Jump to content

SU Carb Questions


tuko
 Share

Recommended Posts

Hej guys,

I know we have gone down the path on the SU carb conversion for the 2.25L motor, but I'm making progress and I have a few questions.

The bore in the old Weber carb is 36mm and the bore in the SU carb is 44mm, a jump in 8mm of fuel/air mixture to the motor, can somebody tell me how much of a gain in CFM to the motor have I generated?

The "elbow" I'm making, the inner bore is also, 44mm. the elbow will be bolted directly to the inlet manifold, but the bore opening on the manifold is 42.6mm, which is going to create a small lip around the bore, when the elbow is installed. Is this lip going to create any problems or should I get the dremel out and file it down ?

Then there is the crankcase emission valve..... On the Weber carb, the valve is connected to the adapter under the carb. On the Land Rover originally, the valve was t junctioned, one hose going before the carb and the other going after carb. ( My landy runs like a cow, with this removed, so I want to keep it ) So, I'm wondering, do I use the original set up ? Can I just connect it before the carb? OR Do I do as it sits today, connect it after the carb. Which would be more work, having to bore/tap or weld a small piece of pipe onto the elbow.

Also the very few pictures that I've found on the internet, people have installed the SU carb with the external fuel bowl over where the throttle linkage is, the HIF I'm using, doesn't fit there. Then there was a picture posted on the old forum, where the guy had the carb over the motor next the valve cover, but It's tight there (very tight, actually there is contact there, if you don't severely cut the angle on the elbow)

So, I'm going to face the carb to the rear, which will bring the throttle linkage in the same location as present. Since the carb is going to be over the manifolds, should make a heat shield and place it under the carb?

OK, enough of the questions for now....suggestions/ideas/comments?

Todd.

Link to comment
Share on other sites

The bore in the old Weber carb is 36mm and the bore in the SU carb is 44mm, a jump in 8mm of fuel/air mixture to the motor, can somebody tell me how much of a gain in CFM to the motor have I generated?

not sure on this one but you are not comparing like with like. Weber have fixed size bore whereas the SU bore varies as the piston goes up and down. 1.75 inch (44 mm metric) is a good size for 2.25 l engine.

The "elbow" I'm making, the inner bore is also, 44mm. the elbow will be bolted directly to the inlet manifold, but the bore opening on the manifold is 42.6mm, which is going to create a small lip around the bore, when the elbow is installed. Is this lip going to create any problems or should I get the dremel out and file it down ?

For best results, keep the bore after the carb as smooth as posible so out with the dremel. I doubt you will have any serious problems if you don't do this.

Then there is the crankcase emission valve..... On the Weber carb, the valve is connected to the adapter under the carb. On the Land Rover originally, the valve was t junctioned, one hose going before the carb and the other going after carb. ( My landy runs like a cow, with this removed, so I want to keep it ) So, I'm wondering, do I use the original set up ? Can I just connect it before the carb? OR Do I do as it sits today, connect it after the carb. Which would be more work, having to bore/tap or weld a small piece of pipe onto the elbow.

there should be 3 pipes pressed in to the carb body; one is for fuel in, one for fuel overflow and last one is for ventilationtion.

Since the carb is going to be over the manifolds, should make a heat shield and place it under the carb?

A heat shield would be a good idea; MGB used one, a piece of sheet steel with two asbestos pads - you may want to leave the asbestos out and use some other insulator. I made my own from sheet steel with no extra insulation and it ran fine.

Link to comment
Share on other sites

Todd,

FWIW, my 109 didn't have that fancy setup for the crankcase ventilation, even though it had a "modern" 5 brg engine. Just a hose from the engine to the elbow on top of the carb. Maybe your engine runs like carp because you keep some of the stuff only?

Also, is it registered as a light truck? In that case you don't have to worry about breaking any anti pollution regulations, they are only valid for passenger cars.

BTW, picked up my new 265/75s today and made a trial fit. So my LR will look as a J**p too. :rolleyes: B) :D

Link to comment
Share on other sites

Lars L,

"BTW, picked up my new 265/75s today and made a trial fit. So my LR will look as a J**p too."

OH MAN,

do I ever have ammunition now! :lol::lol::P

Even registereed as a light duty truck Lars, they still tested the emmissions and there was a limit of 3.5%, that I had to stay under. ....buggers! :angry:

02GF74,

Thanks for the reply and advice.

On the SU carb I'm going to use, it has only one press fitted tube on the body (fuel in line). The fuel bowl vents at the air filter and there is no overflow tube. The SU carbs I have here, are from early Volvo cars. The reason why I went with these, is because the cars have the same size motors and the cars weigh approx. the same. Therefore I'm hoping that I'm not going to run into many problems with the jet needle.

Todd.

Link to comment
Share on other sites

Finally, got some time in front of the computer.....

I installed the SU carb with the elbow adapter to the 2.25L .

Instead of placing the carb, tilted towards the valve cover (I did trial fit it there, but there was nearly no clearance between the valve cover and the bottom of the carb), I placed it so that it tilted towards the rear of the motor (lots of clearance there). Also placing it there, the throttle linkage lined up, including the chock cable.

SU01.jpg

Would you believe it, the engine started, the first attempt at starting it. :huh:

Ran Ok, to boot, but the carb is not set up and the motor ran really rich. I did play around with the carb for a few minutes and changed needles, but it made no difference, it still ran rich.

I ran power to the solenoid valve and that worked, increasing the rpm's by about 100rpm.

Overall, everything worked to this point, but it's not ready to be installed just yet.

First, I need to figure out the correct needle for the carb, set it properly, so it's not running so rich. (lean is better, or at least, that's what I want)

The spring on the carb, for the throttle linkage, is way too strong, so it needs to be replaced with something weaker. You really have to push hard on the gas, in order to get it to respond. (I already have another spring to change in) You would never be able to drive it as is, because you either have no throttle, or everything at once.

A weaker spring, will make the throttle more gradual.

It was suggested, to remove the emmission stuff, well, the motor did run OK with it blanked off, BUT oil started to leak out all around the valve cover breather. So, the emmission stuff will be reinstalled. I'll drill out the elbow and weld a small piece of pipe, to solve that.

A heat shield will be made/installed under the carb, tilted up towards the elbow, to help stop freezing up. I couldn't believe how fast it took for the elbow to get frost on it. Using a heat shield and also running the emmission stuff into the elbow, I'm hoping they will solve any further freezing issues.

Oh I forget to mention, what I thought..... What a difference! It was like unleashing a wild animal, there was an immediate response from the motor, no hesitation or stumbling and it seamed to rev more freely. I'm impressed so far and I expect it to get even better, when I get everything finished.

BTW, I'm still looking for the adpter assembly for the carb, which I have listed in the classified section.

Todd.

Edited by tuko
Link to comment
Share on other sites

You wrote that you blanked off the crankcase ventilation, no wonder that the oil started seeping then :o Leave 'em open, man! ;):)

And, isn't there some gizmo in the bottom of the carb where you can adjust the fuel mixture. Changing needles is AFAIK (and that may not be the full truth) if you like to change the fueling "along the line". For example if the mixture is OK on idle, but insufficient at higher revs. Anyway, this applies to Zenith-Stromberg and perhaps also to SU.

Link to comment
Share on other sites

You wrote that you blanked off the crankcase ventilation, no wonder that the oil started seeping then :o Leave 'em open, man! ;):)

And, isn't there some gizmo in the bottom of the carb where you can adjust the fuel mixture. Changing needles is AFAIK (and that may not be the full truth) if you like to change the fueling "along the line". For example if the mixture is OK on idle, but insufficient at higher revs. Anyway, this applies to Zenith-Stromberg and perhaps also to SU.

Lars, I have similar recollections of the needles, Change the needle (for one of a different profile) to get the right mix at different speeds and but IIRC you raised/lowerd the needle in its mount to alter the idle mixture.

Link to comment
Share on other sites

Yupp,

you guys are correct, but of course, I was unaware of that, when I had the carb on the motor. I plan to play with the carb this weekend and I'll set it up according to the manual. (I just downloaded it and have some form of reference to read and follow)

Two further questions,

1. What was the needle that you guys have recommended for the SU carb on the Landy? The search engine appears not to work correct at LRA forum, so I can't find the info.

2. The manual says " Adjust the mixture screw ....rich or weak to achieve a CO reading of 0.5% to 2.5%"...... Without sounding dumb, but, turing the screw clockwise or anti, what does what? .....Clockwise is richer or leaner?

Todd.

Link to comment
Share on other sites

Instead of placing the carb, tilted towards the valve cover (I did trial fit it there, but there was nearly no clearance between the valve cover and the bottom of the carb), I placed it so that it tilted towards the rear of the motor (lots of clearance there). Also placing it there, the throttle linkage lined up, including the chock cable.

Would you believe it, the engine started, the first attempt at starting it. :huh:

Ran Ok, to boot, but the carb is not set up and the motor ran really rich. I did play around with the carb for a few minutes and changed needles, but it made no difference, it still ran rich.

I ran power to the solenoid valve and that worked, increasing the rpm's by about 100rpm.

First, I need to figure out the correct needle for the carb, set it properly, so it's not running so rich. (lean is better, or at least, that's what I want)

The spring on the carb, for the throttle linkage, is way too strong, so it needs to be replaced with something weaker. You really have to push hard on the gas, in order to get it to respond. (I already have another spring to change in) You would never be able to drive it as is, because you either have no throttle, or everything at once.

A weaker spring, will make the throttle more gradual.

It was suggested, to remove the emmission stuff, well, the motor did run OK with it blanked off, BUT oil started to leak out all around the valve cover breather. So, the emmission stuff will be reinstalled. I'll drill out the elbow and weld a small piece of pipe, to solve that.

A heat shield will be made/installed under the carb, tilted up towards the elbow, to help stop freezing up. I couldn't believe how fast it took for the elbow to get frost on it. Using a heat shield and also running the emmission stuff into the elbow, I'm hoping they will solve any further freezing issues.

Oh I forget to mention, what I thought..... What a difference! It was like unleashing a wild animal, there was an immediate response from the motor, no hesitation or stumbling and it seamed to rev more freely. I'm impressed so far and I expect it to get even better, when I get everything finished.

BTW, I'm still looking for the adpter assembly for the carb, which I have listed in the classified section.

Two further questions,

1. What was the needle that you guys have recommended for the SU carb on the Landy? The search engine appears not to work correct at LRA forum, so I can't find the info.

2. The manual says " Adjust the mixture screw ....rich or weak to achieve a CO reading of 0.5% to 2.5%"...... Without sounding dumb, but, turing the screw clockwise or anti, what does what? .....Clockwise is richer or leaner?

Todd.

comments on the above.

thinking outside the box here by tilting it back towards the rear of the engine!! Cna't swee why that should make any diference.

Ofcourse it started first time - it's an SU - crude but robust and reasonably reliable!

what playing round did you do? almost any needle will run but not well; you choose the profile to optiimse power/ecomony/emissions .. sounds like you're almost there.

no idea what this solenoid valve is - sounds like you have one of the last type - elctric choke is it?

The spring on the carb is way too strong? the one on the carb for ahte accelerator? coz usually there is one or two more springs. also look at the accelretor pedl movemnt - the carb cams may require a different amount of movement of calbe and pedal. the spring is unlikely to be the problem unless it is rusted solid or smothing else is jamming, if anything the spring will be weakened with age.

A weaker spring, will make the throttle more gradual. <-- comment about calbe legnth,s levers etc.

removing the emmission stuff is not good; there should be a tube to the carb to such in the fumes and burn them. I don;t think plumbing ointo the manifold is good - I need to take a look at SU to see if this goes before or after the throttle vavle, if aftern then your plan is good.

won't putting az heat shield under the carb stop heat from warming the manifold? some sort of water jacket is needed. Also all SU set ups I have seen have an insultor between manifold and carb; yours does not appear to have one.

2. The manual says " Adjust the mixture screw ....rich or weak to achieve a CO reading of 0.5% to 2.5%"...... Without sounding dumb, but, turing the screw clockwise or anti, what does what? .....Clockwise is richer or leaner?

check the needle is fitted correctly in the piston; if too high, then it will run rich.

the mixture screw that is noramlly blanked off as you are not supposed to mess with it!! raisesd or lowers the jet. anti-clockwise raises it. there whould be a bimetalic strip that affects the jet height according to temperature to adjust for change in fuel density - can't remember whcih way round it goes but would say it rises when hooter. check that this is working - it is quiteobvious and from memory the distance moved is a few mm; (8 mm?)

to start with, once engine is at tunning temp, wind out the screw so the top of the jet is flush with the carb bridge, the screw in 3.5 (three and a half) turns. (steps 12, 13 and 14 of the manual you are reading :rolleyes: )

Link to comment
Share on other sites

Of course, leaving the crankcase ventilation open is not an "approved" method, but just for testing I can't see any problem. And for avoiding a pressure buildup in the engine during the tests.

It should be routed so the fumes are burnt up, but plugging into the manifold sounds like a bad idea and will probably disturb the adjustment of the carb. Somewhere before the carb but after the air filter would IMHO be the best solution.

Edited by Lars L
Link to comment
Share on other sites

"Somewhere before the carb but after the air filter would IMHO be the best solution."

The Weber carb setup, that's on the engine now, has the plumbing is "after" the carb, into the manifold. If it works fine with the Weber setup, then why not with the SU carb ?

I just stripped the SU carb and found that the needle seat was seized. That's why it was running so rich. Everything is free and full of movement. ;)

The seat is adjusted according to steps 12-14, be giving it another go during the weekend.

BTW, the two needles I have are BCJ & BDJ. Still wondering what was that other needle that was recommended earlier........ :huh:

Todd.

Link to comment
Share on other sites

The Weber carb setup, that's on the engine now, has the plumbing is "after" the carb, into the manifold. If it works fine with the Weber setup, then why not with the SU carb ?

don't they work on different principles? SU vary fule delivery using the vacuum to pull up the piston whereas webers open orifices to allow fuel to pass controlled by a lever connected to the throttle?

(I don't know much about webers/dellortos but will need to learn at some point :( )

Link to comment
Share on other sites

Perhaps if they also incorporate some valve thingy that shuts off or opens at some specified circumstance... :unsure: Anyway, routing some kind of "open hose" into the manifold doesn't sound right to me. The PCV valve on your engine was later discarded on the Series engines and "something else" which looks like a tee piece replaced it. From what I've been able to find out from the Green Bible...

If I was in your shoes, I would simply take the hose from the engine block and shove it into the air cleaner. That's how it worked on my Volvo engined GAZ and on my SIII. Without any problems, I might add. ;):)

Link to comment
Share on other sites

02GF74,

It's all Greek to me! :D

just checking the manuals, both Volvo and Land Rover plumb their PCV with a T junction, one leading before the carb and the other after the carb into the manifold.

Todd.

dunno about that. forgot to bring in to scan qthe SU manual but there is a sideways picture showing the 3 pipes mentioned and they are all before the throttle disc. Maybe the needle/springs etc. are set up to take that into account?

On the V8, there is a crankase breather so air is deliberatley drawn into the engine and suck out in to the carb.

I cannot see how you can correct vauccm for the carbs to work properly with a system opened to air like that.

I guess try both ways as see which runs best.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy