jovial_cynic Posted July 31, 2008 Report Share Posted July 31, 2008 I'm not sure how typical this is of SU carbs, but I've noticed that if I let the car sit without running for over a week, the engine cranks FOREVER before it fires up... like it's not getting any fuel. Is there a priming or choke mechanism that I'm missing? Quote Link to comment
Phlebmaster Posted July 31, 2008 Report Share Posted July 31, 2008 A can of starting fluid. lol :lol: Is your accellerator pump working? Do you have a leak in your supply lines? Quote Link to comment
datzenmike Posted July 31, 2008 Report Share Posted July 31, 2008 :lol: SUs don't have those!! (sorry) SUs have a variable venturi which adjusts to the demand of the motor. The Hitachi has to transition from idle to a huge primary barrel and it's difficult to get the air moving when the throttle is suddenly jammed open. The engine sucks air and runs lean before the vacuum builds and draws in gas. The accelerator pump fills this lean spot with a squirt of raw fuel. Think of the SU as a carburetor that becomes bigger as you need it. There is a slide or piston that is drawn up to enlarge the barrel in direct relation the manifold vacuum. Thus the venturi has a constant (or nearly so) vacuum and delivers gas at any speed and sudden throttle opening. Josh, there is a choke, of sorts, but not like other carbs. The slide is raised slightly by a linkage. Raising the slide pulls the tapered needle valve out of the jet allowing more gas to enter the air flow and make it richer. Does the same job as a choke but in a different manner. Quote Link to comment
zerow Posted August 1, 2008 Report Share Posted August 1, 2008 :lol: SUs don't have those!! Good call!!:lol: I would suggest starting fluid, as a last resort. SUs rely upon vapor at the needle, and using starting fluid only disrupts the fine mixture at the needle. Kinda like starting a lawnmower with starting fluid, a good VROOOM, then you're starved for fuel until the float bowls fill up. jovial_cynic, are your choke cables in place? If so, you'll have one option: pull the coil wire, apply choke, crank without spark until your float bowls fill. These carbs are hard to flood because of their construction. Once you have cranked sufficiently, you'll be able to re-connect the wire and start the car. This method allows for the carbs to do what they were designed to do, draw in a mixture into the combustion chamber, without the detonation. Quote Link to comment
Pacific coast Datsun Posted August 1, 2008 Report Share Posted August 1, 2008 I have th esame issue on my 620. It cranks for awhile until it fires. But..on mine they have a manual choke. If i pull the cable halfway it fires up faster. I had to make a bracket to connect both chokes... Quote Link to comment
DISLEXICDIME Posted August 1, 2008 Report Share Posted August 1, 2008 get an electric fuel pump and a regulater at 3 to 3.5 pounds no more cranking :):) Quote Link to comment
ppeters914 Posted August 1, 2008 Report Share Posted August 1, 2008 SU's are simple, reliable beasties, but cold-blooded. If they're adjust correctly, you'll still have to choke the $h!t out of 'em to get 'em started. Once warmed up, they'll purr. Quote Link to comment
Phlebmaster Posted August 1, 2008 Report Share Posted August 1, 2008 :lol: SUs don't have those!! (sorry) SUs have a variable venturi which adjusts to the demand of the motor. The Hitachi has to transition from idle to a huge primary barrel and it's difficult to get the air moving when the throttle is suddenly jammed open. The engine sucks air and runs lean before the vacuum builds and draws in gas. The accelerator pump fills this lean spot with a squirt of raw fuel. Think of the SU as a carburetor that becomes bigger as you need it. There is a slide or piston that is drawn up to enlarge the barrel in direct relation the manifold vacuum. Thus the venturi has a constant (or nearly so) vacuum and delivers gas at any speed and sudden throttle opening. Josh, there is a choke, of sorts, but not like other carbs. The slide is raised slightly by a linkage. Raising the slide pulls the tapered needle valve out of the jet allowing more gas to enter the air flow and make it richer. Does the same job as a choke but in a different manner. By phlebmaster at 2008-03-11 Silly me. lol Quote Link to comment
DISLEXICDIME Posted August 1, 2008 Report Share Posted August 1, 2008 SU's are simple, reliable beasties, but cold-blooded. If they're adjust correctly, you'll still have to choke the $h!t out of 'em to get 'em started. Once warmed up, they'll purr. min start with out the chokes on :P Quote Link to comment
FJDatto Posted August 1, 2008 Report Share Posted August 1, 2008 min start with out the chokes on :P So do mine, but I have mine set up rich for a reason. Also, I had a Z-18 Quote Link to comment
jovial_cynic Posted August 1, 2008 Author Report Share Posted August 1, 2008 I have th esame issue on my 620. It cranks for awhile until it fires. But..on mine they have a manual choke. If i pull the cable halfway it fires up faster. I had to make a bracket to connect both chokes... Perfect - thanks! This will help quite a bit. Quote Link to comment
DISLEXICDIME Posted August 1, 2008 Report Share Posted August 1, 2008 So do mine, but I have mine set up rich for a reason. Also, I had a Z-18 i bet we run them rich for the same reason lol i got lucky at a garage sale at altered motives and got one of these twin pull choke cables http://www.datsunparts.com/1243 Quote Link to comment
jovial_cynic Posted August 1, 2008 Author Report Share Posted August 1, 2008 Twin choke cables, eh? It seems like having a single cable with a bracket would be easier to cobble together... Quote Link to comment
zerow Posted August 1, 2008 Report Share Posted August 1, 2008 Twin choke cables, eh? It seems like having a single cable with a bracket would be easier to cobble together... It is, and I had the same problem, but my fear was not drawing both chokes at the same time. If for some reason one carb had a bind at the choke, it could cause the one carb to run richer than the other, and performance issues as you might think two cylinders were running poorly. You search ignition while this thing makes an unhappy face O / O, instead of O - O. Perhaps mounting the choke cable center **could** alleviate the problem, but it is only a guess. I have an idea to run a 240Z choke cable set and fabricate a knob assembly under the dash. No twisted cable, as the Z cable is solid wire, and the bracket comes with the assembly to pull two cables simultaneously. Quote Link to comment
MikeRL411 Posted August 1, 2008 Report Share Posted August 1, 2008 Face the carb. On its left, the front of the car, below the casting you will feel a spring loaded pin. It's on the front, the choke cable is on the cabin side [unless you have a much later left hand / right hand pair.] That's the prime which is built into the carb. When you let the car sit for a long time fuel evaporates. There is still some in the bowl. Poke the protruding tip about twice on each carb and it will start up as though you had just shut the engine off 30 minutes ago. By the way, the 2 choke cables come originally with a double choke cable assembly attached to the choke knob inside the car. Roadsters and RL411s use this setup. Might still be available? Quote Link to comment
zerow Posted August 1, 2008 Report Share Posted August 1, 2008 Face the carb. On its left, the front of the car, below the casting you will feel a spring loaded pin. It's on the front, the choke cable is on the cabin side [unless you have a much later left hand / right hand pair.] That's the prime which is built into the carb. When you let the car sit for a long time fuel evaporates. There is still some in the bowl. Poke the protruding tip about twice on each carb and it will start up as though you had just shut the engine off 30 minutes ago. By the way, the 2 choke cables come originally with a double choke cable assembly attached to the choke knob inside the car. Roadsters and RL411s use this setup. Might still be available? MikeRL411, I'm not so sure about that. The small spring-loaded pins (lifter pin) on the sides are for verifying the proper operation of the suction chamber and piston movement. It's effectively the method to check for the infamous 'metallic clunk' for the suction piston. Both carbs have them, and if no clunk is heard, then the relationship between the piston and the needle is not in proper alignment, and needs removed and repaired or replaced. Maybe it gives the carb enough movement to provide a small amount of fuel vapor for starting, but then again, I have choke cables. Quote Link to comment
ggzilla Posted August 2, 2008 Report Share Posted August 2, 2008 So to summarize: 1. Make sure your chokes are working. Choke it to start a cold engine 2. Maybe the fuel is leaking out of the fuel bowl overnight. This is really common with regular Datsun carbs. Solution: Fix the leak. Installing electric fuel pump will mask the problem yes, but doesn't really fix the leak. Fuel should stay in the bowl 3 days at least. After a week or two maybe not. Quote Link to comment
zerow Posted August 4, 2008 Report Share Posted August 4, 2008 So to summarize: 1. Make sure your chokes are working. Choke it to start a cold engine 2. Maybe the fuel is leaking out of the fuel bowl overnight. This is really common with regular Datsun carbs. Solution: Fix the leak. Installing electric fuel pump will mask the problem yes, but doesn't really fix the leak. Fuel should stay in the bowl 3 days at least. After a week or two maybe not. I would use a short length of coat hanger wire to check for fuel in the bowls before I would rule out a leak. Run it down the overflow tube(s). I have the small overflow tubes at the top, easier to check. If you are running the long length of steel tubing for overflows, then you will obviously have to remove them to check. Do you have the brass floats? Be extremely careful not to punture them, you're just checking for fuel. Better yet, use a Q-Tip, It will confirm fuel w/o the worry of puncture. Quote Link to comment
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