Hello and Tiger 90 backfire problem.

[Phil_Mart]

Hi to all,

Been a longgggg time since I last had a bike (c70s).

Anyway I've just gone and got hold of an old, 71, Tiger 90. The chap I got it from has rebuilt the engine to 500cc but it has developed a nasty misfire on the right hand cylinder. Also I notice that on tick over that cylinder is running cooler than the left one.

The carbs are Amel concentrics, I've changed them over but the problem remains on the right side. I've swapped over the plug caps and the problem remains on the right side. This just proved that the twin coil unit, a Duciller, is Ok.

The bike is fitted with an electronic ignition unit, with a pick up housing replacing the distributor, my last 5ta had a standard distributor set up. Could it be that there's something wrong with this unit?

Any help or advice would be welcome.
TIA.

[L.A.B.]

Hello Phil,

As these electronic ignitions (which type is it? Boyer? more info would help) generally work on the wasted spark principal (both plugs spark each time although every alternate cylinder's spark is wasted) as you have a double outlet coil fitted I doubt that the ignition system is to blame, if it fires one cylinder then it would also fire the other as well at the next revolution.
So if it isn't the plug lead, plug cap or plug that is faulty then the problem is likely to be elsewhere (carb or engine).

[Phil_Mart]

Hi,

Thanks for the reply.

I think its the Boyer ignition, but then as I've swapped the plug leads over and the problem remains I guess that rules out the ignition system then.

But then I've also swapped over the carbs and the problem remains. Soooo, its looking like an engine fault.

Apparently new valves springs and bronze guides have been fitted to the head. I'm wondering if the exhaust valve could be sticking?

[L.A.B.]

Apparently new valves springs and bronze guides have been fitted to the head. I'm wondering if the exhaust valve could be sticking?

Yes it could be a sticky valve.
But you do seem to have a collection of different parts? I'm not sure any T90s had a distributor? And no twin carbs? The T90 model also being discontinued after 1968. It sounds as if the engine could have been built up from an earlier model?  

[Phil_Mart]

Things have moved on!

Just to confirm the history of the bike, it was originally a 1958 3ta, then modded and re-registered in 1971 as a Tiger 90. It has now had the engine upgraded to a 500cc spec.

The carbs, Amal concentrics, were in a mess, different needle settings etc. One with no filter in the banjo, no filter on the tank tap and of course both full of debris. So they have now been fully stripped and cleaned out.

Then on a short run, with still the usual back fires from the left (when the throttle closed) the left side packed up all together. Investigation revealed a coil gone down. However, as the coil is a Duciller 12v twin coil unit I will have to replace the whole thing.

The ignition is a Boyer unit and as the compression ratio is 9.5 : 1 I'm tempted to follow Boyers advice and use two 6 volt coils in series. Unless that is someone knows better  

Once the sun shines I'll try and get some photos posted to my photo site and get a link to em  

[guinnless]

I run my bonnie on 6v coils and it starts and runs better that when it had the 12v ones. It pops and farts like mad if the plug gaps are too big (on overrun)

[L.A.B.]

The ignition is a Boyer unit and as the compression ratio is 9.5 : 1 I'm tempted to follow Boyers advice and use two 6 volt coils in series. Unless that is someone knows better

2x 6V Lucas (or similar type) coils should be the ideal setup, although there is no reason why another double coil (12V) should not work equally well? Just ensure that it has a primary resistance of 3-4.5 Ohms. I've heard that many American Boyer users have had good results using a Harley Davidson (Sportster?) double coil unit.