750SF valve timing/advance and compression numbers

TeddyG

TeddyG

Hero member
Location
Portland Oregon
Hello all,
Now that I've got my 750SF running, the engine overall performance/acceleration is poor, w/ bogging at throttle, and struggling past 50-60 mph. The carbs have been rebuilt (gaskets, slides, cables, choke plunger, K2 needles, float height checked) valve clearances are good, the points and gaps are good, the ignition timing on both cylinders are good. A leak down test gave no hissing and initial compression is 150/130. The bike starts immediately w/ steady idle and will rev freely. It has a Malcolm Cox exhaust w/ no cross over pipe and the engine is stock as far as I know w/ 15K miles. The header pipes are golden color and I've had constant issues w/ fouling plugs even after going down to 55 pilots and B7 non-resistor plugs. Since the bike starts immediately w/ minimal enricher, I'm thinking it's too rich, but I've run out of possible causes.

So, how involved is a valve timing test w/ a strobe light and what are the steps- I don't see it covered in the Green Book. What are the advance numbers and what's a good compression amount to expect?

Thank you
 
Make sure the choke isn't hanging on, the choke cable has free play where it enters the carbs and the rubber tips of the choke plungers are nice and soft. Check the floats are set right, Just touching the plunger. Not compressing that tiny spring on the plunger with the carb sitting flat and horizontal and at 18mm. Check you're getting advance on both cylinders, physically check carb sink. Finger in the throat. It's going to be something dumb, like a fuel tank breather. Good lick.
 
Thanks- I installed new cables and plungers that have free play and also checked the float height, and synched the carbs. The bike struggles to reach 50 mph, so something's wrong here.

I'd appreciate anyone who can answer my question about strobe timing and compression numbers, thanks
 
Cam timing isn't done using a strobe - I think you meant ignition timing advance? Easily checked by running a strobe and checking that the PM mark moves steadily clockwise (when looking from the left) as revs increase. Mark on case and A mark (= full advance at 40 deg BTDC) should be aligning at something like 4500rpm (someone may have the details).

Struggling to reach 50mph (80kph) is seriously underwhelming! An SF should be pulling hard to well over double that. Fouling plugs is uncharacteristic of an SF - and B7s are VERY hot plugs! I've never run anything bu 9s and have never fouled a plug. So could well indictae something with fuelling is amiss ... or a weak spark.

If you wanted to check cam timing you'd need to drop the motor, remove the top cover and ensure that the mark on the cam sprovket aligns with the mark on the head at TDC.
 
Have come across one bike in the NSW club that was put together with the cam 1 tooth out, would start and idle, but no good higher up. Drop the motor remove the top plate and check. Don't trust the TDC mark on the pulley, keys shear.
 
TeddyG said:
<snip>
I'd appreciate anyone who can answer my question about strobe timing and compression numbers, thanks
Click to expand...
150 & 130 psi, no great problem there.

(As others: strobe is for checking the ignition advance, nothing to do with valve timing).
 
wit the comprehensive information provided on what has been done, it does sound to me as though the cam timing is one tooth out, check this first and if its bang on, at least you have proved the point, another possible cause (much harder to see) is that a valve spring may be broken, but has enough left to close the valve onto its seat, but without any great deal of pressure, or speed of return.
CLEM
 
Apparently, if you didn't squeeze them to the max, don't wring them out or they tear the foam, Unifuilters and remove as much filter oil as possible they choked the engine like Andy said. But your problem is pretty extreme.
 
Thanks guys for all your helpful info. The engine was supposedly inspected w/ new piston rings and a valve job, but uncertain when this was done. I'm assuming that if the mechanic was at all competent, the valve timing would've been checked or set, but that remains to be a question.
I installed DNA air pods and there's no difference w/ or without them- the bike accelerates horribly and struggles to reach any speed. It vibrates quite a bit and I've checked the mounting bolts. I also installed premium spark and coil wires and tested the coils.
A friend is going to help me check the timing and I'll report back w/ the results.
However, it is a pretty garage ornament!
 

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Always a bit of a lottery buying something 'someone else' has done 'major' work on, Teddy. i repaired an SF motor for a friend after it destroyed a piston and a rod (bent 180 degrees!!) and the splines in the inner web because the very expensive rebuild had included line-boring the crank bearing journals and fitting sleeves - SFs are renowned for spinning the mains in the cases. Loverly machining work ... unfortunately the sleeves the 'experts' fitted inadvertently covered the oilways to the inner mains, which is also the feed to the big ends and cylinders!!

So don't assume a 'competent' engine builder has done it right. In the scheme of things it's no big deal to drop the motor and check the cam timing. No need to degree them unless you are looking for an extra few percent.

Definitely check that the TDC-PM mark on the starter freewheel aligns with the mark on the cases by using a degree wheel and a piston stop. You may like to fashion a plate that fixes to the cases that gives you a closer indication of TDC. Mine bolts to the two top sidecover threads in the cases and is a simple piece of unequal alu angle and small L-brackets with a curve to fit closely (but not touch) the freewheel).
 
All these machines are over half a century old.
Don't ever assume anything unless you know the owner.
Shiny photos and words ain't worth nothing. Imagine what it would cost if we did with pictures of women in girly magazines what we did with pictures of bikes in advertisements.
Paul
 
Paul Marx said:
All these machines are over half a century old.
Don't ever assume anything unless you know the owner.
Shiny photos and words ain't worth nothing. Imagine what it would cost if we did with pictures of women in girly magazines what we did with pictures of bikes in advertisements.
Paul
Click to expand...
And from experience even then unfortunately....
 
A quick way to check if cam timing is a tooth out is to use your thumb or finger to seal over the spark plug hole and feel for pressure before tdc compression and vacuum after it (whip off your finger and it should "pop" sort of. One tooth out and either of these will be missing. I have had SF with a tooth out either way and this test exposed it. One way and it is lean as shit at idle and only goes with higher revs. The other way and it feels like a diesel that won't rev out at all.
 
Yes, I have no illusions about the previous work done to this bike after seeing some ham-fistedness. It was kept pretty but NEVER serviced.
Thanks Tippie and Pirhana- I'll try your suggestions next. A friend checked the ignition timing w/ a strobe and said it was spot on. After doing all the above mentioned work and tests, I've run out of ideas for possible causes. I've also noticed that the cam chain tensioner bolt was never fastened w/ the locking nut, so perhaps it slipped a tooth.
Again, the engine fires right up, holds a steady 800 RPM idle and revs freely. But once underway, it runs like...
 
Highly unlikely it would jump a tooth with the tensioner bolt undone Ted. I've had a tensioner arm break in half and the chain didn't jump. Sounding more and more likely it's cam timing. Tippie's quick test sounds like a bewdy - I know the 'pop' on the thumb well when i'm doing a quick check for TDC comp or exhaust.
 
All the bad characteristics described are similar to how I would summarise my SF. Mine not so bad, but certainly 'that way inclined'. Be careful not to chase perfection.
 
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