timing, backfire, and 93 octane

[Braden]

ok so i have a 77 620 with l20b, open chamber head, stock cam, fresh carb, no smog,, new plugs wires, cap, rotor and points, and new exhaust,my question is i had been running 87 octane, i stared running 93 and it runs like shit, i have adjusted the idle, choke and mixture correctly, and i checked the timing today and found it to be at like 5 degrees before top dead center?!?!?!?!?!?!?!?!?! so i am getting ready to try and adjust the timing to about 12 degrees, i know some of you run 15 degrees? why do you run 15 degrees and could my timing problem and 93 octane be whats causing my truck to run like total shit? it has no power when i try and take off from a dead stop, (sometimes it quits when i try and pull out and ill be in the middle of the highway)and it sputters when i romp on it, it also backfires badly even though the mix is a little lean (2 turns out on the mix screw) ps, it backfired and sputtered when i was running 87

[datzenmike]

Check the plug wires are in this order, don't assume...1342. Mismatched wires can still run, but poor acceleration and sometimes backfiring.

 

Check your valve lash. A tight valve will not close properly.

 

Timing should be at about 12 degrees at idle with the vacuum hose connected. There is no vacuum advance at idle so all you have is the static 12 degrees. After setting the timing leave the timing light on and increase the rpm. The timing should immediately jump up as vacuum is applied just above idle. If not, remove the hose from the carb end and suck on it. Observe that the rotor on the dizzy turns clockwise slightly and holds there until you release the vacuum. If not, there is a vacuum leak in the hose or the vacuum advance on the dizzy. Poor or no vacuum advance will cause poor throttle response at low speeds.

 

Octane level has little effect on a stock motor.

 

Is this a fresh (as in new) Hitachi or Weber? If Hitachi, the idle mixture has no effect on running mixture. Engine off, look down carb and work throttle. You should see a strong squirt of fuel from the accelerator pump into the primary barrel. No, or wak accelerator pump can cause a sudden lean condition off idle and can cause poor throttle response and possible backfire.

 

A new carb may have a blocked primary jet causing a very lean mixture and resultant backfiring. An incorrect jet size can also do this.

 

 

Pull some plugs and look at them... if they are bright white, this is an indication or running very lean.

Edited December 15, 2009 by datzenmike

[Braden]

plug wires are in order, carb is a fresh rebuild, jets are stock jets, accelarator pump is good, i have gone through everything except timing, coil, and valves, the valves are adjusted from the factory as this is stock motor with 102k on it

[datsunaholic]

Running 93 in a stock L20B is just draining your wallet faster. The lower compression of a 1970s smog engine doesn't make running higher octane in any way desireable. At best it would be a band-aid for a highly carbonned-up engine (since the lower octane fuel would detonate faster due to the buildup). Running higher octane when not needed LOWERS power and LOWERS fuel efficiency.

 

So running 93 is just a way to spend money faster. If you've got the extra cash, great. I'd stick with the 87.

 

As for the backfiring, I don't remember what you're running for ignition. My current '76 had a backfire that got worse and worse quickly over a period of a couple weeks, and turned out to be a plain worn-out points distributor (shaft wobble) that ate the points cam rider and was firing the coil randomly. Swapped distributors and problem disappeared entirely.

[banzai510(hainz)]

what datsunaholic said!!!!!!!! possible worn distributor.

Happen to me also with my 510 I thought it was the carb.

 

PS if you chainged to a High out put EI coil it could fry the point s faster. but hopefully you still have the stock coil and ballast in there.

 

condensor???????

[datzenmike]

Free dizzy!!!

 

http://forum.ratsun.net/showthread.php?18077-Free-L16-carbs-distributors