Engine 17580 Reincarnated but...

E

Evan

Full member
Location
Texas
It was 4/2/2021 @ 3:15pm CST when I fired it up. I enriched the mix this time where as before it couldn't get much out of it @ 1 1/2 turns out. Ran Ok but I had to tune carbs. I believe I have the mixture @ 3 turns out or more. Working both screws I now have it smooth. Grn Bk says 1 1/2 out on mix but also says Bosch drop keys need replacing & then a pic of how to hold the engine when loosening a crank nut Primary side? (page 73... metal rod wedged between sprocket teeth) Never did either of that...
17580 was rebuilt w/ the Ross Pistons (80MM I think?) So my problem I believe was more volume inside the cyls & it was too lean.
Then it also cut out stopped a few times. If this is a rich mixture wouldn't only 1 cyl stop 1st?
I did remove the Voltage Reg & replaced w/ new grommets inside w/ extension screws. Seems to be charging. All other electrical remained in tact.
Runs great otherwise. Its an Aries!
 
80mm is standard bore for SF. What size pilot jets are in it? If the ignition timing is correct at idle, and valve clearances are correct you may need one size up (5 numbers) pilot jet. Generally 45 or 50 is ok. Also make sure all the manifold sealing surfaces are perfect - either side of the insulator piece that the o rings sit in.
 
Tippie said:
80mm is standard bore for SF. What size pilot jets are in it? If the ignition timing is correct at idle, and valve clearances are correct you may need one size up (5 numbers) pilot jet. Generally 45 or 50 is ok. Also make sure all the manifold sealing surfaces are perfect - either side of the insulator piece that the o rings sit in.
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Standard pilot jet is 55.

Paul
 
and I got a new Air Filter. This one is the green foam not the brown fiber like the originals. I cleaned my old original one & it looks Ok. I could put it back on. Runs fine but I don't know if the original would be better....
 
Evan said:
It cuts out when riding. Not @ idle.
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Check there is air flow into the tank at the hole in the cap, or open the cap, you may be starving it of fuel if air cannot get into the tank as fuel tries to flow out.
Not sure what you mean by "Bosch drop keys need replacing", please clarify if you think it's something you need help with.
 
I doubt it would stop suddenly with no rough running beforehand if it was a fuel issue - although at 3 turns you definitely need to look at your carburation (but that's a different issue). If both cylinders cut out you need to look at your electrical system, contacts, earths, dodgy wires/wiring. Have you gone over the whole bike, set the points and ignition timing, removed, thoroughly cleaned the carbs, checked the sealing of the intake manifolds (as mentioned, o-rings on both sides of the black nylon insulator-spacer).

What's the history of the bike? New to you?
 
If it cuts out when riding it is not idle mixture. There should be no difference to running with or without air filter, leave the connector tubes to the airbox off while you fault find. As said, try riding with the fuel cap open, or a bit of thick wire in across the flats to stop it sealing. It sounds most like a bad elect connection. Do all the electrics die at the same time, or just the motor?
 
Bosch "drop key" as I call it is code # 76110987 75 on page 93 of Laverda 750 Operation and Maintenance Spare Parts List. Never had a problem w/ it. Don't know why Grn Bk says replace it, I believe I have an electrical issue on it cutting out while riding. I bought 15538 back in 1984 as a parts bike for Sf8787 which I rode until 1987. Sf 8787 I originally got in 1981 traded a Yamaha 350 Rd. Rode it for 8 yrs. In the process of redoing it. Then I bought Sf2 17580 in 1987 which I rode until 1995. Sf2 did cut out when riding. Never could figure it out. Didn't cut out all the time. but it had the connector box under the seat. Just rebuilt 17580 w/ Ross pistons & lesser head. Sold a lot of parts in the 90s off it. Sold sf2 frame w/ title 2011. Still have odds & ends from Sf2 but I always liked the original racer...so much for history.
Will look into the electrics more...
 
A classic for your symptom is the ignition switch, either the contacts inside or the soldered wires on the back of it. Many of us have fitted a mini relay so that the switch doesn't have all the bikes power going through it.
 
Evan said:
Bosch "drop key" as I call it is code # 76110987 75
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OK, I did wonder if that might be what you were referring to but could not see it on 17580 which would have the later, very reliable key switch.

It's not the push in key itself but the contacts on the switch mechanism inside the headlamp shell that will need care - likely they're corroded or may even have bits broken off. If cleaning up all the contacts doesn't resolve the cutting out issue then replacement of the whole switch mechanism with a modern key switch mounted wherever is convenient is probably your best option.

The Bosch switch as on the earlier twins was also used on BMWs like R75/5 of the era and you could try to replace with same if you want it to be as original as possible.
 
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