Author Topic: 750SF3 Please Help  (Read 295 times)

Offline marcocompano

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750SF3 Please Help
« on: August 16, 2019, 02:58 »
So pleased to have found you all.
I have a '76 SF3 that suffers from serious under performance; compared with passed memories of 2 SFs that I owned in the UK many years ago. I have a friend in Nottingham UK with an SF3 and got to ride it June of 2018. It was as it should be. Free revving and no hesitation to 100 mph and beyond. However mine was dyno'd a couple of months ago at 44 HP at the rear wheel and struggles to reach 70 mph and 80 is not really an option. It runs as if it has a towel stuffed up each of the Campbell pattern silencers.
I have adjusted the valves, the points, the timing. I sent my Del Ortos to Wolfgang Haerter in Canada for refurbishing. I even tried running it on 100 octane gasoline (at $10 per gallon), but all to no avail.
I live in Los Alamitos CA USA and desperately need a competent mechanic or existing SF owner to help me out.
Thank you.

Offline Laverda SF

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Re: 750SF3 Please Help
« Reply #1 on: August 16, 2019, 04:03 »
GEEZ - That's an old bike. Trust the casing has been dissembled inspected, all gaskets replaced - Including weak Bearings - Hone job, Valves refaced and new rings... etc ?

Carbs have to be perfect with No Manifold leaks and Distributor working.
« Last Edit: August 16, 2019, 04:09 by Laverda SF »

Offline Laverdalothar

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Re: 750SF3 Please Help
« Reply #2 on: August 16, 2019, 06:44 »
Hello Marco,

have you checked the compression of the bike?Are both cylinders equal and with high readings? Is the cam-timing correct? 44 HP is way to less, it should read something like 60 at the rear wheel (at least that was what a friends SF1 did, and that engine is very similar).

What exhaust does the bike have? The original one with the big tube under the engine or a 2-1 or so?

thanks
750 SF(0) 1971 #9934 *sold*
1000 SFC Classic 1989 #3367 *sold*
1000 SFC 1985 #2934 currently under restoration
1000 SFC Preproduction bike 1985 #0001 *sold*
1200 Jota 1981 #7474
750 S-Formula 2000 #1602
1000 Motoplast - currently under construction

Offline Paul Marx

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Re: 750SF3 Please Help
« Reply #3 on: August 16, 2019, 06:49 »
Breathing
Petrol
Advance


Paul

Online Ernesto

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Re: 750SF3 Please Help
« Reply #4 on: August 16, 2019, 06:52 »
Hi, Marco.
Did you already check, if it reaches full advance?
40 at 2800-3100rpm
Ernesto

Offline Tippie

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Re: 750SF3 Please Help
« Reply #5 on: August 16, 2019, 07:02 »
Check what Lothar has said, and as Paul says, check full advance with a strobe.
If it has the exhaust collector, it will be fine to test without mufflers, as that collector quitetens them heaps by itself.
44hp at the wheel on a proper dyno is about normal and the bike should go fine, mid 40s was what we used to get on a water brake dyno back in the day. If you are getting that at low revs, as you say it cannot get over 120kmh on a flat road which is only about 4500rpm, it should increase plenty when you get it revving freely.
I fitted some mufflers once which totally strangled the motor, I was pissed off because it barely pulled 160, when it did 190 with the previous rusted out contis.
Another thing is to remove the inlet rubbers to the airbox if it has one, or the filters if they are fitted direct to the carbs, will run fine to test with just the carbs and no bell mouths or stubs.
Other things to check are brakes not fully releasing - I had a friend wanting to remove and give me just his motor to strip and rebuild as it had no power - I got him to drop off the bike, his problem was binding front callipers, the engine was fine.
Keep us informed as to how you go.
SF2 17483 (race)
SF2 17424 (road)
An Australian living near Oslo in Norway

Offline Paul Marx

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Re: 750SF3 Please Help
« Reply #6 on: August 16, 2019, 08:59 »
Campbells will soon auto destruct anyway.

Paul

Online Paul H

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Re: 750SF3 Please Help
« Reply #7 on: August 16, 2019, 09:46 »
How is it running at lower speed?

Once you have established the compression and the 'timings' (ignitions, valves) are OK, then my next suggestion would be to check for over-richness at larger throttle openings. It can really hold em back. But, be careful as you go - lean will cause a whole set of more expensive problems!

Another thought. Are you sure no one has subbed-in some softer cams at any point during the bikes lifetime?

Online Ernesto

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Re: 750SF3 Please Help
« Reply #8 on: August 16, 2019, 11:10 »
Hi, Marco.
Die the performance drop to the current situation? Or
do you know the bike
like that from the beginning?
My friend had similar symptoms on his SF though not as
bad as on your bike. Turned out, the
camchain was one or more teeth off.

@Lothar:
Is your friend's bike for sale? Must be a hell of a runner.
The (generally optimistic) Italian data state 61hp, but that's
at the engine. So no chance to reach that at the rear
wheel.

Cheers, Ernesto

Offline Paul Marx

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Re: 750SF3 Please Help
« Reply #9 on: August 16, 2019, 12:40 »
Hi, Marco.
Die the performance drop to the current situation? Or
do you know the bike
like that from the beginning?
My friend had similar symptoms on his SF though not as
bad as on your bike. Turned out, the
camchain was one or more teeth off.

@Lothar:
Is your friend's bike for sale? Must be a hell of a runner.
The (generally optimistic) Italian data state 61hp, but that's
at the engine. So no chance to reach that at the rear
wheel.

Cheers, Ernesto

Yep, 44 sounds around normal.
I'd be worried if it was a Norton, wondering if somebody hadn't mixed acetone with the petrol.

Paul

Paul

Offline CLEMTOG

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Re: 750SF3 Please Help
« Reply #10 on: August 16, 2019, 13:34 »
my brother in law's SF was just like that, it turned out to be burnt exhaust valves on both sides, easily checked by the roadside, just pout the bike in top gear and turn the engine by rotating the rear wheel, you feel no resistance and can hear distinct chuffing at all points of rising pistons in the silencers.

back at base a proper compression revealed none at all, makes me wonder how it ran, and all it was in reality, a little bit hard to start and then flat as a witches tit after up to 4,000 rpm, with nothing beyond
CLEM

Offline Piranha Brother 2

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Re: 750SF3 Please Help
« Reply #11 on: August 17, 2019, 04:14 »
So obviously clear that something isn't right, so you'll need to be methodical.

If someone brought it to me I'd start with:

1. valve clearance check
2. ignition advance unit operating correctly (does the points cam advance rotate against the springs by hand (when the motor is not running!), then check timing
2. compression check. If very low, does it go up considerably if you add oil down the cylinders?  = worn rings, damaged bores. If not, remove head and strip head for valve and seat work.
3. drop motor (only have to lean it forward to allow removal of cam cover) and check cam timing against marks at TDC
4. doesn't sound like a carburation issue but try it with no air filter/s. Make sure chokes are fully closing (freeplay in cables at the carbs and plungers definitely seating) - could also remove the plungers and check the rubber seats are OK.
What ... leave it standard??!!