

sulphurdave · « **Reply #95 on:** January 06, 2015, 08:06:17 PM »

...tightening that chain oiler screw

sulphurdave · « **Reply #96 on:** January 06, 2015, 08:23:03 PM »

Pushing up on the chain in the center is about 2" of slack

seanbarney41 · « **Reply #97 on:** January 06, 2015, 08:30:21 PM »

do you know how to tension it?

sulphurdave · « **Reply #98 on:** January 06, 2015, 09:04:30 PM »

I was going to search Google or YouTube but you can fill me in

calj737 · « **Reply #99 on:** January 07, 2015, 03:36:20 AM »

Bike on centerstand, at the end of the swingarm are axle adjusters. Turn them and they in turn push or pull the axle, which in turn draws the chain as the chain is looped over the rear sprocket.

Take note, on both side of the swingarm where the axle passes thru, are "hash marks" so you can insure the axle is evenly spaced. This must be the same. If not, the axle is not square to the chain and other problems will occur.

Didn't mean for you to remove clutch cover, just sprocket cover

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Author Topic: My 73 barn find with 15k original miles! (Read 18563 times)