is my carb undersized for 2F?

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mtj555

mtj555

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Let me first say I am learning as I go. I am ignorant when it comes to carbs. The background on my vehichle is 78 FJ40, 35x12.50 tires and I am assuming stock gears. My FJ40 came with the Weber 32/36 Electric Choke Carb. From what I have been reading it is undersized for the 2F. The better option between Weber carbs would be the Weber 38. From my searches, I understand the OEM carb is a little bigger and when rebuilt by the two carb gods on here OEM seems to be the most popular choice. Considering my Vehicle came with the Weber 32/36 already installed with the OEM Air Filter, pedal and linkage removed. Would it be better for me to scrap the Weber 32/36 and install a Weber 38 since I am currently set up for the Weber. I know these engines aren't designed to produce lots of power. I just dont want to be losing power due to an inadequate carb. I am thinking it would be way more expensive to get an OEM carb and all required pieces to be installed in my truck since all the original carb components are long gone. What would I notice switching from a Weber 32/36 to a Weber 38?. I am not brand loyal when it comes to carbs because I don't enough about them to make a choice of my own.
 
It all depends on what you want to do with it. The original Aisan works best off road and at all angles. It gives good fuel economy too. Weber makes good carbs and a 38DGAS would be more appropriate for a 2F engine than a 32/36. Rochester and Holley/Motorcraft 2 bbl carbs are fine too. They just don't work quite as well off road without some fiddling with because they weren't designed to work at steep angles.
 
An average 32/36DGV series Weber flows right at 300cfm , 38DGAS series is around 380cfm . Italian-built units are slightly higher . The issue here is the fact that although the 38 flows more it's also synchronous. Running on both barrels all the time affects fuel mileage . If jetted right they aren't too bad but never will match the stock carb or THC for that matter . For angles , I've written volumes on how to deal with that , do a search on the web .
Sarge
 
Any idea how the 38/38 w 26mm chokes is 380CFM? The 38/40 w/ 31/35 chokes is about 420 using 'murikan standard holley measuring methodology. Seems like a 40% smaller venturi area would equate to a significantly lower flow rate?

If we could afford to buy & install a 45DCOE, that would be whole 'nother animal. Of cours, that wouldn't address the real problem of an engine having a 89# crankshaft and 42# flywheel, so a carb has to accomodate the 'special needs' of such a slow revver.

I'll try to find the ancient posts comparing the various carb sizes with like math and stuff...
Founded it:
"Weber doesn't make a carb to fit a Cruiser engine. They focus more on the small car and performance market. The Aisan carb is big at the top end (~60% bigger than the common weber swap), but has a small primary barrel for low speed operation. It is designed for the Cruiser engine and it's unusual demands.

From an old post:
Stock FJ60 carb is almost the biggest that is readily available. 75-76 USA carb is biggest CFM available, around 425.

The weber 38/38 that is commonly sold as an upgrade has 38/38 throttles and 26/26 venturis.

By comparison Aisan 38/40 has 31/35 venturis.

Aisan throttle area: 23.90cm^2
Weber throttle area: 22.68cm^2
Aisan is 5% bigger.

The limiting factor to airflow through a carb has to be the venturi. For the carb to work right, there must be a significant delta P right at the venturi, a greater change than anywhere else in the induction system.

Cross sectional flow area at the venturi:
Aisan: 17.17cm^2
Weber: 10.62cm^2
Aisan has 61% more CSA in the critical venturi area."
 
Last edited:
You answered your own question: The DCNF is more expensive cause it's more better. The carb swap kits have to be cheaper than a stock carb to make it seem a good value proposition.
 
Thanks. So if I wanted to go back to a Aisan, how difficult will that be and how much money am I looking at compared to buy a weber 38. I would think if I coul find an OEM carb, I would send it to Jim or mark for a rebuild so I can do it right. I don't have the experience to mess with the jetting on a weber to I that route. I have time so it's not like I am wanting to rip the 32/36 off right now. I could start getting what I need to go back to Aisan. Does an Aisin require the use of the stock air filter housing?
 
A weber 38 is gonna be the cheapest route. I think it can reuse the mounting adapter and air cleaner.

Parts that may be needed to un-hack a foreign carb install:
Carb
Choke cable
throttle linkage
air cleaner assembly
formed steel fuel line
insulator baseplate
4 studs & nuts
misc vac & vent hoses
 
I installed a new 32/36 DGEV (furnished by the PO, it had a Holly 500) on my '72 FJ40 with the F engine and have been trying to get it tuned and running right ever since. It has a solid state Pertronix ign system, the timing is on the mark (I've tried all different settings), and no vacuum leaks. I've changed all jets, etc numerous times and just can't get it right :( It will rev good, but still will backfire through the carb at fast acceleration and after slowly accelerating to 60 mph (2600 rpm), it won't go any faster without coughing, sputtering and backfiring through the carb. I'm seriously considering changing to a Aisan stock carb, IF I can get one. I'm about stumped so am anxiously following y'all's posts.
 
mtj555 said:
Thanks. So if I wanted to go back to a Aisan, how difficult will that be and how much money am I looking at compared to buy a weber 38. I would think if I coul find an OEM carb, I would send it to Jim or mark for a rebuild so I can do it right. I don't have the experience to mess with the jetting on a weber to I that route. I have time so it's not like I am wanting to rip the 32/36 off right now. I could start getting what I need to go back to Aisan. Does an Aisin require the use of the stock air filter housing?
Click to expand...

Here is my track through that process. https://forum.ih8mud.com/40-55-series-tech/531106-going-aisin-details-details-details.html

You can ignore the pages where I fought a vacuum leak. Wasn't overly expensive since all the parts can be found on this forum if you are patient and willing to work with what you buy. I ended up rebuilding my "new to me" carb myself (my 1st experience) via Pin Head's videos and the FSM. In the end I'm very pleased I switched (had both Holley and Webbers previously) back to stock and taught myself how to work on the setup. Best of luck, it is a journey.:cheers:
 
I have decided on going back to Aisan. I am in the Army and my new job will pretty much take up most of my time so I will just start getting all the parts together and once I have it all, make the change. It might take me a year but I think it will be better in the end. I want the carb done right the first time so once I can get a suitable donor, I am going to send it to Jim and have it rebuilt.
 
61n72cruisers said:
I've changed all jets, etc numerous times and just can't get it right :( It will rev good, but still will backfire through the carb at fast acceleration and after slowly accelerating to 60 mph (2600 rpm), it won't go any faster without coughing, sputtering and backfiring through the carb.
Click to expand...

This sounds more like a fuel delivery problem rather than a carb tuning issue. If your tank is full of dirt and rust, you just can't pump fuel fast enough to keep up with any carb.
 
I have an electric fuel pump and it appears to deliver adequate fuel to the carb under all conditions;I also have a fuel reg and have tried setting it at 2, 2-1/2 and 3 lbs pres which is what weber recommends and still no success :(
 
Ok , several things :

One , yes the DCNF is much more expensive - but that said that carb has ball bearing throttle shafts , interchangeable venturis , top loaded jets/emulsion/air sets (no taking the top off) as well as a nice, narrow float bowl . They will have the usual stall at high angles , albeit much better than the DGV series . I understand the whole marketing thing , but Redline is to blame for not biting the bullet on a lot of the conversion kits sold in the early years . Many carbs were seriously undersized and their demand for a mucho-cheap product to sell to the masses resulted in some issue with Weber and hence started the whole Spanish-made crap . I've had Italian castings come through here that were half rotted out - after rebuilding them they would still easily outperform a new Spanish casting . Not to mention , dyno's don't lie and there is a serious power/cfm difference . Keep that in mind as well since all the old cfm numbers are still used depsite the junk being made in Spain . They can be made to run, but it takes work .

For any issues such as 61n72 mentioned , first place to look is the idle jets . These control fuel delivery in the transition stages before the main venturi's take over from idle position . This works for both primary and secondary throttle bores on progressive 32/36, ect carbs . The jet's job is to meter the VOLUME of fuel/air (mixed) to the stepped holes just above the throttle plate in each respective bore . This allows the carb to be tailored to fit individual engine vacuum response and timing . Every engine is different - there are baseline jetting setups for nearly every kit out there but each engine should be tuned to suit it's needs/build . Once correct , these carbs will drive so close to efi it's amazing . Throttle response should be crisp and almost to the point of being a bit "touchy" . If not , it's not jetted right or the timing curve doesn't support the carb .

Ditch any attempt at using a fuel pressure regulator . Unless you want a stepped vacuum controlled unit that costs several hundred dollars just use a return line . If your carb isn't tapped for a second fuel inlet you can instead use a Heep CJ fuel filter that has a return built into the body . This keeps pressure low and still allows proper volume when needed . None of the cheap regulators out there today will ever work correctly , period .

Sarge
 
Sarge, et al,

Thanks so much to everyone for all the feedback and info but I'm not completely comprehending all everyone is saying, except that it sounds like the 32/36 is perhaps just too small for my 3.9 L F engine. With that said, I'll change it to whatever carb will work, but after acquiring my ride for nothing even resembling its original MSRP and then about doubling that again in restoration, I'm hoping to not put too much more in it...Ha! Who said money pit? As for my DGEV, it came with 60/50 idle jets, I've increased them several times with what came in the "jetting kit", and have ended up downsizing back to the 60/50's because it just wouldn't idle smoothly and seemed way too rich with the larger idlers. I've increased the prim and secondary mains which caused it to run better but still have the coughing, sputtering and bf problems upon sudden or sustained acceleration; I've even increased the air correction jets, still to no avail. Too stumped for stump jumping in my 40 :(
 
Did I forget to mention that I've also put headers on her? I'll be getting that HEEP fuel filter and tossing the fuel reg too... I'm determined to get this carb issue resolved!!!!
 
I have taken the fuel line loose at the carb to check the flow and the electric pump put out a steady stream of fuel, but I'll check the volume when I get back in town from eating turkey with family :)
Hope all you out there that are trying to help and all those other cruiser junkies have a wonderful Thanksgiving!!!!
 
mtj555 said:
From what I have been reading it is undersized for the 2F.
Click to expand...

All I have to offer is my anecdotal evidence: I used a 32/36 for a few months on my 2F. It started and ran very well. I never sensed that it was underpowered for my engine, even while pulling a utility trailer. But I never got the kind of gas mileage I wanted out of it (11-12) so I bought a '78 Aisan carb. on Ebay for about $40 and a rebuild kit and followed Pin_Head's videos for the rebuild. I much prefer having the stock carb., but my reasons for replacing the Weber were centered around fuel economy and originality, not a lack of power. $0.02
 
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