SOHC.co.uk Forums > CB750
CR750 Set Up
hairygit:
If you're certain the carb rubbers are good, and tight, check by spraying carb cleaner on the joints of the suspect cylinder, any change in revs will show a leak. After that, possibly a weak or broken valve spring or dodgy sealing of an inlet valve seat.
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K2-K6:
Sounds like you've got a reasonable base setting with the jetting so shouldn't be giving you anything out of the ordinary.
Spitting back through carbs has to have some pre-conditons. Inlet valve has to be open to allow the combustion to go back out that route, this is usually achieved by ignition being too advanced but you've got yours ok at tickover so it shouldn't be a concern.
But as far as I can see, that firing of all cylinders will give this condition. If you look at no1 cylinder coming up to compression it will fire something like 10 degrees before top with both valves closed, giving the power stroke. At exactly the same point, no3 will be on its intake stroke with piston 10degree before bottom and just the inlet valve open. This should fire that mixture straight back out the intake IF it ignites, giving you spitting.
What Boyer won't disclose is the strategy around this, is there a lock out for all plugs firing at the same time? If so is it based on a rev floor limit? Or could it be set on time basis ie if you switch ignition on will it give a time base of just firing plugs conventionally (say 30 secs) after which it expects to see the engine above a certain rpm (perhaps 700 rpm) and return to multiple firing?
These are some of the things you'd find in even fairly old ecu controlled systems, so perfectly possible.
You could try turning on and immediately starting it to see if it spits at you, or pose the question to them
philward:
Hairy - I think that the kick back/spit back is on multi cylinders and quite pronounced - not just a little 'puff' (kick back through the kickstart is quite severe sometimes
Also, forgot to mention, when hot it pops through exhaust on de-acceleration at low revs (below 3k ish)
philward:
--- Quote from: K2-K6 on September 23, 2018, 09:29:17 PM ---Sounds like you've got a reasonable base setting with the jetting so shouldn't be giving you anything out of the ordinary.
Spitting back through carbs has to have some pre-conditons. Inlet valve has to be open to allow the combustion to go back out that route, this is usually achieved by ignition being too advanced but you've got yours ok at tickover so it shouldn't be a concern.
But as far as I can see, that firing of all cylinders will give this condition. If you look at no1 cylinder coming up to compression it will fire something like 10 degrees before top with both valves closed, giving the power stroke. At exactly the same point, no3 will be on its intake stroke with piston 10degree before bottom and just the inlet valve open. This should fire that mixture straight back out the intake IF it ignites, giving you spitting.
What Boyer won't disclose is the strategy around this, is there a lock out for all plugs firing at the same time? If so is it based on a rev floor limit? Or could it be set on time basis ie if you switch ignition on will it give a time base of just firing plugs conventionally (say 30 secs) after which it expects to see the engine above a certain rpm (perhaps 700 rpm) and return to multiple firing?
These are some of the things you'd find in even fairly old ecu controlled systems, so perfectly possible.
You could try turning on and immediately starting it to see if it spits at you, or pose the question to them
That makes sense K2 - K6. I bought the Boyer system based on having done trouble free miles over about 40k miles in the 70s - TBH, I wish I'd gone for another make. I'll put the question to them
Thanks
--- End quote ---
philward:
--- Quote from: K2-K6 on September 23, 2018, 09:29:17 PM ---Sounds like you've got a reasonable base setting with the jetting so shouldn't be giving you anything out of the ordinary.
Spitting back through carbs has to have some pre-conditons. Inlet valve has to be open to allow the combustion to go back out that route, this is usually achieved by ignition being too advanced but you've got yours ok at tickover so it shouldn't be a concern.
But as far as I can see, that firing of all cylinders will give this condition. If you look at no1 cylinder coming up to compression it will fire something like 10 degrees before top with both valves closed, giving the power stroke. At exactly the same point, no3 will be on its intake stroke with piston 10degree before bottom and just the inlet valve open. This should fire that mixture straight back out the intake IF it ignites, giving you spitting.
Have emailed Boyer based on your point - thanks again K2 - K6
What Boyer won't disclose is the strategy around this, is there a lock out for all plugs firing at the same time? If so is it based on a rev floor limit? Or could it be set on time basis ie if you switch ignition on will it give a time base of just firing plugs conventionally (say 30 secs) after which it expects to see the engine above a certain rpm (perhaps 700 rpm) and return to multiple firing?
These are some of the things you'd find in even fairly old ecu controlled systems, so perfectly possible.
You could try turning on and immediately starting it to see if it spits at you, or pose the question to them
--- End quote ---
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