DCH340 FYI - All 3 of my Choke Pulloffs are Bad (& other problems)

[bilzbobaggins]

 

This is how he did it.

 

I dont believe you want to cut yours.  You dont want to buy the adapter.  I dont think you can have your pie and eat it too

[Cardinal Grammeter]

Why are people cutting up OE intakes  air cleaners when there is an specific 3-1/8" (80mm) Datsun specific adapter???

 

 

Edited March 28, 2022 by Cardinal Grammeter

[Stoffregen Motorsports]

Because the adapter lifts the air cleaner and that can cause it to hit the hood.

[Stoffregen Motorsports]

Wait, cutting up intake manifolds? Or air cleaners?

[Stoffregen Motorsports]

And if you use a heat gun to remove the rubber o-ring, you can save it any re-attach it with rubber cement.

[Cardinal Grammeter]

Opps, meant to say "air cleaners" not "intakes"

 

Lots of "other stuff" going on in the L18 intake that may not clear the Weber if intake lowered with welding.

 

 

 

 

 

[throw1932]

debris in the carb could cause a high idle? been having a really high idle problem for a little while. i had my carb off to helicoil the threads that secure the top to the base. I made sure to keep the internals clean, i am very confident that nothing got inside the carb. any drilling and tapping was done with grease to control the shavings. while it was off, I threw a new accelerator pump in as well. once back on the truck, it idles very high. I do not have a tach, but my guess is around 2k rpm. there are no vac leaks, I doused it with brakleen and smoke checked the system. mixture and idle screw do nothing to change the idle either way. choke is operating properly, butterflies and throttle shaft bushings are in good health. when looking down the primary side, it is spraying a lot of fuel at idle.

[Cardinal Grammeter]

2 hours ago, throw1932 said:

debris in the carb could cause a high idle? been having a really high idle problem for a little while. i had my carb off to helicoil the threads that secure the top to the base. I made sure to keep the internals clean, i am very confident that nothing got inside the carb. any drilling and tapping was done with grease to control the shavings. while it was off, I threw a new accelerator pump in as well. once back on the truck, it idles very high. I do not have a tach, but my guess is around 2k rpm. there are no vac leaks, I doused it with brakleen and smoke checked the system. mixture and idle screw do nothing to change the idle either way. choke is operating properly, butterflies and throttle shaft bushings are in good health. when looking down the primary side, it is spraying a lot of fuel at idle.

What year/engine?  If it is a L20b with all the insane pollution controls with vacuum delays, etc. I would have no idea.

 

EGR?  Mine is disabled, but if it opened when it shouldn't, that would be a sort of vacuum leak where it would be sucking from the exhaust.  

 

It idles high when all warmed up?  If YES, then it must be crazy high when cold started when choke is closed and fast idle cam is in play?

 

There is always the BCDD.

Edited April 15, 2022 by Cardinal Grammeter

[throw1932]

it is a '74 with the factory L18. I never had any issues with my carb up until I pulled it off. it was still running perfect and getting 30mpgs while the top of the carb was flopping around as I drove. when the choke is closed and it is sitting on the fast idle cam, it idles at its normal high rpm. once the choke is open and I have kicked it off the fast idle cam, rpm will drop slightly than gradually get higher, until I shut it off because of how high it is racing (slip the clutch to lower rpms.) i have not played with the bcdd yet. trying to find a high quality precision vac gauge, but all I see available is cheap Chinese junk. I did complete a five speed swap while the carb was off the truck, there was a vac line running to the auto that i have capped off. 

 

the thing that really stands out to me is how much fuel is being sprayed from the primary nozzle, it is dumping a ton, as if at wide open throttle.

 

I did take the egr block apart, it was pretty packed full of carbon. perhaps this opened something up? spent months trying to track down a gasket for that, there were three available in the dealer inventory. there are no reproductions available to my knowledge. 

[Cardinal Grammeter]

POWER VALVE & LOOSE AIR HORN THOUGHTS:

 

If the air horn was loose and flopping around, there would be no vacuum signal to pull the power valve piston up which closes it.  So at idle, the power valve would be open.

 

And you know about the hollow base to body screw, right?  That is the vacuum passage to the power valve piston.  If a solid screw is used, again no vacuum and the power valve would be open.

 

Now if the choke is closed, there would be a little pressure drop across it that would push the air horn onto the body perhaps making some seal for the vacuum signal to the PV piston.  Then as the choke opens, you lose the pressure drop and the air horn is free to move around.

 

ADJUST IDLE & MIXTURE TO REDUCE RPM:

 

Were you able to get the idle down adjusting them?

 

 

 

 

My weird DCH340 on my L18:

 

For 8 years mine worked just fine except twice, I had to remove debris once from the main jet and once from under an air bleed.  

 

Both times, the carb behaved just fine afterwards.

 

This last time, the hot idle increased, nothing crazy but maybe 1500 rpm.  AND the primary had gone lean.  If given too much throttle too quickly, engine would pop through the carb.  At first it seemed like a bad accelerator pump, but the primary was clearly lean under power.

 

I figured dirt again got in the car.

 

Took it apart, sprayed the passages, found nothing, back together IDLE WAS CORRECT and Primary was less lean but still lean.  NOTE:  I bent the fast idle link so I was at the top of the cam so I would have more time and more high idle when cold.  Engine seemed to not have enough high idle maybe 2-4 minutes from cold start.

 

I've been driving it like this for some months, and it seems recently, sometimes when it is warm, I'm starting to get some of the bottom end torque back - usually I can pull all the local hills in 4th.  But I can make it pop if I try - and it is much easier when cold and warming up.

 

This was when I discovered the choke pulloff and 2 others I had were all bad.  

 

With a bad pulloff, the choke would have to be readjusted for changes in ambient temp.  So if the pulloff is working, I could tighten up the choke to make it richer and spend more time on the high idle cam.

 

Of course, this doesn't explain why the carb worked good for 8 years?  Could if have been the pulloff failed?  I just don't see it.  Something else must be afoot.

 

 

[throw1932]

I helicoiled the holes for the screws securing the top of the carb. used a new gasket and it sealed up well. I did not touch the base of the carb, however i did notice the hollow screw. I attempted to adjust mixture and idle speed this morning, both screws did absolutely nothing. played with them for a while, bottomed them out (softly not to damage anything), and had them about to fall out. had them both bottomed out together, all the way backed out together, one bottomed out with the other backed all the way out, and then flipped them. either configuration, to my surprise, did not change the idle whatsoever. speed did not change, idle quality did not change. it is a smooth idle, just very high. the truck starts up instantly, zero hesitation. I will get a photo of the inside of the carb when running tomorrow morning if I get a chance. I am not sure if it is possible to post a video on here, I would do that if I could.

[Cardinal Grammeter]

TIP:  There is a Locktite PURPLE which is the weakest strength intended for small screws.

 

You are saying the Idle screw had no effect on rpm?

 

With the Idle screw all the way out, the Throttle Plates are closed and there is nowhere to get air to support a high idle.

 

That leaves:

*) Vacuum leak:  throttle shaft to body clearance which you said is good so that is not a problem.  There is also Side Wear on throttle plates due to the thrust load of using the accel pump bellcrank to serve as the throttle return spring.

*) BCDD:  This is described as a "Little Carburetor" that ....   You don't need vacuum gauges, etc. to set the BCDD.  It's been a LONG time since I played with mine 8 years ago:  I believe all you have to do is blip the throttle and adjust the BCDD so that it hesitates at the "hold point" and then goes to idle.  It is possible that the BCDD will hang at the "hesitation" point. 

 

NOTE:  The BCDD solenoid must be energized to turn the BCDD off.  I believe the L18 had a slow mph switch in the speedometer to engage/disengage the BCDD.  The L16's did not have this cutout/in switch present.

 

Another tip:  the BCDD screw is on a servo diaphragm.  If you are going to screw the adjuster "in", you can also gently push the adjuster screw in and see the immediate result.

 

HISSING SOUND:  Way back I discovered when the BCDD was "working", it produced a loud hissing noise.  I'm not sure if the hissing was working or not working.  But it was instrumental in understanding what the BCDD was doing "back then" 8 years ago.

 

Edited April 15, 2022 by Cardinal Grammeter

[throw1932]

the idle screw has no effect on rpm, and the mixture screw does not change idle quality whatsoever. side wear on throttle plates is not an issue, the carburetor is in great shape. I have taken it to a carburetor shop to see if I was missing something, and they said it was in excellent condition. i agree, this does seem like it would be a vacuum leak, infact a very large one, however I have smoke checked the system and have come up with no vacuum leaks, I have not checked if the intake valves are too tight though. 

 

now, here is the interesting thing. the BCDD is controlled differently between auto and manual trucks (i have been doing more research.) on manual trucks have a speedometer switch that deactivates the BCDD under 10mph. on automatic trucks, the neutral safety switch is used to deactivate the BCDD when in neutral or park. I have also read that to adjust, rev the engine to 3000-3500 rpm, then snap the throttle closed and watch the vac pressure. it should increase to -23.62 inHg or higher, then slowly go back to the set pressure. set pressure for a manual transmission should be -19.69 inHg. 

 

my truck, a '74, is an automatic truck. however, I have since swapped in a '79 280ZX five speed manual. I looped the neutral safety switch wires together so I would be able to start the thing, therefore the BCDD is never turning on now that the truck thinks it is always in neutral. that being said, I do not see how this would affect the idle speed. my only other thought is maybe the vacuum pressure has changed now that the automatic is gone? I am not sure the function of it, but there was a vacuum tube going to the transmission. maybe I am thinking too hard and missing something simple here, hopefully. 

 

the vacuum gauge i purchased finally came in the mail, going to tie it into the system and see where the pressure is at tomorrow morning.

 

also, what do you mean by, "adjust the BCDD so that it hesitates at the 'hold point' and then goes to idle?" I am not understanding what the hold point is. 

[datzenmike]

 

 

What's the idle speed? If above 1,000 mixture screw won't have any effect.

 

Is the choke fast idle cam engaged. This would produce a fast idle.

 

Use a flashlight and look down the secondary. Is the butterfly not completely closed?

 

 

When suddenly closing the throttle at high speed the engine goes lean and miss fires producing emissions. The BCDD allows extra gas and air into the intake when decelerating from a high speed to counteract this. Manuals have a switch in the speedometer (and why it does not read 0 and stops at 10) that turns the BCDD relay off. Autos should be the same. There were so many ways of turning the BCDD on and off, some had a solenoid. Later ones were adjustable and turned on and off by themselves.

 

Look on the middle of the BCDD for a small round rubber cap. Under this is a screw. Turn counter clockwise to reduce the sensitivity of the turn on/off of the BCDD. Keep track so it can be put back if this doesn't help.

 

The neutral safety switch is only to prevent the engine starting with the automatic in drive or reverse. It interrupts the start signal to the starter. That's all it does, has no connection to the BCDD.

 

 

[throw1932]

it is off the fast idle cam. when I first start the truck it idles normally on that cam, and snaps off as it should when I kick it down. after maybe 30 seconds the idle speed will gradually increase well over 1000rpm and does not stop until I shut the truck off. the throttle cable has plenty of slack, choke fully opens, secondary butterfly is fully closed. thank you for the correction on how the BCDD is triggered, I had read on a different forum that the autos were different. 

[Cardinal Grammeter]

What can cause rpm to change:

*)  Amount of fuel ingested by engine

*)  Amount of air ingested by engine

*)  Distributor Timing - the vacuum advance advances timing using manifold vacuum and increases rpm

 

Just some thoughts:

 

What happens if vacuum advance is disconnected (seal the vacuum line)

 

RE:  ADJUSTING BCDD:  Here is a long thread discussing adjustment that has plots of Vacuum vs Time which "reveal" BCDD operation

http://datsun1200.com/modules/newbb/viewtopic.php?topic_id=70961&forum=1&post_id=496870#forumpost496870

 

When you say "well over 1000" do you mean like 1500 or 3000?

 

Did you say you had the EGR blocked off?

 

I'd introduce a small vacuum leak to see what effect that has on rpm - there is a small hose located low in the middle of the intake on the LHS. 

If a small leak raises rpm, it meant fueling was rich - if it lowers rpm, fueling was lean. 

 

If a small leak raises rpm, check for vacuum leaks that increase as the engine warms up.  

QUESTION:  Is there any history of the L-series being prone to intake manifold gasket leaks?

 

Another similar "test" can be performed by slowly manually choking the carb with your hand - as you choke the carb you are reducing air and increasing fuel.

If rpm increases, mixture was lean - if rpm decreases, mixture was rich.   NOTE:  This is test Old Timey mechanics would perform.  Find the forum post of the hundreds of cool tips on the H.A.M.B. @ jalopyjournal.com

 

 

 

 

 

 

Edited May 10, 2022 by Cardinal Grammeter

[datzenmike]

All engines will idle up as they warm up. When warm they vaporize the fuel better and clearances tighten. This is why a choke and fast idle cam are used to keep it running till warm.

 

There is no vacuum advance at idle. If the throttle plate is properly closed.

 

 

Does your BCDD have that rubber cap or cover on it's side????? Top of picture below with the screws around it.

 

 

Remove cap and turn screw counter clockwise to reduce the turn on and turn off.

[throw1932]

I have spent a little bit messing with the truck this morning. I smoke checked the vac system again and still have come up with no leaks. with the vac gauge on the truck, it shows about -20.5inHg at idle. when I increase rpm and let off, it snaps to -23inHg, then quickly bouncing between -22 to -19inHg, settling at -21 and slowly resting back to -20.5inHg. I do not have a tach to know exact rpm, but if I would have to guess it is around 3000rpm. turning the screw on the BCDD made no difference to the idle or vac pressure. the EGR is not blocked off. choking it with my hand lowers rpm a bit, but not enough to make any difference. too much choke and it goes to stall. I will also add, both primary and secondary throttle plates are closed, and the primary nozzle is spraying a ton of fuel. the air has to be coming from somewhere, but like I said, the smoke machine i made does not reveal any vacuum leaks.

[Cardinal Grammeter]

That BCDD introduces an "idle hang" on the way down in rpm when you let off the throttle.  Turning screw in is more hang, out is less.  If the screw is massively out of adjustment it will change idle.  That datsun1200 thread shows plots of that situation if I recall.  But it sounds like the BCDD is working OK.  (Did you notice the loud hissing noise when it is operating?

 

If the nozzle is spraying a ton of air, you mean the booster venturi?

 

At idle, there isn't enough flow velocity to draw fuel into the booster.

 

The only way the booster could be getting fuel is if it is being pressuized from the fuel bowl.

 

Could the float level be set too high?  I'd think the odds of that or a leaky needle and seat are remote.

 

Just thinking... has the carb worked correctly previously? Did you say you took it to a carb shop and they said it was working properly?  

 

There is either a bleed tube or emulsion tube that may have a screw in plug in it - I seem to recall there is a plug that must be screwed into the bottom of one of the vertical tube/wells before you screw in the top bleed jet or emul tube.

 

Is there any way you could put another carb on it?  Even some 1bbl on a plywood adapter plate.  Just enough to run engine with you hand on the carb linkage for throttle.

[throw1932]

I did hear some slight hissing at idle, but it is hard to tell due to how loud the engine is at the rpm it is turning.  is the booster venturi the circular piece right below the choke plate? that is where fuel is spraying from. the float level is good, fuel level is perfect with the line on the sight glass. the carburetor worked flawlessly prior to me taking it off the truck. the screws securing the top of the carb had ripped the threads out of the main body. I took the top off, helicoiled the holes, did a general cleaning (even though it was tidy to begin with) and reassembled. I also swapped out the accelerator pump. I never touched the BCDD. I put the carb back on the truck, and it wouldn't idle lower than I'd say 1000rpm, no matter how I adjusted it. someone that heard the truck idling said they could hear "lean pop" every so often in the exhaust. I checked for vac leaks and found it was leaking where the carb and spacer mate to the manifold. removed it and cut out new gaskets and used a sealant with them this time. after that, the issue was worse. I took it off a third time because I thought the throttle shaft bushings were bad. I brought it to a carburetor shop and the gentleman said the carb was in great shape and he could not find anything visually wrong with it. after I put it back on, the issue is so bad now I do not drive it. i do not have any other carburetors I could run on it unfortunately. it is the factory fuel pump on the truck, all I have done is replace an internal oil seal, it is still the same diaphragm so I do not think excessive fuel pressure is my issue. I can try to take a video tomorrow and post it on YouTube to share here. 

[Cardinal Grammeter]

Did you unscrew all the bleeds and tubes (brass jet-looking things) and blow out all the passages?

 

VISUAL CARB INSPECTION:  This is by no means an indication the carb is functioning correctly.  There are so many bleeds and brass parts that can be completed installed in the wrong body "tubes" and there would be no way to determine this visually.  You'd have to take the carb apart, consult the parts manual and check the stamped numbers on all the brass pieces.  It is a PITA since there are so many and the parts diagram is very hard to interpret.  At least the part descriptions give the stamped number.

 

The Booster Venturi's are the "small venturies inside the main ventury - they are the small "tubes" where the gas dumps into the carb when making power.  If you rev the engine and watch, a tremendous amount of fuel will spray out of those tubes.  Are you saying you can see fuel coming out of them with the throttle plate closed and idle screw backed out?

 

What about the 

*) Idle Mixture Screw - any effect?

*) Transfer slots - a small vertical slot where the throttle plate at idle is somewhat below midway "up the slot" .  There should be a smaller slot opening below the plate and the opening above the plate.  This is where the fueling between Idle and Booster Venturi fuel is accomplished.  With a big cam, if you have to increase the idle setting so much that throttle plate is above the slot, you have to drill small holes in the throttle plate so that it can be adjusted back to it's proper position over the slots. 

 

MYSTERIOUS DEBRIS:  This has been a ongoing issue for me.  Twice for sure and I believe this 3rd time also, massive debris somehow gets past the filter and screen (or comes from inside the carb) and has  on 2 occasions essentially plugged the Main Jet so the primary circuit was getting no fuel or power - would have to get the truck revved up in a lower gear so I could lay into the vacuum secondaries to make enough power to get home.

 

This third time, my carb started acting up again and I was expecting more mysterious debris.  I found non, but I did blow out most of the passages and when I put it on, maybe 50% of the problem went away.  Then a month or 2 later, about a week ago, the carb started working properly.

 

So debris is a major issue but I'm talk about carbs with pre-alcohol resistant gaskets and rubber parts.

 

Back to your "the idle screw has no effect" - I need clarification:  Do you mean it has no effect trying to lower rpm?  - because if you crank it in, it will open the primary throttle plate to increase the rpm as high as you are brave.  I assume you did not mean that it was not possible to raise the idle with the idle screw.

 

VACUUM TEST:  When you start the engine, with Idle screw back off and Mixture screw turned full in,  vary a vacuum leak from a small hose or the big one from the booster.  With a big enough leak, you should be able to make the engine "flame out" and shut off.  If it doesn't that means you are somewhere getting a massive amount of fuel from somewhere.

[throw1932]

I did not touch any of the bleeds or tubes when I had it apart. the idle screw has no effect on the rpm, regardless of how far in or out I turn it. with the idle screw backed off, the throttle plate is fully shut and the the booster venturi is dumping a lot of fuel onto the plate. the mixture screw also has no effect on the idle speed or quality. I can lightly bottom it out, and have it on its last thread, it still idles exactly the same. 

[Cardinal Grammeter]

OK, if you turn the Idle Screw all the way in, it will definitely open the throttle plate - have you confirmed the idle screw is opening the throttle plate?

 

How does the engine behave if you quickly stab the throttle and rev it?  Any "flat spot" or hesitation?

 

If you didn't touch any of the bleeds or tubes, I would be concerned that there could have been debris laying in the bottom of some passage that may have been dislodged when the carb was being handled off the engine.

 

However, the boosters dumping fuel AND the float level is correct - that is a complete mystery:  Does the booster discharge occur immediately when cold OR does it increase as rpm increases and engine begins to warm up?

 

The booster discharge is ABOVE the fuel bowl lever.

The only time there is booster discharge is when there is enough air flow from an open throttle plate to generate enough vacuum to draw fuel up into the booster discharge.

 

Other than this creeping idle increase, is driveability OK and does it make the appropriate power when in primary or WOT into the secondary?

 

POWER BRAKE UNIT:  Have you confirmed you don't have a massive diaphragm failure resulting in a HUGE vacuum leak?

 

 

[throw1932]

just realized I never updated the thread, I pulled the carb off again, cleaned the jets, put it back together and on the truck, issue has disappeared. the jets were relatively clean, but there was some improvement for sure. not sure if that is what fixed it, but I am not arguing. now I am trying to chase down an overheating issue. always something. 

[Cardinal Grammeter]

1 hour ago, throw1932 said:

just realized I never updated the thread, I pulled the carb off again, cleaned the jets, put it back together and on the truck, issue has disappeared. the jets were relatively clean, but there was some improvement for sure. not sure if that is what fixed it, but I am not arguing. now I am trying to chase down an overheating issue. always something. 

I've had to tear down mine 3 times because of mysterious dirt - twice the black stuff was once a "lump of coal" and the other a "long shard" which stuck right in the main jet hole.  The 3rd time, there wasn't much of an improvement, but after driving for a while, it "cleaned itself out" and worked properly.  Now it seems like there is a tiny bit of "dirt behavior" going on - nothing dramatic but definition a "Change in the Force."  Where this stuff comes from I have no idea.

Edited September 11, 2022 by Cardinal Grammeter