I have a straight bore NH that seems to be rich at take off from stop. It has that momentary blubber-studder upon openning the throttle after sitting at a light for a while. I wonder if I should set the float a bit lower even if that puts it out of spec? Anyone have this issue and what did you do?
I doubt it will help since you are going to have to open the mixture screw some to compensate.
When you say "..sitting at a light for a while" it sounds like the engine isn't actually idling rich but instead, fuel is pooling up in the manifold and you are picking it up as soon as you open the throttle.
If this is the case, warmer intake air will be your friend.
Eric, Check the air vent to the float bowl for obstructions. This can cause that sort of issue on the straight thru NH's.
If your float is set anywhere even close to correct, the issue is not your float height.
Adom Doleshal is correct you need to check your vent hole to see if its open. I also take a NEW 1.8" Drill bit and drill out all PLUGS in your low speed passage. Take one of the old all wire WHITE coat hangers and place it in all three passages. I use white or yellow so its easier to see the wire. Then take a very small wire or drill bit and make sure that both air holes on either side of your choke butter fly is open into the REAR SLOW SPEED PASSAGE. Remove the coat hanger then plug the front two passage holes with a 1/4" brass welding rod. I sharpen the welding rod.
NOTE; The plug hole on the middle plug on the side of the NH is very easy to crack when driving the plug in. Be careful.
After you plug the front two holes take a can of WD 40 or some other pressurised can with the long orange tube and place it in the rear hole. (the one in the flange). Force some WD 40 into the passage way and see if it runs out the bottom of the carb.
When I set my float, I hold the carb upside down and set the float horizontal. This will not quite be up to speck but it works for me.
I always use the Neopreme Float Needles (Viton). They always leak on me the first day, but after that I never have one leak. I have some that have been in over 30 years. I NEVER shut my gas off. I bought a bunch of Grose-jet needles one time and did not like them.
I wonder if you have to set the mixture needle to compensate for a sloppy throttle shaft in the carb body ??
Extra air "leaking" past the shaft bores/shaft will need a bit more gas at idle, and mixture will have to be tweaked leaner at speed
Bob, this carb has all new shafts with no leak around them and the holes were all cleaned out at rebuild. I never have to readjust the needle.
Dave, I suppose there could be a new obstruction in the vent hole. I will check it out as soon as I can. I hope that's all it is. For an in car test, I could remove the bowl, spray the WD as you suggest into the vent hole (the one next to the choke shaft?)and see if it comes out the bottom. I do like the viton float needles.
Seth, it feels like that is exactly what is happening. Is there just not enough airflow to get decent atomization at idle? I could try the hot air intake too.
Once it gets past that first, after idle stumble, it runs great. That makes me feel it is loading up gas in the intake.
Any other ideas?
Well, I checked the vent and it seemed checked clear. The symptoms I described were noted while running the engine in the garage (doors open) after a long time of not running. Today I got out for a long over due drive and the carb limbered up and worked well. Maybe I had some inlet valve seating issues as I had run the carb dry before the long rest in the garage. Whatever it was, it is no longer an issue. Thanks all for the willing assisstance.
I am a spark guy so what do I know? But here goes.
I have a friend who studied the straight through NH carburetor as opposed to the subsequent sway back carburetor.
He believes as the throttle is opened the intake manifold vacuum drops (due to the large opening) and the atomization of fuel slows down and this causes the stumbling you refer to. He also believes this is why Ford soon dropped this carburetor.
Ron the Coilman
He also believes that is the penalty you pay for the better top end performance of the straight through NH Model T carburetor.
Ron the Coilman
Erich, do you have a hot air pipe installed between the carb intake and the exhaust manifold??? If you don't, that certainly could be a problem. With todays fuels, it works best to use a hot air pipe all year round.
Ron, that sounds very reasonable and I always figured Henry changed away from that carb for some good reason. He must have figured it was not the compromise he was after. I am finding I am adapting my driving style around this carb and am liking the overall performance. If I am a few cars back at the light, when it turns green, I slowly open the throttle and that works well. Once in motion, all is smooth and normal. It starts and idles well too.
I run a straight through NH that is even reamed out another 1/8" on a standard motor, no hot pipe and have no trouble at all, gives me more torque for the hills.