Coil over plug conversion query for 750 Slabby

[Macduff]

I thought it would be a good idea to do a Coil over plug conversion on my 750 slabby as it misfires at the slightest hint of damp on the leads. I located a set of coils from Eblag for a 600 oil boiler and when I tried to stuff them down the plug hole they seemed to sit a few inches proud of the top of the cam cover. Is this normal or had I not pushed them in far enough?. Next question when using coil over plugs do the spark plugs have the top nipple fitted or not? My plugs don't have the nipple fitted for the normal plug leads.

 

Edited September 24, 2017 by Macduff
more blah

[Oilyspanner]

 

Got to be careful which coils you buy/how many ohms they'll give you when fitted in series Mac. Slabbies' cdi boxes are made to have coils that have 3 - 5 ohms of resistance, Slingshots are made for 2.4 - 3.2 ohms (there will be a bit of tolerance though). If the coils you've bought don't have enough resistance, you will risk frying your ignitor box. The early GSXR K1 - K5 type stick coils are fine for Slingshots/B12s and are about the right length - the srad stick coils for example have a lower resistance. Make sure you measure the resistance at the terminals of the mini-loom you make for the stick coils, if it's in the ballpark then you're good to go.

I did a bit of research and measurements before I did my conversion a few years ago, the new version works well - but be careful which coils are used and measure what you've got before starting your bike. I used the dust covers from the top of the original plug caps, opened them up a bit and fitted them to the stick coils - this keeps the coils from rattling lose. I don't use the screw on cap on my plugs.

 

[Macduff]

Cheers that’s v’useful .

[Macduff]

Measured the primary resistance of the coil over plugs and they are 1.4ohms so 2.8ohms in series. Will that be too low for the slabby cdi and fry it due to pulling more current? 

[Oilyspanner]

That's close Mac - might  add a bit more resistance via the loom  .....  no worries for a Slingshot, anyone done this to their Slabby ? - would be best to get input from someone who's put some miles on the earlier version . There's scope in everything on our bikes , you'd think that would be okay - someone ?   - I have heard of someone fitting a resistor to give a middle reading.

[Macduff]

A further thought question. Has anyone fitted a slingshot CDI to a slabby? I'm thinking possibly a longstroke 750 slingy should have similar settings to the 750 slabby and it could just be a case of fiddling with connectors and possibly swapping ignition pickups over (or hopefully using slabby ones)?

 

[richyrich]

Or put a resistor in the sub loom somewhere maybe to use what youve got,  dont ask me what one  

[Macduff]

Well I've done the deed and fitted the coil over plug ignition. I wired them in series with a 10w 2.2ohm resistor to get close to 5ohms total. Bike starts pretty much the same as before i.e a bit shit  until warmed up , I  really must sort out my carbs idle ccts / balance. The resistors get fekking hot so I've taped them to the frame to try and use that as a heat sink for now . Working up the courage to take my handiwork for a short test ride and hoping it doesn't break down melt the cdi. 

[suzook12]

I would imagine that the resistor will fail eventually. 10W gives less than 1A current. With 5 ohms and a 12.6V supply (allowing for losses) gives you a 2.5A draw. You need a higher rated resistor, somewhere around 35-40W.  Ballast resistors are pretty cheap and can handle pretty high loads.

If you go for a slingy CDI, you will need back plate, pulse coil and rotor as well as these work with a single trigger. They may work by just using the wires from the 1-4 pulse coil, they may not, never tried it personally. The plugs for the CDI's may need looking at as well, been a while since I've had a slabby ignition although may have a dead one kicking around....

[Macduff]

Yup agreed on the resistor wattage, it was all I could get at short notice, ordered 100w 2 ohm ones

[davecara]

1 hour ago, suzook12 said:

I would imagine that the resistor will fail eventually. 10W gives less than 1A current. With 5 ohms and a 12.6V supply (allowing for losses) gives you a 2.5A draw. You need a higher rated resistor, somewhere around 35-40W.  Ballast resistors are pretty cheap and can handle pretty high loads.

If you go for a slingy CDI, you will need back plate, pulse coil and rotor as well as these work with a single trigger. They may work by just using the wires from the 1-4 pulse coil, they may not, never tried it personally. The plugs for the CDI's may need looking at as well, been a while since I've had a slabby ignition although may have a dead one kicking around....

I'm using a slabby twin pickup back plate, a B12 rotor and a MK2 bandit loom and CDI. Seems to work fine with the 2/3 sensor unplugged. The existing plug for 1/4 was the same as the bandit one.

ive got a set of COPs to try on mine so I'll keep my peepers on this!

[suzook12]

15 hours ago, davecara said:

I'm using a slabby twin pickup back plate, a B12 rotor and a MK2 bandit loom and CDI. Seems to work fine with the 2/3 sensor unplugged. The existing plug for 1/4 was the same as the bandit one.

ive got a set of COPs to try on mine so I'll keep my peepers on this!

Fair enough, I weren't sure so didn't want to say yay or nay. I knew the plug for 1=4 pulse coil was the same, I have a feeling the harness plugs may be the same as well, allbeit, the slabber has connection tails whereas the slingy plugs straight in to unit. Just been such a while I can't remember for definite...

[Macduff]

The 2.2ohm 50W resistors arrived this morning so made a little loom and mount to connect them into my existing CoP wiring. I used the  bracket from between the original coils to mount the resistors to. this will hopefully act as a little heatsink for the reistors heatsink covers. This will be bolted back on to the frame in its original position.  Will try to fit later but probably wont get a chance to try it out until Saturday.

[Oilyspanner]

Top man Mac ! - your patience and work should pay off with a neat , trouble free conversion.

[Macduff]

Fitted the mini resistor loom and took a moment to tidy up the wiring as I think previous owners have had this bike in bits and stuffed the loom into the gaps when it was rebuilt. It works and seems to have filled in a fluffy/flat sport when running on needle though still tricky to start (which it was on old coils, so need to sort crabs ). I've done about 70miles and seems to be holding up, fingers crossed it stays that way.

[Macduff]

Quick update . I’ve done a further 100 or so miles on these and they haven’t give me any grief. Starting is getting better since I richened up the idle mixture, though I think the choke cct isn’t working as intended as it’s seems to prevent starting more than help . 

[Macduff]

Quick update, in my search to solve starting issues I revisted the COP conversion and thought I'd visually check the spark at the plug. With the resistors in the cct the spark looked a bit white which I think is a bit weak, without the resistors the spark was a nice fat blue. Unfortunately I also pulled  a wire from the connector to one of the coils, so nned to rewire that one before I can test further.Once I fix the wiring issue  I've decided to run without the resistor pack and if the CDI fails due to driving a lower than it expects resistance I'll just replace it with an ignitech unit.

[Oilyspanner]

Sounds like a win : win MacD - fatter spark or failing that an ignitech,  a good result either way !  

I rechecked my coil connectors when I found one  of the wires coming adrift, no issues since .