Vacum Advance

[Murrayf]

due to some work i am doing i cannot run the vacuum advance

do i need to do anything to the base timing

or can i just leave it off ??

what are the ramifications

thanks

[jetmechmarty]

This sounds tricky. I believe you have to have the vacuum advance or run an aftermarket ignition with electronic advance. I can't remember how many degrees advance that device advances, but it happens after the engine lights off. The mechanical advance comes in much later.

I assume you have somehow disabled the #2 carburetor. You're going to need another vacuum source. The intake port will not do.

This may be what you're looking for: Sachse Digital Ignition ZDG3H

[Murrayf]

i do not have a pre venturi vacuum source so it is not an option

i will look in the manual and see what it really does

i can recurve the mechanical advance if i really need/want to

[MPittma100]

Advance

i do not have a pre venturi vacuum source so it is not an option (quote)

You need a "total timing" advance - a combination of centrifugal + mechanical. Both advances are pretty much zero when starting the engine. If timing becomes a fixed advanced, you will experience hard starting (harder for engine to turn over).

You are well advised to keep the timing as is designed. Drill a hole in the carburetor and add a nipple to retain ported vacuum advance. Otherwise you may put unnecessary load on the starter and battery if you "fix" the timing.

Mike

[Murrayf]

it is not possible to get the type of vacuum from the induction system i am

running period there are no butterflys

its ok i will figure it out after i map the timing under load with and without

the vacuum advance just thought i could get a few answers

[Crooked_Fender]

I am running mikuni rs36's flatslide carburetors that have no butterfly. I'm running vacuum advance. It can be done. You just need to restrict the port properly. Not perfect, but serviceable.

[natemoen]

Explain to us what it is you are doing so that we can maybe help you more. Information is needed to solve problems.

[TopCatGr58]

Hey MurrayF,

Okay, you do NOT need the Vac. Adv., it is NOT a critical timing function...sorry Marty!

The Vac. adv. mostly gets involved only when you're in CRUISING MODE and have the carbs/butterflies only open enough to give maintaining fuel levels, which while cruising are relatively LEAN...and the vac. adv. kicks in to provide additional timing adv. beyond the max ~35 degrees Mech. Adv. to take advantage of the lean fuel to provide more combustion time to get the most out of the fuel for fuel economy!

When you crank the throttle open more for power...the vac. adv. RELEASES it's affect due to drop in vac. levels, and the timing then retards back to the more POWERFUL range of 35 degrees BTC(Max Mech. Cent. Adv.) for the most POWER affect with richer fuel/air mixtures.

SO....with your carb mods...I'm going to assume that you've put on some newer type/brand of carbs that do NOT have a metered vac. port, you can just cap off the vac. adv. module, and will still have the full POWER that you can expect from your bike throughout the rpm ranges. You will just loose a little bit of possible fuel economy from the lack of the increased vac. adv., however with more modern carbs....you may be getting much better atomization of the F/A mixture which may offset any mileage economy loss anyways! And the bike/engine should have a 5 or 10 degrees static timing adv. dependent on year/model which provides adequate starting timing advance.

A member with an XJ had his TCI die...the TCI/Ignition module for the XJ has both the Mech. Cent. Adv. and the vac. adv. curves programmed into the TCI. XJ TCI's are rare and $$, he was able to find an earlier 4RO TCI which only has the Mech. Adv. curve programmed into it....it normally utilizes the mech. vac. adv. device...but his XJ does not have one....just a vac. sensor that electronically signalled the TCI! SO...he ran it with the 4RO TCI, NO VAC. ADV. and it ran perfectly, full power to redline, no problems starting, and really didn't notice much if any loss in fuel economy either!

T.C.

[TopCatGr58]

PS,

Read this article: http://www.xs11.com/xs11-info/articl...uary-1978.html Go down to the 13th paragraph for confirmation of what I've said!

T.C.

[BA80]

I have an 81 SH in the parts pile that burned to the ground.

It has all the SH timing parts on the engine but it had a 2H7 TCI on it.

Just sayin'.

[Murrayf]

top cat

thanks for that i had already found that article

on my primary dyno runs i simulated the vacuum adjust with a hand pump

and saw it made very little diff to power output thru most of the range

it does keep the a/f abit more stable during deceleration and i suspect

that i will have to be careful to keep my exhaust sealed or i may even get

some afterfire in the exhaust or i may use a air cut valve if it becomes a

problem

and i think what i am going to do is recurve the mech advance to give

me another 6-8 degrees total above 6k

[TopCatGr58]

I have an 81 SH in the parts pile that burned to the ground.

It has all the SH timing parts on the engine but it had a 2H7 TCI on it.

Just sayin'.

Hey Greg,

In absence of the 4RO TCI on that 81SH, the bike was ONLY getting some 16 degrees MAX of vac. adv. combined with the static 10degrees was 26 degrees max....just 10 degrees shy of the 35 degrees max for Cent. Adv. curve, so the bike WAS able to run with just the Vac. Adv. and without any cent. adv. affect, it probably didn't have quite as much POWER or top end performance compared to if it had had a 4RO TCI, OR...Cent. Adv.!

I remember guys coming on here saying that their bike wouldn't get above 3-4k rpm, and it was determined that their cent. adv. plates had gotten STUCK/Frozen from rust or such, but they still had Vac. adv.. Once it was cleaned/lubed, they then were able to have full throttle response to redline because they were able to get the proper amount of cent. adv. in the timing.

Next time I take Godzilla out for a little local run, I'll disconnect/clamp off my vac. adv. module and see how it performs. I had it out for a run today for just an hour to blow the cobwebs "Literally" OFF the bike, had been sitting in the garage for 2 months!!

T.C.

[Murrayf]

one of the things i will be careful of is the "advance" system actually

provides for a retarding action upon cracking the throttle to prevent pinging

just by the nature of the vacuum dropping when the throttle is rapidly opened

i will have to be careful of this below the minimum speed of the centrifugal

advance unit

this gets worse if the accelerator pumps dump a bunch of fuel in at the same

instant

[TopCatGr58]

Hey Murray,

Not sure if you have an XS service manual or not, so you may not have seen these, you may find them helpful. The first is the XS11 timing curves, the other 3 are the XJ's timing curves related to the vac. adv. module signals.

The first one shows the ADDITIONAL advance of the Cent. Adv. unit above the static 10%, that's why it says that at 1000 rpm, then at ~2krpm, 1 more degree is added, and then it increases linearly with rpm up to the max at 4750 rpm!

T.C.

[motoman]

I have an 81 SH in the parts pile that burned to the ground.

It has all the SH timing parts on the engine but it had a 2H7 TCI on it.

Just sayin'.

Won't cause an issue like your suggesting though........ case in proof: getting my Venturer back 14yrs. later, it had the early TCI on it. Was a bit slow comin' on the rpms, anda was way 'doggy' runnin', but that was bout all. Obviously it had been on there for many years also.