Jump to content
MapleNuts

L20b power loss/valve problems?

Recommended Posts

to check your distributor if properly set, turn crank by hand and watch for #1 cylinder at compression stroke, either have an assistant stick their finger on spark plug hole of number 1 or you (you will feel compressed air coming), and see if it lines up at TDC or Zero degrees of pulley.  Your two front cam lobes (take off valve cover)  should be at 10 and 2 oclock, my L18 dist rotor will point towards the front and rotor point will also towards the number one cylinder.  You can see this when you put the cap on and rotor is pointing at number 1 of spark plug wire.  Always start with base setting,  1) distributor is set properly, 2) points within range of gap/dwell, 3) carb is set base level and go from there.

Share this post


Link to post

This 'afterfire'.... is it extremely loud explosion? or POPs or just rumbly uneven exhaust sounds because there is an anti backfire valve on L20Bs. If not there or inoperative there will be some exhaust noise on deceleration.

Share this post


Link to post

at TDC the rotor is on 1st cyl but a little to the left of it, idk if it needs to be dead on it or not. And my timing mark is at +12. 

The plugs were black but i was like 2-2 1/2 turns out so it was rich but seemed to have more power. however cyl 1 plug was closer to tan.

the color progressively got worse from cyl 1 to cyl 4.

 

The after fire is mostly on deceleration, it pops, and if i was on the gas hard enough before hand there is an extremely loud explosion. followed by some smaller pops.

 

also i can turn my mixture screw in all the way and the car doesn't die at idle, but is obviously running extremely lean, idk if this means i have the right jets or not.

Share this post


Link to post

15% is not bad, but what cylinder? or all?

 

Do you have a straight through fart can or 'glass pack' or anything that can't really be called a muffler?

 

Has the 'pollution crap' been removed from your engine? Particularly the anti backfire valve?

Share this post


Link to post

Turn your idle mixture 1/2 cw and drive car around at least 5 minutes and pull over and check you plugs, careful of exhaust manifold, very hot.  If your spark plugs are still black, then turn the idle mixture CW another 1/2 turn and do the same and check your plugs.  Your RPM may go down a bit so you may have to adjust your idle RPM, around 800 or so. You should not run your plugs too black, it should be brownish/tan looking color.  If you keep on adjusting your idle mixture CW and then your plugs are whitish color then that's too lean.

Share this post


Link to post

%15 on all cylinders. I have the muffler side off so its just the downpipe with cat at the moment because i plan on straight piping it.

There is no anti backfire valve, my dumbass probably thought it was emissions junk and removed it when i first put the weber on.

also removed the EGR valve. 

I couldn't exactly tell where the A.B. valve hooks up when looking in the service manual. 

does it go to a fitting right by the PCV valve and then to the T in the smog pump hose?

Share this post


Link to post

On deceleration the fuel mix goes extra rich and tends to burn in the exhaust manifold and pipes. The AB valve senses high vacuum and opens to let air into the intake to lean it out.

 

Hey, if this is the cause of the noise, guess what? There is no problem. Put a muffler on it. Or put an AB valve back on.

Share this post


Link to post

Well it was fouling out my plugs with all the gas. cyl4 plug reeked of gas. 

I'm going to try to tune the carb the best i can and put the AB valve back on and see how it goes.

Is there 2 hoses coming from the intake to the AB valve?

a smaller vacuum one and larger one

then one to the carb

Share this post


Link to post

It needs filtered air so one hose from the air filter the other to the intake and a very small one to the fitting with the brake booster hose on it.

Share this post


Link to post

I don't have all the canister bullshit but when my 510 did this it was a worn out motor with 190k miles on it and assume the chain stretch and maybe a worn point lobes and bad gap on the points.

 

Sometimes I just drop another distributor( had 3 to 4 go bad since my ownerships of Datsun in the last 28years) in to make sure than all this fucking around. 99% of my DGVs  just bolt on and runs fine beside fucking with the mixture.

 

webers do wear out also but find the dist is more likely esp a point type as the bushing points and condenser will cause cause issues.

 

 

I canrt see how more gas is on number 4 unless the valve is NOT tight to cause the gas NOT to explode. or maybe the brake tube for the vaccum is doing something but really Im fucking just guessing here.As its usually on the 4 runner

Share this post


Link to post

so the anti-backfire valve did nothing. 

I did the carb tune like racerx said. Got it to run with a 1/4 turn out (the leanest it can go). but even with it that tune i have gunk all over my cyl4 plug, i cant tell if its oil, gas, carbon, or all of it at this point.

It still afterfires and its putting out a lot of smoke. 

Share this post


Link to post

Have u tried measuring ur vacuum at idle?

Share this post


Link to post

I just did off the intake manifold, it was around 20 at idle after revving the engine.

It dipped down to 15 or so at some points but that's because i don't think it liked the tune it was on, so i brought the idle up and did another half turn out on the mixture and it seemed better

Share this post


Link to post

So what color is the electrodes on the plugs now that you turned the idle mixture 3/4 out?

Share this post


Link to post

I didn't check the other plugs because I was running short on time. But cyl 4 is black and wet even at the leanest I could get it

Share this post


Link to post

U have to check all of them so that u can compare. The colors will not be exact but should be similar like brownish/tanish in color.

Share this post


Link to post

Forgot to mention to either install new ones or clean them really good with sand paper. Then drive car for a few and check plugs again.

Share this post


Link to post

Yeah I was planning on doing that, but time. They are new too. When I pulled them before the better tune cyl1 was tan. 2 was a lil black. 3 black. 4 way too back.

Share this post


Link to post

Either too rich still or issue with oil coming up. How many miles on engine? U also said your compression is 120, if u did this correctly then thats low. Was the compression all the same on all cylinders? Did u test when engine was at operating temp? Throttle wide open? And important did u use screw in type of tester not plug in type?

Share this post


Link to post

195xx miles. Was told it was rebuilt at some point when I bought it but who knows. 120 on all cyl dry. Did the screw in type. Held the throttle to the floor. Was at temp if I remember.

Share this post


Link to post

Any thought about running hotter plugs?

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

By using this site, you agree to our Terms of Use.