vacuum advance question ?

[Tonyxs11]

Ive hooked up the vacuum advance on my 79 using the inlet rubber and using an air valve to regulate it, (FJ1100 carbs) the bike runs almost perfect and happy with it but my question is, when i fire up the bike and at idle the vacuum moves almost all the way and sits steady, if i rev the bike theirs a small amount of movement but not much, then when i switch off it returned to it stop.
ive seen a youtube video where a guy is checking the timing and when he fires it up the vacuum advance doesn't move till he rev's the engine. its the opposite of mine !!!!!! but it runs good

[DEEBS11]

Tony, a thought on this issue, the XS11 advance diaphragm must be set to a certain vacuum at a certain RPM in stock form. You obtain the vacuum level and the timing advances to full advance. I'm sure the threshold vacuum is documented somewhere perhaps the shop manual. Or a vacuum gauge could be used on a stock bike to see what level of vacuum the diaphragm starts to advance the timing. That being said, if you could adjust your attenuator to mirror that vacuum level so you would be spot on. Your FJ carb must have a different shaped venturi and it must create a lot of vacuum and your diaphragm is advancing too early. The bike will run but in high heat it will pre-detonate which is not good for the motor.

[Tonyxs11]

thanks DEEBS i'll do some more experimenting with the air valve when the weather dries up ????

[BA80]

You have the hose running to a constant vacuum source, On the XS carbs there is a ported vacuum nipple on the #2 carb just for that purpose, Ported vacuum only allows vacuum to that port when the butterfly in the card moves past the port to allow it to have vacuum. Clear as mud? Google it.

[DEEBS11]

You know he's right. The correct vacuum level is located in between the throttle plate and the CV slide like on the stock carb. It varies with RPM when the throttle plate moves. Brilliant. The real issue of full advance at idle is where your advance hose is connected which is the rubber manifold. The vacuum is too constant and powerful and only gets stronger with RPM like BA80 said. The only way to cure the issue would be to drill and tap a new port carefully on those FJ1100 carbs and thread in a brass nipple to attach your advance hose.

[DEEBS11]

Something like this

https://www.ebay.com/itm/40120853584...Bk9SR66O4IPcYQ

[Tonyxs11]

yes I know about the 0.4mm hole in the carb body and sits behind the butterfly. I thought about drilling the hole in the FJ carbs but it has to be in the exact place and not easy to do !!!!!
as my XS carbs are beyond repair and used ones over here are almost impossible to find, I read that others have used the FJ1100 carbs without the vacuum connected so went down that route as i picked up a good set of FJ carbs. maybe I should have a go at drilling the FJ carbs

[DEEBS11]

These are FJ1100 carbs. I don't have a set here in the shop but these are some possible areas to port. The main objective is to get behind the throttle plate before the CV slide somewhere on that rack. Not sure where those 4 screw holes end up. In any event, you have to hook up the advance hose in order to get the proper timing.

[Tonyxs11]

These are FJ1100 carbs. I don't have a set here in the shop but these are some possible areas to port. The main objective is to get behind the throttle plate before the CV slide somewhere on that rack. Not sure where those 4 screw holes end up. In any event, you have to hook up the advance hose in order to get the proper timing.

yes their is a place that can be drilled on the FJ carbs and i did think about it when i bought them, but when people say they fitted them without the vacuum and ran good I didn't bother. should of gone with my gut feeling
just been looking on the net to see if their is an adjustable vacuum valve that could be fitted ???

[DEEBS11]

OK, let say that drilling is out of the question. This could be done electronically with an RPM sensor with output switch closure hooked up to a 12vdc solenoid that interrupts the vacuum signal from the manifold area. I know its not a linear advance but Harley Davidson uses this method on EVOs with a vacuum switch VOS. HEY! you could use a VOS (Vacuum Operated Switch) from an EVO Harley to operated the solenoid. Probably incorrect RPM range but its a thought. I would prefer the adjustable unit so you can adjust your timing

https://www.ebay.com/itm/30465574684...Bk9SR8rCzofcYQ

[Tonyxs11]

thats an option but then the cost and the set up comes into it drilling the carb is free but needs a full strip down of the carb bank. I think drilling the carb is the way to go

[DEEBS11]

I agree. I would drill. If this works you will pioneer the FJ1100 carb modification for XS1100 use. This is cool.

[Tonyxs11]

I agree. I would drill. If this works you will pioneer the FJ1100 carb modification for XS1100 use. This is cool.

lol never seen me as a pioneer its just the thought of getting the carbs off AGAIN !!!!! not a hard job by any means just a p.i.t.a.

[Tonyxs11]

You have the hose running to a constant vacuum source, On the XS carbs there is a ported vacuum nipple on the #2 carb just for that purpose, Ported vacuum only allows vacuum to that port when the butterfly in the card moves past the port to allow it to have vacuum. Clear as mud? Google it.

you are right but i think not totally, looking at my number 2 old XS11 carb i can see with a light down the vacuum tube that the hole isn't totally closed off when at idle. their is also a small cut out on the butterfly which will let a small amount of air past the vacuum hole. if you turn the idle screw back off where the bike no longer idles the butterfly will cover the hole. but the vacuum will increase as the butterfly opens.
you can see the light in this pic.

[DEEBS11]

Yikes Tony, that hole location looks mighty purposeful. Not sure if you can mimic the exact location of the stock hole and what Yamaha was trying to accomplish. I guess you could try just behind the plate and see how it goes. Maybe you could dial it in with the adjuster tee you have there. What exactly is wrong with your original rack of carbs? Maybe they are still repairable. Might be a better route.

If you drill the carb and it doesn't work right you could always plug it back up. No real harm done.