proform race carb / Idle trouble

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360duster

360duster

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Hi Guys,

i´ve got a 750 cfm proform race carb for my 408sb. I could it get to idle halfway correct only with the idle mixture screws totally out, so far that theres no more effect when turning more. This makes me think if the idle air bleeds should be bigger for my application - or to decrease the size of the idle feed - is this correct?

timing is set at 18° at the moment, i adjusted this with my brothers edelbrock carb - with this one it was idling very good and had good response.

Thanks for info!

Michael
 
after reading some more stuff it seems that i´m wrong - the more out the idle mixture screws are the more rich is the idle mixture. So in this case it needs just more fuel at idle - solution is to reduce the size of the idle air bleeds?

according to the data sheet of the carb it has got "70" air bleeds in stock condition. I just checked it, there are "75" bleeds in the primaries, "73" air bleeds in the secondaries. What do you think, make air bleeds smaller?

Does it make sense to use different sizes in primary and secondary?

I don´t have smaller ones here or i had i tested before.

Michael
 
yes try reducing the air bleeds or plugginga couple so it pulls more fuel.
 
Do you have any .025 welding wire?

If you do, cut a piece long enough to fit in the bleeds and loop around the bowl vent tube.

Sticking the wire in your current .075 bleed will make it about a .070-.071. Observe how the engine reacts. If it needs even smaller, stick another wire in the bleed or a piece of .035 wire. That will make the bleed about a .067-.068
 
Did you verify your fuel pressure and float levels? I have the same carb, similar idle problems, was idling over 1,100 rpm. My fuel pressure was unregulated and was 11-12 psi with my Carter mechanical pump. Plus, the float level was almost at the top of the sight glass. Solved those issues now it's at 850. Never touched a jet or an air bleed. IMO, air bleeds are really only for fine tuning.

Might also check to see if your throttle linkage is adjusted properly.

I think that the jets that come in these carbs are 74 and 84.
 
Might want to check the float level. Inner air bleed should be around the 70 mark If you need to lean it out go over the what you have right now.
But I would follow RMCHRGR step first. 360duster what kind of fuel you running Race gas or Pump?
 
The bleeds in center (High Speed) should be around 36.

The outers (idle) should be around 73. The secondary idle bleeds for a carb that is not 4 corner idle would be in the 36 range.
 
Thanks for your answers!

I´ll try the welding wire trick (already thought about reducing the bleed size, but had no idea how...:newb:). I´ll post how it works.

It´s a 4 corner idle carb, and i´m using pump gas around 90 according to your measurement (it´s 95 here).

The fuel pressure thing makes me think - i also use a carter mechanical pump and did not check the fuel pressure. I´ll check this and the float level tomorrow - will keep you updated!

Michael
 
All the changes are predicated on the carb baseline settings being correct or in range.

If the float level is 1/4 turn high or low, it can make a rather large difference in a/f ratios.
 
I meant to say the outer ones. If you were to have 70 for example you only want to go 3 sizes up to lean it out. If you wanted to richen it go down 3 sizes but that is not the case for you. Also leaning the idle can affect the high speed bleeds. You can find that out when driving at slow rpm.
 
Hey this is quiet interesting as I have been reading around my search results for air bleed tuning and low and behold I find a post about the carb I now have!!

I have also concluded that the idle air bleeds are way too big! I even tried switching the primary idle air bleeds with the High speed air bleeds to test my theory as I was having a very bad lean out condition when just applying light throttle. Of course when I switched the air bleeds and screwing the mixture screws almost all the way in, tip in was awesome but the higher end of the primaries was dangerously lean!! I tried sticking paper clips in the idle air bleeds (after switching them back) to no avail!

Next week I will buy a set of air bleeds, its just hard on my wallet to spend $50-$100 on some air bleeds thats all!

I did run this carb last year but I did not run it on the street so I did not care about the bad lean tip in bog, but I run in the street eliminator series here in Germany and am tired of changing carbs and would like this to run right! Plus I would like a good carb to tune before I run on the bottle!
 
Have you pin gauged the ifr to determine what size they are? If you have to really step down on your IAB to get it in range, I wonder if the IFR is REALLY small. If the IFR isn't at least around .030, they are too small.

Mcmaster-carr for some brass set screws 6-32 por 10-32, and a 61-80 and 1-60 drill set and you can do a bunch rather inexpensive without buying those kits.
 
If the IFR is too small, do I drill it out? If too big? Are they replaceable? Could it be that they are clogged? I haven't looked at them yet.
 
Most std holley metering blocks are not adjustable. You either have to drill them out or remove and tap for a 6-32 set screw.

If they are andodized alum or aftermarket blocks, they are usually adjustable.

I didn't notice if you had 4 corner idle or not, but if you have a 73ish secondary IAB with a 2 corner idle, that's going to be really lean.
 
It has 4 corner idle, I am thinking about richening the secondary side to see if that helps also. It's all trial and error for me as I am used to my old trusty TQ that I can't find parts for anymore.
 
first of all, i apologize for not updating when i was messing with this carb. I tried to get the car on the street, had not the time to fully understand / adjust this carb, lost interest in it and got another more simple carb - so i gave this one to cory for some testing, later we made a deal on it.

Cory: as far as i remember there was a low rated PV in this carb - after checking the vacuum on my car i thought this PV would never open....did you put a better suited PV in it? my engine needs around 8"HG PV.

Michael
 
You should have squared the primaries and idled off the secondaries. That is why the have 4 mixture screws and two idle set screws on pro-forms. You need to idle off the secondaries and only use the square portion on the primary transfer port.
You can get them to work by changing PV and air bleeds but this should be for fine tuning only. You should never have to go smaller on the bleeds from what they came with unless you have a vacuum leak somewhere. Or bad valves or wide seats.
 
I tried idling from both the primaries and secondaries, but could not get it to work properly. The Engine was a new one with good cylinder heads.

Cory: the carb was very clean inside when i got it, so i did not even think about a closed passage in the metering blocks. You should check these again

Another thing to consider: the hose connection on the backside (brake booster) did interfere with my EDE-manifold, couldnt put a hose on. After putting a 1" spacer in everything was ok. Do you have space around this port? This could create a vacuum leak.

Michael
 
I put a more suitable PV in last year when I got it. I will hopefully get to checking out the other issues later in the week.
 
Thanks Michel, I also now have a A/F gauge, which is why I wanted to put the carb back on.

I checked the IFR and they are at .032 all around. As I was putting the bowl back on I stripped out a hole so I will have to order some more heli-coils. The transfer slots were closer to a more rectangular shape, so I will fix that as well. I switched the secondary idle air bleeds to the primaries. I think I will still order some set screws to make some air bleeds so that when I finally do get it right then I can tune it perfectly.
 
You may be chasing your tail if the baseline timing settings are not up to snuff.

.032 is on the big side for IFR's IMO
 
Thanks, I will check that as well, once again. I have always tuned with total but I realize that the initial might be off.
 
Well my initial is 16 and total is at 34 with my magnum heads. Tried her out again today. Seems like the lean out moved up a little now it is not right when I five it slight throttle but a little higher around 2500-2800 (in neutral) it is very lean. After 2800 AF ratio drops back down and runs great! She idles very steady and full throttle response is great. I will hopefully have a little more time to mess with it tomorrow.
 
Don't base anything above idle off what you see in neutral.
 
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