mechanical advance mechanism

[tarzan]

I just got my bike back together after installing new rings and lapping the valves and I have a stumble off idle and then it's smooth and pulls hard from about 1500 on up to about 4000ish , then it stumbled briefly and will run smooth through the rest of the rpm range.

So I was checking out the advance while timing it and it didn't seem smooth
as it moves.I took the vacuum advance off and it seems fine.So I removed the mechanical advance and it seems to have a little play in the bushing.
I have another one from a late model engine.old one is 2H7 & new 3H5.
They look identical.I think the springs and weights even look the same.So I installed the newer one.It seemed to run a little better.
Does anyone know if the two units are different?It figures that if it is a different number it should have some difference.

Coincidentally, I adjusted my idle mixtures and synced the carbs and it runs
pretty strong, but still have a slight hesitation off idle.Then it pulls pretty good after that, but also still has a little bit of stumble at the transition at somewhere between 4000-4500.Then pulls hard again.

I'm thinking I need to pull the carbs and check the idle circuit and maybe
the needle in the slides.

Any thoughts?Sometimes it's best to walk away and get a second or third opinion before proceeding.

[mro]

I'd take a look at the plugs before taking the carbs off agin, rich?
Then double check float height etc...

If all else fails, check for week spark, then pick-up coil wires.


mro
btw, how old are the spark plugs?

btw2
Vacuum advance:
I've got one engine with 10M cases, 80 vacuum advance (pluged) and runs good. I'm slooowly re-doing this one as time allows and plan to see what difference the advance unit makes one way or another.

[trbig]

I've got one engine with 10M cases, 80 vacuum advance

The actual case below the cylinders... are all the same, and the crank & ends are the same as well. It's what you hook onto those that makes the difference.

Tod

[mro]

high jack

Never had an XJ apart... just took a look and they are the same

Mine has an 80 head and cylinders too

mro

[79XS11F]

Tod, I really would not fully assess it until the rings have fully seated. As for the bob weights if your concerned about them being different use the actual weights and springs off the original one on the replaced one.
Rob

[trbig]

Not quite sure what you're talking about, Rob. I was simply addressing MRO's comment.

Tod

[tarzan]

I actually think the advance is working ok at this point.

I did have a plug not firing on cylinder #2 when I first fired it up,
so put in an old plug I had.I had put new plugs before the tear down.
Plugs were fouling before.I'll pick up a fresh set.

As for the rings, I think Rob has a point, let them seat.I only have about 2 miles on them.I think they are going to seat fine though.I have no smoke
out the exhaust, but do at this point have a little out the crank case vent.

I probably have a carb problem as well.The PO gave me reciepts for $900
he paid the local Yamaha dealer to rebuild the carbs and tune up.The dealers mechanic had two idle screws broke off in the carbs when I bought it and the wrong main jets and pilot jets.I drilled the tips out and rebuilt the carbs.I installed stock jets.So I may need to go back in and re-check floats
and everything else to to verify I did it right.

Oh,I almost forgot to mention.I took the 4-2 w/turn-outs exhaust off
for the tear down, and installed a kerker on it now.

Also I have some pods on it while I tune it.Maybe the pods are giving me the hesitation?I wanted to get it running right and then install the stock air box.
Rick

[79XS11F]

Not quite sure what you're talking about, Rob. I was simply addressing MRO's comment.

Tod

Sorry, When I "full" page viewed it your response was at the top so I responded as if you started the thread. Of course the 1/2 jug of RYE I have in me may have been a contributing factor :-) My Bad ...
Rob

[motoman]

Thinkin MRO is on the right track for starts. Your last thread was a clue as to your exhaust and intake setup. Previously had said you installed stock jetting back in, so with that and the mods your still too lean in the mains. The later carbs are a PITA if done accoring to specs(clear hose on bottom of carb coming up side filled with fuel while running checking fuel level of bowls to be at 3/16" below top of bowl), which is a sea level setting. Mine for example are right now at 1/2" fuel level on all carbs(least got the floats all the same) which is 28mm at lowest portion of float for a static measurement. Runs very well at ALL rpm levels, but is lean and plugs show it. Gonna pull the bank off and try those static levels at 26-26.5mm. See if that will raise those running levels to 1/4", re set the idle, idle mix srews, sync, re-set idle mix, etc. See if the 1/16" fuel level drop will give me the results I'm lookin' for here at 5000ft.elev. Re checking levels and making sure each carb is set the same would be a good place to start and someone might chime in with the same carb and mod set-up and have a specific static bowl level that works at whatever elevation and climate they're at. Then see that it accelerates smooth and QUICKLY right up to redline without seeming to labor so to speak at any time. If it does or rpm's seem to come slower from 6K to 8K, then go up a step in mains. It becomes a trial and error thing with some intake and exhaust mods anyways. Hope something here may be of help.

[mro]

pods on it while I tune it

If you’re going to be using a stock air box ... personally would only tune it with it on. Might save you from having to do it again, and again, and again …

mro
BTW, varying float levels more than a mm from recommended always caused me problems. (with stock air boxes and exhaust). If too lean/rich would consider changing a jet or two.

[Pat Kelly]

Coincidentally, I adjusted my idle mixtures and synced the carbs and it runs
pretty strong, but still have a slight hesitation off idle.Then it pulls pretty good after that, but also still has a little bit of stumble at the transition at somewhere between 4000-4500.Then pulls hard again.

I'm thinking I need to pull the carbs and check the idle circuit and maybe
the needle in the slides.

Any thoughts?Sometimes it's best to walk away and get a second or third opinion before proceeding.

The carbs transition from the pilot jets to the main jets around 4200 RPM. I had a nice 'rumbly' idle and the bike ran good until I got above 4000 RPM... then it seemes a turbo kicked-in. I bought a Carb Tune (lets you look in the cylinder while the bike is running. Cyl #3 had spark but no combustion, until 4200 RPM. Removed the bad pilot jet to #4 and problem followed. New jet(s) and no more 'rumbly' idle.