388 ci tunnel ram dual carbs. flat spot fix.

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Annoying Eric

Active member
Joined
Nov 27, 2012
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32
Engine is a 388ci with a high tunnel ram intake with twin 450 cfm carbs. It had horrible backfires, would stall all the time.

I rebuild the carbs end of last summer, installed power valve protection, new power valves and all that jazz. Car runs much much better but i still have a huge flat spot if i just completely put the pedal right to the floor quickly.

Im sure i can go down a bit in jet sizes and might help it. i know tunnel rams are a bitch on the street but does anyone know if a progressive linkage would help much??

Any ideas are welcome. I know alot of you guys have had your fingers in this stuff alot longer then i have so please enlighten me.
 
I wouldn't say too much carb.

Is the flat spot only when you go full throttle from an idle?
How does it run in "normal" driving?
Does it run rich or lean? ( have you checked the plugs?)


When I had a tunnel ram on my 283 I fought with a pair of holley 450's for weeks. Ended up putting a pair of Carter AFB 625's on it. It took about a half day to dial in the Carter's and it ran great for the 8 years I had that set up on it. I'm not sayin' to dump the Holley's, I just found the Carters to be simpler to tune for the tunnel ram.
 
the flat spot is anytime i put the pedal to the floor. Normal driving is great!! i can cruise all around and it runs perfect. If i put the pedal to the floor easy then it wont happen.

When it does happen i can left off and get back on easier and get on it. I did read my plugs after i rebuilt the carbs, probably only 300 miles ago. They were slightly rich (darker) but not rich enough to puff out any black smoke or burn my eyes. lol

Maybe i should throw in some smaller jets first?
 
The backfire is from not enough fuel when the vacuum is lost at wide open throttle off the line.
Leave the jetting for now.
What power valves did you put in them?
For example; If the engine idle vacuum is 10 inches then you need 8 to 8.5 power valves.
Do the carbs have the standard accelerator pumps? If so replace them with 50 cc pumps.
Are they double pumpers or do they have vacuum secondaries?
You need to run 91 octane fuel if you're not already.
Check the initial timing. With 91 octane fuel you can run more timing which will help.
 
if i remember right the 450's have a 1 to 1 linkage and no pumps on the rear. they make a good race setup, but suck on the street. 390 vac. sec. would be the way to go for your setup. i have known people to run the 600 vac sec, as well as the 500 eld. carter afb look a likes and do pretty good. i do know that over at the nationaltbucketalliance.com in the tech pages there is a write up on tunnel ram setups.
 
OLD IRON - i have no backfire after replacing my power valves along with the rebuild. i dropped my power down to a 5.5 i think. i cant remember exactly, ive slept a few times since then haha. its really this hesitation when i floor that is the issue.

I do have standard pumps on it, mechanical secondaries and i always run ATLEAST 93.


26TROADSTER - I wish i would of just bought some 390cfm, i knew they would of been a better choice then rebuilding these but i thought i would just either go with a progressive linkage or small jets, maybe even a smaller nozzle.

what size are standard accelerator pumps? i guess i can google that myself and find out haha...

I knew keeping this setup would give me a hard time. its definitely a HUGE improvement after the rebuild.


These are the carbs im running.

http://www.jegs.com/i/Holley/510/0-9776/10002/-1
 
check the linkage on the carb itself, i think most are 1 to 1 there. a buddy had a set and as soon as you cracked the front butterflys open the back butterflys opened too. if they have a progressive then it is one i have never seen on a 450. you might can find some old 600's to get the linkage from. i'm not sure what jets he used but he was adjusting them all the time. i plan on using two 600's because it is what i have. i'm gonna work with the spring set for the back, may even go too stiff just to see. sorry i can't help much, my days of racing were long ago and we had the 660 center squirters.


OLD IRON - i have no backfire after replacing my power valves along with the rebuild. i dropped my power down to a 5.5 i think. i cant remember exactly, ive slept a few times since then haha. its really this hesitation when i floor that is the issue.

I do have standard pumps on it, mechanical secondaries and i always run ATLEAST 93.


26TROADSTER - I wish i would of just bought some 390cfm, i knew they would of been a better choice then rebuilding these but i thought i would just either go with a progressive linkage or small jets, maybe even a smaller nozzle.

what size are standard accelerator pumps? i guess i can google that myself and find out haha...

I knew keeping this setup would give me a hard time. its definitely a HUGE improvement after the rebuild.


These are the carbs im running.

http://www.jegs.com/i/Holley/510/0-9776/10002/-1
 
got a chance to see the carb, the one in the pic is the 1 to 1 like i was talking about.
 
You need the 50cc pump and the larger squirters. When you open it up, you don't have enough gas coming out of the accel pump, it then bogs until the carbs catch back up.

Standard pump is 30cc, IIRC.
 
bama you are right, i didn't even think about that and it looks like it comes with the 30. i still say he needs a vac sec. carb for the street.
 
I would just go up on squirter size. Normally a lean stumble can be cured with a larger squirter size. Normally a 30cc pump is enough.

Also take the carbs off and look at the relationship of the bottom of the throttle blades and the transfer slot. Only about .020"-.030" of the transfer slot should be showing below the throttle blades. Basically it will look like a square showing, the slot is about .02"-.03" wide so if you have that amount of slot exposed below the throttle blades it looks "square"... if that makes sense to you. If you have the blades open too far just to get it to idle it uncovers too much transfer slot and doesn't have enough to handle the transistion off idle.

The transition slot deals with more gradual changes in throttle position. If it falls on it's face when you matt it, it's probably too small of squirters. It could be a combination of both though.
 
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All the tunnel rams I have run black smoked so bad at idle after engine warmed up I had to take them off.
Surprised you have not had that.I did with the carbs you have.Never figured out why it got so much fuel when warm.[S
However I am going to try again with a TR2X with 2 edelbrock 1409 marine carbs and see what happens on a 454.I already have the 1409`s that is why I plan to go with them.
 
On dual carb setups I will put a wire in the idle circuit to lean the fuel flow to have a clean idle. 50cc pumps with 37 nozzles and drilled nozzle screws.



weld on..........[;)
 
How much camshaft and what is your initial timing set at? Please tell us more about your engine so we can get a complete picture.
 
On dual carb setups I will put a wire in the idle circuit to lean the fuel flow to have a clean idle. 50cc pumps with 37 nozzles and drilled nozzle screws.



weld on..........[;)

that is a trick a old friend used, haven't thought about that in a long time.
 

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