Hot Rod Forum : Hotrodders Bulletin Board

Hot Rod Forum : Hotrodders Bulletin Board (
-   Engine (
-   -   idle issue with a new engine (

gonzo08383 04-23-2012 08:34 PM

idle issue with a new engine
new 350 bored .030 over, proform 4150 conversion main body with a quick fuel baseplate, vacuum secondary. the cam is a thumpr from comp cams with .520 lift and 283/303 duration. 107*lobe sep. just a basic run of the mill sbc really. problem is i had the carb on another 350 before this one and i ran fine. the first engine was milder with a small cam(480 lift 238/242 duration flat tappet). the jetting is 80's up front and 74's in the back i think, thats what it liked on the dyno anyway. so i left the initial jetting alone thinking it couldnt possibly run too bad on a bigger motor.....wrong. cant get this deal to idle worth a darn and the vacuum sucks even tho i was told thumpr cams will keep good vacuum. it will idle at 800rpm but vacuum will be 10"Hg and drop to 5-0 and try to kill unless i bump the throttle. if i manually idle it to 1200 rpm the vacuum jumps to a steady 20"Hg and i'm sure i dont have vacuum leaks but then again all i did was the carb cleaner test. so i'm lost as to what to do next. any ideas?

RWENUTS 04-24-2012 08:06 AM

What's your timing set at?
With that much cam you might need up to 20-22 degrees initial.

dawg 04-24-2012 09:17 AM

it might be a intake gasket issue.
I just had an issue with miss at idle and pinging under load and found the intake gasket shifted and 1&3 inlet ports were sucking off each other.
my vacum was reading 17to 19 and no external vacum leaks, BUT i had a leak internally under 1&3 which was suckin in a bit of oil aswell.
was a pita to find this!!!!

F-BIRD'88 04-24-2012 01:36 PM

Thumpr cams have a lot of overlap and need a lot of initial timing at idle.

recurve the distributor for around 24deg at idle. Shorten-limit the curve to around 10-12deg 24-26 at idle 34-36 @ max advance.

When doing the proform HP carb body swap sometimes the idle air bleeds included in the kit need to be fine tuned.
Look at the origional donor carb body (that the metering block came from) idle air bleeds for inpiration.
The idle air bleeds you need will be very close to this.
A afr gauge is a big help in dialing in the idle air bleeds to a fine edge.
A low cost heated narrow band type is fine for this if you're on a budget.

Carb jetting should be 72-74 pri and 80-85 sec. What you posted is backwards.
get a 4.5" rated power valvel so it will stay closed at idle.
A low manifold vacuum at idle is normal for the thumpr cams.

make these changes and it will run like it should.
It will probabily need a fresh set of plugs too.

gonzo08383 04-24-2012 08:46 PM

dawg-i think i'd have thrown a wrench through the windshield before i figured that one out. f-bird thanks! i called comp cams tech and never thought about the power valve. its gotta be dumping at idle, its a 6.5 now and the tech guy said a 4.5 is what i need. unfortunately my fuel pump seems to have air bounded itself somehow now. fuel bowls ran dry randomly. guess its time to relocate it closer and lower than the tank. any thoughts on electric fuel pumps? mine is a mr gasket from the zone. doesnt suck very well, pushes fine....usually. should i spend the cash on a rotary vane or will this one work fine?

454C10 04-25-2012 04:54 AM

you are going to need more fuel pump than an autozone mr.gasket pump.

rotor vane pumps suck also. expensive, loud, and the motor brushes wear out quickly.

I recommend using a msd 2200 efi pump with a return style fuel pressure regulator for a carb (low pressure). This pump is cheap, brushless, quiet, and very durable. The pump cost $100 and the fuel pressure regulator around $100.

Mount the pump by the tank as low as possible and install the regulator in the front (ideal) or the back (if easier). But the regulator requires a line back to the tank (at least 5/16" diameter).

Yes, I know it is an efi pump, but it works great on a carb engine if you use the right regulator and return line.

gonzo08383 04-25-2012 06:10 PM

ok i was wrong. not the power valve. eyes still burning. and the jets are 78 front/80 in the back. car idles fine at 1100 rpm's once its warmed up a lil. still waaaaaay too rich @ idle. plugs are fine and gapped correctly. cd ignition box says to gap em at 60. still digging for the problem....

F-BIRD'88 04-26-2012 01:10 AM

.060" is way too much spark plug gap.
reguardless of what the box says. Reset at .035"

All a wide spark plug gap does is put stress on the cap and rotor and
promote arcing and missfire and ensures a short cap rotor life.

.035" is just right. more is not better.

The primary jetting is way too fat. reduce to 70-74 jet.
Then increase the sec jet to 84 ( no power valve on sec side)

what is the base idle spark timing. must be 20 to 26deg base.
Fix the distributor timing curve.

What is the diameter of the pri and sec idle air bleeds on the carb.?

What is the size of the idle air bleeds on the origional donor carb body?

Remove the carb and flip it over and reset BOTH the pri and sec throttles opening at idle so .020 to .030" of the edge of the idle transfer slot is showing under the throttle blades.

The PCV must be hooked up to the base of the carb and functional.
The PCV is part of the idle circuit. Eliminating the PCV screws up the idle circuit calibration. ( makes it richer)

Make these changes including correcting the timing curve and idle base timing. These cams need 24++ deg at idle. A stock curve will not work.

.060" spark gap will not work.

454C10 04-26-2012 06:00 AM

not enough timing at idle.

after turning up the timing, turn down idle speed (which closes the throttle blades) and readjust the air mixture screws for best idle.

Its running to rich at idle because the throttle blades are open past the idle transfer slots at idle.

if more timing doesn't do it then you will need to let more air into the engine somewhere else. May need to crank open the secondaries or drill a small hole (0.08") in the front throttle blades

Localboy808 04-26-2012 12:25 PM

Advice on my unknown 350
Hey guys. So I bought this 65 Nova. It's got a 350, TH350 with a shift kit, rear end is a 10 bolt Posi with 3.73 gears.

This is what I know just from a visual. The engine is running an Edlebrock RPM airgap, Edlebrock 600 carb, I installed an Accel HEI distributor, it's got Headman shorty headers into 2 1/12" exaust and 40 series Flowmasters. No x pipe. The heads are 882s.

Engine runs well. Sounds like it has a bit of a cam in it. Although its a bit of a dog off the line at WOT. But when it it's about 2500 3000rpm it pulls hard and chirps 2nd gear pretty hard. I'm trying to Discribe it the best I can because I would like some thoughts as to what I might have inside this engine. Maybe it's impossible to tell at all without tearing it down. The previous owner was a young guy just out of the military. He bought it off his buddy who just shipped out to Afghanistan. He had to sell it because of a nasty divorce/custody. But he didn't know all the details of the build. I asked him to try and find out but he never got back to me.

The car was set up for the track. Heidts subframe, gutted with a single racing seat, Fuel cell in the trunk. Batt in the trunk. All new fuel lines and steel braided stuff. They dumped some money into this car. Oh to be young!

Anyway I'm trying to figure out how I can improve my bottom end without totally taking it apart. Maybe it needs a stall? Wondering if the heads were worked? Any way to tell without taking them off? I'd like to get the Edlebrock RPM cam to match the intake. But if the heads are not worked they will need to be.

How would you guys go about this? Just start taking it apart? Or are there things I could look for to give me hints on what I'll need to do? And how do you think this setup will run with the intake and cam with the 882 heads? I have an extra set of those heads in my garage. And a very good shop in my area who's has been there for 50 years quoted me 335.00 to work them for the RPM cam. That's a rebuild with a 3 angle grind. Stock valves. If I did that should I go with 2.02s? Of would I not see the benifit with this setup? I'm on a budget and am trying to use as much of the stuff I have already. Much of it looks newer. Any advice will be welcome.

Localboy808 04-26-2012 01:56 PM

Sorry. I have no idea how my new post ended up here! Can a mod please move this? Or just delete it? I can't even edit it!

454C10 04-26-2012 07:16 PM

need a 10" 3000 stall converter, better heads, a bigger cam, and lots of initial ignition timing.

gonzo08383 04-30-2012 06:53 PM

mmmkay if i advance the timing anymore it idles way too high with the fast idle screw backed all the way out. timing is about 28*adv right now. air bleeds are whatever came in the carb body, its a proform 750 upgrade. the old carb was a holley 600cfm straight leg booster vac sec 300 dollar from azone. i'm not privy to drilling throttle blades when air bleeds are meant to be changed. just dont know how to size em. throttle response is great and i can roast the tires at a dead stop just fine. i gave it a "vacuum leak" and it leaned out a lil. sound like air bleeds to y'all?

F-BIRD'88 04-30-2012 07:47 PM

The proform HP center body is intended to upgrade a 750 carb.

It will work on a 650-700-750 or 800 carb.
All these use a 1-11/16" throttle bore size.

But will not work on a 600cfm carb because the 600 has a smaller throttle plate bore size. The metering blocks from a 600 will be way off the mark.

Drilling the throttle blades is not required.

You need to actually start measuring stuff. Get the old carb body that the metering blocks came from and look at the 4 air bleeds size.
ya it matters

Start by correcting the jetting.
Should be 72 to 74 pri and 80 to 85 sec for that carb body.
Use a 3.5 or 4.5 power valve.
Air bleeds for that carb should be:

This is for 4 corner idle:

pri idle 75 pri hi speed 35
sec idle 75 sec hi speed 35

For a 2 corner idle carb reduce the sec idle air bleeds to

This is the default tune up from the upgrade kit.

You have to fine tune it from there.

Because your metering blocks did not come from a 750 carb
you need to start your tune up using the same pri and sec idle air bleeds size that is in the origional donor carb that the metering block came from. This is your starting point.
You tune it as required from that point.
A AFR meter is a huge help.

If 28deg timing at idle creates too much idle speed reduce it to 24.
You normaly do not need a extra air leak other than the small air leak created by the functional PCV system.
You do not need to drill the throttle plates.

Just hook up the pCV and use it.

Or get the metering block(s) from a 750.
This is the carb you are supposed to use for this upgrade.

Trying to do the upgrade on a 600 carb is a unknown factor.
The throttle base from a 600 carb is too small for the proform 750 center body. The 600 carb metering blocvks are calibrated different.
This may not be the only factor causing the over rich idle but it is a big can of worms for sure . Look at the ignition system too. Your rotor or cap may already be burned up from the excessive spark plug gap.

Is this a GM HEI distributor with the coil on top?

gonzo08383 04-30-2012 08:52 PM

guess i should have mentioned the throttle plate and the metering blocks are both billet quickfuel parts. afr on my other 350 with this same carb was 12.8 on the dyno. that engine made 309ft/lbs and 240 horse with the jets that are still in there. i realize two different engines are two completely different monsters but the last engine was milder than this one. i figured the carb tuning couldnt be tto far off. i actually expected this deal to be on the lean side. think i should mess with the tiny jets in the metering block? the block came with some extra jets i think for the well.

All times are GMT -6. The time now is 05:57 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright 1999 - 2012. All Rights Reserved.