DCH340 stock jetting & multi-carb jetting questions

[Cyan910]

Hellooooooo,

After searching everywhere I haven't managed to find any info on factory jetting for Hitachi's DCH340 that was used on the L20B. Anybody have anything? Also am I right in saying that some of them had some sort of high-speed enrichment system? My DCH340's both came on NZDM spec L20Bs'. Manual choke, no AC. I'm looking at trying to put one of my carbs back to (or as close to) factory standards after my old man whacked both of 'em together on one of our engines and 'customised' the jetting. :thumbup: 

Another question, when it comes to multiple-carb setups, does jetting have to be adjusted to compensate? For example having one carb per 4 cylinders needs to deliver sufficient A/F for 4 cylinders, whereas say 1 carb per cylinder only has to focus on their own cylinder, so would jetting have to reduce A/F delivery if there is more than one carb? Or does nothing change???

Any help is very much appreciated, thanks!

[datzenmike]

Primary jet around #99-103

Secondary #160-170

 

This will serve most applications. My Nevada emissions L20B was a # 99 but I'm at sea level now and tried a 112 and the mileage and driving got noticeably better. 

 

There is a high speed enrichment by use of the power valve located at the back of the float chamber. If you look on the underside of the carb there is one screw that is hollow. This allows intake vacuum up inside to keep the power valve closed. When the throttle is fully opened the vacuum drops and the power valve opens allowing more gas into the secondary to improve hard acceleration or high speed.

 

 

In theory the total load of air would be divided between say, 4 individual carbs, so less air needs less fuel. Say 1/4 the fuel each. 

[Cyan910]

Got another question, kinda off topic but anyways, on the base of the carb next to the mixture screw is a tube that runs into the bottom of the stock air filter housing. Inside the housing where the tube connects is a little flap thingy. What is this line for? What does it do and is it really necessary to include this in a custom intake?

 

Thanks in advance again.

[datzenmike]

The tube on the mixture screw side should go to the vacuum advance on the distributor. The hose to the 'flappy thing' on the air filter snorkel should hook directly to intake vacuum.

 

 

 

The 'flappy thing' is part of the ATC automatic temperature control system. There should be an insulated pipe from the hot exhaust manifold to the snorkel. The flappy thing is pulled closed by a vacuum motor through a temp sensor inside the air filter. in the closed position only warmed air is drawn up past the hot manifold. The temp sensor regulates the mix of hot and cool air keeping it at around 90-100F. At full throttle the vacuum drops and the flappy thing opens fully and draws in only outside air. In cold weather the air is warmed and carburetor icing is prevented. The best reason is that an engine runs better when tuned for a constant temperature. It should run the same at -10 as it will in the summer.

 

Certainly you can run without this handy device if you are in a warmer climate. If you get down near freezing (or lower) with damp or fog you risk carb icing. The ATC is really handy for eliminating this.

[Cyan910]

Thanks for the info Mike, but your not talking about the switch that is visible on the snorkel, that you can operate manually by hand? It has a sticker that says "Summer/Winter" with arrows pointing in respective directions. My knowledge tells me this switch acts like a choke for the air filter itself and forces the air to come from around the exhaust manifold like you said (despite me not having the insulated pipe, the air filter unit is sourced from a junkyard). From what you said it sounds like this is what you're referring to, correct me if I'm wrong though. What I'm thinking of is a thin rubber hose that runs from the left side of the carb base, up into the air box itself, just like the much bigger rubber hose that runs from the rocker cover also directly into the air box (for recycling gasses and fumes I believe.) At the end of the thin hose inside the air box is the flappy thing that looks like it's suppose to shut off from vacuum from the carb (different line to the vacuum advance for the distributor).

I've been stumbled on what this thing is for so long, even my eccentric-inventor-neighbour-who's-pretty-goddamn-intelligent-and-knowledgeable doesn't know what it is which is very surprising  :lol:

[datzenmike]

Some of the way older Datsuns had a simple lever that you turned for summer or winter driving but the vacuum operated ones worked ALL the time making fine adjustments for all year round ranges of temperatures. Every day is different but the air to the engine remains constant at (around) 100F. The engine then remains more consistent in it's operation.

 

The operating valve selects cold outside or warmed exhaust heated air or a mixture of both to maintain that 100F air temperature to the carburetor. There should be no choke action from this unless these pipes are blocked.

 

Engine vacuum is used so this system works during average load conditions and defaults to full cold air when the intake vacuum drops below 2.5" of mercury. Like full throttle needing maximum performance.

[Cyan910]

I should probably source a vacuum operated one then since it's usually cold where I live in NZ and most if the time I drive around town with the choke on, having said that my Bluebird never came with a vacuum operated one even though she's the 'top of the line' model. Seems fitting when it has no AC and other features that overseas models have lmao NZ for the win.

 

BTW everytime I have a new question that's off-topic the original topic should I make a new thread or just continue discussing in my current one? I don't wanna be a pain in the ass but I have plenty of questions.

 

Is it normal for the rpm to skyrocket when changing gears from WOT? For my engine I must take it easy when shifting gears, if I try to shift quickly the rpm will decide to go for a mountain run! Even if I let off the throttle for half a moment before dipping the clutch it will still jump up momentarily (at WOT, this is). This is only really prominent in higher gears at speed but can still happen in the lower gears too. Is this possibly because of a vacuum leak? Or is this simply the nature of the L20B? I think I remember reading another thread on here talking about something similar to this after searching for the cause.

 

Thanks for all the help too this is an awesome community.

[datzenmike]

Just re-read the first post. Do you have a single 340 Hitachi or Dual SU Hitachi's??? The reason I ask is outside N America the 910 (that we called the Maxima) did come with an SSS package and dual SU carbs. You mention the manual choke and SUs only had manual choke because of their design.

 

Like these?

[Cyan910]

I have the big single dch340 Hitachi, originally equipped with a big round light-blue air box with a short snorkel attached. The hole in the bottom of the snorkel for the insulated pipe is vacant. As we said, manual choke and no AC. Sorry I have no pics atm my phone is too old to even be able to upload any.