Check my carb tune please?

[JerseyJoe]

I have a BLP 850 on a 493" RB stroker. On the highway (50-70 mph/2500-3000rpm) and WOT the AFR's are right where they should be, mid 13's and mid 12's.
Idle and light throttle on the other hand are very rich. I can get it to idle at 13 to 13.5 but the idle screws are only 1/2 to 3/4 turns out. And then it will cruise light throttle at 11.5 to 12.
I've increased idle air bleeds 2, 5, even 8 sizes larger and it only produces a lean spike (15-17AFR) when you crack the throttle (no surprise) and no improvement in light throttle AFR.
What I've never done before is change IFRs or T-slot restrictors. Is that where I should be tuning next?
Carb specs below, timing is 24 initial, 34 total. 10" vacuum at idle. T-slots are barely visible below the blades.

 

[Aron Gleason]

If it's a blp carburetor that's been modified by Dominic thumper I'd give him a call. I have the same card with all the information on it like you do, that's why I'm thinking you have a thumper/ blp carburetor.

 

[Geoff 2]

The fact that [a] idle is rich but mixture screws are only 1/2 turn out suggests the pri blades are open too far. Also more cubes, needs more air, so blades open too far. Too far is when more than 0.040" of T slot shows at idle. Remove carb & check. If so, bypass air is needed.

 

[Geoff 2]

Sorry for the bold...

 

[steve340]

It sounds from your description like the primary main jets are to big.
The primary main jets determine light throttle cruise mixture. Get them right and tune the other circuits from there.
Go back to the standard idle air bleeds.

 

[hunt2elk]

Your idle feed restrictors will impact both idle as well as cruise under 2400 rpm or so. Trying smaller ones would be the first thing I would be doing. All 3 of my motors ended up between 29 - 31 ifr's.

 

[steve340]

The name idle air bleeds gives it away. The idle air bleeds only really affect 2 circuits in the carburettor - predominantly idle and to a certain extent transition. If they really influenced the cruise mixture they would be called cruise air bleeds.
To alter cruise mixture alter the sizing of the primary main jets.

 

[JerseyJoe]

If it's a blp carburetor that's been modified by Dominic thumper I'd give him a call. I have the same card with all the information on it like you do, that's why I'm thinking you have a thumper/ blp carburetor.

I did get it from Dom, and I have talked with him in the past. He’s great with advice but I like to try and figure it out before I call.
I purposely tried to avoid saying “part throttle” to me that could be anywhere. I’m talking barely touching the gas and below 2000 rpm. Just maintaining speed like in traffic or a small, local road (I’m in the suburbs). I doubt I’m getting into the mains at 1500rpm, that’s why I’m asking about IFR and T-slots.

 

[Canadian1968]

I think that's your problem your chasing the wrong circuit. Your trouble shooting the idle circuit to fix your cruising problem. What does it actually idle at? 1500rpm is definitely into the main jets or least it should be. Your car doesn't cruise down the road on the idle circuit .

There is also the accelerator pump cams to take into account as well when you get this close to dialing in your carb !

Your close !

 

[JerseyJoe]

Thanks for the input guys, it idles at 800 rpm, T-slots are barely exposed so there’s not a lot of throttle opening. I’ll get back in the garage in a couple days and report back.

 

[steve340]

You have nigh on 500 cubic inches - the extra suck that kind of displacement and extra stroke will have is it will start to act on the mains circuit fairly early.
The circuits of a carb don't go bang on then off and bang on with the next one.
My words - they all sort of meld in and out as throttle is opened or closed.
The only thing the carb knows is how much air is going through it.

 

[BSB67]

If you reduce your IFR, it will impact (reduce) your cruise AF ratio to about 2200 rpm like hunt2elk stated.

 

[Geoff 2]

A lot of nonsense info in some of these posts.
LIGHT THROTTLE CRUISE IS CONTROLLED BY THE IDLE CCT....NOT THE MAIN JETS.
And particularly in this case with the info provided by the OP. Big engine, big carb.
[1] Big engine: it's large cubic inches means it needs very little throttle opening to keep the car moving [ cruise ].
[2] Big carb: has large throttle bores, probably 1 11/16" or larger. That means the throttle does not need to be cracked open very far to provide enough air for cruising...
[3] Which all means the low speed cruising is done on the idle cct, specifically the T-slot.

The OP confirms in post #1 that high speed cruise 3000rpm/70 mph the AFR is fine. At this rpm, cruise might be on the main cct, might not be.

You need to reduce the T slot restriction size, which supplies the T slot with an air/fuel mixture. You could also try reducing the size of the idle jets, try this first.
Two things NOT to do: change the main jets or the idle air bleeds.

 

[JerseyJoe]

Thanks again guys. Yes, I'm definitely not changing main jets, I can watch the AFR go from high 11's to mid 13's when I get on the throttle. I played with air bleeds to see if it made a positive change and went back to what I had when it didn't solve the issue. I had it out a little bit yesterday after pulling the carb off to check the T slot exposure again, pic has a piece of .035 welding wire to show the slots is even smaller than that. I didn't have restrictors so they're on the way.

 

[steve340]

Thanks for the update. I would be interested to hear what you find changes with the AFR when you change the restrictors if you could do another update.
Every engine combination is different and getting the carb mixture correct can be tricky.

 

[Geoff 2]

Joe,
Just to add....
T slot position is perfect. Some folks would kill for that!
In Carter & other brand carbs, the T slot restrictor is known as the Economizer. The name was patented by Carter. As the name suggests, it has much to do with economy. It regulates the quantity of air + fuel that gets delivered to the T slot. This supplies light throttle cruise. It is my opinion that Holleys & clones get a name for poor economy because this orifice is too large. Might be purposely made large to avoid an off idle stumble for one size fits all carbs.

 

[JerseyJoe]

WOW! What a difference a few thousandths makes. Didn't change anything else, but dropped the IFR from .035 to .032
I did have to turn the idle screws out to a full one and a half turns and it idles at 13.5 now.
Cruising along around 1300 RPM is 14.5ish and my AFR's didn't change at speed and WOT. Amazing. Throttle response is incredibly snappy now too.
I am keeping an eye on the IFR jets I made (last pic), they look to be flush with the metering block gasket, I'm concerned it may not flow freely across that spot. But right now it's running better than it ever has. I can't thank you guys enough
Pics for encouragement to other novices, don't be afraid to give it a try!

 

[RJRENTON]

I think that's your problem your chasing the wrong circuit. Your trouble shooting the idle circuit to fix your cruising problem. What does it actually idle at? 1500rpm is definitely into the main jets or least it should be. Your car doesn't cruise down the road on the idle circuit .

There is also the accelerator pump cams to take into account as well when you get this close to dialing in your carb !

Your close !

Not necessarily......the transition from idle to main fuel feed is dependent on the POSITION of the throttle plate's leading edge in relation to the transfer slots in the throttle bore.....its not RPM dependent.
BOB RENTON

 

[EngineerDoug]

JerseyJoe, I went through a similar exercise using a Promax metering block on my six pack setup. Reduced the screw-in IFR's down to 0.028" and it made a big difference in cruise AFR.

Just a suggestion, however, on seating those IFR jets - I noticed mine (like yours) sat nearly flush with the surface of the metering block. I tapped the threads a little deeper to recess the face of the jet relative to the face of the metering block. Might not matter, I dunno, but I was concerned about the gasket swelling a bit over time and obstructing flow into the jet.

Might be good preventive medicine.

 

[EngineerDoug]

BTW my engine is a 493, so big cubes as well.