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

[datzenmike]

The spring below the plunger is very light weight and serves only to keep the BB below it in place over the opening back to the float chamber. It has no other function. It is not a return spring. The plunger is directly and mechanically pushed down and lifted up as the throttle returns to the idle position. A stiffer spring will only return to idle faster not make the pump work better.

 

 

 

The small spring above the plunger is to cushion the piston when the throttle is suddenly floored. Without this spring the fuel couldn't get out the nozzle fast enough and a hydraulic lock would bend the linkage.

 

[JamesE]

Gotcha. So what is the failure point of the accelerator pump typically?  Degraded plunger? 

 

My accel pump is virtually new having been included with the rebuild kit, but I don't mind throwing $20 at a different one to see if the behavior changes.

 

Thanks again Mike and all.

[Rays74]

Ok James, sounds like we may be onto something :)

In this pic, it shows where the likely cause of your trouble........there's a big slot headed plug covering this hole, remove it. Inside you will likely see a small spring, a brass weight and a ball bearing (in that order). Remove all these and confirm they are clean, the well where they come from is clean specially the very bottom where the ball bearing seats....critical.....confirm too that the squirt nozzle is open and clean by spraying carb cleaner through it. Now upon reassembly you have a chance to somewhat 

"tune" how this squirter works by either using all the pieces as stock, leaving out the spring OR leaving out the spring & brass weight and using TWO of the ball bearings (if you have an extra).

In your case I would prolly leave the spring out and see how it does with just the brass weight and 1 ball bearing.  

 

good luck!

--Ray

[datzenmike]

Do the same as Ray has described with the BB below the accelerator pump also. The BB has to seal the inlet hole in the bottom of the cylinder or fuel will get pushed back into the float chamber. You may have to reach in with something to clean it out.

 

The 'rubber' seal just wears out and does not seal as well as it did. Old plungers (back in the 60s) were made of leather. In fact I heard that they were made of kangaroo skin. Anyone ever hear that?

 

 

 

Something I hadn't thought of before is your idle and idle mixture settings. Costs nothing and worth trying.

 

Warm the motor up, be sure the valves are set or, at the least have some clearance and no tight ones. Check set the timing (12 degrees BTDC)

 

Idle at about 750-850. Turn the idle mixture screw in until idle quality begins to drop. Turn the other way until idle quality drops. There will be a 1 to 2 turn point where the idle improves and may even speed up between these two settings. Just pick a point in the middle where the idle is smoothest and fastest. Reduce idle speed to 750-850 and turn in then out and select a spot about half way between where the idle quality is smoothest/fastest. Repeat as many times as needed to get the best idle quality and can't be improved on... that's it.

[JamesE]

The new accelerator pump will not be in until tomorrow, so I'll try these suggestions tomorrow.  Thanks!

[JamesE]

Sorry for the hiatus.  I finally had an opportunity to try the most recent suggestions.

 

First things first, my carb does not look the same as the one that Ray posted.  He's a photo of mine.  It is slightly different.  Notice the orientation of the accelerator pump, and the primary jet is different.  Is this not a DCH340?  My car is Canadian if that is any indication.

 

 

Second I removed and inspected the accelerator pump action with current and NEW accelerator pump I purchased.  Both old and new pump appear to work well to me, but I'm a carb newbie so interested to hear opinions on this.  You need to watch the video in HD quality to see the fuel stream.  Skip to the 1:00 mark for the best view.

 

 

Next I inspected the accerator pump and nozzle cavity (sprayer).  Everything looked clean.  The only surprise was that in the sprayer cavity there was no brass weight as Ray described.  The contents are as follows below except the ball bearing.  Am I missing something or is this a different design carb?

 

Ball Bearing NOT Shown

 

I put it all back together with the new accelerator pump, warmed it up, and took it for a drive.  Behavior is the same.  As always the hesitation is intermitent.  Hammer the gas and it stumbles as it revs up.. then idle down, and hammer it again but the second time no stumble.  Could it be that it doesn't stumble the second time because the accelerator pump isn't shooting a full shot the second time?  The consensus seems to be that the engine is not getting enough fuel but I'm curious if it could be the opposite and it is choking on too much fuel.

 

Mike had also suggested checking the timing, idle, and idle mixture.  I did all of these things before I started this thread.  I have not checked the valve clearance yet. 

 

Thanks for the continued support.  I'm hoping to get it running great in time for spring!

 

 

[Rays74]

Hi James,

 Yeah that's not a DCH340........it's earlier, DAF328 with an electric choke from the L16, later '72-73. No matter, from the video it looks like your pump shot is weak. I would look for a blockage of some kind in the accel pump circuit, between the pump chamber and the brass outlet. Also, looks like there is a ball bearing under  spring #44.....but has no number? Hmmm, well see if that helps, maybe it's gummed or stuck and causing a restriction. To check it, I would remove that brass squirter assy and then pump the accel shaft and see if you have good flow from the open hole. Be careful, it won't take much force, gas should come rushing out with pressure if the passages are clear :)

 

http://www.carpartsmanual.com/datsun510/Datsun510Index/Engine/Carburetor/L16Manual/Jun71ToSep71/tabid/494/Default.aspx

 

[JamesE]

Thanks for the reply Ray.  Good to know I'm working with a DCH328. I'll update the thread.  The exploded diagram is very helpful!

 

I'm really surprised that you think the pump shot is weak.  I have no point of reference but it looks strong to me.  The brass outlet has a very small ID.  It is much much smaller than I expected considering the OD of the brass nozzle.  The output meets my expectation considering how small the ID is.  What should I expect from a proper shot?  Faster stream?  Thicker stream?  Slightly vaporized?

[Rays74]

Having worked with many of the DCH340 carbs, their "shot" is much heavier than what I saw yours do on the video. Maybe the 328 does have a weaker shot overall then? Hmmmm...they were both used on the same engines......., wish I had one here to play with and compare.

The bog you are describing would not be an overly rich condition.......definitely lean.   

[JamesE]

Is the DCH328 the correct carb for an '73 L16?  This is the carb that came with the car and the car is mostly stock so I don't really have any reason to believe it isn't the original carb.

 

My next thought is to check the accelerator pump linkage.  I'm going to try to adjust it to work the pump sooner in the throttle travel.  The rod (marked as #4 LINK) may have been bent or straitened during dis-assembly.

[JamesE]

I experimented a little more yesterday.  I tried disabling the accelerator pump and (no surprise) the condition got worse.  I completely agree that the root cause is a lean condition.  So is it too little fuel or too much air?  In this video below you can see that the accelerator pump is working, and working as well as I would expect.  But it seems to me that the air beats the fuel to the chamber.  Once the fuel catches up it runs fine.  I'm not sure what else I can do to improve the rate at which the accelerator pump delivers fuel.  It seems to be working as designed.  As a last resort is there any carburetor specialists companies I could sent this to and have them take a look at it?

 

[datzenmike]

Check down in the bottom of the accelerator pump. There is a hole going back to the float chamber with a small BB covering it. Under the accelerator plunger is a large thin spring that bears down on the BB holding it over and plugging the hole.

 

In operation, when the accelerator pump is pushed down the pressure and the spring hold the BB in place over the hole to the float chamber so that gas is not forced back into it. This must seal completely or the 'shot' or some of it will be wasted. When the accelerator plunger rises, in rushing gas from the float chamber lifts the BB off it's seat and gas enters the accelerator pump.

 

It should be noted here that there is another BB out by the discharge nozzle that works the opposite. It allows gas out but prevents air from being sucked in on the up swing.

 

Doesn't take much sediment or crap to prevent the BB from sealing the opening to the float chamber.

[JamesE]

When the accelerator plunger rises, in rushing gas from the float chamber lifts the BB off it's seat and gas enters the accelerator pump.

 

Mike, I think you said something that makes a lot of sense.  As I have explained this problem is intermittent.  For instance if I go WOT the engine will stumble, but a few seconds later after it idles down I try WOT again and it does not stumble.  It could be that in this case the fuel has not leaked out of the accelerator pump chamber yet.  If I understand you correctly the accel pump chamber is only primed when the accel plunger rises (throttle backed off).  Therefore if there is 30 seconds or so between heavy throttle instances and the BB is not sealing off the chamber perfectly, the intake could be getting incomplete shots.  Am I understanding that right?  

 

Thanks for sticking with this.  I'm looking forward to my next evening in the garage so I can look into this.

[JamesE]

I removed the carb last night and disassembled it a bit.  The accelerator pump chamber and nozzle chamber are both very clean.  I checked the check balls and they are both 5/32" as specified.  I still believe the check ball in the accelerator pump chamber to not making a good seal; but what could be done to improve it?  I considered increasing the check ball size to 3/16".  I also saw that some carburetors use nylon check balls.  I'm wondering if a nylon ball would make a better seal.

 

[banzai510(hainz)]

Get a weber DGV and dont look back.

 

the Hitachi isnt good enough for that nice looking 510

[JamesE]

Thanks for the compliment Banzai but with the Weber I would miss that baby blue intake when I open the hood.

 

I am so close I can taste it!  I solved the stumble last night but somehow introduced a new problem.  The stumble was solved by blasting carb cleaner through the accelerator pump chamber and accelerator pump nozzle chamber, as well as the nozzle itself.  Mike and Rays you were right all along!  I didn’t do it sooner because the same was done when the carb was rebuilt not long ago.  But the nozzle ID is so fine I suppose it is easily clogged up.

 

I was so excited to drive it to work this morning but just a mile from home I observed a flutter at high load and low rpm.  It also feels down on power in general.  But the stumble at WOT is completely gone.  Something is not right.  I’ll look over the linkages and vacuum hoses tonight.

[banzai510(hainz)]

I would allways recheak the timming.

[JamesE]

It was an easy fix, but then all of a sudden i'm back to square one. I'll explain. I popped the hood last night and immediately spotted that the electric primary choke/butterfly control was disconnected. Oops. i connected it and took it for a short drive and it was running perfect!

 

With my confidence up I decided to go for a longer drive. After a few miles it started showing some of the old stumble symptoms. Then I went to the gas station and topped of the tank with 5 gallons, and maybe I'm imagining it but it seemed to get even worse still! The previous owner told me he dropped and cleaned the tank but I'm suspicious now that my fuel is dirty and clogging the accelerator pump. But shouldn't my new fuel filter be cleaning the fuel sufficiently?

[banzai510(hainz)]

open up the filter.

But most 510 gas tanks dont get dirty. 521 trucks Yes they get rust and dirt EZ from from the wheel well.

[JamesE]

Maybe so where you are from, but this car has been in the Midwest all its life and is not rust free.

 

I'll open the filter. It is new and has less than 20 miles on it, but they are cheap enough.