Coil Issue not fuel as first thought

[MartinMille BANNED]

The story so far

I had a problem the other day where the bike was farting and carrying on backfiring etc , as I had replaced the coil pick up's wiring not that long ago I first thought one pick up might of been damaged, I started with the simplest item first and that being the plugs ,a new set installed and purring away cool all fixed, rode down the road and same thing again grrrrr.
Off with the plug leads and out with the ohm meter, nope thats all good as well so I started to play around with the mixture and cable settings BAD mistake no Carb tuner at hand ,
I started to give it a go, mind you this was on Xmas day at 9am in the back yard ,waiting for the not so friendly neighbors to start calling round but nothing came of it so I waited an extra day
It was than that I check the fuel yep nothing in there and only one carb getting fuel mind you at this point the was no return as both carbs were now completely out of sync

Anyway started to tune the carbs today with the new carb syc all started of well I got the carbs to balance but there was still a lot of backfiring going on. some time past and work called me away ,when I got back I fired her up again and continued bang followed by carb backfire and smoke, I thought it was just fuel vapor but no it was a badly split ign coil , a Dyna 3 ohm coil was the cause of the backfire. I ran out of daylight and I couldnt be bothered to change the standard coil back over

Another thing How long are these Dyna coil supposed to last ? I read somewhere about 10yrs is about it, anyone heard that before?

as these coils have a date stamp of 2/94 green 3ohm style , would the newer coils be any better ?

What are the main differences between the 3 (green) and 5 ohm (black) coils

Now when it comes to coils on bikes any straight 12v coil should do right ?

what coils are you guys running on your Duc's ??? Standard or otherwise

[BevHevSteve]

Hi martin - the 3 ohm dyna is meant for race engines where the majority of time the engine RPM is very high. These coils get very hot otherwise and even in a race bike they must be out in the wind to keep cool.

I would imagine this is why your coil(s) have died.... I always recommend running 5 ohm coils on all (bevel twin) street bikes no matter what, always.

[MartinMille BANNED]

Thanks for that Steve, I will go back to the std coils for now to make sure that this was the problem for sure. Just the same if these coils run that hot I wonder how they would fair our hot Australian summer surly running the engine say 15min or so for tuning shouldn't kill them or?

If one spark was lower/week in one cyl would this play havoc with the manifold vacuum as well ?

[darren nix]

Martin, i am running 5 ohm dyna coils on my 81 mhr after a top end rebuild didn,t fix the same scenario you are describing with the backfiring etc and i was coming home on one cylinder after 40km loop all the time so we put in the black dyna coils and it completely changed the bike for the better, i also use the silicone leads that steve sells with the kit as well, the coils have been on the bike for nearly three years now , i would also like to know [ this is directed more towards steve ] can i use 5 0hm dyna coils on a 860 gts to increase the spark, the visible difference in spark on the bikes is a lot bigger spark on the mhr to the 860, with the 860 still running the old transducers, has anyone out there ditched their old transducers for dyna coils or something else on a 860

[wdietz186]

I may be cursing myself to coil failure but I've been using a pair of green 3 ohm Dynas on my 750 GT for about 20 yrs. now. Granted I don't ride it a whole lot lately but it has had lots of 4-5 hour rides with no problem. Maybe it is from the 750's general lack of electrical energy to begin with. Could a torsional load in the mounting have caused the split? The potting on the Dyna coils is very thick and would need to be pretty hot to burn through. But as they[The team of expert scientists] say S..t happens!

[MartinMille BANNED]

Ok the old std coils and resisters are back on the bike (for now) and the backfire is gone BUT what a sh!t fight I'm having with the balancing of the carbs. did as per how to tune the carbs written by Steve , back off the cables ,back off the slide screws and mix on 1 1/2 turns out start the bike and get some sort of idle happening and this is where I come to a full stop !

Even with the idle screws fully in I can not for the life of me, get a high idle like they should , yes I'm getting full fuel and I have blown out the all the carb passages and galleries and changed the rubber inlet manifold spacers as well (granted not with a new set, but this is all I had at the time) , I'm at a complete stand still HELP

[Aussiess]

Martin,
have a look at your slides, they may be worn and the years of screw adjustments may have "rounded" the contact points (slide /idle screw). You can swap the slides between carbs and provided this trick hasnt been done prior, you will get better adjustment for a while.
You cant tell people enough how winding on a throttle and allowing slides to "clunk" back into place can damage the slide!
Anyway give it a look and let me know how it goes

[Geoff]

Thanks for that Steve, I will go back to the std coils for now to make sure that this was the problem for sure. Just the same if these coils run that hot I wonder how they would fair our hot Australian summer surly running the engine say 15min or so for tuning shouldn't kill them or?

If one spark was lower/week in one cyl would this play havoc with the manifold vacuum as well ?

I ran on a dual output Dyna 3 ohm green on my 77 900SS for about two years (with Lucas Rita ignition) and then the coil failed suddenly. Had it - and the ignition checked out by Mistral Engineering in the UK (they do the Ritas) and confirmed coil failure. Talked to Dyna (who said they'd never had one reported as failed before - but I was able to tell them about another bevelhead who had had the same problem) and by consensus between them and Mistral ended up with the 5 ohm recommendation (the reason I fitted the 3 ohm originally was consensus from them too mind...) I've now fitted a 1.5 ohm ballast resistor in series with my 'spare' 3 ohm and it runs fine. In both cases the coil is mounted to frame to aid heat dissipation and in a pretty good airflow.

At the end of the day you don't need 12v to fire up a coil and spark; do some research - as I had to - and you start to realise that many coils in the 60/ 70s were 6v items (in cars mainly) that got a ballast resistor for normal running (dropping volts back to 6 across coil to reduce over heating and extend life) and bypassed that same ballast for starting (putting 12v across coil for a short period). Interestingly, as Mistral told it to me, the old pair of Lucas or PVL 6v coils that used to be hooked up on the Lucas Rita were oil filled canister type and apparently they're better at dissapating the heat than these Dyna type so didn't overheat and fail even though they were nominally 3 ohm. Go figure...

Hence I'm happy dropping a few volts over the ballast now to protect the Dyna 3 ohm coil.

As to the ambient temperature, I used to run my SS in Houston Tx in the summer in temperatures well in excess of 90 deg F with the 'naked green' on. I moved to the UK and let's just say it's not as warm there but that's where the coil failed!

Cheers
Geoff