that-son Posted August 6, 2014 Report Share Posted August 6, 2014 hello everyone. got my '81 720 4x4 a couple of months ago and i've been having some fun fixing her up. will post pics soon as i figure out how. here's what i've done so far: 1. change oil, gear oil, radiator fluid and manual transmission fluid 2. replace brake master cylinder 3. replace clutch slave cylinder 4. replace spark plugs and wires 5. connect and replace all vacuum lines!!! 6. cleaned, filled and rustoleumed two rust spots 7. repaired charcoal canister diaphragm 8. patched several muffler holes with tiger? joint and crack patch (we'll see how that holds up) 9. set idle speed, fuel air mix and timing and i think that's about it. i need to pass to smog so i'm focusing on emissions at this point. can't seem to figure out why it's still backfiring. just got a handheld tach to set the idle speed then used a vacuum gauge to set the fuel/air mix. timing was at 5 deg. btdc. i did notice that the backfiring happened after i patched the muffler. would a patched up muffler pass by the way? also my speedometer just spins. is this just a cable or do i need a new gauge? thanks for reading and thanks in advance for the help! Quote Link to comment
wayno Posted August 6, 2014 Report Share Posted August 6, 2014 What does this mean below? "my speedometer just spins" The needle spins around in circles? 1 Quote Link to comment
datzenmike Posted August 6, 2014 Report Share Posted August 6, 2014 hello everyone. got my '81 720 4x4 a couple of months ago and i've been having some fun fixing her up. will post pics soon as i figure out how. and i think that's about it. i need to pass to smog so i'm focusing on emissions at this point. can't seem to figure out why it's still backfiring. just got a handheld tach to set the idle speed then used a vacuum gauge to set the fuel/air mix. timing was at 5 deg. btdc. i did notice that the backfiring happened after i patched the muffler. would a patched up muffler pass by the way? also my speedometer just spins. is this just a cable or do i need a new gauge? thanks for reading and thanks in advance for the help! Posting photos... http://community.ratsun.net/topic/691-how-to-post-pictures-and-keep-online-photo-albums/ Backfire through the exhaust when you let off the gas and coast??? or the carb??? Actually there is a thing called an anti backfire valve on the 720. It senses a too high vacuum in the intake and opens to allow outside air in to keep it at a preset value. But first try adjusting your idle mixture. It's the only fuel flowing through the carb when decelerating, and you made some adjustments. Try turning the idle mixture screw in until idle quality drops, then out until idle quality drops. Pick a spot about half way between these two points where the RPMs are the highest and smoothest. Turn the idle speed down to 750-850 and repeat until idle speed and quality cannot be improved on. See if that clears it up first. 1 Quote Link to comment
that-son Posted August 8, 2014 Author Report Share Posted August 8, 2014 hey! thanks for the replying. yes, the broken needle just spins around and around... lol. the backfire sounds like it's coming from the rear. and it's not a loud bang. it's more like puppuppup puppup puppuppuppuppup. and it only happens between 2000-3000 rpm. i took it on the freeway tonight and when i got back home it seems to have cleared up. even took my neighbor who is a retired mechanic for a ride and he thought it sounded good. no farting sounds whatsoever from the exhaust. think maybe it's because it's fully warmed up? i do have an ab valve that i got from rockauto. they list it as a diverter valve. but it's the same nissan ab valve part number. i must have installed over 20' of vac lines but have yet to check all smog components. some of them required the hoses to be connected which i just recently finished doing. here's how set the idle and air/fuel ratio... 1. check timing... 5 deg btdc 2. set idle speed to 800 using actron tach. 3. set air/fuel using a vacuum gauge. the highest vac. reads 20 in.hg and i'm between 1k-2k above sea level. at first i turned the air fuel screw so the engine is starving for gas. then i gradually turned it up until i got the highest vac reading. however, i read somewhere that that could give you a somewhat rich mix so they advice to turn it between a 16th to a quarter turn back. i turned it back a full quarter today before going on the freeway and it was still backfiring. then it cleared up when i got back home. i guess i'll find out tomorrow when i drive it somewhat cold. the strange thing is... i let the truck idle for 5 minutes, then i drove it around the neighborhood in 2nd and 3rd before messing with the idle speed and air fuel mix. so it wasn't completely cold. will find out tomorrow i guess. thanks again guys! Quote Link to comment
datzenmike Posted August 8, 2014 Report Share Posted August 8, 2014 Yes, all adjustments should be on a thoroughly warm engine. Take top off the air filter and check that the choke plate is fully open when engine is warm. Quote Link to comment
that-son Posted August 10, 2014 Author Report Share Posted August 10, 2014 yep. choke plate opens when warm. so here's an update. today, i checked the timing again and lo and behold! it was not set to 5deg btdc. it was at 15deg.! btdc. fixed that, then readjusted the idle speed and air/fuel mix. it's no longer backfiring bet. 2k-3k rpm. but still backfires bet. 1k-2k. i wonder if my head pipe, egr pipe, and muffler leaks have anything to do with it? tomorrow i will check/re-check all the smog components and vac. lines and see what happens. btw, can the tach. in the cabin tell you if you have any vacumm leaks? Quote Link to comment
720inOlyWa Posted August 10, 2014 Report Share Posted August 10, 2014 Certainly could. I cleaned up some f those problems by replacing the flange gasket, muffler, and tailpipe. muffler baffles make a real difference, of course. Never go to Midas, BTW... Quote Link to comment
datzenmike Posted August 10, 2014 Report Share Posted August 10, 2014 Muffler leaks won't prevent a backfire it will make them quieter and less noticeable. Strange that the timing was so far off. Take the cap off and suck on the vacuum line to the distributor vacuum advance. The rotor should turn clockwise and hold till you release it. Make sure it returns all the way back. It may have stuck advanced when you checked. There is also the chance that the mechanical advance is or was stuck also. Quote Link to comment
that-son Posted August 12, 2014 Author Report Share Posted August 12, 2014 lol... don't like the midas in my town either. i've seen stranger things like... ball bearings inside that vacuum lines and vac lines that go around and come back to where it started so... anyway i think i figured out why it was still backfiring. yesterday, i decided to check all smog components and lo and behold! i forgot to tighten the air cleaner and the hose clamps. major face palm. it's not backfiring now no matter how hard i try. i let go of the gas and downshift but... nothing. i'm getting a new muffler and headpipe on wed. but i think i might need a new thermal vacuum valve and egr. oh, and i haven't check the air injection system yet cause i can't find the darned cac, smog pump and check valves and whatever else is part of the system. i did do that test you suggested, mike and it was good. i really like how she's running now even though the idle speed keeps changing. set it at 800 yesterday and today it's down to 600. maybe it's because i sprayed some pb blaster on all linkages and springs. even the accelerator feels silky smooth! btw, any tips on the head pipe and passing smog? thanks again for all the replies and advice! Quote Link to comment
that-son Posted August 12, 2014 Author Report Share Posted August 12, 2014 also... i have a cracked egr pipe. i tried extreme steel but it turned to dust almost immediately. of course i let cure almost 48 hrs. now i've got a strip of aluminum wrapped around it and secured with a hose clamp!!! seems to be holding up better than the hi tmep extreme steel. Quote Link to comment
JoeCool Posted August 13, 2014 Report Share Posted August 13, 2014 Looks like everyone else answered your questions, so all I can say is WELCOME ABOARD! 1 Quote Link to comment
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