Cutting Out 3500 - 6000

[mobro]

My 79 SF1100 has started something new after sitting for a few months. While giving it pretty hard throttle (80% or more) it is smooth till about 3500, then it runs rough but still gains speed at a slower rate of course. If I back off the throttle it smooths out, but if I hold it it will continue the sputtering until about 6k.

Anything position greater than 80% still gives this result. Of course I'm just estimating the percentage.

Any ideas?

Thanks,
Mobro

[Bill]

Re: Cutting Out 3500 - 6000

Mobro wrote:

My 79 SF1100 has started something new after sitting for a few months. While giving it pretty hard throttle (80% or more) it is smooth till about 3500, then it runs rough but still gains speed at a slower rate of course. If I back off the throttle it smooths out, but if I hold it it will continue the sputtering until about 6k.

Anything position greater than 80% still gives this result. Of course I'm just estimating the percentage.

Check the following tech tip:

The Coil Pickup Fix

and the other tech tips referenced there. Pickup coil failure is a common problem on our old bikes and is easily fixed.

Good luck and keep us posted on how it works out!

XSively,

[mobro]

Thanks Bill, I'll let you know. The information in the tech tip sure sounds like this could be it...

[pgg]

Might be a sticky slide in one of the carbs.

[mobro]

I couldn't get it to replicate the miss by tugging on either of the wires, but I did notice that the advance device did not rotate freely at all. After a little WD 40 followed by some machine oil it now rotates very smoothly.

I haven't road tested it but will soon.

It sure seemed like this should be it (bad connection to one of the coils) because the "miss" is so perfect in time.


Thanks to all, will let you know.

[skids]

I would think pick-up coil or bad pipe/mufflers, or plugged screens in the fuel valve(s) of the carbs if the affect is not immediate, or bad diaphragm or sticky slide(s).

[RS Thornton]

mobro,
Any chance you roadtested yet??? I have exactly the same problem on a 78E that has just under 12,000 miles on it. I bought the bike this summer and got it running a few weeks ago. I do not think this bike is putting out like it should, and it wimps out at 5000, if I ease through it seems to pick up after 6-6500. I was thinking the advance might be sticky but I wanted to have a timing light around before I pulled this apart. I tried pulling on the ignition wires but could not get any rpm change or stall to indicate I found the bad one. I was going to try the test again, but being a little less gentle as I tugged the wires. If you solve this let me know what worked. Thanks

[mobro]

Road Test Results

Still have the problem - although it seems to have moved up to about 4000 before acting up. Next step rebuild / clean carbs. Will let you know.

Ed (MoBro)

[Ken Talbot]

I think you're on the right track, Ed. About the only thing that can really go wrong with the engine when a bike is just sitting is the carbs, and they will tend to act up at a particular rpm range depending on just which passage(s) is(are) blocked.

[RS Thornton]

Well, mine is better.
First the good news, I went through the ignition system, pulled apart all the connectors and cleaned them up. Checked coils and plug caps and had to replace 1 and 4. This got me back to better low end, and my turn signals blink a lot faster.
The bad news was it was still stumbling at the high end, and the plug check indicated running too rich. The big has aftermarket pipe and individual air filters, the main jet was up two sizes at 142.5. I went back to the 137.5 original size and got a small improvement. Still stumbled so I moved the float setting from the 25.7mm up to 26.7mm, got a little better, but still no passing power at 6000. Off came the carbs and the float setting is now at 27.7mm. The bike is better but still stumbles at 6000 if throttle is wide open, back it back even a little and smoothes right out. I read that others have had to move the float setting like this, so I think the carbs are coming off for one more 1.0mm adjustment to see if I can get the stumble to go away. Not sure if I will be motivated enough to do this before the snow flies as the bike is cruising well for fall color trips with SWMBO and I am have dropped behind on some home improvement projects.

[mobro]

I'm sure impressed with your determination! That many removals and instalations for the adjustments. But I'm not very mechanical - just the bare minimum of skills to own a classic bike (oil change, brakes, clutches) so I impress easily!

Mine used to be like yours, hesitate only at the very high ends and only at full throttle. Backing off would give it a boost. Now the problem has migrated down in RPM and at lesser throttle.

My local one-man repair shop in my town of 20k doesn't seem to want to tackle the carb rebuild. He just can't seem to get around to telling me when to drop off the bike. He suggested running 4 oz of STP gas treatment per tank till the 12 oz bottle is gone. I'll let you know the results of that one.

We're going on a 1,000 mile ride Oct 3-5 maybe the STP will disolve something that is plugged or stuck. Otherwise I'll have to take it to a big shop in the city or one of my brother in laws who are not mechanically challenged.

[TopCatGr58]

Hey there Mobro,

These carbs aren't as scary as they seem. I had never touched a bike carb until I got mine, but I had done a few 1-2 barrel car carbs rebuilds. You can break them down without actually pulling them apart from each other, so you won't loose your eyeball or previous synch setting. But, if you want to try the chemical way, then find yourself some SeaFoam instead of STP, or at least some Techroline/Techron, and follow mixing instructions. SeaFoam is designed for marine applications, and so is better for disolving heavy grimes and sludge from ?wintering? boat motors and such!!

Good luck. T.C.

[Duster]

I'll have to agree with the Sea Foam. Sence I read about useing this on the foram I regularly use it to eliminate a stumble I get at 3200 RPM.

Duster

[Redbaron]

Jets Maybe

Had a similar issue, at Wot the bike bucked and sputtered terribly. Could back off a little and all would straighten out. Had 137.5 mains, so, just for grins I replaced them with 120's I had laying around. This smoothed out idle and provided smoother response, no more acting up at WOT. I have noticed improvement across the entire spectrum. Tried many things before I broke down to this, specifically the rejetting info posted here. According to the chart I should be running bigger than 137.5's, with indy filters and Harley Fat boy mufflers installed. Glad I didn't try going larger as suggested. Plugs are light tan and all is running better than it has in a long time. When setting at a long traffic light does your bike seem to load up? If it appears to load up at all I'd suggest trying smaller jets as opposed to the larger suggested in the re-jetting article. This is only my humble opinion and this solution may not help anyone else, but it did help me.