blow through carb trouble!

[eligould]

so we just got the turbo Pontiac put together and rolling, but can't figure out why it goes completely lean after getting full boost for a couple seconds

the combo: 408 Pontiac, 8.3:1, cleaned up 6x heads, arp bolts, single plane, old 750dp, holley hp150, Mallory boost reference FPR, 3/8 feed 1/2 return fuel lines, HEI ignition locked at 20 degrees, GT45 ebay turbo 69mm, homemade passenger side crossover manifold to turbo flange, 4" inlet tubing with a spectre hat, 50mm ebay BOV, 38mm godspeed wastegate.

boost reference for FPR and wastegate is plumbed from the carb hat, bov from manifold.

put floats in carb, sealed choke, drilled pvcr to .059 from the factory .040, jets are now at 66 and 86. vent tubes extended into inlet.

So, If I roll onto it, it is still showing low 10's A/F until it sees full boost (about 8psi) where it settles about 10.9:1. using innovate wideband. part throttle is now acting ok in the mid 12's range.

I stuck a gauge to my windshield and the fuel pressure seems to reference 1:1. I had it set at 6psi and it jumped to 15ish under 8-9psi of boost.

here's my issue. after it kicks ass for a couple seconds, the A/F drops out to lean almost instantly. i'm talking not even 1/8th mile from a dead stop. the only things I can think of is #1, the .110 needle and seats are holding me up, or #2 the pressure above the carb is higher than the 16psi max of my fuel pump.

I ordered .120 needle and seats, but apart from that i'm stumped. any insight from those in the know? drag week registration is sunday! thanks in advance!

[BBO Javelin]

I would check boost pressure at the hat. Sounds like it's running out of fuel due to boost pressure not letting bowls fill when it comes up on boost.

[eligould]

that is what it seems like. my boost gauge comes from under the carb. I don't see it building pressure up against the 750 carb, but I sure could be wrong. it does seem like the fuel pressure climbs a little more than boost at part throttle.

I was also thinking that the vent extensions being up in the incoming airflow may be spiking too much pressure against the bowls. I've read that extensions are a good tool to keep high rpm from going lean, but maybe they are too much on this setup. it's raining again today... hard to test WOT in the rain.

If you think of anything else please hit me with it. running out of time fast to get this sorted out.

[BBR]

That fuel pump is internally regulated to 16 psi. It might be internally bypassing under max boost and dropping your fuel volume thru the floor.

[squirrel]

can you take the regulator out of the fuel pump? or shim it or something? might be a bypass type, eh?

[redneckjoe69]

IDK if this link might help, its very informative.

http://www.hangar18fabrication.com/blowthru.html

i kinda remember something about epoxy on the bowl vents or air bleeds.

hope the link helps and you get it running right.

[Russell]

Can you turn the boost back down to 6-7 psi to see if the fuel pump will keep up? (Discalmer: I don't know anything about carbs and / or turbos)

[squirrel]

might see about putting a little bit stiffer spring in the regulator?

[eligould]

It makes sense that the flow would drop out when the pump hits max and bypasses, but it still has (approximately) 15-16psi at the carb log when it falls out. I would think that if the flow dropped the pressure would drop, unless the needles are pushed shut.

I have read through the hangar18 mods like a bible. we blocked the high speed air bleeds before testing, but we still only got it back to about 10.8-10.9:1 with the high speed bleeds open and 66/86 jets with the .059 power valve channel restrictors.

the vent extensions are something that I've read about a lot of guys on turboforums using.

I bet i could get by shimming the spring in the pump, but the flow is dropping dramatically with increased pressure also. gotta be worth a try. i would order an a1000 today if i knew that would fix it.

and backing off boost.... this is a good idea too. but, this is the 5psi spring making 8-9psi. the other spring made right at 10 psi. the gate was supposed to come with a 5 and 10 psi springs. i'm not sure how to lower the boost without hacking a spring and i don't have any extras. the 38mm wastegate may be the culprit there. i have a manual boost controller that isn't on the car. i suppose i could hack the spring that's in it and use the bleeder to get the boost back later if it works.

i'm going to try the .120 needle and seats first, might yank the vent extensions if that doesn't work, then to the pump. i'm leaning towards thinking the pump pressure isn't enough to keep the needle and seats open, or the vent extension are aerating the fuel in the bowls or something. they have the plastic vent things on the metering plate though... i think it is really only capable of maybe 450rwhp at this boost level, i'm not sure if 120gph will feed that. i'll be back with info later! thanks for the input!

[Orange65]

If you go with an A1000, get the controller too. I think that the 3/8" fuel line will choke that fuel pump and cause it to heat. Or look at a Holley billet pump- you could use one of the twin pumps running one on the highway and both at the track. If you step up on pump, I would also up the line size IMO. Good luck- hope you get it figured out.

[BigAL]

66 jets on the primary seems kind of small, try jumping up to 70's and see if it richens things up.

Just to give you an idea, on the 350 that I just built and had dyno'd, I ran an 830cfm holley and we ended up putting 88 jets front and rear in it and it also has power valves front and rear. We had to jet up 2 jets sizes from what the carb came stock with to get the AFR's right.

[eligould]

If you go with an A1000, get the controller too. I think that the 3/8" fuel line will choke that fuel pump and cause it to heat. Or look at a Holley billet pump- you could use one of the twin pumps running one on the highway and both at the track. If you step up on pump, I would also up the line size IMO. Good luck- hope you get it figured out.

i talked about this exact thing with my brother last night. i've been thinking rigging up a dual in-tank for a while now, i was just thinking of using a pair of 255lph pumps and use the second one under boost. but the way things are going i kinda thought about going with something like a walbro 340 or some equivalent. i don't know how far a single 255 would feed power wise at only 20psi or so.

my brother drove it to work today and no trouble so far cruising. i hacked the wastegate spring and put the .120 needles in. dropped the primaries to 65. as far as the jetting goes, it has been super rich everywhere until it runs out of fuel. just now getting the cruise a/f to 13 and WOT is about 10.9. i haven't gotten to see where the boost is going to hold now. it seems like the leaner we get, the longer it will pull. really thinking we're out of pump. it needs to be a little leaner so i'm hoping to experiment later this evening.

[BlueCuda340]

If your boost PSI is out doing your fuel PSI I can see it closing the needles, going lean, and still reading fuel pressure on the gauge. Sounds like your onto it though! See you sunday

[eligould]

jeg's got me some more parts just in time today. put a 255 BBK pump in the tank, slapped in a new aluminum radiator, welded a section of the intake tube together to reduce couplers to blow off, swapped the spring in the BOV again since I cut too much off the last one.

8pm I finally got it on the street and made a 1-2-3 pull up to about 100mph and A/F sat at 11.2 all the way up with 9 psi on it.

called mokan before I loaded for tnt and it rained out. so it looks like my first strip pass may be either sunday after registration or day 1!

good news is that the pump cured it and it hauls ass! woo hoo! so to every other guy who thinks, "I may be able to get by without a high pressure pump". NO, you can't. and I guess I should have gone with the big wastegate, because I can't get the boost to drop. makes 9 psi with the 5psi spring cut off.

[andy30thz]

BITCHIN!!! Can't wait to hear what you think after you make a pass! Must feel WAY snotty on the street!
Man....talk about getting it done just in time!!!