Lazy #3

Rob

Rob

Hero member
Location
Midlands, UK
This has got me puzzled and thought I'd share my Easter head scratcher with you.

Newly rebuilt 79 Jota. Standard pistons. 4C cams.

Bike fires readily on two cylinders (1&2) from cold. No. 3 joins in after a few seconds. Swapped HT leads over (1-3) - misfire remains with No.3.

Carbs were ultrasonic cleaned and fully overhauled (new seals, gaskets, float valves, needles and atomisers inc new choke plungers). Choke plunger operation carefully sync'd by eye. Float heights correct. Pilot drillings checked twice (!) with carb cleaner sprayed per Stevio method and clear. Pilot jets replaced with new. Mixture screws have new O rings. Similar fuel volumes found in all 3 float bowls.

Valve clearances correct. Timing set with degree disc, strobed and correct. Compression checked and similar on all 3. Leak down test shows similar results on all 3 cylinders.

Redax Sasche ignition with IR triggers (early kit). TEK coils with new copper core HT leads and new NGK 5kohm caps. New Champion N3C. Swapped with NGK B8ES - no difference. No. 1+3 coil and lead swapped for another - no difference.

The only thing I have spotted is some fuel standoff on No.3 (running without airbox and rubbers atm for initial setup).
 
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Hi Rob.

Blimey, there's a conundrum...

Are you starting from cold, with choke? If so check the brassware that feeds the choke circuit. All the same? Not twisted? Not blocked?

HTH, Paul
 
Visit your local church and get some Holly Water. Where is the fault pattern changing? That's crazy.
 
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Reactions: Rob
Haggis said:
Hi Rob.

Blimey, there's a conundrum...

Are you starting from cold, with choke? If so check the brassware that feeds the choke circuit. All the same? Not twisted? Not blocked?

HTH, Paul
Click to expand...
Yep, cold start with choke rapidly turned off. Choke plungers all identical (and new). Smooth operation observed through hole in carb mouth. Cables are new too
 
That sounds quite puzzling, and sounds that possibly quite much was changed during the rebuild?

1. All choke plungers bottom on the mating surfaces when off?
2. I do a sort of double method with current fuels. I clean all of the drillings in carbs with solvent cleaner and small fine brushes. Then I also add carb cleaner (petrol mixable cleaning agent) when the tank is filled for first time. This after every carb rebuild and every spring, or when ever needed.
3. Does Ignitech has correct induction time for the coils? Too short induction time for the double coil? At least on my SFC the automatic setting gave too short induction time for the coils and I measured the corret induction time using an oscilloscope and then manually set it for Ignitech.
A double coil may need a different induction time than a single coil.
4. How is the synchronization of the carbs? Slide heights and idle mixture settings?
5. Checked the connection of HT wires to new caps? With resistance measurement (between the caps for double coil) and by taking them off again to check that the centre pin is located on centre of the HT cable?
6. Accelerator pumps and their diaphragms?

-Jouni
 
One of us here on an RGS had a carb breather hose block, resulting in a misfire. Carb balance hose leaking or a missing balance bolt, that happened to me once. Took me a while to find that
 
MotoJouni said:
That sounds quite puzzling, and sounds that possibly quite much was changed during the rebuild? Engine previously ran OK before tear down (a very long time ago!)

1. All choke plungers bottom on the mating surfaces when off? Yes
2. I do a sort of double method with current fuels. I clean all of the drillings in carbs with solvent cleaner and small fine brushes. Then I also add carb cleaner (petrol mixable cleaning agent) when the tank is filled for first time. This after every carb rebuild and every spring, or when ever needed.
3. Does Ignitech has correct induction time for the coils? Too short induction time for the double coil? At least on my SFC the automatic setting gave too short induction time for the coils and I measured the corret induction time using an oscilloscope and then manually set it for Ignitech. It's a Sasche box. Would be good to know the double coil time setting you use for Ignitech though (fitted to my other bikes)
A double coil may need a different induction time than a single coil. Agreed - but no facility for me to alter on Sasche
4. How is the synchronization of the carbs? Slide heights and idle mixture settings? When #3 joins the party, carbs balanced with SynX tool. Idle mixtures adjusted to optimise smooth running
5. Checked the connection of HT wires to new caps? With resistance measurement (between the caps for double coil) and by taking them off again to check that the centre pin is located on centre of the HT cable? This got me thinking! Was going to say - "Obviated by (1) swapping #1 & #3 leads & caps over (2) replacing HT coil & leads/caps" But I couldn't remember if I had swapped the caps for different items when I swapped the HT coil with new leads. I don't think I did.
I've just checked the two caps - one 5kohm, the other 5.7kohm. More than I expected so will try a different cap tomorrow. Resistance across the twin outlet HT coil (leads fitted but no caps) 38kohm - identical to the spare I tried.
6. Accelerator pumps and their diaphragms? Replaced with new diaphragms & jets. Functioning correctly.

-Jouni
Click to expand...
Thanks Jouni - comments in red above. I'm clutching at the HT cap resistance mismatch and will try another tomorrow. 🤞
New NGK HT caps (as mine are) usually read ~5kohm pretty consistently in my experience.
 
Vince said:
One of us here on an RGS had a carb breather hose block, resulting in a misfire. Carb balance hose leaking or a missing balance bolt, that happened to me once. Took me a while to find that
Click to expand...
Thanks - pretty confident new breather pipes and T piece are good
 
hooksey said:
Sounds like it needs a caining, give it a good flogg'n and report back.
Sticky valve....?
Click to expand...
Yeah - I thought it might benefit from an Italian tune-up if all else fails!
You've reminded me that I haven't checked No1 inlet bucket & valve is free to move 'cos that valve clearance was bob-on. Thanks
 
Nice to hear Rob that the problem was solved.

Any problem in double coil system has an effect on both plugs as the secondary of the coil is connected between the plug caps, causing the plugs being electrically connected in series.

My current hypothesis is that NGK plug caps have an underrated carbon resistor causing them a limited service life.
Have not sawed any open to confirm this.
I have often found used NGK plug caps that have had and still have electrical arcing inside.
It probably happens on the eventually disappeared resistor.
I suggest that NGK plug caps should be probably replaced after every 5-15 years for trouble free operation.

Have recently installed mostly Bosch, Beru and Champion plug caps. Known not to have this problem.

-Jouni
 
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