Speedymaru Posted April 3, 2018 Report Share Posted April 3, 2018 Hello, I have a 78 620 that I just recently started daily driving. I’m trying to get the air/fuel ratio correct, and right now it’s running too lean. The plugs are whitish, and it hesitates when throttle is applied quickly. I have been trying to richen it up a bit using the fuel mixture screw, but I’m a bit confused. I get the truck running, then turn the screw in until it starts to drop down in RPM, then I back it out and the rpms come back up, and I’m supposed to keep backing it out until it drops down again, but no matter how far I back it out it doesn’t ever drop back down. Does this mean I need to rejet it? I’m not a carb expert by any means. Also, which jets would I be replacing? 1 Quote Link to comment
Speedymaru Posted April 3, 2018 Author Report Share Posted April 3, 2018 Also forgot to mention that it’s a 32/36 with a manual choke, and that I have to have the choke pulled about halfway out while driving in order to have decent power 1 Quote Link to comment
racerx Posted April 3, 2018 Report Share Posted April 3, 2018 Been awhile but I think u should not turn the mixture screw out more than 1.5 turns, if no affect then jet maybe too small. What is the history of the carb? Is it new? Have you looked at a diagram of a 32.36 weber to make sure u are messing with the right screw? To get more details on adjustment and diagram of carb check out piercemanifolds webpage. 2 Quote Link to comment
Speedymaru Posted April 3, 2018 Author Report Share Posted April 3, 2018 I don’t remember what the correct sweet spot is but you’re right, it should be somewhere between 1-3 turns out and if it’s more than I think it needs rejetted? I’m not sure about the history of the carb, it was on when I bought the truck. The guy said that he had just picked it up for the truck but I don’t think it was new. And yes, I am turning the adjustment screw. Thanks for the info 1 Quote Link to comment
Loryde520 Posted April 3, 2018 Report Share Posted April 3, 2018 Sounds like a vacuum leak to me, or a plugged idle jet. This is true on dirt bikes I work on. When you have to run with the choke on, it usually is a plugged idle circuit. Definitely running lean if plugs are white. It will be running hot too in a lean situation. Also check for vacuum leaks. Take the carb apart if you haven't already and have a look see inside. They are very easy carbs to work on and there is lots of info on the 'net. 2 Quote Link to comment
Draker Posted April 3, 2018 Report Share Posted April 3, 2018 Search for l20b jetting specs. They are posted all over here. Easy to check. 1 Quote Link to comment
datzenmike Posted April 3, 2018 Report Share Posted April 3, 2018 Sounds like a vacuum leak to me, or a plugged idle jet. This is true on dirt bikes I work on. When you have to run with the choke on, it usually is a plugged idle circuit. Definitely running lean if plugs are white. It will be running hot too in a lean situation. Also check for vacuum leaks. Take the carb apart if you haven't already and have a look see inside. They are very easy carbs to work on and there is lots of info on the 'net. Dirty jet. Maybe not fully plugged yet but some blockage. Can you get it revved up into the secondary? This will usually allow it to run because the other jet is OK. 3 Quote Link to comment
Doctor510 Posted April 3, 2018 Report Share Posted April 3, 2018 Your first question, Yes you should keep backing out the A/F screw until it changes , then turn it in until smooth. All the other info you got is super. Go back to basics. 1 Quote Link to comment
kelowg Posted April 3, 2018 Report Share Posted April 3, 2018 Just pull and clean both prim and sec idle jets. also ck that accl pump is working,fuel squirts from between both barrels. After that, remove top of carb. See what main jets it has,factory is 140/140 2 Quote Link to comment
banzai510(hainz) Posted April 3, 2018 Report Share Posted April 3, 2018 If motor idles when running then the idle jet is fine. YOu can adjust. to get it run best case . Turn mixture in/out till best. then time the motor also Now if you have to have the choke on to run better the I bet the main jet is blocked or gunked up and your sucking gas thru the 2 barrel. 1 Quote Link to comment
G-Duax Posted April 3, 2018 Report Share Posted April 3, 2018 Also check the tightness of the screws holding the top on to the main body. I've seen the Weber body gasket dry out, letting the screws loose tension, and become a major air leak around the center where there are several fuel passages. Have even see the gasket rot, and parts of it are missing. By the way, the mixture screw is only for idle, and has very little effect on anything above idle. So go buy a rebuild kit, and go into it. I'm thinking most likely crud, like Mike says. 1 Quote Link to comment
Stoffregen Motorsports Posted April 3, 2018 Report Share Posted April 3, 2018 then turn the screw in until it starts to drop down in RPM, then I back it out and the rpms come back up, and I’m supposed to keep backing it out until it drops down again, That's not exactly right. Back it out until it starts to rise, then go about another 1/2 turn and call it good. You probably need some jetting done though. The idle jets are on the sides of the exterior of the carb, while the fuel and air jets are inside the float bowl. If the carb has been sitting for a long time, the jets can get hard buildup causing them to clog. This is especially true for the idle jets since the orifice is so small to begin with. You can use a single strand of wire brush to clean them out. 4 Quote Link to comment
noramost Posted April 3, 2018 Report Share Posted April 3, 2018 I like to use small propane bottle with a 5/16 vacuum hose about 2' long to pinpoint vacuum leaks it's much more precise than a can of carb cleaner. We used to find about 50% of the Webbor adapters not sealed properly at our shop. 3 Quote Link to comment
Speedymaru Posted April 3, 2018 Author Report Share Posted April 3, 2018 Okay cool, thank you all for the great info. I’ll tear into it when I get a chance and check it out. Does the carb have to be rebuilt if I’m just cleaning the jets? I know it’s better to, but I’d rather not wait for the rebuilt kit to come in. If it is necessary then I can wait 1 Quote Link to comment
Speedymaru Posted April 3, 2018 Author Report Share Posted April 3, 2018 And I’ve tried to use propane before, but I’ve never been able to have good results with it? I’ll give it another try though 1 Quote Link to comment
banzai510(hainz) Posted April 3, 2018 Report Share Posted April 3, 2018 Fuck it do a Hawaiian tune up. Rev motor till 5 k then paut a clean rag over the carb and hope it sucks the gunk/Use cleas rag as not a dirty one otherwise you just suck up the dirt and plugg it again 1 Quote Link to comment
datzenmike Posted April 4, 2018 Report Share Posted April 4, 2018 If a weber, just jam the choke plates shut at 5,000. 2 Quote Link to comment
noramost Posted April 4, 2018 Report Share Posted April 4, 2018 If you cross the #2 and #3 wires, start it, floor it three or four times you can get it to backfire so hard it will blow the jets out with 150lbs of compression, it's an old circle track trick. 2 Quote Link to comment
kelowg Posted April 4, 2018 Report Share Posted April 4, 2018 Rebuild on these is only a gasket and needle/seat. Some other stuff too. But if u dont damage top cover gasket,ur good to go. Have had top off mine half dozen times,cleaning and jet changes. Also ck inlet filter screen(where fuel line connects) just blast it clean. Idle jets do effect engine above idle speed. To like 1/3 throttle,idle and main jets overlap each other. Remember,BECAUSE DATSUN. so anything is possible 4 Quote Link to comment
Speedymaru Posted April 4, 2018 Author Report Share Posted April 4, 2018 Okay, I think I’m going to go ahead and take the cover off and clean the jets manually. Make sure it’s not too dirty in there too. Is it better to have the carb on the truck or take it off? 1 Quote Link to comment
Stoffregen Motorsports Posted April 4, 2018 Report Share Posted April 4, 2018 To keep the gasket in tact, remove the lid screws and knock the top cover with the heel of a screwdriver. That usually pops the cover off without damaging the gasket. Do you know what carb adapter you have? One piece or two piece? 2 Quote Link to comment
noramost Posted April 4, 2018 Report Share Posted April 4, 2018 And I’ve tried to use propane before, but I’ve never been able to have good results with it? I’ll give it another try though The secret is the regulator, I have 3, 1 has a push button that give a puff when pushed. If yours is just a nob, use a balloon on the vacuum line to barely inflate it to get your adjustment as low as possible. Maybe just buy a smoke machine, it works to find all kind of things 1 Quote Link to comment
datzenmike Posted April 5, 2018 Report Share Posted April 5, 2018 To keep the gasket in tact, remove the lid screws and knock the top cover with the heel of a screwdriver. That usually pops the cover off without damaging the gasket. Do you know what carb adapter you have? One piece or two piece? Yes, remove the carb..... so you can inspect the adapter. The bolts often vibrate loose so check them and the gaskets. Loctite??? 2 Quote Link to comment
racerx Posted April 5, 2018 Report Share Posted April 5, 2018 But don't tighten like gorilla, you can damage it they are made of alum. If u can get the one piece adapter. 1 Quote Link to comment
Stoffregen Motorsports Posted April 5, 2018 Report Share Posted April 5, 2018 I like to use red loctite on the threads of the two pocket screws. I also ditch the gasket in favor of Threebond 1211 (Kawibond, Yamabond, Hondabond, etc). You lessen the chances of warping the adapter plates without a gasket in between. 2 Quote Link to comment
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