300A on 2-bbl carb upgrade to 4bbl with Impco 425

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BigBlockMopar
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300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by BigBlockMopar »

I've upgraded the lowly 2-bbl intake/carb with a 300A on top of it on my 318ci this weekend to a 4-bbl Edelbrock Performer intake with an Impco 425 carb and new aftermarket throttleplate.

Can say the difference is very noticable and the car feels more powerful and snappier to drive with now.
Partly ofcourse because of the 2-bbl to 4-bbl upgrade, giving more throttleplate opening for the same amount of throttlepedal movement, but also because everything is new now.
We had freezing temps this morning, and the car being outside, still started up just like on a summersday and would idle at 350-400rpm if I would let it to.

Still need to get into tuning the 425 as it's a bit rich at idle and too lean at power.


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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: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by BigBlockMopar »

Tried a little tuning today but I'm feeling I'm totally back to square one with the 425.
AFR's are not where I want them to be and they seem 'all over the place', but within a narrow range, in relation to engine rpm and load. Triple checked for vacuumleaks ofcourse, but none found.
I got the idle/low rpm AFR's fairly rich now at around 14-15 as the engine feels more responsive this way, but AFR's at power are still way to lean at aroun 16 to high-17s.
Tried turning the power screw 1 and 2 turns richer which helped slightly but I'm not seeing the mixture get richer when accelerating firmly. Tends to hover in the 16s at cruise and even during WOT.
All in all I'm not seeing not much variations in AFRs during driving like I had with the 2bbl/300A setup.

In my old 300A mixer I had installed a lighter spring to change it's enrichment curve (leaner). I would've hoped I didn't have to resort to these kinda tactics with a 425.
There's still an 'extra' light spring inside the original spring the convertor behind the lever to lean it out a bit, which I will remove first soon, and see if I can get a wider AFR-range.

For some proper tuning I need to fill up the tanks first again as the engine 'falls on its face' when accelerating hard because (I feel) liquid propane is moving away from the tank's pickup tubes and starving the convertor. I'm also seeing AFR's rise fast when this happens.
(One cylinder tank is under the package shelf and round tank in the spare tire well in the trunk.)
Thinking of installing another check-valve near the convertor as well as a filter/storage pot for backup fuel just after that.
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.

geraldm
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Joined: Sat Nov 09, 2013 4:53 pm
Location: Pump handle Saskatchewan

Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by geraldm »

Seems to me I just read an article on your type of problem and it ended up the the screws in the aluminum base plate were to far apart and when tightened down warp at the center enough to create a vacuum leak . I'm sure it was on this forum I read that .. i think it was where the new throttle plate bolted to the mixer apparently the gaskets were paper thin and with a little warp it leaked .. i think the guy just made a thicker softer gasket.. just a thought ..
Now how the hell did that happen ?

BigBlockMopar
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Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by BigBlockMopar »

I've recently wrote another topic myself about this issue but I found the gasket I had in my possession was just too thick and caused warpage and binding of the throttle shafts after tightening down the 4 screws.
I remade the gasket in a thinner material and all was fine then. But it's still something I could look into again.

Edit: here's the topic I wrote;
viewtopic.php?f=5&t=1608
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.

geraldm
Posts: 94
Joined: Sat Nov 09, 2013 4:53 pm
Location: Pump handle Saskatchewan

Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by geraldm »

yea that;s the guy :wink: sorry I knew I read something about this problem . didn't mean to chase you back to your own thread .. 8)
Now how the hell did that happen ?

C3H8
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Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by C3H8 »

Hey Bigblock. Saw your post with the change to the 425. I didn't make any comment as you sounded quite pleased with it. I just noted the new posts describing your dissatisfaction with it. Initially I would say you are on the right track checking for vacuum or air leaks. I would also verify your distributor is advancing properly and smoothly. Also VPV's are very good at helping fuel mixtures in heavier vehicles during acceleration and providing priming at startup.

The other comments I have would relate to some posts I posted several years ago. The very first converted vehicle I drove was a company vehicle. It was a 1981 Dodge D150 1/2 ton with a 318 and a 3 speed auto with OD. The intake was a factory 4 barrel with oversized secondaries. I believe the diff was pretty low for this weight of vehicle (something like 3.23) It had a 425 on it when I got it. The truck drove OK but it's acceleration off the line was abysmal. I swear, :oops: a 5 year old on a tricycle could beat me in the first 100 feet. It drove me nuts. I tried everything available back then. Different gas valves, timing modifications, etc. Starting was only so, so. Adding an primer helped with the starting. Power was great in the mid range and higher RPM's. This truck could actually clock about 170K on a one time test on a lonely stretch of highway. I puzzled over the lack of bottom end for quite some time.

Being pretty new to the industry in 1981 I began to pore over the books. The CFM rating of the mixer compared to the requirements of the engine troubled me. I took off the 425 and went to the 300/50 series with minor improvements. I then went to the 300A series and even had better results. One day I decided to mount a 225 and the change was dramatic. I could finally beat that 5 year old on the trike :twisted: . I'm being silly here. The truck was now very responsive in the bottom end. The mid range acceleration was very good but a lost a little torque as the RPM went over 4000 unlike the 425. The top end also suffered a little but I don't drive at 150K +. I did lose a tiny amount on the economy side but that stood to reason as my mixtures accelerating were definitely richer then they were with the 425 set up. The 225 was probably slightly restrictive at 4000+ RPM as the 225 is only rated at 329 CFM. A 318 running at 5000 RPM needs 345 CFM or more. In reality the 225 is a very good choice based on the operating range of that engine under normal circumstances.

This is all based on the engine being stock. Also not every 318 we did had the same results. I would expect this is due to different engine specs or drive trains. I have never been a believer that one size fits all or that bigger is always better. Engines have specific CFM requirements. LPG systems do not have accelerator pumps like gasoline carbs so in the case of IMPCO systems a slow rising gas valve will result in lean mixtures. Since the gas valve is tied to air flow an engine with less than adequate air flow will result in lean mixtures.

Just my thoughts. Good luck with finding the issue.

C3H8
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Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by C3H8 »

One other thing. You can keep your 4V manifold if you choose to try your 300 back on it. The 4V with the 300 might give you the results you were looking for if you can't find anything wrong.

Last thing. looking at you pics of the carb base plate I see a large hole in the center of the plate. I am assuming the mixer is blocking that hole on the top side.

BigBlockMopar
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Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by BigBlockMopar »

Oh, I don't for a minute want to get rid of the 425 as it has greatly improved low/mid power in the engine performance.
Also the car accelerates much better an faster than with the stock 2bbl carb and 300A on top. Much to my surprise I've found new joyment in this ole smog engine since the upgrade.
It's just that I'm eyeballing the AFR-gauge while I'm driving around and noticing not soo optimal numbers on it, what I did have previously.
This mainly let me to post my concerns actually.

I've been eyeballing the centerhole when I was mocking up the 425 to the throttleplate and remember reading about it here somewhere, but I didn't see any issues here as the area right above the throttleplate is one big space anyway. But... now writing this it occurs to me this hole could act as a simple air/fuel bypass and would need to be closed of indeed...
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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Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by C3H8 »

My mistake. I misunderstood your original statement. I thought you said the low end power had deteriorated.

As for that hole. It must be sealed. If that is a square bore holly the 425 would be mounted directly to it and that hole could be open or leaking if it is only covered by a gasket. Better check it.

BigBlockMopar
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Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by BigBlockMopar »

By chance, on a photo I just found I'm seeing the center hole is blind. It doesn't go through the throttleplate so I'm good there.
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: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by storm »

I've never known a Holley 4 barrel base plate to have a blind middle hole. I'd be pulling it off and checking it just incase.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

BigBlockMopar
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Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by BigBlockMopar »

I believe my own photo's more than my own mind... ;)


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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: 796
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by storm »

I don't doubt you but that looks, to me anyway, like it has been filled in.
There's a crack on the side of the hole that follows the round edge for a bit. I wouldn't be surprised if it is solder or some other hand malleable metal.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

BigBlockMopar
Posts: 394
Joined: Sun Sep 02, 2007 3:29 pm
Location: Netherlands
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Re: 300A on 2-bbl carb upgrade to 4bbl with Impco 425

Post by BigBlockMopar »

The blind hole is fine.

In the mean time I've managed to get the WOT afr's 'down' to around 14.5:1 afr by removing the 'extra' spring I had once installed in the vaporizer to lean it out a bit. But it's still not quite as rich as I want them to be (12s).
Next things to try are removing the restrictor plate (?) in the 425's vapor inlet, and/or find a different spring, and/or maybe re-adjust the lever in the vaporizer slightly higher for s bit more fuel delivery, but then again, idle/cruise AFRs are just fine at the moment at around 15.4-16.7s.

Also, with the 300A mixer, I didn't have these lean issues so I think I'll have to focus on the 425 first for now.
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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