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jesus_chrysler
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PostPosted: Sat Oct 10, 2020 9:57 am    Post subject: 1915 stumble Reply with quote

I was running my new 1915 with dual IDF 40 carbs with 28 vents and it ran great but plateaued like I expected. So I went to 32 vents and now I can't get it to transition without stumbling. I know people run much bigger carbs than this on 1915's so I don't think that is the problem. Moved the main up to 140 and it runs great once it gets to the main circuit. Moved my idles up to 55 then 60 and it got less and less stumble each time but still does it. Carbs are synced, fuel pressure 3.5psi, floats set at 10mm. Any suggestions would be appreciated. Im hesitant on adding more idle jet. Yes, my air bypass screws are turned all the way in.

1915
W120
40mm valve heads
1.25 rockers
9.0 compression
32 vents
60 idle
140 main
F11 emuls
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jesus_chrysler
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PostPosted: Sat Oct 10, 2020 10:41 am    Post subject: Re: 1915 stumble Reply with quote

And Im at 1000 ft elevation.
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PostPosted: Sat Oct 10, 2020 11:17 am    Post subject: Re: 1915 stumble Reply with quote

What is the ignition setup?
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jesus_chrysler
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PostPosted: Sat Oct 10, 2020 11:40 am    Post subject: Re: 1915 stumble Reply with quote

Pertronix SVDA. I tried with and without vacuum advance. Timing set at 34 total.
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PostPosted: Sat Oct 10, 2020 12:25 pm    Post subject: Re: 1915 stumble Reply with quote

Maybe try 30 vents next
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jesus_chrysler
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PostPosted: Sat Oct 10, 2020 12:41 pm    Post subject: Re: 1915 stumble Reply with quote

I was thinking about running 30 vents but I don't see why the 32 would be a problem. I see guys running IDA's on 1915's. As soon as it gets into the main it runs amazing. It also idles smooth as glass.
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Alstrup
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PostPosted: Sat Oct 10, 2020 1:29 pm    Post subject: Re: 1915 stumble Reply with quote

Which IDF´s?
34 timing, - total with or without vacum ?
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jesus_chrysler
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PostPosted: Sat Oct 10, 2020 2:37 pm    Post subject: Re: 1915 stumble Reply with quote

Dual IDF 40's. 34 degrees total without vacuum advance.
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PostPosted: Sat Oct 10, 2020 2:54 pm    Post subject: Re: 1915 stumble Reply with quote

WHICH 40IDF´s?
Might as well ask for a more detailed explanation of the heads too
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PostPosted: Sat Oct 10, 2020 4:22 pm    Post subject: Re: 1915 stumble Reply with quote

They are the HPMX 40's and 043 40x35 heads with cleaned up ports. CB offset intake and linkage kit.
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PostPosted: Sat Oct 10, 2020 6:31 pm    Post subject: Re: 1915 stumble Reply with quote

Why 34° total timing? Sounds like a lot for 9-1 compression.
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PostPosted: Sat Oct 10, 2020 6:54 pm    Post subject: Re: 1915 stumble Reply with quote

OK. Now we are getting somewhere.
Ignition timing @ idle 10-12 degrees. 30-32 max advance. 37-40 at full vacum. - But I doubt you can pull that with non modded HPMXés. I could be wrong since I have not had a set of those in my hands for the last 2-2½ years.
Float height 11,5-12 mm
Idle jet 50-55.
F11 E tubes.
Most likely 200 main air
130-135 main jet.
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jesus_chrysler
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PostPosted: Sat Oct 10, 2020 7:30 pm    Post subject: Re: 1915 stumble Reply with quote

I will try taking some timing out.
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PostPosted: Sat Oct 10, 2020 9:45 pm    Post subject: Re: 1915 stumble Reply with quote

As I understand it you went up in size on the vents and now have a delay between the progression phase and the main circuit.

The main ways that you can tune when the main circuit starts are vent size, float height, air jet size and e-tube.

As you already stated you can extend the progression circuit with a larger idle jet at the expense of running excessively rich on cruise as you go larger.

People are asking for more specifics on your set up to help provide the correct guidance on the next step.
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PostPosted: Sun Oct 11, 2020 3:35 am    Post subject: Re: 1915 stumble Reply with quote

On second thought I would remove the 1,25´s as they are most likely "part of the problem" to why you need so large idles and have poor transition. They also probably don´t do much good apart from making the engine wanna rev high
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PostPosted: Sun Oct 11, 2020 4:51 am    Post subject: Re: 1915 stumble Reply with quote

Smaller air jet, to quicken the transition from idle to mains
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PostPosted: Sun Oct 11, 2020 8:38 am    Post subject: Re: 1915 stumble Reply with quote

What vehicle? 32 vents might be too much for a bus.
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PostPosted: Sun Oct 11, 2020 11:58 pm    Post subject: Re: 1915 stumble Reply with quote

Down on the vent size...start with 28mm, 45.7 idles, 120 main, 200 air corr., F-11 tubes. Redo LBI keep timing down for now. @28*BTDC full advance @3200+ rpm. Idle 850-900rpms

Float height 10-11mm gasket in place ball NOT depressed. Float drop 28-32mm

Fuel pressure 3-3.5psi max

No dial regulators of any kind.
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PostPosted: Mon Oct 12, 2020 3:07 am    Post subject: Re: 1915 stumble Reply with quote

IT is HPMX carbs. They want more float height.
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PostPosted: Mon Oct 12, 2020 5:22 am    Post subject: Re: 1915 stumble Reply with quote

How about addressing the problem directly..

Turn up the accel pump shot...
Do this by turning the 6mm nuts "in" so you have about 3/8" of thread showing.
That will give more volume when you stab the throttle and help bridge that gap.

If it ran good with 28's then it's not anything else wrong here..
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