Twin impco 425 adjustment advice

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liteon
Posts: 21
Joined: Mon Oct 15, 2012 6:07 am
Location: The Netherlands

Twin impco 425 adjustment advice

Post by liteon »

Hi all,

Got myself a new project. A nice 454 chevy which now has a twin Impco 425 setup.
They're running of a regular intake with a large holley throttle plate and the "Y" adapter to mount 2 425's on one throttle plate.
The car has been started for the first time recently, and i'm trying to tune the Impco 425's using an AFR meter in the exhuast.

What I noticed with the factory settings on both Impco's is that it was running lean at idle, and much and much leaner at higher RPMs (19.x+).
I've adjusted the idle mixture to make it run much richer at idle to start with. However, it's still goes to 17.x+ at higher RPM.
Since it goes from rich to lean depending on the RPM's, I suspect that both Impco's have completely different idle mixture settings causing them to run "out of sync".

What would be the best way to tune them identical? I doubt that counting the number of turns on the idle mixture screw is anything near exact.
I saw someone on the internet stating to close one air valve so the system will run on one 425, allowing it to adjust that 425. But sounds a bit weird to me, although it might work.
I'm pretty sure there are people here with more expensive of knowing to adjust twin 425 :-).

Thanks!

storm
Posts: 746
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: Twin impco 425 adjustment advice

Post by storm »

When running 2 carbs, of any type, you need to make sure the base settings are identical before fitting them. Have you set the base plates and checked they are running the same? I suggest you watch this video to show you what I mean https://www.youtube.com/watch?v=SRwZPWxC3Sg

When you say running lean and rich you don't really give much information, you only give 2 numbers 19+ and then 17+ at high rpms after adjustment. What is the engine idling at? what is it cruising at? What vacuum readings are you pulling at the lean mixture areas?

I suggest you sync the air flow of the carbs first and then leave the baseplates alone. Then set your idle as close to 15.5 as you can keeping the mixers as close as you can to the same setting mechanically. Test drive and see what the mixtures are. If they are still not good you may need to remove the flow restrictor in the inlet of the mixer.

@C3H8 and @Franz and @Frank will be able to give much better advice but that is where I would start.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

liteon
Posts: 21
Joined: Mon Oct 15, 2012 6:07 am
Location: The Netherlands

Re: Twin impco 425 adjustment advice

Post by liteon »

Hi Storm,

Thanks for your reply! :-).
Both 425 share the same throttle plate. There is " Y " adapter/splitter on top it. So the airflow through both 425's is always identical :-)

storm
Posts: 746
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: Twin impco 425 adjustment advice

Post by storm »

Ok, just out of interest sake test the flow settings anyway. Maybe there is a flow difference between the 2 that can't be found any other way. If that doesn't shed any light see if the restrictor is in the gas inlet.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

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