Carb tunning/mod question

-
C

Cudafever

Well-Known Member
Joined
Sep 27, 2009
Messages
4,870
Reaction score
1,992
I have rebuild many a carbs but never changed or modified! Holley 750 DP. works grate but had to turn the curb idle screw up to the point that the air mixture screws don't do much for me. Has a 4.5 PV, will probably end up putting in 2.5 PV before i'm done. Idle vac is at 5" at 1100 rpm.(Remember, i live at 6600 foot elevation) Here's the question. Do i drill a small hole in the primary butterfly's? or, maybe all 4 butterfly's?Or, do i drill out the idle bleeds a few thou at a time?

I just realized, i should have started this Thread in Fuel and Air Systems . Mod's could you move this thread over there for me. Thanks.
 
Before drilling holes, with the carb off, turn it upside down, on the secondaries there is a stop screw, works just like the curb idle screw, start there. Back off the curb idle screw. See what happens.
 
Ignition timing, especially initial, first. That has to be right before messing with the carb.

What is the initial timing? If it's not at least 20-24 range, it doesn't have enough initial based on what I recall your build having in it.

Drilling butterflies should be a last resort to tune an engine as is used primarily on cars with not enough initial timing. Get the ignition sorted out. There are a bazillion threads on here about setting initial and total timing.
 
Thanks guys JBurch, don't you mean turn the secondary idle screw up as in opening the throttle blades?

crackedback My Initial is at 22* my advance is a pain in the asss, as i have to weld the slot up and file it back to change my Initial. 22*initial/40* total This carb was rebuild and all the throttle setting were set to spec. After initial start up the curb idle had to be turned up a lot causing my idle mixture problem.
I made a small screw driver so that i could turn the secondary idle screw up a little and in turn was able to turn the curb idle screw down, but it wasn't enough.
Should i turn the secondary screw up/open butter fly up more??? This is not a 4 corner idle mixture screw carb. only the primary are adjustable.
 
At what initial timing point does the engine kickback on your starter? That's where, or very close, the initial should be set.

It takes work to get it right, especially when using the old style mopar distributors.

You can open the secondaries and close primaries, buy, that can cause issues if they are both open too far.

Get timing squared away first.
 
When i went for 15 to 18 to 20 degree Initial you could here the eng respond in sound and rpm. When i went to 22 the rpm didn't seam to change much when i went to 24 (24 initial/42 total) My MPH fell off. went back to 22.
It has never Kick Back on me but when i had it a 24*, and warm, you could hear the starter struggle just a little when i first started to turn it over.

No vac adv hooked up, only springs. Comes in at around 1500 and is all in by 2500 RPM. The secondary idle screw was turned in until it didn't cause the throttle blades to bind at close, then was turn 1/4 of a turn. I then turned it approximately another 1/4 turn. I just when out and look/turned the idle screw. I'm 1.5 turn from closed throttle......Does that seam like to much???? Maybe i have a bad metering block??? I didn't pay attention to how many turns it took to set the fuel slot to a Square?

The other problem I'm fighting is i have 5" of vac and a 4.5 PV. guess i will start with a 2.5 PV, see if it will idle around 800 and then try to adj the mixture screws
 
Do you know where the car currently runs best for total?

Then set the initial where it should be and shorten the slots to hit your total number. If you twist more initial in and it picks up RPM, your initial is not where it should be.

You need BOTH parts to be right to get the carb under control at idle.
 
Rob's correct but I thought I'd add something in reference to one of your questions in the original post. You asked if you should drill holes in the butterflies or drill out the idle bleeds. Absolutely do not drill out the idle bleeds at this time. All that'll do is lean the idle and transition circuits out. It doesn't add air like drilling the butterflies.
 
Rick i wish you hadn't. always looking for ideas.

Today's plan is to get a much larger vac gauge, install a 2.5 power valve and go from there.

crackedback, 40* of total timing was my best MPH which it 22 degrees Initial. with my new vac gauge, i will us it to fine out how much Initial it likes and then try to restart it. then i know how much initial to pull out.

There was a thread....i thing on here, that told how many thousands the slot was = how much inital you had or how much of the slot needed welded up. I used this when i re-curved my distributor for my 340 but cant find it now........

fishy68, in my mind, lean the idle and transition circuits out is a good thing :???: But messing up a good working carb is the last thing i want to do!!!!!!
 
Too lean does't want to idle in gear, gives hesitation, and takes forever to warm
up. But too rich sucks, too. I've used aftermarket metering blocks w/replaceable
restrictions. Only complaint- power valve channel restrictions too small to drill to
what I wanted. But if you had one, you could dial in idle system(idle feed restrictioj
size-probably .002-.004 over current size) and do that to your original metering
block.
 
Drilling the "bypass" holes in the throttle plates is a good way to get control of your idle circuits if there is not enough adjustment in the secondary stop screw. Make sure you set the secondary stop back to base and start with about a 16th" bit. I like to locate the holes over the idle fuel hole to help atomize the fuel as it comes into the manifold with a little extra turbulence. Once you are able to cover the transfer slots and get real adjustment on your idle mixture screws it will be much easier to tell the effects of the recurve on your distributor.
 
Find max initial timing setting before you bust out any drill bits. You should NOT need to drill the plates with what you have. You may be surprised that you get control over stuff by twisting the distributor housing CCW...

Slot chart

http://www.forabodiesonly.com/mopar/showthread.php?t=229240

Timing almost always affects carb, not the other way around.
 
crackedback, thanks a bunch for that chart!!!!!!

Picked up a new large/non dampening vacuum gauge today and installed my 2.5 PV.

Twisted the dist ccw until i got my best vacuum, then when a little bit more and could here eng slow as well as vac drop. turned idle down to 900 and was able to use idle mixture screw......they work now, but still not happy with idle quality.

Pulled out timing light and timing was at 30*initial WOW!!!!! Stopped eng and restarted, no kick back.......REALLY? can i run that much initial timing with dist recurved to only have 10* advance???
Anything over 26 didn't change the vacuum gauge much but over 30 it lost vacuum

At 900 rpm I'm running 6 to 7 Inch of vacuum now.
 
Yep, get to work... LOL

Set it at 26-30* initial and 10-14 in the distributor and go run it. You might build up the inside edges of your slots if you did the outsides before.

Lower rpm and more vacuum is a good thing. Now that you have the idle mixture screws under control, you can get a decent idle tune up on it.

See what the engine pulls for vacuum in gear. That 2.5 pv may be worthless! :)
 
Thanks, I have a pos converter. in gear is 4" neutral is 6-7"

Set it at 26-30* initial and 10-14 in the distributor and go run it. You might build up the inside edges of your slots if you did the outsides before.
Click to expand...
Ok,What is the reasoning behind this?

30 degrees just seam crazy, but there are those that have there timing lock out at like 38 so i guess it will be ok.
New territory, ............

Did my first compression check, 175-185 psi, only one cyl was at 175
 
You must log in or register to reply here.
-

Back
Top