Vacuum Advance .... erm... help ?

[catdna]

Hi All

I've been reading through the forums today getting myself more acquainted with the XS11 engine - I've been having some running problems with my trike (see my 'lumpy' thread).

Now, I checked my timing earlier and it was marginally out on 'F' so I adjusted that. However, the advance was just all over the place - went upto and above 40 with some extra revs.

Pondered on this for a while, then did some more reading here.

I read about the vacuum advance - yes, got one of those Then I read that it should be connected to a port on the carbs themselves.

Now, I'm sure mine isn't - I don't remember disconnecting the tube from the carbs when I last removed them - I didn't build the trike, just helped design it and provide grunt work where possible (that's my disclaimer anyway).

I took a look in my haynes manual, but the 'exploded' carb diagram doesn't mention which hose is the one to the advance - I'm guessing it's number '80' in the diagram.

Does anyone have a picture / photo of where the vacuum hose should be connected to on the carbs ? I'll have a check first thing tomorrow when it's light, but I'm guessing that this could be the cause (or one of them) of my dodgy running ?

Thanks
Chris

[Crazcnuk]

If you look at carb #2 (#1 has the choke lever) you will see a vacuum port on the side of the carb towards the motor. Right next to that is another vacuum port on the rubber intake manifold.

The vacuum advance goes on the carb, and the vacuum line to your fuel octopus goes to the intake manifold.

This is if the motor was from a special. The standard is exactly the same except that there are two lines, carbs #2 and #3 both have vacuum lines going to the petcocks.

All 4 manifolds have these vacuum ports, but the unused ones have rubber caps on them. They are used to balance the carbs.

The timing is supposed to go up when you give throttle. If it is bouncing around, chances are the hose is hooked to the manifold, not the carb.

You may have also noticed a loud clicking noise coming from the advance unit, if it is hooked up wrong.

Also, you should have the vacuum hose off, and the port plugged while you set the timing.

[bikerphil]

The vac advance hooks up here on the #2 carb. These are later model carbs than yours, but the location is the same.

[catdna]

Thanks for the replies and photo's guys - just been to have a look and it *is* connected to carb #2.

I knew it wouldn't be connected to the intake manifolds as my trike runs a brake servo connected to all 4 of those (nothing is 'standard' on this trike - no 'vacuum' for the pet cock - that's all gravity fed, the only 'xs' part is the engine / parts of the wiring loom)

Timing was advancing to nearly 50 yesterday because I hadn't disconnected the hose (thought I had.... oh well..) - now seems to settle at around 36 which according to my manual is near enough correct - tried with the hose connected again and back up to 45-50 it went - is that correct ?

Now back to checking the carbs / floats / pilot circuit to check for more 'lumpy' running at low revs (when I get time - supposed to be 'working from home' today )

Cheers
Chris

[DiverRay]

A 78 has timing set at 5,200RPM, 36 Degree advance with the vacuum plugged. a total advance of 52 to 54 Degree is proper with the vacuum connected. At idle, 950 to 1050 RPM, the timing light should have the "F" mark and the pointer aligned. Make sure the advance weights behind the timing plate are not rusted up!
While you have that cover off, CHECK THE PICK UP COIL WIRES!! The front coil is usually the problem with the '78, IIRC.
I had problems with my first XS1100 back in '78, no fire on 1 & 4 at idle. It was the front Pick up coil wires. The metal that the wires runs through to keep it in place was crimped too tight, and the wire broke there. I repaired the wires, and loosened the crimp just a little so the wire could slide back and forth.
Good looking trike! I hope to get back to the UK someday, and would like to see that in person.

[bikerphil]

I had an occasion where the timing was dead on but the engine got terrible mileage and didn't run quite right. I calibrated the timing pointer by using a dial indicator on the #1 piston and finding absolute TDC, turned out it was 10 degrees off. After readjusting the timing, the motor ran great and gas mileage went up about 7 MPG. Just a thought.

[catdna]

I had an occasion where the timing was dead on but the engine got terrible mileage and didn't run quite right. I calibrated the timing pointer by using a dial indicator on the #1 piston and finding absolute TDC, turned out it was 10 degrees off. After readjusting the timing, the motor ran great and gas mileage went up about 7 MPG. Just a thought.

Damn !

Wish I hadn't read that, because :

1. my engine doesn't quite run right up to 4000 revs, then is fine

2. the trike only gets about 20-25mpg (if I'm lucky) - but then, it probably weighs nearly twice as much as a standard XS

I'm still going to investigate the carbs when I get time (this weekend hopefully) - if just to sort out the random-leaking-when-fuel-tap-left-on which sometimes happens.

Cheers
Chris

[bikerphil]

my engine doesn't quite run right up to 4000 revs, then is fine

Yep, sounds like dirty carbs (pilot circuits). Needle + seat will cause the carb flooding. Take your time disassembling. There are some fragile parts that can break, strip, etc.

[TopCatGr58]

Hey Cat,

Another thing to check is how much vacuum is coming or being pulled by that port on #2. I can't recall the max amount of vacuum the advance is supposed to reach it's max range, but it's something like 5 psi.

Some folks have found that someone else reamed out the small port so that it then pulled way too much vacuum, causing pulsing like you described. They put a pilot jet inside the vacuum line to make a restrictor, and it evened out the pulses and smoothed the operation of the advance unit! JAT!?

Also, the advance is also a retard unit, the higher advance amount of 52 degrees is under cruising low load rpms. When the crank the throttle open more to accelerate, vacuum DROPS to the advance unit, it then retards back towards the 36 degrees which is the MAX Cent. Adv amount, which is the more POWERFUL timing setting for higher load/throttle input.
T.C.