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Hitachi Dch340 secondary not opening


Janksun

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I'm having trouble with the carb on a 79 620 with the stock carb. The secondary isn't opening when I give it throttle and as a result the engine bogs and doesn't want to rev up. I have been told it's vacuum activated but I can't seem to find the problem. I cannot convert to a Weber due to having to smog the truck so that option is out. Any help would be appreciated.

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The secondary can only open when two things happen. It certainly has to rev up and the throttle be1/2 or 3/4s open to release an interlock and there has to be sufficient vacuum produced to work the vacuum secondary. If the primary jet is part blocked or you have an ignition problem the engine would be weak and it might not open. Generally if the secondary does not open, not much happens it just lacks power but will rev up. slower if under load.

 

What exactly does it do when it 'bogs'? and at what speed does it 'bog'?

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And you almost must be under load, as holding the throttle wide open without a load, you can easily go beyond a safe rpm before they open.

 

I had an old Toyota Corona that I would never see the secondary open just by free reving it.

So I drilled out the jet size in the vacuum circuit going to the vacuum pod.

They opened then !

First time I got on the throttle part way through a turn, like I used to do before the mod, I spun the car.

Made a heck of a difference in mid range power.

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Engine off, look down the primary side and work the throttle. What you should see is a strong squirt of gas from the accelerator pump. The extra gas helps richen the otherwise lean spot when the throttle is suddenly opened.

 

Oh, you're in California.

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Well not working at least.

 

Idle has it's own gas supply, it's very small but adjustable. When idling and you suddenly open the throttle the engine isn't moving enough air in the primary barrel, it can't suck more gas it and it suddenly goes lean (too much air too little gas) and it will struggle. The idle circuit cannot supply this. The answer is a mechanical pump like a Windex sprayer to fill in the gap between idle and primary running.

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As said above, it's almost impossible to get the secondary open unless it's on the road (under load).  If it bogs at heavy throttle that's usually a plugged jet, not a secondary not opening.  The secondary gets used so little that the secondary main jet is prone to collecting a lot of crud.

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Also look at how worn the accelerator pump linkage is.

After a while the parts wear out, and you don't get full travel on the pump stem.

It can be tweeked to gain some of it back.

Just watch the pump rod to see how far you have to move the throttle before the linkage actually pushes the pump rod.

And there are two holes in the rocker arm that pushes the pump rod, the one nearest the piviot point gives the most fuel.

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Had a hard time finding anything available with that part number.

 

What about 16151-U6800?

 

http://m.ebay.com/itm/New-Genuine-Nissan-16151-U6800-Piston-/181324169623

 

Or it looks like it's in the rebuild kit from rockauto like kelowg said

 

http://www.rockauto.com/catalog/moreinfo.php?pk=987559&cc=1210150&jnid=426&jpid=0

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