Topic: 76 CB400F Carbs leaking after rebuild

[M 750K6]

On my 750 I leave the rubbers on the head and tighten the carbs into them first. I put the airbox on last, without the air filter section. Gentle heat on the rubbers pays dividends. Initially it was difficult because the air box seemed a fraction too far from the carb intakes, putting some tension on the rubbers. I filed the holes each side on the airbox mounts. This gave a couple of mm and it all fits up easily now.

The 750 is a totally different animal.  It is far easier than the 350F, 400F and 500/550 assembly. No comparison.

Apologies. Hope you fix it.

[Mark1976]

On my 750 I leave the rubbers on the head and tighten the carbs into them first. I put the airbox on last, without the air filter section. Gentle heat on the rubbers pays dividends. Initially it was difficult because the air box seemed a fraction too far from the carb intakes, putting some tension on the rubbers. I filed the holes each side on the airbox mounts. This gave a couple of mm and it all fits up easily now.

The 750 is a totally different animal.  It is far easier than the 350F, 400F and 500/550 assembly. No comparison.

500/550 is an absolute nightmare...

[Kelly E]

On my 750 I leave the rubbers on the head and tighten the carbs into them first. I put the airbox on last, without the air filter section. Gentle heat on the rubbers pays dividends. Initially it was difficult because the air box seemed a fraction too far from the carb intakes, putting some tension on the rubbers. I filed the holes each side on the airbox mounts. This gave a couple of mm and it all fits up easily now.

The 750 is a totally different animal.  It is far easier than the 350F, 400F and 500/550 assembly. No comparison.

500/550 is an absolute nightmare...

Yes it is!

[Deltarider]

On my 750 I leave the rubbers on the head and tighten the carbs into them first. I put the airbox on last, without the air filter section. Gentle heat on the rubbers pays dividends. Initially it was difficult because the air box seemed a fraction too far from the carb intakes, putting some tension on the rubbers. I filed the holes each side on the airbox mounts. This gave a couple of mm and it all fits up easily now.

The 750 is a totally different animal.  It is far easier than the 350F, 400F and 500/550 assembly. No comparison.

500/550 is an absolute nightmare...

Yes it is!

I can do all maintenance on CB500/550 oldstyle carbs, all, with carbs in situ. I admit that having had the rack on the bench/table at least once, helps. And so do the allen bolts which replaced the 16 floatbowls crossheads.

[marsbound]

Okay, I got them all on, but now how is the best way to sync the carbs? At what speed should I have the bike at? What is a good vacuum reading for that engine speed?

[jonda500]

engine idling - use a fan to keep it from getting too hot as syncing can sometimes take longer than expected

vacuum reading number isn't important - you just want them all reading the same number

John

[marsbound]

Copy. I must've messed up the slide too much on one of them, because the bike won't start now. I think this bike is going to end up driving me crazy! I'll try again in the morning, hopefully it'll fire up with some choke on it and I can get it tuned.

On another note, it seemed like when it was idling at the published 1,200 rpm it really wanted to stall out when I twisted the throttle. Is that normal, or is there some adjustment that'll get rid of that?

[marsbound]

She runs!!! But now I can't get her to idle at 1,200. The carbs are all within a few mmHg of each other no matter what the engine speed is, but I can't seem to get her to settle down to a comfortable idle. I am not using the choke, and the fast idle screw isn't touching that cam on the choke shaft. She'll fire up, idle around 1200-1400 for a minute or two, then jump to 2k-3k and stay there until I loosen the idle adjustment screw...then come back down to 1200 and promptly die. I've read that it could be a sync issue (which I think I've eliminated), or a vacuum leak. Where could there be a vacuum leak? Is it just the o rings to the top end?

EDIT: Just adjusted the mixture screws slightly (1.5 turns out instead of 2 turns out) and now the #4 carb is leaking...again...

[Mark1976]

   I don't see where you clear tubed to verify float height (I could be wrong), and the sync adjusters are probably turned too far out, hence why your idle screw doesn't contact the throttle stop.
post a pic..

[marsbound]

Not sure how to use the tubes to check fuel height, specifically what the height should be at. Got any pics or tips on that one?

[Mark1976]

A basic intro as to what your trying to achieve.

Do a search,  there's plenty out there on it.

[Mark1976]

  If your adjusters are out much more than this, you're probably out too far with the adjuster's..

[marsbound]

  If your adjusters are out much more than this, you're probably out too far with the adjuster's..

Alright, I screwed in the adjusters by several turns and tried synching them again and, lo and behold, the bike started running like a dream! So of course, when I pulled the spark plugs to check the mixture the HT leads fell apart in my heads. Drat. Looks like I'll be waiting a bit for the new coils and leads to come from Charlie's place. Any other upgrades y'all think I should do since she probably won't run again for another month??  XD

[marsbound]

A basic intro as to what your trying to achieve.

Do a search,  there's plenty out there on it.

copy, thanks. My HT leads fell apart after I had finally gotten the bike running, so it looks like I'll be tooling around on the bike for a while until the replacement coils get here.

[marsbound]

okay, ht leads are fixed and the bike idles pretty nice, but now she dies when i roll on a bit of throttle. if i really roll it on fast, it'll lug for a split second before jumping up the RPMs. my initial guess is that i've got the carbs pulling too much vacuum, and probably the mixture set too lean? Here's a photo of where the carb adjustments are at. all the mixture screws are two turns out, and here's a photo of the spark plug in the #1 cylinder with those settings.

[Deltarider]

Me personally I have given up reading/interpreting plug noses after idling. I have made it a habit to idle as short as possible. Once the chokes are fully reopened, which is within 30 seconds, I start ridng and maintain a somewhat raised idle at the first two traffic lights. Then I reduce the idle to say 1200 rpm.
With these carbs, which lack acceleration jets, you should focus on a good driveability. I therefore like to think of the slow jets as auxillary jets that happen to double as idle jets. After all Honda called them slow jets and not idle jets. Trying to achieve the right color on the plugs at idle, is quite a challenge. Let's not forget gasoline characteristics have changed considerably since mid 70s.
My advice: take your bike and yourself for a ride and stop endless tampering which leads nowhere.

[marsbound]

Hm. Got the bike back together and running (even at low throttle settings, it keeps running now!). But now, my problem is that if I try to add lots of throttle, the bike just won't keep running. It'll top out at about 5k RPMs and then die unless I release the throttle quickly! Also, and this might be related, but I can't get the choke control to achieve the prescribed 3.5-4.5k RPMs throughout the choke range. past about halfway on the choke the bike just dies.

[Nurse Julie]

Hm. Got the bike back together and running (even at low throttle settings, it keeps running now!). But now, my problem is that if I try to add lots of throttle, the bike just won't keep running. It'll top out at about 5k RPMs and then die unless I release the throttle quickly! Also, and this might be related, but I can't get the choke control to achieve the prescribed 3.5-4.5k RPMs throughout the choke range. past about halfway on the choke the bike just dies.

Interested to know where you got the info '3.5-4.5k RPMs throughout the choke range'?, I don't think I've ever seen or heard that before. If the choke isn't needed, ie, engine is warmed up enough, giving it more choke will cut the engine out.

[marsbound]

Hm. Got the bike back together and running (even at low throttle settings, it keeps running now!). But now, my problem is that if I try to add lots of throttle, the bike just won't keep running. It'll top out at about 5k RPMs and then die unless I release the throttle quickly! Also, and this might be related, but I can't get the choke control to achieve the prescribed 3.5-4.5k RPMs throughout the choke range. past about halfway on the choke the bike just dies.

Interested to know where you got the info '3.5-4.5k RPMs throughout the choke range'?, I don't think I've ever seen or heard that before. If the choke isn't needed, ie, engine is warmed up enough, giving it more choke will cut the engine out.

Sure! I just grabbed it from the shop manual. It's in the front in the maintenance section:

[Nurse Julie]

Hm. Got the bike back together and running (even at low throttle settings, it keeps running now!). But now, my problem is that if I try to add lots of throttle, the bike just won't keep running. It'll top out at about 5k RPMs and then die unless I release the throttle quickly! Also, and this might be related, but I can't get the choke control to achieve the prescribed 3.5-4.5k RPMs throughout the choke range. past about halfway on the choke the bike just dies.

Interested to know where you got the info '3.5-4.5k RPMs throughout the choke range'?, I don't think I've ever seen or heard that before. If the choke isn't needed, ie, engine is warmed up enough, giving it more choke will cut the engine out.

Sure! I just grabbed it from the shop manual. It's in the front in the maintenance section:

Ah, yes, I do remember seeing it now, thanks for the reminder. But, I've never had to make any adjustments to the fast idle on 400/4 carbs so can't advise why yours won't do what it's supposed to.

[Deltarider]

Marsbound, are you sure IGN timing is correct and advancer works fluidly?