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Hello ive been working on a 1978 cb400t for 2 months now, im pretty far along ive torn the whole bike apart took the engine out replaced the gaskets on the top end, cleaned the pistons and faces of the valves (did not disassemble the head) replaced the piston rings (they didnt need it but i didnt want to reuse the old ones). I put the whole mess back together minus the air box which i wont be using anymore and minus the exhaust. I started her up it didnt go and after many failed attempts i checked the timing .. i had installed the sprocket upside down.. so i fixed that put it where it needed to be and tried starting her, she started up almost right away. I didnt need choke but did have to kick start because the battery drained from all the other attempts. I was able to get it to idle but couldnt hold it evenly, it would surge up to 1250 or 1300 than go back to 1200 rpm. i have checked the balance of two carbs they are good, the idle (rich or lean) adjustments i have adjusted from to rich to too lean and everything in between and no difference still wont give more than 3000 rpms. and if i throttle to much it will just die. I put on the two exhasut pipes with out the catalytic or mufflers and the bike finally held an even 1200 rpm at idle but would still not go past 3000 rpms and still dies if i continue to throttle.. When i tried riding the bike it would die almost the moment i began to release the clutch and could only save it by throttling and pull the clutch back out . Ive also tested the battery and at idle 12v and under throttle 14.5 so that checked out fine, petcock is new and delivering fuel on demand. Any ideas would be greatly appreciated i keep thinking maybe the alternator "T" mark and the cam marks are not 100% maybe 99.9% but they seem right ? cant figure this out?
this is her on the other section of this forum http://www.dotheton.com/forum/index.php?topic=54536.msg615602#msg615602
if you are not running mufflers and/or airbox, still on factory-size jets in the carbs....that is your issue. The engine is breathing far more air than the fuel can keep up with if you don't have airbox or mufflers on. Decide on what exhaust and air intake system you want to run and then get your jetting set up to work with them and this issue should go away.
my compression test came back fine, i didnt warm up the engine much but it went to 14. focusinprogress If i put the old exhaust system on minus the airbox will i still be looking at new jets?
Simplify....Cover the basics.
1. Fuel
2. Compression
3. Spark
4. Timing
If each of these is in order this bike should run, pending you put it back together correctly... Buy a Clymer manual and start reading.
my compression test came back fine, i didnt warm up the engine much but it went to 14. focusinprogress If i put the old exhaust system on minus the airbox will i still be looking at new jets?
any drastic change in the breathing characteristics of the engine will require larger jets for more fuel. 3000rpm sounds to me like you also need to move your needle jet clip position. Proper way to dial in your jetting is called a "plug chop"..... you can google that if you need to.
What I would do:
-verify that your timing is correct, and make sure the timing advance plate is working properly
-verify that your spark plug wires are not arcing the current out onto the head or frame (running it in a really dark place helps watch for arcs)
-verify that your valve clearances are in spec
-check camchain tension
-verify carbs are in sync with manometer
-install the exhaust and air intake/filter setup you intend to run
-see if bike still gets hung up at 3000rpms, what about with the choke half on?....if it is still hitting a wall at 3000rpm, look at the plugs....very clean or white deposits on electrode means your super super lean. Bump up the main jet two sizes and see if that helps (I.E. go from say a 110 to a 115, as jets increase by 2.5's). If you get it to rev through the range smoothly, take a a spare set of fresh plugs and go out and do a plug chop. you want paperbag brown coloring on the electrodes for correct air/fuel mix. This is how you dial in the main jet, if you have rideability issues or lack of smooth part throttle, your needle jet clip position needs adjustment to dial in the fueling at part throttle/midrange.
Focuss has good advice, except some doesn't apply to the CB400T, the advance is electronic and in the CDI. the timing, as long as your cam marks line up and your stator rotor is on the correct mark, it will be in correct ign timing from the CDI. Removing the airbox, needs a +1 min jet increase on the main, removing the h box muffler requires another +1 minimun to even think about it running right, that jet kit you posted is over priced, I got the jets to do my 400T from local shop for $2 ea. and you won't likely need to increase your pilot jet. your bike likely has 108 or 110 mains and a step is 5 on these jets so I would get 120, 122.5 and 125's and you should be able to get it to work with those. if the starting jet is higher get +10 (2 steps) +12.5 (1.5 steps) and +15 (3 steps) These bikes are lean from the start and you can ruin the motor if you make them anymore lean.....I found out the hard way with an exploding piston mess. Bright side, found this site and got it back together and riding again.
Thanks Maritime i will contact the shop to get those jets, i did put the old muffler system on last night and wow im not sure if i tried the electric start too many times but it sounds like loose bearings spinning around now and doesnt turn the alternator, I can still start with the kickstart although it takes alot more work now. Should i still expect the bike to run this poorly while just missing the airbox? the mechanic onsite where i work on the bike (does cars only) thought that maybe the alternator needed to be turned over one rotation back to the "t"and flip the cam and sprocket upright thinking maybe the piston wasnt postioned right.. This somewhat made sense as the piston would be in a completely different place on initial start but i wanted to check with you guys if it makes a difference when you rebuild what rotation th ealternators "t"was on when i set the timing?
Sonreir sorry i meant 4, before i pulled the engine out to do the gasket change it hit 4 aswell i wasnt sure if that was good but the engine ran perfectly in comparison to now. I thought it was suppose to go to 9 or so but like i said before i took it apart it ran great rev'ed high as i could twist the throttle. I want to put the old airbox on tonight to test the bike to make sure this is 100% the issue but what do you all think about the rotation of the "t" on the (sorry for calling this an alternator before) ac generator, does it make a difference if you turn it once or twice before setting the cam and sprocket ?
You need the manual. it sounds like the motor may be out, the T has to line up with one of the pistons at TDC On the compression stroke, then the cam chain needs to align etc. I can see if I can scan or copy the pages out of my manual at home for you but I got the PDF off a link I found in this site I think?? Don't run it anymore until you are 100 % on the cam timing or something may break. Also if you turn the rotor backwards the cam chain can slip and throw the bike out of timing as well. Only turn it the one way, there is an arrow I believe but with a socket on tighten I believe is the right direction. I will get you the info if I can tonight. may be later before I get to my home PC. If you can find the free download there is a complete set of instructions on setting the cam timing correctly do that first, then worry about the rest.
Yeah i just checked ... on the compression stroke with the piston at TDC the cam and sprocket are upside down..... i will switch this around shortly and tighten her back up put the airbox on and start her again... Does anyone know what kind of damage i could have done that wont let the electric starter turn the ac generator ? it will grab after a couple attempts, spins it a little than lets it go and sounds like rattling bearings. THank you guys alot for the help
I need the clymers manual i have the honda manual but it leaves alot out ive felt through this process and shows imagery of different years for your specific year and that also is confusing.
I believe I have the clymeer. I'll check and if I can I'll send it to you depending on the size etc. As for the starter, it may correct itself when you get the timing right, I can't remember the connection on it off the top of my head??? hopefully you didn't bang up any valves, if you are 180 out at least the pistons are down when the valves are open.
Yeah i just checked ... on the compression stroke with the piston at TDC the cam and sprocket are upside down..... i will switch this around shortly and tighten her back up put the airbox on and start her again... Does anyone know what kind of damage i could have done that wont let the electric starter turn the ac generator ? it will grab after a couple attempts, spins it a little than lets it go and sounds like rattling bearings. THank you guys alot for the help
ok i flipped that thing upside down although i started confusing myslef on the compression stroke, right now the cam and sprocket are lined up to the "t" mark and if i turn it the cam depressed the exhaust valve... i know this might sound stupid but does that mean its on the exhaust stroke or it was just on the compression stroke and is exhausting ? i just didnt want to try to start it anymore until im 100%
alright just tried starting her after re positioning the cam and sprocket and setting "t" to TDC on what i believe is the compression stroke and it started the 2nd kick and much easier but still no rev past 3k however the airbox is still off so i ill put that on right now and try it once more, .... My confusion is that when setting the timing this time around the compression was built up in the cylinder and it was at TDC ... if i turn the ac generator forward from this point the exhaust valve is pressed in by the cam releasing the compression ... is this the compression stroke being that it is already built up or is compression stroke the turn that will build up the compression right after exhausting it? sorry this is my first rebuild/ build thank you for the support
TDC is after the exhaust valves have opened and closed. All of your timing marks should aligned at this point. The crank should be very easy to turn. Use a wrench or allen key on the most accessible gear in line with the crank. Usually the flywheel but i dont know this motor well enough to say. You should be able to feel TDC by gently rocking the crank back and forth by hand. Use your marks as a guide but you should know when your at TDC by feel.
By the sounds of this thread and the fact your bike started on the 2nd kick, timing isnt the issue, or fuel, or compression at that. Your jetting may be off but it wont keep you under 3K RPM. It would just get hot from running lean. Your plugs will tell this tale. You checked Comp so ur good there. You might have an issue in a coil or rectifier....I'm not the electrical wizard but I'd start looking for faults while the bike was running. See if a coil is dying out when pushed at X RPM. Then track backward through the harness then rectifier, then battery. Think simple. Check for exposed wires grounding to the frame. This will be a witch hunt but you'll know this bike in and out when you're done.
I put the airbox in and the difference was day and night, it took a second to kick in but than it started idling like the day before i took it apart and would rev nice up to 2500-3000rpm but than wanted to die again but with a slightly bigger will to stay on if eased off the throttle. MXseatpro that back tracking thing led me to something .. looking at my electrical, would my bike run right or even start if the Rectifiers yelow yellow yellow cable was disconnected? the reason i ask is i that i cant test it..i just saw it disconnected and while it could have easily been disconnected when i shoved the airbox in but the bike started and reved up to 2500-3k. and sorry the reason i cant test it to now that i was on a good track with the airbox addition i forgot to tork the cam sprocket bolts and sure enough she gave luckily it looks like my valves are undamaged and all other moving parts seem fine only two parts of the cam broke off i found them they dont appear to have caused any damage and sprocket is good so i think i will be looking at gaskets and a cam shaft that will be here next week with of course a valve adjustment and the lesson of patience and double checking everything for ever... Anyways at that last point with the rectifier cable that was disconnected mysteriously that will leave me guessing now until the parts come, would my bike even run with out that?
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