Inital Tuning of Dual Holleys

[Wreckless Warren]

I wanted to kick this out and get others experiences. I have my new 445 on the run stand and after some teething issues I'm going to start tuning the carbs. They are custom QF 600's on a BT Medium Riser intake, ED heads and a Comp 270S cam.

I have read over and over the FORD TSB's on setting up dual fours and it says on the bench to turn the secondary adjust screw in 1/2 turn on both carbs from a closed position and the primary adjuster in 3 turns from a closed position on both. Then turn in the idle air screws in till seated and then back out 1 1/2 turns. This is your base line setting.

But I have been talking to Joe D. Craine and Woods Allen about using the secondary throttles of both carbs for engine idle. With this method you adjust the secondary throttle blades on both carbs so 0.012-0.014" of the transfer slot is showing. The primary adjusters are set to a CLOSED position. The idle air screws are turned out 3/4 to 1 turn. This is your base line setting. If the idle needs to be adjusted up or down it is done by making small adjustments evenly to the secondary's of both carbs, the primaries remain closed at idle.

It was also discussed about the need for the throttle blades to be centered in the bores if not already.

Obviously this is a more time consuming process but eliminates the potential of activating the power valve circuit at idle, especially with big cams.

I'm going to set mine up like this tomorrow and get it dialed in. I was wondering if others are using this method and had anything to share. Thanks. ww

[My427stang]

I start by ensuring the secondaries are not closed all the way as delivered, meaning they don't make contact with the bores.

Then, I set the primaries on both carbs to where there is about .020 of transfer slot showing from the bottom, sometimes a smidge more, but no less

Then if the carbs are 4 corner idle, I set all of them to 3/4 of a turn from the bottom, if it's a standard primary only, I set at 1 1/8 turns from bottom.

Then I fire it.  If it idles real close to what I want, I adjust a/f with all of them matching to best idle.  Then readjust idle speed.

If idle speed is way off, I open the secondaries a little, closen the primaries, then start adjustment again.

Ironically, I would say that historically, I end up pretty much the same spot that Joe told you, except that I typically have not seen the idle a/f that far in at 3/4 to 1 turn

[cjshaker]

You won't have 4 corner idle adjustment with any type of factory style carb with secondary metering plates, unless they're converted to dual metering blocks and turned sideways, which doesn't work well with dual plane intakes.

In my experience, Ford's recommended procedure has always worked well with mild set-ups. Once the cam gets big enough and vacuum drops, I've had to go to the procedure of what Joe said. But every engine is different. With big cams and low vacuum, the sensitivity is increased greatly and very minor faults will cause big issues.

The biggest thing that I've found helps is, if the carbs are used, get them back to the factory setting, based on the carbs type, then start your tuning from there. Most used carbs have been messed with and altered for the previous engine. That can leave you chasing your tail trying to figure out why something isn't reacting the way you think it should.

[My427stang]

Good point, I have never actually used a 4 corner idle on a dual quad, but was giving that as generic guidance that didn't apply...you are 100% correct,, there cannot be 4 corner with met plates

So I'd start at 1 1/8 and it is what I have done with standard 1850s, original carbs and even 450s

I also start stock for everything, on almost every carb.  Super-tuning usually isn't

[e philpott]

that's pretty much how I am doing mine right now Warren ..... finally got my MR 2x4 on with a pair of 1850's ..... I added a bit more clearance  since my cam is 266/276 at .050 and first try was 1700 rpm ( my Dove 1x4 was 1100 rpm) so I took the carbs off and went too far closed now and won't quite idle ..... hopefully third time is a charm ..... but if not , I'm wondering at what point do I drill a hole in the throttle plate due to the big cam ?

[My427stang]

I would beg "never" drill

With 8 barrels, you should be able to get enough idle airflow between primaries and secondaries to idle well without being in the transition circuits

Even with 4, unless your carb is very critically undersized, opening the secondaries will almost always do it

[cjshaker]

I agree with Ross. My stock stroke 427 with a 260/264 @ .050 cam will idle easily at 900 RPM. With a 2x4 intake, it's just like having a 4 corner idle system, you're just using 2 carbs instead of 1.

Are you sure you're not pulling in timing at that speed? That will immediately make it idle faster and cause it to be near impossible to nail down a good idle.

[e philpott]

that's a good possibility on the pulling timing , I am sure it is , my advance has 14 degrees and my engine is a 416 in by 2500-ish....... Ross , my single 4 HP series carb  have hole drilled in the throttle blade and it idles decent for the cam size/cubes ....... this will be my third try and setting the secondary throttle blade air gap so by no means am I ready to drill anything just yet

[My427stang]

I have seen a few Holleys that were drilled from the factory, including the 1000 HP series I used to run on my 489.  When I put the billet base on it though, I got rid of them.

Keep plugging, the idea is to have the blades slightly open to have some vacuum pulling lightly on the transition circuits, it allows them to start immediately when needed.  If you slam the doors shut because there is a big hole in each blade, you may fix idle, but then you may chase IABs and IFRs thinking you have a transition problem

The fact that you were able to idle too high and too low means that you can fix it with what you have, so glad to hear you are going to keep trying, you just have to find the middle

[427LX]

Wreckless Warren... what is the jetting in those QFT carbs? 

Not a 445 FE build,but my healthy 427 Windsor build is right on the tune with 67/68 jets and .073 Idle Air Bleeds in both Holley 1850's.
Are you going to use a A/F gauge for your on road tuning?

[Wreckless Warren]

Here is the data sheet on the carbs from QF. Yes I plan to have the exhaust equipped with bungs for sensors and use an A/F Meter for road tuning. ww

<a href="http://s226.photobucket.com/user/65cobrajet428/media/390PI-445%20Build/qf-04r_zpsonb496vx.jpg.html" target="_blank"><img src="http://i226.photobucket.com/albums/dd31/65cobrajet428/390PI-445%20Build/qf-04r_zpsonb496vx.jpg" border="0" alt=" photo qf-04r_zpsonb496vx.jpg"/></a>

[427LX]

Very interesting!  I'm running 68 jets in the secondary AED jet plate and get 12.8-13.0 for WOT at 5500 in 3rd gear....ran out of room for 4th!
When you start your road tuning post up your results and tweaks. Thanks!

Note if it shows a bit lean at medium/heavy accel on primary only you can try going to an 8.5 Power Valve. Worked great on my setup and keeps the highway cruise in check.

[e philpott]

little up date on my 2x4 with 600's ..... got a little more tweak time as the weather warmed up ..... closed the secondary's a touch more to get the idle close and ended up going from 66's to 68's ..... idles pretty good , drives pretty good , going to get some miles on it this weekend and get a better feel for the 2x4's  , so at this point I'm pretty satisfied with the 2x4's enough to go cruising

[427LX]

Sounds good!

[Barry_R]

I kinda agree with the comments from Ross - and Doug.

You do not want the throttle plates ever fully closed against the bore - they will notch the bore and bugger things up.  But you are allowed to get them pretty darn close.  That's where checking them for square fitment comes in - gives you a tiny bit more latitude in adjustment.  I generally get everything pretty "tight" but not closed, turn mixtures screws one turn (not because its a special number, just because its easy for me to remember), set the WOT linkage position - and light it up.

As noted - no reason to be drilling any holes in a 2x4 deal.  Closed throttle airflow is the term for what goes around the diameter of a throttle plate.  With eight throttle bores you have plenty of closed throttle airflow.  If anything, its sometimes hard to get the mild engines with dual quads to idle low enough.

Doug's comment about timing justifies repeating.  Its pretty common to use really light springs on the advance mechanism.  Often a better idea to use one slightly heavier one to hold the curve back so low RPM timing remains at "base".