Carb help needed

[niles]

Justing getting it on the road. Carb very rich.
Have new 3110-4 750 cfm Holley, on cobra repli. 2200 lbs; 396 CI cleveland stroker eng; 4V heads @ 300 cfm. medium hot cam. Have O2 sensor installed so I can get A/F ratio directly. Float levels are good; float valves are sealing, when running electric fuel pump with sight screws out; fuel level remains right at bottom of opening.

Problem: out of box- very rich; idle screws will effect idle. idle - vaccum about 6 to 7 in hg. at 2000 rpms about 14 in. came with 6.5 pv and 72 jets. o2 reading of 850 to 950 mv. Plugs confirm rich - very black and starting to miss after about 60 miles.

changed jets to 70; same reading on O2 sensor. No better.
Thinking the PV was bouncing open at idle of low throttle; changed PV to a 4.0; Suprisingly went much richer; O2 is up to 1250 mv and fouls plugs in about 1 mi.

This has got me stumped; motor seems fine; no miss; scarey power at 5000 rpm; just very rich; about 7 MPG.

HELP

gn


HELP

[Tongue Pirate]

acc. nozzles could be too big &/or idle too rich. Remember, in a holley the engine will only pull about what it needs from the venturii's. Pushing the throttle squeezes as much fuel out of the nozzles as it can until the primary circut can catch up. Try smaller nozzle sizes.

[Steve Dickey]

niles,
Interesting read. I to am using a O2 sensor and volt meter to sort out my carbs. I'm curious as to how stable your mv readings are while driving. Mine go +/- 200mv and it is very difficult to estimate an average. As for the increase in mv when going to the 4.0 power valve, I have no personal experience, but have read that some power valves do not open anywhere near their rated vacuum. I suspect yours may be opening significantly higher than 4.0. I would either check it or try a new one.
Good luck and keep us posted, Steve

[HighPlainsDrifter]

Hi,
You need power valves that are a couple #'s lower than your idle vaccuum! you have 6.5 pv's so they will open and dump lotsa raw fuel in at that 6.5 vaccuum point. GO down to 3.5 power valves.
Perry.

[DaveR]

niles,

I don't know how to help you, but I am planning to install an O2 sensor and A/F gauge from Summit in anticipation of problems similar to yours. Where did you insert your O2 sensor?

[niles]

perry: I have already gone down to a 4.0 PV and it went richer.

Dave: if your using a "cheap two wire-unheated" o2, it needs to be very close to the head so that it stays hot. Mine is about 12 in from the head. Just weld in a spark plug bung.

Mine seems fairly stable; I would estimate +/- 50mv with no throttle change, when you move the throtle; maybe a 100 mv swing.

I'm going to try another 4.0 pv; hopefully the initial 4.0 is defective.

Guys, keep the suggestions coming; it's so rich I'm afraid to go more that few block for fearing of fouling the plugs so bad , it won't get me home

gn

[Mr.Fixit]

What is your idle RPM and vacuum reading?
Jets won't really affect idle mixture. That is done with the idle mixture screws and sometimes by modifying the airbleeds (on top, next to the boosters) Does your Holley have 4 corner idle circuits, or just the front two barrels?

[BB427]

Check the idle circuit air bleeds on the top of the body next to the acc. pump squirters. Something may be blocking them or they may need to be opened up larger. Mabye .002 or so. (not much)
This can be done with a small wire bit by hand.

What position are the throttle plates in and are the boosters dripping fuel when at idle?
As far as jets go , if you look in the Holley book a great number of jets actually have the same size hole. ???? I have never understood this. you may make a change and not really make a change at all. Buy good aftermarket jets that areflowed and checked then you can make proper adjustments.

[BB427]

Sorry forgot something. Holley is famous for the surfaces of the metering plates and the main body being off. When I take a Holley apart I always sheck it with a light and a billet straight edge. If it is off I place a piece of sandpaper on a piece of plate glass and sand it until the marks are even across the surface. The idle circuit and intermediate circuit on a Holley can be quite troublesome.

[niles]

Mr Fixit;
Two point idle.
At 1000 rpm, vaccum is bouncing between 6 and 7 in.

gn

[Mr.Fixit]

When you close off the idle screws, the motor does want to die, right? How many hurns "out" are the idle screws 1.5 turns is a good starting point for a two point idle on a holley. Try a squirt of carb cleaner into the idle bleeds.
While the car is idling, if you cover the idle bleeds, the idle mixture goes very rich.
You don't see fuel being pulled out of the boosters during idle do you? Has the idle RPM adjustement screw been turned so far as to move the throttle blades beyond the tiny slots in the baseplate?
A carb will not have ideal stoichiometry at all rpm's, you try to get as close as possible, without going lean during WOT.

[HighPlainsDrifter]

Hi,
I would think that if you went richer replacing the power valve then you have a defect or blockage in the metering plate. OR the accerator pump levers are binding, check that you have 20 thou clearance from lever to adjusting screw on both primary and secondary bowls.
Good Luck,
Perry.

[K.Wilson]

Niles,
Based on your description I would get a 3.5 power valve (not the high flow style). However, before I removed the carb from the car I would verify that the idle screws do make the engine stumble (slow down) when you turn them in (clockwise). I would also spray some carb cleaner in the idle bleed air holes with the engine running and see if they are plugged (the engine will stumble when the carb cleaner interrupts the air flow). If it does not there is something blocking the air bleed holes and you need to find out what it is when the carb is off.
If you do not have a power valve blow out preventer now is the time to install one. Also, check to see that the rear throttle plates are closed at idle, if the rear plates are open even a small amount (more than .010) that airflow can be a problem.
You need to check the acc pump clearance at idle (bolt head to lever at least .010 but not more than .020).
Your vacuum at idle seems low even for a big cam. Have you checked for a vacuum leak? Are you running a PCV valve or is it the correct one? (it may be too high a flow) Is there a lot of fuel in your oil? ( give the dip stick a sniff, if you have been running real rich the oil may have enough gas in it to be a problem)
I would not put to much faith in the O2 sensor for now, you need to get it running right so it does not foul the plugs and foul the O2 sensor before you can trust those readings. The sensor must be clean and up to temp. (HOT) before it is accurate.

Hope this helps, Keith

[George Snyder]

From Holley Tech Service.
Hello
The first thing that you need to do is adjust your float level, no fuel
should run out the sight plug with the engine running. and check to make
sure that your fuel pressure is not over 7 PSI. I suggest 6-6.5 PSI Make
sure the acc. pump is properly adjusted. It should be at zero lash at
idle. and have at least .015" gap at Wide open throttle with the pump
lever pushed all the way down. Then try turning in the idle air screws
all the way in with the engine running.
If the engine continues to run with both screws all the way in, then
thecarb is letting fuel into the engine from someplace other than the idle
circuit. We have to find out where from and correct it (with the screws
all the way in the engine should die) Next you want to check the manifold
vacuum with the engine idling in gear. This will tell you which size power
valve to use. If your vacuum is 9" and under divide by two, and that is
the size you need. If it is 10" and over use a 6.5 PV. If the carb
backfired. If it did it probably blew out the power valve, Unless the carb
is 6 years
or newer and has power valve blowout protection. You will have to replace
it with the correct size. If you get all of this corrected and the car
still
runs rich then you will probably have to remove the carb and adjust the
secondary throttle plates open to the bottom of the transfer slots. This
will allow you to close the primary plates down some. If either plates are
open too far then fuel will be pulled in to the engine from the transfer
slots in the base plate. By adjusting those plates like I said, the carb
will get the same amount of air at idle, but it will get it evenly from
the
front and rear instead of just the front. Once you get it so the engine
dies with the idle air screws, then you can continue with the rest if the
adjustments. Next you need to adjust the idle air screws. Turn them in
until they seat lightly then back them out 1.5 turns. Start the car and
let it warm up. Hook up the vacuum gauge to manifold vacuum. Check it at
idle in gear. Adjust the idle air screws evenly until you get max vacuum
(the vacuum will drop off if you go to rich or lean) With that done you
can start working with your jetting Unfortunately I can't tell you how to
jet it. Each engine is different and has different requirements. It is a
trial and error process. The only thing you can do is adjust them until
you get
decent drillability, and then read the spark plug color to find out which
way you need to go. Black is rich and white is lean. Adjust it until you
get a nice light brown color on the porcelain of the plug. when you get
this adjusted . If you have a off line hesitation that is less than a
second Increase the shooter by one size. If it is over a second increase
the shooter by Two. If it makes the hesitation better you can increase
accordingly if it gets worse drop the size.
Thanks Tom

[niles]

Thanks guys for all the help. Here's the results of two days and six carb tear downs/reasseblies, etc.

between 6-7 in vac at 1000 rpm; it goes up to 8-9 at 1200.

idle screws responsive; starts killing eng at about 1 turn; max vac is at about 2 turns out.

secondary blutterflies were almost fully closed. primaries were open mid-way up the slot to get 1000 rpm idle.

the third Holley tech suggested the following systematic approach that I worked today. If you have black fuzzy rich plugs with a new carb; something is wrong and extra fuel is leaking into the engine where it's not suppost to. He suggested to warm up and take off the prim. block. The cavity behind the block where the big end of the PV is should be DRY. That a vaccum cavity. If it wet you have a crack in block, bad gasket or bad PV. I check it was wet, gaskets looked good, changed to a second PV. Went from fuzzy black rich to white lean. Looks like the new 4.5 PV was defective.

want to still explore the vac leak area because still running very rough from 1000 to 2000.
What is the best way to determine in you have any leaks around the intake manifold?

Comments/ ideas appreciated

gn

[chuckbrandt]

I've cracked open a propane torch (not lit) and ran it around the intake and carb. Supposedly if you have a leak the idle will increase when you suck in some propane.

I'm not sure how effective this is for in the valley vacume leaks.

But it is a lot less messy than spraying carb cleaner around your intake.

Chuck

[niles]

Guys Thanks for the help!

turns out it wasnt a carb problem at all. It turned out to be a bad distributor setup. I set the timing at 35 deg max and didnt check the idle advance. I assumed(you know what that implies) that the "custom built" distributor would be right!

Well the distributor only advanced to 5 deg at 1000rpms, allowing the vaccum to drop to 5-6 resulting in an unstable idle, vaccum and the inablity to get the carb to work right. The dist. had a dynamic range of 30 deg; instead of the needed 20-22 deg. Turning up the initial timing(low end)up to about 15BTDC cured all the low rpm carb/running problems. yes I know the resulting 45deg advance is too much.
Such an easy/obvious fix; yet so hard to diagnose.
gn

[aumoore]

Niles
I was advised against running a vacume advance distributor in a performance motor. I am running a Mechanical Advance Mallory Unilight and it works like a charm. The vaccume advance is basically for fuel milage and I can afford to loose a few mpg in my cobra as I do not drive it that much. What Distributor are you running? I had bad experience with the old ford distributor in my old 351 Boss Cleveland. The vaccume advance kept getting where it was throwing the dwell off due to inaccurate bushings. You guys ever set points? Old school.

I am running a vac secondary 600cfm holley. The Holley is a temperamental little b!^@h but works fine once you adjust everything.