Jet Needle Position

Patient: '81 Seca 550. Yoshimura pipe. 47,000 km (@30,000 miles)

Symptoms: 1. Flat spot in the powerband at 5 thou. It's been like that for 30,000 kms, when the pipe went on.
2. When the bike is hot after riding, and then sits for half an hour or so, it will flood itself and need full throttle to start and clean itself out. Once it's running it will idle @ 1200 all day long, hands free. Always starts and runs strong.

Treatment: Would lowering the clip on the jet needle help the flat spot? Can this be done with the carbs still on the bike? Carb work scares the hell out of me! I'd rather leave it as is, than mess it up by trying to work on a major overhaul of the carbs.
Any ideas on the flooding when hot?

 

I ran my Seca with the adjustable Dynojet needles at 1 clip below suggested position (i believe it was 3 from the bottom). I got a bit more midrange, but stil had a flat spot at 5-6K. I believe this is just a tuning flaw with these motors, as all seem to have the flat spot. I put it back to the recommended notch (3 from the top) and noticed a bit more flat spot, but bottom end was improved. Now my Seca is mainly stock, stock pipes, Uni filter and Dynojet kit. I used the jets for modified exhaust and intake (112's) because the 116's made it run worse. I haven't drilled the slides yet and kind of don't want to as i do not want to foul the carbs up. They are pretty hard to come by. Best bet is lower the needle the 1 postion, then do a plug chop at 5000 and see if the plugs look better. Other than that i have no other ideas. You could get a Megacycle cam and improve the midrange, but that would cost quite a bit for a small increase in midrange. cruzerjd

 

I would check the info that came with the Youshi pipe. Most suggest upping the jets 1 or 2 sizes. Since the flat spot started with the new pipe I would suspect the mix is lean due to the main jet size which means you would be better off raising the needle instead of lowering it. Best way to tell is to warm the bike and when you hit the flat spot slightly relax the throttle and see if it runs better. Then go back to the flat spot and open the throttle just slightly and see if that is better. If lean then relaxing the throttle should render a better response and vise versa for rich.

 

Do an experiment.

Find a cheap rack of carbs being sold for rock bottom because they are basically unusable as carbs anymore.

Pull the Emulsion Tubes out of the Parts Carbs and ... using the Welders Tip Cleaning Set ... enlarge the 3 Top Holes around the Emulsion Tube to the NEXT Size ... (One size up)

Then,

Take the Main AIR Jet and Increase that Main AIR Jet to (2) Sizes Over.

Put the Modified Emulsion Tubes and Air Jets in YOUR Carbs.

Now, when the Needle Jet is LIFTED up during acceleration ... as the Needle Allows more Air to Draw-up Fuel ... more Fuel is going to be Drawn-up from the volume supplied by the Main FUEL Jet.

The increase in AIR available over the Main Jet's Fuel Supply is going to cause a slightly RICHER Fuel Supply in that Mid-Range to make the engine put-on more Power. Because the Main AIR is going to bring-up more FUEL.

At WOT -- there might be an additional gain -- getting-up to Flat-out -- but likely not to effect Total Power determined by Main FUEL Jet Volume.

 

Thanks for the response. I still need some clarification. From what I read, if I raise the jet needle(lower the clip) it should make the bike run richer, which I believe will help. If I relax the throttle in the flat spot it improves, as suggested by BlueMaxim, meaning a lean condition.
Questions for Rick. When you say Emulsion tubes, are we talking about the main nozzle Ref # 36 on fiche, and Main air jet is the Pilot air jet Ref # 23?

Can I access the jet needle if I take the tops off the carbs while they are on the bike? I really want to avoid taking off the carbs. I swore enough last year when I replaced the intake manifold boots. (so no air leaks)

On the up side, I'm starting to understand how the carbs work! Thanks again.

 

#-36 Nozzle, Main (also Emulsion Tube)

I quit calling it a Nozzle because it confused everbody.

I don't see a Main AIR Jet on the Fiche. On the larger Mikuni's, it's in the Intake Opening, on the outside edge, to the right of the Butterfly as you look at the Carb.

Those smaller Carbs must share that Passage.
I'll have to get a set and knock 'em down to tell for sure.

 

Alright, tkae my suggestion abour raising the needle and wad it up and pich it in File 13. I have been chasing a midrange stumble and popping on my 82 Seca all week (Yep, vacation week). I decided it had to be running lean on the midrange so I raised the needles. Made it worse, black plugs et al. Upon inspection of the carbs, i found one of the float needle seats had worked loose and was causing #2 carb to run out of fuel, so i tightened all the seats and made sure the floats were good and set proper. I then replaced the fuel line with a new 1/4" rubber line and a large clear filter. I then rode the bike to the store, 5 mile trip, ran great for about 1/2 mile, then pop and sputter, et al. Guy at the store thought it was running lean, so I went home and checked the plugs, not lean, over rich, which is where the story began. So I reset the needles back to 3 from the top, check the pilot screws, 3 1/8 turns out, replaced an o ring on #3 which previously had no effect from adjusting the needle, cleaned the pilot screw on #4, which for some reason had roughh threads and got it adjusted with the others. Now the bike runs nice, just need to get the idle down below 1000 rpm's and it will be perfect. So my opinion on the carbs on your Seca 550 would be to change the main jets 1 size at a time till you get a good compromise. On my 1974 Z1 with a Kerker and pods, I ran mains 4 over stock, I believe I ended up with 127.5 mains on that one. THose were round slide carbs though, not CV. So hope you can get the carbs figured out, cruzerjd (going to bed...YAWN....ZZZZZZZZ)

 

This is the point where I'd lose my patience. I just don't have the patience to deal with all the difficulty engineered into making Carb Mods so difficult.

I could understand playing with Carbs if there was some easy way to get after the Jets and Floats.

So, now-a-days, when a young guy comes in showing me the stuff he wants put into his bike expecting it to run better ... I just tell him he only needs to have the complete set of Larger Main Jet's he'll get when he bumps-up to the next sized ...

BIKE!

Get out of here.
I'm too old for that crap!

 

hahhh Rick i haven't laughed that hard at a final post for ages.

 

you should be able to do it without taking the carbs off
if you can find tiny washers to put under the clips you can change the position less than a full notch on the pin