Topic: 400F Stumble at 4-5k RPM

[jakec]

Here are the symptoms (400F):

1. Slight hesitation pulling off of idle (1,000-5,000rpm). I have read that this is likely due to pilot jet but I just checked all jets last week for correct size and plugged holes.

2. Beginning of blue coloration in 1 & 4 header. Beginning of slight yellow in 2 & 3. (Note this could be caused by the fact that I originally had the bike running intermittently on 2 & 3, causing the cumulative heating of 1 & 4 to be greater? Also note that headers are DSS and not double walled which ma contribute to faster discoloration)

3. Plugs 1, 3, & 4 are grey/white, plug 2 is black.

Totally stock configuration, with Dyna ignition, static timed. I had planned to use a strobe as well as use a carb tune when I did my 500 mile tuneup (I have to borrow those tools).

What I'm wondering is if there is anything I should try to do BEFORE I remove the carbs again.

[jakec]

Carbs are off. Removed all jets and emulsion tubes. Nothing seems wrong, and checked all holes and they are clear. #3 emulsion tune had some calcium type stuff on it that I removed.

All air screws were at 1-3/4 turns out. I set them all to 1-1/2.

Checked all the float heights and made some adjustments. They are all now 21mm when the tang of the float just touches the spring post in the needle valve. Not resting on it.

The only thing I haven’t checked is the needle position because it’s so hard to get to. Should I check it before putting carbs back in?

[mrbreeze]

I would confirm the needle positions and make sure they are the correct part while you have the carbs off. Sure they're hard to get to. Wouldn't be much fun if they were easy! Be careful with your float pins if you go in. You don't want to break a post. Clean everything up real purty while you're in there.
I would also check your ignition system a little deeper.1-4 or 2-3 problems usually point to the ignition system. After all that's done along with valve & cam chain adjustment get the carbs vacuum synced and you should be golden.

[jakec]

Ok I have confirmed all 4 needles are in the stock position (middle groove).

[mrbreeze]

You didn't happen to check the number on them did you? You may want to know that for future reference. If you didn't get the number at least right down what position they're in. You might not remember a year or 2 from now. And you now know how hard they are to access.
The reason I'm telling you this is I have my carbs on the bench right now. I moved from sea level to 7000 ft. 2 years ago and I'm going through my carbs to clean and fix the jetting. Besides that I'm bored with COVID lockdown and getting my bike up and running again! The last time I had them open was 2005 when I moved from 5000 ft. to sea level. I didn't know any better and put the needles in from my K&L kits and moved them from #4 to #2 clip. Here it is 15 years later and I didn't remember what clip I put em to or that I used aftermarket needles. They aren't the right profile and taper as the originals and I'm glad I kept them and am putting them back now. Unless you were the original owner you have no idea what a previous owner did in there!

[scottly]

Checked all the float heights and made some adjustments. They are all now 21mm when the tang of the float just touches the spring post in the needle valve. Not resting on it.

Turn the carbs upside down and check if they still all read the same. Verify the fuel level with the clear-tube test.

[jakec]

I had been hoping they would be stock position, and they were so that's good. For some reason I was intimidated by taking the slides out even though I'd done it before. In the end it was only about 15 minutes each.

I did a crude bench sync and will probably start the bike up on thursday, and do a carb tune with the vacuum gauges. Does anyone know if you should do dynamic timing before or after vacuum sync?

Also that's funny about the elevation thing you mentioned, my family used to vacation to colorado and we would bring all of our bikes and quads and not even consider the altitude change.

[jakec]

Turn the carbs upside down and check if they still all read the same. Verify the fuel level with the clear-tube test.

I was measuring with the carbs upside down already. The spring posts in the float valve needles aren't all exactly the same strength, so if I let the floats rest with the carbs upside down then the heights are all different. I carefully supported the floats until they contacted the post, and took the measurement then.

Of everything that I checked on the carbs tonight, the only place I made a significant change or discovery was the float heights. So hopefully that was the problem.

[mrbreeze]

Do everything before the carb sync. Clean carbs, float adjust, timing,points,cam chain,valves. Even gas tank,petcock, lines, air & fuel filters.

[scottly]

Turn the carbs upside down and check if they still all read the same. Verify the fuel level with the clear-tube test.

I was measuring with the carbs upside down already. The spring posts in the float valve needles aren't all exactly the same strength, so if I let the floats rest with the carbs upside down then the heights are all different.

Exactly, and if you don't take the different strength of the springs into account, the fuel levels in the bowls will also be different. Checking the height with the tang just touching the post is not an accurate measurement. 

[jakec]

I see what you mean, but the spring supporting the weight of the float would be different than the spring supporting the weight of the needle, right? When the carbs are upright and in operation.

What orientation would you do the adjustment in? carbs upside down or upright?

Also Mr. Breeze I have done everything besides the two things mentioned, dynamic timing and vacuum sync.

[scottly]

Given funky needle springs, I would do the initial adjustment with the carbs upside down, and then verify the level with the carbs right-side up using the clear tube method. This can be done on the bench, before mounting the carbs.

[jakec]

I have it all set up with the carb tune right now. #3 pipe is merely warm while the others are hot, and the vacuum reading is way higher as well. At idle the other carbs are reading 8-16 whereas #3 is over 30.

I did bench sync before installing the carbs so I know that slide isn’t wide open or something. But other than that I don’t know what this means?

[mrbreeze]

You know the problem is cylinder is 3 so switch some things one at a time. If you swap the 2-3 plug wires and the problem moves to 2 you have a problem with that coil/wire. Swap plugs in 3-4. Does the problem switch to 4? If the problem stays at 3 then it may be a clogged jet. Have you tried to adjust the slide on 3? See if it will take adjustment. Did the clear tube show equal levels across all carbs? Time to detective up!

[jakec]

Well I had a little panic moment and texted the friend I’m borrowing the gauge from... and he said well just turn the screw in until it goes down dumbass... long story short I got them synced within the spec per manual. I actually wnjoyed syncing! I had a fan going as well as my old IV stand that I use as a coat rack normally holding the aux tank.. everything is good now as far as I can tell! I’m gona ride a little now and then check the plugs.

[jakec]

Pic

[mrbreeze]

Sweet!

[jakec]

First ride yesterday and it seems that the stumbling aroun 4,000 rpm is persisting. There is also a little stumbling in the higher rpm range, although much more subtle. You can barely notice it, but it's there.

[jakec]

Did another ride yesterday with a passenger around town, which meant I was in the bad range more often. It was very unpleasant riding. I want to fix this problem.. . any ideas?

Idles great. Runs great above 6-7k RPM. Just stumbles really hard in between.

I think it may be a good idea to replace my float valves as their spring tension is not all the same. Then I can better set the float level. However an OEM set is over $100. Is there any worthwile alternative? I have never bought aftermarket carb parts, always just used what was inside when i got them.

https://www.ebay.com/itm/Honda-350-400-Carburetor-Float-Valve-Set-CB350F-CB400F-16011-329-004/221503194605?epid=22019489465&hash=item33929e85ed:g:aCsAAOSwA4dWJN-7

[jakec]

#3 a little darker than the rest. Could slight overfueling on #3 cylinder be the cause of the stumble?

[jakec]

Can damaged slide opening contours affect this? I know at least one has chips in it from PO sync attempts. But I have vacuum synced now.

I adjusted #3 float level about 1mm and have been playing with ari screws with almost no effect

[jakec]

I seem to remember I had the carbs off 4-5 times cleaning jets meticulously, and finally found something small in either the slow jet or emulsion tube horizontal holes. The problem never totally went away, but improved by like 90%. Well it's coming back now, worse when the bike is cold. I just had the carbs off a month or two back to fix a pinched gasket, so for now I'm going to try some new plugs (3500 miles) and some seafoam. I wouldn't normally look to seafoam to try and fix a problem, but I don't want to take those carbs off again right now, and I'm riding this bike to work every day.

I should say, warmed up it only stumbles 4-5k. Cold it stumbles from idle to 5k.

[denward17]

Do you have a filter screen on your petcock?

[willbird]

I looked quick and did not see a mention of what bike this is exactly ?

My K2 has pods, Had k&n back then and Mac 4:1 header. It acted like you describe and my plugs were white. I raised needles one position and it seemed to help a lot.

As far as carbs go the throttle opening position is maybe more important than the RPM. In my case back then the imho lean condition was happening at cruise 50-60mph and maybe 1/4 throttle. The idle screw setting also does a lot more than idle mixture IMHO. Did you say what jet size you are running ? Are you using E10 fuel or 100% gasoline ?

Bill

[willbird]

Oh sorry I see the bike type in the topic now, my bad