Topic: 75 cb440F stumble off idle (solved FINALLY)

[Mark1976]

   Good analogy though, not quite like going to the dentist, but certainly not far from it....

[Bailgang]

Thanks to Hondamans suggestion, I do believe I spotted the problem. Take a look at the pick and notice where the blue arrow is pointing, with the float positioned just right, the brass from the float hinge drags on the shoulder of the post and that is the post that was repaired. That repair post was salvaged off a cb350 twin parts carb and its shoulder at the top is slightly bigger in dia than any of the others and it's just big enough to catch the float if the float happens to be in the right spot and that explains a lot. Sometimes it wouldn't leak at all, sometimes it would dribble, sometimes it would pour. I'm rather stoked at the moment because I've FINALLY found something wrong that would explain things but I'm going to contain my enthusiasm for the time being until I get it back together and running and convinced it won't leak again. Purchasing OEM needles and seats did solve issues with the other carbs but that #3 carb obviously had more problems that the OEM needles and seats couldn't fix by themselves.

[Deltarider]

That could very well be the cause. Realise the movement of the float is soooo gentle, the force is sooo soft, that it takes very little to undo it. For instance, if you would watch the float rise when an empty bowl refills, that movement is so gentle, that when the tip of the needle closes the valve, the pin - the little shock absorber - at the bottom of the float needle doesn't get compressed by even a fraction of a millimeter, not even when the bowl was completely empty before. That's why in the Shop Manual Honda prescibes to measure the float height when the float juuust contacts that little pin and no more.

[Mark1976]

    Good catch, who knew.....

[Bailgang]

That could very well be the cause. Realise the movement of the float is soooo gentle, the force is sooo soft, that it takes very little to undo it.

Precisely. The combination of that float post being just a tad bigger in dia at the shoulder combined with a slightly sloppy fit that I had mentioned before where the float pin is gives the float the wiggle room it needed to catch that shoulder and get hung up. The pic doesn't show it but it can just barely catch the ledge of that shoulder but at the same token clear it as well depending on how the float is positioned and all it would take is a bump in the road to make the float go from working like it should to making it leak. I wont have time to get it all back together until this weekend but I'm pretty confident about it now.

[Deltarider]

You may find the video in this post interesting: http://forums.sohc4.net/index.php?action=post;msg=2255040;topic=193245.0

[Bailgang]

I put the carbs back together and no leaks which I can't describe to you how much of a relief it is to clear that hurdle, now back to the stumbling off idle issue. Everything I tried before pretty much got thrown out the window because as it stand now, I have the #40 slow jet in and idle mixture screws out 2 1/2 turn which according to my manual, that's the max and it still stumbles but it's a massive improvement from where I started. It runs perfect when cold but the stumble arises as it gets up to operating temp and being it is responding to me backing out the idle mixture screws, not to mention the plugs are pretty carboned up, it's too rich and the only thing else I can do to address that is checking my actual float height via clear tube method. The reason I haven't checked that already is because I'm trying to sort things out one at a time. I'll report back but in the meantime it seems like the bike is responding well as I sort out all the details.

[Bailgang]

I could use some help from those of you that know carbs better than myself. After doing the clear tube test to verify my float levels, I found only 1 carb in spec while the other 3 were high. That's all taken care of now and it did make a positive difference but not enough. Just for kicks I decided to put  the idle mixture screws back to 2 full turns out and raise the clips on my needles one notch thus lower the needles, it has always been my understanding that needle height really doesn't play a factor until later in throttle position so I wasn't expecting much in regards to my off idle stumble but much to my surprise that needle height adjustment almost took the off idle stumble away, not completely but it is a huge improvement. I suppose I can make it even better by backing out the idle mixture screws again but I really don't like raising the needles unless I have to so the plan is to put the needle clips back to the middles position and try #38 slow jets instead and go from there. Is my thought process correct on this?

[smee]

I could use some help from those of you that know carbs better than myself. After doing the clear tube test to verify my float levels, I found only 1 carb in spec while the other 3 were high. That's all taken care of now and it did make a positive difference but not enough. Just for kicks I decided to put  the idle mixture screws back to 2 full turns out and raise the clips on my needles one notch thus lower the needles, it has always been my understanding that needle height really doesn't play a factor until later in throttle position so I wasn't expecting much in regards to my off idle stumble but much to my surprise that needle height adjustment almost took the off idle stumble away, not completely but it is a huge improvement. I suppose I can make it even better by backing out the idle mixture screws again but I really don't like raising the needles unless I have to so the plan is to put the needle clips back to the middles position and try #38 slow jets instead and go from there. Is my thought process correct on this?

You may have just found your problem. As an example: If you have your idle set too high, you can end up compensating for the jetting issues at idle as you will draw fuel into the pilot circuit from the other jets. Raising the needle will richen up the circuit here with an idle screw set too high.

Sounds like you're a bit lean off idle. Easy test is to take off the air filter. It will make the problem worse. You may have some gunk in your pilot circuit. Make sure your pilots are squeaky clean (and OEM). Aftermarket brass is leaner jetting then stock jets. Also, you may consider renewing any rubber on the airbox and carb boots. These all contribute to increased air in the circuit when they get old and stiff.

[Bailgang]

You may have just found your problem. As an example: If you have your idle set too high, you can end up compensating for the jetting issues at idle as you will draw fuel into the pilot circuit from the other jets. Raising the needle will richen up the circuit here with an idle screw set too high.

Sounds like you're a bit lean off idle. Easy test is to take off the air filter. It will make the problem worse. You may have some gunk in your pilot circuit. Make sure your pilots are squeaky clean (and OEM). Aftermarket brass is leaner jetting then stock jets. Also, you may consider renewing any rubber on the airbox and carb boots. These all contribute to increased air in the circuit when they get old and stiff.

I genuinely appreciate your input but the problem is without a doubt being too rich off idle rather than too lean, amongst other things the spark plugs prove it. When I raised the clips which are now on the second notch from the top, this lowered the needles thus leaning it and that made the biggest improvement of anything I've done. One could say "just leave the needles there and back out the idle mixture screws a bit more" but I feel that lowering the needles like I did just masks the problem at the expense of the engine being lean at part throttle meaning throttle position at say 60 mph because even though lowering the needles was a huge improvement with my off idle hesitation, I did notice a slight decrease on how well the engine pulled when rolling onto the throttle at say 50 or 60 mph which tells me I should try to keep the needles at stock height with the clips in the middle position and address the slow jet instead. Ideally I'd like to keep the needles at the factory height (clip in the middle) and keep the idle mixture screws within the 2 turns out + or - 1/2 that Honda suggests and the only way for me to do that is to try a smaller slow jet. The new jets won't be here till next week so I'll have to wait till next weekend till I have time to try it. Also the airbox boots are new, carbs boots are original but have new o-rings and clamps.

[Herr Supersport]

Air screws out / in - Enlarging / reducing idle jets - Raising / lowering needle jets.

I believe you're addressing symptoms and not the root cause.
I'm still convinced that your motorcycle is getting false air. New O-rings don't guarantee airtightness for you.

Now that the carburetors are removed, it's a good time to address this.

As mentioned before, I took the route via Threebond.
The argument against it can be found here #7/HondaMan:

http://forums.sohc4.net/index.php/topic,191040.msg2223296.html#msg2223296

However, I don't see where the sealing compound should escape – of course, you wipe away any escaping material meticulously, and of course, you only start the engine once the sealing compound is dry.

Alternatively, you could use thicker O-rings or cut and insert paper gaskets.
And: the four screws for synchronization should also be sealed.

Regards, Daniel

[Tracksnblades1]

I could use some help from those of you that know carbs better than myself. After doing the clear tube test to verify my float levels, I found only 1 carb in spec while the other 3 were high. That's all taken care of now and it did make a positive difference but not enough. Just for kicks I decided to put  the idle mixture screws back to 2 full turns out and raise the clips on my needles one notch thus lower the needles, it has always been my understanding that needle height really doesn't play a factor until later in throttle position so I wasn't expecting much in regards to my off idle stumble but much to my surprise that needle height adjustment almost took the off idle stumble away, not completely but it is a huge improvement. I suppose I can make it even better by backing out the idle mixture screws again but I really don't like raising the needles unless I have to so the plan is to put the needle clips back to the middles position and try #38 slow jets instead and go from there. Is my thought process correct on this?

You may have just found your problem. As an example: If you have your idle set too high, you can end up compensating for the jetting issues at idle as you will draw fuel into the pilot circuit from the other jets. Raising the needle will richen up the circuit here with an idle screw set too high.

Sounds like you're a bit lean off idle. Easy test is to take off the air filter. It will make the problem worse. You may have some gunk in your pilot circuit. Make sure your pilots are squeaky clean (and OEM). Aftermarket brass is leaner jetting then stock jets. Also, you may consider renewing any rubber on the airbox and carb boots. These all contribute to increased air in the circuit when they get old and stiff.

I think he posted he raised the clips on the needle, effectively lowering and leaning the circuit more.
But reports the stumble is better…?

[Tracksnblades1]

You may have just found your problem. As an example: If you have your idle set too high, you can end up compensating for the jetting issues at idle as you will draw fuel into the pilot circuit from the other jets. Raising the needle will richen up the circuit here with an idle screw set too high.

Sounds like you're a bit lean off idle. Easy test is to take off the air filter. It will make the problem worse. You may have some gunk in your pilot circuit. Make sure your pilots are squeaky clean (and OEM). Aftermarket brass is leaner jetting then stock jets. Also, you may consider renewing any rubber on the airbox and carb boots. These all contribute to increased air in the circuit when they get old and stiff.

I genuinely appreciate your input but the problem is without a doubt being too rich off idle rather than too lean, amongst other things the spark plugs prove it. When I raised the clips which are now on the second notch from the top, this lowered the needles thus leaning it and that made the biggest improvement of anything I've done. One could say "just leave the needles there and back out the idle mixture screws a bit more" but I feel that lowering the needles like I did just masks the problem at the expense of the engine being lean at part throttle meaning throttle position at say 60 mph because even though lowering the needles was a huge improvement with my off idle hesitation, I did notice a slight decrease on how well the engine pulled when rolling onto the throttle at say 50 or 60 mph which tells me I should try to keep the needles at stock height with the clips in the middle position and address the slow jet instead. Ideally I'd like to keep the needles at the factory height (clip in the middle) and keep the idle mixture screws within the 2 turns out + or - 1/2 that Honda suggests and the only way for me to do that is to try a smaller slow jet. The new jets won't be here till next week so I'll have to wait till next weekend till I have time to try it. Also the airbox boots are new, carbs boots are original but have new o-rings and clamps.

Have you tried going up a long mild grade purposefully holding the throttle in the transition position where it stumbles trying to accelerate?

Or verifying your rich mixture assumption by checking the same throttle transition stumble with the choke just a little bit on.
To see if it worsens or helps the stumble..

[RAFster122s]

I've seen people try to give a motor a full handful of throttle and the motor struggle with the huge change in the butterfly causing a pressure drop in the throat and change in mixture. Rather than roll onto the throttle...
Just because you can drive your car and push the pedal to the floor without the motor complaining about the rapid change if throttle doesn't mean your MC can do the same...

[Bailgang]

Geez I missed all the responses since my last post. Well anyhow the new #38 slow jets are in, needle clips set back to middle position and with idle mixture screws set at 2 full turns out the bike ran the same as it did with the #40 slow jets with needles lowered one notch meaning it's wasn't quite perfect but a huge improvement compared to what I had a few weeks ago. I then backed the idle mixture screws out a bit at a time and it seems happy with them at 2 and 1/4 turns out. It runs so much better now especially at slower speeds like say 30 mph, great throttle response and still screams when I wind it up. My wife had initially gotten some what used to the off idle stumble it had before but now she's having to reacquaint herself to it without that stumble.

I will admit that I may have simply addressed the symptom rather than the root cause but I'll be damned if I can figure out what that root cause could be if there is one. I can see if I had made changes to the bike but I haven't other than the Delkevic exhaust and I've never had to mess with jetting on a stock bike before which makes me wonder what may I have over looked that made me change the slow jet. My 77 F2 may be an exception, I had to raise the needles on it but those are PD carbs and much different than the carbs on the 400. I wish I had and air/fuel ratio monitor to see exactly what's going on. Unless I have an epiphany or you guys beat some sense into me, I'm leaving it for now.

[HondaMan]

The 350F and 400F are both [what I call] emotional bikes.
When they are left alone for very long, they "develop" symptoms of various sorts, I believe to make you feel bad about ignoring them.
If they are ridden frequently they act much better. It's likely related to the tiny orifices that make their carbs work, and their attendant narrow-mindedness for being jealous of the time you've spent without them, doing something else (it's worse if there are other bikes in the same garage with them that get taken out instead, too).

...just my opinion...

[Bailgang]

The 350F and 400F are both [what I call] emotional bikes.
When they are left alone for very long, they "develop" symptoms of various sorts, I believe to make you feel bad about ignoring them.
If they are ridden frequently they act much better. It's likely related to the tiny orifices that make their carbs work, and their attendant narrow-mindedness for being jealous of the time you've spent without them, doing something else (it's worse if there are other bikes in the same garage with them that get taken out instead, too).

...just my opinion...

I've actually wondered the same thing or possibly oxidation in those orifices due to their age and setting too long before I got it that carb cleaner isn't going to touch but I don't know how to clean them any better other than ride it. I've searched the topic and it seems I'm not the only one whose had the exact same issue with their 400F carbs and they too resorted to either lowering the needles or installing smaller slow jets.

I also think I should comment on everyone's suggestions. I just want you to know I haven't ignored them. I've determined it's too rich in a number of ways and yes I have tried applying the choke just a bit when it hesitated and all that did was make it worse. When I said it would bog when rolling into the throttle, I wasn't grabbing a whole handful and cramming it, just the opposite. As far as the new o-rings in the intake manifolds/boots, yes I used some sealant on the flanges and yes I wiped off any excess that got squeezed out inside and out when tightening.. Trust me, it doesn't have a vacuum leak and even if it did I would think that would promote a lean condition rather than too rich and the air filter itself is a new genuine Honda filter. Also any time I made a change I would hook up my vac sync to make sure the carbs were still in sync and ignition timing is spot on and later on gave it a couple more degrees advance to see how that affected it. To put it in a nut shell, I really do appreciate everyone's input on this.

[MauiK3]

My K3 is in the garage near everything else I do out there. I make a point to check it or put hands on it when I walk by. This, I think, helps keep it assured it has not been forgotten in between rides, so far so good.

[Mark1976]

The 350F and 400F are both [what I call] emotional bikes.
When they are left alone for very long, they "develop" symptoms of various sorts, I believe to make you feel bad about ignoring them.
If they are ridden frequently they act much better. It's likely related to the tiny orifices that make their carbs work, and their attendant narrow-mindedness for being jealous of the time you've spent without them, doing something else (it's worse if there are other bikes in the same garage with them that get taken out instead, too).

...just my opinion...

   To Mark's point, the smaller 4's can be rather 'emotional ' from a carburetion perspective.  I've been riding my current 400f for over 30 years with many different variations of exhaust, cams and piston combinations and I learned early on to have a second set of carburetors to literally make the process of jetting quicker and easier. But I'll be honest, each rack has its own little quirks, especially once you get away from the stock settings. The only real variable on your bike is the pipe, I've never used that brand of pipe so I can't attest to how close it is to a stock system. Yet I've never used anything but a 40 as a slow jet in a stock set up. It's interesting I'll give ya that, live and learn.

[Bailgang]

   To Mark's point, the smaller 4's can be rather 'emotional ' from a carburetion perspective.  I've been riding my current 400f for over 30 years with many different variations of exhaust, cams and piston combinations and I learned early on to have a second set of carburetors to literally make the process of jetting quicker and easier. But I'll be honest, each rack has its own little quirks, especially once you get away from the stock settings. The only real variable on your bike is the pipe, I've never used that brand of pipe so I can't attest to how close it is to a stock system. Yet I've never used anything but a 40 as a slow jet in a stock set up. It's interesting I'll give ya that, live and learn.

When I bought a 75 cb550 about 18 years ago, that was my first bike and my first experience with motorcycle carbs and its carbs, aside from being larger they were essentially the same in design as my 400 carbs but those 550 carbs ended up being happy with factory size jets and needle height settings. My 400 carbs though seem to be much more sensitive than I can remember those 550 carbs ever being and having to go with the #38 slow jet still has my head scratching especially when the carbs initially had #42's in them. I definitely wish I had another rack of carbs just to confirm things or put it on a dyno with an exhaust analyzer on it. As far as the bike getting the attention it needs, it's been getting ridden the most with all the test rides I've been doing in fact my wife is starting to get impatient with all my tuning and wishes I'd stop messing with it so she can ride it.