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#1 Benoit_Dupont

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Posted 05 August 2019 - 07:19 PM

Hi Guys, 

 

while my 1st post is being approved to introduce myself, I would like to have your opinion on a little carburation problem of mine:

 

I have an engine that was rebored by the previous owner to 1380cc but the rest is kinda stock: HIF44, stock cooper 91' exhaust manifold, stock intake manifold. stock distributor. the dashpot oil is SU carb oil. 

 

I recently have installed a BBW needle and a K&N cone filter. I am tuning the carburator but I face 2 issues:

 

1. the idle is high (1500 rpm) and not very stable. 

 

2. when I push the accelerator, the engine revs correctly, but when I let it go, it stays high for a while and goes down in rpm quite brutally.

 

for the 2, my guess is that the spring in the dashpot is not strong enough. I have a yellow type of spring. what do you think ?

 

for the 1, I must say I am kind of stuck there. if I unscrew the accelerator idle below 1500 RPM, then it becomes chaotic and just dies on me.

 

Any thought ?

 

 



#2 nicklouse

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Posted 05 August 2019 - 07:36 PM

Lose the cone filter and fit the standard air box but use a K&N element.

Not sure what those inlet and exhaust manifolds are like but if they are the later ones they are good. What is the rest of the exhaust system.

I like to use the same oil in the dash pot as the engine but it depends on what it has been set up with originally. Factory use the engine oil.

And what cam shaft?

#3 Benoit_Dupont

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Posted 05 August 2019 - 08:13 PM

Thanks for the feedback. 90-91 Cooper have a funny manifold. Mine is stock. It looks pretty much like a torquemaster. That's why i'm keeping it.

The exhaust manifold is also stock and funny. It is similar to the LCB exept it starts with cast Iron then 2 downpipes and the Y of the LCB.

The rest is straigt 44mm pipe to the RC40 central exhaust.

#4 Benoit_Dupont

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Posted 05 August 2019 - 08:14 PM

Oh and camshaft is also stock

#5 Retroman

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Posted 06 August 2019 - 01:47 PM

Not sure the needle is correct but it might not be far off try;

 

As a starting point;

The mixture adjusting screw is on the outside of the carb, turn it in clockwise [right] for richer  

Remove the dashpot [top] from the carb [3 screws]... don't drop the piston or bend the needle !

You can then see the brass jet in the middle, the needle on the piston dangles in it.

The jet will have a hole in the middle either 0.090” or 0.100” HIF38 or HIF44 respectively

The top of the jet needs adjusting so it sits 0.080” below the bridge

Before refitting the dashpot piston assembly check the number of the needle...

Small screw on side of piston, undo remove needle...number on thicker part at top [very small you may need binoculars.]

Make sure there is the small spring on top of the needle

When refitting the needle the holder should be flush with the bottom of the piston

The needle should move and is spring loaded slightly

That may be enough to get it running but may be miles off, guaranteed to need mixture adjusting and ignition timing setting.

Assuming you can then have it running

    If you can get it up to temperature that’s good. You may find it coughs and sputters...and even back through the carb, this usually indicates a weak mixture, so turn the adjuster screw in to the right clockwise maybe a turn or more, this is quite a lot and richens the mixture.

Set to a fast idle 1200 to 1400 revs, and with the engine warm….

Lift the piston in the carb [not too quick]...there is a small lifting pin on the side of some (with a small spring on) if not use your finger under the piston from the filter side and only lift about ¼ of an inch.

If the engine dies the mixture is weak (as above to richen) then adjust idle down

If the engine revs up the mixture is too rich (so turn the adjuster screw out anticlockwise) then you may have to adjust idle up

The aim is when the piston is lifted the revs should rise and then settle back. That means the mixture is somewhere near at fast idle

If the engine has a high lift cam this is more difficult and may well have to be done at higher revs.

 

This is just a starting point and remember anything you adjust [timing or tappets] or fitting other parts, including exhaust can influence the mixture.

 

Ultimately you need a rolling road, but a colourtune can help and or a gas analiser...



#6 Benoit_Dupont

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Posted 06 August 2019 - 04:36 PM



Not sure the needle is correct but it might not be far off try;

 

As a starting point;

The mixture adjusting screw is on the outside of the carb, turn it in clockwise [right] for richer  

Remove the dashpot [top] from the carb [3 screws]... don't drop the piston or bend the needle !

You can then see the brass jet in the middle, the needle on the piston dangles in it.

The jet will have a hole in the middle either 0.090” or 0.100” HIF38 or HIF44 respectively

The top of the jet needs adjusting so it sits 0.080” below the bridge

Before refitting the dashpot piston assembly check the number of the needle...

Small screw on side of piston, undo remove needle...number on thicker part at top [very small you may need binoculars.]

Make sure there is the small spring on top of the needle

When refitting the needle the holder should be flush with the bottom of the piston

The needle should move and is spring loaded slightly

That may be enough to get it running but may be miles off, guaranteed to need mixture adjusting and ignition timing setting.

Assuming you can then have it running

    If you can get it up to temperature that’s good. You may find it coughs and sputters...and even back through the carb, this usually indicates a weak mixture, so turn the adjuster screw in to the right clockwise maybe a turn or more, this is quite a lot and richens the mixture.

Set to a fast idle 1200 to 1400 revs, and with the engine warm….

Lift the piston in the carb [not too quick]...there is a small lifting pin on the side of some (with a small spring on) if not use your finger under the piston from the filter side and only lift about ¼ of an inch.

If the engine dies the mixture is weak (as above to richen) then adjust idle down

If the engine revs up the mixture is too rich (so turn the adjuster screw out anticlockwise) then you may have to adjust idle up

The aim is when the piston is lifted the revs should rise and then settle back. That means the mixture is somewhere near at fast idle

If the engine has a high lift cam this is more difficult and may well have to be done at higher revs.

 

This is just a starting point and remember anything you adjust [timing or tappets] or fitting other parts, including exhaust can influence the mixture.

 

Ultimately you need a rolling road, but a colourtune can help and or a gas analiser...

 

allright, so first of all, thank you very much for the info.

 

one thing where I'm not quite sure off:

 

if I screw (so clock wise), I think the carburator makes the mixture leaner:

if I screw in, I'm pushing the pin that lifts the jet up agains the pin, so, less fuel can pass if I turn clock wise. or at least that what the schematic seem to indicate: you screw in, you push the little flat spring inside the carb resersevoir, it pushes the jet up and there fore it closes around the needle and then, less fuel will pass. Or am I missing someting.

 

there are some answers already to your suggessions:

 

the needle I have is marked BBW. it does not seem bent.

 

it sits flush for sure and it has the little spring as well.

 

I'll get your starting point tomorrow, and keep you guys posted.

 

I got a few alternative needles to try.

 

one thing that I also suspect: I serviced the carb, it really needed attention, most gaskets were done for and because I use to live abroad where gas was not that great, and because i ran without a fuel filter, the carb filter was also in poor state, as well as a lot of crap down the reservoir.  so I replaced all seals and gaskets and bits and pieces from the correct SU servicing kit.

 

I have noticed however one thing: the butterfly has this little poppet valve on it. the previous one also had one, but the new one seems weaker and I read that this can cause some idle problem if too weak, as it let gas flow also the butterfly is almost closed, which raises IDLE RPM.

 

Tomorrow, I'll get 2 things by mail:

one plain butterfly, not good for environnement but at least I'll be able to get this out of the equation to start from. I might switch back later if needed.

 

and a green dashpot spring.

 

one of the issue I have faced is the following: even if I get a IDLE without chocke at say, 1500 RPM, if I give it a bit of gas, it goes up to say 3000RPM... and stays there even if I step off the gas. then it drops back to 1500 RPM a few seconds later.

 

2 things come to mind to explain that funny behavior: either it's the damn poppet, that keeps gas flowing although I don't want gas. or the spring I have to too weak, and does not push back the needle piston down fast enough.

 

so I'll try tomorrow without the poppet. it that still fails, I will then move to a green spring and see what happens.

 

I think the SU oil in the dash pot was a mistake: I used engine oil for quite a while but thought it was a good idea to switch to the manufacturer's brand. I think it's way too sticky.

 

==> what do you think  of the above diagnosis method ? 

 

for the rest: 

 

I do have a color tune, but as you mentionned more than correctly:

it's only good if I have a decent starting point and a stable engine. so I'll try your method first and see if I can get it decently tuned.

 

and yes, you are absolutely correct, it will need timing adjustment. I don't have that equipment or know-how, so my goal is to get it road-ready to get it to the Man without having the car die on me every street corner.

 

cheers and thanks for the tip!



#7 Benoit_Dupont

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Posted 07 August 2019 - 01:15 AM

Actually Retroman I take that back. You are absolutely right about the adjustment screw of course: i got confuses because when i serviced it, i worked on the reservoir upside down: the pin actuated by the screw is above the jet support pivot, so when you turn it CW, it lowers the jet and it gets richer you are right.

#8 Benoit_Dupont

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Posted 07 August 2019 - 06:27 PM

Allright, so here is an update!

 

I tried Retroman's method: it did work for a while. I managed to get an idle at 2000 RPM, sounded quite ok, no smoke. so I switched it off to put the color tune in: it won't restart. I did see a spark which is good. but that's it. I swapped again everything. still no start. it's just done for.

 

on the plus side: I did swap the poppet butterfly for a plain butterfly, and the strange issue n°2 on my list is now gone: when I rev it up and step off the gas, it does come back to idle.

 

So, anyway, I'm kind of stuck and I'm now looking for another source of problem.

 

one thing I did see however is that when I dismantled the carb (again) to verify it was working, there ware merely any fuel in the reservoir. which is odd because when I originally took it off the car, there was some fuel. I checked the floater mechanism, it actuates properly, so there should be fuel in there.

 

I did unplug the fuel line and cranked the engine to see if I got any fuel out of it. and the answer is "barely". I know I should not expect the Bellagio fountain of fuel out of this thing but still, I barely got a few droplets. any hint on what should be a normal fuel pump output ?

 

by the way, I checked the fuel filter, it's not clogged. I actually dismantled it first thing to verify it.



#9 DeadSquare

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Posted 07 August 2019 - 06:46 PM

Fuel should squirt, not dribble.

 

Extend the petrol pipe from the tank some how, and blow down it to see if is clogged.

 

Rig up a small gravity feed to the carb and see if it runs OK.



#10 Benoit_Dupont

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Posted 07 August 2019 - 06:58 PM

Fuel should squirt, not dribble.

 

Extend the petrol pipe from the tank some how, and blow down it to see if is clogged.

 

Rig up a small gravity feed to the carb and see if it runs OK.

thanks for the tip! Will try!



#11 Retroman

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Posted 09 August 2019 - 09:52 AM

Its your fuel pump mate

 

It should litterally be a fountain....2 or 3 PSI and about 18 gallons per hour

 

The gravity feed will work well, I use it on every engine I run on the engine stand

 

You can just see the white funnel tie-wrapped to the dash pot on the image, and the green feed pipe...I now use small tank

 

Qq1Ajca.jpg



#12 Benoit_Dupont

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Posted 09 August 2019 - 12:21 PM

thanks for the tip retroman, I'll try it this week end and see what happens. and nice engine set-up by the way :)







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