some advice on tuning my scca l20b please!!

[sam]

first of all i just built this engine and it hasnt been driven yet , i need help or some one to guide and correct me in how to tune the carbs

 

currrent status

 

block

l20b all stock in the bottom with a high pressure oil pump from NISSAN

 

Head

penut head shaved with ZX valves and springs and a mild cam .

 

fuel

dual SUs all rebuilt with sm needles

stock fuel pump

 

 

electronic distributor

chevey alternator

 

aluminum radiator

 

everything is new

 

STATUS; the motor is at 20 degrees on the timing gun and i cant get it to tune it right , what am i doing wrong ??

my grandpa wants to take out the dual SUs and put a four barrel Holley but i want to keep the SUs so please some help before i end up going Holley .lol

 

old picture

 

i have the unisync gauge to help me tune my SUs but i need some guidance on the two top screws and the side screw on each carb and the bottom switch thing on the carb . how should i start and where should i turn all the screws correctly to get a good tune . also am i in the right timing using my gun??? im at 20 degrees on the timing gun.

 

 

thank you . i hope i was spacific

[ggzilla]

20 degrees at idle will be too far advanced at high RPM. Unless your distributor has been modified, 10 degrees is more realistic.

 

To start, you want to set the advance at 4800 RPM. Set it for 34 degrees initially. Then drive it and listen for detonation. Some L20B can run up to 38 degrees.

 

Or just set it 10 degrees BTDC at idle, then come back to this after you get the carbs sorted out.

[sam]

i forgot to mention . when the car is on i noticed that the carb closes to the fire wall stays cooler than the on near the radiator . i dont know if its because that one is near the heat or is that normal ? how do i know when both SUs are set and tuned right?

[sam]

20 degrees at idle will be too far advanced at high RPM. Unless your distributor has been modified, 10 degrees is more realistic.

 

To start, you want to set the advance at 4800 RPM. Set it for 34 degrees initially. Then drive it and listen for detonation. Some L20B can run up to 38 degrees.

 

Or just set it 10 degrees BTDC at idle, then come back to this after you get the carbs sorted out.

 

which one is the ADVANCE ? and how do you set it up to 48 ? set the timing at 34 degrees? im so sorry for so many questions but this is my first datun 510 and carb car, i always owned 280z fuel injected. but i want to learn and thats how i learn i was alot of questions and read read ... thank you and i really appeciate your help and patience

[sam]

BTDC means ?

[ggzilla]

The ADVANCE is what you set with a timing gun. If you don't know how to set the full advance, set it to 10 degrees before you start messing with the carburetors. Just change from 20 to 10, cappucino?

[Str8_69]

BTDC means ?

 

 

Buy The Damn Chilton... :lol: J/K man... Before Top Dead Center.

[Ratty260z]

Did you put oil in the carbs? The screws on top are only for keeping the oil in the suction dome you need 20 wt oil you can tune your carbs with different weight oil but that's getting into it kind of deep. The big screws on the bottom are your fuel mixture screws turning them down towards the ground richens them turning them up leans the mixture. The sm needles may be too rich for that motor I know guys with mild built l28's have issues with them being too rich but you should go for 2 and a half turns down initially then check your spark plugs to see if they are too lean or rich adjust again and repeat the process.

 

Also use the unisync to set the airflow for each carb you want the ball to float around the middle mark set the unisyn on one carb then adjust the other idle screw so that both carbs make the ball float in the middle line you may have to do it a few times to get your idle speed where you want it. You should check the sync after you reset the fuel mixture as well because it can change.

 

Good luck.

[wayno]

Here are some guides for tuning SU type sidedraft carbs. I copied this from another thread.

 

 

 

http://www.teglerize...bs/techtip6.htm

 

 

 

 

 

 

 

 

[ggzilla]

The 4-cylinder carburetors work slightly differently than the 6-cylinder Hitachis. The L20B runs leaner normally than a 240Z, but richer is better anyways for power and safety. After you get them running well, then you can proceed to lean them out.

 

As ratty said the first step is to check the oil level. Remove the oil cap Nut and check the oil level. Do not add oil if it is between the two lines:

 

 

The damper only holds 3cc of oil, so be careful when adding. Just a touch is all that is called for.

 

 

But maybe the first step should be to determine which carburetors you have. They look like HJE38 Hitachis. Do you know what they came from originally? Can you read the inked number on the side of the float housings?

 

[sam]

Did you put oil in the carbs? The screws on top are only for keeping the oil in the suction dome you need 20 wt oil you can tune your carbs with different weight oil but that's getting into it kind of deep. The big screws on the bottom are your fuel mixture screws turning them down towards the ground richens them turning them up leans the mixture. The sm needles may be too rich for that motor I know guys with mild built l28's have issues with them being too rich but you should go for 2 and a half turns down initially then check your spark plugs to see if they are too lean or rich adjust again and repeat the process.

 

Also use the unisync to set the airflow for each carb you want the ball to float around the middle mark set the unisyn on one carb then adjust the other idle screw so that both carbs make the ball float in the middle line you may have to do it a few times to get your idle speed where you want it. You should check the sync after you reset the fuel mixture as well because it can change.

 

Good luck.

 

 

Thanks I will try that. Sounds very easy lol

[sam]

The ADVANCE is what you set with a timing gun. If you don't know how to set the full advance, set it to 10 degrees before you start messing with the carburetors. Just change from 20 to 10, cappucino?

 

 

Ok so 10 degrees is the normal. But why did you say to put it at 34?

[banzai510(hainz)]

you prime the oil pump

 

olddatsuns.com tech section

[I'm BLUE]

Just built the engine you say? Cool! Clean ....

 

This may sound like a stupid question, but is the engine broken in yet? Compression has to be even ... one low or high cylinder and tuning can be a bische ... carb pro rebuilder taught me that a bit ago ... so what are the compression numbers? :)

 

Agreed with ggzilla and ratty!

 

Head shaved and mid cam.... love the engine bay! !! Looks great! !!

 

How much was the head shaved ?

 

Camshaft.specs ?

 

What position number is your cam shaft sprocket gear on?

 

20 Degrees ignition timing, with shaved head ,,, are you having pinging issues at all?

 

Pics of spark plugs? What do they tell you? :)

 

I run a Holley 4bbl 390cfm on my daily 240z with l28 ... i got tired of worn out Su's ... if you do go this route ... go to holleys website and follow instructions to t .... i love su's btw ;)

 

Run 91 octane ... not 87

 

You can shoot me naow lol

[sam]

Just built the engine you say? Cool! Clean ....

 

This may sound like a stupid question, but is the engine broken in yet? Compression has to be even ... one low or high cylinder and tuning can be a bische ... carb pro rebuilder taught me that a bit ago ... so what are the compression numbers? :)

 

Agreed with ggzilla and ratty!

 

Head shaved and mid cam.... love the engine bay! !! Looks great! !!

 

How much was the head shaved ?

 

Camshaft.specs ?

 

What position number is your cam shaft sprocket gear on?

 

20 Degrees ignition timing, with shaved head ,,, are you having pinging issues at all?

 

Pics of spark plugs? What do they tell you? :)

 

I run a Holley 4bbl 390cfm on my daily 240z with l28 ... i got tired of worn out Su's ... if you do go this route ... go to holleys website and follow instructions to t .... i love su's btw ;)

 

Run 91 octane ... not 87

 

You can shoot me naow lol

 

 

Thanks! I love that color

I don't remember the cam details :(

The head was shaved this much

 

 

 

What do you mean pinning issues.

Valves are adjusted intake. 10 and exhaust. 15

And of course 91 octane all the time.

I just finished put the timing at 34 now I have to finish tuning the carbs. Wish I had a wideband

[Ratty260z]

That seem's like it is shaved A LOT, Can't say I've seen a head shaved as much as too start cutting into the nissan stamp. What is your compression ratio? Did you add Cam tower shim's? If you didn't add cam tower shims your going to have a lot of slack in the timing chain which can cause issues.

 

Pinging is when the gas ignites at the wrong time and the wrong place usually happens because of bad gas, Too much compression ratio, Poor cylinder head design, etc. Pinging can destroy a good motor such as wearing down pistons even breaking a ringland.

 

my Buddy has a 510 with a built L20b with a mild cam and the head shaved about .040 thou I think? I'll have to check his spec's sometime the motor was built by the Previous owner in the 80's It has a W58 head with open chambers and overbore dished pistons so somewhere around the 9's for static compression maybe 8's Any more then that and pinging can become an issue.

[Sealik]

I would of thrown a good stock manifold/Weber or Hitachi carb on there.

Get the timing correct....work out the bugs...etc.... and break in the engine.

Read as much as you can about SUs.....and swap them back down the road

You really don't want to be running too lean or rich in the initial stages.

Yeah...a wide band would help

[ggzilla]

34 degree timing is at 4800 RPM.

 

At idle, set it to 10 degrees.

[sam]

So I ended up putting a Holley, tomorrow I will finish tuning it. Found it in my grandpa garage and I bought the rebuilt kit and did it my self :)

 

 

 

 

Total rebuild and cleaning liquid cost me $38! Sweet!!!!!

just put a adapter and it fired up. Tomorrow I will check the degrees and plugs and gap them and tune it, I can't wait

 

[ggzilla]

Exciting ... let us know how it goes.

[banzai510(hainz)]

Let us know how it runs as I dont like the 90 deg bend the air has to go thru (slows it down) thatswhy I like the sidedrafts as the air has less resisitance before gettin in the intake.

[Sealik]

Right on.....remember to check the plugs immediately after a run...not after idling

Should be a nice tan/grey color.

 

Size of that Holley?

[datzenmike]

500 or 600cfm by the looks... way too big for 2 liters not to mention it appears to be on a stock 2bbl manifold so that's 4 into two adapter? A 2bbl 300 cfm would have been big for this.

 

 

What casting number? (the letters are just to the right of this...

 

 

BTDC means ?

 

It means that that you really need to research engine, carburetor and ignition theory. Then ask questions. I know you have to start somewhere, but every question will get you 10 different answers. YOU need to know as much as possible and ask questions sparingly so you can reject the answers that don't apply or fit you question.

 

The stock carb would have been best to get this running and tuned up. Once running well you start swapping intakes and carbs. If it doesn't work you know for sure it's the carb and not the dizzy or high compression or cam or something else. The SUs would be the next best thing for carbs. Sized properly and infinitely tunable for your motor. 4bbl Holeys are for 5 liter V8s.

[I'm BLUE]

Looks cool! Nice work.! :)

 

If it were to be a 500-600 cgm that would indeed be too big ... :(

 

No secondary metering block .... more likely smaller .... 390cfm ? (What did the casting code tell you on the Holley carbon? :) )

 

A few things you need to know about your Holley!

 

Note: make absolute sure throttle shaft bushings are a-ok ... if not ... car will never tune properly .... baseplates can be purchased!

 

(1) adjust fuel level with engine running ( via sight plug holes, one in primary, one in secondary) to where fuel level is just below sight plug hole. ( if stuck or will not adjust, ,, it is sediment or a sunk float, ,,, brass floats are king)

 

(2) test powervalve ,,, by gently turning in idle port screws, ,, all the way, ,, if engine dies it is in good shape ... if not powe.r valve is bad or wrong one.

 

(3) 5-8ft pounds to tighten base bolts ... no more! !!!!!!!!

 

(4) fuel pressure optimum 5-8psi i believe? Holley.com to check for sure.... your mechanical fuel pump should be best for this application!

 

(5) install.clean air filter, ,, hookup vacuum gauge at base of garb, ,,, adjust idle port screws 1/8th turn at a time, ,,, evenly on both sides, ,,, until you reach highest vacuum on gauge. This id optimum.

 

(6) accelerator pump cam's can be swapped out ... google those key words ... very effective .... a box of colored cam gears that give your aceelerator pump different duration and size shot according to what you need ... takes less than 1 minute to change it on the car!

 

(7) measure your engine vacuum at full running warm, ,, than go to holley.com and see exactly which power valve you require ...especially since you have bigger head valves and a hotter cam ;)

 

(8) blue colored gaskets are best for tuning ... and very reusable!... others work just fine!

 

(9) jetting kits available, ,, if you plan to keep ... they are easy to swap .... use ngk spark plugs .... go from there

 

(10) once that is all done, ,, your secondary spring .. can be adjusted ... or swapped to heavier or lighter springs ... i highly recommend the quick swap kit for this procedure ... very easy with it!

 

(11) i run a 390cfm Holley on my l28 6 cylinder ... holleys official site is gold for diagnosing and tuning ... i see a lot of people make quick judgement ... yet half do not mention any of this stuff lol ... I'm not saying holley is best by any means, but for my stock engine it has served me fine when i took the time to do all of this stuff above!

[EricJB]

That 4bbl carb is 550 ish cfm. It's more than likely from a Ford truck or van from the mid 80's from the looks of the bowl vents. I have one that looks just like it on a 390ci ford truck. And yes it's WAY too big for an L20. You cant compare cfm #'s with different manifold design. V8 's have a plenum under the carb, the empty pocket. It acts as a storage unit for the air/fuel charge. Performance manifolds and spacers increase the volume of the plenum and "trick" the motor into thinking it has a bigger carb on it. (visualize tunnel ram here) That is why a 600 cfm carb works on a v8.

 

With an individual runner design you need more cfm because there is no plenum. I have personally run over 1100cfm on a 1968cc 4 cyl, but the 750cfm on my 460ci truck is about right for it. I agree with others, put a stock carb on it, make it run and then experiment from there. Is the motor broken in? Does it have a new cam? You need to break the cam in EMMEDIATALY upon start up. 2000-2500 for twenty minutes. This work hardens the cam and rockers together. And dont let it idle too much for the first 45 min. Retrofitting carbs on a motor, and trying to make it run right needs to take place long after the break in.

 

In your application, I would recommend a Weber dgv 2bbl. No offense, but it seems that sidedrafts may be beyond your tuning capabilities. Take the manifold off, bolt the adapter on, and with a die grinder and various burrs and sanding cones, make a plenum. While it's not as good as a performance manifold, they work pretty good. Good luck.