Thread: 454 stumble

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Hi there. Looking for direction. My fresh 454 run great pulls strong under even slight throttle. No missing no stumble from idle to 4500 rpm. HOWEVER if you are just light on the gas and maintaining at 2300-2500 it feels like a sputter or miss. As soon as you put a little more load on it it's smooths right out. Any thoughts of what i may check?

FYI - Its a 454 flat tops, iron oval port heads 219/188 valves, comp magnum 280H Cam, Eldelbrack RMP Air Gap, Holley 750 Mechanical 2 feed. Primary jets at 75, secodary 80, power valve 65. has full 13" vaccum at idle, no apparent leaks.
MSD 6AL with MSD Coil and Distributor. Timing 32 Deg total advance.

that sounds like a lean surge. turn the idle mixture screws out 1 full turn. this will make idle too rich. but drive it and see if the surge gets better. if it does then you need a larger main jet. another way to check this is get a propane torch. take the tip off and put a long hose on it put the other end into the air cleaner drive the car and try to tune the surge out using the valve on the torch these 2 methods have worked for me many times.....ted

try 36 deg 13 vaccum in gear? if not move to a small power valve ...75 and 80s are about the right size the only time i have pulled timing out is on blower engines could be lean but what curve do you have is it a MDS dist

Increase the main jets. Your a little lean at cruise. I'm useing 78's but thats a little rich. I run 91 octane so my timing is at 28. I also have domed pistons. I run it a little rich to keep it from pinging.

Thank you all. I amd hoping it is a lean mixture issue as I am more confortable around the carb than the distributor.

DennyW, I can only answer the distributor question this way, the weights are what was installed from the factory from MSD. Not sure what power curve that would yeild.

What would you say is the first thing to eliminate? Carb adjustment or timing?

Oh and the float level is good, the pressue is steady at about 7psi. When i adjusted the idle screws i am 1 full turn out from stop. that was where i had most vaccum in gear... it seemed not enough to me but the gauge dipped to about 11-12 when it was turned out more... in that case the power valve would be too big i think.

put the blue spring on and the light sliver spring on the dist and a 5.5 or 6.5 power valve is were i would start try the the springs first that shoud help .i think it may want abit more timing and a faster curve so the springs will help that

thanks Pat. The MSD is a pro billet 8570. i have the instructions with the spring charts. one question that seems to get a lot of different answers . what would you suggest for initial and total timing for the street. i do not race the car. i was shooting for 10-12 degrees plus 21 mechanical advance for about 32 total at 2500 rpm. looks like with the springs in my distributor i was getting no more than about 12 degrees advance at that rpm so i may have the intial way off... might this be my problem?

yes i would start there i would say 12 on the crank and every thing in the dist the ligth springs will help just start out with them springs i told you to use .to get the curve in faster .i would say if you look at your chart with the stock spring you still not curved all the way till past 3500 .i do not have the chart here its at the shop . but i have set up many msd s. i used the springs blue and ligth sliver and the red bushing that works for me run it till 2500 should be all in. pull out or add timing on the crank to get it were you want it from there.i would start at 34 to 36. you could try 32 but my flat top blower engine likes more timming till boost then i pull timming out. the only way your going to know is to try it .IN SHORT GET THEM STOCK SPRINGS OUT

Try the the jets first. Then do one thing at a time. Don't jump in the and make alot of adjustment all at once. Sounds like it is running pretty well other than a lean cruise. If the idle is good leave it alone. If the engine run good from idle to WOT things must be good. With that being said thing can always be made to perform better with the other adjustments.

Originally Posted by Mikej

Try the the jets first. Then do one thing at a time. Don't jump in the and make alot of adjustment all at once. Sounds like it is running pretty well other than a lean cruise. If the idle is good leave it alone. If the engine run good from idle to WOT things must be good. With that being said thing can always be made to perform better with the other adjustments.

and if the engine at 2500 only has less then 20 degs how well is it going to run get your curve rigth or close then the rest will fall in to place

Pat... sent post earlier...did not get posted??? Anyway thanks! Changed springs; set 12 at crank ended up with 33 total at 2500.... reset low speed idle mixture and voila... stumble/misfire GONE!!

Thanks again

Originally Posted by lowntubd

Pat... sent post earlier...did not get posted??? Anyway thanks! Changed springs; set 12 at crank ended up with 33 total at 2500.... reset low speed idle mixture and voila... stumble/misfire GONE!!

Thanks again

great.. see i still have it

So if you had the set the Idle correctly and you had 13" of vaccum and now you changed it and have 11". Could it be that you masked the lean cruise with a rich idle?