theboraxkid Posted December 9, 2014 Report Share Posted December 9, 2014 I need my Datsun to pass California smog now. It seems to run a little rich, and the P.O. rebuilt the carb so I'm sure they didn't get the mixture right. How do I adjust it properly? Also, my clutch slave cylinder is fixed, thanks for all you help! One more thing though, my truck misfires like crazy until it warms up, and then every time I make a right turn. Can anyone help determine the cause? Thank you! Quote Link to comment
theboraxkid Posted December 10, 2014 Author Report Share Posted December 10, 2014 Update: I found out my carb is a Hitachi, and the choke, cutoff valve, and some kind of vacuum thing on the side of the carb were all broken. I tried to fix them, now it backfires and runs even worse. Also, there's a weird dangling lever on the back that appears to go to nothing. Am I best off with a new carb? Quote Link to comment
Charlie69 Posted December 10, 2014 Report Share Posted December 10, 2014 I would search the wrecking yards in your area for a truck the same year as yours. Might get lucky and find a decent carb cheap. Quote Link to comment
theboraxkid Posted December 10, 2014 Author Report Share Posted December 10, 2014 I was at a wrecking yard today with just that idea, all the 720s I see are totally picked over, most don't even have engines in them. Does anyone sell replacement parts for these carbs? Quote Link to comment
datzenmike Posted December 10, 2014 Report Share Posted December 10, 2014 I need my Datsun to pass California smog now. It seems to run a little rich, and the P.O. rebuilt the carb so I'm sure they didn't get the mixture right. How do I adjust it properly? Also, my clutch slave cylinder is fixed, thanks for all you help! One more thing though, my truck misfires like crazy until it warms up, and then every time I make a right turn. Can anyone help determine the cause? Thank you! Float level set way too low. Gas should be at the dot on the round glass on the front. Update: I found out my carb is a Hitachi, and the choke, cutoff valve, and some kind of vacuum thing on the side of the carb were all broken. I tried to fix them, now it backfires and runs even worse. Also, there's a weird dangling lever on the back that appears to go to nothing. Am I best off with a new carb? If that bad probably a new replacement... but they are hundreds of $$ Quote Link to comment
that-son Posted December 10, 2014 Report Share Posted December 10, 2014 check out nationalcarburetors.com i think it's $158 for a rebuilt with a lifetime warranty. very reasonable i think. might even get a carb from them in the future... have you checked all vac lines and smog components? there is an anti-backfire valve but mine passed smog even with it being defective... Quote Link to comment
theboraxkid Posted December 10, 2014 Author Report Share Posted December 10, 2014 Okay, so all my vacuum lines are in order, nothing is disconnected, still backfires and won't rev. I'm just gonna put it back how it was, it was at least driveable that way. How can I adjust the float level to stop the misfiring? It does seem low... Quote Link to comment
datzenmike Posted December 10, 2014 Report Share Posted December 10, 2014 Float level. It was just a possibility. Low level will slosh over to the left side on a right turn and uncover the primary jet. If when idling the fuel level is at or close to the dot on the front glass then float level is fine.Wires. Back firing can be from a spark jumping to the wrong wire. Check the plug wires go to the correct plugs. (it's very easy to get them mixed up so check) Costs nothing and could solve your problem and save hours of needless looking elsewhere. Inspect cap and rotors for wear or cracks. There are 8 plugs jammed on that distributor, all very close to each other. They must be in good condition. Check, clean and gap the plugs too. A stock cap will have the lettering on them for the plug... 1E Exhaust one, 3I Intake three etc. Timing. Z series are very tolerant of too much timing advance and it might not be noticeable at idle. Set timing to 5 degrees BTDC with the vacuum advance hose disconnected. Valve lash. In order for the explosion inside every cylinder to stay in the cylinder all valves must close tightly. For the valve to close all the way there has to be some clearance between the valve and the rocker arm. If too tight, the flame can get past the valve and explode out the carburetor. Set valve lash to 0.013" intake and exhaust. Carb. A partly blocked main jet will prevent the mixture being correct. This can cause a 'lean' backfire under acceleration. Remove and clean is the best remedy. Accelerator pump. The accelerator pump forces a rich mixture when accelerating just off idle. Above idle air flow isn't enough to suck in fuel through the primary jet and the engine will normally go lean and can 'lean backfire'.. The accelerator pump squirts raw gas in to help transition through this low speed area. With engine OFF look down the carb and pump the gas pedal. You should see a strong squirt of fuel into the primary barrel (farthest from the valve cover) Quote Link to comment
theboraxkid Posted December 10, 2014 Author Report Share Posted December 10, 2014 It ran absolutely fine whenever it was warmed up and going straight, the problem was only when it was cold started or around right turns. Now the thing doesn't want to rev at all, and I put everything exactly how it was before I tried to fix it. It starts, runs fine, but after a few seconds it sputters out, and if I hit the gas, it backfires sharply. I'm a college student, so I really have no money for a decent timing gun thing, but everything else except the carb seems to be working just fine... Quote Link to comment
datzenmike Posted December 10, 2014 Report Share Posted December 10, 2014 Cold running. Next time it is cold take the top off the air filter and look inside. The choke plate should be partly closed. Have someone pump the gas pedal or work the throttle cable by hand. You should see the choke plate snap closed. Like this... Closed when choke is on and engine cold... Open when engine warmed up... When the engine is cold the fuel is not warmed sufficiently and turned into a vapor that mixes perfectly with the air. The engine will run lean. (and can backfire) To counteract this, the engine uses a choke to increase the suction and draw in additional fuel. Most chokes are heated electrically and this turns them off in under ten minutes. Quote Link to comment
theboraxkid Posted December 12, 2014 Author Report Share Posted December 12, 2014 Okay! I got it running again! I adjusted my choke and did a little cleaning, now it is running sweet. Still have no electrical connection to the choke and that weird extra lever, but I can pretend those problems don't exist for now. Thank you guys for your help! Quote Link to comment
datzenmike Posted December 12, 2014 Report Share Posted December 12, 2014 There should be an electrical plug near the back of the carb with a Blue/White stripe to the idle cut and a Blue wire to the choke heater. Find the idle cut wire and you can join the choke heater wire to it, to get it working. A picture is worth a thousand words. Maybe something simple to fix this up. Quote Link to comment
Charlie69 Posted December 12, 2014 Report Share Posted December 12, 2014 Try switching the coil wires. I have bought 2 720s with "bad engines" and all the was wrong is the coil wires were switched. Good deal for me bad deal for seller. Quote Link to comment
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