SF2 - flat spot wierdness

mik667

mik667

Senior member
Location
San Francisco, Ca
hey All
  so in the sf2 rebuild its all together and seems to be ok, starts, idles etc but i have a flat spot thats between 2000-3000 rpm that i can't seems to rid myself of so hoping for some advice. Its really exacerbated if i'm going up a grade/hill  - but if i give it a big twist it seems to power though 2-3 k pretty well - just sort of in the cruising at those RPMs it has the problem stuttering and the likes - doesn;'t do it just rev'n it up the garage

  the bike has standard carbs, a sachese ignition, and a bit higher piston (ross). i've tried dropping the needles, raising the needles, switching between curve 4-5 on the selector (both equally bad). tried nudging the rotor a bit forward and a bit backwards - still the same. switched plugs (BP8ES)/closed the gap a bit - nutting.  i think i have the carbs balanced but the manifolds don't pull enough vacuum for my synchs so its eyeballed. I've checked the float level- seems Ok - at my wits end - any suggestions?

thx
mike
 
Does it have accelerator pumps?
If so, are they squirting as they should?
 
At what throttle opening is the trouble at?  You need to determine which carburetor circuit is causing the difficulty.  Have you changed the slides or pilot jets?  If you are using a different slide cutaway, or if the pilot jetting is not right, that could be the cause.  The Dell'orto book would be very helpful to you.

Also, if the needles & atomizers are old, they might need replacing.  Vibration can cause them to wear unevenly, which can cause erratic running.
 
Hi Mike - Don't like to comment on these sort of problems - as can't test it myself - but if you've still got the points assembly - try re-fitting that temporarily as SF's are so non fernickety with the mods you mention except for the ignition that I've seen  cases recently of erratic behaviour  - don't like BP8Es with H/C pistons - Regards - Phil
 
+ 1  :D to what Phil says and have a quick look around the ignition electrics making sure that you don't have a loose wire.
Paul
 
He was talking about ignition curves 4 and 5 so I suspect there aren't any points. Would also expect jets in particular the idle jets. that was my original similar problem. When we got that going then we had high revs problem which turned out to be the main jets too big.
 
I would be a little concerned at the lack of inlet manifold vacuum, may be worth checking the cam is timed correctly.


    Keith
 
Keith is on the money with his reply, there has to be a reason why you have no vacuum.
Check your valve timing and valve clearances, also check you do not have a vacuum leak on the inlet manifolds.
any 750 engine has ample vacuum to drive a set of vacuum gauges.

 
Thx Guys - i do have all the marks lined up on the cam and the crankshaft etc (checked it twice, including pulling the motor once and looking at it it double check) - but maybe there's a problem there, i'll exhaust all the possibilities before dropping the motor again as he lack of vacuum off the manifolds was something i'd seen prior to the motor blowing up when trying to sync the carbs etc and it didn't have that hesitation - so not sure what that means

jetting now is 58 pilots, 140 mains and K2 needle - all new gaskets in the carbs including those around the pumpers - the plugs don't look super lean but aren't exactly sooty etc with the BP8ES (with champion N3C they sure were through) - i have some BP9ES coming as no one carries them around here - also i tried pulling on the choke when the problem occurs and it didn't help - i also replaced the intake manifold  blocks &  gaskets with new ones from wolfgang  - i'll try disassembling the carbs again and double checking everything
thx as always



 
a long long long (even longer) time ago (1973) I rode a running and riding very well, 750 SF up to Slaters, it was a little bit hard to start (not that bad, with plenty of choke) but did make a sort of chuffing sound when cranking, this was my brother in laws bike (NAB312K) which is now resident and restored in Norway.Roger the dodger diagnosed burnt out valves, and I thought that it just cant be,0 since I had been doing 90 and 100MPH most of the way there (and scraping the shinshavers on those last few bends that go into Bromyard) plus it was doing better than 60MPG and maybe even close to 70MPG,but there was a flat spot around 2500/3500, which is annoyingly around about the UK legal crusing speed.

Dodger and Dickie took the bike into the workshops and whipped off the head, both exhaust valves were looking more like threapenny bits, so new ones were found and lapped in, tappets adjusted, carbs balanced timeing checked and away the same day, try doing that in a Kawasaki dealership!!!

I didnt even pay the bill, since it was not my bike, and Nick had not owned it very long, having bought it from Slaters as an ex demo, I dont recall the details but he was very pleased with the whole plot, shame it cant happen now.
CLEM
 
A simple bush mechanic test to check if your cam timing is way out.
Cold motor, remove the plugs, and turn over the motor by your socket on the crank.
Seal lightly over the plug hole with your finger before and after TDC compression.
It should puff out as the piston rises before TDC, and give a good vacuum as the piston goes down just after TDC (whip off your finger and it will "pop").
If it is so far as a tooth out it won?t do one of these.
As said make sure of vacuum sealing in the manifolds.
2 to 3 grand is barely open throttle. Those are mega size pilot jets. Normal is around 45. 
I never use protruding electrodes in my SFs.
Have you strobed it to see what the timing is doing at those revs?
 
Hey Tippie

the green book recommends 60's i believe for pilots for SF2 - i actually have a sparkplug insert thing  w/ marks on it to indicate  TDC - not the 100% accurate method i know but gives me a pretty good idea of TDC. Going to to the timing thing as you recommended this weekend (provided my timing light can be found). going to dissemble carbs and check manifolds etc. If all looks good will get pilots of different sizes v- it had 55s in it i believe but bike was cold blooded to warm up so put 58 ins - didn't think that was the problem but maybe the engine was so worn before it blew up these were masking the problems etc.  Wolfgang also recommended BP8's suggesting that the extended plugs may cause a piston hot spot, but should give the symptoms i'm seeing

Big weekend planned-hoping it will get above 45F in the garage!

thx!
m

Tippie said:
A simple bush mechanic test to check if your cam timing is way out.
Cold motor, remove the plugs, and turn over the motor by your socket on the crank.
Seal lightly over the plug hole with your finger before and after TDC compression.
It should puff out as the piston rises before TDC, and give a good vacuum as the piston goes down just after TDC (whip off your finger and it will "pop").
If it is so far as a tooth out it won?t do one of these.
As said make sure of vacuum sealing in the manifolds.
2 to 3 grand is barely open throttle. Those are mega size pilot jets. Normal is around 45. 
I never use protruding electrodes in my SFs.
Have you strobed it to see what the timing is doing at those revs?
Click to expand...
 
phil37 said:
Hi Mike - Don't like to comment on these sort of problems - as can't test it myself - but if you've still got the points assembly - try re-fitting that temporarily as SF's are so non fernickety with the mods you mention except for the ignition that I've seen  cases recently of erratic behaviour  - don't like BP8Es with H/C pistons - Regards - Phil
Click to expand...
Absolutely agree on the plugs - I've used B9ES in my SF since standard comp (volume checked at 8.9:1) and right through to 10.5:1, touring, racing, commuting - never missed a beat (and never holed a piston  :o)
As for the flat spot, totally agree again with Phil on fitting the points - eliminate that poss (it's easy and reliable).

Pilot jets do sound very large (too lazy to go to garage and check mine - it's midnight) - early SF2s in Oz had too small pilots, which made them excruciating to (not!) set up (Tippie knows what I'm talking about  :P ).

Any chance the bike's ever had the slides replaced? Incorrect cutaway is another poss (especially if those pilot jets ARE too big to boot). Forget about accel pumps as a probable cause - I removed mine 20 years ago and again, bike has never missed a beat as a result. Unless of course they're flooding an already over-rich mixture ...!

But don't exclude even simple things like plug caps (another Tippie story when we were mechanicking in Darwin in '74 - a Ducati Sport that swallowed about 8 hours while he/we tried to source a misfire - unbelievable that it was THAT effing simple  :-\ ). Most probs will only manifest they selves under load.

But again, as Phil says, it's very difficult speculating without the bike to play with!

Good luck and keep us posted.
 
Hey gents
  So after pulling carbs off and rechecking manifold gaskets, float levels, etc and replacing the idle jets with #62s (replaced #58s) followed by putting BP9ES plugs in and retiming and rechecking all the connectors off the EI the bike now seems to run without the stumbling previously observed - at at least for the 5 miles i ran it around today staring through blurry IPA head/eyes from the previous night
many thanks to all - best holiday wishes!
mike
 
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