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Motorcycle Repair/honda cb450k4 idle issues


QUESTION: '71 cb450k4 that has almost 600 miles on top end rebuild. Valves have just been adjusted. timing set and running a pamco ignition. Compression #s are (cold)160ish left and 170ish right. I just put new Honda throttle cable and that has made a great difference as the PO had put an aftermarket that was about 5 inches too short!

If I run bike off of one carb (either side) only it idles perfect, smooth, sounds great and comes down to 1200 quick and easy when you bump the throttle.

Now that I've gotten the carbs sync'd it fires up and idles ....but not smooth.... Kinda "jumps and skips" every now and then. Exhaust pressures seem pretty even. Just has an irregular "beat" coming out of the pipes. Its slow coming down to 1000rpm, sometimes even goes below 1000 at idle but never dies. If I try to adjust idle to 1200.....while out riding at stops it will hang around 2000rpm and never goes below 3000 while moving. Runs great going down the road though, doesn't bog down, pulls even in 5th gear !!!

I've followed the carb sync steps but haven't been able to get any adjustment at all out of the mixture screws. I have new mixture screws, springs and O-rings on both carbs. The right carb mixture screw will adjust slightly and change the idle when running the motor by itself but once I bring up the left carb to balance it doesn't adjust any more.....confusing to me since it runs so good on either carb alone with no adjustment.

I hate to mess with it too much now because it really is running better than ever. Just wondering what you think my plan of attack would be to get idle smoothed out more and mixture screws functioning...maybe passage got blocked and carbs need cleaning again???

Thank you for your time,
Edinburg, Texas

ANSWER: Chad,  I would definitely redo the carb clean, stripping the bodies down completely and making sure that the bowl vent passages, air bleeds in the carb throat are clean and clear. Bit odd that you have a 10 psi difference between the two sides right after a rebuild, but those figures are in the "normal" ballpark, so at least you got the cam timing set correctly.

"Hanging" idle speed symptoms usually related to either air leaks at the intake manifolds or o-rings/gaskets that seal them up to the head or more often a sticking spark advancer. Corrosion can build up between the point cam and the advancer shaft, causing the point cam to hang up on the rev up/rev down modes, which leaves the spark timing advanced and this is what causes the idle speed to remain high.

The carburetor series numbers have specific jetting/calibration and float levels, so be sure that your carbs are set to the suggested settings, assuming that you have stock air filters and mufflers. If those have been changed, then you may have to play with jetting to get it to run well at all rpms. If the mixture screws are not responding at idle, either the carb bodies have some air/fuel metering problems at low speeds, there are air leaks or erratic spark problems.

You could have some grit in the points that is causing erratic firing and/or bad spark plug cap or weak coil. I have started seeing a lot of failing coils recently. Check primary side windings for about 4.5 ohm readings. Look for excessive arcing at the points while the engine is running. Spray WD40 or carb spray around the intake manifold area and verify your spark advancer function to be working fully in advance/retard modes.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: thanks bill for the quick response.....

running pamco ignition so no points or condenser. have good spark. have NOS advancer unit. DO STILL HAVE ORIGINAL COILS THOUGH !!! have stock air filters with uni foam in place of old paper that was nasty and blocking flow. replaced all rubber intake boots and gaskets. mufflers are aftermarket but same length and as close to stock as you can get for this bike. although carbs are from newer model 450 (the 723a's) they are jetted to stock and have been cleaned 2 or 3 times now, new jets and floats.

any recommendations for replacement coils ?

thanks again

Chad, I overlooked the note about the PAMCO ignition. I would still check the advancer, as it could be sticking with 40 year old grease packed inside. Twist the end of the advancer, if you can do it easily and see if it advances against the spring pressure then snaps back.

I checked the book and it looks like all the float levels are the same at 20.0mm and all have #38 pilot jets. Usually the differences in the carb series was determined by what kind of exhaust system was on the bike. CLs have leaner jetting than CBs, in most cases.

I would put a set of Dyna coils on it and call it good. I guess if your ignition system was setup for points-type coils, then replace the coils with something near to the 4.5 ohm OEM coil primary resistance.

Do check it for air leaks, anyway. Did you remove and clean the emulsion tubes placed way up inside the carburetor body, above the secondary main jets? I use chopsticks to push out the needle jet and emulsion tubes from the carb body.

Get the sparks all contained and firing at the right time, then see if there are fuel metering issues afterwards.

Bill Silver

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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