Running rich....To big of a carb or?

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Dusterdude72

Dusterdude72

IN MOPAR MUSCLE MAGAZINE
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Hey guys,

Before I get into this...I hope it doesn't turn into a big debate over brand preference. Just looking for some helpful info to guide me on what the issues might be.

First of all...1972 plymouth Duster, has a 67 318 engine,bored .030 over to a 323 , purple shaft cam (not sure of specs...but has a mild thump to it). Dished pistons,long tube headers,eddy performer dual plane intake,2.5" true dual exhaust, proform electronic ignition, 2500 stall converter, transgo tf2 shift kit,4.56 gears out back,28" tall tire. Street car mostly for cruising....may see an occasional day at the track.

I am currently running a new summit racing 750 cfm electric choke vacuum secondary 4 barrel carb.( basically an old holley design) ( this carb http://www.summitracing.com/parts/sum-m08750vs ).

I know some of you might be saying "to much carb"....but I have seen some running as big or bigger with great results (even on smaller cubed/somewhat stock engines)......but this is why I am here....for some knowledgeable info from the carb guys....not assumptions.

Here is my issue, It starts great, it runs great,it idles great, it revs up great, It comes on like a bat out of hell. BUT it runs realllllllly rich! It will burn your eyes while its running, puffs black smoke out of the exhaust while running ( mostly at idle....but does it during a rev also) if I rev it up a few times and then get out of the car and look behind it I can see wet patches behind the exhaust on the ground where un burnt fuel sprayed the ground (it will also spray anything behind the car). I pulled my spark plugs today and every one of them was pitch black and reaked of fuel. I cleaned them up and re installed them (they only have a few hours of run time on them over the last couple of years since I have been restoring the car).

I am trying to work out all the bugs in the car and get it finished before I deem it road ready (restoration is about complete).

My timing seems to be good, starts good and runs great, my ignition system is all new ....I dont think it is spark related.

My question is....is this just purely because this is to much carb for my engine?.....or is this something I can tune into proper working order?.....if so.....what should I do?. I tried adjusting the floats once before and I got them to about the middle of the sight glass on each feed. I have tried balancing the carb out before...I still cant get the rich issue to go away.

pretty embarrassing when you hop in your nice restored "new" car and fire it up and it looks like a tired jaloppy puffing smoke out the tips lol.

Id hate to go and buy a new carb if I can no doubt fix the issue some how instead.

again....I know this can be a touchy subject....I know everyone like certain brands and can list of part numbers for all sorts of carbs they recommend. But I dont want it to turn into a debate thread lol. just looking to see if my current set up can be tuned in or if I should be looking at down sizing to a lower cfm carb. It runs great is the thing....no bog or hesitation. it just runs rich....I also am worried that if I downsize carbs to say a 600cfm to get rid of a rich condition...that I will loose some of that seat of the pants power it currently has.

sorry for the long post everyone. Just trying to explain it all to try and get an accurate as an answer as I can. Thanks in advance
 
sounds liek you need to find someone who tunes carbs...

lightly seat the idle mix screws and back out to 2.5 turns.

what jet is is it?
 
Ive already went through the steps on adjusting the mixture screws....still no luck.

Here are the factory carb specs " Fuel:Gasoline
New or Remanufactured:New
Number of Barrels:4
Carburetor Flange:Square bore
Choke:Electric
Secondary Type:Vacuum
CFM:750
Fuel Inlet:Dual
Primary Jet Size:77
Primary Power Valve (Hg):9.5 Hg
Primary Discharge Nozzle Size (in):0.035 in.
Secondary Jet Size:79
Secondary Power Valve:6.5 "
 
Timing is good... always a favorite comment of mine.

What is it and what is the initial and total split?
 
I run that same carb on my 318 except in the 600cfm. It runs great. Maybe too much carb.
 
Mechanical fuel pump? If so, do you have a fuel presure gauge? I was running a Carter mechanical fuel pump and itwas running rich, eye burner. Found out he pressure was at 9 psi, way too much for a holley. Had to put a fuel pressure regulator on it and adjusted it to 6.5 psi. No mo richness.
 
You need to re-jet the carb to run leaner. Then re-time the engine. Purchase a jet kit from Holley a d start tuning.
 
Mostly same with everyone else here....

Get the timing squared away first and foremost. Plenty of threads on the board about this. Are you pulling 19 inches of vacuum at idle? Your primary PV should be half of that, if not you may be dumping fuel from that circuit. Do the idle screws have authority? Will the engine start to die when screwed all the way in? It should. Then adjust out to highest vacuum or highest idle and stop!! there. Could be a turn and a quarter, but just don't set to a definitive rotation count. Jets seem a little large! I am running a radical 408 with a 750 HP DP and don't have much larger jets than you currently have. However, at idle you should not be pulling from the main feed circuit. Take one thing at a time and fiddle with it. You'll get dialed in. Also, I am not familiar with the Summit unit. Does it have transition slots like the Holleys. If you are not getting the authority with the idle screws, then the slots are too open at idle and you are feeding fuel from this circuit. You should be idling off of only the idle screw circuit. Check into that. Holleys have 5 fuel sources, idle screws, transition circuit (just off idle), Power valves, Accel. pumps and main feed (Main Jets.)
 
I don't think it's too much carburetor. It may be too much jet. But the carburetor can certainly be tuned to run on that. You didn't specify compression ratio or what heads.....unless I missed it. So, with dished pistons, I would have to guess 8.1 at the most. That doesn't help burn any additional fuel, but still, I really think the carburetor can be tuned. However as others have said, I would really make sure all of the other factors are in top tune first and then look at the carburetor. Might even look at the plugs. You sure they are the right heat range?
 
aengineguy said:
Mostly same with everyone else here....

Get the timing squared away first and foremost. Plenty of threads on the board about this. Are you pulling 19 inches of vacuum at idle? Your primary PV should be half of that, if not you may be dumping fuel from that circuit. Do the idle screws have authority? Will the engine start to die when screwed all the way in? It should. Then adjust out to highest vacuum or highest idle and stop!! there. Could be a turn and a quarter, but just don't set to a definitive rotation count. Jets seem a little large! I am running a radical 408 with a 750 HP DP and don't have much larger jets than you currently have. However, at idle you should not be pulling from the main feed circuit. Take one thing at a time and fiddle with it. You'll get dialed in. Also, I am not familiar with the Summit unit. Does it have transition slots like the Holleys. If you are not getting the authority with the idle screws, then the slots are too open at idle and you are feeding fuel from this circuit. You should be idling off of only the idle screw circuit. Check into that. Holleys have 5 fuel sources, idle screws, transition circuit (just off idle), Power valves, Accel. pumps and main feed (Main Jets.)
Click to expand...

I'm on board with these thoughts. If you've got a big cam and low vacuum, it can be touchy and (in my opinion) may never "not" burn your eyes @ idle. Too big a carb?......no, it's all about the jetting and power valve and if you're actually on the idle circuit.
 
Take a vacuum check at idle . Divide that by 2 and put that number power valve in. Also remove the Idle circuit air bleeds and thread the holes for replaceable air bleeds. Using a larger air bleed will lean it out at idle. The circuits are a sideways T The leg at the bottom sucks fuel. The leg at the top sucks air the more air it sucks the less fuel it gets. Works like putting a hole in a straw. Most 4 corner holleys come with replaceable air bleeds

1. shows idle air bleed
2. is the high speed

This is on my carb it came with the but I did many like yours including the 500 2bbl on the SC 2.2 below. I also had to boost reference the power valve for boost.

These can be purcheced at any speed shop. very east job and worth the time. do all four it makes tuning a snap . leave the jets as they came. They would be the maximum size for that CFM. air bleeds fine tune without removing the carb after this mod is done
 

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Sorry guys, on my phone right now so I'll try to answer some of the questions as best as I can.

I have not checked fuel pressure yet, it's a factory mechanical style. But I would assume its not a pressure problem because it seems like I'd have issues with flooding my bowls... And I can look in the sight glasses and the fuel level is sitting right in the middle of each of them.

Spark plugs are what the engine called for... Champions( can't recall part number off hand ryc14 something or another I believe). Gapped at .035 as per recommendation.

I am not sure of compression ration (previous owners did most of the engine work. The heads have been shaved some and the block as well I believe. Factory 67 318 heads. Can't remember off hand the valve sizing, I'd have to dig through some of my old build thread to refresh my memory on that.

I did some tweaking on the timing and it seemed to like running closer to zero or at lower numbers (faster throttle response). But still had the rich fuel issue. I currently have it set at about 15.5-16* base at idle. And at 2500 rpm it peaks at about 34*.

With my vacuum connected ...at 2500rpm it peaks at about 49*.

I currently don't have a vacuum gauge so I haven't been able to measure vacuum.

My thing is ....... Do I spend the money and time on a jet kit and power valves and doing little tricks to it just to get it dialed in (if I can get it dialed in). Or would it make more sense going with a lower cfm carb and selling my current carb to recoupe the money spent on a new one (or atleast a good portion of it).

I think my timing and ignition system are good..... So it's got to boil down to being to much fuel.... I just can't get it to stop running so rich (atleast from basic tuning). I've tried adjusting mixture screws, tweaking the idle,tweaking the floats, messed with timing. Etc...etc.... Still no luck .

Also it does not appear to have the slots in the barrels like was asked in one post.

Any ideas guys?
 
Try this, turn the initial timing up until it lugs the starter, then back off 2* and retune the carb at that point. Don't worry about total at this point, get the idle squared away. It might clean up that rich odor a lot.

I run stock 340's at 14-16. A 318 with any type of cam is likely going to want more than 16.
 
cudavert said:
DOES it shut down with the idle mixture screws turned all the way in?
Click to expand...

Yes I can turn the idle mixture screws in and cough the engine out.

However, while running at and idle around 750-800 rpm. If I turn the mixture screw in until it stumbles, and then back the screw out a quarter turn and repeat the same step on the opposite mixture screw. It runs good.... But I checked the position of the screws and at this point both screws are a half turn away from being fully seated. Not sure if that's right or not. But if I back them out any further than a half turn than i believe it will richen the mixture even more. And I'm aiming to lean it out.... Not richen it up
 
Also, look down the barrels while running. Are you getting any dripping from the boosters? You should not have any and this would mean you are pulling fuel from the main jet circuit. Check what cudavert said on the idle screws. Granted you probably are a little big on the carb, but I believe you can tune out of it to some degree, especially if you are getting crisp giddyup.
 
xMOPARx said:
Yes I can turn the idle mixture screws in and cough the engine out.

However, while running at and idle around 750-800 rpm. If I turn the mixture screw in until it stumbles, and then back the screw out a quarter turn and repeat the same step on the opposite mixture screw. It runs good.... But I checked the position of the screws and at this point both screws are a half turn away from being fully seated. Not sure if that's right or not. But if I back them out any further than a half turn than i believe it will richen the mixture even more. And I'm aiming to lean it out.... Not richen it up
Click to expand...

See post # 16 above.
 
With vacuum unhook and plugged, I advanced the distributor to the point that the starter labored almost like a dead battery. I fired it up and at about 800 rpm I was getting a reading of about 45* . I backed it down to 43* . Seemed to start ok and run ok. It may have been ruining a little leaner. But holy advance!. Lol.
 
IMO, if you want some curve in the distributor, put 24-26 initial and the rest in mechanical.

If it will start with 43*, you could lock it out as well at 34-36 or whatever.

You could also see where initial falls using a vacuum gauge.

Make sure the fuel level is correct, no dripping of fuel in the boosters, no excess fuel pressure
 
Well thanks for all the help so far guys.

It's looking like even if I dial the timing in its still going to run rich, I've messed with the timing and retreating the carb over and over and still no luck.

Looking like to much carb for the car and I'll either have to do a bunch of tuning to it or downsize to something like a 600cfm.

What do you think?. Am I right in my thinking on this?.

I just don't want to over do it on my timing to try and band aid a fuel issue.

If I can get a carb that's more suited for my application and paired with a good base time I'm thinking that will fix the issue.

But then again.... I posted this thread for the opinions of those who are more knowledgable on the subject than myself lol
 
Yes, it probably sounds like a good plan. But make sure you check all the other issues, and it pretty much sounds like you have been doing this, before you proceed with the smaller carb.

Good Luck!!
 
A carb is never to big. and the jets are usually right. Air bleeds and power valve is all you need to change. I have run a 850 on a stock stroke 340. A six pack is 1300+ on a stock 340. Air bleeds are your answer.

Timing could be locked a 36 and it would run the same. Big cam/ motors make less vacuum so big carbs work with the original bleeds. a small cam/ motor makes more vacuum so you need to open up the bleeds for lower RPM mixture with a bigger carb.

You could add an X pipe . That should suck the extra gas right through. LOL
 
Jetting any carb to the intended usage is a good deal. Its the difference between a car of cobbled together parts out of boxes or a fine tuned machine. I would be willing to bet 90% of the professional mechanics on here have jetted their carb to their car. The other 10% have fuel injection.lol I run a 1150 three circuit Dominator on my car. Really easy to see what difference jets and airbleeds make.
 
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