Tuning a Holley Carburetor on a '73

[moparfreak]

Alright folks. After a decade my '73 SE is just about street worthy. As I work through resolving some minor cooling/brake issues, I want to go through an adjustment/tuning process on the carburetor to make sure it's running right. Engine fires up OK, seems like it runs a bit rich. The carb is newer and in good shape as far as I can observe. It's a Street 4-barrel 750 CFM Holley.

Are there any good resources or guides (here on the forum, elsewhere online, or books, etc.) on how to properly set & tune a Holley carb on a Mopar engine? This is a 400 cid that is relatively stock.

Thanks,
Adam

[XH29N0G]

Adam,

I can get the ball rolling with my experiences.  I am very new to this forum and have a different carb (holley 670 cfm vacuum secondary) so my advice may be trumped by someone else.  I have received some very good advice from the people here.

I started with what Holley provides, which I found to be quite good:

http://holley.com/data/TechService/Technical/Carburetor%20Tech%20Info.pdf  There are also other online resources.

In your case if it is just a matter of being rich at idle, the adjustment should be straightforward with a vacuum gauge.  I am assuming you know this, but will write it anyway.  As I understand it, you disconnect the vacuum advance, set the idle at ~850 RPM and adjust the idle screws (what you do to one you do to the others on the same metering plate - I do not know if you have two metering plates whether you do all four together or not) 1/8 run at a time to maximize the vacuum.  It is important to check the idle screws to make sure they were all at the same setting (i.e., you did not turn one clockwise and the other counterclockwise 1/8 turn).   I also learned the hard way that before doing any of this I needed to make sure my ignition, timing, etc... were set correctly and the car was warm.  This makes a huge difference to the way the car idles, and to what the exhaust is like.

After that, the issues I had  were related to the way the car responded.  Is there something your car is doing that is not right?  (Hesitation, bog, backfire, etc...).  If so, there are other changes that are worth looking at next.

Changing the various parts is surprisingly easy, but also led me down a road to a solution that was not optimal.  To avoid this, what I recommend is to make changes in small steps and systematically.  Then ask the folks here for feedback. 

I found that I could fix a problem like hesitation or backfiring by changing only one thing (like jets or accellerator pump), but then realized it not optimal.  So I described what I did and asked the list.  They advised;  The car went from one where everything ran smoothly with no hesitation to one that was altogether different, in a good, faster way. 

James

[Chryco Psycho]

Running rich at idle is often just a result of the cam choice , if it lopes , has low vacuum , it may not be mixing fuel efficiently & when it lopes it is misfiring sending unburnt fuel out the pipes .
I am not saying that tuning will not help just that the rich may not be entirely a carb problem .
Start by setting float levels just at the bottom of the threads of the sight plugs ,. set mixture screws turn them in until the engine drops RPM & back them out until the rpm levels off , Make sure the timing is set around 12-16 * at idle & 36-38 at 3000 rpm , check idle vacuum in gear , if it is below 8" you will need to change the power valve to a lower value than the 6.5 it comes with to make sure it stays closed at idle . Make sure there is no play in the accelerator arm , any play will cause a bog off idle , you may need to change squirter or cam if it stumbles off idle .

[moparfreak]

This is really great information, thanks! I'll print this all out and give it a shot!

Thanks,
Adam

[moparfreak]

Running rich at idle is often just a result of the cam choice , if it lopes , has low vacuum , it may not be mixing fuel efficiently & when it lopes it is misfiring sending unburnt fuel out the pipes .
I am not saying that tuning will not help just that the rich may not be entirely a carb problem .
Start by setting float levels just at the bottom of the threads of the sight plugs ,. set mixture screws turn them in until the engine drops RPM & back them out until the rpm levels off , Make sure the timing is set around 12-16 * at idle & 36-38 at 3000 rpm , check idle vacuum in gear , if it is below 8" you will need to change the power valve to a lower value than the 6.5 it comes with to make sure it stays closed at idle . Make sure there is no play in the accelerator arm , any play will cause a bog off idle , you may need to change squirter or cam if it stumbles off idle .

I know this is an older post, but I have the car street worthy now. The carb ran crappy for awhile, but I traced it back to vacuum leaks due to the rubber caps being dry rotted and cracked. Anyways, the car starts and idles nicely. There is one problem, however, and it is related to a hesitation/bog that occurs when it sees any type of acceleration event. I checked the accelerator pumps when the engine was off by traversing the throttle linkage, and it seems to squirt in there pretty nice and strong down the carb throat, so I'm not sure what should be checked next?

[Cooter]

Tuning a Holley to get over the dreaded "Bog" when you step on it is trick, but can be done. As stated, you need to sneek up on this one thing at a time. Extended Squirters, along with being larger can help, as well as Different Accel. Pump cams.(Little piece of plastic under accel. pump lever on carb, usually color keyed pink,green,blue,red,etc. This controls the timing of accel. shot).

Try one thing at a time to see if any difference.

[moparfreak]

Even in an acceleration such as coasting down on a residential street at 10 mph, and then lightly touching the throttle to get back up to 25 mph, the engine hesitates before picking back up. The question I have is, if it's squirting OK visually, could the accel pump still be the issue? Can I replace accel pump components with the carb still installed?

Thanks,
Adam

[XH29N0G]

Many of these things can be done with the carburetor still on the car.  Changing the cams is straightforward.  Changing the squirters is a little more tricky an depends on geometry and tools.  Care needs to be taken when changing squirters not to drop anything into the carburator.  I stuffed paper towel into the barrels.   For those changes that cannot be done on the car, removing the carburetor is also not a big deal.  I removed mine to do most of the work with squirters and jets just because it was much easier to work on. 

James

[moparfreak]

I drove it again yesterday evening to get more feel for it, and it seems like an off-idle type hesitation. Maybe a power valve issue? I had heard that intake backfires could potentially damage the power valve and while I was getting the car started up weeks ago I had a big flaming backfire w/ the A/C on, maybe that could have caused damage??

Thanks,
Adam

[XH29N0G]

Someone else will need to speak to power valves, because I don't know how robust they are.  My experience with my car was that the pump cam has a big effect, and that the squirter and jets also made a difference in not only the bog and backfire, but in the general performance. 

[Chryco Psycho]

the power valve has to stay closed at idle , in gear if auto , so you need to measure that & go 1.5 -2" below the vacuum reading you get , most carbs come with 6.5 power valves , if you are making over 8" in gear at idle leave it alone

[Ronnman]

Moparfreak - check the accelerator pump shot.  The shot needs to be INSTANT!  As soon as you breath on the throttle.  If it is not, you will experience hesitation.  You can review the procedure to adjust the accelerator linkage at the Holley site.
Ron

[moparfreak]

Ronman,

It looks pretty instantaneous to me, I think the squirter is right on. However, I can still stall out when I mash the throttle, which to me is a safety concern if it happens at the wrong time while in traffic. I will visit Holley's website to revisit the setting procedure for the linkage.

I'll measure vacuum per Chrycho's instructions below in gear, which should tell me if that power valve diaphragm is leaking.

Thanks,
Adam

[c00nhunterjoe]

Just because you see it squirting doesn't mean its squirting enough. Buy the accerator pump camand squirter kits. They are relativly cheap.

[moparfreak]

Only place I could find an pump tuning kit was Holley's website, and it's $125. is there a better source? I couldn't find a kit with just the plastic cams, I would think that wouldn't be too expensive.

Something I also noticed yesterday, I can have the car parked in idle, and mash the throttle and it won't stall/bog. But, if I'm out on the road, and mash the throttle then it will stall/bog. I'm still wondering if it's power valve.

Are there any good online or print tutorials out there for a full tuning exercise (home version w/o dyno, of course) on a Holley Street Avenger carb? This is my first time going through and doing it, and I'm assuming if I go through all the right steps properly to get the carb & the timing & advance set properly most of these drivability problems would go away.

Thanks,
Adam

[XH29N0G]

Others can chime in if I am off mark.  I think all that is being recommended is an Accelerator Pump Kits and Service Parts which is a selection of Accelerator Pump Cams like PART #: 20-12 which is $20-30 at Summit or Jegs or other places (for a start) and then adjusing the squirter nozzles.  These can be bought individually.  If you change the pump shot size to 50 ccc that is another matter.  If you get into Jets, then it is another matter, but they can be bought in large or small sets.  What I found with my application was that it was a combination of cam and jets.  I changed the squirter size, but then changed it back once I had the Jets set better and everything worked better in the end.

[c00nhunterjoe]

Fine tuning will take a little while but is well worth it. For most applications to just get rid of the bog buy the cam kit as stated, they are only a few bucks. On a mild low cmpression engine you can probably get away with the stock 30cc pump. You can go up several squirter sizes before having to switch to the 50cc pump. Try the cams 1st, if its still there go up 2 squirter sizes.

[cdr]

with the throttle closed all the way feel if there is any slack in the pump linkage where it contacts the springloaded bolt to the little pump lever,it should have no up & down play,if it does loosen the nut & bolt to make it longer by about 1 turn at a time to remove play