Topic: Hard starting '78 cb750k leaks fuel, sounds like a can of rocks

[distincthead]

Hi all,

I just bought a '78 CB750K yesterday. Rode it home 85 miles fine, but it has issues. First and foremost is that the kid I bought it from had the carbs 'rebuilt' recently. I noticed the carb closest to my right food leaks from the bowl because the gasket got swollen and now there's a gap. I figure I'll take the bowl off, and try to get it back on. I'm not sure if this is related, but the bike is almost impossible to cold start, especially if it's been sitting for more than 8 hours up to a few days. It starts right up no problem once the engine is warm, even with the kick start. But once it's cold, it just turns over endlessly, sounding as though it wants to catch but just wont go. You can smell gas pretty bad when trying to start it like this too. If you pump the throttle a few times while trying to start it, it makes even more promising "want to catch" sounds but still wont go.

Once it's started, it sounds like the transmission is full of marbles. It rattles pretty bad, less so if the clutch is in, but still audible. You can't hear it up past, say, 2500 RPM. The gears also whine when accelerating and they make a very loud CLUNK when you shift them in. I've also noticed that the bike surges a bit when you engage the gears, especially the low end, almost as if the clutch wasn't fully engaged. What's all that about? I've read there is a common "clutch rattle" but this sounds worse than other descriptions I've read. Perhaps the carbs are so out of sync it sounds worse (the engine vibrates way more than other 750's I've seen at low idle).

So that's where I'm at. The bike has 19k on the odo, is in otherwise gorgeous shape for a 30-year-old bike and the engine pulls really strong even with the carbs being obviously out of sync pretty bad (or at least that's my assumption).

[seaweb11]

Is your choke operating fully? Perhaps it has slipped and not covering carb completely.

[shoemanII]

sounds like the PO of my bike had his paws on yours too.

if compression is good, i'd suggest a standard tune-up.  someone here has a preferred order, but something like: 
adjust valves
gap points
check for carb boot leaks
new plugs
set pilot screws 1.5T out from fully seated
clean/new air filter
adjust cam chain tensioner

there's more i think.  once sorted out, these bikes go like stink.  at least compared to other bikes of their time.
bobp       

[BobbyR]

Hi all,

I just bought a '78 CB750K yesterday. Rode it home 85 miles fine, but it has issues. First and foremost is that the kid I bought it from had the carbs 'rebuilt' recently. I noticed the carb closest to my right food leaks from the bowl because the gasket got swollen and now there's a gap. I figure I'll take the bowl off, and try to get it back on. I'm not sure if this is related, but the bike is almost impossible to cold start, especially if it's been sitting for more than 8 hours up to a few days. It starts right up no problem once the engine is warm, even with the kick start. But once it's cold, it just turns over endlessly, sounding as though it wants to catch but just wont go. You can smell gas pretty bad when trying to start it like this too. If you pump the throttle a few times while trying to start it, it makes even more promising "want to catch" sounds but still wont go.

Once it's started, it sounds like the transmission is full of marbles. It rattles pretty bad, less so if the clutch is in, but still audible. You can't hear it up past, say, 2500 RPM. The gears also whine when accelerating and they make a very loud CLUNK when you shift them in. I've also noticed that the bike surges a bit when you engage the gears, especially the low end, almost as if the clutch wasn't fully engaged. What's all that about? I've read there is a common "clutch rattle" but this sounds worse than other descriptions I've read. Perhaps the carbs are so out of sync it sounds worse (the engine vibrates way more than other 750's I've seen at low idle).

So that's where I'm at. The bike has 19k on the odo, is in otherwise gorgeous shape for a 30-year-old bike and the engine pulls really strong even with the carbs being obviously out of sync pretty bad (or at least that's my assumption).

I have the same bike and I hear ya, here is some soothing advice;

The leak you will have to fix the normal way.

They all bang into 1st gear a bit, HDs do it also. At low idle it is quieter. If you still have two cables you can force the idle lower by twisting the grip and kick it into 1st.

They are cold blooded but not to the extent you describe, the choke may be not closing enough.

The clutch basket rattles, there is a fix, but it is somewhat involved. If the carbs are out of synch, the rattle is worse. Well synched it is not bad at all.

I am not sure what you mean by surge, but they are jumpy in 1st and 2nd gear.

When you are looking at the choke, there is also a fast idle cam which deploys when you pull out the choke.

The carb synch on these carbs has much less effect on overall performance that earilier models with multiple cables. The slides are locked together so wide open is wide open on all slides. (I read this in Vintage Bike Magazine).

You have abit of sorting out to do, but compared to some of the horror stories we get here, you seem to be in good shape.

 

[speedy gonzalais]

if the gasket bowl has swolen it wouldnt prize the bowl away from the carb, it sounds to me like th po probably done the carbs himself and didnt have much of a clue and as said above the rattle is normal but will be made much worse if carbs aint synced my 650 sounded like crap wen i opened it up it used to sound like the crank was gona break up then after i stripped the carbs unblocked the idle jets had them tuned and synced it sounds lovley.
as for cold start are u sure its running on all four ? if find the easiest way to check is pull all the plugs give them a clean and run the bike for a while take them back out again and check they aint still clean also the should be light brown, If there black then its def a carb prob

[distincthead]

Ok, so I checked for vacuum leaks and WOW. The clamps for the carb boots were all so loose that I could spin them. Tightening them up changed things drastically, I will see tomorrow how differently it cold starts. I sprayed carb cleaner on part of the case and the idle jumped, so I'm thinking I need to gasket that too.

[eurban]

You have purchased a 30 year old bike with a maintenance / repair history of unknown quality.  Based on your running problems and your more recent findings, I would probably be wise to go over the bike from head to toe. Be particularly wary of the carb "rebuild" as it requires some effort and attention to detail which I would be very suprised if the kid's "rebuilder" was able to muster.  The evidence is not in the rebuilders favor.  Do some searching from the "Tech Forums" page on 78 carbs, pilot jets etc etc as there are a number of posts on the ins and outs of getting these carbs working well. . .Here's a link for the 78 factory repair manual:
http://www.mediafire.com/?5ttzdg9yyuz
Have fun!

[distincthead]

You have purchased a 30 year old bike with a maintenance / repair history of unknown quality.  Based on your running problems and your more recent findings, I would probably be wise to go over the bike from head to toe. Be particularly wary of the carb "rebuild" as it requires some effort and attention to detail which I would be very suprised if the kid's "rebuilder" was able to muster.  The evidence is not in the rebuilders favor.  Do some searching from the "Tech Forums" page on 78 carbs, pilot jets etc etc as there are a number of posts on the ins and outs of getting these carbs working well. . .Here's a link for the 78 factory repair manual:
http://www.mediafire.com/?5ttzdg9yyuz
Have fun!

Yeah the receipt from the "rebuild" says it was done at the local Honda dealership, but I don't know. He must have screwed with it somehow. I can't imagine a dealership leaving gaskets bulging out of the float bowls, "forgetting" to tighten all the clamps for the boots, etc. I'm of a good mind to completely remove the carbs from the bike and go over them individually myself. As for the rest of the bike, I'm going to do as much as is possible in the realm of a tune up. I've already got plugs, wires, oil, filter sitting around. I'm going to grab some misc gaskets, air filter, etc. I might try to fix the rattling clutch issue, and while I'm at it inspect the engine a little on the inside. I've gone around and lubed all the cables, checked hoses, fittings, etc. New battery as the old one looks horrible.

All in all, I'm actually pretty satisfied with my new daily driver given she's in such good cosmetic order, and the engine doesn't sound screwy now that I've got the vacuum leaks sorted out well...

I appreciate the help, that link was great! 

[seaweb11]

Well 7 hours went by, did you look at the choke?

[eurban]

30 year old air cooled engines are going to be noisier than modern stuff.  Rattling clutch noises are not going to go away completely.  Your 78 has the updated clutch which should be less prone to rattle than the earlier bikes.  Brand new parts might be a bit quieter but it still won't go away completely.  Also some of the noise may be attributed to the primary chains.  A smoothly running engine will help a lot so after the proper tune up, including a carb synchronization, you should find that the noises are reduced.  Basically I would suggest leaving the clutch alone for now unless you know for sure that there are performance issue with it.  The factory manual has a good bit of detail on the carb rebuild but you should read through as much info as possible on this site on the ins and outs of the 77/78 carbs.  Use the search function!!   Pulling and cleaning the pressed in pilots (factory manual will tell you not to do this) is a must and it is very doubtful that the Honda dealer did this.  Check for accelerator pump function, adjust the float levels carefully and then do a reality check of the fuel levels using clear tubing on the bowls overflow spiggots.  Bench synchronize the carbs while you have them off. . . .Oh and you will find that the bowl gaskets expand in the presence of gasoline.  Let them air out for a few days and they should shrink enough to fit properly in their grooves.  PO probably had difficulties with this.  New float needles are cheap and would be advised . . . .

[distincthead]

Ok, new info...

I was thinking it was flooding because I smelled gas when trying to start it. Well the way it kept trying to catch made me think that the gas smell was solely from the leaking float bowl, and that the engine itself was really starved for gas. So I sprayed some starter fluid into the airbox and it started right up. I had to hold the throttle open to 2k RPMs until the engine got warm enough or it wouldn't stay running. After that it would run on it's own. After it warms up, it starts right up with no issues, no starter fluid, etc. So basically when the engine/carbs are cold it acts fuel starved. The gas tank on the inside is quite clean and pretty much rust free, but that doesn't mean crap didn't get into the filter or something. What should I check next?

[BobbyR]

Ok, new info...

I was thinking it was flooding because I smelled gas when trying to start it. Well the way it kept trying to catch made me think that the gas smell was solely from the leaking float bowl, and that the engine itself was really starved for gas. So I sprayed some starter fluid into the airbox and it started right up. I had to hold the throttle open to 2k RPMs until the engine got warm enough or it wouldn't stay running. After that it would run on it's own. After it warms up, it starts right up with no issues, no starter fluid, etc. So basically when the engine/carbs are cold it acts fuel starved. The gas tank on the inside is quite clean and pretty much rust free, but that doesn't mean crap didn't get into the filter or something. What should I check next?

Check your choke when you have time. I installed an inline filter to avoid getting crap into the carbs, I have a clean tank, but I can't see everywhere. When you lay the bike up for the winter, go over the carbs. Most dealerships hire kids who really don't know these older carbs, and some of them are sloppy to boot. Speaking of boots, the carb boots get hard with age and new ones are not that expensive.

[eurban]

Whats wrong with the previous suggestions?  Visually confirm that the choke plates are completely closing the carb intake when you pull up the remote choke knob.  If the choke is operating properly then it is time to properly clean and adjust your carbs, particularly the idle circuit.  As I have said before it is unlikely that the carb overhaul was done properly, particularly the cleaning of the idle circuit.  Your issues are consistent with choke problems and or impaired idle circuits. Also, it is foolish to start a bike with a leaking float bowl; if you haven't fixed this issue do so ASAP

[distincthead]

Whats wrong with the previous suggestions?  Visually confirm that the choke plates are completely closing the carb intake when you pull up the remote choke knob.  If the choke is operating properly then it is time to properly clean and adjust your carbs, particularly the idle circuit.  As I have said before it is unlikely that the carb overhaul was done properly, particularly the cleaning of the idle circuit.  Your issues are consistent with choke problems and or impaired idle circuits. Also, it is foolish to start a bike with a leaking float bowl; if you haven't fixed this issue do so ASAP

Nothin's wrong with the previous suggestions I was just adding new info in case it changed those suggestions in any way.

Unfortunately I ride during the winter so there's never much down time to work on stuff as it's my primary transportation unless it's snowing real bad. I'll check the choke plates today. I didn't realize how dangerous/foolish it was to start it, I just figured I wouldn't ride it. Then again gasoline vaporizing and heating up around all that hot metal can't be good 

Thanks for all the ideas.

[BobbyR]

Whats wrong with the previous suggestions?  Visually confirm that the choke plates are completely closing the carb intake when you pull up the remote choke knob.  If the choke is operating properly then it is time to properly clean and adjust your carbs, particularly the idle circuit.  As I have said before it is unlikely that the carb overhaul was done properly, particularly the cleaning of the idle circuit.  Your issues are consistent with choke problems and or impaired idle circuits. Also, it is foolish to start a bike with a leaking float bowl; if you haven't fixed this issue do so ASAP

Nothin's wrong with the previous suggestions I was just adding new info in case it changed those suggestions in any way.

Unfortunately I ride during the winter so there's never much down time to work on stuff as it's my primary transportation unless it's snowing real bad. I'll check the choke plates today. I didn't realize how dangerous/foolish it was to start it, I just figured I wouldn't ride it. Then again gasoline vaporizing and heating up around all that hot metal can't be good 

Thanks for all the ideas.

You can knock off the carbs on a Saturday/Sunday afternoon. If the carb has been cleaned of the crud even faster. You need a strand of wire from a lampcord or an old high E guitar string. Just rod it through all if the openings and spray the debris out with some spray Gumout. Check to see if the accel pump is pumping, from that point on you are good to go. 

[seaweb11]

Forget checking the choke.....I have found sprinkling pixy dust over the entire bike gets rid of most of tte PO problems


or check your choke

[speedy gonzalais]

Ok, new info...

I was thinking it was flooding because I smelled gas when trying to start it. Well the way it kept trying to catch made me think that the gas smell was solely from the leaking float bowl, and that the engine itself was really starved for gas. So I sprayed some starter fluid into the airbox and it started right up. I had to hold the throttle open to 2k RPMs until the engine got warm enough or it wouldn't stay running. After that it would run on it's own. After it warms up, it starts right up with no issues, no starter fluid, etc. So basically when the engine/carbs are cold it acts fuel starved. The gas tank on the inside is quite clean and pretty much rust free, but that doesn't mean crap didn't get into the filter or something. What should I check next?

starter fluid or easy start tut tut
just pull the carbs and check them fuel shouldnt piss out of the gap in the bowl as its not ment to get that high, and ever heard of the text book mechanic cus thats what u will find in most dealerships

[troppo]

Forget checking the choke.....I have found sprinkling pixy dust over the entire bike gets rid of most of tte PO problems


or check your choke

So where do you get your pixie dust from?
I cant find it anywhere here in Aus, i can find rocking horse schit but not pixie dust

[seaweb11]

Trust me, finding pixy dust on a Island full of 55 yr old hippies is easy

It's amazing stuff, perhaps I should export it

[Hush]

You can send some to us Kiwis too, we only have Taniwha dust and that usually just slows stuff down.

[KingCustomCycles.com]

I always wipe the dust off when I get a bike, I had no idea......

[distincthead]

Okay so the choke wasn't closing all the way - good call! Bike cold starts MUCH better now. Took the carbs off to fix that leaky bowl. It seems the rim of the bowl is warped or something and it doesn't sit all the way flush with the carb body. I guess I need a new one. Anybody with a source?

I checked all the jets that I could see, but was hesitant to completely dismantle the carbs today as I needed the bike this afternoon. Everything looked great, not a spec of crud or varnish anywhere. It still doesn't want to idle below 1500. The lower you go the more the tach jumps and if you bring it to 1k or under it'll stall itself. I checked the accel pump and it looked ok, everything moved freely, no passages clogged, but it still stalls out if you crack the throttle from idle in neutral.

So I'm thinking, just for peace of mind, that I'm going to completely dismantle and clean the carbs once I get a few more tools to do it right. Should I get a complete rebuild kit for the accel pump? Do they make 'em? The carb jets look good enough to just clean up, don't need replacing (they're still shiny new brass).

Can someone point me to a nice how-to or detailed thread on carb rebuilding? I've got my manual but sometimes real life advice comes in handy. The search function didn't turn up much for me. This is a 1978 and I found a lot of stuff on the k6 and earlier carbs which I know are different.

Aside from that, the rattle is still there and loud as ever especially when cold. It also makes a weird deep ping sound sometimes immediately after I drop the clutch into a shift - not a clunk, you can't feel it, just a resonant, medium pitched sound that I can't describe. It's almost like if you were to tap a wrench on a long pipe - somewhat musical and definitely metallic.

Final question: Can carbs be so out of sync that it wont idle below a certain RPM?