Dunstall GS750 eating spark plugs

[jock]

Hi there - I have a problem with my 1981 Dunstall GS750. It runs dyna ignition, Accel coils and HT leads and gets through NGK B8ES (also tried BR8ES) plugs at a fair old rate. The problem starts with the engine running on 3 or less cylinders and its always an arcing or weak spark plug to blame, fitting new standard plugs always cures it and it runs like a good un for a while. The engine was tuned from new at the Dunstall factory and is fitted with 29mm Mikunis from VBS (Pip Highams tuning shop in Manchester) and higher compression ratio. It goes like stink when its sparking properly.

Is it possible that the standard plugs aren't up to the tuned ignition / engine ? Can you recommend a higher rated plug that will work better ??

Thanks in advance for your help

Cheers

[hampshirehog]

Hi Jock. Is every thing set up right: ignition timing, valve timing, carb settings (plug colour) etc.

And what do the plugs look like? Are they burned away etc?

Oh - and are the valve clearances OK? They can throw things.

[srsupertrap]

I think spark plugs are merely a symptom of something else which is likely wrong

1) Dyna Coils? Do they have the correct resistance . . . Verify by measuring with a DMM ( I believe 3 Ohm is correct)
2) Any mods done to the wiring harness leading to the coils . . . Ballast resistor or some other mod
3) HT Leads? Swap existing wires with another set . . . problem perist?

Cheers

[tom203]

I think spark plugs are merely a symptom of something else which is likely wrong

1) Dyna Coils? Do they have the correct resistance . . . Verify by measuring with a DMM ( I believe 3 Ohm is correct)
2) Any mods done to the wiring harness leading to the coils . . . Ballast resistor or some other mod
3) HT Leads? Swap existing wires with another set . . . problem perist?

Cheers

How about high compression and low octane gas!

[chef1366]

Are the plugs fouled?

[simon kuether]

Forget the plugs .
I'm chasing a pitcha of yer bike .
A Dunstall GS has got to be very unique .

Cheers , Simon .

[hjfisk]

I'm with Simon I want to see PICS

[jock]

Dunstall GS750

Thanks to everyone for the advice. To answer a few of the questions - the valve shims are all in tolerance, the faulty plug always looks normal and isn't burned away at all, the carbs run k&N pods and are balanced (it ticks over at 1000 perfectly). I will check on the other points like coil leads and the Dyna settings over the weekend and report back.

I have created an album to try and show you guys some pictures of the bike but it wont allow uploads - just keeps saying "upload failed". I have contacted the administator to try and figure it out.
Its a beauty in its Heron colours and pretty rare - I dont think Dunstall made many GS750s cos the Dunstall GS1000CS came in and stole its thunder. I,ve seen a few Dunstall GT750 Kettles about but no other GS750s.

[hampshirehog]

If she's all on pat then it's probably worth putting some colder plugs in there - NGK B9ES or Champion N2 - and see what happens. If she starts fouling plugs then that's not the answer.

[Don-lo]

Which plug is it? I would switch the plug leads with it's mate (1-4, or 2-3) and see if the problem follows the plug lead. If so, then that wire is bad. I have also seen wires arcing to the frame, so rule that out too. One other possibility would be a twisted crank, if 1 or 4 is the problem.

[Suzuki_Don]

A test you can do is when a plug is not sparking hold it about an 1/8th" off the cylinder head and see if the spark jumps from the plug body to the head. If it does then get back to us with the results.

[Big T]

Start with the basics

12V or better power to the coils?

Swap leads as suggested

Do you have the HT leads with yellow wires/red boots? One of mine fried itself at the boot, had to pull the boot and cut the wire back to a clean surface, 30 years of arcing there can play hell with that crimp fitting

[Billy Ricks]

Here's a photo of a Dunstall Kettle I found.



And a GS750.

[jock]

Thanks to everyone for the input - I'll put in some garage time at the weekend and report back (I'm fitting spoked wheels at the same time cos I think they look better than the alloys - just had them rebuilt with stainless rims and spokes). Still can't upload photos

[hampshirehog]

Thanks to everyone for the input - I'll put in some garage time at the weekend and report back (I'm fitting spoked wheels at the same time cos I think they look better than the alloys - just had them rebuilt with stainless rims and spokes). Still can't upload photos

Keep your garage locked Jock. Although I'm a long way south I'm still tempted to come up and pinch those wheels..... You can't beat a GS with spoked wheels.