More Carb Issues & Emco filters

[woodstoc]

I've looked over the site and haven't seen this specific question unless I've missed it.

This ? is on a set of 80 SG carbs on a 79 XS11.
I have been playing with various jetting changes and I'm way closer than I was but I still have a stumble off idle. There is some minor backfiring through the carb at this point as well. I have been operating on the assumption that the idle circuit is running rich, but I may be off base. I don't have air leaks that I can find. From 3000 grand on up, bike is strong and not too lean even though I am up to 132.5 on the mains (stock is 110 outside and 115 inside) I tried smaller stock mains and the bike lost power past 5000 RPM. I am now running 37.5 pilot jets, needle jet and emulsion jet is stock. I also moved to smaller air jet as well, don't remember the size off hand. The carbs are clean and I run inline filters as well as tank filters. Fuel level is at 23mm on all 4 floats. All carbs are sync'ed.

BTW, I had purchased a set of the EMCO cone filters like I saw on another post and these have a shoulder restriction that completely blocks the air pilot as well as the other air holes on the 80 and 79 carbs. I would not recommend them to anyone for their XS1100 even though the dealer shows them listed for these bikes.

[Mike Hart]

Stumble off idle is sometimes indicating lean. When you blip the throttle (in neutral), do the revs stay up (at whatever rpm they get to) for a second before settling back down? If so, also says lean. If it pauses before it revs (with a blip), that says rich. I assume you've synched the carbs?

Don't know who else has put later yr carbs on a 79. I wonder what happens if you put 79 sizes (137.5 mains, 42.5 pilots, and the right size pilot air jet to go with?

EMCO? I (and others) have Emgo (with a 'G') cones, and they have no restriction. Cones, or ovals? The EMGO 54mm Cones definitely fit and have no restriction. Can't vouch for their ovals.

There is some discussion of ovals with restriction at the end of pg5 & cont'd on pg6 of this thread

[woodstoc]

The carbs are synched. I have a 4 tube mercury unit that makes it easy. A blip in my throttle has hesitation which says rich to me. I was going to try the 79 jets but the top end seemed good and idle seemed to be rich so I went smaller with the pilot jet. I have a set of 42.5's, I guess I'll slip them in. I was considering moving the needle jet up a little to lean it out. I am kinda new to the carb part of this, all my cars are TB's and TPI's so I haven't fooled much with carbs, they have always been a mystery to me. What do you think the backfiring is a symptom of? I would initially think air leak but have pretty much ruled out one with the carb cleaner trick.

The air filters are Emgo cones, silver with the rubber intake. I saw them on another bike on the forum and ordered them thru my local Yammy dealer. These definately have a large internal shoulder, I would guess narrow it down from 54mm to maybe 51 or so and they cover the air holes and part of the air pilot jet as well. I'll check the part # when I get home today and post it. Maybe they make more than one, but they do say for 78-82 Yamaha 1100's on the box as well as other applications.

[Mike Hart]

Since you mentioned the shoulder on the Emgo cones, and I was changing jets again today anyway, I took the filter off the rubber adaptor that goes on the carb to get a better look.

I saw a shoulder too, which at a glance seems like it would interfere with the pilot air jet and the other passages, but when you put on just the rubber part without the filter, you can see that none of the passages are even close to being blocked. I am satisfied that they work fine. Try just the rubber, you'll see what I mean, the shoulder part does not contact the air passages, they get plenty air.

Did you mean move the clip up on the needle? That would lower the needle, leaning the metering of your mains. Needle jet is the emulsion tube, the pressed in part that the jet needle (or just needle, for clarity) rides up and down in.

Another thing to check is float height - spec is 24.7 to 26.7mm. Sounds like you're adjusted way rich there - maybe sucking up too much fuel??? Fuel level too low can also bog the motor when you apply throttle.

If top end is good, but idle is rich...hmmm, have you checked the idle screws to make sure the tips aren't broken off? How many turns out are they? You can lean the idle mix by going out up to 4 turns each, more than that means get a smaller pilot.

Spitting back thru the carbs - is that only when engine is cold, cool, barely warm? In Bandit-land, that means lean. If it's only when you first start it up, and never when engine fully warm, I wouldn't worry about it, it might go away when you get the rest of the puzzle solved.

I'm real interested to hear more reports about this...

[woodstoc]

Thanks for your response Mike,

I'll check the shoulder on the EMGO's the way you said and see where it falls. I pulled them off last night and still had the stumble issues anyway so they at least aren't the total problem.

I am going to try and bring the needles down a little bit and see if that works. I have the plastic spacers in the 80 SG carbs and have to do a little sanding work to accomplish this. Hopefully this will get me past this part.

I am also going to make a float change to what you suggested. I'm not sure where I got the 23mm from but I know the 79 carbs were different on the height than the 80 carbs were.

The weird part is the plugs look like they are about perfect. I may try to let it idle for a while, then check them to see if they soot up any.

The backfire thru the carb happens regardless of hot or cold. Doesn't seem to get any worse when warmed up.

[Mike Hart]

yikes, you are right, the float level on the 80 is indeed 23mm +/- 0.5mm - I was quoting 78/79 specs, sorry.

Backfiring thru carbs (more like a sneeze than a bang, right?)... I'll have to think on that, and hope someone else can shed some light on it.

[ratbyk]

Carb Back Fire

A back fire out of the carbs as you crack the throttle as it comes off of idle is definetly a lean indicator. If it runs well in all other situations and has the right plug colour, I would simply raise the float level 1 mm. Should check to see if the back fire occurs of off the same carb everytime. If so just raise the float level on that carb, not all of them.
Ken/Sooke
78E Ratbyk

[woodstoc]

Thanks ratbyk. I'll check into that as a possibility. I had originally thought I was running rich, but now I have did various runs at different RPM levels, hit the kill switch and checked the plugs out on the roads. I'm looking pretty good at all throttle openings with the exception of right off idle, and I really can't get a reading in that area due to time spent there.

Lean causing the backfire is good info.

[ratbyk]

Spit Back

To determine if only one carb is causing your spit back, remove all of the cone air filters. Run the engine at idle then crack the throttles. Make it spit back or pop out of the carb/s. While doing this observe the mouth of the carbs. As one pops back you will be able to visually see which one is the trouble maker. Repeat the test several times to determine which is/are the offenders.
From there you have isolated the trouble maker and know which one needs some messaging.
I would richen that one up just a bit by simply raising the float level. Just make a small adjustment. At this point big steps are not better.
Ken/Sooke
78E Ratbyk

[Jerry]

Uh...just a comment...make sure the exhaust gaskets (head to pipes) are not leaking. I had a 'backfire' on my XJ when closing the throttle while going down hills, though it was a carb issue. Replace the leaking #3 gasket and-whala-backfiring disappeared!