Tuning Holley 670 street avenger for a 383 - are my observations nuts?

[XH29N0G]

 I am new to this, so I think what I am about to describe is a little bit of a comedy of errors, but it also might be useful.

Last december I replaced my original manifold and old Holley 4 bbl with an Edelbrock performer RPM and a Holley 670 Vacuum secondary avenger. The car had a significant stumble off idle, backfired, and did not respond the way it should have.  To correct this, I changed the squirter from an 031 to an 035 and the orange accelerator pump cam to a blue one.  This mostly solved the stumble, but it was still not quite right - there was still a delay at 1500-2500 rpm and it sometimes backfired.  Upon someone else's recommendation (he argued the plenum was larger on the intake and may need larger jets), I changed the jets from 65/68 to 68/70 primary/secondary, and it ran very well.

In the past month, I changed the heads to OTB edelbrock RPM heads and changed the cam to a Lunati 60302.  The bog and delay returned, so I upped the jets again to 70/74, but I noticed that the car smelled rich at idle and low speed, and still had a slight bog up to about 3000 RPM.  I suspected that I might have gone too rich, but I decided to try something that might have been nuts.  I returned the squirter  to a smaller 032 and the accelerator pump cam back to the original orange one.  The slight bog went away and it seems to work well.

NOW I am reasonably sure that my air fuel ratio is not correct.  I have not checked it, and this will be my next step (to return to working with the jets). 

My question is:  Could my earlier issue have simply been that the carburator was running too lean, and the proper fix have been the jets rather than the squirter and accelerator pump cam?  Or did I just violate some basic rule?

Are there any out there who have dialed in a similar set up and might be able to advise me?

Thanks,

James

[c00nhunterjoe]

My opinion the carb is to small for a 383 with heads, cam and intake swap. A 750 would suit you much better.

To fix the current set up you might want to swap back to the 68\70 jet and go another step larger on the accel pump cam. Most likely the pump is too small, need to upgrade your accell pump. All this work and upgrades to a carb that is to small for your combo, swap for a 750 and tune that. You will see more power as well.

[firefighter3931]

It sounds like your idle mixture is too rich so try leaning it out. Use a vacuum guage and start with one turn out on the idle mix and see if it's still rich. Using the vacuum guage adjust for maximum vacuum. Once that is done your idle circuit is now properly tuned.

The bog and backfire is a lean condition when going to wide open throttle. You need to jet properly on both the primary and secondary side of the main circuit. From your description it sounds like you just went up and down on the primaries but did nothing to the secondaries ? You're gonna need more jet (secondary) to eliminate the lean bog 


Ron

[XH29N0G]

Hi Ron and c00nhunterjoe,

Thanks for the advice.  I just recently tuned the idle to maximum vacuum, and will look at the jets.  There are a few things that puzzle me.  My understanding is that I am supposed to disconnect vacuum advance and then tune the idle at 825 rpm rather than 625 which is where I usually set it.  I notice a small if any difference in the settings at 825 and 625 rpm.  Does it matter? 

I also find that the setting I get is with the idle screws in quite far ~1.25 turns from the end.  I am assuming this is OK, but maybe it is telling me something. 

I also find that I when I remove the vacuum gauge (which creates a temporary vacuum leak) that the RM increase slightly.  The setting where they do not change with this is about .25 turns more lean.  Is this telling me something?

I am assuming I should ultimately move to a 750 double pumper as I have seen on other threads, but I bough the 670 vac secondary before knowing this.  I also do not yet know exactly what direction I am headed with the engine.  It may be that you will find me asking about building a 432 or larger in the next year or so. 

I am enjoying learning about this car and these engines and really apreciate the feedback I have received from the forum.

James

[firefighter3931]

Personally, i like to idle any BB mopar at 800rpm....the reason being that the cam/lifters are splash lubed by the crank/rods flinging oil up there. The more oil the better ; especially at idle 

If the idle mixture is set at 1.25 turns from fully closed (bottomed out) then you're good to go 

When removing the vacuum guage from the port you are creating a vacuum leak and rpm's will increase...quite normal 

The carb might be slightly small but it should still run fine and have fantastic throttle response. The new cam and E-heads require more fuel/air so you need to tune for the increased induction requirements. The sparkplugs will give you some feedback if you know what to look for. Start off by going back to the 68 primary jets and take it out for a nice 20 minute cruise. Do not open it up....keep it running on the primaries only. Come back and pull a plug for inspection ; what you're looking for is a nice tan/light brown color. Dark brown/black is too rich....light grey to white is too lean. Work on that first and once the primaries are dialed in...move onto the secondaries.


Ron