Stuttering while accelerating?

[hondakid11]

if you are rich at wot, move down a size on you main. your needle should be in the middle groove and shouldnt need to change that. i didnt change my needle adn i have k&n pods and open exhaust and im at 142.5 mains on my 79sf. i origionally put 145 in and it was too rich.

[motoman]

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.

May sound counter productive to YOU, but an actual TOO low a fuel level will give a rich condition since it takes more more venturi velocity to draw the fuel thru the pick-up tube.......and then flow comes all at once, and too much of it, BTDT. Wouldn't hurt to go back thru and check that those 8 float levels are for sure set at 23mm.

[Graman]

I am also going back in to the carbs to adjust the float height again. Bike is running well and a pleasure to ride but I still have that stutter at about 4500 rpm. I have mine set to the middle of the range given in the service manual. 25.7mm +/- 1.00mm. I think they were around 26mm. A little towards the lean side. 23mm sounds very rich to me but mine is a 79SF not SG. Carbs may be different. I probably should of adjusted them to about 25mm when I installed the larger 142.5 jets. I am thinking that when those main jets start to really open up the fuel level is too low in the bowls and it's starving fuel. Worth a try.

If you go back into your carbs make sure the c-clip on the main jet needle that runs through the diaphragm and slide is in the stock position. Mine is the centre notch. If yours is in a lower position it could be opening the main jet too much at higher rpm's.

[TopCatGr58]

Hey Graman and RobSwind,

Graman, you've got the earlier carbs...with adustable vac. slide needles. Rob, if your's is stock 80G, then it should have the later model carbs with the FIXED/NON-adjustable vac. slide needle, slightly larger/wider vac. slides, and separate pilot and main jet towers with NO shared tunnel.

The float heights are different for the 2 series of course, along with the MAIN JETTING....137.5 for early, 110-120 for later since it doesn't have to feed the pilot....it feeds directly from the float bowl, not thru the shared tower.

In the early part of the 80's production..there were some leftover bodies that were Bastardized by Yamaha....they have the shared tower but have the later sized jets....go figure!?

Just wanted to clear the air about carb tuning suggestions and having to keep in mind the different styles of carbs, jets, needles, so one adjustment may not be possible on the other type!

T.C.

[RobSwind]

You guys are the best. I took my carbs apart and checked out the main jets. Like I said, I have an '80 model, which comes stock with 110 main jets. But what was installed were 137.5 jets. I contacted the previous owner and he said he rebuilt the carbs a few months before he sold it to me and thought he put the stock jets back in it. So obviously he bought rebuild kits for the wrong year.

Just finished installing some 120 jets today. Those should compensate for the pod filters and the open baffle without fouling out my plugs.

[jetmechmarty]

120 should be OK. I've had them in mine with airbox.

[motoman]

120 should be OK. I've had them in mine with airbox.

Not on the SPECIAL carbs Marty.........different configured metering rods (and numbering) IF in fact those carbs are off an 80-81 Special. 80-81Specials= 110's across ALL four.

[jetmechmarty]

Yes I know, but without airbox and open exhaust, it should run OK with 120. Certainly better than with 137.

[RobSwind]

Motoman, what size would you recommend for an '80 SG with pod filters and baffles removed?

And yes, this bike has the original carbs. It only has 12,686 original miles on it. There'd be no need to Frankenstein parts from another bike. The last owner just didn't know what he was doing when he tried to rebuild the carbs, apparently.

[motoman]

Yes I know, but without airbox and open exhaust, it should run OK with 120. Certainly better than with 137.

Not.........

[motoman]

Motoman, what size would you recommend for an '80 SG with pod filters and baffles removed?

And yes, this bike has the original carbs. It only has 12,686 original miles on it. There'd be no need to Frankenstein parts from another bike. The last owner just didn't know what he was doing when he tried to rebuild the carbs, apparently.

ALWAYS start stock, in this case 110's, Genuine Mikunis, NOT aftermarket. Tune and ride, check plug color, etc.......What mains your trying to use now will NOT work, unless you just like smellin unburned fuel out exhaust.
Individual float settings HAVE to be spot-on AND indentical. I can guarentee you though, the actual fuel levels will NOT be the same in each bowl even with that. Those ACTUAL fuel levels ARE critical if you don't want one cyl. 'carrying' another, so on and so on......right down to the idle mixture tuning.

[motoman]

Not.........

Sorry Marty......just messin' with ya'.........and yea, better heading the right direction.....by the same token, Genuine Mikunis AREN'T cheap to wanna be playing that game.

[RobSwind]

So far, with the riding I've done today, the spark plugs are firing much better. No more fuel smell like I had with the 137.5's. The bike runs smoother at all RPM's and even cranks much better.


I've readjusted the mixture screws and will probably fiddle with them the next couple rides to fine tune them a bit more.

[Graman]

You should also make sure to balance the carbs after working on them or modifying them in any way. Just taking them on and off the bike can through them way off. No point in setting idle mixture until after they are balanced. If you don't have a balancer you will probably pay a shop $80 to $100 to do it. I bought my own off eBay for around $100. Pays for itself with 1 use. Mine is Motion Pro and works great. There are plenty of resources on this sight that explain the procedure. Very important if you want your bike to perform at it's best. Glad to hear you found your problem. Probably not the previous owners fault if he gave the parts dealer the correct year and model. He was probably sold the wrong rebuild kits for the carbs. Probably just trusted they were correct. Should of educated himself on what they were supposed to be before installing them though.

[RobSwind]

I have the vacuum gages. Already balanced them.

The previous owner did the work himself, but could have possibly ordered through a dealer and the dealer got the sizes wrong. But if I was going to do the work myself, I'm going to make damn sure the sizes I'm putting in are definitely correct.