tecktinkershop Posted January 9, 2015 Report Share Posted January 9, 2015 Howdy ya'll! I'm new on this site and just joined the datsun owner clan in September when I bought my 1980 Datsun 720 pickup which came with a freshly rebuilt L20B engine. First vehical I've ever owned that isn't a GM product so I'm a little in the dark on a few things. The main problem I'm having with this thing is when I step into the gas any at all, it falls flat on its face. It runs at idle perfectly and if I slowly ease into the gas, it will run just fine. When I try to take of or put it under load it backfires back though the carb and to keep it running I have to pump the gas. I have checked all the ignition components including testing the pick up coil in the distributor and the ignition coil, all checked out fine. I have checked valve lash and timing which both seem fine. I have also rebuilt the carb, but when I pulled the carb apart, there were a few parts that weren't in the carb that were on the exploded view. Now the lady I bought it from was very knowledgeable on this rig and she told me her and her late husband had bought the truck brand new in 81. She told me the only thing that they had done to the truck (outside of regular maintenance) was to remove all the emissions related equipment( or at least unwired and plugged it off), front wheel bearings, and replacing the fuel pump. When I noticed the "missing" parts in the carb, I called and asked her if they had ever had the carb on this truck apart, and she told me that had never so much as had it off the engine. It has the stock hitachi carb on it and the operation on this carb is something that still confuses me. I'm sure you all have seen them, but it has the primary barrel who's throttle plate is hook to the gas pedal, and the secondary barrel which best I can see is operated by a diaphragm. Now when I rev the engine up and look down in the carb, I have never seen the secondary plate move at all. I'm not completely sure about the whole operation principle of this carb so I don't know if it's supposed to or not. Now I've read a lot of the posts on here and everyone says to pitch the stock carb and put on a weber. Now I've looked into that and would love to be able to do that, but I'm paying my way through college so money is pretty tight and this truck has become a budge restoration project. So that being said, I need to try my best and make do with what I have. Does anyone have any thoughts as to why I have this problem? And could anyone be able to explain the theory of operation behind this carb to me? Any help would be much appriciated. 1 Quote Link to comment
wayno Posted January 9, 2015 Report Share Posted January 9, 2015 Welcome :) Wish I could help you, but downdraft carbs are a mystery to me. 1 Quote Link to comment
datzenmike Posted January 9, 2015 Report Share Posted January 9, 2015 Hi ------- (fill in name) The secondary is vacuum operated and works best under load which is hard to do revving up in neutral so for now assume it works. A few questions... The Hitachi carb has three main fuel delivery circuits. The idle, the primary and the vacuum secondary. If the gas pedal is floored from an idle the sudden opening of the throttle allows the engine to gulp air but because it isn't moving very fast it can't form much of a vacuum at the venturi and suck enough gas in with it. The engine runs suddenly very lean and this will cause an aptly named, 'lean backfire'. The idle circuit is just that and it can't supply enough either. To get a carb engine through this critical lean period until the engine can get on it's feet and start moving air, an accelerator pump is used. This is a mechanical pump worked by the throttle cable that squirts raw fuel into the primary to richen the mixture. Engine off, look down the primary barrel and pump the gas by hand. You should see a strong squirt of fuel jetting into the opening. This is the accelerator pump. If not working it will need fixing. Another possibility is a partly clogged primary jet. I say part clogged because you can run if you open the throttle slowly. 2 Quote Link to comment
arizonajones Posted January 9, 2015 Report Share Posted January 9, 2015 always worth it t put a fresh fuel filter in, too. could be crudded up 1 Quote Link to comment
tecktinkershop Posted January 9, 2015 Author Report Share Posted January 9, 2015 Ah, I guess I forgot to put in my name lol Thanks for the welcome ya'll. Anyhow, that makes sense as to why I don't see the secondary plate move at all. And I've replaced the fuel filter and all the lines as they were dry rotted from sitting so long, and the accelerator pump has worked good from day one. As for the jets being clogged, I cleaned the carb spotless and polished the jets good when I rebuilt it, but I guess it's possible something might have got in there to clog one up, so I'll oder another seal kit and take another crack at it. I'll get back to you tomorrow on the results of that. Thanks a lot for the help 1 Quote Link to comment
datzenmike Posted January 9, 2015 Report Share Posted January 9, 2015 On to the next.... Lets check the valve lash. An over tight valve may not be closing tightly and the flame may squeese by and backfire. Lash is 0.010" intake and 0.012" exhaust. 1 Quote Link to comment
Charlie69 Posted January 10, 2015 Report Share Posted January 10, 2015 Ah, I guess I forgot to put in my name lol Thanks for the welcome ya'll. Anyhow, that makes sense as to why I don't see the secondary plate move at all. And I've replaced the fuel filter and all the lines as they were dry rotted from sitting so long, and the accelerator pump has worked good from day one. As for the jets being clogged, I cleaned the carb spotless and polished the jets good when I rebuilt it, but I guess it's possible something might have got in there to clog one up, so I'll oder another seal kit and take another crack at it. I'll get back to you tomorrow on the results of that. Thanks a lot for the help so just curious, What is your Name? LOL Quote Link to comment
tecktinkershop Posted January 11, 2015 Author Report Share Posted January 11, 2015 Alright, so I put in the carb kit last night and tried it. First question, I noticed that the secondary barrel had a larger jet then the primary barrel is, is this supposed to be this way? After I put the carb back on, I tried to fire it up. Now it will not even idle. I have to tried to adjust the idle mixture screw with no luck. Even before this rebuild, I could turn the idle screw all the way out or all the way down and it made no difference in how the truck ran. I checked the valve clearances next and all are set nearly perfect. Any other Ideas? Also I was going to put a picture on here of the diagram that came in the carb kit with the parts I am "missing" and see if I actually needed these or they aren't put in this carb. But, I'm not to tech savvy, how do you post a picture on here? lol and name's Wesley 1 Quote Link to comment
datzenmike Posted January 11, 2015 Report Share Posted January 11, 2015 Hi Westley. Usually 100/150 primary/secondary jets. Secondary barrel is larger... needs a larger jet.Is your idle cut solenoid connected and is it working? You chould hear a light click sound whe ignition is turned on off on off.If idle speed is too high the idle mixture screw has no effect.Missing parts are not good.Posting photos.... http://community.ratsun.net/topic/54932-photobucket-ratsun/ 1 Quote Link to comment
arizonajones Posted January 12, 2015 Report Share Posted January 12, 2015 Post which parts you are missing. Sometimes the lists on your rebuild kit may not be identical to your carb : US Fed, Cali, & Canada carbs are not identical in every way. Quote Link to comment
tecktinkershop Posted January 12, 2015 Author Report Share Posted January 12, 2015 Alright, good to know the jets are in order, makes sense. Same with the idle speed then. As far as the idle cut solenoid, it is there but the needle in it has been removed. Thank you arizonajones, that helps a lot. I am missing 62, 22, and 23. Also my carb has #27, and does not have solenoid 24, but there is no spot for it to be there. 2 Quote Link to comment
datzenmike Posted January 12, 2015 Report Share Posted January 12, 2015 62 and 23 are part of the BCDD adjustment. This is a small carburetor inside the carburetor that is activated when decelerating. Sometimes when on they won't shut off. About all you can do is disable it so it isn't working. Find part 27, a large round thing with a small rubber cap on the side. Remove the rubber cap and turn the screw inside counter clockwise as far as it will go. This should set the threshold for activating the BCDD so high that it won't come on. This may not work but give it a try. Now you may be able to set the idle and the mixture screws. 1 Quote Link to comment
Charlie69 Posted January 12, 2015 Report Share Posted January 12, 2015 Thank you! Welcome Wesley! Quote Link to comment
tecktinkershop Posted January 12, 2015 Author Report Share Posted January 12, 2015 Alright, well I failed to mention that my carb has the #27 for california models, I don't see a screw to adjust on it 1 Quote Link to comment
arizonajones Posted January 12, 2015 Report Share Posted January 12, 2015 Alright, well I failed to mention that my carb has the #27 for california models, I don't see a screw to adjust on it I'm pretty sure my carb matches yours. My carb doesn't have 22 either (just a plug on top where it would be - but in my Chilton book, it doesn't list that part) But #27 goes to a vacuum motor and boost control valve, which I think has a similar function to the BCCD but is more spliced into your emission controls. Is all of your stock emission stuff intact? 1 Quote Link to comment
datzenmike Posted January 12, 2015 Report Share Posted January 12, 2015 Well I guess try to get the missing pieces replaced or a replacement carb? 1 Quote Link to comment
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