Topic: #1 Cylinder Not Firing 400/4

[grumpy56]

Have a 1976 CB400F with less than 7000 miles. Runs great and all cylinders fire immediately at start-up when warm or hot. If stone cold, the #1 cylinder will not fire for between 30-60 seconds after the other 3 ignite. This is with the choke on of course. Any ideas as to cause? Thanks.

[bender01]

Sticking float?

[scottly]

What does the spark plug look like?

[grumpy56]

Attached are a couple of pics of plug #1 and also one of plug #4. I think I will try smacking the #1 float bowl with my plastic hammer when I first fire it up cold and know that cylinder #1 is not firing and see if it comes to life.

[BenelliSEI]

1. Try a new spark plug.
2. Shorten the plug wire by a 1/4” and reattach the cap.
3. Ensure the idle jet is not plugged.

[HondaMan]

I suspect a small vaccum leak. The intake manifolds have an O-ring at the head, and it got hard sometime around 1990. Now it is like plastic, not sealing much.
Honda's part is
16173-253-004
O-RING (27X2.4)
and are expensive at $4.50 each: if you decide to change them then PM me, as I have them made for about half that much locally.
The Post Office can't decide if they want to charge me 1 stamp or $5.95 postage (they keep changing it), so that's the postage part of it (not known until shipped!).

[grumpy56]

Changed plug, re-installed lead, and smacked the float bowl around. No difference. #1 fired up after about 30 seconds on choke (RPMs go waaaaaay up). Before it fired, I shut it down and opened the drain screw and I had gas in the bowl. New plug appeared dry.

I rebuilt and re-attached the carbs about 2 years ago. I don't remember replacing those O rings. I need to check and see if I did.

Thank you all for the help.

[scottly]

How does the bike run after it's warmed up and off the choke?

[grumpy56]

Runs perfect idle to red line except for a slight hesitation just off idle when warm/hot if you are just easing on the gas. If you give it good throttle, no hesitation.

[scottly]

Don't fix it if it ain't broke.

[M 750K6]

Agree. We get used to modern carbs and fuel injection. I've found my 750 is cold blooded. Needs full choke for a cold start and sometimes runs on three until I pull away, but idles at lower revs at my junction 600yds later, then idles fine within a couple of miles.

I cleaned everything scrupulously, float levels, checked choke flap adjustments, balanced carbs, did plugs, points, timing, valves etc. Just the way it is. Runs like a beauty as soon as it's mildly warm. I just make sure no one's around when it gets it's first start of the day!

[grumpy56]

lol,,,that sounds like good advice!!! I also make sure it is warmed up before my riding buds arrive!!!

[grumpy56]

Is there a way to confirm that intake 0-ring is bad without taking off the carb bank? The tried and true WD40 method probably won't work since the o-ring is inside the flange.

[Ozzybud]

Is there a way to confirm that intake 0-ring is bad without taking off the carb bank? The tried and true WD40 method probably won't work since the o-ring is inside the flange.

Spray starting fluid(Ether) at the flanges. If the rpm climbes it is sucking in the Ether.
And it evaporates immediately leaving no oily mess.

[grumpy56]

continuing along......sprayed starting fluid on the intake manifolds at the head and no evidence of leaks. When started cold today, # 1 was not firing, but when I fully opened the choke and then raised the lever to again close it, #1 immediately started firing. hmmmmmmm. Will try that again when the bike is stone cold as that is the only time it does not fire on start-up.

[Ozzybud]

All of my Sohc4 bikes do that occasionally only on startup for a few seconds or so.  I just know they are carbureted not fuel injected . It takes a while for the proper fuel atomization to happen. All my bikes run amazing at this time. Last week I had all 10 out and around town. Including the Ct70's and Z50. The Z50 is the only one that is not street legal.Who is going to pull over a 60 year old dude on a Z50?

Ride it!

[grumpy56]

That is a fine selection of scooters Ozzybud!!!

[Ozzybud]

That is a fine selection of scooters Ozzybud!!!

Thank you!
Just one more... promise.

[BenelliSEI]

continuing along......sprayed starting fluid on the intake manifolds at the head and no evidence of leaks. When started cold today, # 1 was not firing, but when I fully opened the choke and then raised the lever to again close it, #1 immediately started firing. hmmmmmmm. Will try that again when the bike is stone cold as that is the only time it does not fire on start-up.

Did you clean the idle jet?

[grumpy56]

I did not, yet. Any chance of getting that pilot jet out of the left hand carb without removing the entire carb bank? 

[Ozzybud]

I did not, yet. Any chance of getting that pilot jet out of the left hand carb without removing the entire carb bank?

Yes you can get the bowl off without removing the entire bank. It is a bit tricky with finger manipulation  upside down. I use a 1/4;ratchet with a 1/4 inch socket and a small JIS bIt.
The tricky part is getting the small jet out without buggering it up. Getting a small flat head screw driver up there and keeping it solid to the flats can be challenging.
I have had all the racks. On and off so many times I usually go that route.
This bike idles fine after warm right?  I'd leave it alone and ride.
All of my bikes have different quirks when starting. I know all of the characteristics of my different bikes.  If you came over and tried to start my bikes you may have a hard time at first until you learn their behaviors.

[scottly]

You might try adjusting the idle mixture screw in or out a 1/16th of a turn, but dropping the bowl and pulling the idle jet is not warranted by the minor cold starting issue.  

[grumpy56]

Already tried changing the mixture screw a half a turn each way (currently at 2.0 turns out), but no difference. I may just live with it until I need to remove the carbs for some other reason.

BTW, dealing with these carbs is a, what do they say, a picnic in the park compared to my early model Suzuki GT380. That thing has 3 separate carbs, 6 cables, and NO room to get that air box and one piece hard rubber manifold on. Someone on this forum had a great idea to make a little tool to push on the 400s air box from the inside which works great.

Thank you all for your help.

[BenelliSEI]

Next time you are on the bike (after long enough to warm it up), go along at fairly high revs, in a low gear, and pull the chock full on for a few seconds. On occasion this will clear a partially blocked idle jet……..

[grumpy56]

will try that....thank you

BTW.... The only other problem I have with this 400 is a slight hesitation right off of idle....well whatever I did fussin with this issue cured that problem.    went for a nice ride yesterday and absolutely no hesitation off of idle...no clue why but am not complaining

[BenelliSEI]

Get some fresh gas in it and ride it. Best tune up of all! Most fun too.

[seanbarney41]

+1 these bikes really like to be ridden...and don't baby 'em!  They can get really lazy and ornery when not used.  The more you ride, the happier they are.

[HondaMan]

+1 these bikes really like to be ridden...and don't baby 'em!  They can get really lazy and ornery when not used.  The more you ride, the happier they are.

Boy, if THAT ain't the truth!
My 750 feels sleepy every Spring until we find some wide-open road somewhere...