Topic: 500\4 carb adjustments

[Kevnz]

Does anyone have the size of the mainjet O Ring, please?
Can I use a standard circular crossection O ring ; looks like it may be rectangular.
The existing jets won't fall out of the holder under their own weight, but neither are they a tight fit.
Possible cause?

[dave500]

Sooty plugs are caused by too much fuel, but still enough spark to light the fire, Dave.

excessive resistance in the hi tension will also leave a sooty plug,sure it lites the fire but the arc dosent burn hot enough to clean the electrode,ive seen this a few times and correct value caps have been the fix.

[Kevnz]

New plugs, caps, coils, everything ignition related.

[dave500]

ok,Id still check the caps resistance to be sure,d7ea plugs?

[HondaMan]

Does anyone have the size of the mainjet O Ring, please?
Can I use a standard circular crossection O ring ; looks like it may be rectangular.
The existing jets won't fall out of the holder under their own weight, but neither are they a tight fit.
Possible cause?

I have some O-rings that are slightly oversized for these carbs. Many of the carbs have been slightly damaged in the jet-holder area by the years of MTBE in our gasolines. This damage is enough that the normal standard O-ring doesn't seal very well (or at all sometimes). Jamming too large of an O-ring into the holes will then later crack the housing, ruining the carb itself.
Original O-rings were (approximately) 1.2x3.5mm, but reporting varies on how big they actually were, which seems to be a closely-guard (or lost?) secret from Keihin. I had some made that are 1.25x3.8mm. PM me: I'll send you a set of these O-rings.

[Kevnz]

PM'd you. Offer much appreciated.
FWIW, these are before and after shots.

[Ozzybud]

My recent struggle with a sooty fouled cylinder on my CB360T and CB350F this summer # 1 would get sooty and start stumbling , cleaning the plug or installing a new plug would clear it up for about 15 miles.

I tried everything up/down in /out hotter plugs... etc Leak check etc
One important thing to consider is to clean or replace the plug after every change or adjustment before testing again.Once a plug fouls they don't unfoul themselves.

The fix for me was replacing the needle and the needle jet/emulsion tube. They were worn ever so slightly right in the area between 1/4 throttle and idle. Where we have the throttle position during most of our rides.

Both bikes would run all day at 4-7k rpm.it was just when  cruising at 35mph where they would foul.

I also do an initial vacuum sync after any needle adjustment.  When loosening the linkage you will never get it in the same spot.

I am not believing that the o- ring on the main jet has anything to do with your issue. The main jet only comes in to play from half to WOT. If the needle jet and orifice have the correct clearances. Only that amount of fuel can get by.

The CB 360T took me over 3 years with the carbs on / off a dozen times. Plus every combination of jet sizes. Which made zero difference on the way it ran.

[Ozzybud]

Beautiful bike..
 Have you been nominated for Bike of the month?

[Kevnz]

Ozzybud, Thanks for your thoughts, helps to confuse me more!
My thoughts are, perhaps fuel can get sucked up past the loose O ring and bypass the needle, although I don't know if they are that loose, or whether that could be the problem.
Presumably you could eliminate the theory of a worn needle by dropping it and having it operate in a different part of its range?  Unfortunately, when I tried that, the bike ran so badly that I didn't run it long enough to find out whether it had affected the sooty plug issue. How did you identify the worn needle as the culprit?  Just eyeballing it?
These things are obviously ultra- sensitive, because the differences between the 550 carbs and 500 ones are pretty small, but make a huge difference in performance.
At any rate, the richness is much reduced, but still not quite where I want it to be.
Thanks for your input.

[jaytee-nz]

Kev, I've had similar issues with a couple of CB500 / 550's as well. However, in all cases the problem was worn needles. Dropping the needles did help so that pointed me in the right direction. However, the fact that yours ran worse when they were lowered points to something else. I definitely think changing those main jet o rings is worth doing, however if the bike runs better but still not right, then I would go back to the needles (not that there are many new Keihin ones around).
Good luck - just persevere and you'll get there.

[Kevnz]

Thanks JayTee,
Some years back I had an issue with my newly rebuilt 500 that turned out to be a carb and ignition fault on the same cylinder, so I spent a lot of time chasing my tail. Once I finally sorted it, the bike gave me 10 years of good service, so I know they are great once sorted. Shoulda kept it, with this one I'm not really ahead! Live and learn.
Was that your 550F on TM some months back? Looked typical of your quality workmanship, impeccable. Did it sell? Not really a good selling environment at the moment.
I believe my old 550's engine is, or was in your possession for  refurb. Odd bike that, '74 frame, '76 engine but first reg new in NZ 1980. Weird.
Cheers.

[jaytee-nz]

Yep, that was my 550F - didn't sell but I'll try again soon. You're right though, not much is moving at present.
I didn't realise that Ross bought that 550 off you. I refreshed the engine for him but then he decided he wanted the whole bike done, so the rolling chassis got shipped up from the bottom of the South Island so I could put it all together !
Cheers - JT

[Deltarider]

Kevnz, the tiny O-rings around the main jets are, apart from the floatbowl gaskets, the only O-rings I had to replace. Yes, they can cause trouble. Here is what to do. After removing the main jet, inspect the bore it sat in. The bores on mine after 49 years and over 140.000kms are stil immaculate, but I have read that in the US, where they have always opted for the cheapest option and so have degraded fuel with extreme high doses of MTBE, they can be damaged. The original size of the O-rings, 4,2x1,1 is good. I did not have to go bigger. Just push the main jets with their new O-rings in. Do not be tempted to twist the main jets whilst pushing them in. This to avoid their O-rings becoming damaged. I have the original O-rings by Honda and 5% ethanol gas is no problem, 10% could be problematic.
I'm sorry that in this thread some posters still mix up the oldstyle carbs with the later PD carbs. You have the old style Keihin (either 627B, 649A, 022A, 087A or 069A) and they have airscrews in the sides. For once and for all: turning them in, enrichens, turning out, leans. Small increments of 1/8 of a turn can already make a big difference. Use the settings Honda communicated as a starting point if the air intake is original even if you have aftermarket exhausts.
The PD carbs on the K3 models have a different concept. Let's not discuss them here.
A common mistake people make, is that they apply a method which in itself is sensible for many cars and even motocycles: aiming for a highest rpm at idle, by adjusting the airscrews. But... the carbs on those vehicles will have special acceleration jets. Yours and mine have none. So if you do that practice on your model, you will no doubt arrive at a very nice idle - the engine will pur like a kitten - BUT driveability is gone, meaning: acceleration will be poor. An idle mixture on your and my model needs to be on the rich side to guarantee a good driveability. That's what I mean when I say the slow jets also function as auxilary jets in acceleration. I have composed an overview of setting data for these carbs that so far has not been amended. It's about the oldstyle Keihins only.

[jonda500]

An easy thing to try would be putting the #38 pilots back in? (with cut shorter stubby screwdrivers it can be done in situ)
John

[Deltarider]

Jonda is right. I can do it blindfolded. You could opt for replacing the 16 crossheads that hold the floatbowls by ss allen bolts, which are much easier to handle.
When in doubt whether the PO has maybe poked with something hard inside the slow jets and so has reamed them, replace them by genuine Keihin. They are not costly. CMSNL has them.

[Kevnz]

Hey  All,
Thanks for all the input, with your collective knowledge I'm sure I will eventually get there.
Hondaman has kindly agreed to send me some O rings, so I will go from there. I want to keep all the brasswear standard at this stage so I'm starting from a know base point.
Delta, thanks for the spreadsheet, already have it and have referred to it many times, invaluable. It's how I established I had the wrong carbs.
Jaytee, that's the 550 as my avatar. Freedom Green was it's original colour so I kept it that way, although I prefer Boss Maroon. Another one I should have kept.

[Ozzybud]

Ozzybud, Thanks for your thoughts, helps to confuse me more!
My thoughts are, perhaps
Presumably you could eliminate the theory of a worn needle by dropping it and having it operate in a different part of its range?  Unfortunately, when I tried that, the bike ran so badly that I didn't run it long enough to find out whether it had affected the sooty plug issue. How did you identify the worn needle as the culprit?  Just eyeballing it?

I had the needles all the way down and It was still too rich.
DId you clean/replace plugs after adjusting the needles before testing?
I measured the needles and the orifice with digital calipers.

[HondaMan]

I got an email today that promises the O-rings will be here tomorrow. I'll send a couple of sizes: some of these carbs have wear in their mounting holes for the mainjets (dissolved by nefarious fuel additives in the past), so I'll send 2 sizes to try.