Spitfire: carb popping, stuttering engine- help?

[greeneggsnsam]

Hi all, hope someone will be able to help.

Today I had a go (first time) at balancing the carburetors on my Spitfire 1500 ('76). I decided to balance the carbs as a bit of routine maintenance. Unfortunately it's not had the desired effect- it's made everything worse



I took off the pancakes fitted to my car (by previous owner) and then used the Mintylamb guide and used the fast idle screws and a length of pipe to gauge the hiss of the carbs. I got it idling at ~700rpm as smoothly as I could.

Then, looking in my haynes manual, I adjusted two nuts, one under each carb. I can't be sue as the drawing was quite hard to match to the carb but it should be the jet adjusting nut. I moved them to make the mixture less rich but I am not sure that I did it correctly! Perhaps this is the issue?

Then I moved on to the pistons, I noticed that when I pressed the throttle the back carb's piston was moving much more than the front carb. I also noticed that these two pins (arrows) weren't making contact at the same time.



I moved the circled one so that both push down at the same time when I press the throttle.

After this I went for a little drive and upon putting power on the engine stutters and the back carb makes a fairly loud pop. I tested this with the bonnet up when I got back and saw a small puff of smoke as it popped.

I'm terrified to do anything else as I feel like I've broken the carb! Can anyone help me out?

[jms700]

hello,

the manuel will give you a corect measument for the carb linkage i think.

did you use the piston lifting technique to get the right mixture ?

did you turn the nut clockwise or anti ?

are the airfilters back on the right way around ? some have air passages that need to match up on the carb.

thats a few ideas
not sure if its any help

[greeneggsnsam]

jms700 said:

are the airfilters back on the right way around ? some have air passages that need to match up on the carb.

Thanks for the help, I will try your suggestions out. Could you give me a bit more info on this? The pancakes I've got weren't fitted by me so is there a way to find this out? It didn't look like there was any special way they go on.

[jms700]

im not sure because i dont have pancakes on mine, i just know that mgb filter housings have to go on a certain way. best bet is to take them off and have a look if theres any holes that line up to holes on the carbs. i nthe photos on mintylambs guide i can see two holes at the top which could be what im thinkign of. its a long shot.



are you sure you turned the adjusting nuts nuts the right way , what colour are the spark plugs now?

theres little to no chance you have broken the carb so just relax and go through each process again.. you could simply try retuning them again.

[greeneggsnsam]

jms700 said:

im not sure because i dont have pancakes on mine, i just know that mgb filter housings have to go on a certain way. best bet is to take them off and have a look if theres any holes that line up to holes on the carbs. i nthe photos on mintylambs guide i can see two holes at the top which could be what im thinkign of. its a long shot.



are you sure you turned the adjusting nuts nuts the right way , what colour are the spark plugs now?

theres little to no chance you have broken the carb so just relax and go through each process again.. you could simply try retuning them again.

Don't think there are any holes like the ones circled on my car, but I will check again tomorrow when I re-adjust. The spark plugs are carbon fouled but that's because the mixture was rich in the first place (part of why I attempted to tune in the first place!).

Thanks for the help!

[jms700]

no problem, its also worth getting some feeler guages out and setting the throttle bits to the right gap. should describe how in the manual.

update if you find any thingl

[jith]

greeneggsnsam said:

Hi all, hope someone will be able to help.

Today I had a go (first time) at balancing the carburetors on my Spitfire 1500 ('76). I decided to balance the carbs as a bit of routine maintenance. Unfortunately it's not had the desired effect- it's made everything worse



I took off the pancakes fitted to my car (by previous owner) and then used the Mintylamb guide and used the fast idle screws and a length of pipe to gauge the hiss of the carbs. I got it idling at ~700rpm as smoothly as I could.

Then, looking in my haynes manual, I adjusted two nuts, one under each carb. I can't be sue as the drawing was quite hard to match to the carb but it should be the jet adjusting nut. I moved them to make the mixture less rich but I am not sure that I did it correctly! Perhaps this is the issue?

Then I moved on to the pistons, I noticed that when I pressed the throttle the back carb's piston was moving much more than the front carb. I also noticed that these two pins (arrows) weren't making contact at the same time.



I moved the circled one so that both push down at the same time when I press the throttle.

After this I went for a little drive and upon putting power on the engine stutters and the back carb makes a fairly loud pop. I tested this with the bonnet up when I got back and saw a small puff of smoke as it popped.

I'm terrified to do anything else as I feel like I've broken the carb! Can anyone help me out?

Dear me Sam, you're in a right pickle!

By a happy coincidence I just finished a carb rebuild on a Spit that we sold today; it's going to its new owner on Saturday.

What you did with the balance bar between the carbs is spot on. They must be synchronised for smooth running.

The spitback is cause by the mixture being too lean.. However, besides adjustment, this could have 3 other causes. The pancakes you have fitted will alter the vacuum at the mouth of the carbs and cause them to run lean. You will have to either set them richer than normal, which is not desirable, or change the needles to compensate.

The biggest problem with SUs on these cars is wear on the throttle spindles. This causes a very poor idle as the carbs are drawing air passed the spindles and leaning off the mixture. If you can lift the spindles up and down where they enter the carb bodies, they are worn. Nothing will work properly other than new spindles and bushes.

The last is thermo jets fitted to some Spits and Midgets. If you look at the bottom of the jet under the mixture adjusting nut you will see either a vertical red plastic end or a flat, pancake shaped end. If it is the latter these are jets fitted with internal thermostats that are supposed to control the mixture according to engine temperature. They are an unmitigated disaster; the only cure is to change them to the standard type.

In the meantime you can enrichen the mixture by UNSCREWING the adjusting nut;i.e turn it anti-clockwise slowly, perhaps a quarter turn at a time. The idle should improve and the car should pull without popping through the carbs. But it means that you are running too rich. In the long term this causes high fuel consumption, and increased piston and bore wear.

Good luck,

J

[greeneggsnsam]

jith said:

The spitback is cause by the mixture being too lean.. However, besides adjustment, this could have 3 other causes. The pancakes you have fitted will alter the vacuum at the mouth of the carbs and cause them to run lean. You will have to either set them richer than normal, which is not desirable, or change the needles to compensate.

As I said I didn't fit them, the previous owner did- so for all I know he replaced the needles as well. Is there an easy way to tell?

jith said:

The biggest problem with SUs on these cars is wear on the throttle spindles. This causes a very poor idle as the carbs are drawing air passed the spindles and leaning off the mixture. If you can lift the spindles up and down where they enter the carb bodies, they are worn. Nothing will work properly other than new spindles and bushes.

What is the throttle spindle? I will check this tomorrow but wear isn't a likely problem as my car is low-mileage.

jith said:

The last is thermo jets fitted to some Spits and Midgets.

This is fine, the jets are standard.

[jith]

greeneggsnsam said:

jith said:

The spitback is cause by the mixture being too lean.. However, besides adjustment, this could have 3 other causes. The pancakes you have fitted will alter the vacuum at the mouth of the carbs and cause them to run lean. You will have to either set them richer than normal, which is not desirable, or change the needles to compensate.

As I said I didn't fit them, the previous owner did- so for all I know he replaced the needles as well. Is there an easy way to tell?

jith said:

The biggest problem with SUs on these cars is wear on the throttle spindles. This causes a very poor idle as the carbs are drawing air passed the spindles and leaning off the mixture. If you can lift the spindles up and down where they enter the carb bodies, they are worn. Nothing will work properly other than new spindles and bushes.

What is the throttle spindle? I will check this tomorrow but wear isn't a likely problem as my car is low-mileage.

jith said:

The last is thermo jets fitted to some Spits and Midgets.

This is fine, the jets are standard.

The Spit we have just sold had only done 50K, and the spindles were completely worn out. The spindles are the parts that the balance brackets you just adjusted are mounted to.

All SU needles have the type letter stamped on the base, but you have to remove them to read it. You would have to look up the standard needle for your car and carbs and see if they have been changed.

J

[ARH]

just a couple of questions.

did you disconnect all the linkages before balancing?
are the jets centered correctly.

My guess is with worn spindles, this will make it impossible to set up nicely.

[greeneggsnsam]

Hi all,

I have tried what was suggested- move the jet nuts all the way up and then move downwards until it hunts. Annoyingly the popping is still present, in fact it's probably worse than ever. The car idles beautifully, but any power at all and the back carb pops and sometimes smoke even comes out the exhaust pipes.

I kept moving the jet nuts down so the mixture was more rich but the pops continue, they are currently much further down than they were when I first came to fiddle with them.

I've not had the domes off to look at the needles so perhaps this is the next step. Is there something I'm missing?

The crucial thing to remember is that the carbs were essentially fine before I fiddled with them, so it's not an issue of wear, just that they are set up wrong- surely?

[caziques]

Excellent assessment from Jith.

A couple more pointers - early 1500s have ABT needles, later ones ADN. It's stamped at the top of the needle. All 1500s run lean, the air filters fitted mean you have to change the needles to get the mixture correct all the way up the rev range.

The "waxstat" part of the jet can be removed and replaced with 3 1p pieces. It stops the carbs leaning out too much and means you don't have to use a watering can to get the car to start after a long run in hot weather. (Been there, done that).

[greeneggsnsam]

Good news- the problem is solved! (Well, until I discover that it isn't )

In case anyone is having the same problem in the future and comes across this thread, the way I fixed it was to take the domes and pistons out of both carbs, clean them up, add oil to the damper pots, set the jets flush with the carb bridge, and then set the fuel mixture from there. With the nuts all the way up the jets were nowhere near flush and so the pistons had to be out in order to set it properly. Minus the cleaning bit all I did was follow the Haynes manual section on tuning. Worked a charm, car is running better than it was before, even.