Stuttering while accelerating?

[MPittma100]

Jets

I finally had a chance today to look at the pick up coil wires. Couldn't find a problem. The wires are in good shape. I pulled on them looking for any sign of breaks and even pulled on them in both directions with the bike was running. No change to the idle at all. The advance seems to be moving smoothly up when I rev the bike. Still it seems to miss while idling and the shuddering is still there as you approach 4.5 to 5 K rpm. As an experiment I tried pushing it past the shudder in 4th gear and it did start to smooth out some as I approached 6 K rpm. About a month ago I rebuilt the carbs and increased the size of the main jet from 137.5 to 142.5 because of the K&N air filter and custom exhaust. Maybe I needed to make some adjustment to the float bowl heights. I was trying to eliminate a flat spot I have always had at around 5 K rpm. I am tempted to put the old jets back in.
Sorry to go on so much but I'm just trying to give as much info as possible. Not sure what I should check next.

When going back in to your carbs, make it your goal to assure that all of your jets and the needles are genuine Mikuni. I cured a midrange-higher gear stutter by removing K&L carb parts and installing Mikuni replacements. Do go back to the OEM jet size.

[Bonz]

Yes, DiverRay, I am talking about the advance unit, nothing to do with carbs in my posting, thanks for noting that.

Question: What makes an iridium plug more prone to fouling if mixture is off?

[jetmechmarty]

Mixture screws

OK. I am officially confused. There are guru conflicts.

One says adjust to highest vacuum.
Another says turn towards closed until the RPM drops, then open 1/8 turn.
Third says adjust to highest RPM.

These all came from the most trusted and experienced sources on this board.

As a side note, I'm having economy/black plug issues. I had mine set by the highest vacuum method. That turned out to be 1.5 turns out on each. Thankfully, I don't think this setting is very critical because after review of what the most trusted gurus say, I don't know. 1.5 to 2.0 turns out is probably good enough.

[DiverRay]

Marty, We don't always agree because we all learned from different people. Highest idle and highest vacuum are both VERY CLOSE. The bike will run well set with either method, just chose the one you know best.
I do think if he has K&L jets, that could be part of the problem. There IS a difference in the jets, and Yamaha and Mikuni had things correct from the factory. Changing jet sizes should be just that, one step up or down from what is in there. The size is based of flow with the mains, and MIXING THE FLOW with the pilots. If they were not produced correctly, they will not work correctly, IMHO.

[CaptonZap]

They are all correct.
What each of those methods is trying to get to is the smoothest, slowest idle speed with the least amount of gas going into the engine. At that point, the engine is putting out the most power, (creating suction), with the smallest throttle opening, (fuel burned).
1: When using highest vacuum, you usually end up reducing the idle speed a time or two to get to highest vacuum and correct idle speed. This is the best way, in my book.
2: The second method assumes you are at lowest idle speed, and you choke off idle fuel flow until it stumbles, then open enough to smooth it out, and re adjust the speed up to specs.
3: The third assumes that the idle speed is correct, and you adjust the mixture screw to get highest idle speed, which means you go back and set the idle speed down and adjust mixture again, and again, etc. And there you are, back at two.
Just be sure that all others aspects of the engine are correct. Timing, valve clearance, ignition condition, plugs, compression, etc.

CZ

[jetmechmarty]

Ray,
You're right, of course. I'm running rich when I used to be running lean. My jetting is nearly stock and all Mikuni. I used to be lean across with 110-120-120-110 that was in the bike when I bought it in '03. Now, it's 110-115-115-110 and rich. Mix screws at 1.5.

Folks are too quick to mess with jets when they should be left alone. As for me, my solution may yet be the spark and possibly my #4 float as I has a broken float post welded.

Point is, I'm not going to mess with the jetting and most folks should do the same, IMHO.

Back to the topic. Thanks!

[Graman]

I finally got back to the bike this week. Because of the fowling plugs on #1 and #2 and not #3 and #4 I decided to re-sync the carbs. When I hooked up my Motion Pro balancer it went nuts. Vacuum was extremely high on the left side. When I tried to balance it over the idle started to climb drastically. By the time I got the idle down it was too late. About half the fluid in the balancer had been sucked out. After doing some research on the web it turns out I probably had an air leak in the o-rings of the balancer. I took it apart and cleaned and re-seated the tubes and O-rings. I washed all the remaining fluid out. I had read in some forum somewhere that the fluid is glycol based and you could use auto antifreeze as a substitute instead of paying the rather pricey cost of the pretty blue replacement fluid from Motion Pro. I figured nothing to loose by trying it. So I refilled the balancer too the correct levels and it works great. Just like new. If anything the fluid seems a bit more stable. Less bounce in the readings. I re-balanced the carbs. Adjusted the idle mixture by ear and she is running good. Idle nice and smooth. But I still have the stutter @ 4500 rpm. Dam. Researching other possible causes. Maybe carb float level. Maybe main jet needle setting. I have never done a valve adjustment since I owned it. That's the next priority.

[DiverRay]

I have never done a valve adjustment since I owned it. That's the next priority.

You NEED to adjust the valves and check the timing BEFORE trying to sync the carbs! I would start with a valve check, then check the timing and advance per the Yamaha manual. After that, you can check the sync. That may NOT solve the problem, but it WILL get you closer to it.

[IanDMacDonald]

You NEED to adjust the valves and check the timing BEFORE trying to sync the carbs! I would start with a valve check, then check the timing and advance per the Yamaha manual. After that, you can check the sync. That may NOT solve the problem, but it WILL get you closer to it.

By simply swapping-out one shim size on two intake, and two exhaust valves, it threw my sync settings off quite a bit (also made it noisier). Every season I check the valves. It may be a pain, but I don't like to neglect something so easy. Just be prepared to buy another gasket for the valve cover. Spray it down with PAM prior to install.

[jetmechmarty]

I think once you have checked the valve clearance, you will no longer be intimidated by the task. The same goes for swapping shims. I believe there may be a lot of these machines out there that have never had the valves checked or adjusted, due to the intimidation factor.

[MPittma100]

"Back in the day"

OK. I am officially confused. There are guru conflicts.

One says adjust to highest vacuum.
Another says turn towards closed until the RPM drops, then open 1/8 turn.
Third says adjust to highest RPM.

These all came from the most trusted and experienced sources on this board.

As a side note, I'm having economy/black plug issues. I had mine set by the highest vacuum method. That turned out to be 1.5 turns out on each. Thankfully, I don't think this setting is very critical because after review of what the most trusted gurus say, I don't know. 1.5 to 2.0 turns out is probably good enough.

Back in 1975 when catalytic converters and the little plastic limiter caps on the fuel/air mixture screws invaded our automotive lives, there was an OEM prescribed method of adjusting these screws. It was called the "lean drop" method.

The procedure was simple - adjust screws to best vacuum, then turn each screw back in to drop the idle RPM by 50 RPMs (GM cars and trucks). The goal was to keep exhaust emissions in check (lean). I'm guessing that Yamaha was required to do same.

[motoman]

Back in 1975 when catalytic converters and the little plastic limiter caps on the fuel/air mixture screws invaded our automotive lives, there was an OEM prescribed method of adjusting these screws. It was called the "lean drop" method.

The procedure was simple - adjust screws to best vacuum, then turn each screw back in to drop the idle RPM by 50 RPMs (GM cars and trucks). The goal was to keep exhaust emissions in check (lean). I'm guessing that Yamaha was required to do same.

Actually, that was set to highest in. vac., gradually back in for 50rpm drop, THEN back out 1-1/2 turns.
That same 'lean drop' method, if choosen is done same way on our Mikunis.
Once going back in with mixture screw listening for slighest rpm drop(low rpm tach is better), back mixture screw out 1/8 to 1/4 turn at maximan.

Having used this procedure for decades in the automotive world, it was brought to my attention several yrs. back by a local bike tech. here who was ironicly factory trained on the XS11 series back in the day, that clued me to the above procedure.
He now has his own shop with a large backlog of nice looking customer classic scoots to be worked on, and is very particular bout the work he does.
Knowing him for almost twenty yrs., and now having his own shop made me lazy enough to just pull my fairing, leave it there to cool overnite and let him swap out shims for valve adj. Best part, is he doesn't disturb cam journel caps changing shims.

[RobSwind]

I'm having a very similar issue with my 1980 xs11 (SG). But I think mine is due to fuel starvation, though I can't exactly figure it all out.

First off, my bike has K&N pod filters. Cleaned within the past week so air should be flowing well. I did open up the carbs shortly after I bought it a few months ago and they were super clean. I mean, I'd eat off the surfaces, and no corrosion on the jets or other brass components. I also chopped off my exhaust shortly after I bought it.

I did NOT, however, think to look to see what size jets were installed, thought the previous owner said he was almost certain they were stock.

Now is when I'd expect someone to say "Well you're getting too much air due to the added flow from the chopped exhaust and pod filters, you idiot!"

But here's the kicker. At higher RPM's, I'm fouling out plugs. I have to turn the mixture screws almost all the way in just to keep from fouling them out at highway speeds (rich), but at idle, the exhaust is popping (lean). Does this make sense to anyone?

In my opinion, if I'm fouling out plugs only at high RPM's, it seems I have dirty filters, but I just cleaned them using the proper method.

Oh, and even though I'm getting the seemingly rich burn at higher RPM's, I'm still getting that stuttering problem when accelerating at highway speeds that the OP has described.

Thanks for all the help, in advance.

[hondakid11]

I'm having a very similar issue with my 1980 xs11 (SG). But I think mine is due to fuel starvation, though I can't exactly figure it all out.

First off, my bike has K&N pod filters. Cleaned within the past week so air should be flowing well. I did open up the carbs shortly after I bought it a few months ago and they were super clean. I mean, I'd eat off the surfaces, and no corrosion on the jets or other brass components. I also chopped off my exhaust shortly after I bought it.

I did NOT, however, think to look to see what size jets were installed, thought the previous owner said he was almost certain they were stock.

Now is when I'd expect someone to say "Well you're getting too much air due to the added flow from the chopped exhaust and pod filters, you idiot!"

But here's the kicker. At higher RPM's, I'm fouling out plugs. I have to turn the mixture screws almost all the way in just to keep from fouling them out at highway speeds (rich), but at idle, the exhaust is popping (lean). Does this make sense to anyone?

In my opinion, if I'm fouling out plugs only at high RPM's, it seems I have dirty filters, but I just cleaned them using the proper method.

Oh, and even though I'm getting the seemingly rich burn at higher RPM's, I'm still getting that stuttering problem when accelerating at highway speeds that the OP has described.

Thanks for all the help, in advance.

your mixture screw wont cure you being rich at "highway speed" it is either your needle (1/4 - 3/4 throttle) or you main jet (3/4 to wot). mixture screw is for idle, and pilot jet for idle to 1/4 throttle

[RobSwind]

Ah!!! Gotcha.

So my main jet would be too big, or needle too small? If I open it up and find everything stock, I would think that going bigger wouldn't be the solution, if I'm running rich.