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Tuning carbs on Tunnel Ram

General engine tech -- Drag Racing to Circle Track

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dave v
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Tuning carbs on Tunnel Ram

Post by dave v »

I know these might not be the best carbs for the aplication but this is what the customer is using. 408 inch Cleveland 11.5 cr, AFD SP4VR heads, Weiand tunnel ram,
solid Comp Cams roller 260 @ .050 .640 lift 110 deg lobe. carbs are 4777-7 650 cfm jetted 64 primary 70 secondary 4.5 power Problem is even though carbs are jetted small motor seems to still run rich by the plugs looking fat, dark browny black. Should I be jetting the carbs square and removing the power valves?
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Re: Tuning carbs on Tunnel Ram

Post by F-BIRD'88 »

A "rich idle" is not the jets. It mostly a lack of spark timing at idle.

If the distributor is not locked out, do that. Then readjust the carbs.
Be sure all 8 throttles are set even at idle
(idle fuel transfer slot exposure) if the throttles are too far open at idle
The idle and off idle transfer will be out of wack. (lack of timing @idle is the cause)
Use 3.5 or 4.5" power valves. Now it will idle cleaner.
replace the spark plugs.

The idle screw setting will be dfferent when set on a dead cold motor than when set on a motor that has warmed
up. (unless you enply a choke on the carbs)
Let it warm up then reset the idle mix screws. evenly.

Now you can restore the main jetting so the AFR is right at (40+mph) cruise driving and WOT.

Do not allow the engine to idle slow when first started.
Keep the R's up until the engine gets warmed up some.

The idle and off idle throttle response will now be much better
and the plugs will stay clean.

You can fine tune the carbs idle circuit further if required.
minor correcting the idle air bleeds and or idle feed restrictions is sometimes required.
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jmarkaudio
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Re: Tuning carbs on Tunnel Ram

Post by jmarkaudio »

With that much csm the timing needs to be locked to full advance if it isn't already but that isn't all. Idle and transition circuit calibration will make a significant difference in how it runs. You need to make sure no t-slot is exposed with two carbs. The idle feed restrictions and idle air bleeds need to be adjustable, they both need to be smaller to get the correct amount of fuel and duration of operation on the transition circuit since you have 8 holes feeding fuel. Just guessing I would expect .003-.006 smaller on the IJ, .010-.015 smaller on the IAB. Then you get to work on the main circuit...
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Re: Tuning carbs on Tunnel Ram

Post by F-BIRD'88 »

If you were to upgrade the 650's using a holley or proform HP style center body kit, you will gain carb air flow cfm and venturi signal, +downleg style boosters, (a higher flowing more responsive carb) replaceable IAB's and HSAB's. Now you can dial it in real sharp.
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Re: Tuning carbs on Tunnel Ram

Post by cv67 »

Subscribed
Would like to add a tunnel ram one day just for the wow factor tuning duals so they are perfect is a little intimidating removing the bowls a million times etc.
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Re: Tuning carbs on Tunnel Ram

Post by Bob Hollinshead »

F-BIRD'88 wrote:If you were to upgrade the 650's using a holley or proform HP style center body kit, you will gain carb air flow cfm and venturi signal, +downleg style boosters, (a higher flowing more responsive carb) replaceable IAB's and HSAB's. Now you can dial it in real sharp.
F-bird, do use the 650 metering blocks when doing the 750 mainbody conversion?
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Re: Tuning carbs on Tunnel Ram

Post by F-BIRD'88 »

Bob Hollinshead wrote:
F-BIRD'88 wrote:If you were to upgrade the 650's using a holley or proform HP style center body kit, you will gain carb air flow cfm and venturi signal, +downleg style boosters, (a higher flowing more responsive carb) replaceable IAB's and HSAB's. Now you can dial it in real sharp.
F-bird, do use the 650 metering blocks when doing the 750 mainbody conversion?
ya, the air bleeds need minor tuning when using metering blocks that are different from the 750 ones.
Look at the AB sizes in the original donor carb body for a baseline.

most any 650 to 800 DP will work.
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Re: Tuning carbs on Tunnel Ram

Post by rfoll »

The 650 metering blocks have small power valve feeds.
So much to do, so little time...
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Re: Tuning carbs on Tunnel Ram

Post by F-BIRD'88 »

rfoll wrote:The 650 metering blocks have small power valve feeds.
this may or may not be a negative on this tunnel rammed motor.

The smaller PVCR means a larger main jet and a smaller difference or amount of added enrichment when the PV opens
under throttle load. Not to worry about until you get it up and running and see what it wants for jetting at WOT and part throttle.

You can adjust the PVCR size, main jet size balance, later.

Start by getting the motor to idle correctly. and run at just off idle, correctly (up to the point the mains start flowing) Timing at idle, throttle opening position. IAB, IFR if nessessary.
A afr meter and manifold vacuum gauge is nice to have.
if you are willing to work thru it you will have some sharp running tunnel ram carbs. And a powerful engine that runs clean and consistent.

#if using the stock 4777 carb body start with stock default jetting 67pri 76 sec approx-

If going to the proform HP bodys start with the main jets supplied in the kit.
Then start with the idle air bleeds in the kit, but have a look at the stock carb body
air bleeds and keep that in mind.
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Re: Tuning carbs on Tunnel Ram

Post by jmarkaudio »

jmarkaudio wrote:You need to make sure no t-slot is exposed with two carbs. The idle feed restrictions and idle air bleeds need to be adjustable, they both need to be smaller to get the correct amount of fuel and duration of operation on the transition circuit since you have 8 holes feeding fuel. Just guessing I would expect .003-.006 smaller on the IJ, .010-.015 smaller on the IAB. Then you get to work on the main circuit...
I posted this because the idle and transition circuits can dump a bunch of fuel making the plugs dark, and then you try to jet the mains to compensate and you run the engine too lean at WOT. With 8 barrels opening you stay on the t-slots longer, even if it's a progressive linkage 4 opening at once will make it richer than it needs. You need to make sure your idle and transition circuits are calibrated right before you can use the plugs as a guide for WOT.


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Re: Tuning carbs on Tunnel Ram

Post by Matt Gruber »

bump for uglydog56
i'd follow jmarkaudio's advice
.
.
tame a lumpy cam for the street, more street torque! see my article, archived in the waybackmachine.
https://web.archive.org/web/20130707064 ... TGRU/carb/
Great manners equals more fun.
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Re: Tuning carbs on Tunnel Ram

Post by Uglydog56 »

Thanks!
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Re: Tuning carbs on Tunnel Ram

Post by Bob Hollinshead »

Jmark, do you prefer to set them up with 1:1 linkage and have cruise speed fuel coming from all 8 transfer slots or is it engine specific? Any positives or negatives?
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Re: Tuning carbs on Tunnel Ram

Post by mopardave »

I would be interested in the answer as well.
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Re: Tuning carbs on Tunnel Ram

Post by jmarkaudio »

1 to 1 is ideal, it just makes the throttle opening more sensitive unless you work to make the linkage pulling the carbs progressive.
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