overlap with webers

[347]

i am trying to eliminate a flat spot under 3000rpm
i have 50 idle jets
i had 150 mains,175 air correctors,f11 emulsion tubes
i changed to 145 mains and left the others which slightly decreased the flat spot
tonight i changed the air correctors to 200's which made it better again but still not perfect
do i need to try something else or does the overlap of my cam cause this rough patch?
cam specs......solid flat 560lift 248@.050 110 lobes 294adv

i have 44mm idf webers,engine in a 347

thanks

[427Aggie]

Flat spot probably isn't from the cam...I don't know the specifics on the F11 tubes but Cal can probably tell you...most of the guys I know run F7's...I know one question that I always ask for the 48IDA's is what bypass valve are you running? Cal do IDF's have those?

That can make a HUGE difference...I know on the IDA's we have chokes that make a big diff too...I went down to 37MM from 40's and man did that make a difference.

Matt

[Mike I]

347,
I have to agree with Matt, it's not likely a cam problem. A flat spot under 3000 RPM is probably a too small idle jet. Try going up to a 55. Also, the idle bleed adjustment should be about 1 full turn open on all barrels when properly set up. Webers operate on the idle circuits up through approx. 2500 - 3000 rpms where they transition to the mains. Don't mess with the air correctors to cure the symptoms you've described. BTW, F11 tubes are fine. My 48mm IDFs are set up similar to yours. When I was setting mine up, I had some popping and flattness below 3000 and all it took to smooth it out was opening the idle bleed adjustments to 1 full turn. Good luck.

Mike

[347]

buy putting 200 air correctors in would lean out the mains a little?

cal metal and another local guy said 200 air correctors for the street?...i am confused


it has 50 idle jets and its very fumey at idle.....bigger idles....wont that make it worse?
please help!!

[Mike I]

347,
Your problem isn't with the main circuits. As I said in my previous post, a flat spot below 3000 rpm is a symptom of idle circuits not providing enough mixture through transition into the main circuits. Air correctors are for upper end adjustments, changing them will not help your problem. Don't expect Webers to be environmentally friendly at idle, or above, especially with an agressive cam.

I'm attaching a link to the Inglese website that has some useful technical and troubleshooting info. The HP book on Weber induction systems is also a good source of info.

Mike

Weberology 101

[cobrajeff]

Webers run on the idle jets up to around 3000 rpm, when the mains take over. I think with a 347, your 50 idle jets are too small. Most guys I know are running 60's or 65's. Sounds to me like you are running out of fuel before the mains can fully take over.

regards,

Jeff

[427Aggie]

OK someone with IDF's help me here...there is an bypass valve on the IDA's...do the IDF's have the same thing?

If so we need to look at several things...any one can lead to problems.... I know on mine it ran like crap until about 3 grand because I had 0 bypass valves which meant I was pushing every bit of the gas out to the cylinders and she didn't like it...which also made it idle rich.

Matt

[Mike I]

Matt,
IDF's do have the same type of bypass valves as the IDA's. I believe I also have the 0 bypass valves in my carbs, but they seem to work fine for my application. They wouldn't have anything to do with your carbs running rich at idle though, they are only in effect during acceleration.

Mike

[427Aggie]

Correct Mike,

347 stated that he had a flat spot...I know I had a bad one because when I put my foot in it all the gas went to the motor....By changing out the bypass to a .50 all the stumble went bye bye......

Matt