Blow through HS8 issue
Turbo Mini Forums

Hi Guys. I'm a brand new member here on Turbo Minis and think you guys can help me with a fuel issue I'm having.
So, I have fitted a NEW HS8 Carb to a race motor I have built , Its VW.
I Have it in blow through mode.
Driven to the MOT station for a ticket. This has given me 1st chance to see how it runs.
Well , as soon as I get into boost the engine dies. AFR gauge shows very lean.
So looks like I have no fuel on boost.
I have a fuel regulator that increases pressure when on boost, The signal for this is positioned at the carb mouth.
I also have a pipe from the carb mouth feeding boost to the sealed float bowl lid.
I made a restrictor which is 37mm in size. This is before the carb mouth.
I have thick oil in the dashpot. this has fixed the off idle flat spot.
I also have the biggest spring fitted to the inside of the dash pot to stop the piston lifting too soon.
What should I do to stop this on boost no fuel flat spot occurring.
Regards Garrick

have your fuel pressure reference on the boost pipe, then restrictor, then carb mouth.?

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

This should help... no one here can snap their fingers and say the root cause, but do some tests and it'll narrow it down to one area and allow you to isolate it.
http://www.turbominis.co.uk/forums/index.php?p=vt&tid=6298
And we need pics

Edited by TurboDave16V on 16th Sep, 2017.

Hi Guys , Yes the fuel pressure signal is from the boost pipe , before the carb mouth restrictor.
Dave I'll have a look at the link and see what it say's .
Thanks.

you say HS8, is that with the seperate float chamber on the side?

you will need to reference the chamber against boost coming in, and then add a restrictor ring to get fuel pressure to track against boost.


sorry, re-read and it seems you have all that covered, I would look at plumbing in some gaiuges to check the signals look right, pllenum vs folat chamber vs fuel pressure vs manifold pressure.

Edited by Joe C on 16th Sep, 2017.

Well today I checked the pressure reg.
I used the prime button to activate the pump. I had the fuel pipe end in a petrol can , fuel flowed into the can at 3 psi. I pumped air into the ref port with a hand pump, gauge showed up to 15psi. The fuel flowing into the petrol can did increase in pressure but wasn't like wow look at that.

Yes, flow won't increase, but pressure will. The flow in the case you are describing (free flow into a bucket) is simply governed by the ID of the pipes or regulator, which ever is smaller...

Well good start at least; your regulator "appears" to be rising correctly with boost when free flowing. But about the only thing you can really know from this test, is it your diaphragm on your regulator is not split…
Reality is, the test you did, tells you very little - try plugging the pipe to the carb so all fuel dumps back to the tank if you insist - that is at least more representative of reality...
Really though, you need to have a gauge within line of sight to your boost gauge, and watch what happens while you try driving.

Edited by TurboDave16V on 16th Sep, 2017.

ok. I will rig a gauge up next to the boost gauge. I will get the boost gauge hovering at half a bar on the gauge and see were the fuel pressure is. The fuel pressure should be around the 10 psi range , am I right thinking this.
Cheers Dave.

Don't take this the wrong way, but it seems to me that you're trying to run before you can walk ... The link above had pretty detailed instructions which have helped a lot of folks out over the years.

Connect the gauge up, and simply watch it.

From the link above:

Now you need to check if the fuel regulator is increasing the fuel pressure relative to boost pressure. Go for a short drive. You'll notice as you approach the 'boost transition point' (ie - ZERO) on your in-car boost gauge the fuel pressure on your 'wiper arm' gauge will rise. If all is well, it should have increased from 3/4 psi at tickover to 8/9 psi at the 'boost transition point'. Therafter; it rises proportionally with boost (ie 5psi on the in-car boost gauge = approx 13/14psi on the 'wiper arm' gauge). If it does - all is good with the regulator plumbing and the regulator.

Edited by TurboDave16V on 17th Sep, 2017.

Roger that Dave. Be back soon

Ok , so I've fitted the fuel pressure gauge in the cab to check the pressure while driving.
I have the pressure set at 5 psi at idle. The carb doesn't spill fuel out or flood. When the boost gets to zero the fuel pressure gauge needle starts to shoot round. I only have a 15 psi gauge and the needle goes off the dial. The boost pressure was at 7psi the fuel pressure was over 15psi.As the boost started to rise so did the AFR numbers.

I have a fat needle at the top were it fits into the piston , then it tapers to a sharp point at the bottom

Well, looks like you simply need more fuel. We have to assume everything else is OK with your carburetor as we haven't seen any pics of it that might point out something else with its configuration incorrect. That was a lot of information in the link above about how the carb will be connected, so if youve got all that then I guess all is well...

Firstly though, I'd limit yourself to just 5psi of boost until you've at least baselined that - it'll give you a buffer to work with as you won't kill your pistons as quickly.

We still don't know more information about your Engine, but folks will normally start with a BDD needle and start filing away from there.
Search for posts on filing needles, and research that. This will only take a few hours to get pretty close, but will take a long time to perfect.

Edited by TurboDave16V on 17th Sep, 2017.

Hi Dave . Thanks for the advice.
The carb is a new HS8. I have fitted a 125 jet to increase the fuel.
If I have this bit right, at the mouth of the carb there is 2 holes next to the mounting holes. I have boost going into these holes .
Sorry I'm rubbish at putting picks up on sites. I'm also using the standard copper float in the bowl. These to me are quiet large and take up a lot of space in the bowl. Can anything be done to reduce the size of the float or increase the size of the bowl. Are the float bowls made from a good alloy that can be welded. I may be able to increase its size.

I just checked what you wrote, and something jumped out at me... if you only have two "port" holes in the plenum face, that is your problem. Take a look at the plenum in this image:
https://www.worthpoint.com/worthopedia/mg-m...metro-166239497
You will note that there are three port holes - the top two that are standard, but you mentioned nothing about the port hole that actually pressurises the float chamber.
Now, I didn't mention this before, as i assumed you'd already have read it in the original link i posted above, but here goes:
http://www.turbominis.co.uk/forums/index.php?p=vt&tid=2103
Read this - and understand how the carb works in blow-thru operation. It really will help you understand what is going wrong.
Also, as you're trying to "blow thru" what I am sure is a nat-asp carb, you will need to block off the connections outlined in the image in the original link - or it'll NEVER work.

Finally, I'm not sure why you decided you need more fuel and put an oversized jet in there, but ok - that's your choice...
But messing with the float bowls to get "more fuel capacity" is simply a waste of effort that I'm not even going to discuss it any more. You simply don't need to do that - and you may not even need your oversize jet either...
Burt then again, this huge SU could be blowing through a 541 stroker attached to a VW transaxle, so it may well need the oversized jet... But if this is anything in the 1200 to 1400cc range, you probably don't need it. You also may find that running a huge SU like this is a negative compared to a smaller 1-3/4" - especially as regards the plenum restrictior. Remember that the pukka "turbo" HIF was used on the monty / maestro and metro turbo's, which over the years have had many, many examples making the bananas, often on huge boost pressures. The HIF44 and factory plenum restrictor profile is simply hard to beat no matter the engine size, boost pressure or power.

Edited by TurboDave16V on 18th Sep, 2017.

Hi Dave thanks for the above info. Yes my carb only has 2 front ports on the plenum face. I have drilled and tapped the float bowl lid and feed boost pressure into it. I have sealed the float bowl vent too.
Engine is 1911CC
Sealed the butterfly shaft with O rings too.
Ok, I'm going to read the link you put up for me, cheers

its possible you need a more dynamic pressure feed to the fpr and the bowls ,and also that your needle is in need of a good filing .

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

Hi Robert. I'm using a bosch injection pump.
84 psi from the pump to a fpr014 malpassi reg. From the reg its 5psi to the carb ,reg has a return to tank for the excess fuel. fuel pressure is way above the boost pressure when driving.

The air feed to the fuel reg is tee'd off from the feed that supplies boost to the float chamber and the meth controller.

So fuel reg, float chamber, meth controller all get a signal from 1 pipe which is positioned just in front of the restrictor.

Edited by GARRICK.CLARK on 18th Sep, 2017.

i meant a moe dynamic pressure reading Garrick , for example connect your boost signal line to a point in the inlet where it is pointing into the flow , maybe also before the interooler ,so the flow velocity is added to the pressure .

also , it may be the needle you have in the carb is too thick ,and needs making thinner in the areas you are lean . this can be done with a file.

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

the reg thing dosent make sense to me,

normally people just run the metro turbo style reg set to 4 psi ish, this then tracks boost pressure, so pressure before the reg will be boost pressure + 4 psi,

I agree with Joe,

These are simple spill back regulators designed to run at relatively low pressures (compared to a normal EFI regulator).

You simply should not be seeing 84PSI anywhere (unless the return line is badly blocked) because the spring in them, even with added boost, simply isn't capable of such a high pressure and I don't think there is enough thread on the adjusting screw for you to wind it so high that the spring becomes coil bound and stops it moving hence blocking the return.

Post up a picture of how you have it plumbed in and how far the adjuster screw is set.

Schrödinger's cat - so which one am I ???

The reg is not the issue though. the issue is the float bowl not rising in pressure as boost rises.

I'd be interested to see the union to the float bowl to feed the pressure.

I'd also try feeding the pressure to the float bowl from the turbo compressor housing rather than the plenum; even though the restrictor should in theory give sufficient difference between the pressure at the jet and the float bowl.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

Sorry guys I need to explain again.
my pump has a rating of 84 psi . the reg I have, can reduce this 84 psi down to very low pressure. I can get my idle fuel pressure down to 4 psi if I need to. I have the idle pressure set at 5 psi. When I feed boost to the reg, the fuel pressure rises .

Hi Wil.

What I did tonight is this.

I teed off the waste gate pipe and went direct to the float bowl lid.

My float bowl lid is just a regular one. It has 1 inlet for fuel, has the tiny vent hole blocked by me, and a barbed fitting fitted to the top of the lid, this barbed fitting is to pressurise the float bowl.



The mouth of the carb and the boost pipe is 2 inch.

The feed to the float bowl is now only a 3 mm pipe so I now should have a higher pressure going into the float bowl.

Edited by GARRICK.CLARK on 19th Sep, 2017.

that feed has to go to the fpr too .

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