Author Topic: My 73 barn find with 15k original miles! (Read 19024 times)

sulphurdave · « **Reply #50 on:** December 21, 2014, 06:38:47 PM »

The tank has been rebuilt and being shipped back to me now. I have a brand new petcock waiting for its return. The carbs have been all cleaned up, rebuilt and put back together. Hopefully I have the floats adjusted correctly. Drained the Marvel mystery oil and replaced it with Honda 10w40 and a new oil filter, changed the spark plugs with genuine Honda plugs, I'll be replacing the fuel lines and fuel filters when I get the tank back. The battery has a full charge so I think I'm pretty close to firing this thing up for the first time ::praying::

sulphurdave · « **Reply #51 on:** December 22, 2014, 05:30:36 PM »

Does anyone know the size screws that hold the fuel petcock to the tank?

sulphurdave · « **Reply #52 on:** December 25, 2014, 06:09:09 PM »

Rebuilt tank and carburetors back on the bike.

sulphurdave · « **Reply #53 on:** December 31, 2014, 04:51:04 PM »

Alright I've had the bike since September and have VERY slowly worked on it here and there in my spare time and today I finally got to hear it run... kinda. I rigged up a temporary fuel tank and this is what happened.

I was very shocked it fired up but here's some pointers that I noticed.

• It would not start when choked initially
• Turned the choke off and it fired up but RPMs went to 3k steady
• Once it was running, closing the choke 100% killed it. Partially closing the choke brought the idle down but once I opened it slightly RPMs jumped back up to 3k
• Made sure idle mixture screws were 1.5 turns out. Adjusting them didn't audibly do much.
• My first thought was an air leak from the intake boots since they are really hard and don't bend much at all without heat and the 2 and 3 boots didn't slide on the intake ports as far as 1 and 4 did
• I sprayed carb cleaner around the intake boots and sure enough the idle changed slightly
• Then something happened while blipping the throttle in excitement and the RPMs jumped extremely high probably around 7-8k and wouldn't go down so I freaked out and turned the key off
• If I start it up the RPMs immediately rev extremely high again to redline

It got dark and it's time to pop fireworks with the kids so that's where I left it for now.

Thoughts?

BPellerine · « **Reply #54 on:** December 31, 2014, 05:01:51 PM »

check for sticking cables or linkage?bill

sulphurdave · « **Reply #55 on:** December 31, 2014, 06:08:15 PM »

Yea I'm gonna hook up the air box tomorrow and tinker with it some more. It was hard to tell if it was stock Honda parts in the carbs or not. What specific specs are you referring to? Floats? Idle mixture screws? I wanna know if I missed anything.

sulphurdave · « **Reply #56 on:** December 31, 2014, 08:33:43 PM »

I ordered the same size jets that were in there 110/40s because I'm running the stock air box and for now the same pipes that are on it which look and sound pretty restrictive. The only parts I reused in the carbs were the needle jet because it looked fine after the carb dunk cleaning and I reused the idle mixture screw/jet because it looked better made. The rest of the parts I used from the rebuild kit. So the needle position is in the same location as it was. Previous owner said it ran fine when the old man had it before him. I think the crack head that I bought the bike from didn't have the know-how or the money to get the bike running.

I think I found out why it all if a sudden started revving to redline. I just went back to look at it out of curiosity and noticed the throttle linkage jumped out of the bracket on the bottom. Whoops forgot to tighten it in all of the excitement. I'll hook up the air box and tinker with it tomorrow and report back.

Happy New Year everyone!

sulphurdave · « **Reply #57 on:** January 01, 2015, 07:28:43 AM »

I got the ones from Vintage CB750

sulphurdave · « **Reply #58 on:** January 01, 2015, 05:08:36 PM »

Alright here's a video from today. Let me know your thoughts.

sulphurdave · « **Reply #59 on:** January 03, 2015, 01:44:18 PM »

Nothing?

BPellerine · « **Reply #60 on:** January 03, 2015, 03:21:43 PM »

when your bike is at idle when warm and you have it set low,let it cool and use the choke to warm it up and see if it will idle back down,if you start it with no choke when cold you have to keep it going with the throttle or turn the idle up it seems,a carb sync wouldn't hurt.bill

sulphurdave · « **Reply #61 on:** January 03, 2015, 05:18:13 PM »

I will be doing a carb sync at some point I just wanted to see how close I could get it running correctly without doing much at all and just learn what it's doing and not doing. Basically I want to see what kind of running condition it was in before I got it. So far I have not touched the points, haven't adjusted the valves and even though I bought new spark plugs, when I pulled the ones that were already installed they looked brand new as if it hadn't ran at all since they were put in.

Currently if on a cold motor it won't crank if it's choked. Open choke and it starts up just fine and idles low where it should. After it warms up is where it's inconsistent. Sometimes when I rev it it'll idle down slowly. Other times it'll hang up around 3k and if I wait long enough it'll idle back down. That's of today which has only been the 4th time I've cranked it and went through the warming up cycle. Still learning what it's doing.

sulphurdave · « **Reply #62 on:** January 03, 2015, 09:02:45 PM »

From earlier today..

ozpacman · « **Reply #63 on:** January 04, 2015, 03:56:53 AM »

She's sounding pretty good to me Dave - well done!

Russ.

seanbarney41 · « **Reply #64 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 #65 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 #66 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?

sulphurdave · « **Reply #67 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 #68 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.

seanbarney41 · « **Reply #69 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 #70 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 #71 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 #72 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 #73 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 #74 on:** January 06, 2015, 04:26:51 PM »

did it leak before you changed the oil?bill

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Author Topic: My 73 barn find with 15k original miles! (Read 19024 times)