Ladies and Gents need your help here... for one more time. My 650 has that rattle sound of pre-ignition when it gets hot at 4th gear and under load. I have try to reduce the advance but with no success. Measured compression is at about 110 psi so i don't think is comes from there . I have the powerspark ignition installed. Also checked spark plugs and they have a medium to light brown color which surprises me and i am afraid that mixture might be lean. Carburetor main jet size is 115. Any tips about how to come up with this?
Is it dangerous to damage the engine until i fix this?

Thomas

Hi Thomas,

Isn't it quite hot in Greece at the moment? High ambient temperature can afaik encourage pre-ignition.

Also, what type of petrol are you using? Over here, in winter, there is an extra additive in the petrol to allow easier starting in cold weather, this winter fuel is not suitable for use in hot weather. I don't know if this also happens in Greece but maybe some fresh fuel might help (if you haven't already tried this).

I wouldn't be too concerned if the plug colour is light to medium brown, I think this is about right. If it was white, I'd be worried. Of course, plug colour isn't telling you how the mixture is throughout the rev-range, just how it is overall and particularly just before switching off the engine.

That is why on motorcycles, when checking main jets, you're supposed to run at WOT (wide-open-throttle) for a while, then close throttle, switch off engine, de-clutch and coast to a stop to avoid contaminating plug readings by slowing down and running on the idle jet for a short time.

If you're concerned that the mixture might be weak at some rpm's maybe use a little 'choke' (mixture enrichment device) to see if the rattle sound goes away?

As a short term measure and as a test, can you get hold of some aviation gas (Avgas?), I believe this still contains lead as an Octane booster? Alternatively, could you try some lead substitute in your fuel. If this stops the possible pre-ignition, it will protect your engine while you figure out a more permanent cure.

Regards,

AL.

F123C said:

If you're concerned that the mixture might be weak at some rpm's maybe use a little 'choke' (mixture enrichment device) to see if the rattle sound goes away?

Alternatively, could you try some lead substitute in your fuel. If this stops the possible pre-ignition, it will protect your engine while you figure out a more permanent cure.

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Hi AL, i am already using (not always) lead substitute, problem was yesterday that it was a quiet hot day but not a real Greek summer 38 Celsius day. Nevertheless i was in a 80 kilometer trip with the local 500 club at the highway but then again this is not supposed to happen. I used three different fuels to i am pretty sure that this is not because a bad quality of fuel (unfortunately here in Greece not all fuels are good)
I didn't think to use the choke so thank you that might help.
How about using 100 octane fuel like shell v power racing?

Thomas

Hi, have you checked the distributor advance mechanism for sticking? Perhaps it's partially seized and not giving full advance?

Unfortunately my timing gun is dead. But last time I visually checked it before installation it was ok. But I will check it again and buy a new timing gun.
As for now I have retorque the head because of the new gasket and I found all valves close to nothing tolerance, not sure if this matters. "Flight" test to be performer soon.

Definitely get a new timing gun

Yes, you want to have the correct valve clearances. If the clearances are too tight the valves will be open for too long and you could end up with burnt valves and the performance may suffer. I don't think the 2 cylinder engines benefit from experimenting with different valve clearances to increase valve overlap, but that's another story.

Whenever possible I use "super" petrol in my tuned 652cc engine---luckily for me, all 3 of my nearest petrol stations (Esso, Morrison and Tesco) all now have a "super2 pump---so far, no problem.

gordinir8 said:

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--------------How about using 100 octane fuel like shell v power racing?

Thomas

Click to expand...

Hi Thomas,

I only suggested using a higher octane fuel to safeguard your engine until you have figured out what is the problem. 100 octane fuel should be fine for a short trial but shouldn't be normally necessary unless you're running a very high compression ratio. I mentioned Avgas because you work on aircraft - afaik, Avgas is available up to 115 octane :yum:

You later mentioned that you subsequently discovered that your valve clearances had closed up to almost nil. Was this before or after you had retorqued the cylinder head?
Tight valve clearances, as well as risking the burning of the valves and valve seats, also results in the valve heads running hotter , especially the exhaust valves (inlet valves are cooled by the air/fuel charge passing over them). This can cause pre-ignition.

Pre-ignition (as you probably know, but others here might not) is caused by something igniting the fuel air mixture before the spark plug fires.

Typical ways to prevent pre-ignition :-

Use a higher octane fuel or additives to raise the octane rating.

Reduce the temperature or pressure conditions inside the cylinder by :- retarding the ignition timing, making the mixture richer or reducing the compression ratio.

Use water injection or water/alcohol injection.

Decarbonise the engine if there is excessive carbon build-up.

Remove any sharp edges within the combustion chamber. - sharp edges can run hotter than surrounding metal and possibly ignite the incoming mixture.

You might find that setting the valve clearances to factory specs cures the pre-ignition

Regards,

AL.

Unfortunately I don't have access to Avgas. Valves where closed before I retorque the head, now are adjusted to 20-25 as per manual. Head is clean inside and compression is normal as I mention. I am also using the 500 spark plugs, are those the same with the 650? Today I am going to test it again and see what happens.

FC123 rightly listed the common causes of "pre-ignition", but left out the most common one: too much advance.
Gordinir8 mentions fitting an Accuspark, but having no working strobe light. If it's the case that the ign. timing was not reset with a strobe, there's next to no chance of it being correct.
Pre-ignition causes (in addition to damage) not so much a rattle, but more of an intermittent clicking noise at WOT, audible on a motorcycle with your ears a foot away from the engine, but I imagine hardly audible in a car.
High octane fuel like avgas will allow use of compression ratios in excess of 10:1 (not found in a "cooking" 500 engine), but that has not much to do with a compression tester reading, and it will not cure over-advanced timing.
Bottom line Gord8R, make your first test ride a strobe light shopping one

You are all correct, tommorrow I will probably have a a strobo. I removed the distributor and I am not sure about it. I think that it is interchangeable with the 500. Anyway strobo test first togeather with 126 "colder" spark plugs, (500 that I am using are hotter) and we will see.

giardini said:

FC123 rightly listed the common causes of "pre-ignition", but left out the most common one: too much advance.
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I wasn't trying to list all the common causes of '"pre-ignition" but including in the list I gave (post #9 ) of ways to stop pre-ignition was :-

Reduce the temperature or pressure conditions inside the cylinder by:- retarding the ignition timing, making the mixture richer or reducing the compression ratio.

AL.

So you did, and you being both Irish AND motorcycle-connected, I highly respect your qualifications :worship:
I was trying to keep it simple by making a distinction between "pre-ignition" (in quotes) and... too much advance, not quite the same thing.

A small update, distributor cap and rotor found worn and replaced with new. Timing adjusted with strobo light. At list advance mechanism is ok. I am not sure about my final setting and I will have to experiment the next days because I have some backfiring. Also colder B7 spark plugs installed but I need a long ride to see if preignition in still there. I think biggest problem was the worn distributor rotor due to worn distributor cam but I kind of improve it.

Funny thing is that engine operate better at a setting far beyond 28 degrees!!! Maybe more that 30... The closer i take it to 18 the worst it gets, it just starts to backfire from carburator. It idles at about 18 degrees. Valves are adjusted. I am starting to thinking about a 123 system alhough I am not sure if problem comes from the distributor.

gordinir8 said:

Funny thing is that engine operate better at a setting far beyond 28 degrees!!! Maybe more that 30... The closer i take it to 18 the worst it gets, it just starts to backfire from carburator. It idles at about 18 degrees. Valves are adjusted. I am starting to thinking about a 123 system alhough I am not sure if problem comes from the distributor.

Click to expand...

30 would be OK, especially with the ethanol in modern fuel and as you say, maybe poor quality fuel sometimes in Greece. Remember, 18 degrees is the amount that you add to the ten at static advance, so 28 on high revs is the spec.
My car has been running perfectly for 6 of the last 12 months on the original contact-breaker system and 6 months on the 123. There is no difference in starting, idle or running performance between the two. The 123 still has mechanical parts that can wear and you don't avoid the distributor-cap or rotor arm, which are potential weaknesses.
The 123 is a fabulous item and guarantees a strong spark and very even timing balance between the cylinders, but my experience is that it is that generous advance in the timing that makes the biggest difference to performance.

I don't know Peter, it gives me a hard time but I hope in the end I will manage to adjust it. Idle is not that stable also and engine dies sometimes. I think I will try some more settings without the timing gun and let the one that works better. I will also check again valve clrarances.
Thomas