750sf low compression readings (was ignition question!)

Mr Vegetable Trousers

Mr Vegetable Trousers

Junior member
Location
London, UK
1973 750 SF
Carbs rebuilt and serviced by Ducati Paddy. throttle slides in sync and they "snap" back to the closed position.
Cold start o rings are new.
There are no air filters, just the original bell mouths.
Ignition statically timed to the PM mark (it had been timed to the A mark !)
Valve clearances correct.
New battery.
Fresh fuel
The bike starts quite easily, sounds sweet, and I can warm it up and turn off the cold start..
It runs well at about 1500 rpm until I rev it. It revs cleanly to, say, 5-6,000 rpm but when I shut off the throttle the revs drop slowly to about 2-3000 rpm and then it dies unless I manually hold the throttle open a little. Screwing in the idle stop screws sometimes makes a difference, sometimes not...

Im wondering if the auto-advance mech is at fault. i.e. it is "sticking" somehow? and keeping the revs up?
 
Exmoor beast said:
Check for Air leak around inlet manifolds - holding revs and dying could be your trying to bypass the idle circuit with high tickovrer-did you set the mixture screws- do they make any difference if wound in or out?
Click to expand...
Yes, the carbs clamp to steel stubs so I dont think its that but will double check. Also the pipe between the two inlet stubs is new and clamped tight... I have not touched the mixture screws yet. I will try the "squirt some WD40 at the join" trick to see if the revs change.
I will check the pilot screws and report back.
I will also check the float bowls in case there is a load of crud in there...
 
yes, easy to confirm advance is working (up and down with the revs). You can't tune idle, pickup off idle and general smooth running without setting the idle mixture screws. Screw the in and see if revs rise , then drop idle if possible. You basically want the highest idle you can get by adjusting the mixture screws - ideally between 1 1/4 and 1 3/4 turns out. If not possible you may need to go for a larger/smaller pilot jet (in the carby bowl) - larger if you have to go more than 2 turns out and smaller if you are less than 1 1/4 out. On SFs with pumper carbs it's often necessary to go an extra 1/4 turn out (slightly richer) to get revs to drop quickly back to idle.

Have you checked to float height? Eliminate the advance and the possible air leaks, then tune your idle mixture settings. Hope this helps.
 
Brake cleaner or carby cleaner spray can for air leaks, best is Aerostart or similar in your country. WD-40 is low grade white spirit mostly, can attack many grades of plastics. High idle and stall reads like lean pilot mixture, as the many good folk indicated. j
 
slow to reducde engine speed on a shut throttle? means that the motor is getting a tiny amount of air from a place that it should not, very often the O rings on the inlet manifold (each side of the heat reducing spacer which I hope is still in place?) 4 number in total has at least one of them out of place from its groove, remove carbs, remove inlet manifolds and check. You must use a scientific tool to balance the slides, what item do you use? If your 750 will not tick over for a fortnight, your carbs are not balanced, very often you see 750's that will idle for about a minute and then die, the owners "rectify" this by "catching" the stall and blipping the throttle, at trafiic lights on red you should be able to sit back, fold your arms, light a fag, eat a sandwich etc and have the motor idle FOREVER, this is also not good practice for although it demonstrates that your bikes is bob on tuning wise, the oil pump does not circulate oil very well on tickover and can even cause cam lobe damage. (ask me how I know, 3 years of London commuting.)
CLEM
 
RH carb slightly loose an not pushed all the way on. How did I miss that!
Runs much better now but still weirdly the revs go up and down without me touching anything… spraying carb cleaner at the various inlet joins makes no difference.
 
wait a minute, the cold start lever... I move it anti clockwise (pulling on the cable) for cold starts and release it (slacken cable) when warm. Is that right? Or is it the other way round like an Amal air slide on one of my old Brit clunkers....
 
CLEMTOG said:
slow to reducde engine speed on a shut throttle? means that the motor is getting a tiny amount of air from a place that it should not, very often the O rings on the inlet manifold (each side of the heat reducing spacer which I hope is still in place?) 4 number in total has at least one of them out of place from its groove, remove carbs, remove inlet manifolds and check. You must use a scientific tool to balance the slides, what item do you use? If your 750 will not tick over for a fortnight, your carbs are not balanced, very often you see 750's that will idle for about a minute and then die, the owners "rectify" this by "catching" the stall and blipping the throttle, at trafiic lights on red you should be able to sit back, fold your arms, light a fag, eat a sandwich etc and have the motor idle FOREVER, this is also not good practice for although it demonstrates that your bikes is bob on tuning wise, the oil pump does not circulate oil very well on tickover and can even cause cam lobe damage. (ask me how I know, 3 years of London commuting.)
CLEM
Click to expand...
I will check those O rings tomorrow. Yes the heat reducing spacers are in place.
 
Cold start/choke lifts the plungers in the carburettors. Although your bike sounds like it is lean, check that there is freeplay in the choke cables. Are the tiny o rings in place on the idle mixture screws?
If you take off the manifolds to check condition, also make sure the alloy surfaces the o rings sit on are perfect, see lots of them with corrosion. Also see bikes with insulator o rings that are too thick and deform things.
 
The choke plungers open an extra fuel supply when up, so up, cold start, down, normal running.
The mixture screw is a fuel, not an air screw. The more you screw it out, the more fuel you're letting in.
Paul
 
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