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HPMX Jetting? Again - Sorry.
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PostPosted: Wed May 25, 2016 4:00 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

also understand that the idle circuit tune is completely different for engines with SVDA, and engines with cent only timing. So the idle jet used for each is DIFFERENT.
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XR6T
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PostPosted: Wed May 25, 2016 4:12 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Thanks John,

I am going to pull the main stacks and try 55's even though I was told that for my small capacity engine they would be too big.

Am I correct in thinking that I should not concentrate on the readings but on how it drives?

I don't want to mask a problem with an overly rich mixture.
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PostPosted: Wed May 25, 2016 4:15 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

You have to be concerned with the idle mixture, because only if you are doing that will you know that the circuits are behaving properly.

If you have a cracked casting which is leaking fuel into the idle circuit, you will never know that if "11:1 idle A/F" is "acceptable". It's not, it's a sign that something is jacked up (either the tune, or the carb(s)).

You need to leave the main stacks out, and get the idle and progression circuits handled first.

The second red flag is the seeming lack of a response with the idle jet changes. That means the fuel is coming from somewhere ELSE. So, where and why?

"Lean" A/F can be triggered by ignition problems, or exhaust leaks, in addition to carb issues. But RICH A/F are always fuel problems, ALWAYS.

XR6T wrote:
Hi John, I am aware of what the idle mixture screw does and am not too concerned with the idle mixture, but the other readings appear too rich in what I like to call the cruising circuit (what the size of the jets control) and the huge flat spot that has developed by going leaner.

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PostPosted: Wed May 25, 2016 4:34 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Thanks again, more head scratching coming up.
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PostPosted: Wed May 25, 2016 6:25 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

55 Idles
205 Air
120 Main

Not concerned with the slightly rich readings 12.3 as the improvement is huge.

Followed the remove main jets and run on idles, we have a short driveway of about 25metres and a 3:1 slope. Car went up driveway with no hesitation, first time ever.

Ever so slight hesitation sometimes on acceleration, will now wait for some F7 Emulsion tubes.
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PostPosted: Wed May 25, 2016 7:59 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

keep the main stacks out and drive it for a week. Also, idle jet changes should be done in increments of "2.5". 55s are far too big for 40s, FWIW.

If that engine needs 55s to get out of it's own way, I think you'll find the engine has mechanical problems resulting in low compression, OR the engine was poorly designed and has low CR out of the box. Low vacuum means you need over-sized idle jets.
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PostPosted: Wed May 25, 2016 8:45 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Drove it about twenty miles this afternoon and it drives well. Down low and idle it is a little rich, but jogging along between 40-65mph (2500-3200rpm) the fuel reading are certainly in the ballpark. (12.5>15.0)

I think you are correct in the low compression comment. I initially seem to remember being informed that it was 7.5:1 then when asked again told it was 8:1.

I should check the vacuum readings.

I could not imagine it using anything below the original 52.5's will go back to them again for comparison.

Thanks for the help, it has been very encouraging.
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PostPosted: Thu May 26, 2016 1:13 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

[email protected] wrote:
keep the main stacks out and drive it for a week.


A week?
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PostPosted: Thu May 26, 2016 1:21 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

porkchop-rob wrote:
[email protected] wrote:
keep the main stacks out and drive it for a week.


A week?


Yes! You drive as much as possible with the jet stacks removed to get a feel for what the car is doing by driving on the idle circuits ONLY! The idle jets and the idle circuit are working much more than people think. You must read the wideband thread with over 71 pages of info that explains this in detail.
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PostPosted: Thu May 26, 2016 1:23 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

XR6T wrote:
Drove it about twenty miles this afternoon and it drives well. Down low and idle it is a little rich, but jogging along between 40-65mph (2500-3200rpm) the fuel reading are certainly in the ballpark. (12.5>15.0)

I think you are correct in the low compression comment. I initially seem to remember being informed that it was 7.5:1 then when asked again told it was 8:1.

I should check the vacuum readings.

I could not imagine it using anything below the original 52.5's will go back to them again for comparison.

Thanks for the help, it has been very encouraging.


Are you doing all this with the jet stacks out? If not you are wasting your time
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PostPosted: Thu May 26, 2016 2:28 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Yes. While doing this play around with idle jet sizing, go as small as you can while retaining "manners". A little laziness is ok, because if you want more power you hit the pedal more (with main circuit operational).

A couple hundred miles will give you a good feel for the idle and progression circuit, and then you don't have to re-visit the idle jet sizing issue at all, you can focus 100% on the main and air.

porkchop-rob wrote:
[email protected] wrote:
keep the main stacks out and drive it for a week.


A week?

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PostPosted: Thu May 26, 2016 2:47 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

It may "just be advice, etc."but it damn good advice. Thanks

From the many miles with the 52.5's and 50's I have a feeling that I may end up with the 52.5's or awash in the 55's. Here in Tasmania were we live, we call any straight road a mile long a freeway. It is mountainous and windy, a sports/performance car or motorbike mecca.

Now the question becomes IF I resort to the 55's to combat the low compression engine, is there any risk?

The 52.5's were almost liveable apart from the stumble, which was more annoying than anything else. Could that stumble, which was caused by a lean mixture from lack of air (vacuum) on hard acceleration at low rpm, be overcome by increasing the amount of fuel given by the acc. pump? If so this would be the way to go.
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PostPosted: Thu May 26, 2016 2:51 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

you just have to give it the proper amount of fuel. It that means a 42.5 idle, or a 55 or 57.5 idle, so be it.

Your same carbs on your same engine at 9.5:1 compression would want much smaller idle jets, to deliver the same amount of fuel through the idle and progression circuit. This fact confuses a lot of guys, wondering why a jetting "table" doesn't work well.

The main circuit doesn't get affected by vacuum (compression) like the idles do.

Just see which idle jets YOURS wants. But don't rush it.
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PostPosted: Thu May 26, 2016 3:20 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Thanks John and to others who have helped me start to understand a big thank you.

Will take my time and put a few miles up experimenting to get the best set up.
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PostPosted: Sat Jun 11, 2016 6:11 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

OK, done the miles and done the testing.

Jonh's comments on give it what it wants at idle regardless of size, hit the spot.

The air fuel ration gauge has been a God send, allowing me to see what was really happening when I depressed the accelerator in real time.

The end result is:-

Vents 28
Idle 55
Air 200
Main 115
Em. F7

The accelerator pump 3 turns in.

Timing 30 degrees max advance at 2800RPM

Car runs like a dream, no flat spots and am able to accelerate from 1500RPM under load using top gear.

THanks to those that responded and a special thanks to John for his words of wisdom, no doubt gained over many head scratching sessions and time.
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