Topic: CB400f running rich. Can't figure out why. Any ideas?

[kfleschner]

1975 CB400f
MAC 4into1 exhaust
Pod filters
Stock size (new) jets, air screws set to 2 turns out.
Non-resistive spark plugs (new within 1yr, minimal riding). Assuming stock plug wires and rest of electrical.
Dyna S ignition; timing set while bike was running with timing light.
Carbs pretty well balanced with engine warm at 1200rpm and idle screw in just a touch.

The bike has been running rich for some time now. I thought it was the carbs, so I swapped in bigger jets. That didn't work so I went back to stock. I put pods on mostly because I got tired of pulling the carb body in and out so many times and reattaching the air box. Short story, I've done just about everything I know to do and I've still got fouled plugs and white smoke puffing out the exhaust on idle. Do I have an oil leak somewhere I'm not checking? Is there something else I can check or adjust? I'm open to just about any idea people have on getting the bike running well again. Thanks!

[Bodi]

The simple answer is to put the stock airbox back on.

You probably need one size smaller pilot jet, and a needle adjustment - and will need to reprofile the needles a bit to get really good performance with pods.

You have to approach carb tuning methodically. Have all internals sparkling clean, especially the main jet emulsion tubes. Do some WOT plug chops to determine how the main jet is doing, adjust main jet sizes to get tan/brown pugs at WOT. Then move to about half throttle and raise/lower the needles to get good plug color there. Finally work in low throttle - you can first change the pilot jet to get that looking good. Lastly adjust the airscrews for max rpm at idle. This will leave you with some flat spots and surging, pods are not so easy to tune for.
To properly tune the carbs for pods you next mark the throttle grip at ~1/8 travel intervals and do plug chops to see if any are too rich: adjust the needle heights until some are lean and some are good but none are rich: record the throttle settings and the result ie good, bit lean, very lean. You then set the throttle to a "lean" position and mark each needle where it enters its main jet, remove it, and chuck it in a drill or drill press. With a cloth and brasso/solvol pinch each at the marked point while it's turning, to thin the needle a bit (and richen the mixture). Then repeat the plug chop until that spot is correct ... and move to the next lean setting. Of course you can only make the needle smaller, so don't overdo it.
If that gets you good plug color at all throttle settings and you still have surging or flat spots at low throttle... the slide cutout probably has to be changed. Not easy. You can increase it (with difficulty) but reducing it is not practical.
This takes a LOT of iterations and is a super PITA. Putting the OEM airbox and filter back on is much easier.

[kfleschner]

I put the original airbox on the carbs with a new filter. Rechecked the air screws and timing. Still have white smoke coming out of the pipe. I ran it until it was warm and pulled the plugs to see how they looked. I must not have run it long enough because the plugs looked fine. But I think I stripped out part of the threads trying to get one back in, so go me. I'll have to get a tap and chase those threads out unless anyone has a better idea to get a plug in right. Anything else I should check for the rich smelling exhaust?

[Bodi]

You mention new jets - Honda OEM Keihin jets, or aftermarket (Keyster etc) Aftermarket jets are renowned for not working properly. Why did you replace the jets? These don't wear out very often. If you used non-Honda jets then try putting your original ones back in.
Otherwise, carb float height is one likely issue, the main jet and/or pilot jet emulsion tube cross holes being blocked is another.

[kfleschner]

I was having issues getting the engine to idle correctly and running rich. The carbs looked clean, but I swapped out the exhaust (original was rusted through in a spot) and read you needed to rejet. So I figured while I was in there I'd clean up the carbs and put new hardware in. I've still got the originals floating around somewhere. I'll pull the carbs (yet again), put the originals in, check the float height, etc and get back to you on how it's running. Still worried about those threads in the header for the spark plug.

[harisuluv]

1.  Use the stock size pilot jet.  (40)
2.  Move the clip position on the needle jet to 4th from the top (stock is middle position).  THIS IS IMPORTANT.
3.  Go to a 90 main jet.

Done.

[Scott S]

1.  Use the stock size pilot jet.  (40)
2.  Move the clip position on the needle jet to 4th from the top (stock is middle position).  THIS IS IMPORTANT.
3.  Go to a 90 main jet.

Done.

 Just curious....Is this a combination that you've found works for the 400F or ??

[harisuluv]

Yes it seems to work well!

[kfleschner]

Harisuluv, where do you get your #90 mains? I keep seeing #80 and #98 but nothing inbetween.

[harisuluv]

you can get them at a lot of places, 4into1 has them:

http://4into1.com/keyster-keihin-press-fit-secondary-main-jets/

you select the size you want from the drop down menu.  they are keyster, but they should be fine.  Always best to go with OEM if possible, but this jet is not available as a honda part anymore.  If you go to jetsrus you may be able to find some keihin parts that are correct but they will be about 3x the cost.

[MoMo]

harisluv, that drop down menu doesn't have 90, size i think my 400 may need...larry

[harisuluv]

Yeah, your'e right.  I think it is perhaps an out of stock issue.  They also sell them at siriusconinc.com

http://siriusconinc.com/pro-detail.php?pid=&product_id=3821

Also, jetsrus.

[kfleschner]

Jetsrus doesn't have the 90's either. Just checked.

[Don R]

 
I use a piece of rubber fuel hose slipped over the plugs as a starter wrench to start my plugs. I always screw them all the way in with that then tighten with a socket. I have one hose with a plastic T fitting on it for a handle. It always slips before doing harm.

[kfleschner]

I took a look with a little flashlight and the threads are shot. Not stripped all the way, but some are squashed and there's nothing on the top to get the plug started. Ugh.

[MoMo]

I took a look with a little flashlight and the threads are shot. Not stripped all the way, but some are squashed and there's nothing on the top to get the plug started. Ugh.

at least the head can come off with the motor still in the frame.  good luck with the stripped hole...Larry

[harisuluv]

There is some stripped spark plug thread tool someone bought recently and used it with success.  Forgot what it's called but it exists.  Probably best not to mess with it, without the tool you'll probably just get metal shavings/bits in your cylinder.

[Tim2005]

Is it a thread at cylinder 1 or 4, the easy to get at ones?  If so you might find a local engineers shop that will helicoil it in situ- it's probably not the best way, but so much easier than removing the head, you must blow it out a lot afterwards to try to get rid of as much swarf etc as possible. I've heard of people getting away with this before.

That said, is the bike using much oil? How well does it run- what's top speed like etc? I'm wondering if there is significant compression problem- yours wouldn't be the first 400f to break top rings.

Finally, I'm very surprised to see 90 mains suggested, 75s or 78s are the most I've ever used on a stock engine and airbox, I've only ever gone as high as 90s with bellmouths.

[Bodi]

The best fix for partially stripped plug threads is to remove the head and chase the threads with a tap, starting from inside.
If you can get a tap to start true from outside you can try that - and avoid head removal - but risk ruining the threads the whole way through if it "cross threads" and cuts new ones, leaving insufficient metal to hold the plug securely: that means a helicoil repair. To retap from the outside you coat the tap with chassis grease - this holds the chips so they don't drop into the cylinder. Once tapped, clean and regrease the tap then run it through the hole to pick up loose chips (a couple of times).

[kfleschner]

Took the head off and got myself a 12 x 1.25 tap to go over the plug threads. When I took the head off I saw the piston heads and heads covered in about a 1-2mm of carbon. So I gently scratched some of that off and saw the 4 cylinder had a piston with a 1 on it. Pretty sure this would be OK as the 1 and 4 fire at the same time, but wanted to check this was OK? Is there an accepted best way of cleaning the heads and the tops of the pistons off? I was just about to lift the cylinders off as well when I realized I don't really need to do that and would just make reassembly more complicated. While I've got the head off, would it be worth it to pull the cylinders and check the pistons or rings? Is there anything else I should be thinking of doing while the engine is half apart?

[MoMo]

cam chain and tensioners.  Everytime a part is removed an invitation is sent out to go a bit further.  If you plan on keeping the bike, remove the cylinders and inspect...Larry

[Bodi]

Numbers on the piston crowns just identify the mold used or something similar. The pistons are identical and not marked for which cylinder they are in.

[camelman]

OP, the white smoke you mentioned is just condensation coming out as water vapor. It'll clear up, but can take much longer than you'd expect. TwoTired always used to point that out. Excess fuel shows as black smoke and burning oil shows as blue-ish smoke. One exception for excess fuel being white is if the fuel is not combusting at all in the cylinder, in which case it can come out as white vapor. It would have to be a lot of fuel though... along the lines of having a main jet fall out.

[kfleschner]

Well I've got a pretty long checklist of things to do now. The head's off as the threads for plugs need to be chased regardless of the carbs. While the heads off I hear I should check the rings, cam chain adjusts and slipper. Not sure if I want to crack the case to replace the cam chain actual. Can't seem to find a replacement in stock anyways. The piston crowns and heads need to be cleaned of 40 some years of carbon buildup. Not sure if I dare touch the valves. Carbs need have the jets checked, floats remeasured, etc. Then hope it all goes back together again without a spare bolt or two left over.

[Redline it]

I'm going through the same odd changing ways of cb400f good running days. I've been finding out that the highly engineered fast reving little power plant can run nearly flawless until its time to fade all happens at once. No matter how it has ran in the 30,000 miles of rich and lean, the more I look the more I find it's worn out, throughout. I could go crazy(er) by wanting to fix one thing carbs, jets, bad gas bent back to work. I know now I have a little less pressure in getting it back together. Having another 400 works, having a spare box of motor, or an experiment to play with. I'm thinking vent it with inline pair of Mikuni's 38mm bailing wired on. Add some No2 for kicks. That's what trying to figure out what's wrong with it has done. Good luck.   

[MoMo]

Check the valves by pouring a liquid into the chamber.  If none leaks out, you should be good to go.  You can replace the cam chain with a linked chain so you do not have to split the cases...Larry

[kfleschner]

Larry, are you saying to put liquid in the head while it's off the rest of the engine, or does it need to be reassembled to test it under pressure?

[MoMo]

Larry, are you saying to put liquid in the head while it's off the rest of the engine, or does it need to be reassembled to test it under pressure?

When the head is off.  If the seats are bad, liquid will seep out of the seats. I use WD40, heard of people using gas or water...Larry