• 1 Vote(s) - 5 Average
  • 1
  • 2
  • 3
  • 4
  • 5
issue with tuning holley carb/ 351c running rough
hey guys finally got my car started up and running sort of. been tinkering with it for a few weeks and having issues getting it to run smooth

1.) im new to carbs, before my generation but im slowly beginning to understand the basics.

2.) my engine is a 351c 4v closed chamber heads comp cam hyrdraulic flat tappet cam. intake yadda yadda holley 750 cfm mechanical choke vacuum secondary carb brand new. everything in fuel system is brand new.

3.) after playing around a bit i get the car to run roughly with choke half open. as soon as i close choke completely car dies out shuts off. ive played with the mixture screws according to holley, all the way in. then 1.5 turns out. ok. pulled my plugs before the adjustment they are all carboned up. cleaned them put them back in. few days of messing with it. pull one again carboned up. ok so carboned plugs = rich mixture? my plugs are autolite 24. gapped at .35. now my timing should be very close intitally had it set at tdc and advanced it a bit. my timing light is broken and waiting on a replacement. but i also cant see how to see the timing marks on the pulley past all the accessories etc. most cars ive had ive tuned by ear and feel of throttle. ive tried advancing the timing either way a bit and no real changes occur. if anything doesnt run as well. if i give the car throttle and hold it higher seems to run a bit better but still some missfiring/ roughness. i have a mallory coil, msd plug wires, and mallory unitlite dizzy with new cap rotor and pickup module

4.) ive checked for vacuum leaks and cant seem to find any. ive put my hand over carb and engine dies out i was told if i had a vacuum leak it would smooth out and run ok.
You might switch out to the 1 step hotter autolite 25 plugs and try the 1 1/2 turns out on the holley, fine tune it from there with a vacuum guage.

351w - Ford racing GT40X 178 cc aluminum heads - Ford racing(crane) 1.7 roller rockers - Comp Cams 280H magnum cam .544" / .544" lift - ARP hardware - hedman longtubes - magnaflow exhaust with X pipe - Duraspark - MSD digital 6al box - MSD TFI coil - optima red top battery - tuff stuff 140 amp alternator - weiand stealth intake  - edelbrock 1406 600 carb  - march pullies and brackets - Be cool fan controller - derale electric fan - FMX trans - motive 4.11 gears - traction lok - lakewood traction bars.                                            

have you ever gotten to the point where the engine is fully warmed up?

what aftermarket intake is on the engine?

start with the basics, you need a compression check to make sure your not chasing your tail here. you will need the compression gauge to screw into each spark plug port and take a reading for all 8 to make sure you don't have a worn out engine which would run like hell.

vacuum gauge is a must.

timing light is a must.

the plug gap is .035" just to be clear .35 would be really big.

you know she starts so i doubt it timing.

If it was me, i would take those MSD ignition wires off and throw them in the trash and get an OEM set of carbon core wires. I have seen MSD wires do exactly what you are describing.

possible weak spark, how is the duraspark hooked up, did you bypass the resistance wire.

Did this car every run before you started swapping parts?

how old is the fuel you are attempting to start the engine with?

the carb could be miss adjusted with the butteflies too open requiring the choke to stay on to push more gas down the throat, blown power valve could also be in this case.
could be internally clogged up holley which would need a rebuild and flush out.

i'm leaning towards a weak spark right now, but you can have a combination of issues.

you can also still have a vacuum leak. for testing you will want to Cap off all vacuum accessories. if this is an A/C car cap off the vacuum canister. cap off the brake booster line from the carb or intake.
disconnect the vacuum advance(could be blown). automatic transmission cap off the modulator line. you can still have a massive leak at the carb base or the intake manifold base. however you are stalling the engine putting your hand over it.

possible spark plugs are contaminated. i would also install a fresh set of plugs autolite 25s gap them at .035 and retest after throwing the msd wires in the trash.
Everything on this car is new. I purchased the engine from another member who I've become good friends with was fresh from the machine shop. I put the heads on it etc. Gas is new fuel lines are new tank is new nothing got dirt in it. The wires I'm gonna replace anyways since they are way to long. I have a mallory coil with the built in resistor wired up correctly.
You checked the float levels right?

73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
I did not. Assumed it was set from the factory

Just checked float. Fuel is at proper level

I just read that mallory only should use carbon core wires. Are concourse ones carbon core? Where can I get a set of these
87fox72mach;244001 Wrote:I did not. Assumed it was set from the factory

Just checked float. Fuel is at proper level

I just read that mallory only should use carbon core wires. Are concourse ones carbon core? Where can I get a set of these

Vacuum in carb is 15 when idling
yes the stock wires are carbon core.


there were concourse wires sold by martiauto works as well
but i think he discontinued them

scott drake only sold pre 1970s wire kits.

I would also go with autolite 25s if you have any special plugs like duel spark, platinum, etc ditch them, you want just stock plugs.

If it is the same issue i have seen with MSD wires, what will happen is you put the new wires on, and check timing, go try and drive the car, the cylinder heads will clear out the carbon build up and your timing will change when you come back, recheck the timing you should be ok after that.

a volt meter to check the coil voltage would not hurt either.

Also new stuff isn't always good stuff. have a spare coil to try and swap in if you still have a problem.
I got 25s I will order those wires. Hope they fit my cap. I tried 2 different coil
In your original post you said, "close choke completely", did you intend to say "open choke completely"? Your combination is not going to run properly at any timing at or near TDC. Get the timing light, set the timing at about 16 degrees before TDC, limit total mechanical to about 36 and vacuum advance to 8-10 degrees. Get a vacuum gauge and set the "mixture" screws for highest vacuum at the rpm you want for idle. There is nothing magic about 1.5 turns CCW on the mixture screws. However, anything less than about .75 and the adjustment becomes very sensitive to any changes. What is the LIST number of the Holley and what is the current calibration? Chuck
Share Thread:  

Possibly Related Threads…
Thread Author Replies Views Last Post
  Hydraulic clutch issue Bruce180vf 6 156 05-26-2020, 05:35 AM
Last Post: Tnfastbk
  71 mustang 351c adding clutch fan 71Grande 24 1,587 05-24-2020, 10:31 PM
Last Post: jspears
  Need a good carb eventhorizon 5 214 05-20-2020, 09:18 AM
Last Post: Heavy Metal
  Engine running crummy jpaz 64 2,492 05-15-2020, 04:48 PM
Last Post: Stanglover
  New Holley Street AVenger 670 idling problems klinton994 26 662 05-09-2020, 08:27 PM
Last Post: klinton994

Users browsing this thread: 1 Guest(s)