i need help on where to start! (carb setup)

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zinser72

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this is my combo:
'72 318, .030 over, kb167's zero decked, scat i-beam rods, forged crank, fully ported RHS heads with 2.02 intakes, comp xe285hl cam:

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=632&sb=2

10.1 static compression, solid lifters, roller rockers, 978 dual springs, msd street fire box with electronic ignition, rpm air gap intake and the suspect problem, a holley 750.

the problem:
i broke in the cam no problem and added the inner springs. whats happening now when i try to set the idle it wont run under about 1600 rpm. its acting like its running out of fuel because if i touch the throttle the accelerator pump squirts and it revs up for a couple seconds then starts to die again. timing was at about 22-25* at 2000 rpm and plugs looked new after a half hr breaking in the cam. the carb is from a fellow member. its a holley 750 dp with mechanical secondaries. it has billet metering blocks and a proform main body so no choke and 4 air bleed screws. im tearing it apart tonight to see whats in it but theres so much going on with this thing i dont want to just guess and check.

any help on a good baseline setup so it will at least idle and be driveable?? thanks in advance for the help, looking forward to finally getting it back on the road after a long winter
 
Have you got any vacuum readings? Possible vacuum leak?
 
Solid lifters on a XE285HL? That's the first thing that jumps out at me. How did you lash them?

Make darn sure you don't have any vacuum leaks, double check float levels, etc.

If you have 22* at 2200rpm, you don't have enough initial timing in the thing is my guess. It's going to want about that much timing at idle speed. It's already in the advance at 2K rpm. Put a timing light on it and see if it's dropping timing out as you reduce rpm. if it is, keep turning it back in as you reduce idle speed. See the bottom paragraph for another way to do it as well.

If it runs at 2000 rpm and no vacuum leaks, turn the timing up at that point to about 35*, then see if you can adjust the idle down.
 
lash was set at .008 from what i was told by comp (yes they know my cam/lifter setup...they recommended it to me.) i was also told by keith black not to exceed 32* total with those pistons. im still learning and i trust real-world experience more than engineers though. just trying to figure it out, i will check for leaks and mess with the timing. it wanted to backfire on startup today so i pulled timing out, was that wrong? float levels look good, at least in the sight glass...primaries a little above half way. oh and no vacuum readings yet, im going to mess with it now and will report back later. thanks for the input so far!
 
i adjusted the timing and it helped a little. still wont idle and i couldnt find any vacuum leaks. vacuum pulses with engine between 5 and 10 at about 1500 rpm (lowest i could get it to run) and raises to 15 at 2000 rpm. throttle response is sluggish and the rpm lingers a while before dropping fast after i give it a little pump. i took apart the carb and this is what i found:

mains 73, secondaries 82, power valve stamped 45 im assuming this is 4.5 in/hg? all 4 air screws are 1.5 turns out. secondary jets are on standoffs, primary throttle plates are drilled either 1/8" or 3/32". primary bowl level is 2/3 up sight glass and secondary level is halfway.

to me it sounds like its rich so i dont know how its acting lean. i would upload pics but i dont know how to post them here. any ideas would be greatly appreciated, im frustrated and confused :banghead:
 
Your jetting and PV are within a reasonable range. Floats are high in my opinion......what's your fuel pressure?

Reason I ask on the fuel pressure is, if it's within the 5/6lb range, I think your actually lean at idle with the high float level covering for the drilled primary plates. If that's the case, I would lower the float level to the bottom of the site's and open the A/F screws a 1/4 turn and try to lower the idle screw. If it's better, back the screws out another 1/4 and try to lower it again. If there is continued improvement, then you found your issue.

Once you can get the idle down enough, you can work with the initial timing to get it where the engine is happy. Trying to play with the timing numbers idling in the 1500+ range really isn't gonna work, unless your only concerned about the total.
 
no idea on fuel pressure, no way to tell either. i do know its running a mechanical 340 high volume pump and i think -6 AN fittings and line. it ran fine last year until the motor went down due to oiling issue. it had a different carb and was mostly stock though. sorry i dont have more info

do you think a high float level mess with anything to cause my issue? they can be adjusted right on top of the bowls correct?
 
zinser72 said:
no idea on fuel pressure, no way to tell either. i do know its running a mechanical 340 high volume pump and i think -6 AN fittings and line. it ran fine last year until the motor went down due to oiling issue. it had a different carb and was mostly stock though. sorry i dont have more info

do you think a high float level mess with anything to cause my issue? they can be adjusted right on top of the bowls correct?
Click to expand...

No problem. Yes you can adjust the floats up top. I added some thoughts to my previous post. Check it out and see if it sounds like i'm on the right track?
 
Not trying to be confusing...:D. But you're either too rich...High floats or too much pressure. Or your too lean...Too much air with the drilled plates.

Just need to figure it out, one way or the other. We just need to get that idle down to be able to work with the initial timing settings.
 
crackedback said:
If you have 22* at 2200rpm, you don't have enough initial timing in the thing is my guess. It's going to want about that much timing at idle speed. It's already in the advance at 2K rpm. Put a timing light on it and see if it's dropping timing out as you reduce rpm. if it is, keep turning it back in as you reduce idle speed. See the bottom paragraph for another way to do it as well.

If it runs at 2000 rpm and no vacuum leaks, turn the timing up at that point to about 35*, then see if you can adjust the idle down.
Click to expand...

You may be right on this. If it's a quick curve, then I could see the initial being 0* or less.

My main concern was the floats up in the sites pretty good, and the blip of the throttle responding, but nothing after that. Just sounded lean too me.....who knows at this point.
 
you said it ran fine last year with a different carb, do you still have that carb?? I would try that even if it's smaller and see if you can get the idle down so you can set the timing, then try the 750 just an idea...
 
Set the throttle blades to expose about .025 of the transfer slots when you look at them with carb upside down, then screw in all 4 idle mix screws till they bottom out and then unscrew them all out 3/4 turn each...then disconnect the vac adv if u use one at all.and plug the vac line at the carb.... then roll the motor over with a breaker bar with the key on and have a friend with dialback timing light set to 20 degrees watch for the flash as the balancer lines up with zero mark. Do that and adjust the distributor till you get the 20* initial. Snug the disttibutor down some and then start it up and see what u get..
 
.008 sounds really tight on valve lash. I have never heard of running that tight. I'm at. 022 on my solid cam
 
younggun2.0 said:
.008 sounds really tight on valve lash. I have never heard of running that tight. I'm at. 022 on my solid cam
Click to expand...

"lash was set at .008 from what i was told by comp"

says lash was set at .008 from what comp told him...not lash was set at .008..
 
It's a hydraulic cam - just happens to be using adjustable rockers.

"Set the throttle blades to expose about .025 of the transfer slots when you look at them with carb upside down, then screw in all 4 idle mix screws till they bottom out and then unscrew them all out 3/4 turn each...then disconnect the vac adv if u use one at all.and plug the vac line at the carb"

If you have trouble estimating .025 in the transfer slots, make them look square. .020 to .025" is a good number. If you have a micrometer, measure a piece of wire and use the wire for a gage. Otherwise use your eyes to make 'em look square.

Now, to be really smart - mark and write down the position of the slot in the adjustment screw. This way you'll know where .025" is without taking the carby off again.

Timing - the advance is hard to read in the middle of the slope! Take readings all the way up until it stops advancing. Probably between 2500 and 3500 rpm. You should also be able to take the rpms down (when oil is warm) until it stops retarding, but w/o knowing the inside of the distributor that can't be guarenteed. In any event write it all down and then you'll know the advance curve. Once you know it, then you can make changes.

Carb insides - If you really want to get into tuning it.
a. Get a copy of Urich and Fisher's Holley Carburators & Manifolds
b. Using miniture drills as gages, measure the Idle Feed Restriction (IFR) and note whether its been placed high or low, the Idle Air Bleed (IAB), the High Speed Air Bleed, and the emulsion holes along with how many there are and where they are placed. Also, whether the throttle plates have been drilled with air holes - if so how big. Might as well measure the Power Valve restrictions too while you're in there.
 
zinser72 said:
this is my combo:
'72 318, .030 over, kb167's zero decked, scat i-beam rods, forged crank, fully ported RHS heads with 2.02 intakes, comp xe285hl cam:

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=632&sb=2

10.1 static compression, solid lifters, roller rockers, 978 dual springs, msd street fire box with electronic ignition, rpm air gap intake and the suspect problem, a holley 750.
Click to expand...

Mattax said:
It's a hydraulic cam - just happens to be using adjustable rockers.
Click to expand...

See the bold from his first post.
 
thanks for all the responses. i dug out the old 650 holley from last year and it started immediately, ran for about 6, 7 seconds and died. it didnt even try to restart...didnt even pop. i tried pumping the carb after cranking a while after it died. nothing. my initial timing was at 20* for all of this. i will follow all your suggestions on the 750 but now im almost thinking ignition...i have spark and it wasnt flooded when it didnt restart. and i know it had fuel to the carb after the swap because the accelerator pump was working. oh and comp told me to lash at .008 so thats what i did...could that be too tight? would extremely low compression cause this issue as well? im sure its something simple and stupid but for now im still confused...ill get back to you guys with more info when i have it. i will try everything mentioned, thanks
 
What RPM are you calling this initial advance reading. If it's above 1200, it's not initial, it's got some mechanical bleeding in.

What distributor is in this?
 
If it started with the 650 then died, and wouldn't restart, I might be suspect of the fuel pump, you need a fuel pressure gauge, personally I would want to know what your getting for fuel pressure before goin any farther.
 
412 Stroker said:
If it started with the 650 then died, and wouldn't restart, I might be suspect of the fuel pump, you need a fuel pressure gauge, personally I would want to know what your getting for fuel pressure before goin any farther.
Click to expand...

the accelerator pump was working though, i wouldve thought it should at least pop a bit after pumping fuel into it. ill look into a gauge
 
crackedback said:
What RPM are you calling this initial advance reading. If it's above 1200, it's not initial, it's got some mechanical bleeding in.

What distributor is in this?
Click to expand...

mopar performance electronic distributor that came with an orange box, since replaced with street fire msd. timing dropped to 20* as it was dying and stopped there. it was running about 25* at 1500 rpm
 
well ive never been so happy to admit im an idiot. the coil tested bad so i changed it out. it ran better but not nearly perfect. i let it warm up so i could re-lash the valves and what i found made me feel really stupid. i dont know how but one intake valve was was tight (about -.020 clearance so valve never closed) and two exhaust valves the pushrods werent fully seated in the lifters so those were hanging open too. when i added the inner springs i just pulled the shafts off and put them right back on planning on setting the lash later. i checked that all the pushrods were in but i mustve bumped a couple out when putting them back in. so its finally running the way it should :cheers: i will follow all the suggestions fine tuning it now but im so happy it runs!!! thanks you guys for all the help :prayer: cant wait to get it on the road!
 
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