Ok so i gave up on my old tune. I could get it to idle and wot good. But my part throttle. Was 9.0afr. And i cant live with that. Seems like vent tubes just nake everything to fat. I was 70 71 jets squared no power valve. So now i want to start a more traditional tune with a power valve. Its a street car. Sbc 355. Ebay gt45. So right now. Im 89 and 99 on jets. No vent tubes. And 9.5 pv. Pvchanels are still small. Right now it it idles fine but. Once i get around 9.5 vac. It goes full lean. If. I force it past. Seems like the pv opens and. To 17afr till 0vac where it full leans again.
Is there a way to get these blowthru carbs to work with out vent tubes. Once i get vtes to work in wot. Its to rich down low. Then i get down low leaner its to rich up top. Help

 

what size carb?? jsut curious we have a 355 with a 600dp with extensions and it works good,, i think 67 fronts 79back .0860 pvcr and a 4.5 or 5.5 pv 1103 summit cam

id stick with the tube extensions

 

My timing is locked at 20* i get ping any farther.

Blowthru demon carb 850cfm. I know its big for a 355

 

I can get it dialed with vent tubes but its rich at 0vac. Like 9.0afr. Then it setles at 11.7 in boost. But its killing plugs and gassy oil

 

Thats with pv pluged. The vent tubes do a good imitation of a pv. Cuz it gets rich quik too rich

 

fuel regulator and where is it referenced ?? fuel pump??

 

Aromortive 13301. I think and ref from the hat. Bosch sumthing big. 300gph i think. Ive run high boost on this set up. Like 14 psi no lean out

I could get the carb to run at idle-14.1 afr and wot. 11.7 afr Its just to rich in transison. 9.0afr With pv pluged. That was 70 and 71 jets. I was only running 12psi. Today i even took out the hi speed air bleeds out all 4 cornners and it was still rich right before boost

 

I think im gonna try tommarrow. To
Use vent tubes all the way to the inlet of the carb hat.
Use the smallest jets i got in front. 64 and a pv. 9.5 the. Use the 72 jets in back maybe open the pv chanels there like a 5/64 drill bit

 

What do you have for squirter in front and rear? What cams and where are they set ? Are you getting this fat spot with pedal movement or a few seconds after? Next what heads are on it and how much valve lift ? Last what is your vac readings at idle and at cruse? Are you sure that you have no leaks?

 

37 squerter pri and 31 sec. White and pink cam 1 setting its no the pump shot. Thats to rich. Camel hump heads 461. .530 lift. 17idle vac. Crusing is about. 15 vac

 

Ok so you have a 9.5 pv and 850 carb with 461 heads . You get fat at almost 0 manifold vacuum. And you have vte . So let's get this in one spot and try to sort it out . You have to base line it so first make sure you have no leaks. Next block both power valves . Get it running. Set timing . With vac gauge set idle screws. Then drive it at cruse speed and jet it to drive(main jet in primary) no wot at this point. Get like you want it to just drive it not getting in to boost at all . Then take note of the front jet . Now install a jet about 12 numbers bigger . Put same jets front and rear try it wide open and if it goes lean get out of it jet all 4 up till it runs wot . Now figure out how jet sizes you went bigger . Calculate the jet area and make the pvcr match the difference re install the cruse jet and drill pvcr to size calculated. Install a 6.5 pv and try it if it goes Ritch lower to 5.5 try it if you get it to transition and then it seems lean you may have to swap to brpv and use a crutch or drive pressure to operate it .

 

With those small cubes, and an 850 carb, you are fighting a losing battle. Get yourself a 600 or 650 main body, put the vent tube extensions in it and start tuning from a little above stock jetting for a 650 (say 70s and 80s) and tune from there. You don't need any more than a 650 to make 1000 hp on blowthrough.

And DUDE, make just one thread about your tuning problems and keep all the info you find in one place!!! Otherwise, trying to help you is gonna be like trying to herd CATS...

 

Should i do this without vent tubes

 

I loved using the 1:1 linkage on my carb . AFRs very consistent throughout . Annular boosters primary and secondary . Jetting squared . PVCRs squared . two BRPVs ( that work ) .

Never needed more fuel out of boost or in transition .

No vent tube extensions !

If I remember right .....lol , jetting was either 68 or 70 , and PVCRs were either .078 or .081 ...........

 

carb is too big. why did u go so big/

 

I got the carb for a deal that why its big. So i got that part throttle richness problem on the leab side

Heres what i did. I put my carb back to where i had it drivable. And wot afr good. The only that sucked was 9.0 afr from -7 vac to 3psi. Heres the setup

71 70 jets. Blocked pv. Took out the high speed air bleeds completely all 4. I got vent tubes.to the inlet of carb hat Someone on a pontiac forum. Said to drill small holes in the vent tubes. To bleed signal So i put abut 15 on each

Afrs
Idle 14.3 to 15.1.
Throttle blip. Quick throttle smach. 12.5
Crusing. 17.1 to 14.2 engine seems to run fin leaner
Part throttle. 12.5. This was before 9.0
Boost 8psi 11.8 -12.5.
Around 10 psi. 13s I start. To lean out now

The turbo spools a bunch fast no bog. When i get on it. The afr flash to 12.5 then around -4 vac it flashes back to 13.6-14.1 range and then snaps right to 11.8 and stays there till it leans out around 10psi. At about 4k rpm


Im gona put the 8psi wastegate spring back
in tommarow. Currently it has a 14psi on

Then i might put the hsabs back in at a big size and slowly get that wot afr down

 

Reality is your probably going to find that the upper hole in the emulsion package will need to be enlarged a size or maybe 2 and the lower one will need to get a little smaller if yours are screw in then ok if not don't even try to jack with them . I agree that your carb is to big and it being a blowthru carb from the get go they made it with different fuel curve than if you had a box stock carb . So all that works against you. If you make the emulsion hole to the top bigger it makes the early transition leaner but also leans the entire curve if you make it smaller it fatten the early part and the entire curve so you have to tune all the way around it. The lower holes do the same thing but at different times bigger is leaner smaller is fatter so if it runs good up top now and you are fat on the boost after the pump shot is in then you enlarge the top hole it's going to lean out the whole thing from there up . So you have to shrink the lower holes to off set the curve once you get past the fat point. It's a lot of bowls off and on but it takes that sometimes. And get a note book and keep notes so when you get in left field you can reset . Notice how I said when not if right. Cause its going to happen you will make break through and through and then you will keep going then you go what the fuck and you back up a step and it was really 5 steps ago you lost your number.

 

Ok you guys i got a wierd one. So i put the 5 psi wastegate spring in and i was getting the afrs where i want em. And. I was tunning about. 35% throttle and afrs 11.8 - 12.0. And is spikes to 6 psi. And stays there and i could hold 35% throttle to 6k at 6 psi. Fine. So once i got comfortable with that i started going wot. Like 45-70% throttle. And once i hit 3300k rpm where the wastegate opens. The rpms just stay there and wont go higher. Even if i push the throttle more. Afr is at 12.0 and boost is a steady 6psi. And my tach justs stays at 3300 to 3500k. If i go back to 35% throttle. Its goes up in rpm.

 

Like a bad tach or the engine won't rev ?

 

Like the engine wont rev feel power loss. But if i go 35% throttle. It will ho past it. Only at wot it does it. I feel like the waste gate has to do with it

- - - Updated - - -

While its there at 3.5k if afr is 12.0 and. The boost gauge is still at steady 6psi