Topic: Interesting slow jet find

[Scott S]

 I mentioned this on my "idle hover...carb tuning" thread, but not everyone is following that one and I'd really like to get some insight on the differences in these slow jets.

 Carbs are 627B carbs, both sets from CB500's. One set is cleaned/rebuilt and on the bike but giving me problems. The other set I just pulled out of the junkyard yesterday.

 I pulled the bowls off the parts carbs today and immediately noticed that the slow jets were different. They're physically longer, have more holes and the holes are larger than the other sets I have.
 They are pictured on the left. The middle set is what came in my carbs and are in there now. The ones on the right are from an aftermarket rebuild kit.


 Notice, too, that the smaller jets have less and smaller holes and the spacing on the holes differs even between them.
 The parts jets are marked with a slanted star looking emblem and "40". No markings on the middle jets. Aftermarkets marked "40".

 What affect would the jet on the left being longer, and thus deeper into the float bowls, have? How/what does the size and spacing of the holes have, if any?
 Lastly, which one should I use?

[Deltarider]

The one on the left is the genuine #40 slow jet.

[Scott S]

 I cleaned them thoroughly today and will be re-using them. I pulled the main jets from the parts carbs and have them soaking. I'll see how they clean up and what markings they have.

 Hopefully, this is all a step in the right direction for my idle issues!

[Bodi]

Genuine Keihin jets should have their stylized K logo, it's visible in you pic on the left one. There are a few styles of jet for different model carbs but that big one looks like what I would expect in a 550 carb.

[Scott S]

 I can also see the Keihin logo on the mains and they're marked "100". If they clean up OK, I'll use them, too.

[shortside750]

My 750 has those same #40 slow jets with the Keihin logo stamped on them. Those longer ones are definetly genuine. I would trust and use those ones first.

[mystic_1]

The holes in the sides of the jets are "emulsifier holes" and that region is called the "emulsifier tube".  The purpose is to allow air to mix with the gas and form a sort of foam, that is easier to atomize when it's pulled into the carb throat.  The main jet has a similar arrangement but it's a separate part.

The size, number, and placement of the holes determines how foamy the foam is. 

mystic_1

[Scott S]

 I installed them today and will get the carbs back on the bike tomorrow. I'm hoping that this had something to do with my low end tuning issues.
 If the aftermarket jets had too few and too small holes, there may have been nothing I could do to stop it from being too rich.

[ekpent]

I usually yell from any roof top that most aftermarket carb parts are junk nowadays especially if they are from China.Some of the old vintage kits from Japan are descent if you can find them. Clean and save the old and just buy or make a gasket if you have a leak er. I have seen some real crap on the 750's--Anybody want to sell their "old" stuff 

[Scott S]

 The kicker is: I pulled this bike out of a barn where it had sat for 28+ years. I THOUGHT the slow jets that were in it were stock. They looked similar enough to what came in the rebuild kit, right?

 Only when I opened up the parts carbs and found the genuine Keihin jets did I realize what was going on.

[Old Scrambler]

That's what I alluded to in your first thread on the 'hover' issue...........and NO......I am not selling any of my original carb parts!  A good source though is swap meets for off-model carbs........many jets are interchangable between models.  Clean those little holes but don't use anything that will alter their size or edges. I will sometimes use a wooden toothpick.

[zanorak]

I had the same problem. The rebuild kits must have the right size threads on the jets or there was a mixup or something because it happens.

Pilot jet is 7mm or 8mm too short right? So there's no fuel going in it if the fuel level drops. Caused all sorts of problems for me.

You can order jets... I can't remember which right now but i have it written down at home. N424-24 I think? I'll repost later.

[Scott S]

Pilot jet is 7mm or 8mm too short right? So there's no fuel going in it if the fuel level drops. Caused all sorts of problems for me.

 Yeah, at least that much shorter. Even more telling, look at the size and number of holes vs. the Keihin. There's no way they would allow as much air. That HAD to be partially to blame for the richness I couldn't tune out.

[zanorak]

They were probably drilled because the PO realized how lean it got after the bowls emptied but thought he had the right jets. Mine would run until the fuel level went down, then I'd be on mains only - bike was already lean and I'd overheat and get stranded.

I installed N424-24 jets from www.pjmotorsports.com in the proper size (pods and open exhaust - 42 and 115 just slightly rich..) and I've never looked back. I think my total investment was 30 bucks.

Sudco stayed on the phone with me for 45 minutes to an hour to help me figure out which jets I needed. Then they didn't have the jets so he transferred me to PJ! It's like miracle on 44th street...

Good luck!