L16 Stumbles at WOT with Hitachi DCH328 with Video. Suggestions Please!

[JamesE]

Car: 1973 CDM (Canadian) Datsun 510

Engine: Stock L16 with 80,000 miles on factory build

Transmission: 4 speed Manual

Carburetor: Hitachi DCH328 (stock) Similar to DCH340

 

I'm struggling to determine why my engine is stumbling at wide open throttle from idle. 

 

Symptoms

• Stumbles at WOT from idle.  Video below.  Engine is up to temperature in the video
• When driving, occasionally the engine will die when you coast to a stop, and in one instance was hard to restart.  This was before I correctly adjusted the idle and throttle screws on the carb so that may be to blame for that instance.
• When driving, occasionally the tach will go crazy and sweep top to bottom a few times before finding the RPM again. (unrelated?)

 

What I have done

• I was hoping a carb rebuild would correct the stumble.  I had it rebuilt by a friend of mine that was a professional carb rebuilder back in the day.  I oversaw the job.  We didn't find anything out of the ordinary.  I am quite confident that the rebuild was done properly.

• Spark plugs have less than 100 miles on them.  It seems new spark plugs will improve the issue for a short time.

• I have checked and double checked the accelerator pump.  It is assembled correctly with the correct spring orientation and BB.

• No vacuum leaks that I can find in the rubber hoses.  I've tried the spray carb cleaner method and the engine speed doesn't change.

• Considered throwing the Hitachi out in favor of a Weber 32/36, but I would really like to keep the Hitachi if possible.

Any suggestions are appreciated.  Thanks in advance for reading the post.

 

Reference links

• Hitachi DCH 340 explosion / rebuild instructions
• Another Ratsun thread on the same topic
• Hitachi vs Weber discussion

[dr.feltersnatch]

i assume your still running points, have you replaced those recently?

[banzai510(hainz)]

on vid looks like you just reving up the motor to me.

You change the fuel filter?

Coil getting HoT?

 

This SSS tach is it a Parrallel type or a In series type hook up.

'The inseries type might be the proplem of soemthing breaking down. But Im not a SSS tach expert.

 

 

have you tried reving it up then putting your hand over the maine side of the carb so the 2nd opens up (you know its not stuck closed)

Or Rev it up place hand over the main barrel and open the 2ndary up with your finger and it still still stay running.

[banzai510(hainz)]

as above ck the distributor shaft wiggle also whne cking the points

[datzenmike]

Looks normal. There is often a slight bog when jamming the throttle wide open from the sudden drop in vacuum and lack of fuel. Try adding more squirt from the accelerator pump by moving the plunger to a different hole.

[JamesE]

Yes I'm still running points.  I have not changed them but they "look" good.  Is there an inspection/test procedure or is it easy and cheap enough to just replace them?

 

Fuel filter also a good suggestion.  I know the tank was cleaned just prior to my ownership but not sure about the filter.  I'll pick one up.

 

Yes the video was taken from a standstill in neutral.

 

Banzai.. all good suggestions.  I'll check and report the results.  I didn't realize this was an SSS tach, and I'm not familiar with the type.  I'll see what I can find.

[JamesE]

Looks normal. There is often a slight bog when jamming the throttle wide open from the sudden drop in vacuum and lack of fuel. Try adding more squirt from the accelerator pump by moving the plunger to a different hole.

 

Thanks Mike.  I was hoping to get your input.  You seem to be an expert on this topic from the searching I have done.  I've been tempted to try that other hole on the accelerator pump.  I'll give it a try.

[datzenmike]

Idle circuit supplies the fuel for idle but not enough to get above 1,000 RPMs or so. The amount of air at idle is very small and when the throttle is suddenly opened it takes a second for the air to fall in and start moving. It's moving air that causes a vacuum at the primary venturi which sucks in gas into the primary barrel. The engine gets air but not enough fuel and goes lean and stumbles or bogs. The accelerator pump mechanically sprays raw fuel in to richen the mixture and helps the  transition from idle to primary barrel use.

 

You may never get rid of all of it, but when in gear you won't notice it because the motor is slower to rev with a load on it. In other words it will have more time to transition to off idle

[JamesE]

Very well written and easy to understand Mike.  This is the only Datsun I have ever driven so I don't know what is "normal".  Everything you said makes perfect sense but it stumbles when accelerating the vehicle from a stop as well so I believe this condition I am experiencing is not normal.  I will take everyone's suggestions into consideration and report back.

[banzai510(hainz)]

Is there an inspection/test procedure or is it easy and cheap enough to just replace them?

 

So you must not have had this car long?????????

Point ignition is/was the biggest killer of Non mechanically inclined Datsun owners beside over heateing and no oil.

 

I would become a expert on points ignitions and or when possible get a Point conversion or a Matchbox distributor. this will eleminate fure proplems.

[banzai510(hainz)]

Just thought of this.

If you got the stock carb there will be a sight glass.

Rev it up and ssee if the float lever sinks(If you have the carb with the big glass) I dont know about the later carbs if they have the big sight glass where you can see the float lever and float.

 

if tank was clean buy a new filter and cut open the old one. But since your car idles Ok it propable not that dirty.

 

Try when motor is cold and choke is closed see if it revs up then before it warns up. This will eleminate the Tach issue for sure. This might point to a jetting plugged issue(maybe Im just making this up)

[JamesE]

So you must not have had this car long?????????

Point ignition is/was the biggest killer of Non mechanically inclined Datsun owners beside over heateing and no oil.

 

I would become a expert on points ignitions and or when possible get a Point conversion or a Matchbox distributor. this will eleminate fure proplems.

 

 

Haha. I would consider myself mechanically inclined.  I just happen to be an under-car guy (Chassis/Suspension/Brakes) more so than an under-hood guy (Engine/Trans).  This is the first carburated car that I have owned so the learning curve is steep.

 

Just thought of this.

If you got the stock carb there will be a sight glass.

Rev it up and ssee if the float lever sinks(If you have the carb with the big glass) I dont know about the later carbs if they have the big sight glass where you can see the float lever and float.

 

if tank was clean buy a new filter and cut open the old one. But since your car idles Ok it propable not that dirty.

 

Try when motor is cold and choke is closed see if it revs up then before it warns up. This will eleminate the Tach issue for sure. This might point to a jetting plugged issue(maybe Im just making this up)

 

Yes the carb has a sight glass.  I'll try it.

[datzenmike]

Look down the carb with the motor off. Pump the gas by hand and you should get a very strong continuous stream of gas from the accelerator pump.

 

Just for the hell of it. Pull dizzy cap off as well as the vacuum advance hose off carb. Suck on hose and rotor should turn clockwise and hold until released. There is no vacuum advance at idle but there is just above idle. Probably not much vacuum with the throttle wide open but a weakly filled cylinder needs all the ignition advance it can get. May help to check this.

[JamesE]

Thanks again everyone for the suggestions.  I spent some time in the garage this afternoon.  The only thing that looked a little suspicious is the points...

 

datzenmike

 

• Accelerator Pump:  With the car off and throttle on you can hear a steady stream of fuel.  The pin was in the top hole so I tried switching it to the lower hole.  There didn't appear to be any change to the stumbling behavior.

• Distributor:  I removed the cap and sucked on the vacuum advanced hose.  Just as you stated the rotor turned clockwise.

 

banzai510

 

• Fuel Filter:  I replace the fuel filter and gutted the old one.  Photo is below.  It wasn't terrible but probably due to be replaced.  I did notice that my idle jumped up to 1,000 rpm today versus 700 rpm a couple days ago so possibly it is getting fuel more easily.

• Coil: The coil did not feel hot after the engine got up to operating temperature.
• Secondaries:  I checked the secondaries with the method you suggested.  They seem to be working fine.  A video is below.

• Points: The distributor guts look new, but there is possibly some wear on the points.  I'm not sure what to look for so I'm attaching a photo.  Your thoughts?

-

• Float:  I'm not sure what is the preferred behavior of the float.  I saw almost no movement of the float when reving the engine.  A video is below.  Maybe you will see what you are looking for.

• Cold Engine:  It is cold here so I started the engine with the choke, but after only 30 seconds of running gave it WOT and it stumbled the same as it does when warm.

[JamesE]

After putting it back together I took it for a short drive to confirm it still stumbles under load, and indeed it does.  It is hard to tell on the video because the camera shakes when it stumbles...

 

 

What's next.  Replace those points?

[dr.feltersnatch]

make sure you points gap is set properly .018-.022

if you dont have a manual http://www.davidcmurphy.com/olddat/data/Service_Manual_Model_L16_&_L18_Series_Engines.pdf

this will help.

[datzenmike]

Everything looks normal. Points should have a gap of 0.022" or the thickness of a matchbook folded double.

[Rays74]

Hey James, I only run Hitachi's, my DD is running one........I've dealt with this before, it was an accel pump issue both times. 

Take a look down the throat of your carb at the area in the pic, using a good flashlight jazz the throttle (eng off :)) and confirm there is a GOOD STRONG squirt from the accel pump discharge nozzle.

I would wager not much or a weak spritz.

The way your motor is hesitating on throttle tip-in points right at the accel pump ......to me anyways :)

--Ray

 

[racerx]

when you said that new spark plugs imrpoves the issue temporarily, does this mean that they get plugged up with carbon and are they black..maybe getting too much gas.  Check your plugs after driving the car and pull over to see their condition.  If you have adjusted the carb properly and still running the way it is maybe it's time to get a new one. Not sure what the price difference is between Hitachi and Weber 32/36 but I am kinda partial on the weber, installed these on 3 cars and no problems. I created my own by tigthening the plate too much.

[zed]

just in case, replace the capacitor - they have made a fool of me more times than I like to remember. The aftermarket ones can fail within 50 miles, so I use Nissan now

[datzenmike]

Hey James, I only run Hitachi's, my DD is running one........I've dealt with this before, it was an accel pump issue both times. 

Take a look down the throat of your carb at the area in the pic, using a good flashlight jazz the throttle (eng off :)) and confirm there is a GOOD STRONG squirt from the accel pump discharge nozzle.

I would wager not much or a weak spritz.

The way your motor is hesitating on throttle tip-in points right at the accel pump ......to me anyways :)

--Ray

datzenmike

 

• Accelerator Pump:  With the car off and throttle on you can hear a steady stream of fuel.  The pin was in the top hole so I tried switching it to the lower hole.  There didn't appear to be any change to the stumbling behavior.

 

 

 

Well seeing is believing, not hearing. Take a look with a flashlight like Ray said. You have to confirm a strong shot of fuel when the accelerator arm is moved. A thin stream is no good nor is a spray half way down the pump travel.

[JamesE]

Thanks for the comments.  I think you are all on to something with the accel pump.  I finally see where the fuel is being dumped into the primary.  The nozzle was hidden behind the butterfly.  Previously I could only hear the fuel flowing in.  Now I can see it.  I would not describe it as a squirt.  It is more like a steady thin stream.  It is as if instead of being pushed out, it is simply falling due to gravity.  Based on the comments I think this is insufficient. 

 

I have to say I'm surprised because the carb rebuild kit came with new accelerator pump bits; spring, bb, and gaskets if I recall correctly.  I checked my part store's website and the accel pump they show a photo of appears to be a totally different design.  I'm going to pick this up tomorrow and give it a try.

 

[datzenmike]

There are actually two BBs in the pump system. One at the bottom which is obvious. Just make sure the spring holds it in place before installing the plunger. This BB prevents gas from being pushed back into the float chamber and is a one way valve. If this isn't sealing some gas will be pushed back into the float chamber instead of the nozzle.

 

The other BB is between the plunger and the output nozzle. It allows gas out but closes when the plunger moves up to draw fuel in from the float chamber and prevents air being sucked in.

[JamesE]

I was wrong about the above accelerator pump being a "totally different design".  I forgot that there are two springs in the system.  I read just moments ago at the 510realm a user commented that his return spring was too weak following a rebuilt and this caused intermitent problems.  Perhaps this is what I am experiencing because I did the same; replace the accel pump with rebuild but I believe the return spring was reused.

 

Source:  http://www.the510realm.com/viewtopic.php?f=3&t=18211&start=105

 

It is the accelerator pump return spring, the one under the plunger. The rebuild kit had a new plunger, but no new return spring, as it turns out, my original return spring was WAAAAYYY to soft compared to the plunger spring. Suffice it to say that the accel linkage will push the pump down and give you one squirt, but nothing after that. This is where my 'intermittent' accel pump issue comes from.
I ended up putting the 'original' one back in and stretching the return spring by about 100% to increase it's spring rate while I try and track down a replacement.

[JamesE]

I believe this confirms it.  Rebuild kit did not come with a new accelerator pump return spring...

https://buy.walkerproducts.com/carburetor/components/hitachi/hitachi-15474b-carburetor-components.html

 

Where could I get a return spring?