New to the forum, Hello and thank you in advance. I have followed the turbo forums for years but finally joined as i see there is some very helpful people in here. Ive done a lot of searching and dont want to start changing things based on other peoples builds.

First off, 355 single gt45 turbo. no intercooler, demon 750 bt carb, aluminum 2.02 1.60 heads. eagle crank, eagle h beam rods, wiseco pistons, multi layer head gaskets, arp harware throughout. Pretty much all brand new due to catastrophic failure of the first engine with about 1 hour run fun time on cast pistons. went hard lean and it was over quick. Got the new engine built and now Im back in the tuning stages. (actually just pulled engine back out to check everything as i currently have about an hour of run time on the new setup with a couple lean pulls.

Idles best at 13.x afr, tip in throttle goes dead rich but cruising and part throttle are running great around 12.8 to 13.0. As soon as it sees any boost gauge leans out to around 14.5 to 15.0 and I let out of it. wastegate is on spring pressure at 5lbs. I am assuming my issue is in the secondary side but im not sure. had to jet down the primaries to get part throttle a little leaner. Ive got 72 jets in the primary side with bprv and 90 jets in secondaries. Not sure where to go from there. Any suggestions welcome. Thanks again and sorry for the long post.

 

Need to include what fuel system and how it assembled (line sizes, type reg, etc).

 

also include hat type
do you have a visual fuel pressure gauge?

 

Also, when you are just sorting stuff out like this....throw some race gas (or 100LL avgas) in it until you get things in the ball park. This will lessen the likelihood of hurting parts during tuning mishaps.

 

I have a magnafuel 275, fuel cell with 10an lines to magnafuel boost referenced pressure regulator. 6an outlets to both bowls. I do not belive its fuel sytem related. I have the spectre bonnet. Its only seen 4psi boost so far. And yes visual fuel pressure gauge set to 7psi idle and raises correctly with boost.

 

With that hat and carb combo, it needs vent tube extensions.

 

Thank you for all the replies so far. I will pick up some 110 fuel to make sure im safe.

I ran vent extensions to test that theory but it caused it to go super rich cruising. I do feel like it was better under a couple psi boost but everywhere else it was fat as could be. I understand i should jet it down to fix that with the extensions but i was told by an old head that vent extensions were not needed and that if tuned correctly the carb should produce enough fuel regardless. I will run vent extensions and see what happens. Should have the engine back in tomorrow after work.

 

About how long should the vent extensions be? Im finding many mixed answers searching

 

I always had good luck with them starting 1-2 inches in from inlet with them centered in the inlet diameter.
But I have also found that if I made them tall enough with a 90 deg bend to aim them directly at air flow, they worked too. But then you must lock tight them in place or something. Any way you do it, make sure they cant come off.

 

The old head is correct in one aspect. Carby expects the same signal everywhere. That means, in the bowls, and each booster, at the air bleeds. When any one gets more or less pressure, the carb doesnt function properly With carb hats, if the bowls dont get the same pressure as the booster, then the venturi effect doesnt function to its fullest so the proper amount of fuel doesnt get pulled through. This can happen if the vent tubes are just short enough that the fast moving air flows right over them.
This can amplify the issue by causing a vacuum and lower the pressure in the bowls even more.

Same thing can happen in NA setups with forward facing scoops.

If its getting super fat on the primaries it could be that your bprv now needs to be adjust to come in later.
Or, try a jet extension in the rear 1st and see what happens. If just trying it in the back, I suggest you keep an eye on the plugs for front to rear enrichent.

 

Ok that makes sense. I will look into it tomorrow afternoon and try what you say. I'll let you know how things go. Again, i really appreciate the knowledge. I can't wait to get it running safely and turn it up little by little.

 

Ohh ok awesome information. Thank you again for all the help. I will try these things tomorrow evening and keep posted on my results. I can't wait to start turning it up safely little by little. It's in a 4500lb truck but pulls very nice. Im excited. And then I'll lose a few pounds hopefully once the engine is happy and i can move on to other things.

 

Well its back together and running. Got the vent extensions installed and she is running amazing out of boost. I did have to jet down to get part throttle afrs a little leaner as it was just dead rich around 11.5 to 12.0 cruising. Ive got it cruising at high 12s now and tips into 13.2 occasionally. Feels great. Plenty of power down low but it still seems to go lean when i tip into boost (2psi ish). Its on a gate pressure right now at 5psi max until i get it all figured out. I did get upset a bit and stabbed it for a second to the floor. It seemed to richen up, i believe due to the secondaries coming on but im still stumped. can i unhook the secondary linkage and tune just the primaries until i get them working? and how can i get more fuel under boost but not richen up the cruise mixture?

 

Do you have a standard vac power valve or brpv?


One thing about tip on carbs when tuning, or if calibration just isnt correct yet, tip in doesnt tell you anything about WOT. quick WOT 1 or 2 second hits will tell you if afr is correct.

 

You can drill the pvcr orifices. That will give you more enrichment when whatever condition is required to open the PV. Whether that is boost or lack of vac. Depending on the type of PV you have and how the carb was set up.

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I do have a brpv in the primaries. Guess i will pull the metering block and have a look. What is a decent size to start at? Or do i measure the current holes and enlarge 1 size?

 

Measure and enlarge

 

When having issues, I have unhooked the secondaries, and tied them shut. It's nice to work on one end at a time. You'll know if your pri/sec fuel is equal too. Win-win.