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L20b, Weber 32/36, 93 Octane Timing Help


Soundline

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My books don’t cover this, so I’ll toss myself to the mercy of the Ratsun tuning Gods.

 

I have a stock L20b out of a 77’ 620. Stock heads, rods, and stuff...

 

It’s got a Weber carb on it, one of the Spainish ones. The distributor is a points type converted to a Pertronix. I’d like to run 93 Octane and I’d like to know if those things change the timing requirements.

 

I dunno what the carb is jetted at, or if this changes things.

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I dunno what the carb is jetted at, or if this changes things.

 

Sure does.........

Too lean, and the engine will ping, just like if you have too much timing.

 

I feel we are not getting the full story here.

Like the reason to run higher octane.

Was it pinging on regular?

 

What do your plugs look like after a hard pull at full throttle.

And to do this correctly, you have to kill the ignition, and hit the clutch at the same time, without closing the throttle,

otherwise your plugs will show fuel mixture during deceleration, and idle.

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Actually the engine has been cutting out and dying under throttle. I’m changing things in stages starting tomorrow to see if I can identify the problem.

 

I think the cracked fuel line was part of it and so was having the wrong distributor. So I’ll start there and work my way forward in this thread.

 

I’m assuming low octane might be a cause. Given the choice I prefer 93. But... I’m a car dork. So correct me if I’m being fucking retarded.

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It should run on 87 just fine, mine does. 

Any 4 cylinder distributor will work, some maybe a shade better. Pertronix are just breakerless. If replacing get a '78 or up EI distributor and matched coil for higher output spark.. 

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Is this problem accompanied by a stutter or backfire? Maybe a spitting noise? If it were electrical, it could make a large or small backfire noise.

 

Depending on the age of the carb, things in Weber DGV's do wear out. The auxiliary venturis get loose in the bore and can flop around, causing a disturbance in the fuel flow at high RPM's. Check this by jiggling them with your finger tip. If they move at all, then they should be shimmed with a soda can on the end without the feed hole. Here are new ones if you decide to replace them - https://www.pegasusautoracing.com/productdetails.asp?RecID=4147

 

I've also seen the tube inside the aux venturi become loose.

 

Your float level could be severely low. Your engine could also be using more fuel than the needle and seat allow. Stock needle and seat is 175, you could replace it with a 200 size for more flow - https://www.pegasusautoracing.com/productdetails.asp?RecID=4125There's even a 250 size if desired - https://www.pegasusautoracing.com/productdetails.asp?RecID=4122

 

A modern float with less area will allow more fuel in the bowl also - https://www.pegasusautoracing.com/productdetails.asp?RecID=4127

 

If your engine is built to the point where you are running past the capabilities of the carb, you may consider sending it out to have it performance modified, but it's likely that you have other issues causing your problem.

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Also the distributors will wear out. The vacuum advance plates are particularly prone to sticking, so the actual advance can vary widely, causing very carburetor fault like effects. You can wiggle it by hand to see if it sticks or you can look at the timing with a light. All the movements need to be smooth. The L20b is more timing sensitive than the smaller motors, I found that knock occurred first at 5000 rpm, far above the audible range for "ping". It also liked much slower advance curves than the stock distributors were set for, so there was power in re-curving to a slower rate, otherwise you just have to live with retarded maximum timing settings, like 22* maximum, which is optimum at the 3500rpm or so that the distributor maxes out at. That overadvanced ignition may feel like a misfire or stutter. It is a bad thing.

Dennis

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The distributor has a wiggle in it. Another Ratsun member hooked me up with a fix for that. I ordered the gasket and I’ll swap distributors when it comes in.

 

Testing this... my radiator finally let go. The tank separated at the top of the core. It’s being rebuilt so I’m going to have to wait until that’s done. In the meantime I’ll run new fuel lines because there’s multiple issues going on here.

 

I really appreciate the help and suggestions here Gentlemen. I’ll be looking at those carb pieces after I fix the rad, distributor, fuel lines, and I’m getting new plugs and wires. That way I’ll limit the issues as much as possible.

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The auxiliary venturis get loose in the bore and can flop around, causing a disturbance in the fuel flow at high RPM's.

 

I just shimmed mine a couple of days ago.

One took .003", the other took .006" to snug up.

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Also the distributors will wear out. The vacuum advance plates are particularly prone to sticking, so the actual advance can vary widely, causing very carburetor fault like effects. You can wiggle it by hand to see if it sticks or you can look at the timing with a light. All the movements need to be smooth. The L20b is more timing sensitive than the smaller motors, I found that knock occurred first at 5000 rpm, far above the audible range for "ping". It also liked much slower advance curves than the stock distributors were set for, so there was power in re-curving to a slower rate, otherwise you just have to live with retarded maximum timing settings, like 22* maximum, which is optimum at the 3500rpm or so that the distributor maxes out at. That overadvanced ignition may feel like a misfire or stutter. It is a bad thing.

Dennis

Don't L series like about 32-34 degrees total advance when revved up? Or are you not including the static 12 degrees to start with?

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I just shimmed mine a couple of days ago.

One took .003", the other took .006" to snug up.

It happens. They get old and the vibrations kill them.

 

Don't L series like about 32-34 degrees total advance when revved up? Or are you not including the static 12 degrees to start with?

Yes, he's referring to mechanical advance total. Add in the initial timing of about 12 degrees and you get 32-ish degrees. Others say that more than 32 is ok. I have found that 32 is the safest place to be.

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32 total is a great place to be if your compression test results land around 160-180 psi (healthy).  If you come in at 125-150 (lots of wear) you can go up to 35 or so.  All of this is based on a good distributor.  If the springs are shot, you may have to set it lower to prevent pinging.  

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32 total is a great place to be if your compression test results land around 160-180 psi (healthy).  If you come in at 125-150 (lots of wear) you can go up to 35 or so.  All of this is based on a good distributor.  If the springs are shot, you may have to set it lower to prevent pinging.  

Right. On street driven Datsun L motors I shoot for 10:1 CR, so if your CR is less, then you can run more timing.

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Higher octane fuel is a bad idea unless you NEED it.

 

Didn't read the thread, but to educate others I'll spill the truth.

 

There is a huge misconception that high octane = high performance. In reality high performance requires high octane.

 

By definition (in layman's terms) octane is an increment of measurement for volatility of fuel a.k.a "how fast and violent does it burn".

 

Basically the lower the octane the faster and easier the fuel will burn. Higher octane number means how slow it burns.

 

Let's go a step further... Compression, timing and combustion chamber temperatures are most defining of required octane.

 

To put this is very basic logic -- too low of octane and you'll ping. Too high of octane?? Here's the surprise. You'll end up burning a valve due to the slow burning nature of the high octane. The fuel burns too slow and will infact still be burning when exhaust valve opens, causing it to over heat the valve.

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Agree high octane only if needed. My take is that octane is a resistance to auto ignition. Higher number is harder for it to ignite from excessive compression or engine heat. I don't agree it burns slower it just allows you to light it exactly when you want it to light and not ping or explode too soon. IF you run higher octane the timing should be advanced to take advantage of it. If you leave it slightly retarded than needed, yes the exhaust valves may get hotter. If your engine is stock (and the vast majority are) and pings, you need to eliminate what's causing this first rather than going to higher octane. It may be running rich and is carboned up, maybe too hot or wrong make of spark plugs, timing too advanced or engine running too hot. 

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Unfortunately people have the stigma of "bigger is better" so they believe higher octane is better... This could also be due to the USA markets calling high octane 'Premium'.

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