carburettor woes

[scdrtramp]

Hello all, I have a 78 650 special that sat in a garage for 30 years. I has 8000 miles on it.. when I got it the carbs were a serious mess, due to the vac petcocks staying open. I also have an 83 Heritage that I'm building. the 83 had the 34 mm carbs that were ready to go so I ran those for a couple summers, but that bike is coming together so I'm going to need them. So I went through the 38mm carbs that were on the 78, put kits in them, and the seem to work great sitting, but when you take off it stumbles bad right off idle. I have had them off and and everything looks right. When running it with the air boxes off the slides work when running, I don't know if I need to go a bit bigger on the pri jet or what. I have backed out ther mixers to 3 turns, but not much change..
Anyone had this happen??? thanks

 

[Jim]

Did you compare, number for number all the jets as you swapped 'em out? Never used a kit myself, but I've read nothing but bad about 'em here in the forum. There's a possibility you installed the wrong size main, needle and/or pilot jets. From what I've read, kits "usually" have the wrong sizes of at least one of them.
Hopefully you kept the old brass that came out of them? Swap just the jets and needles... new out and old back in. See what that does.

 

[scdrtramp]

Actually none of the jets are marked. The originals had no markings, or they wore off I have no Idea. the jets in the kits had none either. I used the original pri jets. It idles fine and revs off idle great, but between 2 and 3 K rpm it stumbles. I can get it thru the stumble and it seems to rev on up. I have only had it on my side road. I kept the original needles, kit did not have them. I'm going to pull them back off tomorrow and go through them again.

 

[5twins]

Well, having jets with no markings is no good, lol. It sounds like you're too rich in the idle to midrange transition area. If the carbs were all gummed up and sat that way for a long time, the gas varnish can actually eat away the brass jets, making them larger and richer. In particular, this may have happened to your needle jets. It's kinda hard to tell by looking into them because the metering orifice is so large to begin with. You really need a new or known good one to compare to side by side. Here's one of the originals from my '78 (on the left) compared to a new one I replaced it with. Side by side like this, it's pretty easy to see the wear .....

I would also invest in some genuine Mikuni mains and pilots. A jet is a precision part and there's no telling if the re-pop jets are flowing the right amount of fuel, even if they are marked as the correct size.

 

[Its been a long time]

Actually none of the jets are marked. The originals had no markings, or they wore off I have no Idea. the jets in the kits had none either. I used the original pri jets. It idles fine and revs off idle great, but between 2 and 3 K rpm it stumbles. I can get it thru the stumble and it seems to rev on up. I have only had it on my side road. I kept the original needles, kit did not have them. I'm going to pull them back off tomorrow and go through them again.

Echoing what's been said about jet kits being poor. Given where you are I think I would be tempted to read the carburettor guide here and put the carbs back to what they originally were using Mikuni parts. That way you're are working from a known reference. Presently it seems you don't know exactly what you have in there because nothing is marked.

FWIW, I have to use a magnifying glass to see the markings on carburettor parts. Mikuni parts have a small square within a square logo on them as well as the size indication. HTH.

 

[5twins]

Yes, the Mikuni trademark "square-in-square" emblem. You should find it on all their jets, needles too .....

 

[gggGary]

 

[scdrtramp]

n but the jets had been drilled and were way to big, it ran but just too rich. The jets out of the originals have no markings, so I bought the kits. I did finally find numbers on them they are 27.5 pri, and 135 mains. I don't think is rich or it would smell rich when idling right?? The air box has new filters, but the pipes have been modified. they are not open but drilled inside. not sure if that would make a difference??

 

[5twins]

27.5 pilots and 135 mains are the stock sizes for a '78 carb set. Stock needle clip slot is in the middle (3rd from the top).

 

[gggGary]

n but the jets had been drilled and were way to big, it ran but just too rich. The jets out of the originals have no markings, so I bought the kits. I did finally find numbers on them they are 27.5 pri, and 135 mains. I don't think is rich or it would smell rich when idling right?? The air box has new filters, but the pipes have been modified. they are not open but drilled inside. not sure if that would make a difference??

prolly not just off idle.

So it was running well on the BS34s? That (probably) eliminates a lot of wutabouts.

Work the throttle in that "runs poorly range" keep it running there as long as you can, now shut down and check the plugs.
Wet, dry?
Have you visually comfirmed you can see through the pilots? Do the slides drop slowly on the drop test?
if not that can be a choke plunger not sealing issue. (rich)
check the rubber seal on the end is smooth no gouges a smooth ground groove is fine.

"Carbs were really gunked up", have you confirmed all the idle ports are open in the bore near the butterfly? (lean)
butterfly is removed from this body for clarity. there are three separate holes right where the butterfly sits at idle. Idle mix screw removed, shoot in carb cleaner, should spray out of all three.

have the needle jets been removed and cross drillings are clear?

This is going to come into play a bit higher than "just off idle"
orifice in float bowl a popular plugged item.

 

[gggGary]

Lets not forget the "obvious"
vacuum ports are BOTH plugged or hooked to petcocks?