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Guest Message by DevFuse
 

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Dual Carb setup


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20 replies to this topic

#1 Guest_JimN_*

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Posted 03 October 1999 - 11:00 PM

Hi all! Anyone out there have any experience with dual carbs? I'm fitting some to my car and are having some problems. My main question is, coudl the primary fuel jets be different from one carb to the other (left to right i mean)? I've done something while cleaning them up so that now one has too much fuel going into the main venturi. The fuel bowl levels are spot on and i'm sure I havn't messed up any of the other jetting. Could i have soemthing to do with the slow idle bleed jet? Any other ideas? I'm almost out of ideas myself. btw they are off an EA81 engine, with hitachi's installed.

#2 Guest_Frank B_*

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Posted 03 October 1999 - 11:15 PM

Maybe only one carb has the idle circuit??? just a thought.

#3 Guest_JimN_*

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Posted 03 October 1999 - 11:58 PM

Thanks Frank. What do you mean when you say idle circuit? Only one has a vacume line going to the top front of the carb. Is that what your talking about?

#4 Guest_Frank B_*

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Posted 04 October 1999 - 01:40 AM

The idle circuit provides fuel to the cylinders when the throttle is closed. Which is why carbs have an Idle mixture adjusting screw. The fuel usually enters below(after) the throttle plates, which are nearly closed at idle. it gives the engine just enough fuel to stay running. I don't know anything about Subaru dual carbs but in most other multiple carb set-ups, only one will have an idle circuit and choke. Ever see an old Dodge six-pack??? Three two barrel carbs on a V8. Only the second(middle) carb has the idle circuit and choke. The other two are actuated by linkage from the middle carb at a set point to "come on" so the engine doesn't get too much fuel at low RPM's, kinda like a secondaries on most two barrel carbs. Look for an idle adjusting screw on the carbs. Do both have a choke? Maybe that's what the vacuum line is for??? They also might not be syncronized. If the thottle plates don't open exactly together the carbs will deliver different amounts of fuel because of the difference of air flowing thru them. The linkage between them should be adjustable??? A syncronizer can be found at a motorcycle or VW shop for $20 to $30. Good luck Jim.

#5 Guest_camosuba_*

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Posted 05 October 1999 - 05:39 AM

when you say there is too much fuel what do you mean? are the carbs balanced?

#6 Guest_JimN_*

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Posted 05 October 1999 - 08:14 AM

yeah the carbs are balanced, as close as i can get them anyway. By too much fuel i mean that it is so much at idle that it's not vapourising totally as it comes out the jet. The same symptom as when the fuel bowl it too full but it's right on the dot.

#7 Guest_doctor_*

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Posted 05 October 1999 - 09:21 AM

I have a friend that has some experience with dual carbs, but not on Subarus. He says that the primary fuel jets ARE probably different. I don't know how to check this out, but if you could provide enough details, I will ask a local dealer. The setup sounds interesting and something I would consider. Did you rebuilt the Hitachi's or just clean them up? Mine had a couple of clogged jets(primary slow air bleed which was hard to clear up), but didn't have the symptom you're describing. Good Luck

#8 Guest_JimN_*

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Posted 05 October 1999 - 09:38 AM

Thanks Doctor. I'll pull them off the car again and check out the jets to see what sizes they are. If they are different it's just a matter of switching them. I also just cleaned them up. I was very careful about where the top restricter went but then I relised I had to pull the primaries off, and assuming they were the same in each carb I didn't even look at them. The carbs themselves look like standard hitachi's, just some of the vacuum piping is different between them, and of course the linkages are different to the singles. Both carbs have chokes. As it seems i have to take them off again i'll take some pix and post them at some point. In cleaning these ones up the hardest part was actaully keeping the little tubes for the accell pump clear. They seemed to clog very easily if not every spec was cleaned out of the fuel bowls.

#9 Guest_YUVAL_*

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Posted 11 October 1999 - 01:28 AM

it is not a reply but more of a question as i have never seen a dual carb set up in israel . is it original or an added one any information will be welcomed.

#10 Guest_JimN_*

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Posted 11 October 1999 - 07:55 AM

As far as I can tell the dual carb was only available in japan. Origianlly designed for the subaru rally team or something like that. It is rumoured to have also been used in some light trucks. The engines that they come standard on have larger exhaust ports (oval shaped) and tweeked cams. The combination apparently puts out around 110HP.

#11 Guest_JimN_*

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Posted 11 October 1999 - 08:23 AM

Thanks to Frank and my Bro i am 1 step closer to getting these damn things running. it turns out i didn't do a good enough job cleaning out the idle circiuts. It now idles smoothly but still has an airleak aroud the EGR valve. At the moment it is still running lean and won't idle at less than 1600 RPm no matter how i fiddle with the idle and mixture adjustments. So while i'm waiting for a plate to block off the egr outlet it's off with the tops again to make sure everything is actually clean as a wistle. Also it turns out the main fuel jets are the same size, so there may be something else up with the main fuel flow.

#12 Guest_pete_*

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Posted 11 October 1999 - 10:42 AM

Wish I had a dual carb set up.... You'll get 'em going man. She's gunna be a mean one aye???

#13 Guest_Skip_*

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Posted 12 October 1999 - 02:48 AM

Jim, my expierience with dual carbs comes from Blokemobiles, Triumphs & AustinH.'s There is a air flow meter to balance them, very important for smooth low idle, are you using a short length of garden hose at least for the balance operation?

#14 Guest_JimN_*

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Posted 12 October 1999 - 08:11 AM

I'm using a bit of pipe and my good ole ear to balance them. I'm not sure if there is a direct link between the carbs to balance them out. I do know what your talking about though as I own a 4 carb motorcycle. Once I get them running reasonably well i'll take it up the road to get balanced properly.

#15 Guest_YUVAL_*

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Posted 12 October 1999 - 05:17 PM

IT IS NOT CLEAR TO ME WHAT KIND OF INTAKE MANIFOLD YOU HAVE . BUT IT IS VERY HARD TO BALANCE ANY TYPE OF MULTI CARBS SET UP WITH SEPERATE MANIFOLDS. YOU CAN MAKE IT EASY ON YOUR SELF BY CONECTING BOTH INTAKE PIPES TO EAVEN THE UNDER PRESSURE.

#16 Guest_camosuba_*

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Posted 17 October 1999 - 06:13 AM

it appears i have the same problem jim

#17 Guest_JimN_*

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Posted 17 October 1999 - 07:16 AM

Which problem, the high idle or the excessive fuel?

#18 Guest_Frank B_*

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Posted 17 October 1999 - 06:37 PM

Are the carbs the type with glass fuel level sights on the float bowls?

#19 Guest_JimN_*

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Posted 18 October 1999 - 06:57 AM

Frank : yeah they are, i've checked the fuel levels though and both are right on the dot. The thing with the fuel olny happened after i'd cleaned then out a bit so perhaps I loosened some gunk that somehow caused the problem later.

#20 Guest_camosuba_*

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Posted 24 October 1999 - 05:45 AM

the blocked jet problem which i have finally cured yahoo!! how is your set going?

#21 Guest_JimN_*

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Posted 25 October 1999 - 04:16 AM

Finnaly got the damn things running. All I needed was a plate to replace the EGR valve as it was leaking air, adjustment of the throttle cable, and some CLR. For thosewho don't know CLR is an all purpose cleaner that removes calcium lime and rust. About 5 mls in each carb, then let it sit for a minute or 2, resulting in a totally smooth idle! Amazing stuff.




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