cruise mode surge

[Chryco Psycho]

leave the hose disconnected  , you may also need lighter springs under the metering rods to keep them down in cruise mode

[john h]

leave the hose disconnected  , you may also need lighter springs under the metering rods to keep them down in cruise mode

can you please explain that to me.    I thought the more available vacuum the heavier stronger spring would be required because it would need more help to push the metering rod up.  Maybe I'm thinking of this all wrong.  I am currently running the Pink spring for 7" hg.  I thought  since I was surging I was lean and needed the heavier spring to push the rod up allowing more fuel through the jet. wouldnt not using vacuum advance leave extra vacuum in the plenum?
I have not been able to mess with any of this.  between kids sports and rain I have not been able to test.

I really appreciate the help and ideas.

[Chryco Psycho]

All engines make vacuum , depending on the cam , RPM , throttle opening etc the vacuum will vary .
 there are 2 types of vacuum & they work opposite so as one rises the other becomes lower .
 Intake vacuum is very high at idle with the throttle plates closed but drops off as the throttle plates are opened up .
 Ported or venturi vacuum works opposite & is drawn from below the venturi inside the carb , there the vacuum is very low at idle but rises with air speed through the carb .
 Ported vacuum is what you want to use for vacuum advance so the advance increases with engine speed but this is also accomplished with the mechanical advance & to shorten the curve so the engine is more responsive off idle I generally leave this disconnected .
 with manifold vacuum as rpm increases & or throttle is opened up the vacuum will drop , the springs under the metering rods will push the rods up allowing in more fuel , this is needed for accelerating  but not for cruise so the added fuel may increase RPM which will increase the vacuum if the throttle remains in the same position which will pull the rods back down decreasing the amount of fuel which would drop RPM & vacuum where the springs may push the rods back up creating the surge you are experiencing so by reducing the spring pressure the rods stay down

[V02Barracuda]

I'd like to make it clear that I'm NOT here to override other people's opinions and knowledge.  I'm only expressing what works for me.  2 of my cars (70 Cuda 340 stroker and a 74 Firebird Formula 400 stock) that have the vacuum advance hooked up to the manifold vacuum port on the carb.  It works great. 

I tried to find the article regarding this topic but I must have deleted it.

Anyway, try it.  It won't cost you a dime.  If you feel it doesn't work for you, change it back.

Good luck.

[Chryco Psycho]

That will certainly negate the advance curve as you will have 14* advance at idle & it will retard with rpm . every time I have seen it connected this way the engine runs hot unless the vacuum pot is counter rotational so it will retard at idle , I have no clue how to make this work !

[john h]

All engines make vacuum , depending on the cam , RPM , throttle opening etc the vacuum will vary .
 there are 2 types of vacuum & they work opposite so as one rises the other becomes lower .
 Intake vacuum is very high at idle with the throttle plates closed but drops off as the throttle plates are opened up .
 Ported or venturi vacuum works opposite & is drawn from below the venturi inside the carb , there the vacuum is very low at idle but rises with air speed through the carb .
 Ported vacuum is what you want to use for vacuum advance so the advance increases with engine speed but this is also accomplished with the mechanical advance & to shorten the curve so the engine is more responsive off idle I generally leave this disconnected .
 with manifold vacuum as rpm increases & or throttle is opened up the vacuum will drop , the springs under the metering rods will push the rods up allowing in more fuel , this is needed for accelerating  but not for cruise so the added fuel may increase RPM which will increase the vacuum if the throttle remains in the same position which will pull the rods back down decreasing the amount of fuel which would drop RPM & vacuum where the springs may push the rods back up creating the surge you are experiencing so by reducing the spring pressure the rods stay down

I should probably read this a few more time to make sure I understand what you are saying, but, if my springs are pushing the rods up at cruise, I would be getting more fuel and that fuel could be causing a rich surge?  I thought surge was an indication of  a lean condition.  I have pulled my plugs after a long run at moderate speeds.  they tend to be almost grey so I assumed lean.  now that being said, I live in an area that has a long run of residential after you get off the county roads and it's a slow 25 MPH run through the neighborhood and then up the drive into the garage before shutting it off and pulling the plugs.  I suppose that could change the plug color if I was leaner at really slow speeds.
I will for sure put the orange spring back in and maybe the next one down to the stock spring also mess witht eh timing and see what my total mechanical advance is etc.

Thanks again for sticking with me on all of this.  Messing with it all spring so far and it has gotten alot better just need to figure out this last little bit.  35-45 is the average driving speed around here. so it needs to get fixed.

John

[Chryco Psycho]

Simply put the spring are possiiblity too strong lifting the rods too soon & adding extra fuel causing the surging , it only take about 3 mins to swap spring , just loosen the covers rotate them out of the way & lift out the rads & try driving it if it changes that is the problem

[john h]

Update:  I messed with it tonight and I did some testing.  I don't want to jinx things, but I think it's fixed!  Chryco seems to have come up with my fix.  My timing was way too advanced with the vacuum advance hooked up.  I checked it at 2000 RPM mechanical advance only and it sits at 39 degrees.  I had my wife hold the gas at 2000 and 2500.  it never went past 39 degrees.  I set the timing at idol at 9 degrees and kept the advance plugged off at the pod on the distributor and plugged the port on the carb. I also switched back tot he orange step up spring. test drive.... It never surged at any speed. it seemed doggish though.  I hooked the advance back up and the surge came right back.  Went home, unhooked and plugged the ports, re-set the base timing to 12 degrees and went crusing about 65 miles, all different speeds.  no surge! and it's also not dogish off the line.  I really appreciate all the help from everyone.  it's been so long since I enjoyed driving this thing. I have been working on this problem for a very long time and I typically give up.

Thanks guys.
John

[Chryco Psycho]

 

[roadman5312]

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