SF2 main jet PHF 36

it creates confusion between US gallons and (proper) imperial gallons, I think that is the case here.

Zorba, don't fuck about fiddling, just get a dyno run with air/fuel ratios throughout. Bigger main jets have made it worse, you need to know why. But please remember that the main jet will only "come in" from 3/4's throttle to wide open, not doing much below that, so you need (if road testing) a straight bit of highway about 2 miles long, and nail it in third or fourth and SUSTAIN the full throttle opening, changing gear upwards if the load reduces, but that twist grip has to BE HELD nailed, for A good minute and, and engine cut, then coast to a halt and read plug at roadside, anyway plug colourS these days don't mean a hell of a lot and ARE very hard to decipher

I had an SF3 for fun a few years ago, I ran it for about four years, same engine spec as yours, same exhaust etc, it would rev out to bursting point and would even pull very high revs with a pillion, which surprised a few Jota owners

CLEM
 
Very much doubt the cam timing is out 
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Unless there has been a load of extra work done on verifying the cam timing it's almost a cert that it's out - but not by a tooth - the 750's are notoriously bad at getting the same timing for both cylinders. Although this surprisingly doesn't often show in performance it may well contribute to poor consumption figures. - Regards -Phil
 
Ignition timing ok?
Points clearance ok if fitted?
Valve clearances ok?
Spark plugs and caps ok?
HT lead fitment to caps and coils ok?
Air filter ok?
Fuel taps flowing ok? On main and reserve?
Air vent on fuel cap ok?

Then I would think the carbs need to be stripped and thoroughly cleaned. When was this last done?
Could be clogged up emulsion tubes, carb filters, pilots or just general airways.
Have them ultrasonic cleaned and blown through with an air line.
 
65-70mph is not even half throttle so mainjet size (unless blocked) is basically irrelevant. A half throttle plug chop I once did was around 90mph and at those 65mph speeds my bike has barely any throttle at all. You could take the mainjets out and try full throttle, if it doesnt virtually cut out due to richness you have a restriction. I have had numerous bikes come in for service with the mainjets lying in the bottom of the bowl, owners never knew as it is pretty rare to use WOT. I would also remove the rubbers to the airbox to rule out the filter. Has the bike actually gone properly since you put it on the road, is this something that has just developed? Something major amiss and my suspicion is valve timing if the basics check out. Re your 3000rpm strobe, just rev it right up and see where the advance stops.
 
zorba75482 said:
Swapped the 130 for 140 mains but could not tell much difference. If anything, maybe a tad more sluggish and runs out of steam at 75mph. No mean comments please  but that is just seat of pants  and I haven't ridden her for a while  Did about 100 miles at between of 65 -75mph and returned 26mpg.

Just checked timing with strobe light. At idle PM mark is about 1/4" to the right of the case mark. At 3000rpm the "A" mark is also about 1/4" to retarded.

Static is spot on mark and PM is TDC.

Have a call in with Mr Potter but thought I would give you something to ponder in case you were bored. ::)
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1/4" to the right of the case mark tells me the ignition is too retarded at idle, shouldn't idle at all actually.  Check/verify all ignition markings and go from there.  A quick check with a probe through the plug hole will show up any considerable deviation at TDC.  Might be a good idea to check both cylinders, Phil has a good point.  I find many twins to be off by as much as 10-15 degrees from right-left.  Not that you can do much about that...

With correctly timed ignition at FULL ADVANCE, ignition at idle should end with PM at about 1/4" left of the case marking, ie, 5-8 degrees BTDC.

piet

 
, at that speed you are in transition between , coming of needle and onto main I'd take a look at your needle position and the type off ,  and very easy to check ,just to make sure ,🙂
 
Could still be fuel flow restriction. If the taps or the carbs can't flow enough fuel the bowls will be starved and lack power at higher speed.
Regarding cam timing, when did this problem appear and at which point was it running ok.
If the bike has stood a while unused but ran ok when parked then I would think fuel restriction is more likely.
Can you tell us more about this?
 
If it offends you so much just move on.
Telling folk not to post something isn't really what this forum is about.
The metric system was imposed on the UK and many of folk our age are still bilingual imperial/metric.
The US i still imperial although their gallon is based on the Queen Anne gallon.
Litres/100km means little to me and I spent 15 years in Spain.
 
Grant said:
Ignition timing ok?
Points clearance ok if fitted?
Valve clearances ok?
Spark plugs and caps ok?
HT lead fitment to caps and coils ok?
Air filter ok?
Fuel taps flowing ok? On main and reserve?
Air vent on fuel cap ok?

Then I would think the carbs need to be stripped and thoroughly cleaned. When was this last done?
Could be clogged up emulsion tubes, carb filters, pilots or just general airways.
Have them ultrasonic cleaned and blown through with an air line.

All done and carbs cleaned about a month ago but has been like this since I put it on the road with fully rebuilt carbs including new brass
Click to expand...
 
sfcpiet said:
1/4" to the right of the case mark tells me the ignition is too retarded at idle, shouldn't idle at all actually.  Check/verify all ignition markings and go from there.  A quick check with a probe through the plug hole will show up any considerable deviation at TDC.  Might be a good idea to check both cylinders, Phil has a good point.  I find many twins to be off by as much as 10-15 degrees from right-left.  Not that you can do much about that...

With correctly timed ignition at FULL ADVANCE, ignition at idle should end with PM at about 1/4" left of the case marking, ie, 5-8 degrees BTDC.

piet
Click to expand...

Checked TDC with a Dial indicator down plug hole PM spot on both cylinders
 
Grant said:
If it offends you so much just move on.
Telling folk not to post something isn't really what this forum is about.
The metric system was imposed on the UK and many of folk our age are still bilingual imperial/metric.
The US i still imperial although their gallon is based on the Queen Anne gallon.
Litres/100km means little to me and I spent 15 years in Spain.
Click to expand...

(england)
 
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