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HPMX Jetting? Again - Sorry.
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XR6T
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PostPosted: Sun May 15, 2016 9:47 pm    Post subject: HPMX Jetting? Again - Sorry. Reply with quote

I have managed to get the jetting so close and am in the final stage of getting "just right." I hope.

Dual HPMX 40's on mild tuned 1600.

At present I am using 200 Air F11 115Main, wideband at a comfortable light throttle cruising speed of 3,000RPM is 11.5>12.0 now I am thinking of trying 110Mains.

Is the variation between 200 Air and 110 Main to big a gap?

Should I be thinking of changing both Air and Main sizes?

If I didn't have the wideband fitted, you would think the car ran and idled perfect. Incidentally, the idle circuit is way too rich and I am about to change that also. Mostly in the high 10's low 11's.

Thanks for any advice.
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[email protected]
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PostPosted: Mon May 16, 2016 7:57 am    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

your post doesn't tell us much without specifying

venturis
idle jet
elevation (I assume sea level)
Distributor and timing
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porkchop-rob
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PostPosted: Mon May 16, 2016 1:30 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Seems like you would want to fix the idle first....possibly still on the idle jet while cruising?
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XR6T
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PostPosted: Mon May 16, 2016 1:36 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

porkchop-rob wrote:
Seems like you would want to fix the idle first....possibly still on the idle jet while cruising?


OK, I am waiting on some jets.

Will fix this first then see what the readings are.

John. I will be more specific if I have no success once the idle jets are correct.
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XR6T
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PostPosted: Tue May 24, 2016 7:45 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

[email protected] wrote:
your post doesn't tell us much without specifying

venturis
idle jet
elevation (I assume sea level)
Distributor and timing


Thanks John, Can now provide more detailed info:-

Sea level.
Engine 1600cc with 8:1 comp.
C/w Crank, MOFOCO 040 Street Heads Port and polished with standard size valves.
Hideaway exhaust.
Eagle Cam No 2239. Advance Dur. 276Deg. Dur@050=236" Lift at cam .380" Lift with 1:1 rockers .419" I have 1:4 Rockers?
HPMX 40 dual carbies using offset manifolds.
CBP Magnaspark dist. with red stop and light springs, 30deg at 3,000 10deg at idle 850rpm.
28 Vents
Idle jets 45
Main 115
Air 195
F11
Snail gauge reading at Idle 3.5 A/F 11.1 850rpm
2,000rpm 11.0 WOT Cruising A/F between 12>15.5 with mostly AVERAGE OF 13.4
2,500 11.3 WOT
3,000 11.1 WOT
3,500 11.5 WOT
4,000 11.2 WOT
4,500 12.0 WOT
5,000 12.5 WOT

Engine stumbles badly on acceleration from standing start and when turning sharp corners in second gear, also slightly on acceleration. A/F gauge briefly reads in high 15's to 19.

Any help very appreciative.
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PostPosted: Tue May 24, 2016 7:51 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Bigger idle jet
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PostPosted: Tue May 24, 2016 9:15 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

mcmscott wrote:
Bigger idle jet


Bigger Idle jets?

With 52.5
Idle A/F 10.1 at 900rpm, snail gauge 3.5 Mixture screw out 1 1/8th turn
1,500 rpm WOT 11.9
2,000 rpm WOT 10.6
2,500 rpm WOT 11.2
3,000 rpm WOT 11.2
3,500 rpm WOT 11.3
4,000 rpm WOT 11.3
4,500 rpm WOT 11.9

Still has stumble but not quite as bad.

With 50's 120 main 200 air
Idle 12.2 @950
Idle 11.3 @1,000
2,000 12.2
2.500 12.6
3,000 13.0
3,500 12.0
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PostPosted: Wed May 25, 2016 8:41 am    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

did you check your float levels?
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XR6T
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PostPosted: Wed May 25, 2016 2:26 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Float levels checked OK.

mcmscott, I tended to agree with you about bigger idle jets, but unless I am mistaken the figures show rich? 10.5 at idle and consistently in the 11's on the road indicate rich. Am I correct or reading the gauge wrong.?

I will today however try some 55's unless someone else can offer some other explanation.

I have thought of enrichening the accelerator pump?
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PostPosted: Wed May 25, 2016 3:03 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Pull the main stacks and jet the idles by themselves.

Then, we can work on the mains and airs...

From all the reading I have done, this is the best "how to" I have come up with. Most of this is from John from ACN. If I have mis-understood, please correct me.

1. Check spark plug gap and make sure they are clean
2. Check fuel pressure (should be ~3lbs)
3. Check valves / set them
4. Set static timing at 8° BTDC
5. Set idle screw where there is no vacuum coming from ported vac on carb. (1/2 turn or less from just touching)
6. Set lean best idle (plus 1/2 turn or so)
7. Sync carbs using idle screws (don't go past the 1/2 turn mark)
8. Set idle speed using small adjustments to timing and/or bypass circuits
9. Check LBI again
10. Check Sync again.
11. Drive the car gently without any main stacks. If it takes off and drives well without any hiccups... go one size smaller on the idles (readjust the carbs of course). Rinse. Lather. Repeat.
12. Once you get a studder or popping through the carb on acceleration, go back up one size on the idle jet. That SHOULD be the correct jet.

Then......You can install a VERY RICH main jet and begin to set the transition timing with the air corrector.

This is the process I plan to follow next week once I get my weber windows installed and get off of these forsaken 12HR days.
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Last edited by porkchop-rob on Wed May 25, 2016 3:15 pm; edited 1 time in total
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PostPosted: Wed May 25, 2016 3:05 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

idle A/F is not adjusted via the idle jets. You can be idle-rich with a 40 idle jet, and idle-lean with a 80 idle jet.
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PostPosted: Wed May 25, 2016 3:15 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

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 3:16 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

[email protected] wrote:
idle A/F is not adjusted via the idle jets. You can be idle-rich with a 40 idle jet, and idle-lean with a 80 idle jet.


Can you explain this please. You just threw a monkey wrench in my understanding.
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PostPosted: Wed May 25, 2016 3:19 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

Porkchop, thank you for the reply.

I have ALMOST followed that system, but did not pull the main jet stack.

I will follow that routine ASAP sounds good.

Will get back with the results as soon as I can.
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PostPosted: Wed May 25, 2016 3:26 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

the idle mixture is set via the idle mixture screw, NOT THE IDLE JET.

This is covered in depth in the Wideband Tuning thread.

Show me someone who is too rich at idle, and I'll show you someone who either didn't adjust their idle mixture properly, OR has fuel coming from somewhere other than the idle circuit.

XR6T, you NEED to follow the procedure outlines in the Wideband Tuning thread, to prevent getting confused. If you don't, you'll be repeating work, and the main circuit will be throwing you off on your idle jet sizing, etc. The order is very important.

porkchop-rob wrote:
[email protected] wrote:
idle A/F is not adjusted via the idle jets. You can be idle-rich with a 40 idle jet, and idle-lean with a 80 idle jet.


Can you explain this please. You just threw a monkey wrench in my understanding.

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PostPosted: Wed May 25, 2016 3:45 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

[email protected] wrote:

XR6T, you NEED to follow the procedure outlines in the Wideband Tuning thread, to prevent getting confused. If you don't, you'll be repeating work, and the main circuit will be throwing you off on your idle jet sizing, etc. The order is very important.


John, can you point me to the post where the "procedure" is shown?

I went through that thread a few weeks ago (as well as your website) and the above list was what I came up with to try next. If it is erroneous, then I don't want to waste my time or anyone elses. Maybe I missed something.
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PostPosted: Wed May 25, 2016 3:47 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

it's not just following the procedure. It's knowing WHY. That's the main purpose to getting the ignorant to read the whole thread, and not think they know what the hell they are doing by giving them 12 steps....

I hear from guys who re-read that thread years later, and they often tell me they catch something they missed the last time.....
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PostPosted: Wed May 25, 2016 3:52 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

[email protected] wrote:
it's not just following the procedure. It's knowing WHY. That's the main purpose to getting the ignorant to read the whole thread, and not think they know what the hell they are doing by giving them 12 steps....

I hear from guys who re-read that thread years later, and they often tell me they catch something they missed the last time.....


That makes good sense. Ill take the time to go through it once more before I embark on my carbs again.
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PostPosted: Wed May 25, 2016 3:56 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

I don't mean to be a jerk. The procedure is outlined somewhere in that thread, but I don't have time to go dig it out.

It's amazing to me how many guys are trying to get their jetting right, yet they don't know how to do one or more of these

set ignition timing
set floats
check/adjust fuel pressure
clean air filters
clean fuel system
measure jet sizes
don't have a wideband
have the wrong spark plugs installed
etc, etc.

With the folks who contact me for help with their jetting, 80% of them have problems OTHER than jetting. And this is why they get confused that their jetting is "wack" or the engine isn't responding like it should. Only after all that other stuff is fixed, will the engine respond properly.

"Clean fuel system" one is a hoot. Typical guy has an engine which runs like @$$. Carbs are filthy inside and out. Buys new carb kit, installs new carbs, which are immediately filled with junk from their filthy fuel lines and fuel tank. Complains that new carbs are doing exactly what the old ones were doing. No kidding? Laughing

Guy will spend $1k on a new pair of carburetors, but won't change his fuel lines nor clean his fuel tank...... OK!
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Last edited by [email protected] on Wed May 25, 2016 3:58 pm; edited 1 time in total
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PostPosted: Wed May 25, 2016 3:58 pm    Post subject: Re: HPMX Jetting? Again - Sorry. Reply with quote

porkchop-rob wrote:
[email protected] wrote:
it's not just following the procedure. It's knowing WHY. That's the main purpose to getting the ignorant to read the whole thread, and not think they know what the hell they are doing by giving them 12 steps....

I hear from guys who re-read that thread years later, and they often tell me they catch something they missed the last time.....


That makes good sense. Ill take the time to go through it once more before I embark on my carbs again.


I found it helpful to read it once, THEN re read it several times only paying attention to John's comments but ignoring the pages focused on IDA's and EFI since they have extra adjustments. I found his posts that made a lightbulb go off in my head and took screen shots on my phone. Then, out in the garage just re read the basics. I just kept reading the same 6 posts of the basics until I got it
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