Needle jet advice/stock position

Brassneck

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Hey All,

I'm getting ready to address a carb issue that I've been putting off for a while and wanted to get some feedback on my thinking. ('79 special II, stock engine/carbs/exhaust)

Backstory:
a while back I had my bike at the shop to get my new wheels on, etc... at that time I figured I would have them double check my work (I did a half-assed job with my carb-cleaning to get the bike running and wanted them to go through it one more time). I also had found that the PO had torn slide diaphram when putting it back together and while I attempted to repair it, the shop was adamant about replacing it. Which they did.

So, I got the bike back...ran well and all seemed good... although about a month or so later I noticed that sometimes the bike wouldn't pull as hard as it use to...(Kind of felt like I was running on one cylinder but not all the time). A few months later as I addressed some charging issues... I decided to pull the spark plug on the right side (not sure why I didn't earlier) and noticed that it was running really rich on the right side (same side as the torn diaphram)... while the right carb's mix screw was set pretty much stock 2 1/4, the plug was heavily sooted. So I did the dead-cylinder method and found that the right side mix screw way off compared to the stock setting...anything more than 1 1/4 turn would have the bike running too rich on that side...and even fully closed it was still getting a little fuel... so clearly something's off. I've since kept it at that setting so that it's running "better" and enjoyed the summer riding months... but it's bugging me and now that the weather is turning, I'm going to address it.

Note: Bike pulls pretty well through the full range. There is a very (very) little flat spot when cold getting past 1500rpms, but then it's gone and doesn't come back AT ALL after start up...and there is a little tach needle bounce at idle...+/- 100 rpms (Which I've tried to balance/sync out but that's as good as I can get it).

On to my thinking... Since the bike is stock engine/exhaust etc... and carbs were stock except for the replaced slide/diaphram, I'm thinking that the jet needle was put in either a clip or two too high... or there's some other issue....like maybe a float height setting issue...(maybe a very small air leak?) or all the above?

All of which I'll check, but just wanted to see if this line of thinking makes sense?

Thanks all!
 
Yes, inspect the needle. There are spacer washers on it that have to be in the right spot. The thin metal washer goes under the e-clip, the thick plastic "doughnut" spacer above .....

5O2NeedleWashers.jpg


If they mixed these up, the needle will sit too high and run too rich, even with the e-clip in the correct #3 (from the top) slot.
 
OK, so checked the needle and it's sitting correctly (position 3) and spacers/washers in the proper order. :(

So, then I pulled the mixture screw...looks fine, o ring looks good, but I blew it out all the same. Perhaps the o-ring isn't sealing as well as it should? I've got a new one on order just in case.

I did a slide test on both carbs...left side ("good operating side") drops extremely slow with my finger over the intake...The right side drops a little faster...How slow is it suppose to drop--I'm assuming both should be close to each other? The diaphram is good (no tears/holes, etc that I can see...checked in a black room with a flashlight) I suspect the faster drop rate could be coming from the choke plunger...perhaps it isn't sealing as well as it should? Here's a video of the test:

Would that cause a richer condition, with regard to the mix screw setting that is?

Next, I checked the floats and they were set at about 27mm...I adjusted them down to 24mm. Hoping that may make some difference as well.

Beyond that, I'm wondering what else may cause the right side's mixture screw to only turn out 1 1/4 before being too rich? If this doesn't fix it... Would the next step be to raise the clip to position #2?
 
On an all stock bike (intake, exhaust), you shouldn't have to change the needle from stock. Yes, a leaky choke could cause rich running. The choke plungers are the same so you might try swapping them between carbs and see if the rich running follows.

Floats @ 27mm are actually low which would give a lower fuel level in the bowls and leaner running. Do re-visit your mix screw settings now that the floats are re-set. They may need some tweaking. One of the signs of incorrect float settings is little to no response to mix screw adjustments. Usually, you can hear the cylinder you're adjusting start to slow down and stumble once you adjust too far past the "sweet spot" (ideal setting) in either direction. With the floats set wrong, short of closing the screw completely (which will stall that cylinder), turning it every which way may seem to have little, if any, effect.

Something else to check would be the fit of your needle jets. They should be a snug push-in fit into the carb body. They are sealed in there by an o-ring. If they're loose and can be wiggled in the body, the o-ring is shot. In extreme cases, that can allow fuel to pass around the outside of the jet instead of just through it as it should. That can cause rich running.
 
Well, put it all back together...did the dead cylinder on each carb and it's the same... left side 2 1/4 turns, right side, about 3/4 to 1 turn out seems to be the sweet spot...fully closing doesn't even shut down the cylinder...beyond 1 turn and it starts to flood...So I'm stumped on that right carb.

Checked the choke plunger (switched with left side) and it didn't do anything different.
The float settings seems to have little affect (not noticeable).
needle jets fit snug...no noticeable play in them.

I suppose the only other thing is the mix screw itself...and I'll have to wait to see if a new one fixes it.

It pulls fairly strong, doesn't bog down or stumble throughout the rev range...yet still has that little hesitation right at the first pull of the throttle...but that goes away after it's warm. The tach still hovers a bit +/- 100rpms at idle. Setting the choke and letting it rev up, then turning off the choke seems to settle it some...which makes me think there's some kind of leak there...I've got new plungers on the way...so I will change those out when they come too...

pretty baffling...I'm not a carb expert so I may be missing something obvious, but so far I think I've covered everything.

Thanks!
 
its worth checking the carbs over really carefully for air leaks. Even a tiny leak will ruin the carb settings and running.

Recently in desperation I stripped my carbs completey and found several minute air leaks on both the choke assembly bodies where the original gaskets had dried and deteriorated. i also found that an over-tightened vacuum blanking screw had split the threaded hole in one of the choke bodies .I had to replace the whole choke assembly in the end as there was no other way to fix that leak.

On checking my float height settings I realised that the pivot holes in the carb brackets that hold the float pivot pins had worn so badly that the floats were free to move up and down at least 3mm in either direction. This made nonsense of the 24mm float height settings even allowing for the +-1.5mm tolerance .

To reduce the amount of play in the pivot I carefully crushed the brass until the steel pivot pin was a tight press fit into the brass float brackets . There was still more than sufficient play left in the alu carb post holes for free movement of the floats with a +-2mm tolerance.

I guess what I am saying is that these old carbs often need substantial and meticulous overhauling to compensate for all that wear , poor maintenance and neglect over 36+ years before they'll operate as they were originally intended to
 
I'm no carb expert myself, but you might want to check the pilot jet on the poorly working side. If loose it can let excess fuel bypass it.
Leo
 
I'm no carb expert myself, Leo

who is ?:) the more I work on them the less I realise I know about them:laugh:

What rpm does the engine idle at ?
If its higher than around 1200rpm its likely that the vacuum from the engine is drawing some fuel from the main jet as well as the pilot circuit.
try setting your idle a little lower by 100rpm or so and see if that makes a difference to your air idle screw setting on the rich cylinder
 
Peanut, the main circuit NEVER shuts off, even at low idle (just as the pilot circuit NEVER shuts off, even at WOT). That's why changing needle position always forces a change in mixture screw setting and often forces a change of PJ.

Brassneck, the way your problem developed makes me think you may have a leaking float in your right side carbie. Give the float a shake; if it's pretty full you'll feel fuel sloshing around in it. If you don't feel that, go to the next test: hold the float down in hot water and look for bubbles.
 
Peanut, the main circuit NEVER shuts off, even at low idle (just as the pilot circuit NEVER shuts off, even at WOT). That's why changing needle position always forces a change in mixture screw setting and often forces a change of PJ.
.

if the engine is always drawing fuel from the main jet even when on idle ,why are the carbs fitted with a idle jet and idle air screw ?
I thought that the CV caburettor worked by vacuum pressure from the engine and the main jet only contributed fuel when an increased throttle opening allowed a greater vacuum to draw fuel from the main jet as the slide lifted . :wink2:
 

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What rpm does the engine idle at ?
If its higher than around 1200rpm its likely that the vacuum from the engine is drawing some fuel from the main jet as well as the pilot circuit.
try setting your idle a little lower by 100rpm or so and see if that makes a difference to your air idle screw setting on the rich cylinder

It idles well from 1100 and up. I did the dead cylinder test at 800-900rpms to get the engine as slow as possible to hear the change... I couldn't really get it any lower or it would die.
 
Brassneck, the way your problem developed makes me think you may have a leaking float in your right side carbie. Give the float a shake; if it's pretty full you'll feel fuel sloshing around in it. If you don't feel that, go to the next test: hold the float down in hot water and look for bubbles.

I didn't hear any fuel in the floats, but I didn't test them under water...that is a good idea. It looks like one float has been soldered, so it clearly had an issue before...I'll test that this week when I get them apart again.

Thanks
 
Peanut, the reason there are multiple metering devices in a carburetor is to enable it to respond to changing demand conditions. Now in point of fact the main JET has little effect at idle, but the jet needle (part of the main CIRCUIT) has a strong effect; the main CIRCUIT, even at idle, is in play through the slide cutaway. If you do a little more tuning you'll find out what I'm talking about in a hurry. Then you'll use that component-and-range chart for kindling or hygienic purposes and focus on secondary effects of component changes.

Brassneck, if there's a lot of extra solder on the float in the problem carbie, that could be a problem too; if it doesn't bubble, you might want to weigh it against the known good one. If you do need to change floats, spend a little extra and buy an OE float from Yamaha; I don't know of any repops that are anything but trouble.
 
Brassneck, if there's a lot of extra solder on the float in the problem carbie, that could be a problem too; if it doesn't bubble, you might want to weigh it against the known good one. If you do need to change floats, spend a little extra and buy an OE float from Yamaha; I don't know of any repops that are anything but trouble.

Good to know. When I last had it out, I didn't notice it being excessive, but then again, maybe just a bit out of weight/balance may be enough. Probably better to get a known good one if all else fails...or just get the entire set rebuilt so I know it's perfect to start with. I think I saw someone on here that does that...might need to spend the $ to get them running great. As is, the bike runs strong, albeit, not symmetrical with regard to the mix screw. And the OCD in me is hating the fact that the one mix screw is nowhere near the other... lol.
 
Put your mind at rest about OCD--you're right to be concerned. The right cylinder mixture screw setting is just a symptom of a larger problem. If fuel control is the issue the mixture will be affected through the whole throttle range, and you've mentioned a drop in performance. Running too rich doesn't do your motor one bit of good!
 
Put your mind at rest about OCD--you're right to be concerned. The right cylinder mixture screw setting is just a symptom of a larger problem. If fuel control is the issue the mixture will be affected through the whole throttle range, and you've mentioned a drop in performance. Running too rich doesn't do your motor one bit of good!

Yep... got to figure it out...although with it turned 3/4 of a turn out vs. the stock setting at 2 1/4 out (Which is where the left carb is set at)...it runs really strong. Something is off, but with it at this setting, it's not running as rich... plugs look good, bike pulls really hard....but clearly it's not correct. I've received a rebuild kit, so now I'll play with it a bit and see if anything isn't seating well. (Nothing seems to be loose, but who knows.)
 
Your descriptions sound to me like a partially blocked or restricted airbleed.

During carb cleaning it's not enuff to be able to see carb cleaner spritz outta the airbleeds. They've got to flow the proper amount of air, else the pilot circuit on that side will demonstrate richness and delays.

Some folks use guitar strings to unplug jet orifices. I use (carefully and gently) numbered drills of range #60-#80, next size down from the orifice diameter for this.
 
. Now in point of fact the main JET has little effect at idle, but the jet needle (part of the main CIRCUIT) has a strong effect; the main CIRCUIT, even at idle, is in play through the slide cutaway.

Grizld1 I was refering to specifically the idle circuit which is where the op stated there was an idling problem.

The jet needle and main circuit plays absolutely no part in the engine idling .
The only time the main circuit might come into operation at idle would be if the throttle butterfly stops were set too high after carb maintenance which would simulate a slight throttle opening condition and lift the slide slightly. This is why I asked the op what the idle rpm was in case it was running at say 1500rpm+ at idle

The slide cutaway influences initial throttle response but I was discussing the idle circuit only which comprises the air idle screw and the pilot jet

Here is an excellent video representation of a cv carburettor operation:wink2:
 
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