Dellorto On 4k

[toymotar]

Hi guys,

 

I am currently building a 4k with a single sidedraft dellorto,does anybody know what i should do with the vacuum advance line from dizzy?

...and will it run correctly without it hooked up?

 

any help would be great!

[corollaart]

I wouldnt worry to much about it .It proberly hasnt worked for years any how.

 

Ive never used vac advance ,just block it off.

 

rob

[toymotar]

I wouldnt worry to much about it .It proberly hasnt worked for years any how.

 

Ive never used vac advance ,just block it off.

 

rob

Thanks rob! ;)

[seabiscuit]

IF you can hook it up, It would be a good idea, though.

[toymotar]

IF you can hook it up, It would be a good idea, though.

Yeah i would like to,just not sure where or if my dellorto can..?

[toymotar]

This is the carb...may help. ;)

[KENut]

As far as I know there is no provision on dellorto carbs for vac advance.

Not sure if you can hook the line into the same vac line as brake booster line. This may give too much vacuum to the advance unit.

It seems many people just go without vac advance after putting dellortoes on. Hope this helps a little

[toymotar]

As far as I know there is no provision on dellorto carbs for vac advance.

Not sure if you can hook the line into the same vac line as brake booster line. This may give too much vacuum to the advance unit.

It seems many people just go without vac advance after putting dellortoes on. Hope this helps a little

that helps heaps,cheers..no vacuum addvance it is then. ;)

[steve_tm]

hey mate

 

i'd be keen to hear how u go with setting this carb up and how well u get it running. i've got the same single dellorto but putting it on a 5k! havent gotten around to playing with it much in terms of getting it running ok!

 

as for the vacum advance, as said, jst block it off.

[philbey]

No ports on the carby. It's got to be on the downstream side of the butterflies as well.

 

If you have a brake booster line, yes, you can just tap off this. This will come off the inlet manifold, not the carb.

 

I'm not running either vac advance or vac brake booster.

[rob83ke70]

put a small port/fitting into the manifold on the opposite side to the brake booster line, and get a vac advance valve that cuts vacuum below a certain level (ie turns manifold vac into ported vac) and hook it up. Much better fuel economy and drivability with vacuum advance.

 

Failing this, get the dizzy recurved without the vac advance, although they say this is more appropriate for race cars.

 

I run a catch can vented to the atmosphere rather than a pcv system as it actually seemed to suck too much air into the manifold (short little lynx thing for twin su carburettors) and the idle speed would increase to about 1800rpm as the temperature rose.... you may or may not have similar dramas...

 

Robert.

[toymotar]

Thanks heaps guys! ..its nice to know helps not far away :lolcry:

I'm sure ill have plenty of other probs in months to come.

[altezzaclub]

That's an interesting question toymotar..

 

The vac advance unit is there to retard the ignition timing when you floor the accelerator so you get better initial acceleration, and then it advances it back up as you lift off the pedal and cruise. With less air going in on an almost-closed throttle you need more advance to burn it, so more advance drops fuel usage on part throttles. On wide-open throttle the cylinder air density is higher and it burns faster, so less advance. If we take the 'rolla down to Sydney from Orange it pinks at sea level as we can handle more advance at 2500ft.

 

Without it you will not get the full spark advance unless you add more static advance at idle, and so add advance right through the rev range perhaps in places you don't want it. I don't know what it pulls, but this guy says it adds 15degrees, so max advance is taken from 36 out to 50deg. So instead of 8 to 10deg at idle you'd be looking for 25deg to let the mechanical advance get up to 50!! Unfortunately you won't be able to handle 50deg with wide-open throttle at 4000rpm because of detonation, so without a vac unit you will not get as much advance into the system.

 

http://www.thesavoy.de/html/vacuum_advance.html

 

As Rob said, you can get the dizzy re-graphed to suit the carb/cam setup and they will alter the springs and weights under the dizzy base to maximise the engine's potential. You will still not get the effect from spark retarding when you floor it or the fuel efficiency from the vac advance.

 

Where it gets interesting is that the two points for getting vacuum, the carb or the inlet manifold, have completely different results.

 

Early, pre-emission vacuum advance units were typically linked to a manifold vacuum source. This meant that the vacuum was most often taken from a location below the carburetor throttle body. During idle and part throttle operation, manifold vacuum is high. This advances the ignition timing under those conditions and improves fuel economy. When the engine is operated at wide-open throttle, manifold vacuum is low. This means the vacuum mechanism does not advance ignition timing. As a result, there is no chance of detonation (or pinging).

 

In the mid-'60s, vacuum advance mechanisms changed to suit emission requirements. The vacuum source was changed from the manifold to the carburetor venturi. This is called "spark ported vacuum." Spark ported vacuum is lowest at idle, and then increases as the throttle is opened. This is completely opposite to manifold vacuum. At idle, a spark ported vacuum system has no vacuum advance (in contrast, a manifold vacuum advance might have as much as 12-degrees extra timing).

 

So hooking it into the brake booster line will be most interesting to see what that does to your advance at idle. It should suddenly add a handfull of degrees of advance on and you will have to retard the dizzy or lower the idle speed to compensate.

 

I do wonder if all the guys who say their Corolla goes so much better with 12 or 15degrees of advance actually have a broken vac unit, or they disconnected it.

 

What do you think Mr Philbey?