L20b dual side draft 40mm dead spot

[Kingcab]

I just put on dual 40mm side draft webbers and it idles just fine but I cannot figure out why it has  dead spots and hesitation can anyone help please 

Edited July 14, 2019 by Kingcab

[nad015]

Is it when you step on the throttle?

thats usually a lean spike.

cured mine with bigger pump jets

[Stoffregen Motorsports]

The accelerator pumps are adjustable too, so if anyone had ever taken these Webers (one B) apart or made adjustments to the rod, you may need to adjust them to suit your needs. Generally speaking, tuning Webers takes some time, patience and an assortment of jets.

 

You have started down the slippery slope of engine mods, and to get the most out of the new carbs, you will have to make a few non carb related adjustments or changes. Cam timing or new cam, distributor re-curve, new heat range on the spark plugs, larger exhaust...the list is not long, but it does take more than simply bolting carbs onto a stock engine.

 

One massively overlooked area of tuning is the distributor. A $200 rebuild/recurve to your OEM Datsun distributor is worth the money.

[Kingcab]

The dead spots at low rpms high rpms it does fine. What would i need to regap the plugs to and I'm 12° on cam and at 12 mark on dizzy

Edited July 15, 2019 by Kingcab

[Stoffregen Motorsports]

I think you need to make sure you have the right plugs (NGK B6ES is a good starting point). And the timing set at 12 degrees is good, but what's the total advance? No more than 32-33 I hope. You should also have the vacuum advance disconnected and the curve should also be adjusted.

[distributorguy]

My experience with Webers - 40s, 45s, and 50's.  

You will need a minimum of 16 degrees BTDC at idle, no vacuum hooked up to the distributor as your vacuum signal is weak at best.  20 degree BTDc at idle is better.

Total timing will be dependent on engine compression, cam duration, and cranking cylinder pressures.  29-34  at 3000 rpm is going to be correct for 14:1 down to 9.5:1 compression.  Anything lower than that won't ever run well on these carbs.

 

To tune out the flat spot, start with 16 BTDC, and turn your idle speed down around 600 (or lower) to adjust the real idle mixture without the emulsion tubes bleeding fuel into the equation.  Once that is established, then you can turn up the idle speed to figure out which emulsion tubes will work for you, as they are used from 650-1800 rpm in conjunction with the idle jets.  You'll likely find that the ideal setup is half way between 2 different options, so you will solder a couple holes closed on the richer of the 2 choices.  An Air/fuel meter install in the vehicle will be the only easy and affordable way to determine AFR.  Any other option short of a dyno session (or 4) is a shortcut that won't pay off.  Once you get the right idle jets and emulsion tubes, thing get easy.  Choosing the correct main jets is easy, and they cover 2000-4000 rpm driving range.  Above that is the air correctors, also easy and inexpensive.  The mains, idles, and air correctors can be soldered closed and drilled out with a set of jet drills (from ebay or Pierce manifolds) so no big cost there.  The emulsion tubes can get expensive buying 4 at a time, but its worth while as when you get them right, it will drive like you have perfectly balanced SUs, with more top end power.  

All of this is futile if you're trying to get race carbs (DCOEs) to work with low compression, as the air velocity in the carbs needs to be high for them to meter properly. 

 

Follow this step by step in the right order and you'll find your engine running great.  

[Stoffregen Motorsports]

True, it's not uncommon for these street engines to need very small chokes in the carbs. Something like 32-36mm will help build velocity.

Edited July 17, 2019 by Stoffregen Motorsports

[distributorguy]

That's true, smaller chokes build low end power better, but an engine this displacement can run on 36's and you'd never find chokes bigger than that in a set of 40's.  

 

Start by initially setting the float height and drop in both carbs.  That's the easiest way to fix a stumble - add or remove fuel from the emulsion tubes by changing the fuel level.  Google "Weber Float Level" for a 1-page image of how and where to measure.  Between this setting and engine timing, that'll be the only adjustments you can really make a difference with - short of changing jets.