half throttle stumble help!??

[jasonsnova]

ok guys a few weeks ago i finished the topend swap on my nova wich consisted of afr 195 heads,air gap intake and a voodoo 268 cam, well i put it all together and it runs decent but i now have a half throttle stumble,wierd thing is from a stop to full throttle hard run it seems fine ,it only does it when your cruiseing along at part throttle then step on it easy it stumbbles till i get to wot then it clears out. i didnt have this stumble before, so i pulled a plug and i was ridiculess lean! it had 73/78 jets in it, so at 1st i bumped them to 78/82 ran it for a few min and it still appeared lean so i bumped it again to 82/82, then took it for a ride and i still have it. i have checked the float level,and squiter play and fuel pressure as well with no luck. so guys give me some suggestions!!! i really want to go to the drags soon but need to get this straightened out 1st.........any ideas????

[jasonsnova]

sorry guys i also should of mentioned its a 4150 hp 750 cfm. also a little light bulb just popped in my head.... a raceing friend set the carb up for me when it had the auto/highstall conv and bigger cam....he took the powervalve out and blocked it off. could that be my problem?? im thinking that it didnt do it before because it was basicly a drag race set up , but now it is making alot more low end and maybe it needs the powervalve??? i honestly dont know alot on how they work.....

[68Formula]

Put the powervalve back in and start jetting back down. Or always drive at WOT.

[jasonsnova]

Put the powervalve back in and start jetting back down. Or always drive at WOT.

drivin wot....hmmm. lol! anyone know what size power valve i should try????

[6'9"Witha69]

With that cam, you should still have decent vacuum @ idle, so start with a 7.5 and see how that works.

[jasonsnova]

ya im gonna set up a vac gauge, i went on to holleys web site and they said 6.5 12" or over, and anthing thing under divide vac in half ....

[68Formula]

You should be getting at well over 12"Hg vacuum, probably closer to 18" if the tune is right. So 7.5 would probably be a good start.

[jasonsnova]

ok...so any ideas on a good start point on jetting??? its got 82's squared now.

[68Formula]

Where you were in the beginning sould be a lot closer.

[6'9"Witha69]

You should be getting at well over 12"Hg vacuum, probably closer to 18" if the tune is right. So 7.5 would probably be a good start.

That was my thinking exactly with the 268 cam

Where you were in the beginning sould be a lot closer.

I concur!

[My427stang]

Recommend going back to stock setup for all values, including jetting and powervalve (6.5 is stock, but I also agree a 7.5 will work great here)

One of the the things I see most with a Holley is when guys start chasing tuning issues is usually a float level issue

The idle, transition, and main jet circuit are all very sensitive to float level issues, the higher the float level, the easier for the circuit to "energize"

If it were me, I'd return the carb to bone stock, ensure the accel pump was adjusted correctly to .015 of clearance at WOT, and ensure my initial timing was 10 BTDC or greater, then set float level to were it runs out of the float sight plug.

Not "rock the fender and make it come out" but a clear level to the bottom of the hole at idle.

Then I'd run it and see what I had, often an addition 2 "flats" when looking at the adjuster nut higher than coming out the hole will make all the difference

I will say that your jet changes did nothing because it was the wrong circuit, you are fighting a transition circuit issue and you were soliving it with main circuit fuel, apples and oranges

Get it back to stock, verify timing and let us know what you come up with

[jasonsnova]

well heres the latest,i havent drive the car this past week so i havent done much, but i did put in a 6.5 i had laying around and put the jetting back to 74/78 and i dounle checked everything else. it still has a stumble but is much better,i looked at the plugs and it is still a little on the lean side, i picked up a 7.5 power valve just havent gotten around to it yet. gonna try that soon.

[My427stang]

Try raising the float level 2-3 flats, it may surprise you without going inside the carb

Remember, the transition and idle circuits are siphons, if the float level is higher, its easier for the vacuum to pull the fuel in.

[jasonsnova]

what do you mean buy" raise the float level 2-3 flats"??? ive always rewmoved the sight plugs and slightly shook the car and raised till fuel just barely comes out.....

[6'9"Witha69]

You need it higher than that. like 1/3 up the hole. The biggest problem with float levels too low is stalling the car on hard decel. Very hard to manuever in a panic situation w/o a running engine. Never mind trying to rev up while hard braking into a turn!

[My427stang]

69 is right, you need to raise your float level so its running out lightly when idling.

When I say 2-3 flats, I mean the nut. Lets say you adjust it where its coming out. After that, turn the nut an additional 2 - 3 flat part of the nut on the needle/seat assembly. That way you can keep track of how much you turned it and then see what happened and go back easy

However, it does sound like your float level is low.

You may need more powervalve, but I doubt it.

[jasonsnova]

ok guys gonna mess with it tomarrow... i havent had time during the week lately and the last 2 weekends it has rained so i havent had the car out, i do know i have to rock it pretty good for the fuel to come out ,so i think i'll start with the level, and will probally jump another jet size...i want to go to the drags next weds and see what it will turn and id like to use the nitrous so i need to have it right on!!

[6'9"Witha69]

...{snip}id like to use the nitrous so i need to have it right on!!

Damn straight! Too lean then dump n2o will KILL your engine!!

[kamaroman68]

Is it possible you have a vacuum leak???? A vacuum leak will also cause a lean condition as you described. With all the work you did ie heads, intake... you may have missed something very small causing the leak. good luck

[jasonsnova]

well....i went to do some simple carb tuning yesterday and found both head gaskets are leaking anti freeze to the outside in the rearsides!!!! wtf!!! ....i retorked one back down and it stopped,still have to do the other tonight, but i dont think that is my problem, anti freeze wasnt leaking into the oil or the cylinders....wierd i put the expensive mls gaskets and new studs on it too... anyway it still appeared lean so i bumped the primarys from 74 to 76 and the secondarys from 78 to 79 and changed the 6.5 powervavle to a 7.5, but now i have to cure my leaks, then i'll check the lean/stumble issue....oh ya and i did check the float levels again....all good their