L20B backfiring

[moparvwfreak]

tonight on my way home, about 5 miles from home and the truck (73 620, L20B 78 intake with 73 L16 exhaust weber carb,EI w\HEI) backfired while coasting in 4th. odd but no real biggy. happenes every once in a while. But when i got closer to home, it backfired 3 times, all through the exhaust, and almost stalled turning a corner. it acted like it was out of fuel, just like at Golden Gardens. i will be checking valve adjustment, timing, compression test and swapping fuel pump and carb tomorrow as well just to make sure those aren't an issue. But I was wondering if anyone else had ideas?

 

i do have one exhaust leak but its been there since i installed the L20 in my truck in December-ish. #1 cyl at teh manifold. i think the gasket slipped while i was bolting it up but meh... this motor is gettting swapped for a low mileage L20b so its just to keep it going till i can sort out a few unknowns on the new motor.

 

all timing parts are new, O.S.K. made in Japan parts. dizzy is off my 78 and a NEW HEI unit installed in place of the factory remote ignitor system. and all new plugs wires cap and rotor when i bought my 78 last april.

 

 

 

so with those factors in mind, ideas would be great!!! thanks guys!

[hogboy52]

That sounds like a clogged low speed jet.

[datzenmike]

An exhaust backfire on deceleration is simply unburned fuel and air that 'lights off' inside the exhaust pipe. This can happen from an interrupted ignition (like a loose wire on coil or bad dizzy ground) where there is briefly no spark and the fuel and air is pushed through the motor. When the ignition comes back on it lights the fuel in the pipe.

 

The carb could be flooding and spilling raw gas into the cylinders which finds it's way quickly to the exhaust pipe.

 

Idle speed too high or idle mixture too rich. Remember on deceleration the carb is at idle position.

[hessianben]

I had an experience upon the slow death of my l16, where it began to backfire and bog, until it blew a giant backfire and wouldnt' start again. to this day i haven't figured out what caused it- the headgasket looked intact. I changed the hg and timing set, and it fired right up! is it possible to jump a tooth?

[datzenmike]

Hainz asked this too... Never seen a confirmed example. If the tensioner and slack side guide were removed... maybe. They are almost always installed wrong to begin with.

[banzai510(hainz)]

most time to me its been worn out ignition that has been a killer in power(backfiring)

worn dist shaft bushing, (if points) worn out points or bushings(side to side play in dizzy shaft) condensor or condensor grounding.

 

if motor looses a cylinder its a lash pad fell off. adjust all the lash .

 

 

I havenet changed a distributor cap/rotor in over 5 years on all my datsuns. Most have been worn out dizzys!

[datzenmike]

I did the same. Didn't change the cap for years and years until it wouldn't start. The button wore down fell out. Shoved the wire in further to get home.

[moparvwfreak]

Checked the valves and ALL the exhaust valves were out of adjustment. Choke was disconnected, missing an E clip, and the dizzy isn't wobbling. So now I am kinda lost. Why would the exhaust valves only be loose

[Logical1]

My L18 with dual SU's, Matchbox & a MSD box just started tailpipe backfiring like crazy when I let it decel in gear. However I have noticed NO change in the actual engine running or power. I leaned out my SU's a little and it still does it. I dont have a air/fuel gauge but when I stomp the crap out of it sitting in neutral I don't get any black smoke from being to rich. Its weird. I sound like a damn harley when I decel in gear! I am going to do all the same check on mine ASAP. I though it might of been a slipped timing chain as well, so I'm glad to hear this is highly unlikely.

[banzai510(hainz)]

datsunmike wrote;I did the same. Didn't change the cap for years and years until it wouldn't start.

 

I still hoever ck the cap and maybe sand the contactks or rotor.

 

For the valve out of adjustment the valve sinks into the head. then one has to loosen the rocker arm and increase the clearance. Till one day the rocker arm post is turned in all the way and you cant adjust anymore.

 

The choke disconnected????? If eleltric choke you need it hooked up or the flap will always cover the intake and you wont get air in the higher RPMs. One way is to loosen the 2/3 screws and turn it to hold the flap open(if that what you want)

 

 

Logical.

Dial motor up to TDC and ck the timming cam marks when crank is at Zero. If not out of time. maybe try anoother Dizzy (spare). but I bet its more likeworn out SU carbs sucking in gas. Does this diesel when you shut it off?

Su want like 12-14BTDC on elelctrical timming

[moparvwfreak]

datsunmike wrote;I did the same. Didn't change the cap for years and years until it wouldn't start.

 

I still hoever ck the cap and maybe sand the contactks or rotor.

 

For the valve out of adjustment the valve sinks into the head. then one has to loosen the rocker arm and increase the clearance. Till one day the rocker arm post is turned in all the way and you cant adjust anymore.

 

The choke disconnected????? If eleltric choke you need it hooked up or the flap will always cover the intake and you wont get air in the higher RPMs. One way is to loosen the 2/3 screws and turn it to hold the flap open(if that what you want)

 

 

was planning on it. the cap looked ok when i popped it off, a little crap but not much. i will pop it off again tomorrow while i am at my buddys place and clean it up a touch. and run a comp check since my remote start disappeared AGAIN.

 

its missing an E-clip. i will have to pick one up. it WAS electric choke that some idiot put a manual "kit" on. its never worked right. i want my other Weber back on it. but i need to fix it first.

[banzai510(hainz)]

it WAS electric choke that some idiot put a manual "kit" on

 

I have had one of these on my car .they are not bad.

 

My proplem once ai adjusted a eleltric choke. I could never get it quite right. It start up fine but eiehter stay on too long or not enough. So I said fuck it and put one of those cheezzy cap wire pull cable kits. it worked good enough till I found another weber. Most time I just pump the gas a few times and hold /pump the pedal till it warms up.

[moparvwfreak]

it WAS electric choke that some idiot put a manual "kit" on

 

I have had one of these on my car .they are not bad.

 

My proplem once ai adjusted a eleltric choke. I could never get it quite right. It start up fine but eiehter stay on too long or not enough. So I said fuck it and put one of those cheezzy cap wire pull cable kits. it worked good enough till I found another weber. Most time I just pump the gas a few times and hold /pump the pedal till it warms up.

 

 

i have never had good luck with them. have one on my dart, never got it working at all. now this one is just a pain. i am just gonna swap back to my old manual choke weber. it worked like butter. i just had an issue with the throttle shafts being loose.

[racerx]

If ignition system checks out, check your carb (air/fuel mixture), can be vacuum leak...combinationw ith exhaust leak as mentioned can add to the problem, yeh, check your timing too. good luck..

[Logical1]

Checked crank timing, looked good. Swapped plugs, although they looked clean / little lean. Swapped the distro cap, made all the difference! Its about 60% better I would say. If I REALLY rev it up and let it decel in gear downhill I still get some backfiring. Checked my vacum advance and it worked. Next is pulling the valve cover and checking cam timing and valves. Thanks Hainz! Btw Watched your Timing Vid, Great info and dig the outfit :cool:

[moparvwfreak]

If ignition system checks out, check your carb (air/fuel mixture), can be vacuum leak...combinationw ith exhaust leak as mentioned can add to the problem, yeh, check your timing too. good luck..

 

 

the exhaust leak has been there since i put the motor in. and this was a REAL recent occurence. it pulled the same thing at Golden Gardens but cleared up with more gas. so i am lost other than the valves needing an adjustment.

[banzai510(hainz)]

moparfreak

Sometimes a exhausr leak is a blown out lower intake exhaust gasket. or loose intake bolts.

 

Im haveing a proplem with my car the 1st 2 lower bolts alsway get loose.( I smell the exhaust) I tighten them then Im good for a while. Maybe threads are wearing loose since I had that intake on/off so many times.

 

Logical

Once you know for sure the timming is correct(mechanical/electrical) and you believe the dizzy is good(uasually a point dizzy will be suspect) then look at the carb side.

[hang_510]

Im haveing a proplem with my car the 1st 2 lower bolts alsway get loose.( I smell the exhaust) I tighten them then Im good for a while.

studs or bolts?

 

i changed to bolts on the lower, w/lock washers.

[Logical1]

Checked my mechanical timing, When the cam marks are lined up I'm at about 10BTDC:

 

 

So when the crank is dead on 0 The cam gear is just to the right (drivers side) of the mark. My books tells me this is ok. I tried avancing my retarding my timing a bit, it ran smooth but had less pick-up & power. Tried advancing it a bit and she peps right back up. Although idles just a tad rougher. Got called into work before I was able to check my valves, Thats next on the list. However I'm thinking its my carbs at this point. I have been running straight velocity stacks with no air cleaners so I'm sure there is some funk in my SU's....

[moparvwfreak]

moparfreak

Sometimes a exhausr leak is a blown out lower intake exhaust gasket. or loose intake bolts.

 

Im haveing a proplem with my car the 1st 2 lower bolts alsway get loose.( I smell the exhaust) I tighten them then Im good for a while. Maybe threads are wearing loose since I had that intake on/off so many times.

 

Logical

Once you know for sure the timming is correct(mechanical/electrical) and you believe the dizzy is good(uasually a point dizzy will be suspect) then look at the carb side.

 

#1 cyl exhaust port. blown out gasket. from the first fir up. i know right where it is. and i am not happy about it but untill i fix the motor i am not gonna worry about it.

[moparvwfreak]

ok, heres a new twist.... had my heat, wipers, radio, headlights and fogs on and it pulled the same thing. backfire and hesitation like it was out of gas. yet driving home i left off the fogs, and wipers and it ran like a champ. could it be i am pulling WAY to much power and its cuttin out the ignition system???

[datzenmike]

Yes. I had an old VW and the gen. quit miles from home. (back roads) I drove it until it started running really rough. Shut the headlights off and run with just the parking lights... it ran better. When it started to quit I switched to 4 way flashers only and made it home. So yes if the battery is low or you don't have enough juice it will run like shit.

[moparvwfreak]

ok then. sound like the alts on its last legs. time to throw on my KA alt i have layin around.

[datzenmike]

Let me know how that goes. I swapped an '80 200sx 50 amp IR alt into my 710 and though I think I wired it correct, the red light is on and I checked... no charge. I guess it could have been bad.

 

I have an 80/90 amp '89 240sx KA alt that I swapped the pulley onto. Had to go to a junk yard to get the two pin connector for it so I can wire it into the 710 harness. The mount may need mods??? Damn I want this to work.

[moparvwfreak]

Will do. I'll take peeksures when I do it. I have later mounts I can use plus I think the wiring diagrams for going IR are on here somewhere.