Thread: mid to high rpm stumble-no power

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I have a stock bottom end 454 with oval port open chamber heads with crane valve springs and compcams thunder cam
Basic Operating RPM Range: 1,800-5,600
Intake Duration at 050 inch Lift: 227
Exhaust Duration at 050 inch Lift: 241
Duration at 050 inch Lift: 227 int./241 exh.
Advertised Intake Duration: 279
Advertised Exhaust Duration: 296
Advertised Duration: 279 int./296 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.498 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.483 in.
Valve Lift with Factory Rocker Arm Ratio: 0.498 int./0.483 exh. lift
Lobe Separation (degrees): 107
Intake is a old edlebrock TR2X bottom with a TR4X top with a single 4160 750 (brand new) with 72 main jets, 3.5 power valve and 2.5 turns out on idle mixture. Running 10 degrees initial and not sure on mechanical with mallory unilite with mechanical advance and not sure of what springs (Can't tell what color.), advance tab is bent just beyond middle to the advanced side. Car runs very good off idle and low end but when you get into the throttle it cuts out and kinda pops. I am also around 6000 ft or so. I was going to try a lighter secondary diaphragm spring and some bigger main jets with also maybe a bigger squirter (currently a 35). What other things can I try to eliminate this problem.
http://s393.photobucket.com/albums/p...t=P5180188.flv

try looking at fuel psi .fuel pump. if mechanical pump i run a ultra light fuel pump rod .check fuel lines . check fuel tank vent . check and set floats .you need to know how much timming you have. that would help . check that the carb vent tubes are not getting shut down by the air cleaner lid or small air filter

Looked at fuel and fuel pressure is good, float bowls are good and all fuel lines are flowing. Seems like I have been having a timing problem. Replaced the Mallory unilite I had with a new MSD ready to run vacuum advance and new wires and the car runs so much better. Currently have 16 degrees initial with the stock 21 degrees mechanical. Also replaced power valve back to a 65 and changed main jets from 72 to 68 (tried 70's but still too rich). Car is running really well off throttle and midrange but is still stumbling on the higher end. I tried to give it more initial but the starter does not like anything over 16 so I was going to try the 25 degree mechanical bushing and lighter springs. Car pulls really well untill around 3500 to 3800 then starts stumbling. I have read some big block guys are running a total of 40 to 42 timing, so is it safe to go this high? Also the vacuum advance helped out a ton on normal cruising, car runs so much cooler and smoother.

try the red bushing on the msd use one sliver light spring and one blue should only need 10 on the crank to get the job done you can run 40 .did you check at tdc if you pointing at 0 ?

Pat, yeah I did check tdc and it is at 0. I do currently have one blue and one silver (light) springs installed. You think the red (28 degrees) would be the ticket? This motor really seems to like alot of timing. Thanks for your help.

Originally Posted by burnout1969

Pat, yeah I did check tdc and it is at 0. I do currently have one blue and one silver (light) springs installed. You think the red (28 degrees) would be the ticket? This motor really seems to like alot of timing. Thanks for your help.

red that is how i set alot of the bbc up that way

Seems like we get a lot of these on here. Sounds like your timing is OK. I think you may be running out of fuel. Like Pat said, monitor the fuel pressure at the carb while running particularly where it stumbles. Tank vented good? Fuel level in bowls set right? Are you running slosh tubes in the secondaries? That is not a big carb for a 454.

Kitz

on top of that ram i would look at the air bleeds blow air thru them to make sure there is no junk in there i think i would go up on jets and go back to a 3.5 power valve the back of that carb is just a plate no jets ? or power valve ? a double pumper is what you need

The carb is a new 4160 with a plate (instructions said equal to 75 jets) and checked the air bleeds and all is clear. I put the 72 jets back in with the 35 power valve and the car ran pretty good but sputtering on the top end still. So I changed the mechanical advance to the red 28 degree bushing and set the intial to 12 degrees. Car started and ran good as usual just cruising around. Now when I mash the throttle the sputtering and popping through exhaust happens much earlier in the rpm range. Is this a symptom of the engine not getting enough fuel like was mentioned? I am going to try a buddies 750 double pumper or race demon carb to see if more fuel helps. Thanks for your help guys.

Update, so I borrowed a friends 825 race demon and currently have it tuned with 95 main jets, 45 power valve, 99 secondary jets (started with 72p and 91s, then 80,90 and 95 primary with 99 secondary)with 45 squirters primary and secondary. Each increase in primary jet size helped the car pull harder) Car has an off idle hesitation under load(need bigger squirter?) but pulls really well until you open up the secondaries then I get the same condition of popping out exhaust/cutting out and surging. If you keep the secondaries open it surges and tries to smooth out but does not. The car definitely runs better, sounds better and pulls harder then with the 750. Also spark plugs look good and are not real dark. Do you think a bigger carb might be needed? I also have experimented with the timing and currently have the 25 degree mechanical secondary in with about 14 degrees initial using vacuum advance, also used 16 degrees initial with 28 degree mechanical bushing. Timing changes has not affected the high rpm problem. I have also gone back through and re-adjusted the valves. The car will rev (no load in park) using primary and secondaries with no hesitation, bog or stammer.

more duration on the squirters cams ? try a tube squirters and 50cc pumps front and back them long runners need more fuel with one carb

Well, got to do more testing and decided to do a compression test because I am getting a lot of blow by on the driver side valve cover and found some disturbing #'s. The whole even side of the motor is is between 112 and 120 psi and the odd side #1 is only 52 psi, #3 and 5 are about 120 and # 7 is 147 psi. So I guess I will be pulling the motor for a tear down. I did put some oil in the #1 cylinder and retested it and it did go up to over 100 psi but after I fired it up it went back down to 52. Pat thanks for all your help on this. Any recomendations for piston/rings? Was thinking something between 9 and 10:1. Just want a mild street motor.
Thanks again
Scott

you could use the speed pros hyper h426cp 100 dome 119cc/8.5 or 340 dome 119cc/10.2 other then going to a srp piston ,for rings moly i used just about every ones rings for mild builds .hasting s speed pro .je.mahle

Can you just pull the cylinder head on the affected side?maybe just a stuck/bent valve?

Pulled the motor and tranny out and took the heads off and I not only have a flat lobe on the cam it ate the lifter too. Exaust lobe on the #2 cylinder and it must of went flat from first start up. The lifter actually has pieces missing off of the bottom where it mates with the cam. None of the cylinders look scored or marked so I might try to just do a re-ring and shave the heads for more compression.