79sf Jetting

[Shuriken]

Had another chance to ride, lately. This time I did about 100 miles on the weekend.

I've been running my 79sf (with 79sf carbs) with 130 mains and 42.5 pilots. I don't remember the jet needle position I've been running it at. Elevation is 6700ft. It has the stock airbox and stock air filter but has a 4-1 exhaust.

The bike runs with good power but seems to miss under load through the midrange. Personally, I think only when the carbs get on the mains does it run more like it should. I just don't think the bike has the whiplash I think it should. I think I have to open the throttle a bit slow or it bogs a bit worse.

I looked at the plugs today at WOT and MID. Both circuits seem to be rich based on color, so I thought I'd tweak.

I didn't have any 125 mains but have a bunch of 115s and 110s. I installed 115s and tested. The bike ran like crap. Put the 130's back in and tried some 37.5 pilots. The bike ran like crap - would not even idle at all, complete loss of idle circuit.

I don't have my manual, but I think the stock mains were 137.5, so I'm already leaner at 130. But I think the 4-1 exhaust has given more airflow than I thought, because reducing fuel on the mains and pilots has caused terrible performance, even though the plugs look fairly bad.

I'm getting between 30-35 mpg and reasonable performance with 130s/42.5s. I want to dial in the carbs, though, if possible.

What jets do you guys think (just guessing) would be fair for my setup and elevation?

I may dare try something a hair less than 130s, but at this point I'm not convinced that jetting is so much of a problem.

I remember seeing on my old 80g constant black plugs until I installed some decent diaphragms and fixed the pickup wires. Wonder how one could tell if spark was cutting out from low to midrange....

Ben

[DiverRay]

Ben;
with the stock air filter, you probably don't want to stray far from stock. A K&N will flow a lot more air, and with the 4 into 1, probably need to be rejeted. I think you may need to adjust the needle position. Because of the elevation, I would try dropping the needle one notch. run with that and see how the bike feels. This will make it slightly leaner in the mid-range. If it helps, you may want to try and get 125 mains for the WOT test.
Any adjustment you make should be small, and one at a time. The best way is get a notebook, write down what you have now, and adjust things one at a time. You couls also try to reset the float level. if you drop the fuel level in the float bowl, you should make the bike run a little leaner.
Good luck, as carbs are "fun" until they are correct!
Ray

[Shuriken]

I thought about 125 mains. Also, since the mains affect the other circuits, perhaps just a slight decrease in mains might really dial the carbs in.

I have to yank the carbs anyway to take out the 37.5 pilots to replace with 42.5s, that would be a good time to try one size leaner on the mains. I can have the carbs off in 15 minutes now after a few years of doing it.

Ben

[Shuriken]

So I figured out why after trying smaller jets my bike ran like crap. It helps when you remember to connect the vacuum advancer tube to the carbs..... DOH.

So, after discovering that, I repeated the jetting tests - well, at least on the needles and pilots.

So as I mentioned, I was running 130 mains, jet needle in the middle position, and 42.5 pilots. Well, I put in my nice, shiny 37.5 pilots and gave 'er a try. RAN LIKE CRAP. Would not idle.

I thought they were too lean so I kept trying to get it to idle by turning the screws out. Well, you may guess where I'm going with this... Would not run with the screws out 9 turns each, so I put them back at lightly seated and it idled.

Now I was confused because I put in smaller jets and got richer conditions! So then I held the 42.5 and the 37.5 up to the light and lo and behold the 37.5 is larger. Peice of crap jets. They are of no use to me unless I ride 3000-4000 feet lower elevation.

So I put the 42.5's back in and it ran like I expected. Because the bike stumbles in the mid I tried dropping the needle one clip, but that seemed to make things worse.

So perhaps 130 mains, 42.5s, and middle position jet needle combined with my stock airbox/filter and 4-1 pipes is as good as I'm going to get at 6700ft. It just misses a few beats in the mid/full circuit somewhere. Not sure if the revs cause some vibration that bump the spark or if there is a sudden lean condition or what....



If stock pilots are 42.5 and stock mains were like 137.5, and I reduce 5% for each 3000 feet above seal level, that comes out to around 38.25 pilots and 123.75 mains. Would just try a few things. With 4-1 it is a bit trickier.

Wonder what if there is a general jet needle adjustment rule for elevation....

Ben

[pggg]

Ae7, I heard somewhere that for running at high elevations, advance your timing 2 - 3 deg. Supposedly a good tweak for altitude.

[Shuriken]

What is the best way to find out if my midrange flubbing is fuel or spark related?

I don't want to chase my tail.
Ben

[DiverRay]

You may want to try advancing the timing as pggg said. leave everything else as is, advance 2-3 degrees, and give it a ride. It's a quick and easy adjustment that will be easy to put back if it doesn't work.

[Shuriken]

Will do.

Ben

[Shuriken]

Checked some things this weekend:

1. Float level. 8mm brake bleeder screws work well. I set the floats awhile back to stock and they indeed are set up well. Fuel is the same in each bowl.

2. Synch. I tried to synch the carbs again but my guages were fluttering too much. I took apart the guage and put some light oil on the rods and reassembled. Smoothest metering I've yet gotten. Carbs are synched.

3. Spark plugs. New set of plugs.

4. Timing. Just shooting the light indicates I was already a few degrees advanced. In fact, I cannot seem to adjust the timing to get it right on. I cannot retard the plate enough.

Still the bike misses in the midrange. I color tested the plugs on a slight hill at 75mph on the interstate and they look pretty good. But from idle when I get on the bike hard it always misses in the midrange. When the elevation gets higher and I am cruising and want to pass, it really falls on its face.

I think I'm going to try and order several sets of OEM Mikuni jets and go from stock. Not sure what is causing the missing.

Ben

[DiverRay]

Ben;
See if you can have a friend follow you on your test run. Have him watch for black smoke out of the exhaust. This past weekend, on the Reno Ralley, I noticed eliberty was running rich. He had the cross over 2+2 pipes, and 1&4 seem to be VERY rich at altitude. When he jumped on it, he was smoking out of his right pipe.
Just try to get someone you can talk with to watch as you accelerate from different RPM's. They should be able to tell you when and if you have black smoke out of the pipe.
Ray

[Shuriken]

Just got a working tach (replacement). When I get a chance to ride again I'll note at which RPM I'm having the bog.
Ben

[Shuriken]

Bike seems to idle and roll on from low rpms much better after I used a digital caliper on the floats. Tried to set them as close to stock (and consistent with eachother) as possible and that made quite a difference on the low end. Still have the mid range problem, but I want to disassemble the carbs and count the holes in the emulsion tubes to make sure the right tubes are in the right carbs. Also want to check timing.

Ben

[Shuriken]

Lended my bike to my brother in law so he could go get his motorcycle license. When I got the bike back, now it will hardly start and run.

He didn't do anything to it but it's like I've lost #1 and 2 until like 4k or something. Bike has trouble starting. What a PITA.

A week before all of this I rode the bike at 3,000 feet less elevation and it would smoke the rear tire. Ran all around much better.

I wonder if I could rejet/filter, etc. and simulate the 3,000 ft level to get my bike running great.

I guess now I have to find out why #1 and 2 are crapping out. Argghghgh...... and I was going to ride this week.

Ben

[mro]

with the stock air filter?

Don't know if it would help.
Look at the dyno run thread.

Guy had drilled holes in stock air box and it seemed to make a difference.



mro