My draw through turbo

[advinnie]

Well it started of this morning at only 2 degree C / 35 F in your money but it's a massive 13 C (55 F) now

[advinnie]

Lads I need some help. I've helped a friend build a draw through set up on his bug and it's all most working great. It's great when theres no boost, it's amazing when it's on boost but the transition between off and on boost is bad.
This is what's happening, your crusing along at a steady speed and have a high vacuum reading in the manifold and lots of ignition advance, you then floored it the vacuum gauge reading drops the ignition retards but when the vacuum gauge reads zero it stays there for a long time and the engine just doesn't accelerate as it should, you can hear the turbo spooling fast but it just hangs up at the zero vacuum reading but as soon as it hits 1psi boost it's of like a rocket and boost builds fast. Any idea on why it's getting hung up at the zero vacuum reading (100 Kpa) ?

[madmike]

Hey Adam, a photo would help with all the engine spec's? sound like a fuel issue to me

[advinnie]

Sorry I have no photo of his set up but it's the same as mine. His engine is a 1600, C/W crank with a 120 cam and 1.25:1 rockers witch I said would be to big but it works great. He's running a compression ratio of 8.8:1 but only 6psi max boost because of his higher than wanted static compression ratio. He's crusing with 13.4:1 A/F ratio and 12.1:1 at 6psi and the engine loves this ratio. Here's the updated as the manifold vacuum hits zero on the gauge the A/F ratio gauge goes mad it fluctuates all over the place and then sorts it's self out as soon as boost kicks in. So we are slowly getting some where and can definitely say something is going on with the fuel feed between the carb and engine but what?
The carb is a Weber 40 dcoe but a different model to mine cause he two air bleed screws with I don't have.
Yes madmike you was right it does have something to do with the fuel. The fuel pressure is set a 3psi and we have tried fuel pressures from 1 psi to 6psi.

[Dan Dryden]

Perhaps a small leak in the inlet manifold?

[advinnie]

That was my first thought so checked all that and no leaks found.

[woodsbuggy1]

The best method that I have found to check for intake leaks is with an unlit propane torch, the propane burns just like fuel and creates no mess.
Good Luck
Kenric

[madmike]

Can you swap carb's with him needs a bigger 'squirt'

[advinnie]

We have thought about it but mines working to good to take apart
Anyway here's an update.
I think is the fuel pressure regulator that's at fault. Here's the story, we plumbed in a fuel pressure gauge inside the bug to see what it was up to and it was a good call to do so
At idle fuel pressure is set to 3psi, at cruise with a -7psi manifold gauge reading the fuel pressure going to the carb drops down to 1 psi then as the manifold gauge reading drops when he floors it the fuel pressure to the carb starts to rise and rises fast and by the time the gauge reads -1 psi the fuel pressure is up to 8psi and hits 12psi when the gauge reads zero and it's at this point the engine goes flat and heavy and acceleration stops and black unBurt smoke start coming out of the exhaust, then when boost finally starts and reaches 2psi boost the fuel pressure drops right back down to 4psi and all is as it should be.
So long story short the fuel pressure goes through the roof for some reason when manifold pressure gets closer to atmospheric pressure and then sorts it's self out when boost pressure builds.
Hope this makes sense?

[Chip Birks]

Sounds like you need at least a one way valve in the line to the fpr.

[advinnie]

He has one fitted I was the one to fit it. I will text him and get him to pop down the shops and get another and I will fit it tonight but I doubt that will solve anything but it's cheaper than a new regular.

[Chip Birks]

The only thing pointing me there is that pressure drops under vacuum, obviously the one way-ness isn't doing its job. Is the regulator rebuildable?

[advinnie]

Hi all been away camping, anyway yes it was a bad non return valve on my mates turbo bug that was causing all the problems but unfortunately it had broken the regulator as well but it's all been replaced and working fine.

[advinnie]

Well on to my bug now. For the last few weeks I've been running 14 psi boost witch is bloody amazing but as I said in an earlier post to run that boost without get detonation I had to run rich witch mean when of off boost she was well to rich and the bug felt heavy and fat. I tried leaning her off but got detonation even tried regarding the timing but lost power and still got detonation
The happy place my bug wants to be in where she runs good off boost and on is with a pair of 50F9 idles and 1.35 mains. Any larger jets than this the off boost performance goes down and normal street driving becomes a pain. She runs the best with 45 idles and 120 mains but that only allows me to run a few psi boost before to goes lean and detonation kicks in. With 1.35 mains and 50 F9 idles I can safely run 8psi boost and still have great of boost performance and good town driving performance so that's where I'm staying for now. If it ever goes on the track it's only a five minute job to change the jets and wastewater spring anyway. So hopefully this Christmas I may go EFI than I can have controller over the fuel and get back up the 14 psi again
I'm going to have a play with the .70 pump jets next to see if they can be made smaller

[buguy]

I've read that a lot of guys can only run 8-10 psi safely. Sounds pretty normal to me. Your running with an efi crowd here and probably just got your hopes up for more. Save your pennies, go efi, and enjoy the benefits they do.