Topic: Looking for confirmation

[davis5052]

Good evening.  First time poster, long time viewer.  To be brief: I have a 1975 400F that sat for 19 years - with fuel in tank and carbs.  The tank cleaned up surprisingly well but the carbs are giving me fits.  I've gone over all the relevant posts I could find and racted accordingly.  Carbs have been cleaned ultrasonically.  All original guts put back in.  Timing has been set with strobe.  So on and so forth.  The problem is that this thing runs extremely rich, so much so that the plugs foul up with the fluffy black soot rather quickly.  The mixture screws have no effect (very little).  I checked the float levels several times and finally set them to 24mm (no sediment anytime I opened them up).  The float valves seem to be "soft" but do work from what I'm seeing.  The carbs were synched with the Motion Pro manometer.  Here's the big clue - if I leave the brass adapters open when I start the bike it runs better.  I did replace the intake boots with new Honda (a little pricey but it eliminated one potential source of trouble).  If the bike runs better with the synch tubes open does this sound like my carbs are in need of "new" guts?  The emulsion tubes, all jets, everything is clean.  I've worked other bikes before but never had this type of carb issue.  Your thoughts?  Thanks in advance.

[jessezm]

No-brainer but have you checked/replaced your air filter?

[Greggo]

Are you using the stock intake system?  Did you replace the air filter?  Does the bike run better without the air filter?  Are you SURE you cleaned EVERY hole/jet/orifice/passageway in those carbs?  Are all four plugs equally rich?  Have you checked the float level with a clear tube for each carb?  If the plugs are equally sooty, something is clearly choking the system of air somewhere...If all or one are different, you could have individual carb problems, possibly from cleaning/unfiltered gas/something else.

[Greggo]

Jesse beat me to it.  If it's paper, they're only good for one season.  Even if it looks brand new, it can be clogged.

[davis5052]

Thanks to all for the replies - thought I'd address all in one swoop.  Runs the same with/without air filter.  All plugs equally fouled.  As for the carbs - everything was removed and each carb put in the ultrasonic cleaner and then reassembled with original guts.  All passageways pass air and all brass with holes gently cleaned out to ensure "open".  This is what really has had me by the short hairs - I believe everything is correct and the carbs "look" good but sure don't perform like it.  My son has a '77 400F and did the same drill with it and the bike is running great.  I initally thought I was missing something obvious but after reviewing this and slugging it out for a month I think I need to just start over and this time change out the carb guts - even if the parts don't perform like OEM they can't be as bad as what I've got now.  Thanks again

[phil71]

correct me if i'm wrong, but those idle mixture screws are AIR screws on that carb. You mentioned it running somewhat better with the sync holes open. You also mentioned the air screws do nothing. THis would point to at least a few of the carbs not letting air go thru the little passage that runs from the throat of the carb, thru the body, to the seat of the air screw and finally to the outlet side of the carb. We tend to put a lot of faith in ultrasonic cleaning, but it's not an absolute fix for some nasty clogs. I'd get in those passages and try varying sizes of guitar strings.. pull the air screws and start working it from every side you can get at till you can blow compressed air thru them.

[davis5052]

One of the first things I did was explore this avenue - with the screws removed you can actually "see" from front to back.  Some fine copper wire and With that accomplished (I didn't really find anythng blocked) I gave it another go.  Still bad.  I then di dthe float level adjustment to 24mm believing the shutoffs were too soft and allowing an overflow.  This helped a little but still not where it needed to be.  I can take her down the street but it's an awkward ride; rough progression from idle to off-idle to mid throttle.  My big unknown is why I'm not getting air flow if the idle air passage is open - is there something I'm missng that would allow fuel flow that cancels out the air?  I even tried a hotter plug to compensate - no improvement.  With the air filter out and the idle network air passage 'open' I'm still getting too much gas ... any thoughts about the carb internals that would help create a rich condition? 

[jneuf]

Assuming all is well with the carbs, have you checked the points and coils? A weak spark could be fouling the plugs as well.

[davis5052]

I didn't use any scientific method but I attached a plug to #4 and grounded it to the engine while I cranked it over - appeared to be a decent strong spark.  I also did a leak down test on the cylinders and that showed 15-20% leak rate; I read that a fresh engine can have 10% y virtue of piston ring bypass so I'm believing that the cylinders are tight enough.  I triple checked my valves and they are within spec, as is the timing and points.  I guess from what the forum folks that answered are confirming is that either the carbs are completely screwed up or, as I suspected, missing something that is completely obvious.  Thanks for the input.

[LesterPiglet]

Bad fuel? Try a different source.

[scottly]

Have you checked the float level with a clear tube for each carb? 

Do this check!

[TwoTired]

Did you remove the main emulsion tubes for cleaning?

Did you verify the float valve spring pins are free and "plunge" as they should?
Have you verified the actual fuel level in the bowls?

Did you replace the main jet orings?

On some carbs, there are separate slide needle jets.  Are they installed?  (They are sometimes part of the main emulsion tube.)

What slide needle jet setting did you use?

[jessezm]

A few more thoughts:  On reassembling the carbs, it is in fact possible to install the slides backwards so that the cutout sides are facing the motor and not the air filter as they should.  Just make sure this is not your problem.  Second thought--sometimes what feels like a rich condition can actually be an ignition problem.  Have you checked to see that you have good, fat spark, good voltage at the coils, a good battery (I'm assuming you replaced it after 19 years...), and good, clean points that are properly gapped?  And does the resistance across the plug caps measure correct?

[grcamna2]

I hope you replaced all the rubber seals/o-rings in each carb w/ the O.E. carb rubber seal kit's..they sure helped my carbs run clean. +1 on the main jet o-rings !

[camelman]

Did you check the jets to see if the PO drilled them out for "MORE POWER"?  I'd be concerned about those float valves sealing and the slides possibly being in backwards. It wouldn't hurt to check needle clip position and needle size either. The main jets will affect mixture all the way down to "off idle".

Worn out o-rings could easily cause these symptoms too.

[davis5052]

Excellent points.  I'm not so concerned about the PO drilling or trying to get more power as this thing is completely original.  Not original enough to be a show bike but a very clean driver.  With that said I will double check jet sizes and orings - the fact that it is rough from off-idle through midthrottle is disconcerting.  Thanks for the input.

[Rudi91]

Sorry to re-open this old topic but the problem u have described is exactly the problem im facing with my cb350f.
Did u ever solve the problem? If so how did u do it?

[lucky]

Thanks to all for the replies - thought I'd address all in one swoop.  Runs the same with/without air filter.  All plugs equally fouled.  As for the carbs - everything was removed and each carb put in the ultrasonic cleaner and then reassembled with original guts.  All passageways pass air and all brass with holes gently cleaned out to ensure "open".  This is what really has had me by the short hairs - I believe everything is correct and the carbs "look" good but sure don't perform like it.  My son has a '77 400F and did the same drill with it and the bike is running great.  I initally thought I was missing something obvious but after reviewing this and slugging it out for a month I think I need to just start over and this time change out the carb guts - even if the parts don't perform like OEM they can't be as bad as what I've got now.  Thanks again

Maybe you should just pay an expert to rebuild them.
They do a great job and really make them look great.

[Rigid]

Never trust the numbers on a jet to be what it is.  I have drilled my share over the years but have the decency to file off the numbers.  I posted a chart some time ago on here that showed jet sizes and their drill bit diameter equivalent.