CB400F Too much fuel?

[donmac55]

Good Day all of you!

Yesterday was a big day, finally got my 1976 CB400F running after about 15 years of sitting idle. Bike turned over like a champ but is running rich. I have grey smoke coming out the exhaust, and it wont hold an idle. I can rev it no problem but as soon as I let go of that throttle it stalls. Just had the carbs rebuilt and timed.

So what's the next step? Do I try to mess around with the fuel screw on the carbs and adjust those? Look for vacuum issues? Or do I just yank those carbs out and start taking a look at the float bowls, needle jets etc?

Your help is much appreciated. I'm sure this is a pretty common problem...

 

[teazer]

Who did your carbs and what exactly did they do?

It maybe that teh air screws are not adjusted or the slides are not synched, and you can do that. Do you have a manual yet? See what it says about air screw adjustment. I don't have a 400F but the usual setting is to screw them all the way in - gently- and then back them out 1.5 turns. Repeat for the other three.

Check to be sure that there is no fuel leaking out of the float bowls or out of the drain tubes. That would indicate a float valve leaking.

If the carbs were done by a reputable shop, it is quite possible that they are fine. Have you checked header temperatures? It is possible that one cylinder is not firing or maybe even a pair.

Grab a non contact pyrometer like this cheap one http://www.harborfreight.com/non-contact-pocket-thermometer-93983.html or get a good one from Fluke or your local auto store. If one or two pipes are slightly cool, that may be that they are rich. If they are stone cold, look at the plugs and points.

 

[donmac55]

I was reading up on the idle mix screws for each one and I'll definitely try that tonight....it seems pretty straight forward. It was a Honda dealership up in Northern Canada that rebuilt them, so 50/50 chance they were done right I supplied the carb kits and all they had to do was put it together.

I checked each exhaust last night and they all seemed to be good. I just put in new spark plugs last night as the other ones were jet black (again, probably from unburnt fuel.) I checked sparks on 1 and 4 (they were easiest to check.) and they were both firing. I'll check the other two tonight.... I did have some wiring issues but I'm pretty sure they were resolved as it fired up as soon as I fixed them.

That pyrometer is awesome by the way.... I'll be sure to purchase one.

I'll let you know how that goes, thanks for the response.

 

[carnivorous chicken]

Who did your carbs and what exactly did they do?

It maybe that teh air screws are not adjusted or the slides are not synched, and you can do that. Do you have a manual yet? See what it says about air screw adjustment. I don't have a 400F but the usual setting is to screw them all the way in - gently- and then back them out 1.5 turns. Repeat for the other three.

Check to be sure that there is no fuel leaking out of the float bowls or out of the drain tubes. That would indicate a float valve leaking.

If the carbs were done by a reputable shop, it is quite possible that they are fine. Have you checked header temperatures? It is possible that one cylinder is not firing or maybe even a pair.

Grab a non contact pyrometer like this cheap one http://www.harborfreight.com/non-contact-pocket-thermometer-93983.html or get a good one from Fluke or your local auto store. If one or two pipes are slightly cool, that may be that they are rich. If they are stone cold, look at the plugs and points.

All good advice, especially the part about getting a manual and reading up on it.

I would add: there is also an idle speed adjustment screw (one screw that adjusts the idle speed on all four carbs) -- it's the big one under the right hand side of the tank. Your idle speed should be about 1500rpm, and it may be stalling because that's not high enough. And your fuel mixture depends on your air filter and exhaust set up as well. Stock? Pods or aftermarket exhaust? These things matter. And you should talk to whoever rebuilt the carbs. Did they replace the jets with stock size jets? Were they another size when he pulled them out? Did he mess with the needle adjustment?

IF the bike hasn't run for a long time, it's also not unusual for the exhaust to burn up and blow out crud that's collected over the years in the pipe.

 

[crazypj]

..................................and grey 'smoke' is more than likely condensation evaporating.
Running rich will give black smoke (which you generally can't see unless there is very light concrete under exhaust outlet)
If you had a shop clean/fix carbs, it should have taken around 4 hrs,it isn't possible to do them faster (unless you skip a lot of the procedures)
Dunking in an ultrasonic tank without full strip down is just about the worst thing that could be done to them (using any of the 'Autozone,et.al. cleaning buckets is the worst)

 

[donmac55]

I really appreciate your help with this. I came home and played with the idle speed adjustment screw, got it up to 4000 with the choke on, and then it kept stalling as soon I turned the choke off.

Then looked at the exhaust, found number 1 exhaust (left side of bike) was cool to the touch. Pulled the plug and saw a white spark when I tried to start it. And No more smoke coming from the exhaust so I'm good there.

So, it's probably the jet on the number 1 carb I'm thinking? Oh and I meant to say I have stock everything, and the carb kits were from 4 into 1 so I'm not sure about the quality.

The factory setting for the air screws is 1 1/2 turns which I've done, and no issues for any of them except that number 1 not firing.

 

[carnivorous chicken]

Next step is to pull the float bowl from the #1 carb. If it has gas in it (enough to reach height) then it's the jet. If it doesn't have gas in it, then it's the float valve. It could also be float height, if whoever rebuilt your carbs didn't set it properly. You should ask them. You should be able to pull the float bowl without pulling the carb rack with either a small socket with a Phillips end or a z-shaped Phillips.

I'm assuming that you also checked the #4 pipe for heat -- the #1 and #4 cylinders run off he same ignition point and coil. If it's cool too but you're getting spark, you might have your wiring to the coil, point and condenser somehow connected to your 2-3 circuit (very slim chance, but I've seen it). Or your timing could be way off.

And what size jets did the shop use, did they adjusted needle height, etc. You should call them and ask them these questions.

 

[crazypj]

In my experience, pulling carbs off CB400F isn't too bad, but, re-fitting makes you want to slash your wrists ;D
I've never had any luck trying to do carbs on the bike, it's easier to fit K&N and learn jetting

 

[teazer]

I just re-read the thread and the OP mentioned jet kits or carb kits. I would recommend buying 4 new slow jets (pilot jets) from Honda or jets R Us or SUDCO - you will have to check to see which has genuine Keihin jets for your bike. When you pull the carbs to check the float levels, remove the pilot jets and see if they are genuine and what size they all are.

And remember that float levels are checked with the carbs tilted - not upside down.

 

[crazypj]

I think they are same as CB350 with 'spring' holding them in? (haven't been 'inside a 400f since around 1980)
Could be the 'O' rings were damaged or missing ?

 

[teazer]

Or a main jet fell out. That happens more often that we like to imagine. My gut feeling is that the carbs were not 110% clean and jets were not "Jenn you wine" Keihin, but we'll all know when the OP pulls the carbs.

First thing I do when an owners says the carbs were rebuilt is to pull them and inspect them and if they look really clean all I have to do is to check all the drillings with a can of spray carb cleaner to see if they all flow the same.

CMSNL have a graphic that looks like press in jets and the graphic for the jets shows CB750 style jets, so I'm not sure.

Could also be sinking floats.

 

[donmac55]

Good Morning,

So I went and pulled the carbs out and took a look at #1. Looks like there was a gasket pinched in the lower float bowl. sorted that out and added number 2 penatex to it as well. Floats were good (22mm), jet was good. Put the carbs back in. (I don't know how to measure the jets, so I'm going to read up on that and pull the assembly again). I set them all to factory settings on the air screw (1.5 turns)

In the mean time I put the assembly back in, Started up the bike and immediately started running like a dream.... until #4 stopped firing after about 30 seconds. So I think the carbs are good #2,#3,#4 were firing before I took it out, now only #1,#2,#3 are firing. I'm probably going to replace the points tonight once I buy a feeler guage to see if that helps.

I only took #1 carb apart as that was the issue... should I take them back out and open the other ones? If replacing the contact points dont work, I'm thinking of Measuring the slow jets, ordering new Keihin jets and measuring the rest of the floats. Any other pointers/ideas. You guys have saved me thousands in mechanic fees so far

 

[1fasgsxr]

I am cleaning a set off my 350 F and I know exactly what your are talking about.. The o-rings are just a little bit too big it seems..i can see where it would be very easy for it to pop out or pinch when putting the float bowls back on. I just put a dab of sealer in the groove where the o-ring sits and let it sit long enough to get tacky so it will hold the o-ring. Your "slow" jets size should be marked on it...

 

[carnivorous chicken]

Good Morning,

So I went and pulled the carbs out and took a look at #1. Looks like there was a gasket pinched in the lower float bowl. sorted that out and added number 2 penatex to it as well. Floats were good (22mm), jet was good. Put the carbs back in. (I don't know how to measure the jets, so I'm going to read up on that and pull the assembly again). I set them all to factory settings on the air screw (1.5 turns)

In the mean time I put the assembly back in, Started up the bike and immediately started running like a dream.... until #4 stopped firing after about 30 seconds. So I think the carbs are good #2,#3,#4 were firing before I took it out, now only #1,#2,#3 are firing. I'm probably going to replace the points tonight once I buy a feeler guage to see if that helps.

I only took #1 carb apart as that was the issue... should I take them back out and open the other ones? If replacing the contact points dont work, I'm thinking of Measuring the slow jets, ordering new Keihin jets and measuring the rest of the floats. Any other pointers/ideas. You guys have saved me thousands in mechanic fees so far

The #1 and #4 cylinders work off of the same point. So if you're #1 is firing, it can't be your points. Likely another carb issue -- are you running inline filters between the petcock and the carbs? It's possible that whoever rebuilt your carbs didn't do a thorough cleaning job, and the newly circulating gas is picking up some crud off the bottom of the float bowl or from the passages, that is then clogging jets. Easy to pull that bowl and have a look -- starter is on the other side. Get your bike running on all four cylinders well (as well as it can) before you start throwing new jets at it -- you want to make sure you're only trying to solve one problem at a time.

 

[donmac55]

Perfect! Yeah i'll take the float bowl off and take a look. I'm definitely running inline fuel filters just so I could try to avoid this problem!!! I'll take a look at it tonight. Hopefully its another gasket issue.