750 SF engine pivoting

TeddyG

TeddyG

Hero member
Location
Portland Oregon
Greetings all,
My 750 SF purchased several months ago has been running better thanks to your input. After rebuilding the carbs,checking the points and valve clearances and strobing the timing, the bike seems anemic past 60 mph w / much engine vibration. the bike starts immediately and revs w/ no stumbles or back fires- compression test showed 130 lbs ea cylinder and plugs are burning clean. It has a Malcolm Cox exhaust w/o a cross over pipe I’ve heard can affect midrange. another member advised checking the cam chain timing which requires pivoting the motor to remove the cam top. I have the Green Book, but if anyone can offer tips on the process, I would greatly appreciate it, thank you.
 
No need to tilt the motor or remove the top to check cam timing. A degree wheel on the crank RHS and dial indicator in through the tappet covers is possible and gives a definite set of numbers versus just looking at whether the timing marks line up.
The benefit of removing the top would be to confirm what cam type is in there and have a look at the condition of everything. That might instruct regarding expected performance etc, but measured lift and duration should allow to determine it also.
 
Here ya go, I even found a pic...
Improvisations were the degree wheel mount (could possibly use the downloadable CD print on this site, cut a big centre hole and double sided tape it to the sprag clutch housing), pointer (bent coathanger wire), extended thin probe for the dial indicator (?), mount for dial indicator via one of the tappet cover studs (normal mag base might stick a frame gusset somewhere?).

timing.JPG
 
Thank you - I will give that some thought as I’d prefer that over dismantling the motor, etc. Just trying to determine the cause of the poor performance after checking everything else.
 
No way removing the crossover will have any serious effect on midrange, if at all. I ran my SF! for most of its life without the crossover. Does sound like cam timing to me. 130psi not a whole lot of compression but definitely enough to perform better than your description.

If you're using Breganzane's method, you'll also need an accurate reference for top dead centre. Worth doing some googling so you can get your head around it, but basically you fit a piston stop into the plug hole. I made one out of an old spark plug with a strong rod rounded off at the inserted end that can protrude about 1 1/2-2 inches into the cylinder; mine is adjustable and held in place by a grub screw - make sure you have the piston down out of the way of the piston stop before you screw it in!! I also do my check on compression stroke so the piston stop doesn't contact the valves.

Once fitted, gently rotate the crank fwd or backward until the rising piston hits the stop, and note the reading on the degree wheel. Rotate in the opposite direction until the piston rises again and hits the stop. Note the degrees and calculate where the exact midpoint is - adjust your pointer so that 0 deg (TDC) is exactly halfway between the degrees shown at the two piston stop positions. Once you have this TDC exact it's worth making a simple removable jig that bolts to your case with a mark that closely and consistently aligns with the PM mark on the starter pulley, rather than the vague factory stamped slot in the case that's about 2 inches from the pulley.

You're endeavouring to accurately determine where your maximum lift is - almost always midway between quoted opening and closing figures. So you'll need to your cam open-close figures (green book shows std cam varieties and timing for different models). eg, my 5C inlet cam has max lift (IIRC) at 102 or 104 deg ATDC. If you know where max lift is and know where it SHOULD be (according to the figures) you know how close your cam timing is.

This stuff can do your head in if you're new to it. Tippie on this forum also has a proven basic quick check for 'one-tooth-out' that only requires removal of plugs and one thumb. I reckon he'll chime in soon ...
 
TeddyG said:
Just trying to determine the cause of the poor performance after checking everything else.
Click to expand...

FWIW... The bike in the photo above had not too dissimilar symptoms, just didn't pull, vibrated a lot and occasionally would lose power completely. I went right back to square one checking the basics of compression, cam and ignition timing, faults etc but in the end it was simply running too rich with modern fuel and perhaps a different air filter element, etc. It was unexpected as it was a one owner bike which hadn't been altered, so jetting was not my first assumption. Went down several main jet sizes and maybe also lowered the needles one clip and it pulled like a train. Obviously, leaning off the jetting is to be done with great caution, the small valve head bikes can be prone to melting the odd piston.

If yours is an early SF with the heavy GT-type crank, they do not like to rev hard and vibrate a fair bit when asked to. They're much happier riding the torque wave. But they should respond to the throttle and gather speed in an authoritative manner to at least 100mph. The early SF above is my all-time favourite 750 twin, I loved it.

P-Bro above has given good tips on how to dial a cam, do some googling and come back if you need any further guidance.
 
Thanks gents- I will obtain a degree wheel and dial indicator and follow your instructions. BTW, what compression numbers should this engine have- over the 130 lbs I'm seeing? The carbs have been synched and valve clearances are correct.
More info on the bike- I originally thought it was running too rich from fouled plugs, but Scott Potter who had worked on the engine years ago suggested applying the enricher while riding the bike. The engine came to life, which I was told indicated a lean mixture at the upper end. It originally came w 138 main jets, so I went to 142s and increased the atomizer 2 sizes higher. The bike is running better w/ clean B8ES plugs, but the engine struggles past 60 MPH. It also came w the cam chain tensioner loose, and I don't feel much travel from the plunger that feels stuck in place even after lubed w WD40. There is a slight rythmical noise inside the engine that could be a loose cam chain.
So if the compression should be higher, new rings may be in order, and I need to pull out the tensioner to see if the wheel is worn. Its got 15K miles and I understand it needs the primary chain replaced.
 
I'm having similar issues with my 1975 SF2.
The bike stumbles 3500 rpm to 4000 rpm. The plugs are a bit black and sooty even after dropping the needles 1 notch.
The top end was just refreshed. It has 140 main jets. Top end power seems anemic, especially compared to my Guzzi LeMans.
I wonder if a rich running condition is causing all of this?
 
Diodehead,
After my experiences, I suggest doing a thorough servicing to the carbs, making sure they are synched w/the floats properly adjusted, etc, etc. I can give you a comprehensive Dell Orto PDF tuning guide from Bevel Heaven- my email is ted@tedgadecki.com
 
Lots going on there, Teddy. What model SF do you have? 32mm square slide Dells, small-valve head or 36mm and big valve head? Comp test on warm engine with full throttle? Cam chain tensioner is 4 bolts and 10 minutes work to sort. Pull it out and release the locknut and pinch bolt - sprung shaft should immediately spring out. To fit, depress plunger as far into the tensioner as poss, pinch up, fit and release; rotate crank while feeling the plunger rod until you feel it is as far in as it wants to go, pinch up and secure locknut - no more to see here.

Might be worth finding a dyno and spending $150 on a simple air-fuel ratio check. SFs should pull very hard to 7500. As Steve said, any model will be strong to 100mph. I personally wouildn't start playing with atomisers without firm stats on fuelling. Worth noting what Steve said above about modern fuel and a previously std and fine-running moto behaving very badly.

But as he says, you really have to get everything else right, so elminating cam a tooth out is paramount.
 
Would be expecting 165 on your compression test , was this a hot motor when tested ,
Second put the K&N air filter on ,,, square type , make sure your advance and retard bob weights haven't seized or spring failure ,,,may be , even rusted springs , as your still on points , as i am , now use generic points and condensers , did you change your condensers just to rule that out too , they are simple bike don't go to deep to start with . Just my take on things
125 compression turned out to be this ,, but the bike ran great , and the land had been broken for many miles , in-fact about 12,000 miles to be honest , slowly getting worse over the last year …
 

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Simple test for a tooth out cam. With correct timing you can feel compression against a finger over the sparkplug hole, rotating towards tdc on a cold engine, and after tdc you will feel it trying to suck your finger into the engine, easily detected if you whip your finger off. If a tooth out either of these will not happen. It does not give you a reading or a percent or anything measureable so we do not need an argument from the boffins here 👹.
It is an easy check when assembling a new motor.
All this testing I would do without any air filter, there should be no diff with or without. You are sure float levels are correct?
 
Mel said:
Would be expecting 165 on your compression test , was this a hot motor when tested ,
Second put the K&N air filter on ,,, square type , make sure your advance and retard bob weights haven't seized or spring failure ,,,may be , even rusted springs , as your still on points , as i am , now use generic points and condensers , did you change your condensers just to rule that out too , they are simple bike don't go to deep to start with . Just my take on things
125 compression turned out to be this ,, but the bike ran great , and the land had been broken for many miles , in-fact about 12,000 miles to be honest , slowly getting worse over the last year …
Click to expand...
Yeah, I reckon that one's earned retirement to the top paddock :eek: 🤣

I rode my SF1 for probably two years including travel halfway around Australia when there was still plenty of dust roads ... with the airbox and intake rubbers but no air filter. It was still running fine but definitely felt like it could do with a top end inspection. Top rings were in about twenty pieces, bore ridges so deep it had to go straight to 0.6mm OS. Riding the reco'd motor was a revelation and made me realise just how down on power it had been ... creeps up on you, this wear thing.
 
Thanks again guys- Tippie, I tried your test several times and feel pressure on both alternating cylinders as they reach TDC, but don't feel much vacuum pressure as TDC is past the alignment marks. I did another compression test w/ hot engine and again seeing 130 lbs both cylinders. Pirahana, I will remove cam chain tensioner next since the bike arrived w/ the lock bolt loose, and the plunger barely moves out as it's pulled and lubed w/ WD 40. The carb floats were both adjusted for the 18mm travel and carbs have been completely rebuilt w/ new O rings and slides. The bike instantly starts up, holds a steady idle w/ no flat spots while revved, but motor strains past 60 mph. I plan on removing the top cover and check the cam timing marks. Will report what I find soon-
 
My problem reaching full power was much easier to fix, it was likely caused by a much stupider reason. I'm hoping you are not as stupid as me.

I hadn't ridden my Laverda for over a year during a rebuild phase. I was driving my modern bikes though. On my first ride on the Laverda I couldn't get the revs past 5000 or so at full throttle. Took me a mile or so of puzzlement thinking of of all sorts of potential serious problems before I remembered Laverdas don't have 1/4 turn throttles and I needed to do the hand reposition on the grip to get full throttle. If this is your problem don't admit it, invent something more exotically complex.
 
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