patricklav

iTrader: (7)

Hello,
I am trying to get my recently rebuilt engine to run properly.
As its currently set up it struggles to start and once it does it pings at lower rpm and when throttle is opened up it will occasionally backfire out the carb. I am not exactly sure how many degrees its advanced but I would guess about 40 degrees. If I start retarding the timing it will start running sloppy and eventually stall. The car also won't start unless it advanced to about the 40 degree mark. Once its running if i pull the vacuum advance it will immediately want to stall. The carb is a holley 750 that was recently rebuilt.
When I pulled a plug after it was running for about 10 minutes it was all black and fouled.
Anyone have any ideas on what I can try to get her running good?

Here are my engines details
350
Vortec heads
286h cam
Holley 750. #70 jets 5 lbs fuel pressure
Stock hei distributor
Accel plugs gapped at .45

Ransford2987

Do the plugs look black AND wet? That might be a fuel issue. I remember the break in for a 350 I once had and the symptoms were the same you have. Turned out I had too much fuel going into the carb. The pressure wasn't high, but the flow itself was. What fuel pump are you using, and have you adjusted anything on the carb itself? How does the exhaust smell?

I'm not saying this is your issue, but it might be a good place to start.

patricklav

iTrader: (7)

The plugs were black but weren't wet. they were extremely hot, even after 45 minutes. I'm using an in-tank fuel pump with a holley return style regulator, currently set at 5psi. The carb was just rebuilt and doesn't appear to have any flow issues.
The exhaust smells awful, like its running extremely rich

bwiencek

Were the vortec heads cut for the higher lift cam?

Double-check the firing order and conditon of your cap/rotor/plugs/wires. If everything checks out try backing off the valve lash - I've helped several guys that had a hard start/rough run/backfire problems on fresh engines solve it by backing off the lash on the cams - they went way too far in the preload adjustment (they went until the hydraulic lifter bottomed out thinking that was '0' lash and then added the 1/2-3/4 turn on top of that so the valves were barely hanging open)

patricklav

iTrader: (7)

The heads were not cut. I bought A valve train setup that allowed me tk skip doing that. Untried your idea of adjusting the rocker arms with no luck. I also moved the dizzy over another too to see of that would help. Now I can't l get it to start at all.

Orr89RocZ

iTrader: (20)

What jets and idle screw settings are in the carb? I am not a carb guy at all but black sounds rich or just not getting good spark for proper burn, but then i think it would be wet. High timing can help it run and start when too rich sometimes. Especially on a cold start. My motor did same thing, base tune was rich and wouldnt start without alot of timing

bwiencek

Go back to the beginning - pull the #1 spark plug (or valve cover) and rotate the engine over until you're on the #1 compression stroke and then bring the engine to TDC. Make a mark on the base of the distributor that lines up with the #1 terminal on the dist. cap. Then pull the cap and the rotor should be pointing right at the #1 terminal. If not pull the dist. body and rotate the rotor until it lines up with the #1 mark you made when it's stabbed. Once you have that look down at the reluctor wheel and line the "V" of the shaft so that it's just past the pickup on the body - that should get the timing really close to start.

Pull the plugs, clean them off with a propane torch and then re-install them.

Then turn the key and fire it up - should fire right off with that setup - if not you've got a fuel delivery problem - try disconnecting and plugging the fuel line to the carb and see if it'll run on what's in the bowls good once it starts - if it does you're dumping excess fuel somewhere (i.e. floats too high, float sunk, needle/seat bad, etc.) - if it still doesn't run right for a little bit off the bowls then you've got a bigger carb problem.

patricklav

iTrader: (7)

thanks for all the replys so far.

Got back to it today - I did as several had mentioned - restabbed the distributor, cleaned the plugs and loosened the rockers some. Idle mixture screws are initially set at 1.5 turns.

It did start, and seems to idle better but not where I would feel comfortable on the street with, as I had to set the idle screw to keep the rpms up a bit. I ran it for about 10 minutes, the engine seemed to respond best at about 38 degrees advanced. I was getting about 12in vacuum until I then closed the idle mixture screw .25 clockwise and retarded the timing by about 10 degrees and was then getting about 15in vacuum. it seems like the gauge is jumping quite a bit and isn't steady.
Once I shut it off I back an extremely loud backfire out the exhaust pipe.

So now, im backfiring out the exhaust and not the intake, which I guess is good, but im still missing something to get it where it needs to be.

Any other ideas?

thanks Guys!

sofakingdom

iTrader: (1)

Backfiring out the exhaust = unburned fuel in the pipes

Either the thing is RIDICULOUSLY rich, or you have cyls that aren't firing.

Double-check your plug wire order. Pay particular attention to #5 & #7.

patricklav

iTrader: (7)

I rechecked all the wiring and they are correct. I also hooked the timing light up to each wire and each is firing.

I'm using #70 jets right now; could these be too small for my setup? The carb was recently rebuilt, however I did have some backfires out the carb which may have blown the power valve (currently a 5.5). Is there a way to test the power valve before I replace it?

thanks

sofakingdom

iTrader: (1)

Whaddya think?

It's possible to apply suction to it and see if it leaks; but for as CHEEEEEP as they are, it's hardly worth dinking around with "testing". Best strategy would be to simply replace it, and while you're at it, install the "blowout preventer kit" in it.

patricklav

iTrader: (7)

I meant , are the #70's two big, not too small. I'm sure they are too big, but any suggestion on what may be ideal for this setup? Would dropping to #68's make any difference?

sofakingdom

iTrader: (1)

There's no way to know, other than trying it out.

Read the "Holley Tuning" sticky on the Carb forum. It talks all about that, and describes an orderly, logical, step-by-step, process for isolating the various systems of the carb and tailoring them to the engine's tastes.

JaBoT

Follow the holley tuning sticky. Also what jets and powervalve are in it now all around? No, 70 jets are not really big.
Since you said you are popping in the exhaust and the vacuum gauge is jumping around a lot I would look for a dead cylinder. Or your timing is really retarded or it's just dumping fuel.
That's a weird cam selection for vortec heads. single pattern and .560 lift. I hope you had a machine shop set those heads up right. Even with the alex parts spring kit you can only go up to .550 lift.
With the stock hei, did you change the weights and springs? With the stock springs it still advances past 4000 rpm. If you set it to 34 at 3000 you could easily be over 40 up top.

patricklav

iTrader: (7)

thanks Guys for the help with tuning her up, she is doing much better.

Or was I should say, now I have a 'no spark' issue. always something

Jonesyfxr

iTrader: (2)

I had an engine I was working on and had a hell of a time tuning the carb. It ended up having a slipped intake gasket....sucking through the crankcase.