Carbs

[Ozmodius]

I've recently rebuilt the petcocks an repainted the tank on my MNS. While I had all that off I took the oportunity to replace the coils, wires and caps with a set from MikesXS .. (direct bolt on for me). Also the PO had a small hose runing from the back of the bike to the #2 cylinder vac port and the vac advance capped off. I'm not sure what was plugged into it but i took that off and plugged the vac advance back in.

After putting it all back together, and putting gas in I noticed a sound.. a sound not unlike water pouring on plastic. I turn off the petcocks (They work now) and a little inspection the air box has gas in it..

I take it off, dry it out and put it back on I then turn on the petcocks while tapping (very gently) on the float bowls. This seemed to stop the pouring sound.

Ok now I turn it on and thumb the starter.... it roars to life like never before .. litteraly jumped to life and settles into a nice even idle at about 1100 rpms.

I reach down and turn the petcocks on and the engine starts to climb ... all the way to 7k rpm .. when i turn one of the petcocks off it takes a minute but will settle back down into an idle. turn it back on and it goes right back to 7k.

I have a few things i'm gonna try this weekend. Let me know if I'm warm.

1) the Vac Advance line may have a hole it it..the reason I assume this is because. I'll just cap it and the #2 carb vac port off.
a) if a float needle is not seating it should enrichen the mixture and cause a stumble if its not getting enough air.
b) the only thing (i'm a complete newb) i can think of is a float sticking or a needle not seating

2) take the carbs off and ever so gently clean the bowls and anything i can reach from the bowls.

I plan on sending these to 81xsproject for a good cleaning and adjustment, but untill I get the cash I'm hoping to at least get it rideable.

Any advise?

[dbeardslee]

Did you synch your carbs after disassembly/reassembly? Also, when you replaced the coils did you widen your plug gap? .035" should be about right. I don't think the gap is causing the problem but it still needs to be widened to keep from burning up your plugs.

[Ozmodius]

I didnt do anything to the carbs ... just took them off to put in a new airbox and K&N filter. The old airbox had a missing butterfly and a UNI fillter in it.

They did sit for about a month without gas in them while i worked on the other repairs. It ran OK , very very slugish under 4k before i started the work though. I'm not sure now long it sat up before i got it though... it was last tagged in 93.

[tarzan]

Well it sounds like you need to stop the fuel leak.You are on the right track.Pull the carbs off and pull the float bowls.Pull the float pins(carefully)Inspect needles and seats.Could just be gunked up or needles could be shot.If that doesnt look like the problem,maybe just float hanging up on side.You can fix this your self if you take your time and work on one at a time.good luck

[boyat68]

Ozmodius:L

Do you suppose that the tube running from the rear of the bike is the tank overflow vent mistakenly or deliberately put on the vacuum port with the idea that it would suck up any overflow? As to the leaking carbs, you have a sticking float or bad valves, and the tapping on the sides of the carbs suggest that it is sticking floats that most likely are the result of the floats falling too far down in the bowl and hanging up. Correcting the fall by bending the back tang out more parallel. The unhooking of the vacuum advance is practiced by some as there is some thought that all the advance does is help with the conservation of fuel. Also the Vacuum advance could have been replaced by the PO using a different animal. Open the plate on the left side engine and observe the travel of the vacuum advance, now that you have re-hooked it.

[dbeardslee]

Oz - anytime you do anything that affects air/fuel mixture - like a change in the airbox - you need to recheck your synch.

[DavesXS]

needle valves in the bowls not seating.
One petcock supplies 2 carbs, so its easy to narrow it down to 2.

Vacuum advance has absolutely nothing to do with fuel economy.
Your timing fires before top dead center (BTDC) As your motor climbs in RPM's the timing has to advance in order to continue firing BTDC. If there is no advance you'll eventualy be firing at TDC and higher RPMS with have you firing after TDC.
Thats why with an increase of RPM range like 10k+ the common thing to do in racing is to further advance the timing a degree or 2.

If there are people running with no vacuum advance there tearing up there motor, getting way less power, and just plain idiots.

Every automobile has to have timing advance. Weather its vacuum controlled, or ecu controlled.
Do not un hook it!

[Crazcnuk]

Vacuum advance has EVERYTHING to do with fuel economy.

In fact, that is it's only function.

It's the Mechanical advance your thinking of.

[AlanB]

Hi Guys,

Vac advance is the hardest to get a grip on; Bikes don't usually have it at all.... until we get to the EFI era, ONLY the XS11 and the RD200 (go figure!) have Vac advance.

Let me know about any others...... Never worked on one, but I would guess that G' wings might have it??

Almost ALL cars DO have vac advance.

Vac advance adds extra advance when there is vacuum in the inlet tract; look at where the take-off is in the XS if you want to start figuring it out.

If the throttle is wide, the Vac advance does NOTHING...... If the throttle is almost closed, but revs are high, the advance is close to max....

For performance testing, vac advance has nothing to offer, so most bikes never added it. Out in the real world, cruising on the freeway, the extra advance burns much more of the available fuel so gives much greater economy.

One more example of the unique and inherent quality which binds us to these old bikes.

AlanB