SU carb question for those in the know...
Discussion
Hello,
I'll try and be as succinct as possible but a bit of background:
I'm building a three wheeler which uses a Guzzi 750 small block engine. The short version is I want to use an SU as have had these on previous cars and I just generally have a bit of a thing for them.
I've made a manifold from copper (28mm from each cylinder, going into a 35mm tee before the carb), and fitted an unknown HS4 SU which looks to have come from a mini. I put a new jet in the carb (0.9in red) and used new gaskets too. I believe the needle is AAV.
I've got the car running but it's very lean - in fact it only runs correctly with the jet screwed out the max and even then it's still a little lean. My initial thought was an inlet leak but I've even gone as far as making it twice and then covering it in insulating tape. I've sprayed easy start all round and the engine speed doesn't change. The throttle control doesn't have much play either.
I'm beginning to think that I don't actually have a leak and the issue is carb related. Possibly the AAV needle is too lean, but for a 750 engine I thought it would be fine. I also wonder if there's something wrong in the set up of the carb - it'd got dashpot oil and the needle slides freely. Would it be possible to have the needle sat too far down? I.e. could I loosen it and move it up 1/4" so I don't need the jet screwed out to max?!
I'm at the stage where I'm starting to get annoyed it with and chasing my tail. The engine ran fine before on bike carbs and I've just done the valves and replaced the headgaskets etc. just to make sure it's right. Timing is also spot on so I'm sure it's a carburation issue.
Many thanks,
Phil
So you’ve selected a needle/jet based entirely on the fact that the motorbike engine is nearly the same capacity as the Mini engine you think the carburettor might have come off? And you’ve made your own manifold?
I have enough trouble setting SU mixtures on cars they were developed for!
I have enough trouble setting SU mixtures on cars they were developed for!
Sounds like fuel starvation,are you getting fuel,via the needle valve into the float chamber? Is the float level very low?
Most su needles have a very similar diameter at their base,at tickover.If you have screwed the jet down and its still weak,then you either have an air leak ( eg very warn throttle spindle) or theres is hardly any fuel in the float chamber.If you raise the float height too much it will flood fuel out of the jet and drip out of the carb.
Most su needles have a very similar diameter at their base,at tickover.If you have screwed the jet down and its still weak,then you either have an air leak ( eg very warn throttle spindle) or theres is hardly any fuel in the float chamber.If you raise the float height too much it will flood fuel out of the jet and drip out of the carb.
Probably not much help OP, but when I was a young'un messing around with cars (back in the days when people knew what an SU carb was 
), an old mechanic mate of mine used to swear by cleaning everything with brake fluid. He'd completely strip the carb, douse it all with brake fluid and wipe clean with a lint-free cloth and then reassemble. He'd repeat the exercise with the inlet manifold, if accessible.
As I said, probably no help, but eliminate all the obvious stuff first, eh? Or put the bike carbs back on?
), an old mechanic mate of mine used to swear by cleaning everything with brake fluid. He'd completely strip the carb, douse it all with brake fluid and wipe clean with a lint-free cloth and then reassemble. He'd repeat the exercise with the inlet manifold, if accessible.As I said, probably no help, but eliminate all the obvious stuff first, eh? Or put the bike carbs back on?
Give the choke cable a pull. On an SU that moves the jet down so on to the tinnest part of the needle and making the fuel hole the biggest and, in theory letting more fuel through.
If doing this makes things much better then, as you seem to have done everything logical to rule out an air leak, it probably is just too lean a needle, so you're in to the random witchcraft of SU needle numbers to find something better matched.
If it doesn't make any difference then it's probably a fuel supply issue - if you're only getting a dribble of fuel through to the jet you're only going to get a dribble out and into the engine, no matter how big the hole. Give the engine a little 30second run then check the float chamber. That should be long-enough to fill it if everything's good on the tank side of the carb, and if not start working your way back looking for crap blocking the lines or an astmatic fuel pump. If you've got plenty of fuel in the float chamber look the other way, for a blockage in the float->jet pipe for example (and depending on model of SU). In HS cars it's not uncommon for poor fitting or age to get bits of rubber blocking the pipe.
If doing this makes things much better then, as you seem to have done everything logical to rule out an air leak, it probably is just too lean a needle, so you're in to the random witchcraft of SU needle numbers to find something better matched.
If it doesn't make any difference then it's probably a fuel supply issue - if you're only getting a dribble of fuel through to the jet you're only going to get a dribble out and into the engine, no matter how big the hole. Give the engine a little 30second run then check the float chamber. That should be long-enough to fill it if everything's good on the tank side of the carb, and if not start working your way back looking for crap blocking the lines or an astmatic fuel pump. If you've got plenty of fuel in the float chamber look the other way, for a blockage in the float->jet pipe for example (and depending on model of SU). In HS cars it's not uncommon for poor fitting or age to get bits of rubber blocking the pipe.
Are you quite sure you're not just strangling it in terms of flow rate? 5 minutes on Google suggests that Guzzi 750s ran a Dellorto 32mm carb per cylinder. You're running a single SU, presumably 1 1/4 or 1 1/2 inch which is 32-38mm equivalent. All things being equal that's a significant reduction in carb size and presumably CFM air-flow. I'm not an expert of carburation, but could this be contributing to your problem?
I had one of these carbs on my Morris Minor. Now moved to a HIF 44, which is better IMHO.
What is the breather pipe attached to?
Is there enough oil in the dashpot?
The problem you will have is as soon as you take it apart / fiddle with it fuel will fill up the float bowl so it looks like it's not that.
What is the breather pipe attached to?
Is there enough oil in the dashpot?
The problem you will have is as soon as you take it apart / fiddle with it fuel will fill up the float bowl so it looks like it's not that.
Keep it stiff said:
I don’t claim to be in the know but would an SU behave differently with fuel fed via an electric pump compared with gravity fed on a bike?
I would tend to think so but depends upon how high above the carb the tank is... Electric pump on say a Mini is IIRC around 4lb/sq in.To the point above, a HS4 is IIRC an inch and a half bore. (HS2 was inch and a quarter).
If the original set up was two 32mm then you aren't that far away, maybe you need to check the needle level or find another needle. A guessing game there but in the (very) old days we would wet and dry a needle to thin it down a little and increase mixture.....
HIF44 is a nicer carb for the A series 1300, a little more modern (ha ha) and reliable.
Keep it stiff said:
I don’t claim to be in the know but would an SU behave differently with fuel fed via an electric pump compared with gravity fed on a bike?
Not really, no.Most SU carbs are designed to work with an over-supply of fuel to the float chamber, which closes off the inlet with a needle valve when full. This then 'stalls' the fuel pump until fuel is used, the level drops and the value re-opens. And the float chamber is only there to
Considering how slowly the engine actually consumes fuel you won't need much head at all for gravity to fill the float chamber faster than the engine is using it. Pumps, either mechanical or electric are generally used on cars because the fuel tank is below the level of the carbs/float chambers so you need something to force the fuel up hill. They aren't there to 'force' fuel in to the float chamber in order to keep up with the rate the engine's burning it.
Just a thought, as it's 750CC have you tried looking up Austin 7 stuff, (I might be biased here!)
It's a 750cc size and lots of development done over the years, I have SUs on mine but not sure on the size.
It's a 750cc size and lots of development done over the years, I have SUs on mine but not sure on the size.
Skyedriver said:
HIF44 is a nicer carb for the A series 1300, a little more modern (ha ha) and reliable.
Yup, should have said that my Morris has the 1275 engine in it so spot on. Had trouble with it when first bought as previous owner had put a glob of silicon down the breather tube, of course every time I took it apart the seal was broken and the fuel flowed again.Edited by austin on Tuesday 17th November 10:52
The float level must be set correctly,with the fuel level in the jet just below the point that it floods.
The level in the float chamber is NOT just a reservoir, the common SU fuel pressure is more like 1 .5 psi,and generally with an interruption pump ( tick tick style), the pulses will cause it to flood if left on at much more than 3 psi.You can get more with a constant flow pump,maybe 4 to 5 psi .
My Pre War Aston is supercharged,has one 2 5/8 inch SU,needs not to flood on tickover,but consumes more than 2 gallons of fuel per minute at full load.It has 3 large float bowls,with interconnected feeds and exits to each bowl ,with an extra feed straight up the main jet ( which is 300 thou diameter) this will flow 4 gallons a minute throughout the assembly if needed.
So,I do know what im talking about.
The level in the float chamber is NOT just a reservoir, the common SU fuel pressure is more like 1 .5 psi,and generally with an interruption pump ( tick tick style), the pulses will cause it to flood if left on at much more than 3 psi.You can get more with a constant flow pump,maybe 4 to 5 psi .
My Pre War Aston is supercharged,has one 2 5/8 inch SU,needs not to flood on tickover,but consumes more than 2 gallons of fuel per minute at full load.It has 3 large float bowls,with interconnected feeds and exits to each bowl ,with an extra feed straight up the main jet ( which is 300 thou diameter) this will flow 4 gallons a minute throughout the assembly if needed.
So,I do know what im talking about.
hardcastlephil said:
Hello,
I'll try and be as succinct as possible but a bit of background:
I'm building a three wheeler which uses a Guzzi 750 small block engine. The short version is I want to use an SU as have had these on previous cars and I just generally have a bit of a thing for them.
I've made a manifold from copper (28mm from each cylinder, going into a 35mm tee before the carb), and fitted an unknown HS4 SU which looks to have come from a mini. I put a new jet in the carb (0.9in red) and used new gaskets too. I believe the needle is AAV.
I've got the car running but it's very lean - in fact it only runs correctly with the jet screwed out the max and even then it's still a little lean. My initial thought was an inlet leak but I've even gone as far as making it twice and then covering it in insulating tape. I've sprayed easy start all round and the engine speed doesn't change. The throttle control doesn't have much play either.
I'm beginning to think that I don't actually have a leak and the issue is carb related. Possibly the AAV needle is too lean, but for a 750 engine I thought it would be fine.
Its a 750 bike engine, from a Guzzi........that ran two carbs as mentioned.I'll try and be as succinct as possible but a bit of background:
I'm building a three wheeler which uses a Guzzi 750 small block engine. The short version is I want to use an SU as have had these on previous cars and I just generally have a bit of a thing for them.
I've made a manifold from copper (28mm from each cylinder, going into a 35mm tee before the carb), and fitted an unknown HS4 SU which looks to have come from a mini. I put a new jet in the carb (0.9in red) and used new gaskets too. I believe the needle is AAV.
I've got the car running but it's very lean - in fact it only runs correctly with the jet screwed out the max and even then it's still a little lean. My initial thought was an inlet leak but I've even gone as far as making it twice and then covering it in insulating tape. I've sprayed easy start all round and the engine speed doesn't change. The throttle control doesn't have much play either.
I'm beginning to think that I don't actually have a leak and the issue is carb related. Possibly the AAV needle is too lean, but for a 750 engine I thought it would be fine.
Think about it.
hardcastlephil said:
Hello,
I'll try and be as succinct as possible but a bit of background:
I'm building a three wheeler which uses a Guzzi 750 small block engine. The short version is I want to use an SU as have had these on previous cars and I just generally have a bit of a thing for them.
I've made a manifold from copper (28mm from each cylinder, going into a 35mm tee before the carb), and fitted an unknown HS4 SU which looks to have come from a mini. I put a new jet in the carb (0.9in red) and used new gaskets too. I believe the needle is AAV.
I've got the car running but it's very lean - in fact it only runs correctly with the jet screwed out the max and even then it's still a little lean. My initial thought was an inlet leak but I've even gone as far as making it twice and then covering it in insulating tape. I've sprayed easy start all round and the engine speed doesn't change. The throttle control doesn't have much play either.
I'm beginning to think that I don't actually have a leak and the issue is carb related. Possibly the AAV needle is too lean, but for a 750 engine I thought it would be fine. I also wonder if there's something wrong in the set up of the carb - it'd got dashpot oil and the needle slides freely. Would it be possible to have the needle sat too far down? I.e. could I loosen it and move it up 1/4" so I don't need the jet screwed out to max?!
I'm at the stage where I'm starting to get annoyed it with and chasing my tail. The engine ran fine before on bike carbs and I've just done the valves and replaced the headgaskets etc. just to make sure it's right. Timing is also spot on so I'm sure it's a carburation issue.
Many thanks,
Phil
I know very little about SU carbs (od any carbs), but this was exactly what I did to fix the poor running issue I had with the Stromberg on my Merc.I'll try and be as succinct as possible but a bit of background:
I'm building a three wheeler which uses a Guzzi 750 small block engine. The short version is I want to use an SU as have had these on previous cars and I just generally have a bit of a thing for them.
I've made a manifold from copper (28mm from each cylinder, going into a 35mm tee before the carb), and fitted an unknown HS4 SU which looks to have come from a mini. I put a new jet in the carb (0.9in red) and used new gaskets too. I believe the needle is AAV.
I've got the car running but it's very lean - in fact it only runs correctly with the jet screwed out the max and even then it's still a little lean. My initial thought was an inlet leak but I've even gone as far as making it twice and then covering it in insulating tape. I've sprayed easy start all round and the engine speed doesn't change. The throttle control doesn't have much play either.
I'm beginning to think that I don't actually have a leak and the issue is carb related. Possibly the AAV needle is too lean, but for a 750 engine I thought it would be fine. I also wonder if there's something wrong in the set up of the carb - it'd got dashpot oil and the needle slides freely. Would it be possible to have the needle sat too far down? I.e. could I loosen it and move it up 1/4" so I don't need the jet screwed out to max?!
I'm at the stage where I'm starting to get annoyed it with and chasing my tail. The engine ran fine before on bike carbs and I've just done the valves and replaced the headgaskets etc. just to make sure it's right. Timing is also spot on so I'm sure it's a carburation issue.
Many thanks,
Phil
The washer on the needle wasn't flush as it should have been.
Thank you for all the replies and sorry its taken a while.
I know to some putting a single su on an guzzi sounds like reinventing the wheel but I am building a 3 wheeled pick up based on a 1932 Morgan design so nothing is straight forward!
I've taken the carb apart, checked float height and just generally given it a good clean.
The great news is its running well and revving clean, however when I used the lifting pin it's stalling which I believed was a sign in lean running.
Decided to out the colour tune on it and it came up as a bit rich with a bright orange flame! Screwed the jet up to a nice blue and overall I'm happy with it but the lifting pin still stalls it! Have I missed something?
The next move is to get it connected up fully cable wise and then take for a blast as at least I'll get a good idea about fuel delivery when under load.
For ease it's fed by a Morris minor su pump as I felt it made sense to keep the fuel delivery matched.
Interestingly I have a reoccurring revving at idle issue which happened with both the bike carbs and the su (different manifolds) and I believe could be fuel pump related, but that's a seperate matter I think!
Thanks again,
Phil
I know to some putting a single su on an guzzi sounds like reinventing the wheel but I am building a 3 wheeled pick up based on a 1932 Morgan design so nothing is straight forward!
I've taken the carb apart, checked float height and just generally given it a good clean.
The great news is its running well and revving clean, however when I used the lifting pin it's stalling which I believed was a sign in lean running.
Decided to out the colour tune on it and it came up as a bit rich with a bright orange flame! Screwed the jet up to a nice blue and overall I'm happy with it but the lifting pin still stalls it! Have I missed something?
The next move is to get it connected up fully cable wise and then take for a blast as at least I'll get a good idea about fuel delivery when under load.
For ease it's fed by a Morris minor su pump as I felt it made sense to keep the fuel delivery matched.
Interestingly I have a reoccurring revving at idle issue which happened with both the bike carbs and the su (different manifolds) and I believe could be fuel pump related, but that's a seperate matter I think!
Thanks again,
Phil
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