79 XS11 with FJ1200 carbs, ignition troubles?

[bfreehill]

So I could not leave well enough alone and swapped out my stock carbs for a FJ1200 set.
The bike fires instantly, idles excellent, and runs very very smooth and pulls REAL hard untill....
Its about 10 minutes into the ride that the bike starts to choke and stumble. At first I thought it was the carbs running lean but it seems more like the ignition is changing its timing curve.
Let the bike sit untill its cold then hoo-ray another 10 minutes of great riding untill bog city stops the fun.

Could the bike be pulling more vacuum when hot?
any thoughts guys?

I have yet to check the valves or sync the carbs, but both seem well within spec.
Timing looks to be on the money too.

I may try the stock carbs again to see the difference.

[stoked]

So what's the choke set up like on the FJ carbs? It almost sounds like you have the choke on when you fire it up and then it warms up to not need it and it's not being un chocked.

Have you stoped and pulled a plug once its started running bad to see if it's white or powdery black?

[bfreehill]

the choke is similer to the XS... its enrichment circuit.
Im sure the choke is off when Im riding.

The plugs are white-ish.
which is why I was guessing the timing was retarding.

[stoked]

So white ish color. You sure you still got good fuel flow when this happens. Is it possible the tanks not venting good enough to keep the fuel flowing?

Could the octupus perhaps be connected to the vacuum advance port and the vacuum advance connected to the octopus port? If you still have the octupus.

[bfreehill]

I was having the same thoughts about fuel flow.
When this happens I have stopped to see if the fuel filter were still full and yes they were. I also tried leaving the petcocks on prime... this did not help.

The vacuum advance port is a bit of a wild card.
there is no such port on FJ carbs so I am running off the intake boot.
At first when I tried this the vacuum modual would knock or click very hard at idle (at first i thought a valve was about to go)
Basically it was pulling WAY to much vacuum as the port on the XS carbs has a very small diameter.
I tried to replicate this with a small pilot jet inside the adavance line. All knocking or clicking gone and the ignition seems to advance properly. (for 10 minutes)

[hollywoodxs1100]

Aside from setting petcocks to "prime", have you tried actually opening up the tank filler neck to let air in? It's possible the vent on the tank isn't functioning properly, causing a vacuum to be present when fuel is drained out. Opening the tank would release vacuum.

[bfreehill]

I have not tried that but will right now.
i could use a good ten minute ride.

[bfreehill]

unfortunately it does the same thing.
I rode for about 10 mins, ran great then... bog.
Opened the tank cap let it sit for a few then rode more. Bog. tried it on prime with tank open , same thing.
Revs great while not under load.
ugh.

[John]

Stupid question...Has the tank been treated with some type of liner? When Wildkat was bringing her FJ home for the first time she experienced the same thing. It turned out to be the Kreem tank liner had detached from the top of the tank, and the weight of the fuel on the detached liner forced it to settle over the petcock, which had no filter tower, and starve it down. After a few minutes on the side of the road, it would fire up and go a couple of miles, and die again. What a PITA that was.
The FJ has two throttle cables and a cable actuated choke...how did you rig that up to the XS?

[TopCatGr58]

Hey Bfreehill,

If it runs great with enrichener off, then bogs after 10 minutes, sounds like it may NOT be carbs, especially since you put it on PRIME which bypasses the OCTY and feed the carbs directly!

So....back to basics. Need to verify that you have spark on ALL 4 plugs when it's bogging. Revving in neutral/no load is easy even with only 2 cylinders, but hard to drive, develop power under load!

Symptoms could be pickup coil wires, but since it's good for 10 minutes, I don't think so. You still have the ballast resistor, and it has been known to go bad, but it would cut power to both coils IF it was cutting OFF power. So...this leaves us with the possibility of "A" coil going bad, shorting or breaking continuity when it gets HOT.

SO....ride it till bogs, then stop and pull plug wires, have spare spark plug you can stick into lead to lay against engine to test for spark! You'll probably find a pair that has lost spark!?
T.C.

[AlanB]

Hi Bfreehill,

Once it bogs, will it rev Ok with the clutch in?

Just wondering if it might be a sticky brake; not too bad when cold, but coming on an dragging once warm. It would explain a "loss of power", but the engine would behave well with the clutch in.

Glad to hear that someone else has tried modern carbs and found an improvement; I have GSXR carbs on my SG, and cannot believe how good they are.

Stick with it; the benefits are worth it!

AlanB

[bfreehill]

OK... so the tank looks clean & I do not see evidence of cream. But I may just drain it and take out the petcocks just to be sure.

the brakes are working well and not binding.

I rode the bike till it started to bog, stopped & pulled one plug wire at a time. Testing each with my spare plug while reving the motor and had consitant spark on all.
Maybe this is different under load? bad ground?
Sounds like I should get some spare coils...

I did a high speed run down the street while getting it to bog for a while, cut the motor and coasted into the drive. I pulled the carbs to verify fuel level, which was fine and intake boots equally wet.

plugs a bit white and wet.

I tried the stock carbs and same thing but with much worse idle etc... (they need work hence the FJ carbs)

letting it sit for a few hours only gets me a few blocks of good running.
letting it sit overnight gives me 10-15 mins.

[mro]

Might try swapping out the TCI with a known good one.
Can leave the vac advance disconnected too.

mro

[stoked]

Originally posted by mro
Might try swapping out the TCI with a known good one.
Can leave the vac advance disconnected too.

mro

mro beat me to my next guess. Seems you've covered about everything else that this problem could be from.

Check out this symptom I read in a thread about CDI's going bad

"You may instead find that the bike runs great while it is cold and then begins to fail, misfire, lose a cylinder altogether after it has warmed up."

[bfreehill]

Originally posted by stoked

"You may instead find that the bike runs great while it is cold and then begins to fail, misfire, lose a cylinder altogether after it has warmed up."

Wow, sounds very similar.
That would explaint the symtoms.
Interesting that the CDI would not just quit, but progressively get worse.

Ill swap it.
maybe get a spare coil as well.