Author Topic: CB400F Carb issue (Read 3400 times)

stevenclemmons · « **on:** December 27, 2012, 06:48:14 PM »

I'm diagnosing carburetor troubles on my 1977 CB400f and I'm stuck. the

bike idles fine, but as soon as I roll on the throttle, it hesitates

and almost stalls out. I notice with the filter pods off that as the

throttle is rolled on, the 3rd carburetor sprays a large mist of fuel

out the back. I believe that this carb is flooding, at around 1/8th

throttle, causing the bike to lose power. if this is the case, what

needle/jet/function should i be looking into to even out the mix at

this point in the throttle?

harisuluv · « **Reply #1 on:** December 27, 2012, 07:51:19 PM »

Welcome. We will need more info about your bike to begin to try and narrow down your problem. We know it has pods, what kind of exhaust, what is the jetting, when was the last 3000 mile tune up of the bike, basics about the bike (miles)?

A carb spitting can have a few different causes. Bogging down when you hit the throttle often times is a symptom of modifications (pods!). You might need to go up in jet size to compensate for the amount of air the engine is getting.

These are just some ideas. Go ahead and use the search function (the google one, the other one is broken I think) and look up carb spitting or something along those lines. A member here named "twotired" on at least one occasion has gone into detailed explanation so start there, good luck!

phil71 · « **Reply #2 on:** December 27, 2012, 07:54:29 PM »

what tuning did you do to accomodate the pod filters? The 350 and 400F carbs are notoriously unhappy with pods, more so than most.

stevenclemmons · « **Reply #3 on:** December 28, 2012, 05:30:49 AM »

the bike has an open 4-1 exhaust, i rebuilt the carbs a few months ago, the bike struggled to reach high RPM's so I put some #82 mains in it last week and the WOT acceleration improved a bit. the main now is in the low throttle range. replaced the cylinder head recently because the valves were sunk and set the timing and clearances when i did it. the cam chain tensioner seems to be seized however as i couldn't get any adjustment out of it. I had an issue with the jet needle shim popping off and flooding that carb a few weeks ago actually so I'll pull them and see if that's causing the issue again soon

phil71 · « **Reply #4 on:** December 28, 2012, 09:31:39 AM »

It's going to be very hard to help here.The cam chain adjuster doesn't ever really show any change, you are simply releasing the set screw so the sprung plunger can extend more, compensating for wear on the face. If it's worn past its useful life, it will just get really noisy first, then eventually, things will start breaking. If it were destroyed, you'd have seen lots of evidence in the oil, and you certainly would have seen the chain being very slack when you re-assembled the head.
Do you know compression numbers? If you have weak vacuum to begin with, plus pods, plus open exhaust, you're not going to get that thing to run anywhere approaching normal.

Psychonaut · « **Reply #5 on:** December 28, 2012, 10:02:35 AM »

What did you set your floats at?

stevenclemmons · « **Reply #6 on:** December 28, 2012, 02:26:52 PM »

okay so I was messing with the bike today with the help of a more knowledgeable uncle, I noticed, placing my arm behind the carburetors with air pods removed that as I rolled on the throttle and RPM's increased, the #3 carb was forcing a ton of air back out the carburetor. We're thinking bad intake valve? i did lap the valves when i recently replaced the cylinder head with one I found on eBay... My uncle told me to get and hook up a compression gauge and putting the cylinder at TDC, hook up an air compressor and listen to the air escape from the valves and compare the sound to the same thing done on the other cylinders to see if I can notice a difference. I'll be getting a compression gauge and trying this out here soon.

Tim2005 · « **Reply #7 on:** December 28, 2012, 02:55:36 PM »

Could be worth rechecking the valve clearance on that one too, maybe it has tightened up a fraction and the valve is no longer seating well. It should be .05mm with the engine cold.

stevenclemmons · « **Reply #8 on:** December 29, 2012, 10:02:16 AM »

just checked valve clearances, they're good. i got the compression gauge and did a cold, dry compression check (not really sure of the process to do anything else, I'm new to this).
1 - 76
2 - 73
3 - 58
4 - 73
looks like there is definitely an issue with #3, what do i check from here? valve seat? any tips or signs to look for with the cylinder head off? Not exactly sure what to be looking for. and thanks for the help so far!

Tim2005 · « **Reply #9 on:** December 29, 2012, 10:48:14 AM »

2 things: firstly those figures are all very low, something over 100 would be expected, but maybe it is a gauge issue- if your gauge has a longish hose on it the the little 400f cylinders don't fill it so well & figures come out lower than they really are.

Secondly, irrespective of the above that number 3 cylinder definitely looks to have a problem. valve seats on 400s don't tend to be much of a problem, I've never had them crack for instance, but low compression could also be a slightly bent valve, a blowing head gasket or, a broken top ring - that's a very common problem on 400Fs too. If you can gently pressurise the cylinder with the valves closed (attach airline to spark plug hole) you should be able to hear any valve leaks.

stevenclemmons · « **Reply #10 on:** December 29, 2012, 01:21:21 PM »

Couldn't hear any valve leak so I went ahead and removed the cylinder head and jugs, I noticed the #3 intake side valve head had a brownish color to it and the surface of the cylinder gasket had a bunch of dirt and debris on it around the 3rd cylinder.

Tim2005 · « **Reply #11 on:** December 29, 2012, 02:38:37 PM »

That debris is just road muck that builds up in the stud holes & is released as you dismantle it. Number 3 looks a little hotter than the others, which is not at all unusual on a 400f either, but now the head is off you can put a plug in the thread & fill that chamber with kerosene -then look up the valve ports for any leakage past the valve seats.

Also, are the cylinder bores good? What's happening with number 4... the pic looks like some piston scoring?

crazypj · « **Reply #12 on:** December 29, 2012, 06:00:24 PM »

Quote from: stevenclemmons on December 29, 2012, 10:02:16 AM

just checked valve clearances, they're good. i got the compression gauge and did a cold, dry compression check (not really sure of the process to do anything else, I'm new to this).
1 - 76
2 - 73
3 - 58
4 - 73
looks like there is definitely an issue with #3, what do i check from here? valve seat? any tips or signs to look for with the cylinder head off? Not exactly sure what to be looking for. and thanks for the help so far!

You didn't have the throttle wide open when checking, did you\
BTW, Tim 2005, that isn't how you check valve sealing, you stand head on front or back and fill ports pointing up. (one at a time, it' much easier to empty)
Filling combustion chamber just makes a mess

stevenclemmons · « **Reply #13 on:** December 30, 2012, 01:42:14 PM »

thanks crazypj, I did not have the throttle open.. well, now I know for next time. I used your leak check method and that valve seat is DEFINITELY the culprit. I'll try to lap them and see if that doesn't do the trick before I consider resorting to having the valve seats redone. the cylinder tubes look good, I think it was just a bad picture. any tips on getting the pistons back into the cylinders? i don't have a piston ring compressor.

crazypj · « **Reply #14 on:** December 30, 2012, 05:21:14 PM »

You don't need a ring compressor with stock bores/pistons.
Do middle pair at TDC, rotate crank (real careful until outer's get to spigot and wiggle them in using your fingers/nails
I don't think I've ever used ring compressors on stock CB

grasscutter · « **Reply #15 on:** December 30, 2012, 05:27:22 PM »

Make sure your ring gaps are 120 deg. apart.
One of the pics.. think its cyl #3, the gaps are wrong.
Although things can move while you're checking things out.

I'm not a fan of doing a 'home lap' job.
Have a machine shop do it.
Its cheap.

And in agreement, the comp #'s are pretty low! You've got to be @ the bare minimum with those #'s!

crazypj · « **Reply #16 on:** December 31, 2012, 09:40:14 AM »

I never bother about the 120 degree crap, the rings rotate about 7:1 depending on engine rpm.
They also rotate at different rates depending on which ring your talking about (due to different weights and pressure between top, middle , oil control)
Anyone ever looked at ring positions on a motor they stripped?
Total waste of effort setting them 120

Tim2005 · « **Reply #17 on:** December 31, 2012, 10:08:29 AM »

Quote from: crazypj on December 29, 2012, 06:00:24 PM

BTW, Tim 2005, that isn't how you check valve sealing, you stand head on front or back and fill ports pointing up. (one at a time, it' much easier to empty)
Filling combustion chamber just makes a mess

You really should doing it that way, it's much easier than the 4-at-a-time standing up approach, quicker too, and just wipe the chambers out when done.

crazypj · « **Reply #18 on:** December 31, 2012, 12:53:45 PM »

Oh yeah,quicker, easier, DOESN'T WORK

If you can get leakage by 'your' method, you can see light through the bent valve
I taught motorcycle engine reconditioning for almost 4 yrs, seen just about every ;shortcut' you can think of (and probably a lot you've never thought of)
Do it once, do it right

stevenclemmons · « **Reply #19 on:** February 05, 2013, 05:49:05 PM »

update: the bike has good compression (~120 each cylinder) with brand new rings, valves lapped, just checked valves clearances, adjusted timing and vacuum synced carbs. put airbox back on and a more restrictive muffler to fix some fuel mixture issues. jetting is stock pilots (35) and slightly larger mains (80 from 75 stock). the ONLY issue now is after a minute of riding, the bike hesitates heavily (almost stalls in 1st) at 1/4 throttle position. with air screws backed all the way out of working range I'm still idling rich. not a pro at this but I'm thinking i need leaner pilot screws? any idea how much smaller to go? thank you all.

stevenclemmons · « **Reply #20 on:** February 05, 2013, 06:07:29 PM »

i see there is actually another thread open right now very similar to mine (sorry for not checking first). it's worth noting though, that I have had this same issue (bogging down at 1/4 throttle) with factory airbox and muffler as well as with open pipe and filter pods.

crazypj · « **Reply #21 on:** February 05, 2013, 10:41:24 PM »

Get into the other thread and post a link to this one, then post his link in yours so he doesn't feel left out or railroaded

Psychonaut · « **Reply #22 on:** February 06, 2013, 05:09:57 AM »

Quote from: Psychonaut on December 28, 2012, 10:02:35 AM

What did you set your floats at?

crazypj · « **Reply #23 on:** February 06, 2013, 07:20:48 AM »

set float levels 1mm HIGHER than book spec (27mm?)
Your not running stock airbox, stock settings don't work

stevenclemmons · « **Reply #24 on:** February 08, 2013, 05:24:42 PM »

I am running the stock airbox now! I put it back in as well as a more restrictive exhaust. the filter helped a bit, the exhaust didn't make much a difference, but the bogging at 1/4 throttle is still really severe. I ran the bike at 1/4 throttle for about 30sec and checked the plugs, they were sooty black. I have the carbs off now and the floats were at about 22mm(manual says 21mm) I went ahead and adjusted it up to 23mm. any other ideas? this issue has been racking my brain for months!

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Author Topic: CB400F Carb issue (Read 3400 times)