Hitachi DCH 340 Carb Questions/ Problems

[stevensonDNL]

Can anyone tell me what the Hitachi DCH-340 primary and secondary throttle shaft outside diameters are? I assume they are metric measure. I'm away from home, otherwise I could measure it myself with my calipers. Would the throttle shafts on the DCH-328 be the same size(s)? I've searched and couldn't come up with anything. Thanks in advance.

 

Dan

[ggzilla]

You mean inside diameter? There is no outside diameter as they are cast in one peice with the carb body.

 

If you remove the air cleaner and look down you will see a cast number in each bore indicating the Venturi size.

[stevensonDNL]

Thanks for the response ggzilla, but what I'm after (I'm hoping maybe someone happens to have a spare carb laying on their shelf or bench, disassembled) is not the inside diameter of the throttle bores or venturies, but the O.D. of the steel throttle shafts, to which the primary and secondary throttle plates (butterfies?) are screwed/ attached. The shafts that pivot within the carb throttle body. Maybe my idea will prove costly or impractical, but I'd like to see if I could find correct sized bushings to repair my worn out throttle body bores.

 

Dan

[datzenmike]

Glad someone is trying this.

 

The shaft has to stay precisely centered. If it ends up slightly off center when done the butterfly won't seat properly and will rub or lodge against the wall and not close at idle. Anyway good luck on this.

[stevensonDNL]

Glad someone is trying this.

 

The shaft has to stay precisely centered. If it ends up slightly off center when done the butterfly won't seat properly and will rub or lodge against the wall and not close at idle. Anyway good luck on this.

 

I appreciate your support on this, however I'm not entirely committed just yet. I'm just a shade tree mechanic, and definitely no machinist! It's frustrating to me that these throttle bodies become unusable when the throttle shaft bores become so worn that they leak air. I'd rather stick with the stock Hitachi than resort to converting to a Weber. Some of the prominent carb rebuilding companies out there do sell DIY bushing kits for at least the Rochester Quadrajet that contain bushings, specialized reamers/ pilots, and replacement throttle blade screws. Maybe with some research it would be possible to make a kit specific to the Hitachi. From what I've read on line, professional carb rebuilders condemn the use of a conventional drill bit for enlarging the bores, warning of innaccurate, damaging results. I can measure the throttle shaft IDs myself this coming Saturday.

 

Dan

[ggzilla]

I have never personally come across a Hitachi with worn throttle shaft bores. But I keep hearing about them... just suggesting your check yours out, it probably doesn't need it. If you cannot feel any play, then they are fine. Be carefully not to confuse rotary play with busing play (in other words, don't let the shaft turn a bit, or if feels similar to actual play in the shaft.

 

The most common problems I have seen on Hitachis is:

1. Leaking vacuum diaphrams (may cause it to run poorly, regardless of how much tuning/adjusting you do)

2. Bad power valve plunger (which is a vacuum fed system)

3. warped tops

[datzenmike]

I can't see a worn shaft causing all the problems it's accused of.

 

If there is longitudinal slop (side to side) along the shaft length then the butterfly will jam against the wall and won't return to idle. I've had a few that you had to tap the throttle to get it to kick down. A vacuum leak will only affect idle/near idle running at high vacuum. Once revved up the amount of air added is nothing.

[stevensonDNL]

ggzilla: To me, there seems to be way too much play between the primary throttle shaft to it's bore in the throttle body. I have tested both the choke vacuum break diaphram and the throttle vacuum secondary diaphram for leaks and they are good. I currently have the carb disassembled, so I'll check the top for warpage. I bought a rebuild kit from NAPA, and have soaked major carb components in a new can of GUNK brand carb & parts cleaner, rinsed with water, then blew out all passages with GUMOUT aerosol carb cleaner I did not attempt to remove any of the jets or air bleeds, nor did I remove the staked power valve from the underside of the carb top section. I haven't assembled anything yet. I don't have much experience working on carbs. My problem with the engine running was a very high idle. I believe fuel was being sucked in from the transition port just above the closed primary throttle blade. Idle mixture screw had no affect, even when turned fully (lightly) in. I had the idle speed screw adjusted out away from the throttle. It still ran at high idle with the fuel cutoff solenoid deenergized. I searched this forum and tried datzenmike's advice to others with similar problems by checking for vacuum leaks, and applying power to the BCDD solenoid to vent it's vacuum, but no improvement. There is a good gasket between the carb heatshield and intake manifold, the phenolic spacer appears good, as well as the baseplate gaskets above and below the spacer.

 

Dan

[datzenmike]

Check the secondary is fully closed. I started my Z24 motor implant and it had a high idle that would not go away, left it until the spring when there was time to work on it. I tried everything and gave up several time to work on other things. One day I grabbed the rod that goes up into the vacuum diaphragm and pulled down on it and the secondary clicked shut, the idle dropped and stalled because I had everything backed off. Set the idle and mixture and runs great and idles smooooth. Something was stuck. Maybe the throttle shaft, maybe the diaphragm? I don't care as long as it works.

[MikeRL411]

If you really feel that you have air intrusion around the shaft to carb body juncture, a quick check would be to apply a bead of DC4 silicone grease to the shaft to boby area. If there is an improvement, you need to consider a rebuild. No improvement, look elsewhere before spending money on a carb shaft or bushing rebuild. Silicon grease won't last forever, this is a check only. Good Luck!

[ggzilla]

Yes, if you can feel play, fix it.

 

About vacuum leaks, don't forget that EGR and is venturi vacuum, so gets stronger as the throttle is opened and engine revs up. I'm thinking there is another one that is similar. In any case it is a good idea to fix/replace/remove any leaking diaphragms and cracked hoses.

[datzenmike]

Good point ggzilla. The EGR is supposed to be closed or off at idle. If it is leaking due to carbon build up well it's leaking vacuum and idle quality will be affected. I've reached up inside and lifted the plintel with a fingernail and let close again sometimes getting it to seal better. On the L motors it's easy to remove and wire brush clean. Once it is sealing pull and cap the vacuum line and it is de-activated and shouldn't cause any more problems.

[mklotz70]

When the rig is running, you can spray carb cleaner right at the throttle shaft where it goes into the base...if the idle changes, you have a leak. I'd also check that the butterfly is closing completely on the primary...just like Mike said about the secondary. You can hold the base(with butterflies installed) up to the light and you should not see any light around the edges. If you do, you can loosen the screws up, force the butterfly shut, then retighten the screws while still holding the pressure on it. That should realign it.

 

If you end up going through with the bushing idea, these guys should have whatever ream you need. A ream is the only way you are going to get the hole accurate enough to make it worth all the work.

 

https://www.shoprutlandtool.com/catalog/cutting-tools/drills-reamers-countersinks-counterbores/reamers

[stevensonDNL]

Can anyone tell me what the Hitachi DCH-340 primary and secondary throttle shaft outside diameters are?

Upon my return home today, I used my digital calipers to measure the I.D. of the primary and secondary throttle shafts of my DCH-340. If my measuring is correct, both primary and secondary are a nominal 8.0 mm.

Dan

[stevensonDNL]

Check the secondary is fully closed. I started my Z24 motor implant and it had a high idle that would not go away, left it until the spring when there was time to work on it. I tried everything and gave up several time to work on other things. One day I grabbed the rod that goes up into the vacuum diaphragm and pulled down on it and the secondary clicked shut, the idle dropped and stalled because I had everything backed off. Set the idle and mixture and runs great and idles smooooth. Something was stuck. Maybe the throttle shaft, maybe the diaphragm? I don't care as long as it works.

 

While I still had the carb installed on the manifold with engine running, I did push down on the secondary shaft. I'm certain the blade was closed. With the carb on my workbench prior to installation, the secondary shaft/ blade worked as it should. The truck is sitting outside in my driveway, so the cold and rain limits the time i have to work on it.

 

Dan

[stevensonDNL]

If you really feel that you have air intrusion around the shaft to carb body juncture, a quick check would be to apply a bead of DC4 silicone grease to the shaft to boby area. If there is an improvement, you need to consider a rebuild. No improvement, look elsewhere before spending money on a carb shaft or bushing rebuild. Silicon grease won't last forever, this is a check only. Good Luck!

 

Now that's some great advice! I was trying to think what would be a good temporary sealant, all I could think of was RTV, but I didn't think that would work. When I get my carb reassembled and reinstalled, I'll try that.

 

Dan

[stevensonDNL]

When the rig is running, you can spray carb cleaner right at the throttle shaft where it goes into the base...if the idle changes, you have a leak. I'd also check that the butterfly is closing completely on the primary...just like Mike said about the secondary. You can hold the base(with butterflies installed) up to the light and you should not see any light around the edges. If you do, you can loosen the screws up, force the butterfly shut, then retighten the screws while still holding the pressure on it. That should realign it.

 

If you end up going through with the bushing idea, these guys should have whatever ream you need. A ream is the only way you are going to get the hole accurate enough to make it worth all the work.

 

https://www.shoprutlandtool.com/catalog/cutting-tools/drills-reamers-countersinks-counterbores/reamers

With the carb installed on the manifold and engine running, I had no carb cleaner to spray at the throttle shafts/ throttle body, but I did try WD-40, though it had no affect. I adjusted the primary throttle blade as you recommended after backing off the idle speed screw, and even with the blade centered and fully closed, I can still see a thin, concentric ring of light surrounding the blade's outer edge. Also, Thank you for the Rutland Tool website link!

 

Dan

[stevensonDNL]

Yes, if you can feel play, fix it.

 

About vacuum leaks, don't forget that EGR and is venturi vacuum, so gets stronger as the throttle is opened and engine revs up. I'm thinking there is another one that is similar. In any case it is a good idea to fix/replace/remove any leaking diaphragms and cracked hoses.

 

As you and datzenmike suggest checking the EGR valve for leaks, I have to inform you that my EGR valve is missing/ broken off from it's mount since before I owned this truck, and the holes have been sealed with some kind of cement. A real mess, but I'll recheck it for leaks. I'll try to post a pic of it tomorrow in the daylight. I do have a spare 1974 L18 truck engine with it's EGR valve still intact if I need it, but it's not identical.

 

Dan

[stevensonDNL]

Dow Corning DC4 Silicone Compound:

http://images.google.com/imgres?imgurl=http://cpc.farnell.com/productimages/farnell/standard/42422540.jpg&imgrefurl=http://cpc.farnell.com/1/1/42871-silicone-compond-dc4-2793695-dow-corning.html&usg=__w1gcjKOUbnZMp9sUTmMJ1EEw5ow=&h=173&w=320&sz=9&hl=en&start=15&um=1&tbnid=tapK1W_Y1G1FyM:&tbnh=64&tbnw=118&prev=/images%3Fq%3Ddc4%2Bsilicone%2Bgrease%26hl%3Den%26sa%3DN%26um%3D1

[stevensonDNL]

When I was taking my carb apart for cleaning/ rebuild, I found two internal parts missing (per the parts diagram supplied with my carb rebuild kit)from the accelerator pump discharge cavity which the tip of a ball point pen points to in the below photo:

 

 

This next photo shows the five parts (borrowed from a spare carb) that should be intalled in that cavity: 1.) Plug, 2.) gasket, 3.) spring, 4.) weight, 5.) check ball. The spring and weight were missing, and there were TWO check balls in there. Could this cause fuel to be syphoned through the accelerator pump discharge port into the primary throttle bore, resulting in high engine RPM?

 

 

Dan

[datzenmike]

There should be a check ball directly below the accelerator pump piston and held down by a small coil of spring. This is a one way valve to prevent reverse flow when the accelerator plunger presses down and forces gas into the discharge nozzle in the primary barrel. This is likely the second ball you found.

 

The spring and weight that were missing is a one way valve to prevent reverse flow when the accelerator plunger return to it's up rest position.

 

If gas were to siphon, which I doubt, it would not increase idle but would affect the economy.

[ggzilla]

Definitely it will cause flooding problems. The ball and the spring are a one-way valve. If the spring is missing fuel will leak past it.

[stevensonDNL]

There should be a check ball directly below the accelerator pump piston and held down by a small coil of spring. This is a one way valve to prevent reverse flow when the accelerator plunger presses down and forces gas into the discharge nozzle in the primary barrel. This is likely the second ball you found.

 

The spring and weight that were missing is a one way valve to prevent reverse flow when the accelerator plunger return to it's up rest position.

 

If gas were to siphon, which I doubt, it would not increase idle but would affect the economy.

datzenmike: Actually, the check ball beneath the accelerator pump piston, held in place by the spring coil was present, installed properly, and I would get a strong spray mist from the discharge port into the primary bore when I manually excercised the pump with engine off. There really were two check balls as I described. This carb has a very questionable/ unknown history. Thanks again!

 

 

Definitely it will cause flooding problems. The ball and the spring are a one-way valve. If the spring is missing fuel will leak past it.

ggzilla: I will be robbing that spring and weight from the spare carb and installing them as I continue the rebuild. With the parts installed correctly, I believe the fuel under accelerator pump pressure should force the check ball up against the spring and weight, allowing the shot of fuel mist into the primary bore. The vacuum created by the accelerator pump as it returns up to it's resting position should unseat it's check ball, drawing fuel into the piston bore to refill it for the next "green light". I'm hoping this will help. Thanks!

 

Dan

[mklotz70]

I'm pretty sure that the purpose of the ball is to hold the fuel in the chamber above it, so that when when the accel pump starts to push fuel, it doesn't have to fill the cavity back up....also keeps the fuel from being pulled back into the pump cavity. The second ball was probably to hold the first one down, but wouldn't be near as effective as the spring and weight. I don't think the fuel would be siphoned through the accel jet, but, if your float valve wasn't holding back the fuel pump pressure, maybe it was getting a slight push. ???????? Pretty much a guess on my part :) Besides...it still needs air to rev higher....so even if it was being siphoned, there still had to be too much air flowing through the primary bore to pull the gas, making it rev higher.

 

 

 

edit.....the link took me to page one and I posted without seeing Mike and ggzilla's postings. anyway.

[Coryellj]

Does anyone know of any after market air cleaners for the hitachi dch340 or how to make one work?