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Car Forum / Honda Cars / August 2006Honda MC question
Hi there, I know this isn't the motorcycle group, but I am looking for some general info on how to tune the carbs on my 79 Honda CM400 Twin. 395CC engine, 5 spd trannie. I have searched the net up and down to no avail. I think my only option will be to break down and try and find a manual at a decent price. So far no luck on EBay.... There is a Clymer one, but I wouuld prefer an actual Honda manual. So what I am looking for is A) Advice on how to tune up the carbs on this bike OR B) Link to a website that shows diagrams and a step by step procedure on how to do this. The current symptoms include: 1. Brown smoke coming from the exhaust, indicating that the bike is overfuelling and running very rich. 2. Intermittently runs on one cylinder, indicating overfueling and fouling of one cylinder. 3. Gas occasional pours out of the overflow of both carbs, indicating a problem with the floats or the fuel shut off valve. Thanks for your input. t > I know this isn't the motorcycle group, but I am looking for some general > info on how to tune the carbs on my 79 Honda CM400 Twin. 395CC engine, 5 > spd > trannie. news://rec.motorcycles.tech Don't know if you have actually visited news://rec.motorcycles.tech, but it SUCKS. A helpful link, PLEASE. t >> I know this isn't the motorcycle group, but I am looking for some general >> info on how to tune the carbs on my 79 Honda CM400 Twin. 395CC engine, 5 >> spd >> trannie. > >news://rec.motorcycles.tech > Don't know if you have actually visited news://rec.motorcycles.tech, but > it > SUCKS. > > A helpful link, PLEASE. Yeah, I read it often. There was a discussion just last week regarding tuning carbs. Don't like my link, don't follow it, but the data you seek is there. > Hi there, > [quoted text clipped - 26 lines] > > t well, your symptoms certainly /do/ point towards float problems and/or needle valve leakage. you need to strip and clean to see what's up. needle valve failure/leakage is very common on older high mileage carbs. regarding subsequent adjustment, i can't say because i've not stripped those particular carbs, however you can balance between chokes/idle circuits with an old pen and a glass bead. pens usually have a slightly conical center [aids mold removal during manufacture] and therefore a glass bead trapped therein will rise according to airflow from bottom to top, if you see what i mean. that and a rubber tube and a pin [to prevent accidental bead ingestion] and you can now balance relative airflows between carbs. it doesn't give you a setting for what the actual flow rate /should/ be, but it allows you to balance them perfectly and costs almost nothing to make. or you can buy the tool that does the same job! I guess I am reserved to buying the book, every resource I have looked at so far requires that you measure the baseline before disassembly. IE Float tab angle, air screw and fuel screw settings. I am pretty sure that they are all wrong, probably been taken apart already and messed with. Without a vacuum gauge, I guess syncing these carbs will be difficult. I have pulled the jets and all removeable gummed up parts and let them soak in carb cleaner for a couple hours. That includes the float needle, which seems to be working much better now that its clean. However, I hope I didn't wreck it by letting it soak in solvent, there are probably rubber seals inside it. I gave it a shot of some high quality assembly prelube to help lubricate any gaskets inside. The carb has 2 jets, #76 and #112. Which one would be have a bigger jet? Also, can I assume that the one with the bigger hole is the main jet? Thanks for the response! t >> Hi there, >> [quoted text clipped - 17 lines] >perfectly and costs almost nothing to make. or you can buy the tool >that does the same job! Also, with regards to the pen tool, good idea! I will try to make one. perhaps more questions to come! PS Do the jets get screwed down all the way? or is there another setting there to help atomization of fuel? Or does that occur only at the needle? t >I guess I am reserved to buying the book, every resource I have looked at so >far requires that you measure the baseline before disassembly. IE Float tab [quoted text clipped - 21 lines] >>perfectly and costs almost nothing to make. or you can buy the tool >>that does the same job! > Also, with regards to the pen tool, good idea! I will try to make one. > perhaps more questions to come! > > PS Do the jets get screwed down all the way? depends on the jet. if it's simply a fixed orifice, yes. if it's a needle jet, with a fine thread on the adjuster screw, no. usually, they get screwed in GENTLY to the stop, then backed out a number of turns. but what that correct turn number is is anyone's guess. before disassembly, see how many turns it takes to the stop for each jet. if they're both roughly the same, that's probably what you want to use again [and balance from there]. if they're way different, you're back to guess work. > or is there another setting > there to help atomization of fuel? Or does that occur only at the needle? jets and floats are the big things. some floats puncture and stop floating :P or their float level has been "helped" into the wrong position. one trick for cleaning jets is to use copper wire to clean them out. copper is softer than the brass of the jet, so won't abrade and enlarge the hole, but is sufficiently strong to bust out most grime that may have accumulated. if you have a magnifier, check the jets for signs of previous damage. if these carbs have been stripped and "helped" before, there's a danger someone's damaged them. sorry, can't help on the settings - no idea for this machine. the book is the way to go. it may be expensive, but it sure is cheaper than paying for a shop to do this stuff. and it has resale value. > t > [quoted text clipped - 23 lines] >>> perfectly and costs almost nothing to make. or you can buy the tool >>> that does the same job! > The current symptoms include: > [quoted text clipped - 6 lines] > > Thanks for your input. It seems unlikely that both float valves would be hosed, but maybe some dirt found its way from the tank and screwed them up. Pull the carbs and remove the float reservoir bottom end caps. Dependiong on the design, you should have instant access to the float and valve(s). There is probably a simple pin to pull/slide that will release the floats. Pull them, watching for jumping valve components as you do so. Check the valve seats and the needle valve for wear, dirt, corrosion, etc. -Greg |
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