Topic: 75 cb440F stumble off idle (solved FINALLY)

[Bailgang]

This is my first cb440F so I don't know what to expect. After replacing a Tytronic electronic ignition that only lasted 20 miles and getting the bikes charging sys to charge like it should (which may have been the reason the Tytronic failed) I'm now sorting out carb tuning issues. I initially rebuilt the carbs with kits from 4into1 using the factory needle set in its mid position, the main jet, slow jet and emulsion tubes that came with the kits as well as the float needles/seats that came with the kit. At that time it still had points ignition and ran OK but had a real nasty flat spot or stumble when coming off idle or another example, when slowing down to make a turn at an intersection, it would literally fall on its face while rolling back into the throttle after finishing the turn. The needles/seats that came with the kit turned out to be junk, I generally have never had an issue with aftermarket needles/seats but I couldn't keep the floats from sticking, the original ones were long gone so I had to get a set of genuine Honda needles/seats, set float height at 21mm and that took care of that although my wallet took a hit. While I had the carbs off I decided to set them up as close to factory as possible before making any changes so still used the factory needles with the clips set at middle position, the original emulsion tubes, original #75 main jets (the mains in the kit were #76) but I did however use the #40 slow jets from the kit. The reason why is because everything I've researched so far says my carbs should have #40 slow jets however the original ones I pulled from the carbs are #42. It still has the factory air box so I don't see why a previous owner would've messed with the slow jets so I'm inclined to think that's what the carbs originally came with. After getting it all back together and taking a long test ride, the bike for the most part absolutely rips however it still has a slight stumble when coming off idle or exiting a turn. It doesn't fall flat on its face like it did before but it's still there just not nearly as bad. My question is could that #40 slow jets that came with the kit be the culprit and should I put the original #42's back in even though factory specs say it should be a #40? Any thoughts?

[robvangulik]

You could first try if small adjustments to the Idle Mixture Screws make it better, takes just a few minutes to turn them 1/8 turn out, or in again if it makes it worse. It can make a real difference with these small carbs.

[Deltarider]

What Rob says ^. Also you may consult p.17 in the CB400F partslist. Honda shows various options for jetting, including #42 slow jets. https://www.honda4fun.com/dwnload/Part-List/CB400/CB400-F-F1-F2-Parts-List
Are you sure Ignition is correct and the advancer does what it should? Original air intake ?

[Bailgang]

What Rob says ^. Also you may consult p.17 in the CB400F partslist. Honda shows various options for jetting, including #42 slow jets. https://www.honda4fun.com/dwnload/Part-List/CB400/CB400-F-F1-F2-Parts-List
Are you sure Ignition is correct and the advancer does what it should? Original air intake ?

Thanks for the link. So far timing is correct and it has the original air intake. Now that I know that the carbs originally had #42 slow jets, I'm going to put them back in and go from there. I'll report the results later.

[Bailgang]

I put the #42 slow jets back in and the stumble was still there but not quite as bad. I probably could've gotten away with the #40's but being it seems the carbs originally had #42's, I figured there must be a reason why and without knowing what the reason was, I felt it prudent to put the #42's back in. After that, I went with Rob's idea and started messing with the idle mixture screws and that indeed made a difference. I started at 2 full turns out and ended up at 2 1/4 turns out, I tried a little more but that's when the exhaust started to back fire just a bit on decell so I know I went too far and put it back to 2 1/4. There is still a very slight stumble but it's a huge improvement and I can live with it. Thanks for the input guys.

[Herr Supersport]

First try with AI /ChatGPT.
I am surprised, how well that went. Translation of my german-text is:

If I'm not mistaken, the topic was discussed a few weeks ago. I apologize for retelling the story - perhaps it's helpful.

It's possible that your motorcycle is drawing in false air (air leak).
When the warm engine is idling, you can turn in or out the air screw on carburetor #1 until the engine stalls. Then, you set the screw back to its normal value and do the same with the other carburetors.
If one carburetor reacts differently from the others, there might be the leak.

In my case, one cylinder was drawing in air between the intake flange and cylinder head, despite having new flanges and O-rings.
I sealed the issue with Threebond, and since then, it's been quiet.
Side note: Hondaman [I really appreciate your comments!] explicitly warns against this procedure.
From that point on, the vehicle was driveable and the problem was solved. I still had a very slight delay in throttle response when opening the throttle from fully closed. After a clean synchronization, this problem also disappeared.

Regarding the question "what can I expect": a very simple and enjoyable ride on the motorcycle. Compared to my Kawa H2, it lacks power and torque, but the handling makes up for it.

Best regards and good luck,
Daniel

[Bailgang]

It's possible that your motorcycle is drawing in false air (air leak).

That's a possibility I haven't ruled out to get it perfect. It still has the original intakes but o-rings and clamps are new. What stumble there is isn't there when engine is cold but appears as the engine comes up to operating temp. I've sprayed carb cleaner around the boots and got no response from the engine but I never directed the spray where the intake flange meets the cyl head. The carbs were out of sync so after I got them synced and then followed Delta and Rob's suggestions, the stumble is now minimal but there is obviously1 more thing I'm missing in order to get it perfect. In the meantime though it's rideable.

[robvangulik]

I still think that 42 jet is too large, if you put the 40 back in and the mixture screw back to 1 full turn out it'll be better, and you're less on the edge, you can adjust to both sides, in or out. 2 1/4 turns out is a bit too much, like I said, on the edge.

[Bailgang]

I still think that 42 jet is too large, if you put the 40 back in and the mixture screw back to 1 full turn out it'll be better, and you're less on the edge, you can adjust to both sides, in or out. 2 1/4 turns out is a bit too much, like I said, on the edge.

I may eventually try the #40's again but I'm really not looking forward to it. Getting the carbs on and off with the factory airbox in place is an absolute PITA although a heat gun made things much easier the last time .... but still no fun. I've tried pulling the bowls with the carbs on and that wasn't really any easier. Pulling the carbs on my 750 is a breeze compared to the 400.

[Mark1976]

   I agree, it's  no fun pull the rack on these bikes (400f), but the fact that the stumble becomes worse once hot is indicative of a rich condition. If you have a set of 4 keihin slows, clean `em good and stick them back in, the aftermarket brass is garbage. Once you're got them out clear tube 'em just to dbl check the float height, if your floats are a little high you can have a similar condition.

[TwoTired]

Is the exhaust system stock?

[Bailgang]

Is the exhaust system stock?

It has a Delkevic exhaust with all its baffles in place.

[Bailgang]

Right now I've got the #40 slow jets in with idle mixture screws at 1 1/4 turns out and it seems to run pretty good however I'm still having issues with the floats sticking (#3 carb) and until I get that sorted out, I really can't fine tune it any further and I'm at my wits end. Installing OEM needles and seats solved one issue because I initially had all 4 carbs leaking randomly, now it seems to be the #3 and even then it doesn't do it all the time. Tank is clean and I'm not seeing any sediment in the bowls so I think I can rule fuel out but the one thing that was different about the float in the #3 carb was that the tang that the needle rests on was dimpled more than I thought. Some careful filing and then polishing with 1000 grit sand paper and I thought I solved it but after taking a few test rides, it's doing it again. I've gone through so many posts relating to this and I've done everything I can think of to the point that I just ordered another float for it. Removing/installing 400 carbs is a PITA but I've done it so many times now that I'm getting good at it.

To make my weekend even worse, I was messing with the master cyl on my cb350 twin and unknowingly got some brake fluid (hardly a drop) on the fuel tank and didn't realize it until the next morning and of course damage to the paint was done. Not a good weekend for me.

[Mark1976]

   Once you get the new float, before you install, make sure you have a good needle spring. If its a bit soft that may be contributing to your problem, if your needle and seat are aftermarket its a common issue. Yeah, carb issues suck. I just did a rack for a 400f today, attention to detail, can't rush any of it. If it still leaks its the needle and seat, I'm pretty sure i have a spare kehin needle and seat. I'll look and see.

[Bailgang]

   Once you get the new float, before you install, make sure you have a good needle spring. If its a bit soft that may be contributing to your problem, if your needle and seat are aftermarket its a common issue.

The aftermarket needles and seats that I initially installed were junk, all 4 carbs would randomly leak. it now has new OEM needles and seats and I thought that took care of it but the #3 carb is still being picky. As far as the needle springs, it's been my experience that OEM needle springs are much softer than typical aftermarket ones. I'm running out of ideas and it's driving me nuts, there has to be something I'm missing.

[Deltarider]

A sticking valve needle and/or float swivel pin can indicate things aren't as clean as they should. An additive, usually called fuel system cleaner, can help. Also I'd check for a correct fuel flow to all 4 carbs and that the carb vent tubes as well as the overflow tubes are all cleared and end where they should.
If you use my method for checking the fuel level, each float chamber when drained would produce 46-48mm of gas.

[Bailgang]

A sticking valve needle and/or float swivel pin can indicate things aren't as clean as they should. An additive, usually called fuel system cleaner, can help. Also I'd check for a correct fuel flow to all 4 carbs and that the carb vent tubes as well as the overflow tubes are all cleared and end where they should.
If you use my method for checking the fuel level, each float chamber when drained would produce 46-48mm of gas.

Thanks for the input Delta. It will be a few days for the new float to arrive so in the meantime I'll go through the carbs again to make sure they're clean and vents are clear. Right now I'm kicking myself for not swapping the float from the #3 carb with one of the others to see if the problem persisted with the #3 carb or moved to the carb that I swapped the float with. One thing that is different with the #3 carb compared to the other 3 is that when we first got the bike, one of the posts where the float pin mounts through was broken and long gone, the previous owner had been running the bike with a broken float post with the float pin just hanging onto the the remaining post. I repaired this by cutting the float post off a parts carb from a different bike and my brother in law carefully welded it in place. It's height is correct with the other post so the float sets level however the hole for the float pin on the repaired post is just ever so slightly larger making the float pin a bit sloppy. I can't accurately measure the hole because it's so small but I'm guessing it's maybe .002 inch or .051mm larger than the hole in the other post. I don't know if that is a factor or not but so far it seems the float moves up and down smoothly. It's driving me nuts because it wont do it every time I turn the fuel on in fact I rode it for a bit as I was fine tuning the idle mixture screws and I thought I had solved the float sticking issue but it did it again on the last ride.

[Deltarider]

Good you mention this ^. I don't think it is much of a problem. It could however, if that swivel pin is, after reassembly, not rightly postioned between the two whatever they are called in the floatbowl. You may want to see the insert at 27:39 in this video:

All who use Keyster float valves and needles should know there's a difference with the original Keihin. In this video it is shown at 04:00

Video's courtesy of Gruzzel

[Flyin900]

I still think that 42 jet is too large, if you put the 40 back in and the mixture screw back to 1 full turn out it'll be better, and you're less on the edge, you can adjust to both sides, in or out. 2 1/4 turns out is a bit too much, like I said, on the edge.

AFAIK : The factory setting is 2 turns out +- 1/2 turn. My experience is the fuel today burns leaner in these bikes slightly, so a little extra 1/2 a turn out has worked for me on various older Honda bikes. These carbs are listed with a stock #40 secondary jet, at least the #054 series.
It is likely someone in the past replaced the #40's and ordered the original style Keihin #42 jets to try and solve this or another issue they were experiencing at that time.

[Deltarider]

[...] AFAIK : The factory setting is 2 turns out +- 1/2 turn. My experience is the fuel today burns leaner in these bikes slightly, so a little extra 1/2 a turn out has worked for me on various older Honda bikes. [...]

Out? But... an extra 1/2 turn out on these carbs will make them leaner...

[Bailgang]

Right now until I get the float situation sorted out, I'm not going to fuss with the slow jets and idle mixture screws because with the #3 carb giving me issues with it leaking, it pretty much messes everything up. As it stand so far with the original Keihin #42, it seemed to like the idle screws at 2 1/4 turns out, with the aftermarket #40's it so far is OK at 1 1/4 turns out, however that all my be moot until I get the floats sorted out and can verify I have correct fuel level. Once I get that sorted out, then I'll play with slow jets and idle mixture screw adjustments. Excellent vid Delta, I just wish I could understand German but I am able to sort out what he's trying to explain.

[Deltarider]

Excellent vid Delta, I just wish I could understand German but I am able to sort out what he's trying to explain.

Gruzzel's videos are the best. If there's a particular part you don't quite understand, I might help.

[HondaMan]

If you're having troubles still with the #3 overflowing...
-in the 750's roundtop carbs it is quite common for them to develop a tiny ledge on the 2 posts that hold the pivot pin for the floats. This makes the floats 'hang' or 'stick' and float valve is then non-responsive to the bowl depth. It both can cause overflow, and low-bowl levels, and in the same carb! The cure is to look closely at the posts where the pins pass thru for a 'burr' that gets lifter by the sharp edges of the brass float stampings. I have seen it mostly in the 750, but also in the 350F. I'm supposing the 400F is similar.

[newday777]

If you're having troubles still with the #3 overflowing...
-in the 750's roundtop carbs it is quite common for them to develop a tiny ledge on the 2 posts that hold the pivot pin for the floats. This makes the floats 'hang' or 'stick' and float valve is then non-responsive to the bowl depth. It both can cause overflow, and low-bowl levels, and in the same carb! The cure is to look closely at the posts where the pins pass thru for a 'burr' that gets lifter by the sharp edges of the brass float stampings. I have seen it mostly in the 750, but also in the 350F. I'm supposing the 400F is similar.

"The cure is to look closely at the posts where the pins pass thru for a 'burr' that gets lifter by the sharp edges of the brass float stampings."

Do you file off the burr or just look at as you said Mark? I figure maybe you left out the fix after you look at it???

[Bailgang]

I have seen it mostly in the 750, but also in the 350F. I'm supposing the 400F is similar.

From what I understand, the 350F and 400F carbs are pretty much the same so I'll definitely be taking a close look at the posts. I was in a dentist chair yesterday so right now I'm trying to get motivated to pull the carbs again.