Hi Guys,
After what seems forever trying to resolve a carburation problem on my custom built big bore street scrambler I've decided to see if anyone on here can offer help.
So, I have a 270 rephased motor with 750 big bore kit, running a pair of VM34's, with an after market 2-1 exhaust and a UK made electronic ignition. After experiencing very poor performance I checked the profile of the cam, only to discover it had 93 degrees of overlap, so most of the fresh fuel was disappearing out of the exhaust. To correct this, Smedspeed in the UK made me a new cam (their 3.5 'hot' street cam). After installing the cam and carefully static timing the new electronic ignition to the recommended 38 degrees BTDC fully advanced (Smedspeed and ignition makers recommended setting- subsequently strobe timed to check advance operation - appears to be working as it should) the bike now suffers from MASSIVE misfiring from half throttle upwards. I'm running B7ES plugs and they look extremely lean, being very white at the electrode.
I have tried changes to the main jets (150 through to 190s), pilots (20 through to 25s) and gone from the fitted 159P6 needle jet to the next richer P6. I've tried ever combination of needle position with the various jets too, but no matter what I change there is virtually no difference in the degree of misfiring or where its ocurring, which is half throttle upwards, right through to full throttle. I'm running 2.5 slides, which are quite rich, but you'd never guess looking at the plugs.
A friend of mine who is somewhat of a Triumph guru and has been in the business building and tuning customer Triumph T120s, T140s and every other form of Triumph motor, has been working with me on this for a long while now and it is perplexing him. He suggested trying a pair of MK1 Amal concentrics, Amals being his specialist knowledge carb-wise. So we swapped out the Mikunis and went for more and more richer jet set ups with the Amals, right up to seriously rich 250 mains, but the misfiring was no different and the plugs always look extremely lean.
I am now having serious doubts about whether the problem is the carbs or the aftermarket 2-1 exhaust. I understand these exhausts can cause significant problems for performance cammed motors. So I am hoping that after reading this, someone out there with a wealth of experience in tuning these rephased motors with VM carbs can give me their thoughts on what is going wrong here. With the new cam, new rockers, new electronic ignition I've invested heavily in this motor but getting no benefits, so I'm hoping someone can help before I throw in the towel and sell the bike, something I'd rather not do.
After what seems forever trying to resolve a carburation problem on my custom built big bore street scrambler I've decided to see if anyone on here can offer help.
So, I have a 270 rephased motor with 750 big bore kit, running a pair of VM34's, with an after market 2-1 exhaust and a UK made electronic ignition. After experiencing very poor performance I checked the profile of the cam, only to discover it had 93 degrees of overlap, so most of the fresh fuel was disappearing out of the exhaust. To correct this, Smedspeed in the UK made me a new cam (their 3.5 'hot' street cam). After installing the cam and carefully static timing the new electronic ignition to the recommended 38 degrees BTDC fully advanced (Smedspeed and ignition makers recommended setting- subsequently strobe timed to check advance operation - appears to be working as it should) the bike now suffers from MASSIVE misfiring from half throttle upwards. I'm running B7ES plugs and they look extremely lean, being very white at the electrode.
I have tried changes to the main jets (150 through to 190s), pilots (20 through to 25s) and gone from the fitted 159P6 needle jet to the next richer P6. I've tried ever combination of needle position with the various jets too, but no matter what I change there is virtually no difference in the degree of misfiring or where its ocurring, which is half throttle upwards, right through to full throttle. I'm running 2.5 slides, which are quite rich, but you'd never guess looking at the plugs.
A friend of mine who is somewhat of a Triumph guru and has been in the business building and tuning customer Triumph T120s, T140s and every other form of Triumph motor, has been working with me on this for a long while now and it is perplexing him. He suggested trying a pair of MK1 Amal concentrics, Amals being his specialist knowledge carb-wise. So we swapped out the Mikunis and went for more and more richer jet set ups with the Amals, right up to seriously rich 250 mains, but the misfiring was no different and the plugs always look extremely lean.
I am now having serious doubts about whether the problem is the carbs or the aftermarket 2-1 exhaust. I understand these exhausts can cause significant problems for performance cammed motors. So I am hoping that after reading this, someone out there with a wealth of experience in tuning these rephased motors with VM carbs can give me their thoughts on what is going wrong here. With the new cam, new rockers, new electronic ignition I've invested heavily in this motor but getting no benefits, so I'm hoping someone can help before I throw in the towel and sell the bike, something I'd rather not do.
