Weak Spark - Roundcase

[Crashfangio]

Time to warm up the chrystal ball again.

I'm having on-going issues with my 74 750GT electricals. Basically the spark is consistantly a weak, yellow-orange splutter across the tip of both plugs, as opposed to a nice clean blueish 'crack' that is sought.

The background story is this; This has been a constant issue with this bike since I first bought and restored it in 2001. I have never been able to get more than say an hour or two's riding out of the bike on one set of plugs. The bike has a fresh engine and is running 32 Dellorto's that are jetted stock standard. They have both been kitted and cleaned recently.

Through a process of elimination, I have:

Fitted a complete new loom and quallity fuse block,
Fitted brand new regulator
Rewound stator that is producing over 13 volts above idle
Dyna S ignition
Dyna 5 ohm coils
Dyna silicone leads

In other words the entire system has been replaced with brand new quality items.

I attempted to start the bike yesterday using a connected and fully charged car battery and brand new fuel. With used but clean plugs I kicked the thing until I nearly passed out. The best I got was the odd 'thump' from the vertical cylinder. The plugs were removed and were both wet. I checked there was spark with the above result (weak). I put a brand new pair of plugs in and it started first kick, although It was spluttering and coughing a bit.

This example is typical of the bike and even though every single electrical componant has been replaced I have had no improvement.

I realise it is difficult to guess without doing the right tests, but where would a good start be?? Any ideas welcome...

[Lumpy]

I could be wrong here, I have never had the oportunity to check it out for myself but I was told that silicone leads are a no no. I beleive they have resistance built into them as supressors. I recntly changed the coils in my SS and was advised to use wire leads. If memory serves I ended up getting my leads from a mower shop cause the auto store did`nt have any.
Now before you all jump on me and tell me you`ve run silicone leads for years, this is only what I`ve heard.

[wdietz186]

If the silicone leads are resistor type they could be one cause of your problems. Do you have resistor plug ends on the wires?[NGK style ends] That combo would probably reduce spark output quite a bit. You can tell the difference between the resistor type wire and the solid core by looking at the conductor in the jacket,the solid core is stranded wire[usually stainless] and the resistor looks like [and is]carbon impregnated fiberglass strands. The Dyna components are of very high quality and very reliable.I've had them on my 750 for over 20yrs. without a problem.You might want to check that the grounds for everything are clean and secure,a star washer between the frame and the ground wires of the harness ensure good contact.If your frame was powder coated you might not have good contact between the engine and frame and a wire from the battery or good frame ground to the engine would help.I know a seperate wire from the batt. to the headlight ground really helps with light output as the orig. wiring is almost too light a gauge to handle the current needed.

[Crashfangio]

Thanks guys,

I'm probley going to look at the grounding first up, although I'm fairly sure this is ok after my brother threw an ohm meter over the coils a few weeks ago. I might just need a second look at other parts of the loom. Ps my frame is painted.

I guess the main point I would like to make is that after changing all these componants, the problem remains. I have actually tried three different types of plug lead with no change, so I'm sure the good folk at Dyna are not to blame. For info, these leads run the flexible, single black carbon core.

If anyone else can suggest any factors that may contribute to weak output from a brand new coil please feel free to chip in.

If I dont fix it myself it will be off to an auto-elec. which is somthing I like to avoid

[wdietz186]

Next time you have it running check the charging voltage and the voltage to the coils. These bikes just won't run if the battery is below about 10.5 volts.Is the battery good? If it has a shorted cell it could show 12v or so until it has a load on it then it can drop to 10v or so.High resistance at the grounds or looseish fuses and/or bullets in the fuse block[if stock style] can also cause a voltage drop enough to lower the available voltage to the coils and a weak spark.It sounds like you have resistor type ign. wires but the coils should produce enough voltage to give a nice fat spark. While you have the voltmeter out you should also check for voltage drop across the ign. switch and the kill switch. Poor contact at the switches will reduce voltage to the coils too.

[rwhc80]

G,day Crashfangio,

If you decide to visit an Auto Elec, make sure they have some experience with old Ducati's You can spend a fortune having someone chase their tail with these old marvels.

If you're a member of the DOCV try Peter Shearman, he's a bit of a guru with old bevel electrics.

Cheers
Rick

[Crashfangio]

Hi Rick,

Peter actually fitted a new blade type fuse block and ran the loom. I got the bike back with his wiring diagram and fitted off all the terminals at the end of each branch. I agree, he is fantastic with his craft and I'm sure that any problem will be my fault and not his. I'm not a member of DOCV (yet) I was put on to him a little while ago when my original loom disintergrated.

I am sure that the bike is wired correctly as everything works as it should, It just (just??) has a really weak spark.

I'm currently up in NSW on holidays but when I return I am going to have a good look at all the earth points on the frame around the coils. I haven't scraped off any paint since the frame was painted near the coils. I simply assumed (assumptions being the mother of all...) that the attaching bolts would provide a good enough ground through the threads in the frame. I'm strongly suspecting that this will be the issue.

I can't stress enough that EVERY componant has been replaced and replaced with good quality gear with all connections soldered and not just crimped.

Thanks for the input, I'll report back next week.

[Crashfangio]

Slowly getting there. Reviewed the ground point near the coil and have seen a slight improvement with the spark, however the problem is still there.

I have run all the voltage tests between the distributer and coils, all seems to be functioning correctly.

Is anyone able to point me in the direction of how to check or set the static ignition timing with Dyna S ignition systems? I have the factory instructions and it doesnt really explain the process all that well. PS I have a degree wheel, dial indicator etc and know how to find true TDC.

I suspect that the ignition may be over advanced.

Cheers, David.

[Lumpy]

Dunno if advanced ignish would cause an issue with weak spark. Usually if they are advanced you can tell by them kicking back when kickstarted and maybe pinging when you grab a fistful of throttle. For what it`s worth I would still go to the mower shop and try some good old fashioned wire leads. Probably set you you back 10 bucks and lay the issue to rest once and for all.
Do you run ballast resistors?? Maybe try without. The other area that could be suss is the 12V feed to the coils. If thats not top notch it can cause weak spark. Old Jap bkes used to be famous for it. That is weak battery or charge system. Is there an improvement if you kick over with lights on or off??
Last thing is maybe plug gap. Try closing it up a poofteenth and see if that helps

[Crashfangio]

Yeah, I thought the question about the timing would raise some eyebrows, I'm aware that it wont alter spark quality. I just didn't want to start a new thread on ignition timing when there one may already exist. I've checked and can't find one.

Anyway, the bike kicks like a pregnant dog and is poping back through the front carb. Hard to tell whether it is being caused by the fouled plugs or incorrect timing. Either way it needs to be checked. I recall the guy who installed the Dyna S ignition told me that he had set the advance at 12 deg...

I don't run resistors, Dyna S arn't supposed to. I have also run solid wires with no difference. All my test show the coils are getting 12 V and are working OK. Maybe I'm being over critical of ignition performance and should be looking elsewhere?

[Lumpy]

Hmmmmmm, it`s a toughie alright. Have you had a timing light on her?? Might shed some light on the popping front pot. Another area worth a look might be cam timing. I`m not too sure if it was runnng well and then started mucking up after the new ignish in which case the cam timing is sweet. I`m not too familiar with this system. Mine are Ducati electronica and Bosch ignish. I`m guessing you`ve got a system that has done away with the points set up. Do you still have the old points set up?? If you could chuck them back in at least that would give an indicaton if it`s the coils or their feed. I think thats maybe how I`d tackle it, put old components back on one by one till it makes a difference and points to which one`s giving you grief. Thats assuming you have the old components and they`re in fine fettle.

[machten]

Dyna's are actually pretty easy to set up. You should be running 10 degrees BTC statically, and bear in mind you're timing off the other pot with Dyna.

I have heard of Dyna's as delivered not working with dynamic advance up to 32 degrees BTC, but it hasn't happened to me. The Dyna system uses the existing ducati bob-weights that are used on the points system for advance. You need a timing light to test this - preferably zenon.

It is actually far easier to set up timing for both pots with Dyna than points, where both points are on the same advance plate. It was a common mod to cut the advance plate in half so that you could adjust the points advance seperately - something you can do stock with Dyna.

I think BH has a document on the web site that describes it pretty well.

Kev

[wdietz186]

Crash, It's been 20+ years since I installed mine but did you lift the dist. housing and retard the drive 1 tooth before installing the Dyna rotor [trigger magnet]? It's in the instructions and if it is off I think the timing will advance too far.The reason it needs to be offset like that is the trigger signal is generated after the magnet passes the pickup and that uses up some number of degrees of crankshaft rotation.That wouldn't cause a weak spark but it could explain crappy running.

[Crashfangio]

OK, I've been doing a bit more tinkering and have some interseting developments.

Firstly, I have double (triple) checked the spark situation (again) and have found that the vertical cylinder spark aint that bad after all. The horizontal (the one I tend to check the most) is still poor. Guess which one the electronic tacho runs from?? I noticed that when the horizontal cylinder was 'firing', the tach needle would flip to the maximum position. Does this indicate a fault?? Either way, I disconnected the tach and low and behold, the spark has improved in the horizontal out of sight. It now resembles a nice blue colour and now seems equal to the vertical.

The other interesting thing I have found is at the exact moment that the ingition switch is switched 'off' (engine not running, ignition just switched on), both plugs fire. This occurs regardless of what position (degrees of rotation) the engine is in. The plugs are obviously supposed to 'fire' when the energy in the coil collapses but is this normal when the engine is not in the right position to be firing??

I have also checked the static ignition timing and so far from my best effort at finding true TDC, the ignition is gossly over advanced. If my calculations at finding TDC are correct , I have been running 14 - 15 deg. advance.

Thanks to all for the advice and suggestions to date.

[wdietz186]

Could be the tach partially grounding that 1 cyl. [I don't really know I've got the Smiths mech.tach] but the plugs firing when the ign switched is normal.I've had the bike sit for a week and it has popped when the ign. is turned on! Try to get in touch with the guy who installed the Dyna and ask him if he repositioned the distributor gear.That might explain it kicking back so hard.Also when checking and setting the timing remember to check with the engineturned in its normal direction.If you go past the desired point and turn the crank backwards the cumulative clearances of the gear drives can give an error of a few degrees.Remember that the crank turns clockwise[as viewed from the left side of the bike] in effect backwards to what most other engines we are used to.