- Joined
- May 15, 2009
- Messages
- 283
- Reaction score
- 50
- Bike
- Aprilia 650 Pegaso
Hi folks.
Some time ago I decided I needed a project, something that had been poorly maintained. If it was still running, what would be the point of a rebuild? It was meant to be a learning experience, something different to occupy my hands and mind.
In that vein I purchased an abused and maintained-by-monkeys 1981 Suzuki gs450. It was stripped and disassembled end to end, and the engine was rebuilt with new pistons and valves, fork stanchions chromed anew, etc. Some replacement carbs were thrown on as the originals were stripped and broken. Whilst I re-assembled the engine myself, following the clymer manual, the cylinders were bored for the new pistons (2.5mm oversize) by an engine rebuilder.
This weekend past I attempted to start her up and for the life of me could not figure out why she wouldn't start.
At this point it's probably prudent to confess that I have no mechanical experience starting/diagnosing ignition troubles, nor am I a mechanic of any kind by profession. I am, simply put, enthusiastic to find things out and learn something new.
What's has actually been replaced:
The entire ignition system was burned out, along with the wiring loom looking like a electricians electrical tape vendors wet dream. Lots and lots of little bits of wire manually twisted together and "protected" with electrical tape.
This was replaced with a Motogadget mUnit and BoyerBransden electronic ignition. The wiring loom was rebuilt and manually checking the various bits of connectivity confirms working lights, horn, etc. Starter replaced with used original starter and turns the bike over without any issue.
The replacement carbs are the nameless copies of 34mm round slide Mikunis. Helpfully, the jets provided are not numbered at all, because that would be too easy. At least the original jets (at least the one I could get out from the original carbs) do fit, sometimes with a little editing.
Of course, the original air box was missing, so I only have some foam filters on there for now
So now it would be nice to get some pointers and tips on how to proceed. As I understand it, the first things to check would be:
- TDC is where it should be? Confirmed by seeing the top of the piston through the spark plug hole in conjunction with the alignment mark on the timing plate.
- Valves are closed for the compression stroke: to be confirmed, apparently verifiable by having the lobes on the camshafts pointing away from each other?
- There is spark at the spark plug: confirmed
- There is fuel: well, I put fuel into the carbs
- Carb pilot air screw turned to 1 1/4 turns out as per factory spec: confirmed
What actually happened whilst trying to start:
After adding fuel and verifying all bolts were tightened down, the motor would swing over but there as no coughing as if the engine was at least igniting some fuel. I opened the chokes on the carbs and immediately got a backfire and lots of smoke coming out carb intakes. If I understand this correctly this is a result of too much advance/incorrect ignition timing causing spark to occur before the exhaust valve has closed?
Thereafter followed an afternoon of verifying and reading instructions from Boyer Bransden (the ignition component manufacturer), including finding their updated instructions that contradict the original supplied instructions.
At the end of the day, it was pretty much more of the same. Swing over with no starting.
Things I tried:
- Adjusting the pilot air screws, located on the air filter side, in to try and enrich the mixture a little more but it didn't make a difference.
- Cracking open the throttle a little. Sound changed but no joy.
- Using the choke, which caused a puff of smoke and loud bang out the carbs
After all this I figure it is time to sit back and rethink what's going on. I believe the problem lies with the carb and/or ignition setup. The mechanical side I reckon is as it should be, although I will triple-check this when I have a bit of time again.
To that end, can anyone provide a basic how-to on ignition diagnostics? Given my lack of knowledge, there is almost certainly mistakes being made on my part with the setup. So some basic theory and problem finding approach would be really useful.
Thanks
Some time ago I decided I needed a project, something that had been poorly maintained. If it was still running, what would be the point of a rebuild? It was meant to be a learning experience, something different to occupy my hands and mind.
In that vein I purchased an abused and maintained-by-monkeys 1981 Suzuki gs450. It was stripped and disassembled end to end, and the engine was rebuilt with new pistons and valves, fork stanchions chromed anew, etc. Some replacement carbs were thrown on as the originals were stripped and broken. Whilst I re-assembled the engine myself, following the clymer manual, the cylinders were bored for the new pistons (2.5mm oversize) by an engine rebuilder.
This weekend past I attempted to start her up and for the life of me could not figure out why she wouldn't start.
At this point it's probably prudent to confess that I have no mechanical experience starting/diagnosing ignition troubles, nor am I a mechanic of any kind by profession. I am, simply put, enthusiastic to find things out and learn something new.
What's has actually been replaced:
The entire ignition system was burned out, along with the wiring loom looking like a electricians electrical tape vendors wet dream. Lots and lots of little bits of wire manually twisted together and "protected" with electrical tape.
This was replaced with a Motogadget mUnit and BoyerBransden electronic ignition. The wiring loom was rebuilt and manually checking the various bits of connectivity confirms working lights, horn, etc. Starter replaced with used original starter and turns the bike over without any issue.
The replacement carbs are the nameless copies of 34mm round slide Mikunis. Helpfully, the jets provided are not numbered at all, because that would be too easy. At least the original jets (at least the one I could get out from the original carbs) do fit, sometimes with a little editing.
Of course, the original air box was missing, so I only have some foam filters on there for now
So now it would be nice to get some pointers and tips on how to proceed. As I understand it, the first things to check would be:
- TDC is where it should be? Confirmed by seeing the top of the piston through the spark plug hole in conjunction with the alignment mark on the timing plate.
- Valves are closed for the compression stroke: to be confirmed, apparently verifiable by having the lobes on the camshafts pointing away from each other?
- There is spark at the spark plug: confirmed
- There is fuel: well, I put fuel into the carbs
- Carb pilot air screw turned to 1 1/4 turns out as per factory spec: confirmed
What actually happened whilst trying to start:
After adding fuel and verifying all bolts were tightened down, the motor would swing over but there as no coughing as if the engine was at least igniting some fuel. I opened the chokes on the carbs and immediately got a backfire and lots of smoke coming out carb intakes. If I understand this correctly this is a result of too much advance/incorrect ignition timing causing spark to occur before the exhaust valve has closed?
Thereafter followed an afternoon of verifying and reading instructions from Boyer Bransden (the ignition component manufacturer), including finding their updated instructions that contradict the original supplied instructions.
At the end of the day, it was pretty much more of the same. Swing over with no starting.
Things I tried:
- Adjusting the pilot air screws, located on the air filter side, in to try and enrich the mixture a little more but it didn't make a difference.
- Cracking open the throttle a little. Sound changed but no joy.
- Using the choke, which caused a puff of smoke and loud bang out the carbs
After all this I figure it is time to sit back and rethink what's going on. I believe the problem lies with the carb and/or ignition setup. The mechanical side I reckon is as it should be, although I will triple-check this when I have a bit of time again.
To that end, can anyone provide a basic how-to on ignition diagnostics? Given my lack of knowledge, there is almost certainly mistakes being made on my part with the setup. So some basic theory and problem finding approach would be really useful.
Thanks
