Bogging Down
Turbo Mini Forums

Hi Everyone,

looking to see if anyone out there has some suggestions on a bogging down issue I'm having.

Car's spec is -

1275
stock head, crank and pistons.
Metro turbo T3
Stock turbo regulator
Sytec fuel pump
HIF 44 Turbo carb recently rebuilt by AC Dodd with 5w-30 dashpot oil. balance pipe blocked off and part throttle lean off connected.
Lucas electronic 65D distributor

The problem I have is any sharp movement in throttle and the engine bogs down/goes lean. But if you slowly apply throttle it will rev up.

I went out for a drive the last time I looked at the car 2 weeks ago and recorded a video of a fuel pressure gauge i have between the carb and the regulator. its set a 3.5psi at idle but when you apply throttle quickly you can see that the pressure is dropping. I thought that must mean the pump was on the way out as the regulator is a brand new one from mini spares, so I replaced the pump with the Sytec unit and still have the problem.

Looking at some threads with people having similar problems, people seem to put the issue down to timing. So today I have replaced the old turbo dizzy with the 65D and set the timing to 6 degrees at 1500rpm with the vac pipe disconnected. I still have exactly the same problem, any sudden application of throttle and it bogs down and goes lean.

Does anyone have any ideas? I've changed so much on the car now that im starting to think there is some other underlying issue with the engine its self despite it and the turbo having been rebuilt less than 500 miles ago!

Any suggestions of what to check or do are really appreciated!

Cheers!

Edited by Glun34 on 29th Mar, 2020.

do you have a boost pressure signal pipe connected from the inlet box on the carb ,to the fuel pressure regulator ?

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Yes, got a pipe going from the plenum chamber to the pressure reg for boost signal

coolio , well the pressure should deff not be dropping ,so i would focus on the fpr , do you have a piccy of how its connected up ? and of the carb ?

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

As Robert says. I wonder if the fpr reference is coming from the wrong place? It needs to come off the plenum not the inlet.

Would dropping fuel pressure cause the issue above though? Wouldn’t it need to empty the float bowl before a problem surfaced.

Try with the part throttle pipe blocked off (both ends).

Also, is the float vent pipe blocked off?

Send a photo showing all your carb connections.

In the 13's at last!.. Just

Thanks for the replies everyone.

here's a few photos of my carb setup. At the moment I've got the part throttle lean off connected and have tried as well with them blocked off at both ends and still have the same problem. The fuel pressure reg that is on there is a brand new one from mini spares. Its hard to see in the photo but I have the feed line from the tank going in to the right, return at the bottom, top left is going to the plenum pressure signal and the lower left connection is to the carb. I thought that replacing the fpr would solve my problem, but it does exactly the same thing with this brand new one! Starting to think there is an air leak somewhere but im not sure how I could narrow that down. I also do have the correct turbo gasket between the plenum and the carb as I know its a common issue to put an NA one in there by accident.

Attachments:

3.31mb

2.7mb

I should also say theres a standard BDD needle in there!

what needle is in the carb , its hard to see is that blue pipe from the top of the fpr going into the corner stub on the plenum ? what do the plugs look like .

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

With a pressure gauge inline inbetween the regulator and carb, what happens to your 3.5 psi base pressure if you apply pressure (say 10 psi via an air line or foot pump) to the boost reference on the fpr?

Shane

Edited by shane on 30th Mar, 2020.

Long time ago, sure had similar on my turbo van, was the vent solenoid not closing, from memory, but that was 25 years ago.
Sorry not much help.

its the standard BDD needle in there at the moment. I'll have a go testing the regulator and see what happens. I think it must be working though as on boost the afr is good so the fuel pressure must rise with boost, its just any slightly quick movement with the throttle and it goes lean and bogs down.

Hate to sound silly but where is the vent solenoid located? its not something I've played around with yet.

Cheers for the ideas guys!

starts to sound like the needle needs to meet mr file :)

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Ok I'll try some ticker dashpot oil like 20w50 and see what difference it makes.

I also have another dashpot spring that isn't marked but much stiffer than the yellow one thats in there at the moment, so that also might be worth a try.

Cheers for the suggestions everyone!

only try one thing at a time otherwise you won't learn what has cured it.

Shane

Ok, put some thicker 20W50 in there this afternoon and it had the same problem again! Any reasonably quick movements with the throttle and it would lean out straight away. I did just out of interest put some 3 in one into the dashpot after that to see what would happen and again of course I had the same problem.

So now im at the point were I have a rebuilt carb so I know thats good, brand new pressure regulator so in theory should be good, new fuel pump so I know thats not the issue and I have checked the plenum and it all looks fine and unblocked.

My 2 theories now are the fuel pressure from the regulator isn't constant and fluctuates between 4psi and 3psi, so is there a block in the fuel system? That however wouldn't make sense from the way I understand it is that the float bowl would need to be empty for it to be totally lean and bog down? Second theory is that I have a leak somewhere after the carb. Has anyone had experience of a leak from the servo connection on the inlet manifold? what is the best way to check for a leak in general?

Unfortunately it seems I have a more underlying issue that just dashpot oil or needle profile.

Cheers for all the input, its much appreciated!

Edited by Glun34 on 1st Apr, 2020.

Don’t rule out needle profile. It could certainly still be the culprit.

You could pull/cut off the servo hose and block it to test. These can be prone to cracking.

In the 13's at last!.. Just

Ok, so today I tired blocking off the servo connection on the manifold to see if it was leaking, but still the same issue. Goes lean at any sight quick throttle usage. Then got out some vacuum gauges I have for balancing bike cabs and connected one to the dizzy vacuum advance connection on the top of the carb. Saw no drop in vacuum on throttle so I can only guess from that there’s no leak in the system.

Should I try the next needle up from a BDD? Does anyone know what that would be? Is it also worth trying a stiffer dashpot spring?

what fuel pump are you using?
what size fuel lines?

My build thread..

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=542985

Did you fit a pressure gauge between the regulator and the carb' and prove that the fuel pressure rises with boost from the 3 /4 psi base pressure?
Is the restrictor ring still present in the plenum to carb port?
Is the original over boost valve in the plenum sealing correctly?
When you rebuilt the carb did you set the float height at the correct height?
What throttle disk did you put in the carb when you rebuilt it? Solid or one with the poppet valve (curiosity only).
What preload do you have on the turbo actuator / proved opening pressure with a foot pump?
is your idle mixture and ignition timing correct?
Also as per JT, fuel pump type and line size?
Just trying to ascertain that you have a decent datum to go from.

Shane

Edited by shane on 3rd Apr, 2020.

Have you tried blocking off the part throttle lean off pipe? It causes this exact problem when playing up. You can test the theory by just clamping the pipe and going for a drive.
Put some hose clamps on the pipes that haven't got any as they could be leaking, and you don't want the pipes blowing off under boost and start spraying petrol around.

Mine was the choke O rings in the carb

Okay. My question is this what are the fuel tank feed and return line bore sizes?

I had much the same issue and it took me over a year to resolve. I ended up copying the metro system completely into my car and it turned out I had the wrong diameter tank feed pipe.

from my personal experience the outlet from the tank needs to be of equal or greater bore size to that of the inlet of the pump if not you get the exact problem your having.

If this is the case there is a bandaid fix and that is to fit a resistor to the fuel pump as done on the metro turbo.

Edited by Mr Joshua on 21st Mar, 2023.

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