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HSC40 Jets ??

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HSC40 Jets ??

PostAuthor: Coyote251 » Thu Sep 07, 2017 10:10 pm

I just finished rebuilding a set of carburetors and decided to put a jet kit in the bike while I had it apart. They sent me 3 pilots jets (30,35,40) and 2 main jets (50). The instructions said to replace the pilot jet in the rear with the 30 and the front with the 35, then it says to replace both primary jets. The primary I am assuming is the main, which is the one that screws into the emulsion tube, right? The question I have is does this carburetor have a pilot jet that can be replaced?? The parts breakdown shows one (#48) from the Clymer, But my carburetor only has a post where it shows this jet. Picture 1 is my carburetor and picture 2 is the parts breakdown out of the Clymer.

Picture 1
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Picture 2
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Re: HSC40 Jets ??

PostAuthor: Flyingdog » Fri Sep 08, 2017 2:31 am

Yep..that's the difference between Version I & Version II carbs. Actual jet was replaced with press in tube. Forget what the actually years or models (think it was the 920/1000..84 & 85), doesn't matter, cuz Yamaha did it. So...all ya have to do is concentrate on making sure the pickup tube is good and clean (ALL orifices for that matter), pick which jet size you need and screw it in the emulsion tube (aka: needle jet). Guess there has be others who've tried varying this pilot system, but I ask: "why?". When ya got soo many other ways to vary the amount of fuel/air needed to run the engine. Sometimes you just have to "Forget the Manual". They don't always show or tell ya what changed. Kinda like cook'n. Some go by the recipe..ends up tasting like crap. Others wing it, tasting as you go.
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Re: HSC40 Jets ??

PostAuthor: Coyote251 » Mon Sep 11, 2017 7:42 pm

Float height was off by like 5-6mm so that explains why I had so much gas pouring out. I have them both set now to 15.8mm from the gasket surface to the top of the float. No gas pouring out anymore so that's good. Figured out the jet kit as well, I just need to learn to read better I guess. It was the mains that needed to be changed so now i am running a 135 jet in #1 (rear) and a 130 jet in #2 (front). OEM jets are 132 in #1 and 124 in#2, so this is a bit strange that the larger jet is in the rear according to the jet kit instructions, but I triple checked that I was reading it right this time. Got everything back together, got the idle set and the carbs synched at idle and 3500rpm. Started it up and she was smoking like a chimney, it was still spitting some fuel out of the exhaust, but I was hoping that it was just left over from before. Went for a quick ride and she died on the side of the road. Somehow it had gone through about 5L of gas in the time it took me to tune it and go for a little rip. The good news is that there is no more backfiring. Hitched home, went and got some more gas, put it in and the bike started right back up. It seemed like it was only running on one cylinder, but halfway home the other cylinder kicked in and she seemed to be running great. It was really hot and I had smoke coming out of the exhaust when I parked it, so I am thinking that it is running a little rich. I have the pilots turned back to about 1.5 turns off the soft seat, sounds better there. I will have to wait till tomorrow night to go for a ride and see how much of a difference that adjustment made. Might re-check the carb synch again as well.

Overall I'm making progress, i think!
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Re: HSC40 Jets ??

PostAuthor: Flyingdog » Tue Sep 12, 2017 2:16 am

OEM jetting is: #1= #124 (rear)...#2= #132 (Front) 15.8mm is kinda a rich setting. Have you done the clear tube check on that fuel level? And, I'd almost bet if your setting floats at 15.8mm with the carbs laying flat on the bench (which most people do), instead of holding carb at a 65° or so angle when checking. Setting float height with carbs flat is wrong. Because, setting the floats like this will compress the float needle's shock pin. You'll be getting to much fuel in the float bowls when put back in service.
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Re: HSC40 Jets ??

PostAuthor: Coyote251 » Tue Sep 12, 2017 4:32 am

Flyingdog wrote:OEM jetting is: #1= #124 (rear)...#2= #132 (Front) 15.8mm is kinda a rich setting. Have you done the clear tube check on that fuel level? And, I'd almost bet if your setting floats at 15.8mm with the carbs laying flat on the bench (which most people do), instead of holding carb at a 65° or so angle when checking. Setting float height with carbs flat is wrong. Because, setting the floats like this will compress the float needle's shock pin. You'll be getting to much fuel in the float bowls when put back in service.


My mistake, wrote it backwards in the post. I did have the 124KO in the rear cylinder. The instructions with the jet kit had me put the smaller jet in the front and the larger in the rear, which is backwards to the OEM setup. I have read a few other posts were they are suggesting having a larger jet in the rear, their explanation was that more fuel would keep the cylinder cooler. I'm not sure if this is the jet kit manufacturer's reasoning or not. I have asked them but haven't got a reply yet.

When I set the 15.8mm I didn't have them sitting flat, I did put them at the same angle as they sit in the bike. I made up a template when they were mounted, and then mimicked that on the bench. I suppose using the clear tube, to check wouldn't be a bad idea. I'll do that tonight and see where they are at. Thanks for the suggestions.
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Re: HSC40 Jets ??

PostAuthor: Flyingdog » Tue Sep 12, 2017 4:49 am

Yea...I've heard/read sooo many different theories on which cylinder should get the larger jet. Even manufaturer, such as K & N, Factory Pro Jet..etc..differ on the subject. Differing as to the rear runs hotter (true), so needs larger jet for cooling. Guess that's where I throw the BS flag and go by what works. Right or wrong, I just feel rear cylinder runs hotter (hot air is less dense). So why would you dump more fuel into that cylinder, upsetting the air/fuel ratio.

Theories...aren't they fun? Kinda like the one on spark plugs. That's a good one...some just don't have a clue..(or they got hit in the head by a flying spark plug of a certain brand). LOL

You can also bench test the fuel volume in the float bowls by the same (Tube) method. Stick em in a vise, connect your tubing and fill the carb up with gas via a syringe or squeeze bottle or what ever. Could even use the wife's turkey baster, just duck when ya give it back to her.
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