Hitachi DCH340 Question

[Phatjimmy]

So I have a question about my DCH340 carb on my 75 620. I got the truck from the original owner but it had been sitting for 8 year. According the PO, only his nephew had ever worked on the truck.  That being said, I am gathering the 20 year old nephew (who is of course a mechanic) didn't know what the hell he was doing.  So, I pulled this carb apart and noticed that the hollow base plate screw for the power valve was in the wrong location, red flag #1. Then I noticed that the spring in missing from the top of the accelerator pump check ball, red flag #2. I dip the carb, ultrasonic clean all the small parts and give it full overhaul. New float (set for proper level) and secondary diaphragm, the works. Had a small spring from a weed eater carb that fit perfectly on the accelerator pump check ball.  Get it back on the truck (FU*& that A.I.R. rail by the way!) and get it dialed in.  Truck idles perfectly. Lite acceleration down the road it kind of surges a bit but stabilizes at a nice cruise speed. Part throttle is solid and engine pulls good.  WOT I get a little stumble then the secondary opens and it pulls hard BUT.... It is blowing brown/black smoke like crazy the whole time it is WOT.  Looks like a diesel rolling coal.  This is a problem.  I checked the TAC and it was open letting cold air in, choke was open all the way, float level was right on the dot. I did adjust the valve lash hot and timing is at 12 BTDC.  So my question is what are the actual jet sizes?  I'm thinking this dumb kid drilled out the jets to try and solve his power valve and accelerator pump issues because you know as any Holley guy will say "MORE FUEL"......  lol.  So using an index drill bit set, can anyone tell me what the primary and secondary jet size is so I have a base line?  Or do any of you fine Ratsun folks have a spare set of stock jets you would like to part with?  OR am I looking in the wrong place.  Can the power valve be part of this problem?  I can honestly say of the large number of carbs I have rebuilt, this is my first Hitachi.

 

[banzai510(hainz)]

order a weber 32/36 and be done with it

 

cant say whit the black smoke is. Maybe varnish gas????? ck oil level often then you know its oil cunsumption

 

whats a power valve?  I know accel pump. other wise its all vacuum .

[datzenmike]

It's possible that the primary and secondary jets were swapped. Primary jets can range from 95 to 112 depending on the altitude or state. Secondary jets are in the 150 to 170 range. California emissions run lower numbers to lean them out. Higher numbers closer to sea level where air is denser.

 

I would say 103 and 160 would be fine. (this is from the '78 FSM) To be perfect you would need a wide band and maybe it could be tweaked up or down a few numbers but you would never know the difference.

 

My 340 had the hollow screw in the wrong place too. What a difference when you nail it at high speeds. Also it had a 99 jet for 5,000 foot Nevada elevation. It's original home. I'm at sea level so I put a 112 in and WOW the acceleration was noticeably improved. Gas mileage actually improved too. I guess I was running way too lean.