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Weber vs. Hitachi - '79 620 w/ L20b


fizrat

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I get the feeling I'm opening up a can of worms by asking this question, but here goes.

 

I need to put a new carb on my 620-based RV (1979). I can't get the idle down below 1200 RPM (unless you manually push the throttle down), it hesitates and almost dies when accelerating from idle, and when accelerating at higher speeds it stumbles. It's the L20B engine, 4 speed transmission,  with what I believe to be a stock carb (DCH328, I think). I think I'll take the old carb and attempt a rebuild mostly as a learning experience, but it'll be my first try at rebuilding a carb, and I'll want to take my time on it, thus I want to install a carb in the meantime. The question, of course, is: which carb? A Weber 32/36 or Hitachi DCH340?

 

I'm pretty new at working on cars. I've gleaned some information on the carb question from the forum, but didn't see it all in one thread. Also I wanted to post in the hopes that folks could confirm or correct what I think I've learned and also to get more input. 

 

Here's what I've picked up so far:

 

Weber

pros: better performance at high RPMs

simpler - fewer vacuum emission control tubes to deal with

 

cons: dirtier emissions

not as easy to tune (have to be tuned by changing jets)

 

 

Hitachi

pros: better MPG?

can be tuned using adjustment screws

 

cons: slightly less performance at high RPM

 

 

What I'm dealing with is an L20b that's working pretty hard, lugging around a GVW around 2000 kg. On my recent road trip it couldn't really get above 50-55 mph and needed down to downshift on any significant uphill grades. I'm willing to accept that might just be the limits of this vehicle, but I'm also wondering whether the performance advantages of the Weber would be relevant to this situation or not.

 

 

And since people always ask, here's a picture:

http://www.flickr.com/photos/53954929@N07/8305861178/

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L20B did not come with the small DCH328, but only with the large DCH340 which is a bit larger than a Weber 32/36. DCH340 has 24/31 venturis, Weber has 26/27 venturis.

 

If it does have a DCH328, that's the problem. Otherwise you can adjust the carb you have now to idle good. It's all in the adjustment.

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When you say you can't get the idle below 1200 unless you manually push the throttle down, doyou mean:

 

1) you push the throttle lever on the carb and it drops down and stays,

2) You push the throttle lever on the carb, idle goes down but back up when you let go

3) You have to kick the pedal to get it to drop down? 

 

For all these, there are causes for that, nor requiring a rebuild. 

 

For #1, Usually it's an overly tight throttle cable- which on later 620s is adusted at the pedal, under the dash. Earlier 620s that adjustment was at the cable bracket.  Either way, you want the cable to have a little slack at absolute idle (when warmed up).   Another cause is sticky linkage, which requires cleaning all the linkages so they don't bind up, and another cause is too weak of a throttle return spring, which is common on older cars as the springs lose tension over time.

 

For #2, that is usually a slow-idle speed screw set too far in, raising the idle,  Focring the linkage causes it to drop briefly but to goes right back up when letting go.  It's not a big drop though.  This can also be caused by a choke that's not opening fully, which leaves the carb in a fast-idle setting.  With that scenario you could get it to drop into regular idle by forcing the choke open, then working the throttle to release the fast idle cam.

 

For #3, that's almost always a choke set so that it doesn't open all the way, or possibly not getting power to the choke heater.  Either way, the choke stays partially closed, but kicking the throttle opens the choke, and the throttle returns before the choke closes, putting it in normal idle.  But release the throttle slowly, it will go back to fast idle.  This also can be sticky linkage. 

 

 

As for the poor performance, well, you're hauling a lot of weight.  You should be able to hit 100kmh though, on a flat road.  Hills are going to always be a problem with that weight.

 

It's possibe a choke problem could cause that lack of power, but the carb is designed to open the choke at anythink over 1/3 throttle anyway, so that shouldn't be it.  It's possible the vacuum secondary isn't opening at all, a problem that comes with age.  The kits don't usually contain the vacuum bellows so a rebuild won't fix that.

 

So to compare the Weber to the Hitachi, I've always had better power with a Weber, BUT I have not been replacing a good Hitachi with them, always an either worn out or corroded one that was sticking.  Mileage, I've had Webers get better mileage, and ones that get worse.  That has to go with jetting.  Generally it's been better but not by much.  Once was over 10MPG better, which tells me the Hitachi was in VERY bad shape.

 

Oh, and a 620 should always have a DCH-340.  The 328 wasn't used after 1973 as far as I know, and some '73s had 340s too.

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My question would be is the carb you are using in the meantime new? If so either one would be good in my opinion, but if you are swapping on a used carb I would go with the weber because it is simpler. I was messing around with a hitachi today and it is much more complicated than a weber 32/36. I have had good success with used webers needing only minor adjustments to run good.

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Thanks everyone for the info.

 

I'm not sure where I got the idea it had a DCH 328... maybe I misread the Haynes manual I have, which goes back to '73. I'm not sure how to tell by looking at the carb... there is a red tag on the top that might have had that info, but it's worn off.

 

What's going on with the throttle is that when I back the throttle adjusting screw out in idle the RPMs don't decrease. When I push on it at the lever it will decrease (though not all the way down to where it should be RPM-wise, even if the screw is backed out all the way). So #1 on your list. I did adjust at the accelerator pedal to no avail. Though now that I think on it more, I don't feel confident I've eliminated the sticky linkage possibility.

 

But anyway, I don't think that would account for the stumbling while accelerating. Talking it over with a mechanic, he thought the carb was at the root of that problem, and that it was probably an air leak at some seal/gasket.

 

One thing I'm happy about, though, is that even with less than optimal performance, it still gets 17-18 mpg hwy... not bad for a camper!

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Take some pictures of the carb, from a few different sides, and somebody on this site will probably identify it for you.  I am working on some Datsun carburetors now, for 521 trucks, and they do use the Hitachi DAF 328 carburetor.  

 

I know you have messed the the throttle linkage, but do this anyway.  Disconnect it from the carburetor. and see if the carburetor returns to a reliable idle with out the throttle linkage hooked up.   This will eliminate a sticky throttle linkage as the problem.  While poking arounf the carburetor, make sure the secondary side closes completely.

 

 

I have four 521 trucks one was built in 1969, the other three were built in 1970.

The 1970 trucks had a solenoid on the side of the carburetor, that was activated when coasting against the engine, with the clutch pedal up,  The transmission in a forward gear, and the gas pedal up.   This solenoid opened a passage in the carb, that allowed a little extra gas and air into the manifold, for emission reduction when coasting.   To test this system, you jump past the switches, and the engine idle speed goes from about 700 to 1,100 or so RPM.

 

I have a Hitachi DCH 340 carburetor.  It has a vacuum operated device on the side of the carburetor, that does the same thing as the switches and the solenoid on the DAF 328 carburetor.  The device is called Boost Controlled Deceleration Device, (B C D D) This object on the side of your carb may be out of adjustment, if your vehicle has it.

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If it is about the long run and not keeping with stock-I would always go with the Weber. More people running that carb means parts availability will be better for longer. IMO the hardest part of owning older vehicles is close availability of parts. As far as emissions goes, tune it right and pay attention to your vehicle-not the distractions like music, passengers, etc. I've witnessed 30 vehicles two to three decades apart, all on the same testing equipment and the oldest one ran cleanest. It was my 1967 Dodge Coronet 318 2bbl in a class with newer Hondas, Fords, you name it... It just goes to show that paying attention to your tune is what matters most.

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