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Any experiences with adjustment of Jet Adjusting Nut on BN4?

blueskies

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My question has to do mainly with the Longbridge BN4 that is now back on the road after about 50 years disassembled and then under restoration for the past six years or so.

About four years ago, early in the restoration, I rebuilt the stock SU H4 carburetors, including installing new shaft bushings and various wear parts. Memory being what it is, I did not recall the bench setting that I used for the Jet Adjusting Nut. The car first went back on the road last week. When the car was not running well these past few days (due to a faulty condenser it turns out), I rechecked various carburetor and other adjustments.

I discovered that I had backed off the Jet Adjusting Nuts six flats, or one full turn, from all the way up. The Healey manual that I have mentions backing off the Jet Adjusting Nuts twelve flats, or two full turns as a starting point. So I tried that too. And I tried nine flats. I find that, now that the car is running well again, it doesn't make a lot of difference to the operation of the engine whether the nuts are backed off 6, 9, or 12 flats.

The engine does seem to idle faster with the nut backed out 9 or 12 flats instead of 6 flats, but I am wondering whether that is the only difference that I should expect. The engine has great power and sounds amazing, but I haven't driven it enough with the various settings to say much about differences in performance between the jet adjusting nut settings.

Wondering what other people know about this. For these relatively small SU carburetors, what setting might work best? And what difference will the settings make?

Learning as I go. Thanks in advance.
 

Dandare

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First of all the air flow to each carb needs to be confirmed to be equal. And the engine hot.

The 12 flat setting is always going to be rich so turning the nuts up will weaken the mixture towards the correct setting.

As you approach the correct setting the idle speed should increase...so it looks like you are getting there between 6 & 9 flats. At this point you really need to find out at what point the idle stops increasing and then set it back even a fraction of a flat. Having an external tacho attached really helps here. Do each carb alternately.

Make all adjustments slowly waiting 10 seconds after each before moving to the other carb or making another adjustment on the same carb.

The nut position will not necessarily be in exactly the same position for both carbs when correct so some small corrections may need to be made to each at the final stages.

This is a quite simplistic scenario assuming there are no underlying problems else where.

Danny
 

vette

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I will confirm much of what Danny said. The ideal setting of the jets can usually be ascertained by the idle speed. But you will want to make sure both carbs are drawing the same amount of air. This is not rocket science but it can sound complicated in the verbage. Make sure both your throttle plates are completely closed together when the idle screws are not touching their stops. ( throttle shafts Not Locked together). Turn the idle screws down till they touch the stops then turn them 2 turns more. This is your initial idle speed. For time being lock your throttle shafts together. Set your jets to the 12 turns. Engine should start up and run smoothly. After warm up set your idle speed screws to get idle to between 600 and 800 rpm. Nows the time to check the mixture of the jets. Know that any engine is going to run at its happiest when the fuel mixture is at its best. It's best to use an external tachometer like Danny mentioned but if one is not available use the one in the dash. But pay attention to it closely because the fact that it is not that responsive means that any hint of speed change is what you are looking for.
So doing one carb at a time turn the jet towards leaner and see if the engine speed picks up. If it does that means you are going in the right direction. (Obviously if the speed drops off you are going in the wrong direction and will need to turn the nut towards rich again) If the engine speed keeps increasing as you lean the jet then keep going until it no longer increases engine speed. Do the same with the other carb. The idea here is to adjust the jets to get the fastest engine speed from each one.
Some will say that after achieving fastest speed from each jet that you should turn the nut back just a tech to get a smoother engine. Some engines like that, some don't. After you get the mixture right on the jets you should check that both carbs are balanced, ie, sucking the same amount of air. You can do that by listening to the end of a hose with the other end stuck in the throat of the carb or you can use a Unisyn or you can watch the suction pistons to see if they are at the same level and rise together. If they are not the same then loosen the throttle shaft unions and adjust the throttle separately to get then balanced then reconnect the unions. The final parting words are: adjust the jets for the best speed.
Having said all of that, I 'm going back to what you said that you don't notice much difference between your jet settings. Maybe not between 9 and 6 but I would think you would between 12 and 6. But without a tach you might not be noticing much. But what you are really seeing is that at the 12 flats position your engine is running pretty rich. At the 6 flats position your engine is running pretty lean. In the lean setting you might be experiencing loss of power, hesitation and possibly also some bucking at low rpms. In the rich end of things you will be sooting your spark plugs and possibly seeing some black smoke from the exhaust. In the rich end of things your engine probably will not sit there and idle all day. It will soot up and stall. A properly adjusted carb system will have the cylinders burning as cleanly as possible and will burn clean, not soot up at idle and idle all day long without stalling. Good Luck.
 
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blueskies

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Thank you, Dandare and vette. I hope to get to this again soon. It's running well, but I do want to get it adjusted as well as possible too.

I forgot to mention why I think that I set the jet adjusting nuts to six flats when I rebuilt the carbs. Although I don't recall exactly, that is probably what they were set at when I disassembled them, so I probably went back to the same setting. Also, I have several spare carbs, which I checked this week for their settings. They also are set to about six flats. And I have a spare engine from another Longbridge BN4, that I pulled out of a "future" project. Those carbs have not been touched for decades; one was set to six flats, while the other was set to about five flats. This led me to wonder whether the H4 used on the less powerful and smaller loghead engine might require a higher jet setting than the later 100-6 and 3000 engines. It will be interesting to see what the final settings will be when I complete the adjustments on this car.
 

Dandare

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I don't think you can really compare jet settings in terms of adjusting nut position with the later engines as the size of Carbs changed as soon as the Gallery style head was replaced with the 6 port head.


Danny
 
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