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38/38 downdraft webber
- Thread starter Robbie
- Start date
alpine_64
Donation Time
A progressive carb will be much more efficent at lower speeds, the synced carbs use both barrels and will be heavier in terms of fuel consumption (yes in theory they could be close if you use smaller thottle openings... but who does?) They may also bog down a little more than the progressive carbs (a problem even some DGV's have) when flexing the right foot from lower speeds. I believe some peolpe have used them to good effect.. but you really need them to talk about it on here.
64beam
Donation Time
Hi Robbie,
The 38/38 carburettor was used on some of our big six engines here in Australia (4.2 straight six) and I believe on some imported V6's. As Michael pointed out they would definately need rejetting to suit the Rootes engine (unless highly modified or maybe even a racer). The DGV in most cases works well with little or no rejetting out of the box.
Regards, Robin.
The 38/38 carburettor was used on some of our big six engines here in Australia (4.2 straight six) and I believe on some imported V6's. As Michael pointed out they would definately need rejetting to suit the Rootes engine (unless highly modified or maybe even a racer). The DGV in most cases works well with little or no rejetting out of the box.
Regards, Robin.
V_Mad
Donation Time
Rob
The 38/38 is suitable for larger engines (approx 3 litres and up). For road use on the 1725 it will be difficult to get the right mixture for low speed driving. So it might not be fuel effi cient or have optimum power/driveability at low/average driving speeds.
Without going into detailed theory of venturis etc a simplified explanation is that, at low engine revs, the air flow into the engine is obviously low. With a 38/38 carb, air will be flowing in both chokes, whereas, with the 32/36 (or 28/36) air will only be flowing through one choke, so it will flow at twice the rate, which is better for accurate fuel metering.
Low flow rate means low depression (or vacuum), which is what makes it difficult to draw fuel into the intake. A higher depression is better for accurate fuel metering. I hope that helps. Cheers
Jeb Stuart
Platinum Level Sponsor
I have a 38/38 on my SIII. The motor is stock except for a little work on the head. It does idle a little rough but the performance is way way superior to the 32/36. I'll have to play w/ jet size. The problem I've had w/ the 32/36 was that the second would only open when the pedal was almost completely depressed - for passing. It was good for economy but I'll tell you the 38/38 will blow the 32/36 away. I've have my old 32/36 if anyone wants to buy it. I did just buy a manual choke kit for it but the instructions came in Italian only.
64beam
Donation Time
Hi Jeb,
I'm surprised that you did not have to jet the carb down at all and not being a progressive carby your lucky it does not flood the engine. Do you have to nurse it until the revs get up a bit? The head must flow pretty well.
Regards, Robin.
Jeb Stuart
Platinum Level Sponsor
64beam
I bought the 38 off of ebay used for $100.00. If I remember correctly it has 160 main jets and I can't remember the air jets - 200 maybe. I bolted it on, adjusted it the usual way and it runs fine. I had to cut out the manifold to one big opening. It does run a little rich. The carb came w/ a box of jets and I'll switch to smaller jets one of these weekends and see how that goes. However it still runs better than the 32/36 - much more power. I drove the old Alpine about 500 miles through the GA-SC-NC-Tenn mountains and few weeks ago on a two day trip and it worked great. It works great w/ the car in O/D too. I usually cruise about 3,500 RPM in O/D. I don't know how fast I am going because the speedo is not O/D but I am guessing 75. On my 32/36 I even tried the sync-link kit from Pierce Manifold that opens the second barrel much sooner but I couldn't get it to work right. I am not a good mechanic. I'd love some side draft Webers but for the price the 38 will have to do for now.
I bought the 38 off of ebay used for $100.00. If I remember correctly it has 160 main jets and I can't remember the air jets - 200 maybe. I bolted it on, adjusted it the usual way and it runs fine. I had to cut out the manifold to one big opening. It does run a little rich. The carb came w/ a box of jets and I'll switch to smaller jets one of these weekends and see how that goes. However it still runs better than the 32/36 - much more power. I drove the old Alpine about 500 miles through the GA-SC-NC-Tenn mountains and few weeks ago on a two day trip and it worked great. It works great w/ the car in O/D too. I usually cruise about 3,500 RPM in O/D. I don't know how fast I am going because the speedo is not O/D but I am guessing 75. On my 32/36 I even tried the sync-link kit from Pierce Manifold that opens the second barrel much sooner but I couldn't get it to work right. I am not a good mechanic. I'd love some side draft Webers but for the price the 38 will have to do for now.
64beam
Donation Time
Hi Jeb,
That's great; it's not often you get a win like that. Did you sacrfice very much fuel economy with the change? I'm not sure what the 38 jets normally are, but the 32/36 normally has a 140 main. Who knows maybe the 38 is a better choice.
Regards, Robin.
Jeb Stuart
Platinum Level Sponsor
I pulled the 38/38 off and installed a manual choke kit I ordered from Pierce Manifold. The choke(starter valve) works great but I could not figure out how to make it increase the RPMs - but I am working on it.
Since I had it off I switched out the jets:
Choke side idle from 60 to 50
Main from 160 to 150
Air from 210 to 190
These are the smallest jets I have. The car does idle better and the power is about the same. I can now set the idle lower than before too though the generator light comes on. I have the choke side idle jets only 1/2 a turn out which probably means the jets are too big.
I am not a good mechanic and seem to cause more problems than I solve so if someone has jetted a 38/38 for an Alpine please speak up.
What I'd like to see is a 32/36 where it idled on the 32 but the 36 would come on evenly w/ the 32 shortly past an idle. The one I have is all 32 then when the pedal is almost completely depressed wham the 36 opens. I like to drive fast (or at least feel like I am) tacking it, up double clutching, hearing the over side exhaust bark between shifts - just having fun. The Alpine likes it too. I tried Pierce's Sync-link kit fpr the 32/36 but as I said I am not a good mechanic and could not modify it to do what I wanted. That's when I moved on to the 38/38.
Since I had it off I switched out the jets:
Choke side idle from 60 to 50
Main from 160 to 150
Air from 210 to 190
These are the smallest jets I have. The car does idle better and the power is about the same. I can now set the idle lower than before too though the generator light comes on. I have the choke side idle jets only 1/2 a turn out which probably means the jets are too big.
I am not a good mechanic and seem to cause more problems than I solve so if someone has jetted a 38/38 for an Alpine please speak up.
What I'd like to see is a 32/36 where it idled on the 32 but the 36 would come on evenly w/ the 32 shortly past an idle. The one I have is all 32 then when the pedal is almost completely depressed wham the 36 opens. I like to drive fast (or at least feel like I am) tacking it, up double clutching, hearing the over side exhaust bark between shifts - just having fun. The Alpine likes it too. I tried Pierce's Sync-link kit fpr the 32/36 but as I said I am not a good mechanic and could not modify it to do what I wanted. That's when I moved on to the 38/38.
64beam
Donation Time
Hi Jeb,
I actually saw a sync kit on Ebay for the 32/36 while browsing. I assume they are supposed to act the way the 38 does. Alot of us are'nt great mechanics, but we love to tinker and will give most things a go. A bit of patience and some common sense gets you through most things, but having siad that some things are also better left to professionals. Very interesting topic you brought up and another performance option for the Alpine.
Regards, Robin.
I actually saw a sync kit on Ebay for the 32/36 while browsing. I assume they are supposed to act the way the 38 does. Alot of us are'nt great mechanics, but we love to tinker and will give most things a go. A bit of patience and some common sense gets you through most things, but having siad that some things are also better left to professionals. Very interesting topic you brought up and another performance option for the Alpine.
Regards, Robin.
husky drvr
Platinum Level Sponsor
Jeb Stuart,
FYI, installing a smaller air correction jet will make the top of the RPM range more rich - not less. If your engine is basically stock, then an idle speed of 800 -850 RPM would be about right and the idle mixture should be slightly rich ( but not excessive ).
This link might be of some help as well.
http://www.redlineweber.com/html/Tech/38_dgas_tunning.htm
Good luck,
64beam
Donation Time
Hi Don,
Is that idle speed about right for all of the Alpine engine's?
Thanks, Robin.
husky drvr
Platinum Level Sponsor
Robin,
I believe so. 800 -850 RPM is the speed listed in WSM 124 for the sIV with Solex carb and also in the 1725 WSM for the twin CD150's.
If your engine is modified from stock, then the best idle may be different.
HTH,
64beam
Donation Time
Hi Robbie,
Where are you sourcing the manifold from? Is the manifold new? One thing I noticed when installing a 32/36 on my Alpine, is that when I opened up the secondary hole to accommodate the bigger secondary there was not much room to go any further on the stock manifold. I am not sure about after market manifolds though. My point is, just make sure the manifold can be taken out to 38mm safely.
Kind Regards, Robin.
Hi Bill and Robin
I am leaning towards using the 38/38 weber. I am negotiating for a series 4 manifold. I am hoping to get it for under $150 since I can get one from sunbeam specialties for $175. I am not sure what the difffereces are between the manifolds. I have been told by a few people that the series 4 manifold can be modified to take the 38/38.
Thanks for all your help,
Robbie
I am leaning towards using the 38/38 weber. I am negotiating for a series 4 manifold. I am hoping to get it for under $150 since I can get one from sunbeam specialties for $175. I am not sure what the difffereces are between the manifolds. I have been told by a few people that the series 4 manifold can be modified to take the 38/38.
Thanks for all your help,
Robbie