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Author Topic: Should I keep the 42 idle jets?  (Read 192 times)

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Offline shads

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Should I keep the 42 idle jets?
« on: October 09, 2018, 02:13:14 am »
I cant get my idle jets clear!  Ive tried carb cleaner, pine sol soaks, ultrasonic cleaning, hours of boiling, poking with guitar wire and nothing works so ive given up!

Ive found what appear to the correct OEM ones in Oz for a decent price so im just going to buy new ones.  Stock is 38 but my old jets are 42, which seems to be a quite a common thing from what ive read on here.

I realise its almost impossible to answer specific jet questions due to the variables, but my question is - is there any reason to change from 42 back to stock (or something else)?

Changes from the stock CB550K3 setup include: pod filters, 4 into 1 Delkevic exhaust, 0.5mm rebore during engine rebuild (not sure if that affects anything, probably not).  Main jets on the bike are current 100, with two of them being genuine Keihin and two being aftermarket!  Bike was previously in Utah and is now in Sydney.


   

Offline calj737

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Re: Should I keep the 42 idle jets?
« Reply #1 on: October 09, 2018, 04:11:58 am »
42 is pretty large when looking at the pilots. Your bike is already setup from Honda to be rich at idle, so increasing to 42 would only pour more fuel into the motor, likely fouling the plugs after extended low RPM riding.

You don't indicate what needle clip position you're running. Pods make things leaner, so typically you will increase the Main jet, maybe also move the needle clip down to enrich the mixture. And you didn't indicate the air screw mixture settings? The 4:1 Delkevic won't effect diddly. The rebore, unless paired with a healthier cam shouldn't effect too much either.

If the bike were setup for riding in Utah at elevation, it would normally be setup leaner not richer given the thinner air. I'd also like to see 4 stock jets in all 4 carbs, as aftermarket jets are NOT the same as stocks.

If your carbs have a mixture of stock and non-stock components, especially between them, you will forever chase your tail tuning the bike. Forever. Pull the carbs, pull the brass, replace with all stock brand components. Maybe order a set of 105 MAIN jets to address the pods. 38 or 40 Pilot jets will be fine. 42s, not so desirable.
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Offline TwoTired

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Re: Should I keep the 42 idle jets?
« Reply #2 on: October 09, 2018, 08:00:32 am »
I'd go with 38s.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline shads

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Re: Should I keep the 42 idle jets?
« Reply #3 on: October 09, 2018, 06:11:56 pm »
Thanks Cal and TT!  Sorry should have been more descriptive - will definitely be replacing the old main jets, but my plan was to get the bike going with the brass I have, see how it runs, and then get new main jets on that basis.  Obviously my idle jets mean that wont work so ill get some new 105 mains when I order the 38 idle jets.  Carbs are currently dismantled and mostly at the platers getting a nice new coat of yellow zinc.  I didn't check what the air screw mixture was at before I pulled them apart (whoops) and I haven't taken the needles out of the slides as of yet so not sure of their setting.  However, looking at the screws in the bottom they look a bit mangled so someone has had a go at them.

That's interesting re the rich idle setup.  I thought I read on here that the factory setup was lean at idle for emissions reasons, and that dealers were replacing the 38s with 42s to improve things back in the day.  How come so many people seem to have 42 jets if its already rich with 38s?

On a side note, after having recently read through the Carb FAQ thread and lots of TT posts, im now somewhat regretting grinding all the airbox tabs off and going to pods.  The airbox and side covers were already missing so at least I have that on my side.  Also, the more I work on the old girl the more I feel a little bit guilty for chopping her up and not keeping things stock!  Ah well.  ::)

Offline TwoTired

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Re: Should I keep the 42 idle jets?
« Reply #4 on: October 09, 2018, 09:02:24 pm »
Ah.  I was thinking you had an earlier bike with the non PD carbs.  My bad. The PD carbs have an IMS pilot screw to regulate total pilot circuit delivery, and the 42 should be fine in that carb set up.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline shads

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Re: Should I keep the 42 idle jets?
« Reply #5 on: October 09, 2018, 11:56:42 pm »
Uh oh.  38's are already on their way!  Should I swap them for 42s?

Offline shads

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Re: Should I keep the 42 idle jets?
« Reply #6 on: October 10, 2018, 02:14:31 am »
Ok, the plot thickens.  Ive just looked at the needles and number 1 carb is on notch 1 whereas the rest are on notch 2!  They are all genuine Honda needles however they are all E2350F ones which the manual says are from the '77 CB500, as opposed to E2349F needles from the 77' CB550.

So the PO has moved the needles up from notch 3 to notch 2/1 (making it leaner, right?) and also increased the main jets from 90 to 100 making it richer.  I don't know what difference the CB500 needles would make.

Im quite a while off starting the bike, but any suggestions on where to go from here would be most welcome!  Ive got 38 pilots and 105 mains on the way (can easily change that order though).  I guess I just need to chose a needle notch to go with and then make any changes once the bike is running...

Just to clarify as I never said  ::), my carbs are PD46A ones.   

   
« Last Edit: October 10, 2018, 02:16:20 am by shads »

Offline calj737

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Re: Should I keep the 42 idle jets?
« Reply #7 on: October 10, 2018, 03:24:50 am »
Yep, higher notch=leaner mixture. You’ve got a fine mess there, Alice.  ;D Lloyd would know better, but I would suspect that the 500 needles are richer than the stock PD needles. So you may have “good internals”  to get a decent mix with pods.

No matter the combination, pods will require tuning efforts.
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*** Currently experiencing life at 60 WTFs per hour ***
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Offline TwoTired

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Re: Should I keep the 42 idle jets?
« Reply #8 on: October 10, 2018, 08:50:13 am »
I thought you said you already have 42 pilot jets?  I suspect the IMS setting can tune to needs with either jet size,  But, stock book IMS setting won't be likely in either case.  Just a starting point to springboard off from to alter along with your engineering changes of exhaust and filter.  It will run with either.  Plug deposits or a dyno map with determine actual needs.

With less displacement, the cylinders suck less on the carbs, so 500 needles ought to run a bit richer on a motor that sucks harder, like a 550.

I don't know if the needles have a differrent taper profile  as well as a diameter difference.  With both an exhaust and filter change of unknown parameters relative to stock, it's going to be a test and try endeavor, even after a "best guess".  Hope you have dyno access or a test track.  Tuning based on dreams is often problematic.

Counting notches from the top of the needle as installed in the bike, top notch is leanest, bottom notch is richest.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

 

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