resdoggie

I'm tuning my 650 DP and started playing with the secondary idle circuit. Out of the box, the secondary idle screw was turned all the way out i.e. secondary butterfly valves closed. I set the primary idle mixture and idle speed as per normal without issue. When I went to fine tune the secondary mixture screws they were virtually ineffective and could only be turned out about a half turn each. More than this and the vacuum dropped. Turned all the way in from half a turn out had little effect on vacuum.

Removed carb and turned in secondary idle screw to open up the transition slot. Replaced carb. Idle was way up. Brought idle down to ~1000 rpm. I have a 4-sp. Now the secondary idle screws could be turned out to about 1 and a 1/2 turns to attain best vacuum but the primary idle screws now had little effect on idle and when turned all the way in, it wouldn't stall the engine. Remove carb and adjust primary and secondary idle speed screws to expose the transition slots just a wee bit. Replaced carb. Now all 4 idle mixture screws have some effect when adjusted. The car idles smooth at 1000 rpm and vacuum is ~15-16", steady needle.

Here's my question. Do you need to adjust the secondary idle mixture/idle speed when the out of the box setting will run just fine by only adjusting the primary side?

Merry Xmas all!!!

TCracingCA

Holley sucks at sending a tuned mechanical secondary carb. I doubt you can get a stock sold unit out of the box and have it right! They are decent on the vacuum secondary units (because it is vacuum actuated). Also they tend to be from the past, over jetted!! Thus to answer, not being able to adjust the secondary mixture screws to have an affect seems like a problem to me!!!!! And bringin it in with the primary is just pure wrong! Thus playing with the throttle blade openings is the logical move, based on what your engine is actually asking for. Thus from your first test, you had a balancing act, tweek this, then go back and tweek that. Personally I don't like to tune at idle (primarily because I don't really have an idle and the tuning is the overall timing and advance, not just speaking of the carb!!!!! But watch out-- because of all of the book tuners and guys that think precise instructions have to be followed is the way, they will come on here and jump my case. Therefore, just think about where you want the car to perform. Where you want the punch, how you want to bring it in depending on the cam and engine demands and characteristics!!!!!

I was playing with A/R measurements the last time I was tuning more so than vacuum!

PS the so called know it alls, never put up an answer first on this type of thread. But will wait for someone else and then try to look knowledgeable on slamming your observations, without any original thoughts being shared of their own!!!!!! The other experts will tell you to read the instructions to prove their expertise!!! The last half haven't ever tuned a Holley!!!!!!!!

PeteZO6

Nice reply TC. I run a 4150 DP with 2 corner idle and always wondered what was so hot about a 4 corner idle carb. After reading your post and then looking at photos of my engine (I'm not at home) it seemed that the primaries are closer to the front 4 cylinders, so it follows that a 4 corner idle carb would distribute the idle mixture a little more evenly. Probably not important on my engine, it could help a highly tuned engine idle better.

Following that logic, it seems like doggie figured it out the same way I would have. Trial and error until it works best


Pete

diehrd

I would say each case is different , your adjustments to the transition slots was exactly what you should have done , IMOP with the idle set it is time to make sure all throttle positions and movements of it result in as good results as your idle set up

cv67

@TC and

TCracingCA

Therefore I have found about the closest tech write up on what I was trying to say above. Ideally it would be nice to put the car on a chassis dyno and put a load on the engine. This is for guys trying to run racing double pumpers on basically racing engines in a street enviroment!!!!! And this is a little more complex than just taking the instructions provided by Holley on performance tuning out and following them!

427Hotrod

You got it figured out...so that's good.

I think Holley does fine. Unless a carb is specifically built for a specific known engine combo, there is no way to just bolt one on out of the box and be perfect on EVERY engine. Carbs are generally very forgiving by nature...but the better stuff has a lot more adjustability to allow you to play with them.

Depending on cubes,cam, intake etc...an engine needs a certain amount of air to idle. Take one of a perfectly good running 350 and drop it on a 427 and you'll need more air. You can open the front blades more, but depending on how much air you need, you might be way into the transfer slots and start pulling fuel through them too early. It's always been a trick to flip the Holley over and crack open the secondary's a little bit to let some air in...but it's easy to get too much through there to the point the idle circuit on the primaries can't feed enough. This is where the 4 circuits really shine. You can get enough airflow through all 4 BBLs by having them all slightly open and the idle screws can control them. There's the other method of drilling the throttle plates to let in some more air...and some new carbs have a separate idle air screw down deep in the hole under the air cleaner stud. This allows air in without cracking the throttle blades too much. It's all about keeping the idle screws functioning properly.

My experience with the Street HP's is that they are leaner out of the box as compared to the regular "race" carbs. The main reason the idle circuits are rich on "race" Holley's is again..they have no idea what we're going to put them on...and since race engines typically have big cams and low idle vacuum, they need the larger orifices to allow enough fuel in at those lower vacuum levels. Stick one on an engine with higher vacuum readings and a milder cam..instant fat running and you might need to lean the idle circuits a little (not the main jets).

Gotta start somewhere......


JIM

TCracingCA

You nailed it! Good follow-up and add to the thread!

Solid LT1

4 corner idle carbs perform well on intakes with big plenum/runner volume if you have a more street oriented intake or a dual plane you probably don't need the features of a 4 corner idle carb unless your cam is really stupid big on duration.

TCracingCA

Thus this thread is kind of interesting, in that I thought of stepping to the 4 corner idle to help that. I have the big air gap GM Bowtie Single Plane unit that has runners like a tunnel ram! I have the full race double pumper, and I am guilty of stupid levels of duration and will put it on the street and will scare everyone that I encounter! My favorite thing is setting off all of the car alarms as I drive by up the street, and have set off entire parking garages full of alarmed cars also. But my cars have never had manners! But I did buy a Brodix Turtle prior to the invention of 4 corner idle!!!!!