14 replies to this topic

[AlexP]

RSP:
1990:
Standard electronic dizzy:


At idle, should I be able to detect any change in engine revs when sucking on the vac advance hose? It can hold a good vacuum but makes no difference to idle speed or noise.

Is this right?

Edited by AlexP, 03 March 2010 - 09:22 AM.

[lrostoke]

Not sure about electronic dizzy, I presume its same principle. But on any I've dealt with, disconnecting the vac pipe pretty much stalls the engine at idle.
So yes there should be a difference.

[minimender]

Should advance it loads at idle. The engine should speed up considerably when you put the pipe on or suck on it.

[AlexP]

Hmmm, this could be the answer to my rough and inconsistent idle.

Would a replacement vac unit fix it or is the dizzy fubar'ed?

[Dan]

If the unit holds vacuum but doesn't affect the timing, it's probably just become disconnected from the inside of the dizzy.

[AlexP]

Would a bad (detached?) vacuum unit cause trouble starting, inconsistent idle and low rev lumpyness?

[SolarB]

Would a bad (detached?) vacuum unit cause trouble starting, inconsistent idle and low rev lumpyness?

Yes, without the vacuum advance working the timing is several degrees out.

[AlexP]

Is there a way to detach the vacuum unit without remove the dizzy from the car? I cannot check to see if the advance is working due to the fact that I have a 65dm4 dizzy with nothing under the cap except the rotor.

[bamby]

Is there a way to detach the vacuum unit without remove the dizzy from the car? I cannot check to see if the advance is working due to the fact that I have a 65dm4 dizzy with nothing under the cap except the rotor.

have you checked the pipes to and from ? Mine were cracked and casues lousy idle

[Surfbluegarage]

on the 64dm4 dizzy you take off the cap and rotor arm, undo the screw on the outside of the body which holds the advance unit to the body. but you will still have tro take the base plate out beneath the rotor arm to get to the rod that comes out of the advance unit. plrob easier on a bench to be honest.

[AlexP]

have you checked the pipes to and from ? Mine were cracked and casues lousy idle

The pipe's not the problem it's either the vac unit or the dizzy itself - but thanks for your suggestion.

on the 64dm4 dizzy you take off the cap and rotor arm, undo the screw on the outside of the body which holds the advance unit to the body. but you will still have tro take the base plate out beneath the rotor arm to get to the rod that comes out of the advance unit

I tried to take the vac unit off today by removing the screw and pulling it off, but the arm is captive it seems. So i screwed it back on again. I'll try taking the baseplate off soon, but i'm worried all the gubbins of the dizzy will all fall out! Will they???

[Surfbluegarage]

hmmm interesting.. if you say you cnt pull it off its still attached them.... its only held on by a small hole in the arm coming from the unit inside on a pin... no theres nothing really t worry about on a 64dm4 dizzy. when you take a lucas 25 dizzy apart then you start to worry bout bits lol.

[Sprocket]

Simply saying that there should be vacuum operating the vac advance at idle and that there should be noticeable difference in idle speed is wrong.

There are two types of vacuum take off on the HIF carbs, and, depending on the engine/ fuel type will determine which vacuum take off is used.

Manifold depression is the full vacuum that the manifold is subject to, after the throttle.

Throttle edge depression is a partial vacuum, its is neither manifold vacuum or atmospheric.

Throttle edge depression, I know for a fact, is used on all non Cooper 1275 engines fitted with a HIF38 carb. At warm idle there will be little or no advance. When the engine is cold and the choke is in operation or at part throttle cruse there will be some vacuum advance. This is because the vac take off on the carb is located right at the throttle edge and as the throttle moves over the take off port, the vacuum changes.

Why reduce the advance at idle and increase it on choke or cruse? Its all to do with ecconomy and emissions. Reducing the ignition advance at idle helps rduce HC emissions and increasing advance on choke helps rich fuel mixture burn at low throttle openings. Increasing the advance at cruse increases engine reponse and ecconomy. It also has a lot to do with the charachteristices of unleaded fuel, as well as the carbon emissions canister purge. The throttle edge depression shuts off the vacuum valve on the carbon canister, preventing fuel vapours being drawn into the engine at idle.

I looked into this and spoke with Southern Carburettors and Burlen almost 14 years ago

As for the Cooper RSP 1275s with a HIF44, I cannot comment.

All I am getting at is that not everything is what most say it is

Oh and I forgot to say that I would be surprised if you could draw as deep a vacuum with your mouth as the engine draws at idle

AND, im not suggesting there is nothing wrong, there might well be, but on the other hand, there might not

Edited by Sprocket, 04 March 2010 - 10:53 PM.

[Surfbluegarage]

sprocket is very right. i only jst found this out aswell when sorting through carbs at work. turns out that mini and metro hif 44s are different too in the same manner sprocket talks about.. mini hif44s advance take of sf pre manifold i.e jet side of the butterfly whereas metro hif44s have an angled andvance bordering on the manifold side of the butterfly.

[AlexP]

cheers guys, my RSP has the JCG 'S' conversion and twin HS4's. the vac take off is near the butterfly on the drivers side carb.

Does that mean that there won't be much vacuum at idle?

Also, is it correct that the fiercest vacuum I can pull with my mouth doest alter cold idle with a little choke out at all? I thought that it should make some difference to idle speed, no?

Edited by AlexP, 04 March 2010 - 11:24 PM.