4000 rpm stumble

[LoHo]

Gas gauge problem solved...loose connector from man-handling the tank so much...

[labrat99]

Loho -

Did the "Plasti-Dip" fix work? The problem that you described sounds exactly like what my bike does. Thanks.

Labrat

[LoHo]

A couple of months ago, my bike started to run very poorly.

As a long time member of this list, I was aware of the laundry list of things to check.

I looked at the wiring for the vacuum advance. It stretched where it shouldn't have. I did a crappy soldering job, but it was the best I could do. The bike ran better, but not great.

I hadn't ever adjusted the valves. I finally did that. The bike ran better, but still not right.

I checked the vacuum diaphragms, and reported on that above.

Today, I rode to Los Banos to meet with some XSives. Once again, the bike idled well, ran rough in the mid-range, and okay in the higher RPMs. I used three and a half gallons of precious fuel to go 90 miles, a much worse consumption than usual. I decided that I must finally tear into my carbs, a job I've dreaded.

As I was figuring out how to pull the airbox and looking over the hoses I'd need to pull, I noticed something interesting. The vacuum hose that works the advance comes off of the #2 carb. The hose near the carb had a crack in it, just longer than the nipple it attaches to. I clipped it shorter and reattached it, but it is too late for a test ride tonight. It would explain the sudden onset of troubles and the fact that very low vacuum (idle) and very high vacuum (near WOT) were less affected than the mid range. Also, she pinged when I tried to roll on the throttle for passing. Not fully advancing the spark might explain the excess fuel consumption and the pinging, and some muffled back-firing that was also a new experience.

We'll see tomorrow...

[corey]

Alright... this post is pretty much dead, but I'm going to get it going again... as my problem is still persistant. :|

As mentioned above, my 79SF still performs poorly in the 4000-5000 RPM range. However, I recently observed that this doesn't happen all the time. For instance, I'll be applying quite a bit of throttle in the low gears, and I see no sign of this power loss. But then, say in 4th and 5th gear, when a heavy dose of throttle is applied, the power loss/stumble sets in again. However, when I slowly apply throttle, the bike works its way past the 5000 mark and runs as normal in the upper revs.

Does this give any one any more hints? I tried all of the above (pickup wires, new advance unit, stretching the springs, checking the vaccum house, etc.) to no avail.

So frustrating...

[TopCatGr58]

Hey Corey,

Let's recap: 80carbs on 79SF
Pickup coil wires okay,
Diaphragms intact=no pinholes,
Vacuum lines intact,

Did you fix that "catch" in your vacuum advance/timing plate rotation?

What are your jet sizes for the mains?

What are you float heights set at?

Sounds like it's a tad rich, but in lower gears the engine can easily handle the extra fuel and rev quickly obsorbing the extra richness during the transition from pilot circuit to mains, but in higher gears, the load is heavier, and so when you crank open the throttle too much, the slides rise, dumping too much fuel down the carb throat, and so it bogs a bit until the revs catch up?

Have you done the throttle chop to check your plugs, how do they look? You may need to go a little leaner in either the jets or the float heights to lessen the fuel surge during the transition from pilots to mains!?

Or....just don't run below 4K rpm in 4th and 5th gears!!
T.C.

[corey]

Recap is correct. I did fix the snag in my spark advance by replacing the canister assembly.

As far as jet sizes go, I followed the jet size guide and bumped up the main jets three sizes (+1 for Jardine 4/2 exhaust, +3 for cone filters, -1 just because) and the pilot jets one size(for the increase of three main jet sizes). This resulted in 45 pilots, 117.5 in cylinders 1 and 4, and 127.5 in cylinders 2 and 3. The floats are set at the stock (for 80) height of 23mm. I haven't done the clear tube float height test, however.

While we are discussing jets again, I should mention that before rejetting this bike, the mains were 110 in all four, with 42.5 pilots. The air jets said 85, however, suggesting the carbs originally came with the 110/120/120/110 orientation. Thus I merely assumed that the previous owner had messed with jetting, resulting in the 110s all across. Do you think i figured this wrong?

Haven't done the throttle chop test yet, but I think I will soon. So far your suggestions sound right on.

[pggg]

Corey, I reckon it's gotta be better all round to have all jets the same size. Yamahas idea of having different jets in different cylinders seems like B.S. to me. A few degrees in temperature variations would mean diddly squat in an old air-cooled lump like this. I reckon these uneven sizes are half the trouble. Good idea to check out all 4 plugs for some explanation as to what's happening .

[TopCatGr58]

Hey Corey,

Yeah, even though the manuals quote the staggered jets, I found mine were 110 straight across on my81SH when I tore into them!! So....that could be part of your problem, too rich on the inner ones. I'm also running 117.5's in mine for similar intake and exhaust mods, but only my 42.5's pilots, cause a couple were so corrosion welded that I left them instead of trying to drill them out. I just turned the pilot screws out another turn to about 2.5 turns, and they work just fine!

So....like PGGG says, getting them all even will probably help you get closer to solving this stumble!
T.C.

[Shuriken]

I'm trying also to fix the midrange stumble on my 79sf. Almost identical symptoms but I don't have a working tach to tell you at which RPM it happens.

I have had trouble with jets - trash K&L jets I would assume (I wish someone could get rid of all the K&L crap jets out there) - so I want to get genuine OEM somehow, start back at stock configuration, and then change only one thing at a time.

Since I have a 4-1 exhaust, in stock configuration, that would be the first thing changed. Midrange is critical on this engine because it isn't much fun under 3k and drops out on the top end. The mid is critical because that's where the nasty punch is, and my bike is falling over....

Ben

[corey]

So I swapped out a my middle two jets. Now I'm running 117.5 mains straight across, with 45 pilots. Also, while I was in the carbs, I adjusted the float height (more on that in a second).

So my problem is less obvious now, for sure. The stumble isn't present at all in first, second, or third gears. It only slightly becomes apparent in fourth, while it is still fairly apparent in fifth gear, if you don't roll the throttle back hard. Again, when I go wide open the thing gobbles up the MPH like crazy with no stumble at all. I'm thinking its a vaccum problem now. Does that make sense?

As for the floats, I wasn't quite sure what I was doing. I knew to meaure the height from the round top (round part) of the float to the body of the carbs. I was looking for 23 mm. What I didn't know about was whether I was to measure the distance from the gasket surface, or from the little lip that surrounds the gasket surface. ...or is it from the gasket surface with the gasket on?! I went ahead and measured from the lip. Input?

[TopCatGr58]

Hey Corey,

Well, you've set your floats a little leaner than you were aiming for, s'pose to measure from the carb base where the gasket fits, so measuring from the lip, you've set them further down. You'll want to do another throttle chop check later.

Have you checked your timing with a light?! Also, you've got the vacuum line connected to the Carb nipple and not the synch port don't you?

Before you said if you roll on the throttle slowly that it wouldn't happen. Now are you saying that it won't happen if you hit WOT in 5th!? Are you still trying to roll on from 4k and it stumbles at around 5K? Sounds like you're on the right track since your stumble has moved up a few gears and gone away in the lower gears!
T.C.

PS, you've got 4-2 aftermarket pipes, but I can't find what type of intake? If you're running the stock box, then your 117.5's may be too much jet still! Guide for single mod is 2 sizes up from stock, which I've found to be 110's on the later year carbs, so you may only need 115's!?

[corey]

Sorry TC... I was inconcise. The stumble is only present between 1/3 and 2/3 throttle in 4th and 5th gears. If I baby it through or rip it open it will work as normal.. except with maybe a slight hestitation when giving wide open throttle.

As for the intake, I have the cone pod filters, so the 117.5s should be right on, right? Also, I checked the rubber boots of the air cleaners to make sure it doesn't block the air pilot jet... and it sure looks like it doesn't.

[LoHo]

I finished rebuilding the carbs and synchronized them with my new dial vacuum guage, and she ran better, but still not right. I decided to go back over the things I've done from the beginning of the current troubles.

I examined my vacuum advance soldering job. All of my connections were intact...brown to brown...blue to blue...grey to pink...GREY TO PINK!? I redid the soldering, connecting the wires correctly, and now she goes like a bat out of hell. If I'd done the wires properly at first, I never would have fixed my carbs and carb boots or fixed the diaphragms, so guess my error was a blessing in disguise...but it was an excellent diguise!

[xj11john]

Corey where does it stand now?

I am interested in your outcome Corey since mine has a slight burble or stumble from 3.5-4k in 3rd - 5th gears, thanks in advance for the follow ups.

ride on.

[Shuriken]

With stock everything except a 4-1 and at 6700 feet elevation, my bike is stumbling through the entire midrange. It's less obvious if I roll onto it slowly.

??

Ben