425 rich after installing 1" spacer

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BigBlockMopar
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425 rich after installing 1" spacer

Post by BigBlockMopar » Fri Jul 29, 2016 4:45 pm

Since I recently installed a 4-holed spacer on my 318 engine, the AFR-values have become richer than I want.

But I can't seem to lean out the 425's idle mixture any leaner then 15.0 AFR.
With transmission in Drive and engine turning 600rpm, engine vacuum is steady around 20".
Before I mounted the spacer, the idle AFR's where around 16.0 - 16.5, right where I wanted them to be.

Also, the AFR's during accelerating are more rich, and more quickly.
Usually it took pretty much full throttle to get the AFR in the 12's. Now this happens already at 3/4 throttle.

Anyone encountered this before?
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.


storm
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Re: 425 rich after installing 1" spacer

Post by storm » Fri Jul 29, 2016 11:06 pm

You haven't changed to a smaller air filter have you?
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

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BigBlockMopar
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Re: 425 rich after installing 1" spacer

Post by BigBlockMopar » Sat Jul 30, 2016 6:34 am

No, actually a larger one. I had a 10" paper filter-unit before and now using a 14" K&N unit.
I suspected the same, but with or without an airfilter, the AFR's stay similar.
But I've run the 14" filter as an open unit when I installed the 425 at first, that was before the spacer-installation.

I also had a thought about the airfilterhousing wingnut, could be putting too much force on the airfilter stud in the 425, may have skewed something inside.
Seems far fetched but I do have another new 425 which I could swap and try out.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

BigBlockMopar
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Re: 425 rich after installing 1" spacer

Post by BigBlockMopar » Sun Jul 31, 2016 2:47 pm

Swapped the 425 for another one, idle mixture can be set properly now again, as lean or rich as I want.
Opened up the idle valve on the 'rich' 425 but couldn't find anything wrong. Looked nice and clean inside.
Only thing I noticed inside the 425 was a slight oily residu, which wiped off easily.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

C3H8
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Re: 425 rich after installing 1" spacer

Post by C3H8 » Sun Jul 31, 2016 9:46 pm

Your reasoning is pretty accurate. The 425 is a great mixer with three issues to consider during its life.
1. The cap is the weak point if an air cleaner nut is over tightened. Over tightening the nut can cause the lid to warp upwards changing the spring tension. A company called Schwans in NA designed a hold down system that used the outside screws on the lid instead of the center of the cap just because of this problem. They made a triangle shaped plate that was held down by 3 of the lid screws and a welded stud in the center of the plate. Longer screws with spacers between the plate and the mixer lid were required to attach the plate. This allowed tightening of the air cleaner without applying upward pressure on the center of the mixer cap. I would expect this plate could be also diamond shape or even round and use all four screws.

2. The lever inside the idle plate can be deformed if the idle screw is over tightened. This usually results in a lean condition though.

3. The air gas valve wears on the side of the mixer body eventually leading to a rich condition. Its hard to put an average life span on a 425 but approximately 60K to 100K. I've seen them last more but they are usually showing signs of wear by then. This can vary greatly depending on the mixer orientation. A brand new 425 will generate greater then 5" WC under the gas valve at normal cranking speeds. If the inches WC is less then 5" the mixer is quite worn. Basically it means additional air is passing down the outside diameter of the gas valve between the mixer body and the gas valve instead of through the center venturri, This causes the vacuum to decrease lessening the lift of the gas valve. You would expect a lean condition due to this but usually it means the center gas inlet gets worn from the side to side movement and passes more gas due to the wear.

BigBlockMopar
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Re: 425 rich after installing 1" spacer

Post by BigBlockMopar » Mon Aug 01, 2016 6:27 am

Thank you for your reply and confirmation.

I had a look at the idle lever and the large center valve but there were no signs of wear.

During the change from the open to a now closed airfilter housing, I used a large airfilterhousing from a '77/78 Dodge 400ci engine.
That housing was a little deformed which caused its top cover to require some extra force to close up the housing again.
Before installation I already went over the housing and cover and put in back in shape.

But obviously it turned out the cover still needed some extra force to close up properly, whi caused too mich stress on the 425's cover.
I'll look into fabbing a new holddown for the 425 on the 4 post bolts to prevent warping the cap in the future.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

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kikkegek
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Re: 425 rich after installing 1" spacer

Post by kikkegek » Wed Aug 03, 2016 4:11 am

hmm...good info.

I installed an open 2 inch phenolic spacer a while ago and didnt notice any change. problem I do have is that idle mixture does not respond any more to the idle screw...

Image

Image

I was hoping for a little more torque down under. but then again...even if I would have gained some with my small block on my 2700kg Suburban...I probably would never notice it. hahaha
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

BigBlockMopar
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Re: 425 rich after installing 1" spacer

Post by BigBlockMopar » Sun Apr 02, 2017 5:52 pm

About a year later, and my 'second' 425 has become a zombie-mixer also again.
Pretty annoyed about it.

With engine idling at 500rpm in Drive, the mixture becomes pig-rich at 11:1 AFR.
Upping the RPMs (in drive) to 600 or 700rpm, the mixture leans out.
Driving away with normal or firm acceleration, the mixture leans out tremendously to 17-18:1.

The idle mixture wcrew has become totally unresponsive at idle.
Only at off idle, the mixture seems to change a bit.

The 425 now seems to do the exact opposite it's supposed to do.
Also tried changing/resetting the lever height in the vaporizer, but that had only marginal effect on the idle AFRs.
Look into the overtightening issue of the airfilter housing, but to my findings if this is really the case, the mixture should actually be leaning out at idle I think.

Not quite sure what to do next.
I'm pretty much 'done' with the IMPCO 425 carbs (if those are the culprit in my case).
But I may try replacing the vaporizer first for a new one I have on the shelf.
What quirks me is that at the time when I installed the first 425, everything seemed fine.
This also seemed the case with the second 425 I installed on the engine. But twice now, the mixture settings seemed to harder to adjust and control, to the point where the mixer gets a mind of it own and doesn't follow the engine load at all anymore.

See attached screenshot of what's happening during mild acceleration;
(White = RPM, Red = Engine Load/kPa, Green = AFR)
Attachments
425LeanOut.jpg
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

BigBlockMopar
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Re: 425 rich after installing 1" spacer

Post by BigBlockMopar » Sun Apr 02, 2017 6:16 pm

For those who are interested and have MegaSquirt's "MegaLogViewer", here's a logfile (5.6MB) of my car driving during some mixed conditions (city & highway);
https://www.bigblockmopar.nl/msextra/20 ... .33.17.msl
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

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

Re: 425 rich after installing 1" spacer

Post by storm » Sun Apr 02, 2017 9:02 pm

BigBlockMopar wrote:Not quite sure what to do next.
Have you tried a new gas valve?
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

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kikkegek
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Re: 425 rich after installing 1" spacer

Post by kikkegek » Mon Apr 03, 2017 4:24 am

Hi Herman,

I recognise those number from my 425 mixer.

I found 2 problems:

1. my idle membrane had gone bad, had to be replaced

2. my start-assist valve was causing a leak. Took it off, found it not fully closing anymore, thus causing a leak to the converter and a piss ruch idle.

fixed both and my number are awesome.

idle at about 14.5
cruise at about 15.5-16.0
WOT at about 13.0
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

BigBlockMopar
Posts: 344
Joined: Sun Sep 02, 2007 3:29 pm
Location: Netherlands
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Re: 425 rich after installing 1" spacer

Post by BigBlockMopar » Mon Apr 03, 2017 4:58 pm

I don't have a new gas valve (to try), but since both 425 mixers were brand new when installed, I "refuse" to believe something is already worn inside after only 1 year of service from each of them. I did open up the 425's some time ago to look for anything 'wrong' with them, but couldn't find any wear marks whatsoever.

Idle membrane is not torn, squeaky clean and simply still looks brand new, just like anything else inside the mixer.

Don't have a start-assist.
The engine always starts within 3 seconds of cranking.

I plan on installing an OHG X-450 on the engine, just to see how that works.
But I also want to prep the engine bay for future vapour injection and keep both fuel systems operable, where I want to install a new IMPCO vaporizer at the same time.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

storm
Posts: 565
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: 425 rich after installing 1" spacer

Post by storm » Tue Apr 04, 2017 1:05 am

BigBlockMopar wrote:I don't have a new gas valve (to try), but since both 425 mixers were brand new when installed, I "refuse" to believe something is already worn inside after only 1 year of service from each of them. I did open up the 425's some time ago to look for anything 'wrong' with them, but couldn't find any wear marks whatsoever.
"Refusing" to believe something has been the downfall of many "mechanics"and their diagnosis'. Always check and double/triple check the obvious things first. AFRs don't increase, or become non responsive, without something going wrong. There is very little that can actually go wrong in these things and what can go wrong is the idle diaphragm and gas valve. A gas shop in town (near where I live) keeps known good parts for this purpose so that they can verify if an increase in AFR is because of the mixer or something else (like ignition issues or engine wear).
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

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BigBlockMopar
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Re: 425 rich after installing 1" spacer

Post by BigBlockMopar » Tue Apr 04, 2017 5:07 am

True enough.
Haven't looked closely into the gas valve mounting uet, but could a simple very thin washer be used under it to alter it's setting?
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

storm
Posts: 565
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: 425 rich after installing 1" spacer

Post by storm » Tue Apr 04, 2017 4:28 pm

Post a pic of what you mean.
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

Fuel flow requirements calculations viewtopic.php?f=5&t=1638

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