Author Topic: my first bike build, 1977 CB750K (Read 6169 times)

apope930 · « **on:** August 10, 2015, 10:14:02 PM »

Hi guys, been reading on here for the past couple weeks but just now registered. Seems like this is the biggest online community for these bikes. I've done a fair share of wrenching on older air-cooled VWs and many dirtbikes but never on a real motorcycle. So I'm sure ill have some questions along the way. Anyways I picked up this gem this pasted weekend from a buddy that needed money. He bought the bike last winter and immediately replaced the battery, chain, and hub along with "cleaning the carbs" with intentions of getting it on the road but unfortunately it sat under his shed. I gave him $700 for the bike with a clean title. Soon as I got it home, I threw a charger on the battery, drained the tank and poured fresh gas in it and tried to start her up. To my surprise it spun over strong (i have $hit luck) but obviously wouldn't start, next I pulled a plug to check for spark and we're good there. Tried one more time and was able keep it running on starting fluid. All the research I've been doing about disassembling and rebuilding the carbs have been the earlier style however these are the "keyhole" type which I'm skeptical about however I'm gonna try them out. I have a carburetor synchronizer otw I ordered from Amazon along with a rebuild kit and some different main jets. My question is can someone point me in the right direction as to going about dissembling the whole rack? I pulled them from the bike (they are filthy) and removed the bowl, manifold plate thing, and the bar that holds the choke cable. I feel that I need to mark the position of the throttle linkage thats on the bar before removal can anyone confirm this? Thanks a lot hope to post more progress soon!

enwri · « **Reply #1 on:** August 11, 2015, 02:45:48 AM »

Unless O rings on the fuel lines between the carbs were leaking, splitting them completely apart isn't really necessary, it's the insides that need to be cleaned, jets pulled out, emulsion tubes and all the internal passages from the mouth of the carbs, air to the idle screw hole, pilot jet hole, to the little holes in the throat above the idle screw hole, the emulsion tube air supply passages.
Floats and needle valves , float levels set by clear tube when they're going back on after you've cleaned them.

calj737 · « **Reply #2 on:** August 11, 2015, 03:35:15 AM »

To add to that, do not use replacement brass from rebuild kits unless the brass is Keihin brand. These items typically don't wear out but do require complete cleaning. And there should be no need to re-jet the carbs simply because it won't start. Clean the internals to surgical level and enjoy the bike.

Also, do not overlook cleaning the inside of the tank. It too needs to be super clean. Any debris in the tank will be pulled into the carbs and undo any cleaning you've done.

apope930 · « **Reply #3 on:** August 11, 2015, 06:17:10 AM »

Thanks for the input guys, I planned on getting a new pump diaphragm and a carb rebuild kit from vintagecb750. Is this not a good kit? Also I'm gonna run the cycleX 4 to 1 sidewinder exhaust and K&N pods that was my reasoning for bumping up the main jet to a #120. Seeing most guys would also increase the pilot jet also should I leave mine alone or drill it out?

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apope930 · « **Reply #4 on:** August 11, 2015, 06:56:53 AM »

And the factory fuel tank is definitely dirty, also has 2 larger size dents in it. For tuning/getting it going I'm using an auxiliary fuel tank. I'm only 5' 7" 140lbs so I hate how large the factory tank is. I'd like to source something a little smaller and then I'll have it boiled out and sealed professionally.

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enwri · « **Reply #5 on:** August 12, 2015, 01:44:20 AM »

I can't say anything about that particular kit, but what I've read it seems some kits, the rubber parts, needle tips, O rings and gaskets react badly to the ethanol fuels and swell.
The brass parts aren't always the same size as what is stamped on them, so people generally reuse their old brass, it rarely wears anyway, damaged by improper cleaning is more likely (reamed out with hard wire,)

This will bump it back up top where someone who has used that brand will see it and say if its ok or not.

apope930 · « **Reply #6 on:** August 12, 2015, 08:15:35 AM »

Lucky I have a few gas station in town that sell non ethanol fuel most are just 87, but one has ethanol free 93. I soaked the bowls, jets, needles,slides, and the bottom piece for the pump diaphragm (I'm having trouble removing the little ball in the bottom, spring and Teflon piece are removed) in a simple green/water mix over night with a aquarium air pump in the bottom of the bucket to agitate the solution. Pulled it out this morning and sat all the parts in soapy water, I was in a hurry but everything seemed to look very clean. I think I'm gonna try the boiling lemon juice/water to clean the carb body's and finish them off with soda blasting. Pics to come when I get home

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apope930 · « **Reply #7 on:** August 12, 2015, 01:11:55 PM »

So everything looks pretty good, well a lot better than it did. However after blowing the needles off I noticed they kinda wobbled inside the slide, I don't remember them doing this during removal. In the pic provided you can see a c-clip but want you can't see is a spring that looks awfully crooked. Was there some type of rubber down there I didn't know about that could have disintegrated causing this to happen?

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calj737 · « **Reply #8 on:** August 12, 2015, 03:12:19 PM »

Be careful using lemon juice on your carb bodies. The acid will turn the alloy dark grey very quickly.

apope930 · « **Reply #9 on:** August 12, 2015, 09:07:46 PM »

The bent spring I was speaking about earlier. Can I just straighten it out by hand or where can I source a new one? I ended up boiling the diluted lemon juice then just pouring it into a bucket and let them sit frequently shaking it. Didn't let them soak longer than 30 minutes. It really ate through some $hit though. I then transferred them to boiling water with just a dab of soap.

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apope930 · « **Reply #10 on:** August 13, 2015, 04:20:03 PM »

Just waiting on the carb rebuild kit now. I also have an issue where the exhaust pipe on the #1&2 cylinder rattles. The bolts there are tight but I can wiggle the exhaust pipe.

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calj737 · « **Reply #11 on:** August 13, 2015, 04:32:44 PM »

Did you install new copper crush gaskets between the header and port?

apope930 · « **Reply #12 on:** August 13, 2015, 04:49:44 PM »

I have not, but I'm assuming that's what I need? I just noticed how lose it was while removing the factory exhaust.

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SOHC4 Cafe Racer Fan · « **Reply #13 on:** August 13, 2015, 05:21:51 PM »

I was going to suggest that but I just assumed . . .

calj737 · « **Reply #14 on:** August 13, 2015, 05:34:48 PM »

Quote from: apope930 on August 13, 2015, 04:49:44 PM

I have not, but I'm assuming that's what I need? I just noticed how lose it was while removing the factory exhaust.

That would be an "opps" moment

enwri · « **Reply #15 on:** August 14, 2015, 05:57:46 AM »

Quote from: apope930 on August 12, 2015, 01:11:55 PM

So everything looks pretty good, well a lot better than it did. However after blowing the needles off I noticed they kinda wobbled inside the slide, I don't remember them doing this during removal. In the pic provided you can see a c-clip but want you can't see is a spring that looks awfully crooked. Was there some type of rubber down there I didn't know about that could have disintegrated causing this to happen?

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Is that a crack in the overflow tube in the photo of the dirty bowl?

Also, did the pilot jets come out and get cleaned as well? they have their own little emulsion tube thing that also gets dirty.

apope930 · « **Reply #16 on:** April 12, 2016, 11:28:43 AM »

Made a lot of progress on this in the past couple months, will update when I get home with pics

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apope930 · « **Reply #17 on:** April 15, 2016, 07:35:52 PM »

For some of you old timers, I laced my rear wheel for the first time, ik I have the pattern right however with just about 2 threads showing on each spoke the inner spokes are loose enough to wiggle around however the outers are tight due having to bend them in

apope930 · « **Reply #18 on:** April 15, 2016, 07:36:09 PM »

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apope930 · « **Reply #19 on:** April 16, 2016, 06:23:39 AM »

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apope930 · « **Reply #20 on:** July 16, 2016, 03:36:53 PM »

made a little progress the last couple weeks

apope930 · « **Reply #21 on:** July 16, 2016, 03:53:42 PM »

who do you guys recommend sending the carbs to to get them rebuilt? I have the kit but my new job requires me to work 60+ hours a week and I don't have time or anything good enough to clean them with. Ideally i'd like to have them soda blasted or ultrasonically cleaned

SOHC4 Cafe Racer Fan · « **Reply #22 on:** July 16, 2016, 04:14:24 PM »

Quote from: apope930 on July 16, 2016, 03:53:42 PM

who do you guys recommend sending the carbs to to get them rebuilt? I have the kit but my new job requires me to work 60+ hours a week and I don't have time or anything good enough to clean them with. Ideally i'd like to have them soda blasted or ultrasonically cleaned

James aka harisuluv (http://forums.sohc4.net/index.php?action=profile;u=16522) is one of if not THE forum's best carb rebuild vendors. I believe he's in Arizona. His work is first rate. Your carburetors will come back looking and functioning like new.

SOHC4 Cafe Racer Fan · « **Reply #23 on:** July 16, 2016, 04:17:52 PM »

I like the front end, but I am not a fan of Firestone tires. Hipsters (who give zero F's as to ride-ability) like them for "looks" only. They are somewhat dangerous -- terrible edge grip. Have fun pushing through a turn and dragging you side on the pavement. Unfortunately, what you gained in handling and braking with that modern front, you just tossed away in tire choice.

Sorry, just my $0.02!

apope930 · « **Reply #24 on:** July 16, 2016, 04:33:21 PM »

Thanks for the info on the carbs.

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Author Topic: my first bike build, 1977 CB750K (Read 6169 times)