SK racing carbs

[Ladashnikov]

Hi, my name is Hubert and I'm from Poland.

I do not own any model of "Z" unfortunately (but I've seen a 240Z in my life and I surely won't forget how it looked, sounded and speeded down the racetrack), but I have two SK 40 DCOE Racing Carburetors mounted on my Vaz Lada 2107 car

I don't know if I'm posting my message in a good place (if not, then please redirect me to where I can get suitable information).

Engine specs of my Lada are:

4cyl OHC 1.3L shortrodded to 1578cc.

High compression (though not measured)

Sporty camshaft 316/304 (valve timing set up according to tuner's info)

Harder valve springs.

Electric fuel pump 0.25bar 65L/h

4-1 freeflow exhaust designed and made to get use of 316/304 cam.

Engine starts up ok, idles somehow good (engine shakes a bit, yet has constant revs).

Real problems start when I rapidly open the throttle... takes more than a second to rev up and when engine finally revs it blows a cloud of black smoke through the exhaust. When I open the throttle gently it's all ok.

Another bigger problem is at high revs... I don't even reach them, because no matter how wide throttle is opened engine won't rev more than 4500rpm... only roars louder through the air horns.

I somehow tuned it "by ear", but I know it's not the way it should be... and it didn't solved problems mentioned above. I even don't have any manual on how to tune it and link in this thread doesn't work

Below there's a video I made to somehow illustrate my problem.

Please don't mind clanging of the valve levers... Engine runs the same even when gaps are precisely set up.

Some people told me, the ignition timing is off and vacuum ignition accelerator is connected to wrong place; that fuel pump delivers too little fuel for high revs; that carbs are dirty and/or damaged; that jets, choke sleeves and other replaceable stuff doesn't match the engine... all this just makes me crazy, cause I don't know how to diagnose the real problem sources and how to deal with them. Visiting other forums and sites clears my thoughts on how a sidedraft racecarb should work, but brings even more useless information (or no information at all) on how to deal with certain problems.

Your forum looks like the only one, that has competent users... not wizards that even don't know there are such carbs, like SK/OER.

Ok, here's the video

I'd really appreciate any help... and any way to obtain certain parts that could match my carbs to my engine.

Cheers .

Hubert.

[240260280z]

Welcome Hubert!

What are the internal component sizes (jets, choke (venturi), air correctors, emulsion tube)?

What is your fuel level?

Do you have a way of balancing air flow?

Read this to get up to speed on how side drafts work: http://www.classiczcars.com/forums/carburetors-s30/44095-understanding-weber-side-draft-carburetor-through-fictional-supposition.html

[Ladashnikov]

Oh, that explained to me a lot more, than other sources, that were way bigger. Thanks, now I know how things are named

I don't know how to measure the fuel level... If someone would tell me what distance should be from the top cover (not the jet cover) to the fuel level, than I would set it to 29mm as it was earlier posted in this thread.

Err what's balancing air flow? You mean synchronising carbs using vacuum meter? I don't have a VM, but I will buy myself one soon.

I took off the jet cover and unscrewed them.

Accelerator jets are numbered "40".

I somehow measured the venturis, but it's a +\- 1mm tolerance, cause carbs were still mounted to the engine and there was no way to measure it with a caliper.

All the elements have the same symbols, none of them is different, but right carb had emulsion tubes already unscrewed and one jet helder was in the float chamber (!), while the second one was angled and the screwthread on the jet block was somehow damaged, but I repaired that.

I didn't run the engine after I bolted back stuff togetger, because of what I found during camshaft gap regulation...

It terrified me... Those were new parts and the car ran only 11km after engine rebuild.

[Ladashnikov]

Other thing is I don't know If I should link my ignition vacuum advancer before or after the throttle? There is a small pipe near the progression holes' screw, but I don't know if it's for regulation or for the ignition vacuum advance purposes?

Oh and I found that those accelerator jets are squirting fuel like mad... even when slow throttle opening. I have to almost unscrew the pump spring screw completely for the engine to not overfuel it.

I also didn't unscrew the Back-Bleed/Spill Jet, because I didn't take the whole carb apart... I'm afraid I'll loose some kind of small yet vital part and I'll make the carb useless.

Edited May 9, 2014 by Ladashnikov

[LeonV]

Cool car! Your camshaft, however, is toast and needs to be replaced before doing anything else...

[240260280z]

The carb internals seem ok for 1.6 litre. The 50F8 idle jet could be made a bit leaner 50F11 if it runs rich at cruise.

29mm fuel height is a good initial target. The 115-F11-150 should also be ok for a starting point.

The 32mm main choke could be dropped to 30 or even 28 to give you more low end.

Before tuning the carb you need the cam and rocker sorted as well as measure timing.

Here is a great page that goes over tuning webers: Weber DCOE triples, L28s, and Datsun Zcars

Edited May 10, 2014 by Blue

[240260280z]

Here is a Lada racing build with delorto side drafts that may have useful info. Here is a carb bage from the thread: http://forums.finalgear.com/post-your-car/my-lada-2107-racecar-project-5986/page-15/#post856061

Edited May 10, 2014 by Blue

[Ladashnikov]

Cool car! Your camshaft, however, is toast and needs to be replaced before doing anything else...

Thanks, I also think my car is cool I'll fit the old stock camshaft, rockers and springs... then I'll think about buying myself another 316/304. Harder springs had almost no clearance between coils when valve fully pressed (on cold engine after gap regulation), so I think that was the main reason why cam got grinded.

Hey Blue... amount of Your knowledge is just astonishing. Almost instant replies with just all the valuable information! If I'd live near You I'd buy You a crate of realy good beer for that info!

[240260280z]

If I'm in Poland I'll look you up! I have many Polish friends here in Nova Scotia so we should get on well. Keep throwing out questions and all of us on the forum will do our best to help.

[Reverend]

Haha, a Lada enthusiast in Z site Those little boxy cars show up here now and then. They are the last remains of Communist Soviet Russia..

Speaking about carbs. I've done pretty well with my setup but i wonder could i accomplish a little more in the high rpms by tinkering with jets? I have F11 emulsion tubes, 115 main jets and F9 idle jets... Suggestions? Cant remember the air correctors.

Edited May 13, 2014 by Reverend

[240260280z]

• Further, it can be taken that the main affects the total range while the air corrector has more and more say as the revs rise, though at no time is its effect negligible.

• In general, it can be said that large main jets with large air jets give a setting rich in the lower and middle range, while small mains with small air jets achieve the opposite.

• A very large pump jet or high-speed bleed can enrichen the top-end mixture to such an extent that a larger air corrector may be needed in order to prevent over-richness. This may, of course, in certain contexts achieve the desired mixture slope but, on the whole, large pump jets or opened-out high-speed bleeds making up for the main system weakening off too much in the top range cannot be recommended.

• Using the same venturi in a 40 vs. 45, the pressure experienced by the charge will increase more in the larger 45 after passing through the venturi and promote more precipitation of fuel from the charge.

From the great Passini

So you can have many combinations of main and air that will work for you but having the larger orifice combos are not target to aim for. Likewise you can choke a 40mm and 45mm to the same 30mm venturi but when the fuel/air charge exits the 30mm venturi, it experiences a bigger pressure increase in the bigger 45mm carb throat than in the 40mm carb throat that can cause more fuel to precipitate.

I have been studying carbs for the past few years and here is where my tuning process has currently evolved to:

Make sure valves, vacuum/vacuum leaks, fuel pressure, electrical, rings, sealing gaskets, etc. are all in good order. Ensure all carbs are the same with same internal components and are in good order.

Step A: SET Timing

1. set ~ 15 to 20 at idle
   
2. set ~ 34-40 at max
   
3. 
   

Step B: Synchronize Mechanical Linkage

1. inspect and remove all slop
   
2. ensure throttle valves open fully and that accelerator pedal stop is adjusted correctly
   
3. set all idle stops for same air flow at idle (800-1000rpm)
   
4. set 3000rpm air flow to be the same
   

Step C: Disable Cold start system (choke) assembly

1. ensure cold start plungers are closed and sealing.
   
2. disable choke cable mechanical
   

Step D: SET Progression and Fuel Bowl Level

1. get an a/f wideband O2 meter
   
2. remove main jet assemblies and close off jet to accelerator pump (at bottom of fuel bowl. A metric bolt will do it)
   
3. balance all carb's idle screw mixtures and air flow to get the motor to run evenly
   
4. note mechanically where the throttle valves and foot pedal are just exposing the progression holes (cruise)
   
5. try to drive around at this "cruise" point in different gears and note the a/f ratio in each gear
   
6. adjust the fuel level up if you are running lean at cruise (aim for ~ 14-15 at cruise)
   
7. adjust fuel level down if you are running rich at cruise (aim for ~ 14-15 at cruise)
   
8. repeat 2 to 7 above until you are happy (check air flow balance and timing). NOTE If you can't get the A/F right at nominal fuel levels then change the idle jets to leaner or richer and repeat steps 3 to 8
   

Step E: SET Idle

1. this is partially done in step 8 above
   
2. adjust idle enrichment screw to where you want the idle to be. Most find richer idles ~ 12.5 to 13.5 to be a sweet spot but it can be stinky. If 13.5 to 14.5 works for you then that may be better.
   
3. re-check A/F at cruise to ensure the idle progression has not impacted the cruise a/f too much. If it has then change the idle jet accordingly and repeat all of the above.
   

Step F: SET Mains

1. reinstall main jets
   
2. make runs on a long gentle incline in all gears from 3000 rpm to 5000 rpm recording the a/f mixture at each 1000 rpm increment
   
3. change main jet as required to ensure correct richness between 3000-5000 rpm Target would be 12.5 to 13.5
   
4. Once the main jet is selected, repeat step 2 from 3000 to 6500 and note the a/f ratio at the higher end. Change main air corrector accordingly if it starts to go too rich or too lean as you approach 6500. Same target of 12.5 to 13.5 should be aimed for
   
5. Recheck cruise to ensure it is not too rich. The fuel height affects were the mains come in. If the fuel is too high, the mains will be in at cruise and richen the mixture. If the fuel height is too low then the mains will come in too late and you will get a lean spot in the transition.
   
6. If you can not get an appropriate a/f at cruise (mains in early and too rich) then go back to step D above and lower the fuel level, change the idle jets and repeat the all the fun
   

Step F: SET Accelerator Pumps

1. reinstall accelerator jet at bottom of fuel bowl
   
2. make runs on a long gentle incline and punch the gas to accelerate violently and note a/F ratio spikes when you hit the gas and how long it takes to stabilize
   
3. repeat in all gears
   
4. adjust accelerator pump jet size and fuel bowl back bleed to optimize. starting at lower accelerator jet size is best.
   
5. if a lean spot from cruise to rapid WOT can not be resolved with the accelerator pump circuit then raise the fuel level and go back to the drawing board ... STEP D
   

Have Fun. It is actually fun and iterative. A good co-pilot reading the A/F and recording in the passenger seat is a must.

Edited May 13, 2014 by Blue

[mildsquare]

I'll get a video of her running. still dialing her in.