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Carb tuning on a tunnel ram

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Carb tuning on a tunnel ram

Postby Casper393W » Tue Jan 10, 2017 7:58 pm

Ok I have a question for the carb guys that have dealt with tunnel rams.... Ok Yesterday I was out messing with the Ole truck and I had my friend over checking out my new tunnel ram setup.. While he was here I had him help me check for WOT.... Guess what I was only getting 3/4 throttle...I have dialed this thing in where it drives amazing. Awesome response...it is running low 14's cruise with idle coming in at right at 15... I had to drill the PVCR's to get 12.5 at WOT.. Now here comes the weird part. Now that I have fixed my linkage... And I get full throttle the engine lays over and has this weird hesitation around 4800 rpm and up.....when I let up on the throttle it runs unreal again???? What is going on here? I have checked fuel pressure, put whistle vents on the metering blocks I put smaller needle and seats..in at a complete loss I thought about the secondaries so I blocked them off..until I get this fixed. Carbs are Holley 1850's with stepped downleg boosters...

Thx
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Re: Carb tuning on a tunnel ram

Postby Casper393W » Tue Jan 10, 2017 8:26 pm

I do have an idea....I still have a 30cc accelerator pumps installed. They have red cams and 28 squirters it revs clean but maybe it is running out of pump shot?
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Re: Carb tuning on a tunnel ram

Postby pamotorman » Tue Jan 10, 2017 8:35 pm

do you have the secondary vacuum chambers tied together with a vacuum hose ?? if not I would start there
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Re: Carb tuning on a tunnel ram

Postby Casper393W » Tue Jan 10, 2017 9:23 pm

Yes I have the pots connected....but like I said once I got WOT... And found that I have an issue. I tried to isolate the problem and it seems to be on the primary side...

If I can't figure it out I could always put my linkage back....haha...
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Re: Carb tuning on a tunnel ram

Postby Tuner » Tue Jan 10, 2017 11:18 pm

So does it run OK on the primary alone when you disabled the secondary?

What is your current calibration?
MJ size
PVCR size
Primary IJ and IAB
Secondary Main jet orifice
Secondary IJ and IAB.

Are the carbs sideways or straight ahead?

Are you sure it maintains fuel pressure at WOT?
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Re: Carb tuning on a tunnel ram

Postby Casper393W » Wed Jan 11, 2017 5:33 am

I did verify that I had fuel pressure at wot. I ran a line to my gauge on the wiper cowl. The carbs are mounted in-line and
my main jets in the primaries are 66 with a PVCR that was .046 and increased them to .054. My air bleeds are untouched stock but I haven't measured them yet. The secondary plates have been drilled to .072 I'm running 8.5 power valves...

Thanks Again,
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Re: Carb tuning on a tunnel ram

Postby pamotorman » Wed Jan 11, 2017 11:11 am

make sure the pin the floats pivot on is all the way in the float bowl so the floats can not move back and forth and get hung up.
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Re: Carb tuning on a tunnel ram

Postby F-BIRD'88 » Wed Jan 11, 2017 11:52 am

If you want to , need to add adjustable selectable jetting to the secondary metering plate of a 600VS carb this is how:

You can add tun-ability to the sec metering plate
by adding 2 extra 1/32" drilled holes per side.
Each additional hole added increases the jet size by 3 to 4 jets

eG 134-9 ='s a 64 jet .064" or a 67jet .067" hole
add + 1 (1/32" holes gives a 70 jet
add +2 (1/32" holes gives a 75 jet

EG 134-39 ='s a 69 jet .070" or a 70 jet .073" hole
add + 1 (1/32 hole gives a 73 jet
add +2 (1/32" holes gives a 77 jet

Drill the new holes into the main fuel well just like the existing hole.
Beside it or from the flat face into that passage. Block the holes you do not need.

Why is the idle afr so lean 15:1 ish. Carbed motors idle best usually a bit richer than 14.7:1
Set the idle mixture for best idle manifold vacuum and idle quality and then lean roll just the 4 mixture screws a hair, from there, once the manifold plenum is all warmed up.
Possible vacuum leak.

Looks like fuel starvation at high rpm WOT under load. is it a mechanical fuel pump?

Why smaller needle seats? This limits fuel flow at highest demand //WOT-- higher rpm under power load.
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Re: Carb tuning on a tunnel ram

Postby MadBill » Wed Jan 11, 2017 12:00 pm

Weak ignition is a possible cause, now that you are getting more cylinder pressure. As a quick check, you might close the plug gaps by 0.005 -0.010".
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.
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Re: Carb tuning on a tunnel ram

Postby F-BIRD'88 » Wed Jan 11, 2017 12:06 pm

Check the condition of the ignition system. they do not last forever, and need service maintenance. Especially if the car has been sitting. The secondary side is critical Cap, rotor, wires, engine to chassis grounds.
Check rotor cap phasing alignment. If it is too far out of wack you will surly get misfire under WOT load and higher rpm.

Stale fuel is another possibility... Has this car been sitting un used?
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Re: Carb tuning on a tunnel ram

Postby Tuner » Wed Jan 11, 2017 6:33 pm

Casper393W wrote:I did verify that I had fuel pressure at wot. I ran a line to my gauge on the wiper cowl. The carbs are mounted in-line and
my main jets in the primaries are 66 with a PVCR that was .046 and increased them to .054. My air bleeds are untouched stock but I haven't measured them yet. The secondary plates have been drilled to .072 I'm running 8.5 power valves...

Thanks Again,

Your #66 MJ with the .054" PVCR is in the ball park and has proven OK up to this incident, so no criticism there. On a pair of these carbs for T-Rams I usually use #67 MJ with .046" PVCR so not enough difference to worry about since it has run OK.

The secondary plate I usually use .076" MJ and .040" IJ, so you may be a little lean with only .072" MJ but it shouldn't lay down.

I think you may have a bowl vent situation with air filter or no air filter. Did you ordinarily run it with filters and after adjusting the linkage run it without filters? A couple years ago I coached a pal who was tuning a pair of 1850s on a nearly stock 318 Mopar with a genuine Mopar tunnel ram and he was road testing with and without filters and not saying anything about it, thinking the filters didn't matter. As a result, the erratic situation of "I put in bigger jets, why is it leaner?" and "it bogs today but didn't last week," turned out to be the filters were there sometimes and not others. LOL now, but ..... grr... with the filters on it runs like a million bucks, gets good mileage and everything.
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Re: Carb tuning on a tunnel ram

Postby EWC » Wed Jan 11, 2017 11:27 pm

Have you played with the secondary springs ? Is the ball still there ?
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Re: Carb tuning on a tunnel ram

Postby Casper393W » Thu Jan 12, 2017 7:36 am

The secondary's have the ball installed. when I first started having this problem I blocked the vacuum port to rule out the secondary's. I have checked everything you guys have suggested. I'm beginning to think it is either something clogged in the metering block, a fuel bowl vent issue, or still lack of pump shot..

can the air bleeds cause this?

Andy
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Re: Carb tuning on a tunnel ram

Postby pamotorman » Thu Jan 12, 2017 8:33 am

this is why the L-88 uses this is when raced without a air filter. I tried my 68 Z/28 cowl plenum cold air intake with out the air filter and it had a bad miss fire. this prevents turbulence around the air bleeds. I used them on drag cars with forward facing hood scoops to prevent misfires
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Re: Carb tuning on a tunnel ram

Postby Casper393W » Thu Jan 12, 2017 10:15 am

Makes since, it could be killing the signal to the booster causing it not to meter the fuel correctly! I have been running Edelbrock foam triangle filters because the clear the hood... I have heard a ton of bad things about these filters! I tried running it without any filters last night and you are right the problem got worse...
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