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MG MGA - Improved MPG With New Jets
|I keep a very close watch on my MPG both on single tank loads and the average consumption for the last 11 years. It's been okay but nothing spectacular. Combined with this I have always noticed over the years that it has tended to run run slightly rich regardless of all my tampering. And believe me I have tried everything - well almost.
The carbs were a 'new' reconditioned set when I rebuilt the car in 1997. I assumed (wrongly as it's turned out) that all wearable bits would have been replaced during the reconditioning. About 4 months ago I replaced the only parts that I had never touched - the jets. Guess what? Quite apart from now being able to tune the carbs to the ideal set-up, my latest figures show that I am now getting 4 MPG more.
Very pleasing, but a long hard lesson learned. Looking back I guess the clues were in the price I paid for the exchange set of carbs. The £220 I paid was unlikely to have included £90 of new jets.
|Fully reconditioned H4 carbies cost around $A400 each here (220 pound each). |
Your price per carb seems far more realistic. I traded in my old tatty (seized) carbs with a very well known supplier for a fully reconditioned pair by one of his sub contractors, so I never looked at the jets as a possible cause of the slightly rich running. But when you look at the cost, which was half yours, it has to include profit for both parties, labour, cleaning, seals, bushes etc, leaving little if any capital expenditure available for high cost items such as the jets. OK, I paid the retail price for my jets but they are still a costly element in the reconditioning process.
Once I replaced those jets the difference was so immediately apparent. The ease with which the carbs tuned up was like nothing I had previously experienced. Everything just fell into place so readily; correct colour plugs & exhaust; idling vacuum (used to be 17 inches HG but now just under 19; use of choke; performance throughout the range; and of course after several hundred miles driving in a similar manner as before, the improved mpg.
Of course the supplier will tag them as fully reconditioned. But my advice for anyone going down this route is to try to determine exactly what has been reconditioned. What is original and what is new. The price you pay has to be a clue.
|So which jets did you use?|
Just got my invoice out. I somewhat confused myself. My total bill including the jets, needles and super dry seals was just under £100. I don't know why I thought the jets were the expensive item. In fact they were only about £10 each. The needles are GS. I run the 1800 but have found these to be the best for me, having tried at least 2 other sizes over the years.
|Mike,the last set I had done by Midels was just over A$800. Where did you get yours done?|
Help me out here. Confused, as usual! I would have thought that as the needle should rise and fall centrally in the jet, neither would be heavy wear items, certainly not to the extent that their influence on your performance and tuning problems would suggest. Am I wrong here? If it is possible for the needle/jet combination to wear to that extent then perhaps we should all be taking more interest in them than I suspect is generally the case. I confess to having assumed to never needing to consider these items as expendable!
|Bruce. I did wonder that as well. That was why I never looked at the jets in 20 years. I just tried a selection of needles. I measured the old jets in comparison to the new and their bores were oversize by a few thou.|
My price seems the same as yours (2 x $400 at Wilson Carburettor Service) compared to your $800 at Midel for a set.
|The needles will run up and down in the centre of the jet if they are properly centred. In the HIF4 series, fitted to the 1800s in the MGB, they are supposed to be self centering. Details in the handbooks.|
For reasons everyone will understand, I changed the jets in our V8 every two or three years (Before I fitted Fuel
Good day Roger, from down under---- I note your comment
'[ Before I fitted fuel injection ]"
Your comment interests me---I travelled this rout with an Ex-SD1 Rover V/8 engine mounted into an Ex-Californian MG B body, and fitted it up with an Ex-Land Rover hot wire system---- some 15 years ago.
Did you opt for this Lucas easy swap, also? If so, how did you arrange the engine's height under the bonnet?
Did you make up your own ex hausts ?
My conversion used Ford 2.92/1 diff. gears and 5 speed Toyota gearbox.
If you care to come back on my E-Mail, I'm sure we would have some details to ex change!
I have moved on now with a fabricated EFI system on a 5 bearing engine in an MG A Coupe'. It delivers 37 MPG.
Its 5 port head has been a challenge!!
A Happy Christmas---thank you for your wishes.
|Is Barney monitoring this thread? Hopefully so?
If you are Barney, or anyone else with good technical knowledge, does this mean that my Twin Cam (H6) carburettors’ needles and jets are actually items which should be routinely replaced every few (apparently small number of) years due to wear due to running up and down, even if well centred? Mine have not been looked at in maybe more than 20 years! Only a bit of minor tuning very infrequently. The engine runs well, they don’t leak, probably due to regular useage perhaps, and I have always assumed them to be non-maintenance items. Should I be including them in a more regular replacement programme, say every couple of years?
|When properly centered, jets and needles should never touch or wear. I've been running same needles for more than 25 years, 350,000+ miles, no problem. I don't recall if I ever changed the jets earlier on, might be original factory issue around 550,000 miles.
If they spent some time misaligned in a past life, they might be worn. If the engine runs rich at idle speed and cannot be adjusted lean enough, that is a classic symptom of worn jets and/or needles. As always, do the diagnostic work first. Measure the jets and needles to determine condition before spending money on new parts.
|Think I will leave well alone!|
The issue with me of course is that I did not know the history of the reconditioned carbs. During my 20 years of running them I believe I had them set up the best I could, certainly from the jet centreing point of view. They always slid up and down freely. However, if the conditioner did not change the jets during the rebuild I have no idea how long a life they had in their previous ownership, nor how well the needles were centred.
I can only reiterate the very noticeable improvement after I changed them, starting with the immediate affect on the ease of tuning, leading on to the improved MPG.
This thread was discussed between 17/12/2017 and 24/12/2017
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