Good afternoon, I'm new to the forum and I'm hoping to tap into the club's impressive technical knowledge base, so I hope you're all well and I'm very pleased to meet you!
I've bought a 1951 Comet which is in lovely order, it has a few pattern cycle parts I think but I'm not particularly hung up about that. My problem is I don't just want to look at it (as lovely as it is), I want to get some miles in on it and it's refusing to play ball.
It's a standard 499cc single running a BTH magneto and has been converted to 12v electrics. I've checked the ignition timing and the tappets and all seems good, the magneto produces a nice fat spark and there's plenty of compression. The engine fires up quite readily but when I first went to ride it opening the throttle caused it to splutter and die. The chap I bought it from said it would need going through before riding and he wasn't wrong!
With the ignition and the tappets correct, and with good compression, I thought it must be a fuelling problem so I stripped the carburettor. It's running the standard Amal pre-monoblock 229F/1DV and the jets are standard at 200 main jet and 106 needle jet (they do look new so I'm wondering if this fuelling issue has been around for a while). The throttle slide was a 29/4 so I've replaced it with the standard 29/3 and the needle clip is on number 3 position (from the top). The float chamber has its original brass float and that's clipped to the lower position on the original float needle, this equates to the upper clip position on a new replacement float needle. Fuel flows from the taps quite readily and I've had both taps out to check for any gunge or debris, but the filters on both are clear. I replaced the fuel lines whilst I was at it and also replaced all the fibre washers in the carburettor.
So with everything apparently correct and set to standard the only way I can actually ride the machine is by having the mixture screw screwed right in. Whilst this allows the machine to be ridden it means it has very poor acceleration and when I open the throttle above about an 1/8th to a 1/4 the engine dies. These issues suggests to me a seriously weak mixture (as born out by refusing to run with the mixture screw set at the starting point of 1 1/4 turns).
What could the issue be? I've read so many fuelling threads on here that I've gone crosseyed but they only suggest what I suspect, that the fuelling is excessively weak. The only thing I can think of is that the carburettor body is worn out and is letting excessive air past the slide, it is the original brass one after all. I've checked for air leaks at the inlet stub on the cylinder head and the joint between the carb and stub but I can't find any evidence of air being drawn in.
It's got me stumped at the moment so has anyone overcome a similar problem, and if you have what was the underlying cause? I will greatly appreciate any replies whether they work or not!
I've bought a 1951 Comet which is in lovely order, it has a few pattern cycle parts I think but I'm not particularly hung up about that. My problem is I don't just want to look at it (as lovely as it is), I want to get some miles in on it and it's refusing to play ball.
It's a standard 499cc single running a BTH magneto and has been converted to 12v electrics. I've checked the ignition timing and the tappets and all seems good, the magneto produces a nice fat spark and there's plenty of compression. The engine fires up quite readily but when I first went to ride it opening the throttle caused it to splutter and die. The chap I bought it from said it would need going through before riding and he wasn't wrong!
With the ignition and the tappets correct, and with good compression, I thought it must be a fuelling problem so I stripped the carburettor. It's running the standard Amal pre-monoblock 229F/1DV and the jets are standard at 200 main jet and 106 needle jet (they do look new so I'm wondering if this fuelling issue has been around for a while). The throttle slide was a 29/4 so I've replaced it with the standard 29/3 and the needle clip is on number 3 position (from the top). The float chamber has its original brass float and that's clipped to the lower position on the original float needle, this equates to the upper clip position on a new replacement float needle. Fuel flows from the taps quite readily and I've had both taps out to check for any gunge or debris, but the filters on both are clear. I replaced the fuel lines whilst I was at it and also replaced all the fibre washers in the carburettor.
So with everything apparently correct and set to standard the only way I can actually ride the machine is by having the mixture screw screwed right in. Whilst this allows the machine to be ridden it means it has very poor acceleration and when I open the throttle above about an 1/8th to a 1/4 the engine dies. These issues suggests to me a seriously weak mixture (as born out by refusing to run with the mixture screw set at the starting point of 1 1/4 turns).
What could the issue be? I've read so many fuelling threads on here that I've gone crosseyed but they only suggest what I suspect, that the fuelling is excessively weak. The only thing I can think of is that the carburettor body is worn out and is letting excessive air past the slide, it is the original brass one after all. I've checked for air leaks at the inlet stub on the cylinder head and the joint between the carb and stub but I can't find any evidence of air being drawn in.
It's got me stumped at the moment so has anyone overcome a similar problem, and if you have what was the underlying cause? I will greatly appreciate any replies whether they work or not!
