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30K views 46 replies 5 participants last post by  Alexander Hoar 
#1 ·
Hello
My engine has been running rough lately so I decided to remove the dash pots and have a look inside the carburettor

I did notice that the passenger side carburettor was different to the drivers side in that the little brass round jet bearing I think it's called was raised by about 10mm

I'm not that technically minded with carbs so I don't know if it's correct or not does in raise due to any setting on the carb or choke or is this a fault. ?
Please see picture any help much appreciated
Alec
 

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#2 ·
Hi Alec.
When asking a question it's always best to give your vin number and year model etc it makes trying to help a lot easier!!,
Also I have to say the car was running fine before right? And just started running rough?mmy guess is your carb question isn't the issue (.it must have run well with that jet at that setting) was your dash pots dry? Put them back on and refill the dashpot oil let it rest for a while and see how she runs.
Without knowing the model/year/ vin number etc it would be difficult to assist or advise what to check first,,,,,

Chris
 
#3 ·
Hi Alec.
When asking a question it's always best to give your vin number and year model etc it makes trying to help a lot easier!!,
Thank you Chris!! I used to think that people were being nitpicky when they said this, but after dealing with my two cars and those of others for 9 years now I have to say that having your chassis number is essential.

Based upon the avatar, if that's Mr. Hoar's car, it must be a Shadow II.

Depending on how rough you could have an anti-run-on/anti-dieseling solenoid getting wonky and stuck partially open. My Wraith II (LRK37110) could barely be made to run at idle and was extremely rough. It was eventually traced to a partially sticking anti-dieseling solenoid. It never hurts to take these off to check them. It also never hurts to give them a shot of WD40 or similar in the center to keep things gliding smoothly since exposure to dirt/collection of crud is not an issue. I've seen many revived and continuing to work correctly after a shot of penetrating oil, allowed to soak, and then a good number of on/off cycles by hand to make sure that everything is opening and closing as expected.
 
#4 ·
carbs

:oops:Thanks Chris and guyslp
Yes my mistake Its a silver shadow 2 1978 vin SRH32815
Now for the long story
The carbs have been tuned by su carb chap a while back all working fine
Then suddenly last week I was getting rough / missing idle and lack of power on driving
I only have basic knowledge of these things but willing to learn
So I cleaned all the spark plugs and found they were very oily on the drivers side . I had to remove the inlet pipe that goes to the side of the carb on the passengers side to remove the plugs I replaced the gasket with cut out one ,the type you cut to size yourself.
Still no difference
So I check all HT leads with multimeter and found one lead that had no resistence so changed that
Still no difference
So Blast! I bought new plugs and replaced them all and when I removed the gasket on the inlet pipe I noticed I didnt cut out the breather holes for it
At the same time I checked the pots for oil and found they were empty so I removed the pots to check for any oil in the chamber gave that a clean but noticed that the brass jet was raised up on the same side as the gasket was with no beather hole?:oops:
The other carb is level
So I was concerned I may have now damage this, causing it to suck up with excessive air suction ?
I have just learnt right or wrong that these jets are adjustable in height via the mixture control nut so maybe when it was set this was the correct height at the time
Lack of knowledge for me on this one
I think I have found the cause of the rough idle faulty ht lead ,wet plugs no oil in damper and gasket with no breather holes in
But due to lack of knowledge Im concerned about the height of the jet as the other is level not knowing if the gasket could of caused this to rise or this could just be the result of the carb guy tuning the carbs
I have been told on another forum that it should be level?

Thanks Alec
 
#5 ·
Hi Alec

Yes, the jets are adjustable in height by the mixture control screw. The lower the jet, the richer the mixture. The height of the jet in the pic looks just plain wrong. As you suspect, they should be roughly the same height and slightly below the level of the bridge piece. My car is a 74 SY1 which has entirely different SU carbs so I cannot give any advice about the carbs on your car. If this had occurred on my car I would immediately suspect a failure in the jet/lever assembly. It is possible the one needle on your car is so badly worn that it would require the jet to be set in this elevated position, however if that were the case any person experienced in tuning these carbs would recommend new jets and needles were fitted. If you do not want to strip this carb down yourself, maybe a visit back to the SU guys is in order.

Regards

Geoff.
 
#6 ·
Carb

Hello Geoff

Thank you for taking the time to reply

You have managed to put my mind to rest and have
learnt something new

To strip down the carb and retune is beyond me
So I will have to call in the su carb chap

Would the height of the jet have any adverse effect on the spark plugs

As I noticed one bank of 4 plugs were very wet oily ?

Thank you Alec
 
#7 ·
Hi Alec

The height of the jet would not affect just one bank. This is because each carb services two cylinders on each bank. From the drivers seat as you look to the front of the car, the bank on the right hand side is the A bank and the left hand side is the B bank. The intake manifold connects the right hand carb to the following cylinders - A1 B2 B3 A4 and the left hand carb is B1 A2 A3 B4. I would not like to guess at this stage as to the reason for the oily plugs on the one bank only.

I think you just need to chat to the su carb chap and see what his opinion is.

Let us know the outcome

Regards

Geoff.
 
#8 ·
Hi Alec
Well it does seem something else is at play here and guy and geoff have highlighted a couple of things to think about those weakner valves could always do with a squirt of 3in 1 and it takes no time to do so I would do that and just check the spade connectors one at a time so you don't mix up the wiring whilst your at it.
Something iv done myself is just check your sparkplugs are seated correctly recently when I replaced mine I didn't quite have them fully seated (we all know they are a bugger to fit) so i had to borrow a better wrench set (mine is just a little to short and I didn't "feel" them tighten) and did them all again.
I have however fully rebuilt my carbs myself and had a chap come and balance them on the car once I had refitted them ( my settings was apparently very close) it's not a job I would advise everyone to try but I have to say if your a competent home mechanic with an eye for detail it is possible (if you drop screws use dirty rags to clean bits or don't think making notes is worth it then your not the kind of chap to try refurbishing carbs!!) it's a job for the methodical approach,,,ps I have a 1976 m11 shadow I do have photos of my carb rebuild but alas can't seem to see how to add images using my ipad.

Chris
 
#9 ·
Hi Alec

Quite a curious problem you have there. Even in the worst case scenario, a blown cylinder head gasket, you would only expect one or two cylinders to have wet and oily spark plugs. Worn valve guides would cause the same symptom, but why on only one bank, unless the one cylinder head has been reconditioned and the other not - highly unlikely, I had to smile at Chris's point about seating the spark plugs. I have done this myself, but only on the one plug. The problem is when you are screwing them in they hit tight spots giving the impression they are fully seated when in fact they are not. You do have to be so careful not to overtighten them and strip the threads, which exacerbates the problem. I always use anti-seize on spark plug threads.

I personally would run a compression test on all cylinders and also check the oil dipstick for any traces coolant which shows up as whitish foam. Also check the exhaust is dry - no steam in it.

Regards

Geoff

PS - The one thing Chris forgot to mention when carrying out any work on the car - loads of photos.
 
#10 ·
Thanks Chris and Geoff

Great Help

I have a much better insight .I did read the tech manual on su carbs hif just so I could at least understand how the jets are adjusted and could see how the lever arm works.

Thanks for the advice on the spark plugs

I think I will need the su carb chap in to have a look
I will get a compression test done aswell

Thankyou for all timing the time to help great site

PS How sunning is in Las Vegas not stop rain here in Cornwall uk

Thank you Alec
 
#12 ·
There is no way that jet should be that high. The top of it should be just below the bridge it sits in. In the configuration it is now it will completely shut off fuel to half the engine at anything below about 1500 rpm and will not let the piston drop to the idle position. If it is running like this four of the cylinders will be extremely lean and this could seriously damage the engine. Try the adjusting screw in the side of the carb to see if you can move the jet down. (This will be an HIF carb on a Shadow II) If it doesn't move when the screw is turned then the adjusting lever inside the float chamber has come loose or the plastic tabs on the jet have broken. Turning the screw clockwise (in) should lower the jet. Or perhaps the "SU guy" that last worked on them assembled the jet wrong. Either way, if you can't get the jet to come down flush or just below the bridge the carb has to come off to see what is amiss inside. If it was indeed running with the jet in this configuration then something else is seriously wrong with the fuel system. If your guy adjusted it this high to get it to run he will have been trying to compensate (incorrectly) for another fault perhaps in the weakening system. If he did find another mechanic. Get the jet situated correctly then have it tuned by someone who knows what they are doing.
 
#13 ·
Hello

Thanks Jim

My local mechanic came out to me today

He managed to simple pull out the jet after removing the dash pot

He them removed the carbs and told me the jet had come out of the jet head?

He popped it back in and balanced the carbs

Starts and idles great

He did say that it should not pop out like that so should I buy a new one is the jet supposed to be fix in to the jet head or removable

Thanks Geoff

Alec
 
#16 ·
The jet is #4 in the diagram in the link Geoff provided. The aluminum/brass jet should not come out of the plastic base, I would replace it to prevent a recurrence of the problem. Very unusual for this to happen, probably a fault in the manufacture of the jet or somehow someone has cracked the base of the jet.
 
#17 ·
Thanks Jim

I will get it replaced ASAP should I buy a new needle at the same time

Or just the jet

Also I spoke to soon started her up this afternoon started fine
Idled fine but when I rev her up she seem to nearly die then normal idle
Comes back ?

Would this be a air leak somewhere

Thanks Alec
 
#18 ·
Hi Alec

There are many possibilities for this problem however I would look first at that jet assembly. If it has just been pushed back in then the original fault has not been corrected. What's to stop it coming free again. If the jet moves just slightly it will throw the tuning out. There may be another reason for the fault you describe, but I personally make sure the jet assembly is correct first.

Geoff.
 
#20 ·
Hi Alec

A new float needle and seat would only be necessary if they were causing the float chamber to flood. Maybe your mechanic means a new jet needle. It makes sense to replace the needle as well if you are replacing the jet assembly. I personally would buy a recon kit and rebuild both carbs. However, I do understand that since you are paying for the the services of a mechanic this can get quite expensive as there would be significant labour time involved.

Regards

Geoff
 
#21 ·
Alec, the jet needle can wear as it can touch one side of the jet and rub against it every time it moves. Your mechanic has most likely run his fingernail along the needle and felt some wear so has advised replacement. The plastic base of the jet will not allow the jet to go any higher than level with the bridge inside. As it was much higher than level it must have come loose from the base, pushing it back into place is unlikely to last. If the base is cracked perhaps the crack is above the level of the fuel and is admitting air with the fuel sucked up into the jet, leaning out the mixture. Without knowing the history of repairs I would get two carb kits and overhaul them both, it is possible the float needles and seats are worn too. Do it right the first time, if you just replace the jets six months later the float needles could start leaking then off come the carbs again. The carbs must come off to replace the jets. With the carbs off replacing the float needles and seats would add less than five minutes to the labour. Unlike the earlier Shadows, the later ones with the HIF carbs require removal of the carbs to replace the float needles and seats as well as the jets.
 
#22 ·
Hello Geoff


I need to go to spec savers I replied calling you Jim
sorry

I've been reading the tech manuals for the carbs and decided
to replace the jet myself and while I have both carbs off to fit a
rebuild kit makes sense

Am I thinking correctly to set the jet it should be level with the bridge and then
lowered 1.5 flats to start

Thanks Alec
 
#23 ·
Hi Alec

I think that's a really good thing to do. Here are a few tips. Absolute cleanliness is essential. Always keep the carbs completely separate from each other to avoid mixing parts from the one with the other, Never rest the piston on it's side as it may push the needle out of alignment, I have drilled two half inch holes in a shelf in my garage so whenever I remove the carb pistons on my car I can place them over the holes so the needles poke through. This way there is never any stress on the needles. Always replace the suction chambers in the same position as they were removed. I have centre punched my carbs to effect this. (It may be that on your carbs the holes are asymmetrical, so the suction chamber is automatically fitted in the same position. On my earlier carbs this is not the case, hence the need to mark the correct position).

You are absolutely correct, adjust the jets so they are level with the bridge and then lower 1.5 flats. This gives a start point for tuning the carbs.

You have a really interesting and enjoyable project ahead. Keep us posted.

Regards

Geoff
 
#24 ·
1.5 flats
Geoff & Alec, what is this "flats" measurement in American English?

This is, for me, an instance of "two peoples separated by a common language."
 
#25 ·
Hi Brian

I think this must be a case of "two peoples separated" as the phrase made perfect sense to me even though logically it is meaningless. I took one flat to mean one full rotation of the mixture setscrew. Maybe Alec could confirm this, Logically of course it has no meaning as the mixture screw does not have (on my car) a hexagon head. If it did, the one flat would imply 1/6 of a rotation. Thinking about it, maybe after 11 years in the US I am losing touch with the nuances of UK English. Maybe Alec did mean 3/12 of a rotation.

As I recall, the SY2 manual states the mixture is factory set and the setscrew aperture is sealed. I presumed from one of Jim's entries, regarding the setting of the carburetors, that this statement is now obsolete, given the age of these cars,

As a guide, for SY1 cars, the jets should be first adjusted so they are level with the bridge and then the mixture screw turned two full rotations. This is approximately equal to the lowering of the jet by about 1/32 of an inch. Both jets should be set exactly the same. It seems logical to me the same setting would serve as a good start point for tuning carbs on the SY2. Maybe Jim could correct me on this if I am making a false assumption.

Regards

Geoff
 
#26 ·
The term "flats" comes from the jet adjustment method on earlier SU H and HS series carbs used on many British cars. The jet adjusting nut was an external hex nut, and the adjusting directions were to turn the nut so many flats for the initial adjustment. Directions to turn it one flat were probably more easily understood than saying turn the nut 60° when the carbs first appeared in the first part of the last century.
On the HIF's, the initial jet adjustment is set by turning the adjusting screw in the side of the carb clockwise until the top of the jet is 1/8th or .125 thou lower than the bridge. After they are back on the engine and it is running the mixtures must then be set with an exhaust gas analyzer or the old manual "lift the piston" method until one can get it done on an analyzer.
I can't remember the last time I saw tamper proof plugs in the HIF's, most of them have been removed long before now.
 
#28 ·
Thanks for the thorough history on how the "measurement" term flats came about.

It does make perfect sense once you know that or if you've dealt with an SU carb that has that layout. Otherwise it would remain quite mysterious.
 
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