Topic: Inconsistent Idle, jetting HELP

[Brownc567]

I have a 1977 CB550 with 5000 miles on it. it was a barnyard cleanup which I purchased off a guy.
One thing I cannot tune in for the life of me is the idle, and it seems to be rich on the idle circuit.
Its almost bone stock with a 4-1 exhaust (trumpet style muffler, Not sure if this is a stock exhaust system or not). Stock airbox, running 91 fuel (only stuff around with no ethanol) Last year the work I preformed was;

Swapped in a cb650 larger camshaft,
rebuilt the top end seals, cleaned the pistons,
grinded out the casting marks on the intake runners and the intake ports in the head,
lapped the valves.
installed an electronic ignition and verified ignition timing twice,
set the valve clearances as per Hondaman's recommendations,
Timing chain properly adjusted as per manual.
cleaned and set carbs. verified all ports and jets were free and clean, verified float height set to 22mm, vacuum synced the carbs once all the timing and everything else was done

Here is the issue thats driving me nuts. For some reason the bike does not idle great or handle the chock circuit very great either. The idle bounces from 1000 to 2000 rpm pretty frequently. When i choke start it from cold with full choke, It springs up to about 5500 rpm within 2 secs, and i quickly have to push the choke down to about half way to stop it from screaming up there. It will not hold idle with full choke after a few minutes of warming up. I have to ride it for a few minutes to be able to hold idle at a stop. I find i am constantly making small adjustments to the idle knob for the whole ride. When the bike is fully heated up and at the later part of my ride, when I come to a stop the idle is hanging up around 2500 to 3000 rpm for 4 to 5 seconds and a little twist of the throttle will help bring it down to its idle (anywhere from 1000 to 2200 rpm, climbing and settling every few seconds), or if I leave it hanging around 2500 to 3000, it'll finally settle back to the "normal" range after about 10 seconds... . Sometimes it will stall out at a stop light and starts right back up. As the bike gets warmer, the idle wants to stay higher and I have to keep fidgeting with the idler knob to keep it in a normal range. Going from WOT to full closed throttle, the bike seems to pop and backfire slightly as the rpms come down, leading me to believe the idle circuit is too rich.

Throttle response while riding is snappy, and feels great. I have not changed out any jets and it has the stock settings. half throttle and WOT all pull strong and feel great. Only issues I have are at idle
Main is 90, idle jet is 42 (push in style on the PD46C carbs) and pilot screws are set to 1.5 turns out.
New rubber seals were used in my rebuild (kit from 4into1.com).
Throttle is snappy, moves freely and all slides return to normal postition.
Live around 1000 - 2000' ASL elevation.

My theory is the idle circuit is too rich and i need to go down in size on the idle jets (currently the stock 42). Before I did any of the work on he bike it seemed to idle and cold start like a dream, would only need choke for about 45 seconds then hold a nice smooth idle that didn't bounce around. I didn't need to make any adjustments to the idle knob either.

Any help would be greatly appreciated.

[grcamna2]

I see.
Have you lowered the idle,then spent some time synching the carbs with gauges;did you ever do a bench synch to them ?
Does your bike have the stock  PD carbs ?

[Brownc567]

when I bought the bike, the previous owner had them cleaned and rebuilt by a "professional" backyard honda mechanic (his words). the bike was tuned and running flawless after i fixed an electrical issue. When i cleaned the carbs, i didn't touch the butterfly's or mess with anything other than removing the bowls and jets for cleaning. I didn't bench sync them since they should not have been changed, and i hooked up my vacc sync gauges, and they are all still perfectly synced.

[TwoTired]

Spark plug deposit provide window for combustion conditions.  Reading them might Give a hint on what’s going on.

Proper heat range?

The PD carbs were designed to be lean burn to pass 1978 idle emission exhaust tests. Honda attempted army compliance.

The stock 4to4 mufflers were high pressure to reburn  retained hydrocarbons. This allowed a leaner idle mixture to be delivered by the carbs. Aftermarket mufflers are not usually high pressure,  so stock setup PD carbs deliver lean mixtures.  The tune procedure for the PD carbs was to use a sensitive tach and adjust the IMS screw inward until max idle rpm was attained while fiddling with the idle knob to bring it back down to 1000 +/- 50.  Each carb was adjusted in this way.  Note that the idle circuit must be slightly rich of stochiometric, as when the slides are lifted the mixture goes lean due to carb throat pressure equalization closer to atmospheric which makes all the jets reduce flow.  If it doesn’t begin over rich, a too lean mixture won’t combust, and the engine wheezes with no power until the slide is closed back to make the jets deliver enough fuel to ignite again.  Why Honda didn’t put accelerator pumps on the carbs like the PD50s found on the 650, I don’t know.  Anyway, the idle mixture is tied to throttle twist response on the 550 carbs.  In any gear from idle you should get reliable if slow pick up from throttle twist using up to one half remaining  throttle travel.  If you get pick up with more than sudden half throttle twist, the idle is too rich and it will soot the plugs during prolonged idle And make spark erratic.  This gets complicated by operation on the Jet needle or main when that is too lean, as that cleans the soot off the plugs and they then fire better… for a while.  Without the stock mufflers, the main needs to increase in size, to prevent lean, too hot operation from baking the valve guides, cooking the oil, and guide seals.

Now, The above applies the a stock CB550 motor, which yours isn’t.  I seriously doubt your motor will be happy with stock set up carbs. This applies, to selection of main jet for WOT throttle, slide needle thickness, taper angle, and position in the slide for throttle midrange settings, and lastly the best IMS setting for idle without sooting  plug tips and still having proper response to throttle twist.  The fastest way to find what carb adjustments are needed is to run on a dyno while it sniffs the exhaust while the engine is under load. But, with a test track and a store of clean plugs to install between each test run, you can read the plug deposits after each adjustment and dial in the carbs to match the configuration changes you’ve made away from stock.

[scunny]

 cold with full choke, It springs up to about 5500 rpm within 2 secs.
that's were i would start.

[newday777]

when I bought the bike, the previous owner had them cleaned and rebuilt by a "professional" backyard honda mechanic (his words). the bike was tuned and running flawless after i fixed an electrical issue. When i cleaned the carbs, i didn't touch the butterfly's or mess with anything other than removing the bowls and jets for cleaning. I didn't bench sync them since they should not have been changed, and i hooked up my vacc sync gauges, and they are all still perfectly synced.

Professional backyard mechanic......
My inclination is idle circuits have hardened gas deposits from setting with gas in the carbs giving you a lean mix causing the high idle.
I don't know the 550 carbs, but the 750 PD carbs have an idle circuit that is very finicky to clean from what I've read. I don't know if the 550 PD carbs on the 77K have the same idle circuit.

I read back through your previous posts to try to figure out some clues to what you have done to your bike to cause your idle problems. This would have been better if you had kept it all on one thread rather then starting this new thread on the same continuing problem.
You say you polished the intake ports, do you have experience in this? There is an art to polish and porting intakes on bikes. Not having been taught properly about how to do it can lead to poor results. We don't know you and your experience and training, so all I can do is guess at what you have done.
As TwoTired said, the best way to tune your modified bike is on a dyno. I know a race tuner in the San Diego area that has built many of these 550s with the 650 cam mods and he has told me the tuning is needed on a dyno by someone who knows what they are doing. It's not a backyard mechanics shade tree practice that will get good results. Asking here will only get you marginal results at best, generic replies. You need a race tuner now

[Brownc567]

I appreciate the education on these carbs and the changes they made during this year. The reason I started a new thread was how old the other one was and the information posted was not accurate in which I incorrectly stated I “polished the ports”. This is not the case. I simply grinded down the rough casting marks in the intake runners and intake ports to allowed for smoother laminar flow. As for the “professional backyard mechanic”. This was the PO who took it to a Honda guy (so he says). I have personally and recently stripped the carbs down and re cleaned them all myself so that I could verify the job was done right. I found a few restricted ports and cleared it all up but it did little to change the performance. Unfortunately I don’t have access to a dyno nearby so I’m hoping to get an idea of where I can maybe start in terms of jetting required based on the symptoms present. Once again, thank you guys for the responses. I have not done a proper plug check from clean to engine cutoff with fresh installs, but my new plugs I installed last year have a light tan colour indicating normal combustion. Mainly from what I can tell and what I’ve researched, my issue seems to lie within the idle circuit which I am having the issues dialing in. And since I ride the bike with very little time spent in the idle circuit I’ve mainly lived with the issue.

I’m hoping anyone with 650 can swap jetting experience can advise of the carb changes they needed to perform. Thanks guys!

[ReggieC]

If you have the PD carbs (identifiable by "PD" on the size of the carbs, and the floats are plastic trapezoidal shape vs. round in the earlier carbs) then 22mm of float height is too much (i.e. fuel level in the bowls will be too low). I believe that the factory float height spec for 1977 was 14.5mm.

Maybe the current 22mm float height is causing your erratic idle problems? It's a lot of work, but I would try pulling off the carbs and re-setting the floats to 14.5mm.

[scunny]

do the clear tube test to see what is happening.

[kerryb]

 Had similar issues on a stock 550 motor, the repair was weak advancer springs causing hanging idle.  But don't put the advancer back backwards...looks good but won't run at all!

[HondaMan]

Where to start...?

The #42 is certainly too rich for the 650 cam idle. Try a #40 idle jet, or at least turn the air screws inward about 1/4 turn to lean it out some (yes, inward to lean...) to see if it helps. This will approximate a #41.2 size jet, and might give you a hint.

One thing you didn't mention (and 2 that I will): did you replace the 4 O-rings on the face of the intake runners to the head? These turn hard as plastic after a few years, and once they are subsequently removed they don't seal anymore. They are the same size as the valve caps' O-rings. Second: are the rubber hoses between the carbs and intake manifolds still rubber-y? If they are hard, they also may leak, which will cause uncontrolled airflow to the engine. Make sure the clamps are not closed tightly from bending the clamps' screw hole mounts: maybe consider taking them off and flattening those clamps' screw faces (I usually add a trimmed flat washer to help) so they will clamp a little tighter on the carbs and manifolds. These rubber parts are almost 50 years old...

Try using regular grade gas. Today's premium grade burns far too slowly for these engines. Today's regular burns at about the same rate as 1970s premium. The longer burn times today are intended to light off catalytic convertors, so we can all benefit from using that to run less-expensive fuels (that won't wet the combustion chamber, causing engine run-on issues).

KerryB has a great clue, too: 100% of the spark advancer springs today are too weak, due to the engine heat having annealed them over time. I usually cut off 1 turn of 1 of the springs, or 1/2 turn from both springs, to slow down the spark advancers to match with the stock 550 cam: with the 650 cam this is even MORE important because of its early intake valve opening. If you have installed a Tytronics (or other similar unit, like Dyna S) ignition, check the spark timing carefully with a strobe timing light, as it will not set up correctly by trying to do it "statically" like was done with old 1-cylinder bikes or some twins. Those electronic triggers do not respond ON-OFF statically in the same way they do when the engine is running.

The choke: given the low vacuum at low engine speeds that the 650 cam will provide, don't overchoke or it will make the engine quite rich, and foul the plugs quickly. Normally the 550 takes a few seconds of choke at start, then hold the idle up about 1500 while turning the choke off and give it 30-60 seconds there to settle in (once it's all straight again, that is...).

General rule of thumb, here: the earlier the intake valve opens, the less fuel the pilot jet must provide. The low vacuum caused by the early intake valve opening causes the carbs to "act" like the engine is breathing less air (i.e., the velocity of the air slows down at low engine speeds as compared to the normal cam) so the carbs also act richer with its old jets, which is normal for their design.

[WideAWAKE]

Where to start...?

The #42 is certainly too rich for the 650 cam idle. Try a #40 idle jet, or at least turn the air screws inward about 1/4 turn to lean it out some (yes, inward to lean...) to see if it helps. This will approximate a #41.2 size jet, and might give you a hint.

One thing you didn't mention (and 2 that I will): did you replace the 4 O-rings on the face of the intake runners to the head? These turn hard as plastic after a few years, and once they are subsequently removed they don't seal anymore. They are the same size as the valve caps' O-rings. Second: are the rubber hoses between the carbs and intake manifolds still rubber-y? If they are hard, they also may leak, which will cause uncontrolled airflow to the engine. Make sure the clamps are not closed tightly from bending the clamps' screw hole mounts: maybe consider taking them off and flattening those clamps' screw faces (I usually add a trimmed flat washer to help) so they will clamp a little tighter on the carbs and manifolds. These rubber parts are almost 50 years old...

Try using regular grade gas. Today's premium grade burns far too slowly for these engines. Today's regular burns at about the same rate as 1970s premium. The longer burn times today are intended to light off catalytic convertors, so we can all benefit from using that to run less-expensive fuels (that won't wet the combustion chamber, causing engine run-on issues).

KerryB has a great clue, too: 100% of the spark advancer springs today are too weak, due to the engine heat having annealed them over time. I usually cut off 1 turn of 1 of the springs, or 1/2 turn from both springs, to slow down the spark advancers to match with the stock 550 cam: with the 650 cam this is even MORE important because of its early intake valve opening. If you have installed a Tytronics (or other similar unit, like Dyna S) ignition, check the spark timing carefully with a strobe timing light, as it will not set up correctly by trying to do it "statically" like was done with old 1-cylinder bikes or some twins. Those electronic triggers do not respond ON-OFF statically in the same way they do when the engine is running.

The choke: given the low vacuum at low engine speeds that the 650 cam will provide, don't overchoke or it will make the engine quite rich, and foul the plugs quickly. Normally the 550 takes a few seconds of choke at start, then hold the idle up about 1500 while turning the choke off and give it 30-60 seconds there to settle in (once it's all straight again, that is...).

General rule of thumb, here: the earlier the intake valve opens, the less fuel the pilot jet must provide. The low vacuum caused by the early intake valve opening causes the carbs to "act" like the engine is breathing less air (i.e., the velocity of the air slows down at low engine speeds as compared to the normal cam) so the carbs also act richer with its old jets, which is normal for their design.

I got no skin in the game on this thread, but just wanted to say thanks for posting that!

Great information!