Dodgy idle with edelbrock 500

jefferybond · Post by **jefferybond** » Mon Mar 01, 2010 11:03 am

Hi guys,

I'm having trouble getting a clean idle on my 3.5L RV8 with edelbrock 1404 carb. I've recently fitted a wideband lambda, so I can finally see what's happening.

When started from cold (no choke, just a few pumps of the pedal), it starts and idles nicely at about 13:1 AFR, once it starts to warm up after a minute or two, the idle goes lumpy and the AFR swings between 10:1 and off-scale lean (>18:1). Need to keep prodding the throttle to keep it alive.

Once it's fully warm, it idles OK with AFR about 13.5:1, maybe still slightly lumpy and the AFR does waver a little, maybe between 12:1 to 14:1. It's tolerable though! It runs great as soon as you apply some throttle, goes like hell with no messing about.

Why this funny behaviour during warmup? I've suspected leaks around the throttle shafts in the past, because they are a bit loose, but they were just the same when the carb was brand new. Also, I'm running a hilborn scoop so get very cold air straight into the carb, especially cold in this weather. Could it be icing up? Another thought was air leaking down the threads of the idle mixture screws, is this a possibility?

Carb has a home-made 1/4" heat insulator, and runs msns-extra for the ignition, so sparks should be fine - never have any misfires etc. Engine is mostly stock - stock cam, vitesse valves, 9.75:1 CR edelbrock performer manifold, tubular headers, lightly ported heads/manifold.

Any ideas?

Jeff

kiwicar · Post by **kiwicar** » Mon Mar 01, 2010 12:07 pm

For Idle I would think you are set very rich. An idle mixture when warm of 13.5 is is way too rich bear in mind the carb is set lean at high vacuem, I would think you want more like 16:1. It sounds like you are getting fuel puddling in the inlet manifold as a result of the very rich mixture along with a charge heavily diluted by exhaust under these conditions must be very difficult to get to burn.
Did I mention I think it could be a bit rich

Best regards
Mike

jefferybond · Post by **jefferybond** » Mon Mar 01, 2010 12:21 pm

kiwicar wrote:For Idle I would think you are set very rich. An idle mixture when warm of 13.5 is is way too rich bear in mind the carb is set lean at high vacuem, I would think you want more like 16:1. It sounds like you are getting fuel puddling in the inlet manifold as a result of the very rich mixture along with a charge heavily diluted by exhaust under these conditions must be very difficult to get to burn.
Did I mention I think it could be a bit rich 8) :lol:
Best regards
Mike

The edelbrock carb manual suggests around 13:1 for idle (figure 1, page 2). CO and HC emissions are fine at MOT time too. (2% CO, ~300ppm HC).

Turning the idle screws either way also worsens things, so I think the idle mixture is OK - it's just that it varies a lot as it warms up.

Jeff

JSF55 · Post by **JSF55** » Mon Mar 01, 2010 1:27 pm

what pump and what pressure?

kiwicar · Post by **kiwicar** » Mon Mar 01, 2010 1:41 pm

Have you got Vac advance connected?
Mike

jefferybond · Post by **jefferybond** » Mon Mar 01, 2010 2:11 pm

Two answer the previous two questions:

Pump is a facet red top interruptor, running through a filter king adjustable pressure regulator. Input to carb is set to 3 psi (verified via gauge). Float hight is per manual at 7/16".

Vac advance is connected to the MAP sensor in my megasquirt. Base timing is 12deg at 1000rpm, ramping up to 33deg at 3500rpm, then flat up to the redline. Vac advance adds roughly 0.2deg/kPa (ie. up to an extra 20 degrees at 'full' manifold vacuum), but mostly limited to the 'cruise' area of the map for safety.

Jeff

JSF55 · Post by **JSF55** » Mon Mar 01, 2010 2:58 pm

where's the sensor fitted ? is it reading from both banks at the same time?

kiwicar · Post by **kiwicar** » Mon Mar 01, 2010 3:08 pm

Hi
Try sticking in alot more advance at Idle on high Vac aim for about 30 to 35 degrees total to start with pulling it out rapidly as the Map sensor sees manifold depression drop. At Idle you are trying to burn a mixture that is small in quantity with very poor atomisation, and heavily diluted with exhaust gas, there is basically very little oxygen around the big droplets of fuel, the extra advance starts ignition while the piston is still quite rapidly moving up the cylinder giving lots of turbulance and mixing everything up nicely. I still think you are getting fuel puddling in the inlet manifold by the way, but the extra advance should help burn this mixture. By the way the Vac can on the distributor also puts in lots of advance at idle for the same reason.

jefferybond · Post by **jefferybond** » Mon Mar 01, 2010 4:40 pm

kiwicar wrote:Hi
Try sticking in alot more advance at Idle on high Vac aim for about 30 to 35 degrees total to start with pulling it out rapidly as the Map sensor sees manifold depression drop. At Idle you are trying to burn a mixture that is small in quantity with very poor atomisation, and heavily diluted with exhaust gas, there is basically very little oxygen around the big droplets of fuel, the extra advance starts ignition while the piston is still quite rapidly moving up the cylinder giving lots of turbulance and mixing everything up nicely. I still think you are getting fuel puddling in the inlet manifold by the way, but the extra advance should help burn this mixture. By the way the Vac can on the distributor also puts in lots of advance at idle for the same reason.

Not sure I agree with the above. The vac advance on a 'conventional' distributor will not put in *any* extra advance at idle because they are normally connected to the 'ported' vac port on the carb. This port sees no vacuum when the throttle is in the idle position. Also, how would my idle mixture get diluted with exhaust gas? (I have no EGR).

I also think running as much advance as you suggest at idle will cause problems, especially with starting etc!

Anyway, nearly all engines of all types manage to idle nicely with 10-ish degrees of advance, so why should mine be any different? Personally I'm suspecting the edelbrock carb, which seems a bit cheaply made and not the best quality, especially the throttle shafts and mixture screws.

Jeff

jefferybond · Post by **jefferybond** » Mon Mar 01, 2010 4:43 pm

JSF55 wrote:where's the sensor fitted ? is it reading from both banks at the same time?

Just from one back, after the four headers merge into one.

I have two completely separate exhaust systems, so there is no common point that I could have installed the sensor. I've got no reason to suspect that the two banks are running differently, since all 8 plugs are the same colour, and each barrel of the carb feeds 2 cylinders on both banks anyway.

Darkspeed · Post by **Darkspeed** » Mon Mar 01, 2010 5:06 pm

Are the float heights and fuel pressure correct?

What advance setting are you currently using?

Andrew

jefferybond · Post by **jefferybond** » Mon Mar 01, 2010 5:24 pm

Darkspeed wrote:Are the float heights and fuel pressure correct?

What advance setting are you currently using?

Andrew

See above! ;-)

Jeff

CastleMGBV8 · Post by **CastleMGBV8** » Mon Mar 01, 2010 5:56 pm

Jeffrey,

If the idle AFR is fluctuating as wildly as you suggest it would indicate either an air leak as previously suggested or possibly you have the wrong step up springs fitted allowing the carb to go to step up rich condition with an overfast idle.

What is the idle speed?, with a standard cam the idle should be in the 6-800 RPM range any higher and the slow running jets are probably getting a top up from the primary main jets, the slow running jets according to the manual are supposed to provide 100% of the fuel for idling.

I'm also assuming that the headwork did not include increasing the port runner size which would lower gas velocity and affect idle especially on a 3.5.

Kevin.

kiwicar · Post by **kiwicar** » Mon Mar 01, 2010 7:44 pm

Hi
The charge gets diluted by exhaust gas at idle because you have virtuall no scaveging at idle revs, also there is relativly a long time for exhaust flow to reverse through and into the inlet side. Mixture at idle is always the most difficult to burn as a result.
And yes US 4 barrel carbs are really cheeply made.
Mike

mgbv8 · Post by **mgbv8** » Mon Mar 01, 2010 8:02 pm

jefferybond wrote:
kiwicar wrote:Hi
Try sticking in alot more advance at Idle on high Vac aim for about 30 to 35 degrees total to start with pulling it out rapidly as the Map sensor sees manifold depression drop. At Idle you are trying to burn a mixture that is small in quantity with very poor atomisation, and heavily diluted with exhaust gas, there is basically very little oxygen around the big droplets of fuel, the extra advance starts ignition while the piston is still quite rapidly moving up the cylinder giving lots of turbulance and mixing everything up nicely. I still think you are getting fuel puddling in the inlet manifold by the way, but the extra advance should help burn this mixture. By the way the Vac can on the distributor also puts in lots of advance at idle for the same reason.

Not sure I agree with the above. The vac advance on a 'conventional' distributor will not put in *any* extra advance at idle because they are normally connected to the 'ported' vac port on the carb.

This is wrong for your set up I think Jeff. You need to use the other vac port on the 500 carb. This gives you vac advance at idle with the flaps closed and max manifold vacuum. Then the vac drops off as you open the flaps and the Mech advance takes over.
Just set the timing at 10 degrees with the vac off at 900 rpm, then connect the dissy to the other port and see what happens. Worth a shot eh

This port sees no vacuum when the throttle is in the idle position.
This is the port you would normally use for the vac modulator on an auto gearbox. Why would you want no advance at idle, and then add more advance via vac and mechanical?

Also, how would my idle mixture get diluted with exhaust gas? (I have no EGR).

I also think running as much advance as you suggest at idle will cause problems, especially with starting etc!

Anyway, nearly all engines of all types manage to idle nicely with 10-ish degrees of advance, so why should mine be any different? Personally I'm suspecting the edelbrock carb, which seems a bit cheaply made and not the best quality, especially the throttle shafts and mixture screws.
I'm pretty sure the video I saw that came with my mates new 500 showed the dissy connected to the wrong port??

Jeff