Topic: rejet for exhaust?update/fixed

[72500john]

i have a 72 CB500. all is stock with the exception of the following..
hondaman ignition, uni air filter, and motad 4 into 1 exhaust. bike runs great and plugs look good. only problem is idle has to be set around 1200 to 1300 for a smooth idle. is this normal when going from a stock configuration to 4 into 1.
all of the 3000 mile tune up stuff has been done in the correct order( yes i searched first) and yes new points and plugs. and yes carbs have been completely torn down and everything cleaned..including emulsifier tubes.( sorry not being rude just trying to give enough info)
do i need to change my "slow"/idle jets? if so in what direction. stock is 40 for idle jet and 100 for main. i have verified these are installed. clip on the needles is set 3rd from top.
thanks!

[phil71]

there is not 'should be' when you introduce variables like this, there is only 'what works'.. and to find that, you gotta fiddle. What I do is take some gaffers tape and start wrapping the pods a little bit at a time to see if restriction smooths things out, if it does, you gotta upjet.. if it doesn't, you may get away with some air screw fiddling (out, not in)

[72500john]

thanks phil71,
do you think i should go back to a stock filter to take out that variable? i am sure the uni filter is less restrictive than stock. only went with it to have something renewable so to speak. sorry but i forgot to mention i have turned the air screws too 2 turns out and it has helped a bit.
once again..much thanks.

[Gearheadgreg]

Spend the 20 bucks and step up the Slow jet 1 size, 42, i had to step up the slow jets on both my bikes for the same problem you are having, sounds like you are using a stock air box with a performance filter, but the headers alone will lean out your idle circuit, try the slow jet increase and let us know how much better it runs.   

[lucky]

i have a 72 CB500. all is stock with the exception of the following..
hondaman ignition, uni air filter, and motad 4 into 1 exhaust. bike runs great and plugs look good. only problem is idle has to be set around 1200 to 1300 for a smooth idle. is this normal when going from a stock configuration to 4 into 1.
all of the 3000 mile tune up stuff has been done in the correct order( yes i searched first) and yes new points and plugs. and yes carbs have been completely torn down and everything cleaned..including emulsifier tubes.( sorry not being rude just trying to give enough info)
do i need to change my "slow"/idle jets? if so in what direction. stock is 40 for idle jet and 100 for main. i have verified these are installed. clip on the needles is set 3rd from top.
thanks!

Is the Uni Air filter one that fits into the stock airbox?
Is it one single filter for four carbs or is it four separate filters?
What is the mixture screw set at?
I would increase the stock#40 idle jet to a #42mm idle jet.
Do you have a photo of the "MOTAD" exhaust?

[lucky]

Tap....tap..tap...

[LesterPiglet]

Photo of Motad on the Exhaust thread.

[72500john]

I do appreciate your help lucky, but i work 12 hour shifts on the weekends. thus the looong wait for an  answer...
 yes the uni filter fits in the stock airbox.
mixture screws are 2 turns out..
and hopefully, here are the pics of the exhaust.
if you think a #42 jet will help, where would you recommend to buy from?
thanks.

[Lil Red]

I'm planning on some ofmthis stuff for a 400f...so i may soon be in the same boat. I'd put the stock set p back on with the paper filter. And see if it all straightens out or just runs a bit better. If it al smoothes out, then you know you need to jet it up...and maybe decide to let it all alone...also  from what i've read in similar threads, you may try to move your needles to their highest setting (lowest notch on the needle) as this will richen the mixture a tad throughout the range. Float height is also important i believe.

i' ve got some experiance with similar problems created by exhaust and pods but with dell'orto carbs. I'm looking forward to losing my cherry on these japanese carbs. But if it all srays ok with the stock airbox...I'll leave it be.

Oh and remember the adage...if you suspect carbs...triple check ignition and timing, cuz its true. And the valve clearances.  And there are some throttle linkage baselines as well...the zero stop and the full throttle stop...all those things need to be squared first...then a carb balance...Then jetting etc.

[Gearheadgreg]

@ 72500john, last jets i purchased 3 weeks ago were from the local parts dept from our dealer here,...still only like 25 bucks..or look here>http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1805

[72500john]

i will check with the local honda shop also. the link you sent was close. mine screw in. i "think" these are them. will need more research to be sure.

http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1947

[luap]

did you try oiling the filter might richen it up

[lucky]

I do appreciate your help lucky, but i work 12 hour shifts on the weekends. thus the looong wait for an  answer...
 yes the uni filter fits in the stock airbox.
mixture screws are 2 turns out..
and hopefully, here are the pics of the exhaust.
if you think a #42 jet will help, where would you recommend to buy from?
thanks.

Lets back up a step.
You have not told us if it has a particular problem.
Does it hesitate when flipping the throttle open?


1972 CB500
Stock air box, stock style filter.
4 into 1 Motad exhaust  w/(muffler)
#40 idle jet,(stock).
#100 main jet,(stock).
Needle- middle position
2 turns open on the mixture screw.


Before I would recommend any changes I would want to know if it hesitates when trying to advance the throttle.
Does it backfire on deceleration on the road?
What are the symptoms making you want to rejet???

[lucky]

Spend the 20 bucks and step up the Slow jet 1 size, 42, i had to step up the slow jets on both my bikes for the same problem you are having, sounds like you are using a stock air box with a performance filter, but the headers alone will lean out your idle circuit, try the slow jet increase and let us know how much better it runs.   

I would agree with this.
The reason is that the OP (72500JOHN) said that when he turned the mixture screws "2 turns out" it ran better because  it richens up the mixture on this model. The mixture screws when turned out LOWER the slide giving it less air. Also with a less restrictive air filter it speeds up the air just a little but the stock air box opening is still the same so I think the overall effect is not much.

But I want to hear what made the OP want to make changes? What was it doing or not doing to make him want to make those changes???

[72500john]

the bike runs great, no hesitation, backfires, sputtering,coughing,weezing or runny nose! just have an issue with having to turn the idle up to 1200 to 1300 for it to be smooth. ( as stated in original post). just trying to improve things if needed. will try the #42 jet. will let you know the results. might be a while, going on a well deserved vacation this week.
thanks for your help!

[lucky]

the bike runs great, no hesitation, backfires, sputtering,coughing,weezing or runny nose! just have an issue with having to turn the idle up to 1200 to 1300 for it to be smooth. ( as stated in original post). just trying to improve things if needed. will try the #42 jet. will let you know the results. might be a while, going on a well deserved vacation this week.
thanks for your help!

Let me ask a stupid question, what happens if you just turn the idle screw down?

When it is idling and you give it a quick turn of the throttle will it instantly rev with no problems?


If it does not rev as it should from idle I would move to a #42 idle jet.

If it Does acts properly when revving up from idle WITHOUT babying it, then the #42 idle jet probably will not help.

If the problem is just the high idle, you need to sync the carbs and make sure you do not have a air leak. Also check the O rings on your mixture screws.

[72500john]

no such thing as a stupid question. even though i do ask some .
i will give what you suggested a try. i have synced the carbs and checked for leaks but it will not hurt to revisit that.
as far as orings on the mixture screws..i searched for them for a while only to find in the parts breakdown that there do not appear to be any on this installation.

[lucky]

no such thing as a stupid question. even though i do ask some .
i will give what you suggested a try. i have synced the carbs and checked for leaks but it will not hurt to revisit that.
as far as orings on the mixture screws..i searched for them for a while only to find in the parts breakdown that there do not appear to be any on this installation.

Oh ,thats right it is a 72, Sorry. No O rings the mixture screws on that model control the slide position (air).

[TwoTired]

IMO, the stock carb settings should work fine, with perhaps a tweak on the idle air screws.  (Assuming early Keihin 627b carbs).
Turning the air screws outward will lean the idle mixture (and possibly make the throttle response "wheezy").  Why do you have them turned out to 2 turns?

So, the problem is that the idle "smoothness" at 1100 RPM is objectionable?  Meaning all cylinders are not firing at equal strength?

Is the compression exactly the same for each cylinder?
Is the idle vacuum the same for each cylinder?
Did you do the carb vacuum sync at idle RPM?
Is the idle timing exactly the same between 1-4 and 2-3.
Are their any air leaks between air filter and each carb inlet?

Are all four carbs the same age/use with the slide fit in the bore being equal among them?

Can you identify a weak firing cylinder at idle (header temp differences).


It may be possible to set the air screws differently among the four carbs to smooth the idle, depending on answers to questions above.

Cheers,

[72500john]

i will try turning them in and see what happens.
 as far as compression goes i do not know..do not have a tester..yes i know..get one
 
idle vacuum is the same and did the carb sync at 1100 rpm.

idle timing is the same. ( you do mean ignition timing?) set with a strobe timing light.

no air leaks that i had found before but will definitely recheck this closely.
also will recheck the sync. if need be.

all four carbs appear the same age and did not seem worn when i had them apart.

all cylinders are hot.

why did i turn the screws out 2 turns?   thought that was the correct way to go.
thanks for all the help..got kid duty tonite but will fiddle with it in the am.

 

[72500john]

found some time to mess with it this evening..wife has a late start at work tonite.

warmed up bike. set idle to 1100 and tried to not nurse the throttle..bike dies.

restarted, idle at 1100,sounds a bit better, set idle screws to stock setting 1 1/8 turns, still dies but not as soon.

restarted, checked for air leaks at all the carb boots, at manifold on head, found a big one where the air filter should fit snugly against the front of the airbox. spingclip that holds the filter in place had popped loose. fixed this and it will bog for a second but not die now.

guess i need a #42 slow jet? or should i try turning the air screws in some more?

as always thanks for all your help.

[chris walters]

this entire thread is dejavous of last summer messin with my 750k with pod filters. i ended up taking off the head and replacing the cam shaft and lapping all the valves to achieve perfect equal compression and vacuum through all 4 carbs. then i soaked/simmered the boots from between the carbs and head in oil to soften them up so they clamp down and seal good. with the carbs triple cleaned and larger fast jets installed... then they all syncrod beautifully.  bobber runs perfect accept for it will stall if i dont feather the throttle to bring the motor out of a slow idle. but anyway last summer i knew nothing about motorcycles accept how to ride them and i forgot how much i learned till just now reading this thread. if only i had a computer and this forum last year. i had to figure out all this stuff alone. at one point i even did the tape trick on the air filters

[lucky]

2 turns out on the idle screw.....Yes, if it was me I would go to a #42 idle jet.

[72500john]

already ordered some. guess when i said the bike had no hesitation i never noticed it at normal driving conditions, guess that was from having the idle bumped up and my driving habits where not to get  into the throttle too aggressive at fist.
doing a, shall we call it, throttle dump test..idle to wot really showed the problem.
will replace the jets and recheck the sync and let you know how it goes
..it think it will be better.
thanks lucky, twotired, an all else for your advice.

[TwoTired]

restarted, idle at 1100,sounds a bit better, set idle screws to stock setting 1 1/8 turns, still dies but not as soon.
guess i need a #42 slow jet? or should i try turning the air screws in some more?

Turn the air screws in 1/8 turn at a time and check for effect, both idle and throttle response.  Mark your throttle.
At idle RPM and in any gear or engine loading, you should be able to snap the throttle to 1/2 of total remaining throttle and have the engine accelerate reliably without engine "wheeze".  In top gears it won't pick up fast, but it will pick up reliably.

You have air screws on the 627b carbs.  Turning them in makes for a richer mixture.  I still don't believe you need larger pilot jets than #40s on a Cb500.

Cheers,