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Author Topic: '75 cb400f rough idle  (Read 2187 times)

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Offline rockis4ever

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'75 cb400f rough idle
« on: September 04, 2011, 11:55:50 AM »
Alright guys. I picked my supersport up for a whopping $200.  It sat in a shed for 15yrs, the guy I bought it from, hooked cables to it and she started right up so I knew i had to have it. I've put about 5k miles or so on it in the past four months. I removed all the pitting and polished every chrome piece, replaced plugs, changed the oil, rebuilt the master cylinder and front caliper, etc... (the bike was it really good shape to be honest) So for the past two weeks or so, she hasn't had the pick up that she first had. The idle when hot and i come to a stop light will hang at around 2400 then drop to 12-1400 where I had it set. If i increased the idle above 2k the bike would run better but not optimal. With this happening I said the hell with it, I'll clean the carbs. I pulled the whole assembly from the bike and removed all four carbs indiviually from the bracket. I pulled the bowls and top covers off and went through four cans of carb cleaner. Now i have the carbs and everything back together on the bike. She runs, but sounds more like a four stroke dirtbike, won't go above 6-7k rpm without major hesitation and crackling. I think it may be the gas in my tank... I use 93 octane w/ ethanol treatment and a carb/fuel system cleaner.  I put some gas from her in my push mower and it ran like absolute #$%*. Any Ideas would be greatly appreciated! Thanks
-Ryan
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Love is not between a man and a woman, a man and a man, or a woman and a woman.  Love only truely exist between a man and his bike!  The only complaining you get is when you let off the throttle and she back fires from excessive deceleration. Now that's the only #$%*ing I don't mind!

Offline MoMo

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Re: '75 cb400f rough idle
« Reply #1 on: September 04, 2011, 12:14:55 PM »
Ryan,  when you cleaned the carbs did you remove the midrange emulsifier tubes?  ref #3 of this fiche  http://www.mrcycles.com/fiche_section_detail.asp?section=121666&category=MOTORCYCLES&make=HONDA&year=1975&fveh=2911    ....Larry
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Offline rockis4ever

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Re: '75 cb400f rough idle
« Reply #2 on: September 04, 2011, 12:26:42 PM »
No but i did replace the main jet (75) and cleaned needles and jets.   
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Love is not between a man and a woman, a man and a man, or a woman and a woman.  Love only truely exist between a man and his bike!  The only complaining you get is when you let off the throttle and she back fires from excessive deceleration. Now that's the only #$%*ing I don't mind!

Offline MoMo

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Re: '75 cb400f rough idle
« Reply #3 on: September 04, 2011, 01:39:41 PM »
If the bike was sitting for any period of time, those tubes need to be cleaned.  There are a series of holes that aerate the gas from 1/4 throttle to full.  My experience is those holes are always clogged...Larry
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Offline rockis4ever

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Re: '75 cb400f rough idle
« Reply #4 on: September 04, 2011, 03:30:40 PM »
Is that the inch and a half long brass screw needle looking thing with the multiple ports that screws in by the main jet? If so I miss interpreted the fiche and did blow them out. I made sure the cleaner forced everything out and unplugged the ports. I also made sure to blow out every nook and cranny that i could. You can't really go overboard when you're cleaning 37 yr old carbs. I even made sure to let them sit over night to ensure that all the carb cleaner evaporated
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Love is not between a man and a woman, a man and a man, or a woman and a woman.  Love only truely exist between a man and his bike!  The only complaining you get is when you let off the throttle and she back fires from excessive deceleration. Now that's the only #$%*ing I don't mind!

Offline rockis4ever

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Re: '75 cb400f rough idle
« Reply #5 on: September 04, 2011, 04:46:16 PM »
I took the bike for a five mile ride to see if it may clear up. when shifting threw 1st-4th under light acceleration (5-6k max) the bike sounds helatious. when i get into 5th and sixth with the throttle wide, she runs fine. I did remove all of the carbs from the housing. Now the linkage...I did my best to keep it uniform. (the nut the screws onto the threaded rod with the thin plate connecting carb 1+2 and 3+4) does that matter signifigantly?
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Love is not between a man and a woman, a man and a man, or a woman and a woman.  Love only truely exist between a man and his bike!  The only complaining you get is when you let off the throttle and she back fires from excessive deceleration. Now that's the only #$%*ing I don't mind!

Offline MoMo

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Re: '75 cb400f rough idle
« Reply #6 on: September 04, 2011, 06:12:59 PM »
Quote from: rockis4ever on September 04, 2011, 03:30:40 PM
Is that the inch and a half long brass screw needle looking thing with the multiple ports that screws in by the main jet? If so I miss interpreted the fiche and did blow them out. I made sure the cleaner forced everything out and unplugged the ports. I also made sure to blow out every nook and cranny that i could. You can't really go overboard when you're cleaning 37 yr old carbs. I even made sure to let them sit over night to ensure that all the carb cleaner evaporated


Yes, that is the part.  Read your later post, is the bike running well now?...Larry
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Offline Gordon

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Re: '75 cb400f rough idle
« Reply #7 on: September 04, 2011, 06:26:52 PM »
The emulsion tubes sit on top of the main jets and are what the needle moves up and down inside of.  In clean carbs they'll just fall out once the main jets are removed, but in dirty, varnished carbs they're typically glued into place and need to either be pulled out from the bottom or pushed out from the top.  If you didn't remove these and clean them during your rebuild then it needs to be done. 
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Offline phil71

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Re: '75 cb400f rough idle
« Reply #8 on: September 04, 2011, 06:53:26 PM »
i dunno, that behavior sounds like a vacuum leak, and the fact that it got worse after a re-rack would seem to indicate a shot boot or 2 that got worse from the fiddling.n Spray carb cleaner on 'em when running. If engine speed changes, that's your culprit.
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Offline rockis4ever

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Re: '75 cb400f rough idle
« Reply #9 on: September 05, 2011, 02:22:21 PM »
Its still runnin funky. I did seat all the boots well, but on the intake side there was alot of grime btw the boots and the intake chamber. to be honest i think my issue lies in either the gas, or the mixture in the carbs themselves. It almost sounds like its being choked of air and running really lean. If i keep loosening my air-idel screws, could that richen the mixture some?
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Love is not between a man and a woman, a man and a man, or a woman and a woman.  Love only truely exist between a man and his bike!  The only complaining you get is when you let off the throttle and she back fires from excessive deceleration. Now that's the only #$%*ing I don't mind!

Offline phil71

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Re: '75 cb400f rough idle
« Reply #10 on: September 05, 2011, 02:49:02 PM »
Soooo.. you didn't actually TRY spraying the boots while running. Why ask for help?
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Offline rockis4ever

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Re: '75 cb400f rough idle
« Reply #11 on: September 06, 2011, 11:59:23 AM »
I did not mean to offend you Phil. I didn't want to try it because of all that build up. I'm afraid it will cause an even worse leak than there already is. I resynced the carbs and replaced the gas. Shes running better but still hesitates some.
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Love is not between a man and a woman, a man and a man, or a woman and a woman.  Love only truely exist between a man and his bike!  The only complaining you get is when you let off the throttle and she back fires from excessive deceleration. Now that's the only #$%*ing I don't mind!

Offline phil71

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Re: '75 cb400f rough idle
« Reply #12 on: September 06, 2011, 12:24:21 PM »
Rock, I wasn't offended, but your reasons for not checking the boot condition still don't make any sense. Do you want it fixed, or don't you?
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Offline jessezm

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Re: '75 cb400f rough idle
« Reply #13 on: September 06, 2011, 12:46:53 PM »
If you haven't pulled the carbs back off and cleaned out the emulsifier tubes then I really think you're wasting your time chasing the problem.  Those 1.5 inch brass tubes you referred to in your previous post were NOT what photolar and Gordon were referring to--those are the pilot jets, and also need to be carefully cleaned so you can hold them up to the light and see a perfectly round pin hole.  I use a guitar string ground to a small enough diameter to fit through the hole--it is tiny!

As for the emulsion tubes, your best bet is to tap them out from above.  If you take the slides out and look straight down from the top of the carb, you'll see it directly below in the bottom of the carb throat (venturi).  I like to tap them out with a #2 pencil, but have been guilty of using a metal punch as well, giving it one good whack.  Once you get that out of the bottom of the carb (underneath where the main jet goes), you'll see it's a long brass tube with holes along it's sides.  Clean those thoroughly and clean out the tube it came out of as well.

You also mentioned keeping the linkage uniform.  All of these screws will not be at the same height, and that is not where you want to look to get your carbs properly bench synched.   If you loosen those lock nuts and turn the screws, it lowers or raises the slides.  Just do each one so that the slides are at the exact same height.  You can use an 8th-inch drill bit to set them all the same.  Then do the vacuum sync to fine tune once they are back on the bike.
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