750 SF3 issue any suggestions welcome

S

Spondon440

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Location
Stockport
Fully rebuilt the whole of my 750 SF3, it starts and runs great on tick over, I'm running in a new crank, rebore and piston and am now at about 500 miles, just so you know the spec, its running Kieth Nairns electronic ignition which I believe is ignitec, stock carbs with all new stock jets and needles etc, exhausts are Armour with no balance pipe.

Bike runs great, no issues at all upto 4500 rpm then it hits a wall like its a rev limiter, anyone got any idea where to start?

Andy



















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What's the jetting? Float level okay?

In my experience, Armours pipes are crap. They lasted three weeks daily riding on my SF2 until the flange at the head unwelded itself and Armours never answered my complaint.
Is the exhaust rigidly mounted, if not, check the tightness of the collars at the head regularly.
I should think that the Ignition is an Ignitech.

Paul
 
pilot jet60, main 140, start jet 70, needle jet k2, spray nozzle ab265
the exhaust is on my radar and I've managed to get a balance box and a pair of stock silencers, the down pipes and box needs chroming, silencers are new, the only thing putting me off this is that the wall is at exactly 4500 rpm although initially it was at 4000 then it just changed, I'd expect carbs and exhaust to sort of be different by a small amount.
I set the float levels and although I'll check again but what I would say is that when I change gear it pulls like a train again in that gear.
The armour exhausts are rubber mounted at the rear pegs so can move, to be honest they fit a bit pants anyway, the headers are pretty solid but as soon as I get the stock system chromed I'll be getting them fitted.
I don't know if it would have any effect on the ignition but I know electronic is a bit sensative to volts, the dynamo isn't outputting any charge so the battery discharges during a ride.
 
Spondon440 said:
pilot jet60, main 140, start jet 70, needle jet k2, spray nozzle ab265
the exhaust is on my radar and I've managed to get a balance box and a pair of stock silencers, the down pipes and box needs chroming, silencers are new, the only thing putting me off this is that the wall is at exactly 4500 rpm although initially it was at 4000 then it just changed, I'd expect carbs and exhaust to sort of be different by a small amount.
I set the float levels and although I'll check again but what I would say is that when I change gear it pulls like a train again in that gear.
The armour exhausts are rubber mounted at the rear pegs so can move, to be honest they fit a bit pants anyway, the headers are pretty solid but as soon as I get the stock system chromed I'll be getting them fitted.
I don't know if it would have any effect on the ignition but I know electronic is a bit sensative to volts, the dynamo isn't outputting any charge so the battery discharges during a ride.
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Jets are in the ball park though idle jets are way too rich.
I think that originally, they're 55, I've gone down to 50 on my SF2.
I doubt that battery voltage comes into it. That electronic ignition either has enough juice, or it doesn't.

Sounds like a fuelling problem. If not fitted properly, the pumper arm can prevent the slide from lifting completely. Put a finger in the intakes and check lift of the slides.
Those engines aren't exhaust sensitive in day to day use. You can fit a straight through exhaust without touching the carbs.
Keith Nairn welds a tang to the headers that bolts onto the engine to stop the headers grinding away at the exhaust gaskets.

Paul
 
Ditto to all the above comments, and the suggestion from RGS showed me that my 750 was running lean on the top end, when I thought it was running too rich. If the motor comes alive w/ enricher activated while riding, you'll know it's too lean.

Do a plug chop test- immediately pull over after a high speed run for evaluation- what plugs are you using and are crabs synched? Check for air leaks at carb mounts and make sure exhaust is fully seated. Have you done a compression/leak down test now that it's broken in? Mine came w Malcolm Cox exhausts which are quality built and holding up well.
 
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4 to 4500 rpm is usually where peak torque occurs, how that's relevant don't know. Try putting some marks on the throttle to indicate 1/2, 3/4. That's more relevant to what section of the carb isn't working. An oldie but a goody is to try opening the fuel cap. I had an issue recently with my tank bag pinching off the cap breather hose. Have a look at any carb breather hoses as well, a mate had his carb breather melt on the exhaust on one carb. Hopefully it's something small and easy to fix. Choke on is a great test. BTW, it's possible on the ignitec to set all kinds of parameters when fitted, including the rev limiter, plus the advance curve, all done and seen on a plugged-in laptop. Who set up the ignition?
 
Tank cap open a good easy check. Is the rev wall independent of throttle position? That is, if you use 1/4 throttle or full throttle are the symptoms the same? Will it rev beyond 4500 in neutral? Even without doing a plug chop, pulling the plugs might give a hint if it's overly rich (possibly choke plungers slightly open - need plenty of slack in thew cable at close - or rubber seal at base of choke plungers dodgy and not sealing).

Do you have an MS based PC (ie not a Mac) and can load the Ignitech app (off their website). you'll need to correct cable to attach to the laptop. You could then confirm that the ignition curve isn't awry - unlikely if Keith set it up for you, but always good to double check if there's an issue.

Constant loss battery won't be a prob - if it starts the bike there's heaps of juice for running the Ignitech. Jetting sounds close - agree pilots may be a tad large.
 
I’m a big believer in asking the supplier first, before asking the audience to phone a friend for a 50/50 answer 😁
Maybe get an opinion from Keith Nairn regarding the advance?
 
Andy said:
I’m a big believer in asking the supplier first, before asking the audience to phone a friend for a 50/50 answer 😁
Maybe get an opinion from Keith Nairn regarding the advance?
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True that Andy - but also good to get your head around the Ignitech system using a PC.
 
I once saw someone lose their map by fiddling on their laptop with this stuff. Sadly, it was actually at a bike shop just before the Dino was attached. That was frustrating for all involved that day. I think there is a way to store this data before learning the system as you fiddle for the non-pros out there.
 
Andy said:
I’m a big believer in asking the supplier first, before asking the audience to phone a friend for a 50/50 answer 😁
Maybe get an opinion from Keith Nairn regarding the advance?
Click to expand...
I already spoke to Keith Andy but he's 200 miles away from me and really I think he needs the bike in front of him to really diagnose this issue (I may well do just that yet), he suggested a lot of whats been suggested here but I thought I'd exhaust everything I could before mithering him further.
Andy
 
So where are we up to, checked the fuel taps, plenty of fuel and the taps are brand new, carbs fully open, I have 50 and 55 pilot jets on order already should arrive next week, did a plug chop and they are a decent colour.
The 4500 rpm is all the time whether its on the main stand or being ridden in any gear, just hits it like a rev limiter.
I took it out today are used the choke and it made no difference at all, I checked the exhaust flanges and they are sealed ok and haven't moved from where they were originally and all seems good at the moment, I have run it on a filter (K&N in the filter box) and I've run it without and its exactly the same.
My own thoughts are that its either the coil or the ignitec box with an issue, I guess I could send both back to Keith for testing but the bike is out of commission for a while which I could do without.
Andy
 
This won't be a pilot jet issue at 4500rpm.

Would be good to confirm if it is Ignitech (which Keith prefers). If it is then would be worthwhile checking settings and see what's happening with a strobe too.
I've no experience of what triggers are used on a 750 but did have a problem with the board on my triple. Made worse by its intermittent nature - did show up when the strobe didn't flash briefly.
The other possibility is a broken wire going completely open /short circuit with the frequency of vibration at 4500...
 
Good idea from Paul to check with a strobe. By 4500RPM it should be at full advance of around 35-40 deg, depending where Keith sets max adv. To my mind it's starting to look like a rev limiter has been inadvertently set to 4500, because even if advance is only going to 20 deg, It will still rev beyond 4500. The fact that it won't even spin beyond that in neutral is definitely a tell-tale.

No idea how a motor responds to various advance settings but it could even be that the Ignitech is not advancing at all, so trying to rev when firing at 8-10 deg BTDC ... I'd be looking at ignition for sure. Do you have access to a PC? Once you have the correct cable and the appm loaded it's dead easy to check the map and the settings v- you don't need to change anything to have a squiz.
 
piranha-bro2 said:
Good idea from Paul to check with a strobe. By 4500RPM it should be at full advance of around 35-40 deg, depending where Keith sets max adv. To my mind it's starting to look like a rev limiter has been inadvertently set to 4500, because even if advance is only going to 20 deg, It will still rev beyond 4500. The fact that it won't even spin beyond that in neutral is definitely a tell-tale.

No idea how a motor responds to various advance settings but it could even be that the Ignitech is not advancing at all, so trying to rev when firing at 8-10 deg BTDC ... I'd be looking at ignition for sure. Do you have access to a PC? Once you have the correct cable and the appm loaded it's dead easy to check the map and the settings v- you don't need to change anything to have a squiz.
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Exactly, if it looks, runs and sounds like a rev-limiter, maybe it IS a rev-limiter. Can you reinstall the points and condenser as a test? If it then ran up over 4500 Paul will be jumping for joy!🤭
 
There are occasions where running in a new build, it can help to use a lower rev limiter, but I would expect Keith to tell you. It's a 10-second fix with a laptop if that's it
 
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