Vaccum advance on bike carbs?

[GeezerGarage]

A friend has fitted bike carbs to his engine and it has a vac advance dizzy on it. Someone has told him that fitting an air box and taking it off that will work but that sounds like it won't work to me.

My idea is to take it from a single manifold as it should be the same pressure as if they mere all united right?

Thanks in advance! BDUM!

[ChasR]

I shall research this but a few of the Micra chaps use a collective vacuum box taken off after the throttle bodies IIRC. I shall see if I can find the post on here (it was about a year back it was posted).

EDIT: And found :

retrorides.proboards.com/index.cgi?board=techni&action=display&thread=112162

After briefly researching this, some reckon you can get away with one vacuum take off from a singular branch of the inlet manifold, others feel that you really need all four tracks to be take off from (after the throttle body) to a collector/reservoir complete with a one way valve on the final tube leaving the collector.

[Deleted]

my hunter on twin carbs (1 tube per cylinder) had a vacumn takeoff on one of the inlet tracts.. and it all seemed to work fine.. think theres so little delay between there being vacumn there the dizzy hasnt a chance to respond to individual pulses

[ChasR]

The chances are that your twin carb manifold is connected by a balance pipe between the two carbs (or at least the SU carbs I have come across have one in place).

[RobinJI]

You can't take the vacuum feed off the air-box as it's pre-throttle, so there's no vacuum there to read. Taking it from a single runner seems to be relatively normal, and successful for some people, but without having done it or researched it myself I couldn't say if it's definitely the answer. A feed off each runner into a collective container, with an output from that is the most fool-proof way, if a little messy.

ChasR. I would have thought the one way valve coming off the collector was only for the vacuum feed to the brake servo, and not on the feed to the advance, you wouldn't want one in the advance feed as it would stop the dizzy knowing when the vacuum decreased, which is exactly when you want it to know.

[ianboyd]

It might be easier to change the alternator to a one with a vacumme pump on the end, like on some diesel models!

[RobinJI]

Jul 1, 2012 15:46:11 GMT ianboyd said:

It might be easier to change the alternator to a one with a vacumme pump on the end, like on some diesel models!

Again, that would be goof for the brake servo vacuum, but not the vacuum advance.

The distributors vacuum advance needs to receive a real-time feed of the intake vacuum, post throttle. This is because the intake vacuum reflects the engines level of loading, so the clockwork/electronics can decide when's best to spark.

[popuptoaster]

Switch to a non vac advance dizzy? Thats how it was done old school when switching to one choke per cylinder.

[ChasR]

Sorry Robin, I should have said I meant thata one way valve was to be used for a servo so as to offer a 'reserve' on the assistance.

Going to a non vacuum advance distributor is one way around the issue but if a daily driver fuel economy will most likely suffer as a result by as much as 20-30%.

[popuptoaster]

If your sticking loads of extra carbs on your car fuel economy probably isn't a major concern....

[ChasR]

True, but every little helps with fuel getting pricier as it is .

[ianboyd]

Jul 1, 2012 15:55:12 GMT RobinJI said:

Jul 1, 2012 15:46:11 GMT ianboyd said:

It might be easier to change the alternator to a one with a vacumme pump on the end, like on some diesel models!

Again, that would be goof for the brake servo vacuum, but not the vacuum advance.

The distributors vacuum advance needs to receive a real-time feed of the intake vacuum, post throttle. This is because the intake vacuum reflects the engines level of loading, so the clockwork/electronics can decide when's best to spark.

I meant to say, run the brakes via the alternator pump, and use one take off for the vacuum pipe, therefore avoiding the need to link all the intakes

[GeezerGarage]

That would be good but I think all the diesel models replace the alternator completely.

[Hudzoyne]

You probably wont even get enough low down advance with the vacuum advance standard setup anyway. Bike carbs needs LOTS of advance off the bottom to make them effective I have found. I have had the most success by welding them up and running fixed advance. Does make it a curse word to start though!

[jrevillug]

Jul 3, 2012 16:20:20 GMT Hudzoyne said:

You probably wont even get enough low down advance with the vacuum advance standard setup anyway. Bike carbs needs LOTS of advance off the bottom to make them effective I have found. I have had the most success by welding them up and running fixed advance. Does make it a curse word to start though!

Eh? Vac and mech advance are two separate things. Vac advance doesn't alter full-bore power or starting ability - merely light-load cruising.

In any case, the right advance curve for the engine depends on many many factors, not just the carbs.

I will concede that more advance low down (as you'd get when running with no mechanical advance) will tend to improve the low-end torque (which will drop off with bike carbs) - but there are better ways of achieving that than locking the advance mechanisms completely. That will tend to lead to either pinking at low rpm or poor power and economy at speed.

[pork]

This is what I done with my old k10 micra, aluminium drinks bottle with 6mm push-fit fittings tapped in

Attachments:

[pork]

Then tapped each run on the mani

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[pork]

The finished product

(sorry bout single posts, posting from phone)

Attachments:

[mattpole77]

Jul 3, 2012 17:13:47 GMT pork said:

The finished product

(sorry bout single posts, posting from phone)

I'm sorry to ask, does this set up will work?! Will it'll solved all the idling prob?!

[mikeymk]

I daisy-chained a void through the carbs, using the redundant casting for the water heating, exiting to the dizzy out one end..









On another set of carbs i went onto use the same principal, but going through the bike rubbers instead.

Worked fine, but not really necessary in my experience - having the vac has never really improved anything, still idle's fine and starts instantly with or without it, whatever engine i've fitted them to (1.4/1.6, 8v/16v).