BS38 needle jet leaking w/ new o-rings

[JeremiahL]

Hello all,

I finally have my project bike up and running, and overall it is running fairly well, but I have had some issues with jetting.

I am running the stock '75 carbs with uni pods and a 2-1 exhaust with a baffled muffler (I think it was put on at some point in the 80's).

Long story short, I have been chasing an idle that hangs around 3k rpms once the bike is warm. I finally pulled off the air filters while the bike was running the other day and there was a lot of fuel getting sucked up around the needle jet. I had replaced the o-rings with ones from mikes, and I knew they went in a little hard so I wasn't super surprised.

So I went and got OEM o-rings from my Yamaha dealer, put them in greased up with a little dielectric grease so they went in nicely, and when I started the bike back up the issue looks to be just as bad as it was before!

Anyone have any ideas? The carbs weren't in too bad of shape when I got them, but maybe the carb bodies have corroded and eroded so there won't be a good seal anymore?

Here's a video looking into each of the carbs, hopefully the quality isn't too bad.

 

[TwoManyXS1Bs]

Are you sure it's the needle jets?
Hard to tell, almost looks like the transfer ports dribbling down the throttle plate.

I probably need better glasses.

Edit: Try doing the same thing, but separately cover each of the main and pilot air bleeds, and see which one changes the character of the fuel emission...

 

[JeremiahL]

Ok I'll give that a try, and I'll try to get some better video too.

I was having trouble getting my phone camera to focus, and youtube decided to upload it really low quality.

 

[JeremiahL]

Alight, so I did a little bit of testing today. This time when I started the bike cold I had the air cleaners off, and while the right carb was showing the same problem right away, the left carb (which has the choke plunger) wasn't at all. Once the bike warmed up and I took the choke off both were back to the river of fuel going down the bore.

Covering the main air inlet on the airbox side of the carb didn't seem to have any effect on the problem. When I first tried covering the pilot inlet at idle the right side made it stall immediately, but with the left side covered it still chugged along on one cylinder for a while. When I decided to use my brain and opened the throttle a bit so it was off the idle circuit and covered the pilot inlet again I am pretty sure that the unwanted fuel flow either stopped or slowed down significantly (it's not the easiest to see in there).

Hopefully this video is clearer:

 

[JeremiahL]

Any more thoughts anyone? Could it possibly be float level? I set them within spec and double checked when things were running funny, but I just set it with calipers, not with a sight tube.

 

[JeremiahL]

Here's a quick list of engine and carb work done, probably not complete but it's a start.

Engine: New Pistons, Cylinders rebored, new gaskets and seals all around, new cam chain guides, Pamco ignition with mechanical advance, PMA, cam chain adjusted, valve clearances set

Carbs: Intake manifolds (tourmaster I think?), mikes throttle shaft seals (checked and not leaking), mikuni float needles, seats and washers, mikes choke plunger, one new mikes float (other was tested in hot water and did not bubble), 47.5 pilots (bs30/96 for '75 float bowls), 130 main jets.

 

[TwoManyXS1Bs]

Yeah, this is gonna be a tough one.

...Covering the main air inlet on the airbox side of the carb didn't seem to have any effect on the problem...

...covered the pilot inlet again I am pretty sure that the unwanted fuel flow either stopped or slowed down significantly..

Alright, that simple test strongly indicates that the issue would be:
1. Confined to the pilot circuit,
2. or that the pilot circuit is somehow crossfeeding to the needle jet.

For item 1 to occur, the fuel would be coming from either:
A. The transfer ports, dribbling down the throttle plate, puddling on the floor.
B. That odd looking annular ring surrounding the pilot circuit's "upfeed" pipe.

For item 2 to occur, some wierd cross feed would have to exist between the pilot circuit or its airbleed, and the feed to the needle jet. Quite unlikely, but could check the soundness of the gasketing and bowl/body surface conditions in that middle area surrounding the needle jet. A problem in that area would cause the bike to really crappy anyways. Probably not the issue.

So, we're back to the pilot circuit. Need to figure out that wierd annular ring surrounding the pilot circuit's "upfeed" pipe. It almost looks as though the pipe is sitting in an oversized hole, and fuel is emerging from there.

Dunno, don't have a carb here to compare. There's numerous pics in here that would show that pipe better. Time for a search, and help from others...