73 SF2 Renovation

Bikepirate

New member
Hi All,

This bike has been in storage for a number of years. Bit of a mess when I took it over but have cleaned out the tank and carbies, replaced all seals, O rings, gaskets etc. including new fuel needles + seats and pump diaphragms. Adjusted timing etc. Engine starts and runs OK on choke when cold but when warm and the choke is backed off slowly the engine fails and won't restart without choke. I'm thinking fuel starvation. Any suggestions/recommendations?

Also, the bike is fitted with Champion N3C Plugs. I wish to replace them with new, any recommendations on this?
 
Hi All,

This bike has been in storage for a number of years. Bit of a mess when I took it over but have cleaned out the tank and carbies, replaced all seals, O rings, gaskets etc. including new fuel needles + seats and pump diaphragms. Adjusted timing etc. Engine starts and runs OK on choke when cold but when warm and the choke is backed off slowly the engine fails and won't restart without choke. I'm thinking fuel starvation. Any suggestions/recommendations?

Also, the bike is fitted with Champion N3C Plugs. I wish to replace them with new, any recommendations on this?
I would look at pilot circuit - scrupulously clean, check for broken pilot screw tips in the orifices, grotty pilot screws not sealing in o-rings, then any other air leaks. B8ES plugs, but I keep a set of ancient N3C for rough tuning ‘difficult’ engines so suggest keeping them in until you get things nearly right.
 
Yes, pilot circuit blocked.
You'll likely not be able to get it running properly without carb overhaul and cleaning. Another likely cause is blocked fuel tap filters. Check fuel flow out of the tap.
Paul
 
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Have you removed the inlet stubs? Both the o-rings and insulating blocks between stubs and head are often knackered and need to be replaced.

piet
 
Has the bike been yours for all that time in storage (ie do you know its history?). If not, I'd be ultrasounding the carbs and following all the advice above, then refit after checking for correct pilot jets. Really not a very difficult problem to trace.
 
Has the bike been yours for all that time in storage (ie do you know its history?). If not, I'd be ultrasounding the carbs and following all the advice above, then refit after checking for correct pilot jets. Really not a very difficult problem to trace.
Thanks to all for advice. I will carry out the suggestions. No, the bike was not mine when in storage. It has been under cover since initial purchase but a bit of a mess when I got it. The carbies have been thoroughly cleaned but I will check the pilot jets. What size should they be?

No problem with the fuel taps. They have been cleaned and not leaking. Adequate fuel coming through both.
 
50 pilot jets should be fine, it will definitely run ok with them, and you may be able to go down to 45.
You say thoroughly cleaned, that is not necessarily ultra sound, which is recommended if blockages are suspected. Check the manifolds/insulators first, and that the screws for fitting vacuum gauges on top of the engine side of the carb are properly in place, the little o-rings on the mixture screws as well.
 
OK, carried out all your suggestions/advice. Ultrasonic cleaned all the carby jets etc, checked the timing and fitted new B8ES spark plugs. Mixture adjusted 3 turns out. Engine starts with choke fully on, revs OK. When I back off the choke all good until about one quarter on. After this the engine goes unstable with lots of bangs in the muffler and then cuts out altogether.

Any suggestions?
 
Mixture screw 3 turns out?? thought 1.5 turns to be the base setting.
Being a 750, shouldn't it be running B9ES?? Which is equivalent to N3C plugs??
 
Mixture screw 3 turns out?? thought 1.5 turns to be the base setting.
Being a 750, shouldn't it be running B9ES?? Which is equivalent to N3C plugs??
Agree on all your points, but also with 3 turns out it should idle initially, when engine still cold.
So idle circuit should be checked.

Marnix
 
I might have read this wrong in your post but it's the body of the carb that needs the ultrasonic cleaning not just the jets. The internal carb passageways can have 90-degree corners, they're made with machine drillings meeting from different directions and get blocked with even a couple of months of sitting about and not flowing fuel. Guess how I know, it's a giant pain to fix this. Years ago it was just typically a blocked idle jet only and that's a 5 minute fix. Seen these blocked passageways be very hard to clear out.
 
Ultra lean running at low revs is also a symptom of valve timing one tooth out. If it is lean as fuck it won't matter what range of spark plug you put in it.
 
I might have read this wrong in your post but it's the body of the carb that needs the ultrasonic cleaning not just the jets. The internal carb passageways can have 90-degree corners, they're made with machine drillings meeting from different directions and get blocked with even a couple of months of sitting about and not flowing fuel. Guess how I know, it's a giant pain to fix this. Years ago it was just typically a blocked idle jet only and that's a 5 minute fix. Seen these blocked passageways be very hard to clear out.
Thanks Vince, I am going to bite the bullet and carry out a complete carby strip down and a thorough ultrasonic clean. Also, the O rings in the manifold isolators, are these standard or special?
Regarding the valve timing, the bike was previously running OK, so I can't see this being a problem but who knows...
How does the advance/retard work on these infernal machines?
 
If it's on points, and it probably is, BTW I don't have a twin but the advanced retard is done by bob weights on springs that change via centrifugal force, spinning faster. The tiny springs can stretch, or they can seize and not advance. EIs have it controlled electronically, built into the chip. An easy test is to connect a timing light and see if it changes with rev increase, from a mark on the engine case and one on the flywheel. Test it happens on both plugs.
 
Advance is a very classic centrifugal unit as mounted to all motorcycles since 1897.
Strobe to test. Set timing statically on the PM mark and then strobe, full advance is the A mark.
Paul
 
I might have read this wrong in your post but it's the body of the carb that needs the ultrasonic cleaning not just the jets. The internal carb passageways can have 90-degree corners, they're made with machine drillings meeting from different directions and get blocked with even a couple of months of sitting about and not flowing fuel. Guess how I know, it's a giant pain to fix this. Years ago it was just typically a blocked idle jet only and that's a 5 minute fix. Seen these blocked passageways be very hard to clear out.
Well, per your advice, I carried out a thorough strip down of the carbies, purchased a larger industrial ultrasonic cleaner and completely cleaned each body. After this, I gave them a thorough air blast in all apertures/passageways and of course the various jets etc. Unfortunately, I have the same problem. The engine will only start using engine start spray. When started (on choke) it runs well, revs up and and down nicely. When the engine is warm, I slowly reduce the choke until about 80% out then the engine coughs and spits and dies. The mixture is adjusted two turns out. Timing is spot on to the manual.

Any other clues you can offer?
 
If you have to add fuel to keep it running then either the carbs aren’t adding fuel properly or there’s air sneaking in later. Sounds like the carbs are spotless now. Checking the intake manifolds was suggested earlier but you haven’t confirmed, apologies if you’ve already eliminated that 👍
 
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