'81 CB750F trying to get it road ready.

[Chicago Ted]

Hey everyone. I just joined the forum. After nearly every google search for my problems led me to this forum, I figured it would be worth my while to become a member. I naively thought I would get into motorcycles inexpensively, so I found an '81 CB750F on craigslist. I ran when I bought it, but now that it is in my possession, issues have been springing up. I quickly realized that I am in a bit over my head, and my friend suggested I ask the knowledgeable people on "one of those web forums". So here is my humble plea for assistance. I will list the problems I have cataloged in order of apparent complexity.

1) My petcock leaks. I looked up the proper petcock for my bike (Honda part # 16950-MA4-671), and my new one just arrived today. However, it looks a fair bit different from my old one. This new one has a plastic intake, with filters on it, compared to the plain metal pipe on the old one. Also this new part has larger threads  on the nut that attaches to the tank, and the interface with the fuel line is larger than on the previous one. I went to home depot to look for a thread adapter, but all they had were parts in standard units, and I assume my honda parts are metric, yes? I was reading that the petcocks come in different sizes, but I don't know what the correct one ought to be, if the one for my bike, for my year does not fit. What do you all recommend?

2) My rear brake doesn't work. I can push it down, but it offers no resistance, beyond a bit from the spring, of course. I added new brake fluid, but the reservoir was not empty. I have tried to bleed it, but nothing comes out. When I apply suction to the bleed screw with my mouth (I read the stuff is not that bad for you to get a bit in your mouth), I see just a bit of fluid, like a few drops, eek out, but mostly bubbles, ever after I wrapped the tube with teflon tape, to make a good seal. What would you recommend I do to get the brakes working?

3)When I start the bike, I see a pretty steady stream of fuel coming out of my #4 carb. It is coming out of a hole in the bottom. If I turn off the fuel at the petcock, this stops soon after, but, logically, so does the bike. It appears to be running smoothly despite this, with a pretty steady idle and all that. What could cause this issue?

4)My choke does not have any noticeable effect when I adjust it. It appears to be connected, and when I move it, I see a lever attached to the carbs move, which I assume is correct. Any ideas there?

5) There is a tube, which looks similar to my fuel line, coming out from the inside side of the #2 carb. After I bought it, I noticed it just has a screw jammed into it, not connected to anything. What is this line, and what ought it be connected to? My friend suggested it may be a fuel return line?

Okay, those are my most pressing questions. I am sure that I am likely to have more soon enough. I appreciate all of your help in advance. I will add pictures if requested.

[seestheday]

Welcome to the forum

1. Petcocks are really problematic.  They changed almost every year.  It's also difficult to know if you are running the original tank, so you could have an 81 bike but an 80 tank with a different petcock.
Are you sure that you looked up the part number specifically for your year?  Yes every part on the bike should be metric (unless some aftermarket stuff has been added, e.g. the bolt holding on my aftermarket exhaust is standard).

2. Have you tried to bleed it with a pump like Tools often describes?  Sounds like you have a lot of air in the line and you need to flush it all out.

3. You have a stuck float valves.  Running some seafom through the tank/carbs might help, but you likely need to pull the carbs and clean them (at least polish the float valve seats).  Sometimes giving the carb a sharp tap with a deadblow hammer can get the valve to unstick, but I've personally never had success doing that.

4. No change to how the bike runs when you pull the clutch indicates that you're already running very rich.  It's looking more and more like pulling the carbs and checking/cleaning them will be required.

5. The tube is coming out between carbs 1 & 2 towards the head side of the carbs (vs. the airbox side)?  That is a vacuum vent designed to work with the auto-petcock(a.k.a. the SOS or spawn of satan).  If you're just has a screw jammed into it then a PO likely bypassed it (this is pretty standard).  Does the screw setup give a good seal?  That vent has to be sealed off for the bike to run properly if the SOS is bypassed.

[seestheday]

Oh ya, it seems more and more that Nabble SEO > cb750c.com and cb1100f.net SEO.

[Johnnylevi]

Did you order the 18mm or 16mm petcock? For my 81F I ordered a petcock for a 81F and it came as an 18mm, which did not fit my bike at all. I needed the 16mm! hope that helps. got mine on ebay for $30

2. seems like your master cylinder is gunked up. Clean it up.

3. Float height, sticky float, bad float. or! the tube inside your bowl is cracked.
4. ... do you have choke plates? haha seems like a dumb question, but if its not doing anything... or maybe the plates are getting caught up somewhere.

5. I think you're dealing with the Spawn Of Satan... Take that off and just attach a fuel line directly to your #2 carb. Just make sure to turn your fuel off when the bike isnt running.

We have the same bike and i had roughly the same problems! So feel free to shoot me an E-mail. Although the guys here are the ones who helped me through it all.

[Chicago Ted]

1. The petcock I listed is the part listed on all of the schematics I have seen for my bike. Which one do you think I should try next? I am thinking this one.

2. I was hoping not to have to invest in a single use tool. I don't think I will be bleeding brakes terribly often. I was planning on taking it to a shop once I could get it hobbling, and I figure they would be able to do it easily if I could not.  Any other tricks I could try that would not require several hours of pumping the pedal and twisting the bleed screw?

3. Where on the carb would you suggest I try giving it a whack? I already have some seafoam that I have put in the tank. Should I put it somewhere directly in the carbs?

4. I had kinda guessed that. My buddy and I were planning to give the carbs a cleaning this weekend. Can anyone point me to a good "Idiot's guide to cleaning carbs on a CB750F"? Your theory also squares with the fact that, when I gun it in first, it kinda tops out around 5-6k rpm. My buddy tells me if your mixture is too rich, you will not get full power. (I have been asking my buddy a lot of questions, in case you can't tell)

5. That is where the tube is coming out. Why is the auto-petcock so maligned? I only know it is supposed to allow you to keep the petcock on at all times. Sounds like a good idea...
I am happy to leave it as is. Would this suggest that the bike was built with an SOS petcock, and another owner down the line swapped it out? And yes, the screw is in there darn tight. It may even have some sort of epoxy keep it in.

[Chicago Ted]

I believe I ordered the 18mm, but it was too large, so I am guessing I need 16mm. What other sizes did they come in? can you point me to the one you got?

2. How would you recommend I go about doing that?

3. Okay, I will inspect all of that when I take the carbs apart this weekend.

4. So, I will meet your dumb question with a dumb answer. What's a choke plate, and how would I know if I had them?

5. It appears that is exactly what the previous owner did. I will leave it as is then, with the screw jammed on.

[seestheday]

#3 - get the very detailed carb cleaning manual from here: http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&p=157399#157399

#4 - these are choke plates.  You definitely have them unless your carbs have been heavily race modified (not likely)

[Hoosier Daddy]

Mornin Ted, welcome to the forum! With the knowledgable group you found here, you will get these problems sorted out so don't fret.

1.) This linky will help you figure out what petcock you need. CB750 Petcocks 79-83 DOHC For the life of me I have never figured out why Mother Honda jumped around so much on these bikes and petcocks... so picking up a "81 CB750" petcock isn't going to fit EVERY CB750 that was made in 81. You have to know which one you need and are buying.

2.) I don't want to assume anything, so humor me... you do know how to bleed brakes, right?
 A) open bleeder
 B) depress brake lever and HOLD IT DOWN
 C) tighten bleeder
 D) NOW you can release the lever
Have you made sure your bleeder is clear, sometimes the tiny orfice plugs and you won't get fluid out. You can try removing it completely and putting your thumb over the hole in step 3 above.
 If you have done this properly and still have the issue, I'd try loosening and tightening the hose connection at the Master Cylinder instead of the bleeder in the above process.
You don't need to completely unscrew it, just crack it open a half a turn or so, enough to allow fluid to leak past the banjo fitting. Drape a towel under the master cylinder to cover any painted parts that might get fluid on them as brake fluid is an EXCELLENT paint remover. If you don't get good fluid without bubbles here, your master cyclinder needs rebuilt.
 Once you get good fluid there you can move down to the hose connection at the caliper and repeat the process. Loosen , depress lever, tighten, release lever.
 Lastly, to the bleeder screw that you have made sure is clear...
 Keep checking the resivour level and don't let it run empty or you'll have to start all over.

 3.) you got good answers already, float level, sticking float pivot, a float that is bad (if it is the plastic ones you can shake it and listen for gass sloshing around... if so it needs replaced), dirty / gummed up in needle valve / seat, a cracked or possibly pin holes from corrosion in the tube that's in the float bowl.




4.)  You said you can see the lever move when you pull on the choke. does the lever them rotate the shaft that it is attached to? Make sure that shaft rotates on both the outter carbs (if it does the outter two it is also rotating the middle two)
 These are the choke plates... one on each carb.



 They are on the inlet side of the carbs on the end that faces the air filter box. They will close like pictured to restrict the air flow through the carbs when the choke is "ON" (knob pulled out). They should help on a cold engine only, once the engine is warm they should cause it to run rough unless you have a sever vacuum leak that allows excess air in AFTER the choke plates. Possibly cracked losse carb insulators (the rubber boots that the carbs are mounted to the head with).

5.) Make sure the hose isn't cracked and the screw has a good tight seal. That could be the cause of your vacuum leak and giving you your choke issue in #4 if it can suck in air.

[Chicago Ted]

Well, good news everyone. A lot of my problems have solved themselves. I don't know if it was the whack on the carb and/or the seafoam I put in the gas, the the ol' girl is purring now. Good/bad news on the choke. It does DO something. I started it for the first time that day, and it was sputtering very badly, with tons of smoke pouring out. I ran like that for a bit, but it did not resolve itself. Then, I tried pushing IN  (turning off?) the choke knob. Since I did that, it has started like a dream. My buddy, (who has the proper gear, I just ordered mine yesterday), road it around my neighborhood to put it through its paces today. Runs great, but it is emitting a fair amount of exhaust smoke. So, combined with the rough operation with the choke on, even from a cold start, and the smoke at full throttle, the theory we developed is that it may be running a bit rich, from the baseline. Can you guys confirm or refute this? If so, what is the simplest way to correct the issue?

I will be looking into the petcock. I got it to stop dripping for now, so it is less of an issue, as long as I never need my reserve! I think the one I linked earlier should work.

Hoosier, you assume correctly that I don't know much about what I am doing. I looked into bleeding brakes a fair bit getting into this, and that method, as you prescribe, has not been working so far. If I open the bleed screw, and lay a weight on the brake lever, and come back a half hour later, I may get a half an inch of fluid in my tube, but I can not repeat this process. If I tighten the screw, release the lever, open, and lock down the lever again, no dice, no more movement.

I may try your method with the main hose, but I am planning on bringing it to a shop regardless, just for them to give it a professional looking over. My brakes are something I would prefer not to mess around with.

BUT! I have new questions now.

The backlight behind my speedometer is out. Where can I buy a replacement, and how do I install it?

I am looking to replace the seat, because mine is old and busted and torn. What do you all recommend?

After my buddy gave it a run today, I pulled it into the garage, and saw a small puddle of oil forming. It was hard to find where it was dripping from, but I determined it was likely originating from a hole below my oil filter linked here.


I am not 100% sure what it is. It is the grey hole in the center of the picture. Is it okay that it was leaking oil? Is it okay that it appears plugged with rubbery caulk? This is the first time that we have gotten the bike really hot since it has been in my possession. The good news is that the oil seemed pretty clean.

Thanks for all of your help so far!

[seestheday]

Awesome, I'm happy it's running. That is a very good sign. Congrats :-). What colour is the smoke coming from the exhaust? The grey plug in the center of that pic is your cam chain tensioner plug. Proper fix is to replace it but it requires pulling the head. Is the leak bad? The seal is only $3 or something super cheap, but it takes a fair amount of work to get to it. Yes, pushing the choke in means turning it off.                                                                                                                                      If the bike starts easily with the choke off then it is likely jetted too rich. How long have you run it for in total? Is the smoke issue clearing up or staying the same? I'm trying to eliminate a leaking valve issue.

[Chicago Ted]

The smoke is white to grey, not the darker blueish tint of what I know to be "rich fuel" smoke.

The leak does not appear very bad. This was the first time I noticed the leak at all, but it was also the first time I really got the bike moving since I got it. I will know more once I actually take it on a trip. Would more caulk or other sealant be an acceptable solution?

I ran it for maybe 5-10 minutes. The smoke towards the end was lighter, and only present when I really let out the throttle, but more than I would expect to see from an engine running optimally. The smoke might have gotten better towards the end, but I can not be sure.

It also appears to not rev as high as it ought to. I will have to test this when I have more space, but it appears to start topping out near to 6k rpm. I will be confirming in the next week. My buddy suggests that carbs/jets running too rich would make your "wall" lower than it ought to be.

[seestheday]

Did you put any oil into the cylinders to do a wet compression test?  Do you know if the PO put some oil into the cylinders to store it?

The smoke you're seeing sounds like burning oil, but it could just be the oil that was in there from a compression test or storage burning off.  Once you run it a little longer you'll see if the smoke persists.

If it does then it's likely your valve seals that have given up the ghost and need to be replaced.  You'll need to pull the head to do this.  Do a compression & leakdown test first to check to see if you have ring/valve issues at the same time.

That said, it could be your jetting is too rich.  I can't see/smell the smoke so I can't be sure.  It would likely be easier to fix that problem first before pulling the head.

[Chicago Ted]

Well, I finally got myself a helmet and some other gear and was able to putter around my neighborhood a bit. I just wanted to be confident that I could ride it to the shop alright, possibly tomorrow. I have no idea if there is extra oil in the cylinders. I have not done a compression test, and I do not know much about what the previous owner did to it. Good news is that I did not see much smoke while I was riding today, but, then again, I wasn't looking behind me.

More issues have come up though that I was wondering about. Two main things. When I am sitting at idle, or holding the clutch in for a bit, the engine will die. I can prevent this by tugging the throttle a bit, but, I would prefer it didn't happen. Additionally, and more importantly the bike will backfire fairly often. Sometimes, the sound is not unlike a gunshot, other times its more of a *poof* sound. From what I have read this could be further evidence that my carbs are jetted too rich. Does that sound right?

[Chicago Ted]

So, after calling around to a few shops, it appears that no one is willing to work on my bike until the off season, because it is so old. This is a great pity, as i was really counting on getting some professional assistance. That said, the main thing I need to fix is my rear brake. It looks like I am on my own for this one. I  did a bit more troubleshooting, and removed the main brake line from the master cylinder, on the wheel. I then pumped the brake, expecting fluid to come out of the brake line. This is correct, yes? Nothing happened. The line is wet with fluid. It dripped a bit when I took the bolt out. Still, pumping is doing nothing, apparently. So, what sort sort of brick and mortar store would stock one of those hand pumps you guys were talking about? Is that still the way you would suggest I go with this? I worry that air in the lines is not my issue. My pumping mechanism at the reservoir may be shot.

[TOOLS1]

You need a master cylinder rebuild kit. Check out vintagecb750.com
TOOLS

[cdaiscool]

Given your name, I'm assuming you're in Chicago or the area. I don't know any shops out there, but I'm amazed you can't find an old bike shop that only does the old stuff. Heck, I live in a college town and within 30 miles there's 3 plus a place that can get me parts on top of those others.

If you're in the Ann Arbor region of MI, I'd be willing to give you a hand with it. Otherwise, yes these guys are all correct as to what your issues were/are. Not that I have much wrenching experience.

[Johnnylevi]

I would go ahead and take off my carbs and check them out. It sounds like maybe your idle jets are plugged a bit. Mine would do the same thing. Sit at idle and then slowly die out till I gave em a good clean. You should also probably do a valve lash. Seestheday has a video of it on his website thats pretty helpful

[seestheday]

Also, for the price you would have to pay a mechanic to really get your bike running top notch you might be better offer financially just buying an already solidly running bike.

Old bikes are for people that can fix them, or don't mind spending money for a damn cool bike.

If the bike has been sitting for a while, the carbs likely need to be cleaned.  That manual that I linked to walks you through everything you need to do.  Given what you've posted already, I'm positive you have the mechanical ability to do it yourself.  It will just take you time.

You may end up needing to buy carb insulators if they're cracked ($80) and a rebuild kit ($172 http://randakks.com/products/randakks-own-dohc-four-master-carb-overhaul-kit).  

If you're feeling frugal there is nothing wrong with just taking them off and giving them a good cleaning and putting them back on.  You might be lucky and not need a rebuild kit.  Or you could tear them apart and find out that all of your gaskets/o-rings have disintegrated and you'll be waiting for the rebuild kit to arrive in the mail.

If your insulators are old and rock hard, you might crack them when pulling the carbs off and not be able to tell.  This could result in the bike running worse when you put your cleaned carbs back on.  This happened to me.  New insulators fixed it.

You will definitely need a carb sync tool (~$80) unless you have access to one already.  These are so worth the money.  With carbs out of sync you'll have a rough running beast of a bike, and with in sync carbs it will run silky smooth.

[Chicago Ted]

A bit of "good" news. After great difficulty removing the hex bolts that attach it, I have finally managed to pull out my master cylinder for the rear brakes. Unsurprisingly, the piston is 100% seized. Just filled with rust and goodness knows what else. Right now, I am boiling it, since I hear that that can clear out a seized piston, but I am not optimistic. The rubber seal between the brake pedal and the piston also tore as soon as I have it the gentlest tug. I am expecting that a simple rebuild kit may not cut it in my case. I think I need a new master cylinder. Which would you guys recommend?

On the carb front, I have made some progress. I found that a few of the hoses that connect the airbox to the carbs were not sitting tightly, and had no tension ring on them. I went out and bought some rings, and installed one. However, one of the ring-less hoses is on the #2 carb, and the hose is not seated all the way. I have played about with a screw driver, trying to slip that thing on, but no dice. There is just not enough room to work in there. Any suggestions?

I was happy to find this problem, thinking it could solve some things for me, but I was then thinking that a bad seal on the airbox would let in TOO MUCH air, making things run lean, but I am having backfires, and the any choke will instantly kill the engine, which I understood to be symptoms of running rich. Any idea what problem or combination of problems would cause all the symptoms?

[Johnnylevi]

Is the hose fuel or vacuum? Is the hose running the the front of the carbs or middle? pictures really help. does your bike back fire or acceleration or deceleration? Have you played with your air/fuel mixture screws at all? As far as the rear master cylinder goes, my friend had the same problem and ended up ordering a used one from ebay. I hear that those air hoses on the air box( that top slope-ey T piece) can get filled up with gunk. Maybe check that?