Topic: slow deceleration cb450

[playinsafe44]

i just got my little cb450 running a lot better. i bought a precision ruler and adjusted the floats to where they needed to 14mm (they were so far off that im surprised it even ran...25mm!!). it starts right up now. ive rebuilt the carbs, checked and gapped the plugs, adjusted the cam chain, and adjusted the valve clearence. i put new throttle cables and choke cables on, and put clubmans on it. i tried adjusting the carbs the way the clymer manual states, but the idle mixture screw had no effect on the idle speed. i tried adjusting them one cylinder at a time (take the spark plug wire off of the cylinder im not working with) and nothing happens. it will idle down to about 800 or 900 rpm (going off the mechanical tach) but it hovers around 2500-3500 for a few seconds or until its put under load. the pressure coming from the tail pipes while the bike is running feels really close, and my throttles dont look like they are hanging up on anything (throttle opens very easy and snaps right shut). i put made and installed some new intake gaskets too, and then tested for air leaks using some carb cleaner and it made no change in idle. my only guess would be air filters. i did all of this without the air filters installed, because mine are really dirty and i want to get uni's for it. would that make it not respond to adjustment? i did take it for a ride around the block though and im in love! this bike SCOOTS for a 450!

[playinsafe44]

im thinking its one of three things, and im going to start working on this in a minute.
1. the automatic ignition advance is hanging up
2. it might be missing the o-rings that are supposed to be on the idle mixture screws
3. it wont take any adjustment without the idle mixture screws on

[mlinder]

air leak?

[playinsafe44]

tested for them with carb cleaner

[mlinder]

Decel problems like that are often caused by a lean condition (hence, checking for airleaks, which cause a lean condition.)

[playinsafe44]

i thought it would be a rich condition if anything. i was under the impression that a lean condition would cause backfires on deceleration. since its so easy to start and adjusting the idle mixture screw all the way and all the way out has no effect on how it runs i would think it was rich. im not posotive but i dont think i had to use the choke either.

[playinsafe44]

well im back to square one. i checked the timing advance unit (took it all apart) and it looked fine, so i cleaned it and lubed up all the parts and put it back together. the only thing that might be a concern there is that the springs dont quite retract all the way, well it does at random. it will retract until it is just a hair from being retracted all the way. id like to put some new springs in there but i need to know where i can get some. i also checked the idle mixture screws and they have brand new orings on them. i still cant get any adjustment out of the screws. i dont need to use the choke to start it either. sometimes the idle will come back down to where it is supposed to be but most of the time it just hovers. i dont know what to check next. please help!

[playinsafe44]

ok talking with some people on another forum. they say that the correct float height for these bikes is 20mm from gasket surface. that is information straight out of a clymers manual, but my haynes manual says its supposed to be 14mm from carb base. does anyone here have a factory manual for a 1971 cb450 that can varify what the correct float height is supposed to be? and if it is supposed to be measured from the gasket surface or the carb base.

[jnel500]

I have heard conflicting numbers on the float height, I think it has to do with the tool used and whether it is measuring the height to adjust to or the amount of fuel it allows in. The 20mm number seems to be the one to adjust the floats to from the indentation on the carb body.
I think you can take half a coil out of the spring to "rejuvenate" it. This should help it retract.

Not to be redundant, but have you checked timing and triple checked the carbs for any blockages?

Good luck.

[playinsafe44]

i havent checked the timing, and i checked the carbs twice now (they look brand new). i was going to get the timing figured out after i got these carbs somewhat where they are supposed to be. i dont think the timing should have anything to do with the IMS not responding to any adjustment. right?

[mlinder]

timing, points gap, and valve adjustment should always be done before carb synching.

[jnel500]

I think removing half a coil on the advancer unit could help. Do it with caution and after you set the timing (static and strobe) to see what affect this has. You can also check the pistons in the carb to make sure they are moving effortlessly. The carbs on the 450 need everything to be dialed in for them to run well, but when they are, they are great. Get the timing set and carbs sync'd. You can also check the slow jet again for any obstructions.

[playinsafe44]

yea i really do need to get it timed. but the damn thing idles smooth and revs fast, so i dont think it could be THAT far off. it just decels slow and sometimes hovers. i changed the float levels to 20mm like everyone has been telling me to do. i used to choke to start it this time, but the IMS still were not responsive. i thought for a second that the right cylinder was stumbling just a little bit when i richenend the screw, but i that was probably wishful thinking. haha. when it was running there was a slow drip coming from the right cylinder carb. i was thinking it was my petcock though. im going to try to remove a coil on the the advancer springs, but i would MUCH rather replace them with new/performance ones. i think im going to try to raise the float height to 22mm and see how it runs. to me it does seem like its getting too much fuel, but i am still new at this. thanks for all the help everyone! and keep the advice coming

[jnel500]

Do you think it is getting too much fuel because it's fouling plugs? If you are just letting it sit idle than it is hard to know if it is getting too much fuel throughout the throttle cycle. Excessive idle will seem like it is fouling plugs. I would leave it at 20mm for now set the air mixture screws to about 7/8th-1 full turn out and double check that they are sync'd. Of course all of this has to be with spot on timing. Check your plugs to see if they are fouling or need to be cleaned and make sure that points gap is set. I don't think you can just get the springs for these bikes, you have to buy the complete unit, that is why taking a portion of a link out can be helpful. Let us know how it goes.

[oldfordguy]

I had this problem w/ my 450K3, and I cut 1/2 loop out of one of the advancer springs and bent up the next loop, and it cured the problem.

[playinsafe44]

that what im going to do. the advancer doesnt always retract all the way. thats after i cleaned it really good, and put a little lithium grease on it. would that explain why the carbs wont react to adjustment though?

[hapsh]

I know this thread is a bit old, but I had to pipe in.  I had the exact problem on my 72 CB450 back in the 80s.  I lived with it for 3 years.  It was that way when I got the bike.  I took it to all the MC mechanics in town and NONE of them could figure it out.  They all thought it HAD to be something with the carbs.  They tried everything including; float hight, idle mixture, polishing slides, needle hight, and stock air box.  I finally got a spare set of carbs from a friend and guess what, same thing.  Finally I got it to keep from idling hanging high by just dropping the idle speed so low that it would not idle on its own, I had to blip the throttle when stopped, but this was only a temporary fix.  I kept it that way for 3 years.  The funny thing was that I had checked the timing, at static and advanced but didn't pay too much attention to the timing retard after decel.  Then one day an old timer from Texas told me that the springs on the old CB450s lost their tension and would not allow the timing advance to return.  This is what causes the hanging.  Basically what happens is that since the advancer is slightly stuck advanced the engine wont run off the idle circuit, it needs more RPM to even run, so that forces you to increase the throttle stop higher than normal.  That is why your idle mixture screws don't seem to work.  I am 100% sure this is the case with this hanging idle.  I just want anyone else out there with CB450s to know this so they don't go through the endless headache I did.