tunnel ram tunning help 408sb

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mopar_persson

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hi i have a 408sb with tunnel ram, carburetor I use is 2 x 390 cfm, the engine runs fine on until you press full throttle then it begins to (Counterattack) I can not really describe how it sounds. it feels like it may be gets too little gasoline.
carburettor is restored, and I currently have 54 nozzle in the front of the carbs and 64 nozzel at the back. carburetors are vacuum secundaris,

I'm bad at writing in English / simon from sweden
 
might need alitle more accpumpshot?

feel free to give us alitle more information on your combination and it might be easier to help out:)
 
Also sounds like the carbs might be a little small depending on how healthy that 408 is.
Might be worth a switch to a pair of holley 450's
 
racerhog said:
Are They Holley 4 barrel Carbs?
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He said 2, 390 CFM carbs. In which I agree they could be a bit small depending on the engine combo.

Does it backfire through the carb?
 
your english dont seem to be bad but if you need some help with translation feel free to PM me what you need translated and i will try to help out :)

but like others have pointed to in this thread i suspect your problem is not enough pumpshot or secondarys opening to quick creating a leancondition that makes the engine backfire thru the carbs.the carbs seem alitle small but i doubt the size of the carbs are the problem.
does the engine run good if you bring it up into higher rpms slowly?
 
1Fast340 said:
your english dont seem to be bad but if you need some help with translation feel free to PM me what you need translated and i will try to help out :)

but like others have pointed to in this thread i suspect your problem is not enough pumpshot or secondarys opening to quick creating a leancondition that makes the engine backfire thru the carbs.the carbs seem alitle small but i doubt the size of the carbs are the problem.
does the engine run good if you bring it up into higher rpms slowly?
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okey thanks. the engine dosent backfire thru the carbs,. the enigine runs really good in lower and middle rpm, but when you press the last little on the gas pedal it starts to run bad, maybe att 5000-6500 rpm its runs bad, it still runs bad if i try to slowly increasing rpm

svenska= den bluddrar/motar på fullgas låter de som iallafall.

the engine combo :
360 block
0,30 over probe srs pistons
scat 4" crank
596 heads 1,88 /1,60 vales
mopar performance purple cam P4120231 RPM Range 2,600-6,000
Duration 284/284, Lift .484/.484
weiand tunnelram intake
2 x holley 390 cfm 4-barrels

the engine is new built.

it is 4-barrel carbs.
 
what kind of ignition system are you running? 2 things come to mind........fuel starvation or ignition breaking down.if your running a stock ignition system that could be a problem at high rpm.
 
i think i would translate it into the engine breaking up at higher rpms:)

i agree with 340_8bbl on this one, what kind of ignitionsystem and have you checked all the wiring,connectors and grounds? what kind of fuelpump are you running?

third option would be valvefloat,but i realy doubt that one,since you wouldnt get it to 6500rpm if the valves start floating at 5000.
 
1Fast340 said:
i think i would translate it into the engine breaking up at higher rpms:)

i agree with 340_8bbl on this one, what kind of ignitionsystem and have you checked all the wiring,connectors and grounds? what kind of fuelpump are you running?

third option would be valvefloat,but i realy doubt that one,since you wouldnt get it to 6500rpm if the valves start floating at 5000.
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the ignitionsystem is a new mopar performanc distributors,
and msd 6al box, new 8 mm ignition cables and champion spark plug,
ignition is set at 35 degrees.

the fuelpump is a Carter Muscle Car Mechanical Fuel Pump M6866 120 gph

earlier I had an Edelbrock RPM intake on the engine and a 750 vac carb, I had no problems then,
 
check your float level.pay close attention to the rear bowls.also,how do your plugs look?
 
Don't waste your time with the power valves. putting a 3.5 in only makes them open later which isn't the issue at WOT.

When it starts to run poorly, what occurs if you roll out of the throttle slightly? Does it pick up?
 
crackedback said:
Don't waste your time with the power valves. putting a 3.5 in only makes them open later which isn't the issue at WOT.

When it starts to run poorly, what occurs if you roll out of the throttle slightly? Does it pick up?
Click to expand...
what he said and ill add......when you say 60 nozzles i assume you mean jets.going from 54s to 60s is a big jump.try 56 jets first to see if that helps.
 
Now it runs really good. I put in 60 jets, and power valves plugs.

thank you very much for all the help :cheers:
 
If it runs rich while cruising around, put the PV back in and 52-54 jets
 
what is your vac reading at idle in gear?id select a pv 2 numbers below your lowest vac reading.ive been through all this with the tunnel ram on my 340.ive got 10.5" of vac in gear and i run an 8.5 pv.stock jetting on a 390 cfm holley is 51s.im running 53s.you established it was lean,but it can be fine tuned from what you have now.im not sure eliminating the pv is needed.
 
Power valves shouldn't be needed on a tunnel ram although I think you aren't realizing the full potential with those small carbs. 2 660 center squirters or even 2 750 vacuum secondary carbs would be better and would improve the driveability as well.
 
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