vacuum advance chattering

[hyperballsmcgee]

long time no see, fellas. anywho, my vacuum advance chatters back and forth at idle as if it were receiving vacuum in pulses. the octy has been deleted and all carb boot nipples capped, the advance hose runs to the nipple on #2 carb on the CARB BODY and not the boot. i wanted to clarify that as i have read that having it hooked to the boot is a common cause for this issue. i have hooked a vacuum pump to the hose and pumped it up till the advance plate stopped moving, and it never dropped until i took off the pump, however, revving the bike up the advance does not move until higher RPM, which im assuming is because the mechanical weights are taking over, another issue i have which i believe to be related is coming off idle from a stop and at cruising speed the bike has NO POWER and has a bad stutter but at full throttle it runs like a screamin' beagle. i have yanked and pulled the pickup wires every which way and have not gotten any reaction, and riding with the advance unhooked makes no difference in the engines behavior. the carbs are synced, plugs, wires, coils are good. bike is a 1981 1100 special thats been turned into a rigid chopper type thing. has pod filters and a 4-2-1 exhaust and carbs have been rejetted. any thoughts are welcome as i have been chasing this issue for quite some time now.

[motoman]

Voltage being low TO the coils causes poor burning of mixtures in cyls. which causes erratic and low vacuum and causes the vacuum advance diaphragm to dance and not be consistant. Chack votage at battery and again at voltage feed to coils.....should be same-same.

[TopCatGr58]

Hey McGee,

A few things. Rejetted carbs, but with what jets, both SIZE but more importantly BRAND. We have found that the aftermarket jets in kits by K&L and such are not metered the same as Genuine Mikuni despite being labeled the same, and it's worse for the pilot jets circuit.

Next, POD filters, again what kind? The inexpensive EMGO type have a prominent lip on the mounting surface/ring that often interferes with the air flow into the carbs, the inlet bell ports and even the vacuum slide arced port. See this tech tip on the various ways to adjust for this problem.
http://www.xs11.com/forum/showthread.php?t=396

The idle setting for the carbs may possibly be set too high/big which is exposing the vac. adv. port to engine vacuum too soon. There is a recess on that #2 butterfly valve. Also someone may have drilled thru the vac. adv. nipple into the carb body opening it up too much?? You may need to get the carbs synched better and the PILOT JET Screws adjusted to allow it to idle higher without having to have the MAIN idle screw cranked so far which opens the butterflies too far.

The manual tells you about how much vac. the vac. adv. pot should respond to. If you have vac. gauge, you could put it on the #2 carb nipple and see how much is being delivered, if way above the spec, then see above paragraph. Aside from replacing the #2 carb body, you could try putting a pilot jet into the vac. line to try to help dampen the vac. pulses!

BTW, your 81 special has the mech. adv. curves programmed into the TCI, there are NO mechanical centrifugal adv. parts, just the vac. adv. pot/plate.

T.C.

[hyperballsmcgee]

if there are no weights then what causes the advance to rotate at high rpm? i haven't ridden it with a vacuum gauge but i plan to throw one between diaphragm and #2 port and see what vacuum i get at what rpm. correct me if i'm wrong but you will have maximum vacuum in the lower midrange and start losing vacuum at higher rpms, yes?

[TopCatGr58]

Here's an excerpt from the Articles section, 78E Cycle Mag article!

The Eleven ignition system's second important feature, and one that's unique to motorcycles, is the vacuum-actuated advance/retard mechanism. Actually, there hasn't been an automobile produced in at least 30 years without some form of inter-connection between manifold vacuum and ignition timing, simply because spark advance otherwise has to be compromised between the conflicting demands of power and economy. Engines running under slight throttle openings have a thin, low-pressure mixture in their cylinders, and they don't run efficiently unless the fire is started long before the piston reaches top dead center. Much less ignition advance gives optimum results at large throttle openings. Yet, all motorcycles but the Eleven effectively have a fixed spark advance, with a centrifugal device to retard the timing for easy starting. And their off-idle timing has to be compromised, with enough advance to give reasonably good efficiency under cruising conditions but not so much as to cause detonation when the rider uses full throttle. The Eleven we tested had 10 degrees of static ignition advance, and another 26 degrees from the centrifugal mechanism, which adds up to the 36 degrees other engines of similar configuration have needed for maximum power. But then there's also 16 degrees built into the vacuum-advance device, and that pulls the timing around to 52 degrees BTC when the bike is cruising along at freeway speeds. Wind the throttles open, and it drops back to the power-setting; roll off throttle, and it auto-advances for economy; use some in-between throttle position, and the ignition adjusts itself to suit conditions. It's a big step out of the technological Dark Ages, and one that's long, long overdue.

T.C.

[bikerphil]

The more load applied to the engine, the lesser the vacuum,
Less load, such as coasting and engine braking will produce highest vacuum.

[hyperballsmcgee]

i pulled the carbs and resynced them, first with a feeler gauge, then with vacuum gauges and they are all 4 pulling about 6 inches of vacuum at idle. i guess i gould have titled this thread to focus on my low rpm stutter issue. i will pull the carbs again and remove the bowls cause i can't remember my jet sizes. is it possible that my pilots are too large and causing the mixture to be too rich at low rpm?

[motoman]

i pulled the carbs and resynced them, first with a feeler gauge, then with vacuum gauges and they are all 4 pulling about 6 inches of vacuum at idle. i guess i gould have titled this thread to focus on my low rpm stutter issue. i will pull the carbs again and remove the bowls cause i can't remember my jet sizes. is it possible that my pilots are too large and causing the mixture to be too rich at low rpm?

Refer to post #2. At an idle, vacuum should be 20in. at your near nothing elevation. Here, at my location(elevation 5,000ft.), in. vac. is at 18in. Definitely a low voltage issue AT the coils, in your bikes case.

[red bandit]

Mine has always (13yrs) had about 6-7in. at idle. Valves and everything is adjusted correct. 175psi compression on all 4. Not saying what's right or wrong. Just saying. MHO.

[Geezer]

Dumb question, are you 100% positive you have the vacuum advance hose connected to the correct place?

Tony

[hyperballsmcgee]

advance hose is hooked to number 2 carb body, all manifold nipples are capped. manual petcocks so there's no octy. haven't had a whole lot of time to mess with the bike, but i'm off work for the weekend so i'll try to get a voltage reading at the battery and at coils.

[hyperballsmcgee]

also just to add, i did try hooking it to a manifold nipple and that made it chatter so loud it sounded like piston slap. only ran it for a few seconds like that. didn't take long to tell that DEFINITELY wasn't right. haha

[motoman]

also just to add, i did try hooking it to a manifold nipple and that made it chatter so loud it sounded like piston slap. only ran it for a few seconds like that. didn't take long to tell that DEFINITELY wasn't right. haha

Imagine it would as there's no restrictor. #2 carb brass nipple has the dampening restictor built in.

[hyperballsmcgee]

i'm getting 12.9 at both the battery and the coils at idle. voltage at both ends rises and falls at the same rate through the rpm range. i trimmed my plug wires back some and took it for a ride but nothing changed. how do i test the coils resistance and what numbers am i looking for?

[motoman]

i'm getting 12.9 at both the battery and the coils at idle. voltage at both ends rises and falls at the same rate through the rpm range. i trimmed my plug wires back some and took it for a ride but nothing changed. how do i test the coils resistance and what numbers am i looking for?

Meter set to ohms. Meter leads across coil leads of coils themselves.......should and need to be 3ohms. My cut-off point in this case is 2.9 to 2.8ohms. Even at this point, there is internal voltage leakage and coil replacement is definitely gonna need to happen.....or a long walk, your choice.