rich on one cylinder

O

olly

Senior member
Hi,
I'm having real probs getting my SF1 750 ready for the ILOC meet tomorrow.
It runs very rich on the left cylinder.  I've had the carb, jets out etc and whilst it's worn and a bit corroded in there it's no worse than the other carb which leaves the plug nice a clean rather than caked in soot like the left.

I dropped both needles a notch to make the misxture leaner and it's made no difference.

As it stands, the bike is very lumpy between about 3500 and 4500 (though not particularly smooth anywhere else).  Once it's properly warm it ticks over well enough at very low revs.

Could it be anything other than carb related? Any quick things I can try or do this evening?
Thanks Oliver
 
A few quick things:

Choke plunger is sealing.

Float hasn?t leaked and sunk.

Float valve is sealing.
 
The float floats.  The choke rubber disk has an indent where it has been sitting. I couldn't pick it out to flip it over.  How do I check if it's seating properly please? Same for float valve...how to check it seals? Blow somewhere?
Thanks olly
 
Just don't run it too lean! Says the man who holed a piston. :-[
Especially if you are going cruising as on open road.
 
Paul Marx said:
Check choke cable to that carb isn't sticking somewhere.
Check timing statically.
Paul
Click to expand...
Carb cable all free. In fact I will even disconnect it temporarily to be sure.  Is the timming between the cylinders not fixed i.e. if one cylinder is fine then it must be carb related?
 
SF750-72 said:
Just don't run it too lean! Says the man who holed a piston. :-[
Especially if you are going cruising as on open road.
Click to expand...
Done 30 mins at 80mph today, exhaust not overly blue and plug colour not too fried.  Will prob put needles bAck to middle position tonight though to be safe.
 
olly said:
Carb cable all free. In fact I will even disconnect it temporarily to be sure.  Is the timming between the cylinders not fixed i.e. if one cylinder is fine then it must be carb related?
Click to expand...

Timing is independent, sort of, one cylinder to the other.
If backplate moves, timing will move on both sides. If the independent point moves or if the gap closes on one of the points, only one cylinder will be affected.
Paul
 
While you are looking at the points, check that the advance unit is properly screwed down and that it is free to rotate against it?s springs.
Have you tried with new sparkplugs? Or try swapping them and see if the problem stays on the same side.
Do a compression test.
Did you completely strip the carbs, needle jets out etc and check that compressed air blows through all the fuel and air passages?
When you say "lumpy" do you mean vibration-wise or running rough/uneven? Does it misfire?
 
When you say rich, do you mean it's sooting a plug at idle or when running from mid range on?
Did you read the plugs?
Do you use the bike regularly and if not do you run the carbs dry or empty float bowls before putting it away?

Paul
 
Tippie said:
While you are looking at the points, check that the advance unit is properly screwed down and that it is free to rotate against it?s springs.
Will do

Have you tried with new sparkplugs? Or try swapping them and see if the problem stays on the same side.
Stays the same with new and swapped

Do a compression test.
Will do, but what are you angling at there? Oil being burnt something like that? I'll be very very surprised if compression differs significantly on that cylinder but I will look

Did you completely strip the carbs, needle jets out etc and check that compressed air blows through all the fuel and air passages?
Yup all except the inlet to fill the float bowl

When you say "lumpy" do you mean vibration-wise or running rough/uneven? Does it misfire?
I mean hesitation and uneven speed, jerky, no misfire
Click to expand...
 
Paul Marx said:
When you say rich, do you mean it's sooting a plug at idle or when running from mid range on?
I don't know when it soots them for certain but I've had the colur tune on it and the flame is nice and idle. My guess it's mid range on or maybe mid range only. 3500 to 4500 is the worst bit

Did you read the plugs?
I've not done a proper plug chop as such but since I know the mixture is good at idle and I know it's sooting up elsewhere how would I interpret the plug?  The other plug is fine

Do you use the bike regularly and if not do you run the carbs dry or empty float bowls before putting it away?
I've had the carbs apart, far from perfect but both carbs very similar except one doesn't soot up the plug.
Paul
Click to expand...
 
Hi,

I spent ages last year trying to get one cylinder on my SF 1 to run a bit weaker. Although I was sure the timing was right on both cylinders when I checked again the rich cylinder was retarded slightly. Once the timing was exactly right on both pots the bike was transformed - really smooth with a nice tickover. Surprising how important timing is on the SF. Do the basics first.
 
mikeringrose said:
Hi,

I spent ages last year trying to get one cylinder on my SF 1 to run a bit weaker. Although I was sure the timing was right on both cylinders when I checked again the rich cylinder was retarded slightly. Once the timing was exactly right on both pots the bike was transformed - really smooth with a nice tickover. Surprising how important timing is on the SF. Do the basics first.
Click to expand...

Okey doke - up at 7, will have a bash, got to be on the road at 1ish for ILOC.

P.S. Aren't you quite close to me in Oxfordshire ;) ....working tomorrow morning ?

Thanks Olly
 
Hey Olly,

Very close to you in Oxfordshire...

My advice is to pay the weak cylinder more and if you educate it right then it will become as rich as the other one.

Hope this helps?!@?

See you at Baskers.

J
 
Olly, it sounds like you have covered the carbs pretty thoroughly with not much success.
Maybe it is time to look for a poor connection along the ignition lines.
It could be vibration related poor continuity.
Check all your connections from the points to the coil negs, and the power circuit to the coil plusses. Also the HT leads and plug caps. If someone nearby can help you could try swapping the points plate in case it is a condenser. In fact to isolate it you could try swapping the wires to the points, and swap plug leads also, and see if the problem swaps sides.
This will address rough running but not really related to a sooty spark plug, but you never know. It is pretty quick to check.
 
Some food for thought there. Thanks.

I bought another 1970 BSA A65 the other day (my current one is for sale soon), this new one has had a lot of engine work done, electronic, ignition, new carbs etc.  Except for a tiny tiny weak cut from idle it's made me realise the old bikes can be made to run great.  I want my SF1 better than that :)

On an aside, the previous owner of the BSA had modern linings fitted to the front break and it all skimmed to suit. What a difference! Definitely on the list of things to get done to the Laverda.

Right, so I adjusted the points and timing.  Gaps were fine, the back plate was off TDC by a couple of minutes, the other cylinder by more, unfortunately one of the plate clamping screw threads is stripped so it's only held by one screw.  The eccentric adjuster screw is a bit fierce isn't it.

Set up as best I can and needles returned to middle notch.  No time for more fiddling, pack, lunch and go to Hay.

Thanks Olly
 
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