DCOE's on a L20b, questions...

[AZhitman]

Having some challenges with my 510, hoping some of the experts here can maybe give me some guidance... I "short-circuit" when I'm stumped, and nothing gets done.  :(

 

Here's what I'm working with: A fresh L20b with Isky cam and Cannon manifold, Weber 42 DCOE's, matchbox distributor.

 

Engine has been broken in (on the stand) and I recently pulled the intake and carbs off to do a PVC setup for crankcase ventilation (valve cover port is open to atmosphere).  

 

Reinstalled everything, and now, on startup, the idle is running wild.  Idle screws are dialed all the way out, mixture screws are 2.5 turns from bottomed out, and I'm running open Mikuni air horns.  Nothing is binding, and the throttle arms are fully closed - still, idle is WAY high.

 

1)  Is this more likely a vacuum leak issue or a timing issue?  I dialed back the timing to where the idle dropped, but then it's backfiring through the carbs.  Quick check revealed it was 10 degrees AFTER TDC.

 

2)  Two engine guys (who I respect and learn a lot from) have differing opinions on the use of vacuum advance on this setup.  One says there's no need, and the mechanical advance is sufficient.  The other seems to think that the lack of vacuum to the advance on the dizzy is the cause of my problems.  Tiebreaker?  :)

 

3)  Throttle shafts seem to keep coming to mind.  Is it common for this to be an area of air leaks?  I'm not well-versed on the Webers.

 

I think I've been dinking with this setup for too long without any good success, so I'm kind of frustrated - as such, I'm probably missing something obvious.  

 

Anyone want to take a stab at it?

 

Thanks in advance, guys.  

[EricJB]

If the butterflies are closed, it is pulling air in from somewhere else. Did someone mess with the air bypass screws? Vacuum leak? Did it run OK before the pcv mod? If so, pinch off the pcv hose to see if it changes.

The Air/fuel screws work like a faucet. In is lean, out is fat. 2 1/2 turns is out a lot. The best way to set those is, (once you have the idle under control and the timing set) turn each screw in one at a time slowly, and listen to the engine speed. When it starts to fall off, turn it back out slowly till it comes back, then 1/4  turn out. Repeat.

 

I personally dont run a vac can with dual, dual throats. Find someone with a timing light with an advance feature and set it at 36 degrees total (full centrifical). Then let it idle where ever it ends up. It's far more important to know where it is at 3-6k than at an idle. Good luck.

[Stoffregen Motorsports]

1)  Is this more likely a vacuum leak issue or a timing issue?  I dialed back the timing to where the idle dropped, but then it's backfiring through the carbs.  Quick check revealed it was 10 degrees AFTER TDC.

 

2)  Two engine guys (who I respect and learn a lot from) have differing opinions on the use of vacuum advance on this setup.  One says there's no need, and the mechanical advance is sufficient.  The other seems to think that the lack of vacuum to the advance on the dizzy is the cause of my problems.  Tiebreaker?   :)

 

3)  Throttle shafts seem to keep coming to mind.  Is it common for this to be an area of air leaks?  I'm not well-versed on the Webers.

I'm assuming the setup you broke it in with is tha setup you're using now...right? So did it run ok before you made the changes to the intake? Try plugging the PCV valve to see if that makes a difference.

 

1) Have you verified TDC by removing a spark plug?

 

2) You do not need the vacuum advance, but if you run without it, you need to re-curve the distributor. This entails not only shortening up the mechanical advance, but also adjusting the spring tension for a quicker and full advance.

 

3) Leaky throttle shafts could be the problem, but not likely. A twisted throttle shaft is more likely.

 

So again, did you use this setup on break-in? and if so, how did it run then?

[D52E]

I would tend to agree that it sounds like a vacuum leak.

 

Can you carefully try spraying a small amount of carb cleaner around sealing surfaces to find potential vacuum leaks?  The idle would noticeably change if you find a leak.

 

What PCV valve did you use and where is it installed? - Generic part - could it somehow be installed backwards?

Perhaps you can temporarily block off PCV flow and confirm if PCV system is the source of the vacuum leak?

[AZhitman]

Thanks guys.

 

I used a generic PCV valve.  Large end towards crankcase (same orientation as if it were installed in the top of the valve cover).  I clamped off the line between the PVC and the manifold, no change.

 

SM, when you say "twisted throttle shaft", do you mean internally in the DCOE?  Aren't there supposed to be seals at the outboard section of the shaft (where it exits the carb body?)  I'll double-verify TDC and marks.

 

Break-in was done with a completely different carb setup. 

 

Eric - I was using a popular Weber setup article (that mentioned starting at 2.5 turns open).  Not certain on the air bypass screws, but I will check those as well. 

I'll try starting out a little leaner, and I'll get hold of a total advance light.

 

With this many Datsuns, I'm afraid I'm becoming a "jack of all trades" but, as they say, a master of NONE.  :(

[Stoffregen Motorsports]

I used a generic PCV valve.  Large end towards crankcase (same orientation as if it were installed in the top of the valve cover).  I clamped off the line between the PVC and the manifold, no change.

 

SM, when you say "twisted throttle shaft", do you mean internally in the DCOE?  Aren't there supposed to be seals at the outboard section of the shaft (where it exits the carb body?)  I'll double-verify TDC and marks.

 

Break-in was done with a completely different carb setup. 

You can decrease the orifice size of the PCV valve if you like, simply by prying it apart and installing a washer with a smaller hole. This will help fine tune the system, bringing down the previously discussed vacuum leak possibility.

 

A twisted throttle shaft is just that, twisted. Picture the shaft taken out of the carb with the butterflies intact. Now picture it if you twisted one end of the shaft one direction and the other end the opposite direction. Now visualize that assembly in the carb body. You would have one butterfly closing while the other is hanging open a bit. This is common. Many linkages are not set up properly. When you have a linkage at one end of the carb and a stop at the other end of the shaft (like an idle adjustment) you can overextend the one end with the weight of your foot mashing the pedal to the floor, while the other end is stopped, causing a twist, and thus, causing a "popping" at idle and part way through the throttle input opening. Full throttle is harder to notice this problem.

 

Now carb balance is similar to twisted shaft. It takes some practice to be able to balance the carb synch. I do it by ear, some use a meter.

 

Since break-in was done with a different setup, I would start by looking for obvious vacuum leaks, but keep in mind the balance/twist scenarios.

 

Good luck!

[TENDRIL]

is a mechanical fuel pump going to be used? im going to be running 40's on my l20 and was wondering what kind of FP you have.

 

sorry for thread jack but im going to need all the info i can get on this setup  :geek:

[AZhitman]

Got it!

 

Spent some time with Dr. 510 (Dick Lowell) today... found that the throttle plates were "shifted" somewhat to one side.  Apparently when tightening the hardware on the ends of the throttle shafts, we "pulled" the alignment off.

 

Looking down the carb barrels up to the light, there's a little crescent of light at one side of each plate.

 

Quick adjustment and a re-tune, and FINALLY the Dime is idling properly!!!

 

Stoff, you nailed it.  Thanks a ton for the diagnostic help.

 

TENDRIL - Mechanical (OEM) pump with a FPR and gauge.

[Stoffregen Motorsports]

Got it!

 

Spent some time with Dr. 510 (Dick Lowell) today... found that the throttle plates were "shifted" somewhat to one side.  Apparently when tightening the hardware on the ends of the throttle shafts, we "pulled" the alignment off.

 

Looking down the carb barrels up to the light, there's a little crescent of light at one side of each plate.

 

Quick adjustment and a re-tune, and FINALLY the Dime is idling properly!!!

 

Stoff, you nailed it.  Thanks a ton for the diagnostic help.

 

TENDRIL - Mechanical (OEM) pump with a FPR and gauge.

Awesome. Glad to hear you're up and running.

[cruznude]

Damm, that was good advice from all.  When I had my dcoe's and me not being that much mechanically inclined I would take the ride into the shop at 150.00 a pop and get all the adjustments done. 35 years ago I wish there was a site like this.

[banzai510(hainz)]

yes I try to avoid taken those nuts off the end as Im scared of bending the throttle plates/shafts