MASSIVE backfire....... where am I going wrong?..........

[berni29]

Hi There

I posted last week about my suspect ignition, and that I was going to swap the whole system to see if there was an improvement.

So I have from a previous +2 a Christopher Neil new (modified) distributor (they sold them a few years back), plus modified cap (grooves made in the sides where the screws would have gone), and an optical Luminition.

Before I started playing about the car ran although badly until warm. I have put the new distributor and Lumenition on having tested the system on the kitchen table. I have made sure that No1 sparks at 10 deg before TDC and then nos 3, 4 & 2. I have seen these sparks as I turn the engine by hand with the leads off and spare plugs on the leads.

When I attempt to start the car there is no sign of life until after about 10 revolutions wherupon there is a MASSIVE backfire, which is fuel igniting in the exhaust pipe.

This is confusing the hell out of me. I have checked everything 5 times at least.

Possible clues are that I suspected arcing with the old ignition system and perhaps this has carried over even though everything including the leads is different. The rotor arm might be wrong in some way because I do not know which one came with the CN system. Swaping it for another (different but of seemingly similar reach) results in no spark at all.

Even if the rotor arm is wrong, how can I observe sparks in the correct order with the system together and on the car, but get the results that I do?

It is possible that I have the optical cutter in the "wrong" place/orientation on the distributor shaft, but again, if it sparks at TDC what can go wrong from there? I definately have the leads on in the correct order.

Suggestions please!

Many thanks.....

Berni

[elans3]

Berni, sounds like you have the distributor in 180 degrees out. Especially if you're getting a really huge backfire.
It's an easy mistake to make with the twincam distributor.
Make sure you also have the leads on in the correct order.
elans3

[Frank Howard]

I have made sure that No1 sparks at 10 deg before TDC and then nos 3, 4 & 2. I have seen these sparks as I turn the engine by hand with the leads off and spare plugs on the leads...how can I observe sparks in the correct order with the system together and on the car, but get the results that I do?...if it sparks at TDC what can go wrong from there?

Berni,

Are you sure that #1 sparks at 10 deg before the correct TDC? There are two TDCs. The first is at the end of the compression stroke, and the second is at the end of the exhaust stroke. If you have your ignition set to fire the plug 10 deg before the end of the exhaust stroke, that would explain the massive backfire as the plug is igniting fuel/air while the exhaust valve is still open.

The remedy is to turn your distributor 180 deg or if there is a space limitation, pull the distributor out and turn the rotor 180 deg. Hope this helps.

[alaric]

Hi. When I first ran my twink it did the same as yours - loud bangs and not wanting to run at all, but obviously some firing.

To get my twink running I ended up peering in the oil filler cap hole and observing the cam and valve to check that I was sparking on the compression stroke and not the exhaust stroke. Once I'd established correct timing with the distributor, I put the leads on in the correct order according to the manaul and the direction of rotation of the dizzy. Kept me out of trouble for a few hours, but I'd definately got them all mixed up the first time I tried it. The engine leapt into life immediately with the dizzy in the right place and the leads connected correctly.

Sean.

[berni29]

Hi There

Thank you for the replies. I have not tried it yet but you must be right. I had already reversed the rotor in the distributor 180 degrees, but then I put the leads back on with 1 & 2 reversed. After that I lost the plot completely. I should have realised that the engine must be 180 degrees out.

First thing in the morning I am back in the garage.

Thanks again!

Berni

[nebogipfel]

I think it's always best, if you are in doubt, to whip the cam cover off so that you can see what the valves are doing ......... then you know you are firing on the correct stroke

If all you get is explosions - you know you are a mile out

[curly type 26]

got a sneaky feeling if on tdc no1 if you open oil filler cap no4 xhaust cam lobe to left of vertical round about 10-11 oclock position if my memory correct will save cam cover removal, if not give me some abuse have fun Colin.

[Hamish Coutts]

Berni,

To save whipping off cam cover to find TDC try turning the engine over on the starter (plug leads disconnected) with something that can detect compression pressure, inserted in No 1 plughole!

My preference would be to put my finger over the plughole and wait 'till compression pressure pushes my finger up. Then you know you're on the compression (& ignition) stroke.

Doesn't take long and saves lifting the cam cover and breaking the gasket seal.

Hamish.

[berni29]

Hi There

Thank you to you all again. I rotated the rotor by 180deg, and all is fine now as far as the timing goes. The intermittent ticking is still there, and I just cannot track it down. I wonder if it is a cam bucket rotating?

It definately runs better with this ignition set up so I will leave it on the car.

The next thing is the carbs. I will do a new post for that now.

Thanks again

Berni