Proform Carb Body Air Bleed mod question.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
Proform Carb Body Air Bleed mod question.
I've been tweaking the 406 for a while trying to get a decent idle. At this point it'll idle at 1200 rpms,but if I try to lower the idle in abiout 15-20 seconds the engine will stall. Starts right back up,and if I jazz the throttle I can keep it going. It pulls 10 " vacuum at 1200,and I am in the process now of recurving the distributer(Per several forum suggestions) to allow for a 20 degree initial timing setting,and keep the 36 degree top end (full mechanical) setting.
I've drilled 3/32 holes in the throttle plates,replaced the PV with a 4.5,and jets to 74's when I installed the Proform main body.Throttle response is wicked,and I can easily blip to 7000 rpms....but the idle seems a bit high. I was at a local machine shop last Saturday and explained the problem and he recommended reduced air bleed size for the outer (idle bleeds).
Anyone else run into this?What size bleeds should I use,or should I just buy the whole Proform kit Summit has with something like 11 different sets of bleeds,and try them all.
I realize the cam (Isky Z-35 254@.050 .560 lift) and Vic Jr intake are causing the idle,but there has got to be a work around.
I've drilled 3/32 holes in the throttle plates,replaced the PV with a 4.5,and jets to 74's when I installed the Proform main body.Throttle response is wicked,and I can easily blip to 7000 rpms....but the idle seems a bit high. I was at a local machine shop last Saturday and explained the problem and he recommended reduced air bleed size for the outer (idle bleeds).
Anyone else run into this?What size bleeds should I use,or should I just buy the whole Proform kit Summit has with something like 11 different sets of bleeds,and try them all.
I realize the cam (Isky Z-35 254@.050 .560 lift) and Vic Jr intake are causing the idle,but there has got to be a work around.
Advanced
Joined: Oct 2006
Posts: 68
Likes: 0
From: Smog Central CA
3/32nds is what I did to my holley 950HP,was a mistake,I ended up just buying new throttle blades and idle was steady at 850rpms making 14" of vac on a 455 pontiac when back to stock
.
3/32's is definately larger than what my holley had stock.I drilled a larger than the stock size in the carb,there wasn't a need for me drill for larger size so I made myself pay for it in the end.
.3/32's is definately larger than what my holley had stock.I drilled a larger than the stock size in the carb,there wasn't a need for me drill for larger size so I made myself pay for it in the end.
Burning Brakes
Joined: Sep 2005
Posts: 993
Likes: 3
From: Iowa
How is the idle mixture on this engine? That is probably where the problem is located. How does the engine act when you blip the throttle? Does it come right back down and settle to idle? I would look to fix this because you are right, the cam you have and intake should idle lower so there is something that is not quite dead on yet.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
Last time I ran it the idle does return to the 1200 ish range,throttle response is quick,(not tranny,just flex plate)
I did not know how low the idle with this size cam could go,which is why I posted in the first place. The 406 combo I have is big for a smallblock combo,but small for a big block,so what work's for a big block with ease is a stretch for a smallblock.I feel like I'm close to ideal with the way it's tuned now,but still would like the idle to come down a notch,which the air bleeds may do. Time will tell I guess.
I did not know how low the idle with this size cam could go,which is why I posted in the first place. The 406 combo I have is big for a smallblock combo,but small for a big block,so what work's for a big block with ease is a stretch for a smallblock.I feel like I'm close to ideal with the way it's tuned now,but still would like the idle to come down a notch,which the air bleeds may do. Time will tell I guess.
Burning Brakes
Joined: Sep 2005
Posts: 993
Likes: 3
From: Iowa
One thing I didn't ask that I should have is what type of cam is this, flat tap or roller? It sounds like it could be a flat tap and those tend to idle pretty bad from my experience. Either way one thing you can do is loosen up the lash a little bit and take some of the bark out of it. Just for comparison purposes, the 355 that sits in my 79 vette has a solid roller out of a sprint car engine that hits .740 lift on the intake with a 1.7 rocker and it idles down at 1200 RPM so you should be able to get it down to at least 1000 RPM. Try loosening up the lash to at least the manufacturer’s spec of .016 hot and if that helps you can go to .018 or so. You will sacrifice some lift and upper range power but it should allow the engine to behave better.
Racer
Joined: Jun 2006
Posts: 309
Likes: 0
Don't mess with the idle until you have the initial timing set to 18 or 20.
Don't drill holes in the throttle plates. Get the front throttle plates in the right range for the transfer slots then if you need more idle air use the secondary idle stop. That much cam will require fat idle restrictions probably .040 or greater (if you can't make the idle overrich by backing out the idle screws the idle restriction are too small adjust .002 at a time until you gain control). The idle airbleeds need to be balanced against the high speed bleeds so to start I'd put the original idle bleeds back in. If the cams real ratty it helps sometimes to convert to a four corner idle mixture configuration. Recognize that an idle that hunts (goes up and down, starts high then slows down and dies) is always caused by too lean a mixture.
Don't drill holes in the throttle plates. Get the front throttle plates in the right range for the transfer slots then if you need more idle air use the secondary idle stop. That much cam will require fat idle restrictions probably .040 or greater (if you can't make the idle overrich by backing out the idle screws the idle restriction are too small adjust .002 at a time until you gain control). The idle airbleeds need to be balanced against the high speed bleeds so to start I'd put the original idle bleeds back in. If the cams real ratty it helps sometimes to convert to a four corner idle mixture configuration. Recognize that an idle that hunts (goes up and down, starts high then slows down and dies) is always caused by too lean a mixture.
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Racer
Joined: Jun 2006
Posts: 309
Likes: 0
Originally Posted by cuisinartvette
It should be able to hold an idle with that cam...Wondering if at 1200 you are even operating on the idle circuit ..
