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HS4 Jet issue


cliff.b

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Spitfire been running really well for ages but today very rough, won't tick over and feels like misfiring until throttle opened wider & revs rise. All ok if choke pulled out a bit. Found tickover no worse if plug leads 3 or 4 removed so assume rear carb problem. After further investigation I noticed the jet on that carb is protruding 2 or 3 mm and obviously, operating the choke cures this. This explains the problem and winding it down several turns gets the car running ok but now externally, everything is considerably lower than the other carb. Struggling to work out what has happened 😟

IMG_20230921_154041_916.thumb.jpg.db35e9b80db567edc5865fe6a47177f1.jpg

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Hmm, took both jets out and the rear one does appear to be a couple of mm longer. Might be due to the waxstat "bodge" but the jets are solid in the carrier with no discernable movement. So how did this problem suddenly come about.? 

While looking at this I found the choke operating linkages are a bit sticky and I'm wondering if that jet has been stuck too low for ages and when I got the car running I adjusted around it. So if it suddenly freed up the jet would be too high and almost cut the fuel supply off to that carb at small throttle openings.

Not sure if that is plausible but best I can come up with at the moment.

Edited by cliff.b
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If i'm reading that right richening the mixture, either by opening the choke or winding the jet down sorts the issue - and everything was fine before.  That would have me checking the breather pipes between the problem carb and the T piece.  If the pipe starts to split extra air gets in after the jet, so giving a weak mixture.  Pull the choke and you can richen the mixture to match the extra air.

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its quite common for the jet tube to b a bit too long and this jams the choke /jet return but these are old jets should be ok as long as the tube is

not kinked under the spiral protection .

many fit new and have to trim 3mm off the tube 

and often spare 0 rings /olives are found stuck down the tube hole 

Pete

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In answer to all the above, I did some work on the linkages and refitted the jet so, by eye, the top of the jet was roughly the same height as the front carb. Adjusted further after a quick check with Colourtune then off for a road test which went well, to start with. I stopped & checked the plugs and rear ones were a bit weak, so lowered jet one flat. After this, the tickover was perfect and goes well with foot down, but running rough at slower speeds. Once again, pulling the choke out solved it do progressively richened the mixture and it got better, but not perfect.

Now jet half a turn lower than the setting I started with and barely any difference to plug colour. I will work through all suggested here.

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Hmm, I have just read that the 76 to 79 Spits had FZX1122 carbs and apart from any other differences these were fitted with ADN needles which appear to be leaner across the rev range than ABT's.

ADN is grey line

Screenshot_20230922-2138212.thumb.png.46f8598c5c3b89220d9e9ca9cdafc9ed.png

I have always found that if I set the mixture at home it is always much weaker if I stop in a layby and check the plugs during a run. Guess this might explain why 🤔

Edited by cliff.b
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22 minutes ago, Josef said:

The needles are marked. In tiny letters! So you should be able to check if you have them out again.

Any idea if the springs have any identification marks on them? The comments I read about the different needles in later Spits also said the springs were different. Looking at my carbs, there are some slight differences and I'm not 100% convinced I have a matching pair.

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2 hours ago, cliff.b said:

Any idea if the springs have any identification marks on them? The comments I read about the different needles in later Spits also said the springs were different. Looking at my carbs, there are some slight differences and I'm not 100% convinced I have a matching pair.

I think springs were originally colour coded. But I’m not certain. 

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The paint invariably comes off. The only way is to test the compression force at a regulated distance. Stromberg springs have a weight in grammes at a compression height of 53mm. Easily done with a cardboard cylinder cut to length and a setof kitchen scales. Just compress the spring with something flat to the cylinder and read how many grammes it requires. 

Iain 

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49 minutes ago, Josef said:

I think springs were originally colour coded. But I’m not certain. 

Looking at what is available to buy online, both types say they are Red and 4.5oz but the earlier ones are long and thin and the later ones are half the length but much fatter. Not sure what, if any, affect that has on how they perform.

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9 minutes ago, cliff.b said:

Not sure what, if any, affect that has on how they perform.

Having tried 3 weights in my Stromberg carbs it makes a big difference! It alters the rise of the air piston(valve) which has a large effect on the fuel mixture. 

Generally the lighter spring is a weaker mixture but then there are the needle taper and damper oil effects to consider. I found choosing the correct needle/spring is paramount and damper oil changes acceleration mixture. 

Take a peek at my topic Vitesse needle the story in this section. All fun and games and I've not yet posted my latest and greatest! 

Iain 

Edited by Iain T
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10 minutes ago, Iain T said:

Having tried 3 weights in my Stromberg carbs it makes a big difference! It alters the rise of the air piston(valve) which has a large effect on the fuel mixture. 

Generally the lighter spring is a weaker mixture but then there are the needle taper and damper oil effects to consider. I found choosing the correct needle/spring is paramount and damper oil changes acceleration mixture. 

Take a peek at my topic Vitesse needle the story in this section. All fun and games and I've not yet posted my latest and greatest! 

Iain 

That's interesting. I will dig it out and have a read. I noticed yesterday when adjusting with the filters removed, although I got both pistons to start rising simultaneously, one then moved more slowly than the other. Need to check again that there is no binding or suppose it could be due to an air leak 🤔

Was initially wondering if due to differences between the carbs but that is probably a long shot. Work checking out now I have a better idea what to look for though.

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check the idle stop setting any variance with air flow through the carb will alter the rise heights of the air pistons 

i have a simple gauge that you can set the throttle balance by measuring the air piston lift 

which is just another spin on how to balance a crude bit of kit 

in their day they were just preset with a fixed idle screw amount and bunged on the engine no faffing about with balancers of any sort were rarely ever used 

on production. air flowed control was not devised till the late 80s up till then 1.5 turns on the idle screw was all it took 

Pete

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Ok, spanners out, air box off and another go at it all. Looked at the rear carb and the jet was above the bridge again by a couple of mm. I was at a loss to understand how this could be but took the jet out, measured it and it was longer than it was yesterday lol. Pulled the jet tube gently and it came away from the rest of the jet, which explains everything. Reseated it, squeezed gently in a vice and now jet is 4mm shorter. Feels tight so refitted, adjusted and then went through all the idle/fast idle/choke adjustments, some of which were way out. By eye, the pistons in each carb appear to rise together now, the tickover is perfect and running well on the road. 

Maybe it's time to bin these old waxstat jets and get some new replacements.

Incidentally, the tag on the carb says FZX1258 so if what I have read is correct, either the carbs or the entire engine would appear to be from an earlier car. Unless mine was made in 77 and hung around for a while before being registered.

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