12 replies to this topic

[TwelveSeventyFiveGT]

Turbo Won't boost.

I've just re-fitted the engine after a gearbox rebuild.

Now I just can't get the turbo to boost and I've run out of ideas.

Engine Revs freely and drives in N/A conditions fine. As soon as the Turbo starts to come onto boost (2 psi) the AFR meter instantly goes lean (high 20's) and the engine loses power and no increase in boost.

I think it is fuel related or fuel pressure but just can't come up with any other things to check.

It’s at newly refurbished 1752 Garret Turbo, fusion fabs manifold and plenum.

What I have checked so far...

1. Correct Gasket between plenum and carb to pressurise the float bowl.
2. All pipework from Turbo to intercooler to Plenum (all checked - no leaks)
3. Fuel pressure into regulator 51 PSI. Regulator to Carb 4 PSI.
4. Pipe from plenum to regulator fine and ‘TEE off’ Trick fitted to ensure carb float is pressurised.
5. Carb Rebuild with SU Turbo Carb kit.
6. Timing map on Magajolt still as it was before the engine was removed.
7. Blow Off Valve pipework fine.
8. No leaks from any pipework going to turbo gauge, mega jolt and BOV.
9. Fresh fuel.
10. Fuel Regulator diaphragm perfect condition.
11. Avon bar 14PSI needle and green spring.
12. SU Damper Oil level checked (SU carb oil)
13. The Valve in exhaust that is connected to actuator closed and works fine.
14. No air leaks apparent.

It’s just very strange why the AFR meter runs into the high twenties as so as it starts to boost.

Is it common for the part of the carb that pressurises the float bowl to become blocked?

Any ideas would be appreciated or what I can try / test before I start thinking about buying a new Carb.

Many Thanks

[Ethel]

Could you rig up a test for the pressure regulator to see if the output (carb) fuel pressure goes up when it senses boost pressure? A bike pump or syringe would likely do as it only needs to be static pressure.

[TwelveSeventyFiveGT]

Never thought of that.

If I connect the fuel pressure gauge to the fuel output of the regulator and then increase the pressure (bike pump maybe) into the regulator using the pipe that connects to the plenum. With the fuel pump running I should see the output pressure rise.

Does that seem right?

Thanks

[Ethel]

Is it a pucka turbo carb?

 

Another thunk - If you have an extra, external, boost line to the float chamber as well as the internal one could it blowing round in circles and actually sucking on the float chamber with the Bernoulli effect ? Might be a bit fanciful, but dead easy to eliminate, to rule out.

[TwelveSeventyFiveGT]

Yes, it’s a Turbo Carb.

I only tried the Tee Trick a few days ago to eliminate the float bowl pressure issue. I think I will remove it tonight.

I’m going to test the regulator tonight and see if that possibly is the issue.

Thanks

[TwelveSeventyFiveGT]

Just checked Fuel regulator and the output pressure increases as I increase the psi into the pipe from the plenum. Looks like regulator is fine, to be honest it was new and only seen about 5 miles.

🤔

[mini13]

I would say stuff a new set of plugs in it...

 

on numerous occasions ive had plugs that "go bad" and will be ok running at idle... & maybe cruise, but soon as you give them throttle or boost they bread down and misfire, (also worth noting that a misfire can read lean on a WB lambda gauge)

 

a lot of plugs dont like being wet with fuel and don't really recover from it... i think it gets into the porcelain and leases some kind or residue

[Ethel]

It could still need adjusting. you'll want the fuel pressure to be above the boost pressure, ideally by about the fuel pressure without boost.

 

Does sound like you've eliminated it as the cause of abrupt under fuelling though.

[TwelveSeventyFiveGT]

Good call on the plugs, I’ve had that problem previously. I put new plugs in last week and no improvement.

I’m going to increase fuel pressure via the regulator and see if that makes a difference. I’ll report back.

Thanks again for your replies

[TwelveSeventyFiveGT]

Fuel pressure increased to 4.5 psi and the issue still remains.

Carb back off at weekend for another inspection.

Do none turbo carbs have the ability to pressurize the float bowl from the plenum? Do they have the internal pipe work as a turbo carb would have?

Thanks

[Ethel]

Not that I'm aware of - Easy to spot by the lack of symmetry on the gasket face courtesy of the extra  hole.

 

It does sound like that sort of issue, something blocked (or unblocked when it should be blocked) causing a breakdown in communication.

 

The Turbo carb connections:

 

That internal port from plenum to float chamber

 

The fuel overflow/vent pipe - had the solenoid valve that shut it off when the ignition was on in factory spec. I assume you have it blocked one way or another!

 

The part throttle economy do-dah - the one with hose connecting it to the manifold flange above the throttle

[TwelveSeventyFiveGT]

I’ve changed over onto another Turbo carb, should know by the weekend if it has cured the issue 🤞🤞🤞

[TwelveSeventyFiveGT]

New carb has rectified the issue. Closer inspection of previous turbo carb needed as it seems the internal air pressure channel to float chamber must be blocked.

Thanks for advice responses.