Timing again...
Posted: Fri Dec 29, 2017 10:39 am
Hi
My Big Valve Twink is running like a sack of spuds.
This is partially my fault as I'd closed down the butterflies on one of the Dellortos by tweaking the adjuster. Should have left it alone! Oh well, out with the hand held flowmeter and it's balanced again, near enough. Now I want to check the timing. My first problem is that I cannot see the timing marks on the case. I know they should be visible from the exhaust side of the engine but they are not - I just cannot seem to get a good angle. Any tips? I plan to try and paint the marks for better visibility, or maybe just put a small mirror behind the radiator.
Secondly, I am trying to determine what the timing specification should be. I have a UK spec big valve engine from 1973, which I guess is what Miles calls a "late domestic" big valve with Dellortos.
The book is a little confusing as it first makes a point about distributor RPM being half crankshaft RPM, then shows tables of advance timings with "distributor RPMs" going up to 6500. Which would mean the engine was running at twice that and I do not think that's possible in a road going engine. To further confuse matters, it shows a graph that has crank RPM against advance, and these match the figures given for distributor RPM. So I reckon the advance tables are showing crankshaft RPM despite being labelled "Distributor RPM".
The "which distributor" tables are confusing. I could have a 41225 ("late model domestic Big Valve"), or a 41189A ("all domestic Big Valve"). The second table ("Engine use") states that "All big valve Weber/Dellorto domestic" engines use the 41189A distributor with 12 degrees static advance, so I will go with that.
This means the following advance curve applies (with the 12 degrees static timing added already):
1000 RPM: 12 degrees
2000 RPM: 12 degrees <-- after this point, the centrifugal advance mechanism should kick in
2500 RPM: 14.5 degrees
3000 RPM 16.5 degrees
3500 RPM 19.0 degrees
4000 RPM 21.3 degrees
4500 RPM 23.5 degrees
5000 RPM 26.0 degrees
Hope I got that right.. comments?
My Big Valve Twink is running like a sack of spuds.
This is partially my fault as I'd closed down the butterflies on one of the Dellortos by tweaking the adjuster. Should have left it alone! Oh well, out with the hand held flowmeter and it's balanced again, near enough. Now I want to check the timing. My first problem is that I cannot see the timing marks on the case. I know they should be visible from the exhaust side of the engine but they are not - I just cannot seem to get a good angle. Any tips? I plan to try and paint the marks for better visibility, or maybe just put a small mirror behind the radiator.
Secondly, I am trying to determine what the timing specification should be. I have a UK spec big valve engine from 1973, which I guess is what Miles calls a "late domestic" big valve with Dellortos.
The book is a little confusing as it first makes a point about distributor RPM being half crankshaft RPM, then shows tables of advance timings with "distributor RPMs" going up to 6500. Which would mean the engine was running at twice that and I do not think that's possible in a road going engine. To further confuse matters, it shows a graph that has crank RPM against advance, and these match the figures given for distributor RPM. So I reckon the advance tables are showing crankshaft RPM despite being labelled "Distributor RPM".
The "which distributor" tables are confusing. I could have a 41225 ("late model domestic Big Valve"), or a 41189A ("all domestic Big Valve"). The second table ("Engine use") states that "All big valve Weber/Dellorto domestic" engines use the 41189A distributor with 12 degrees static advance, so I will go with that.
This means the following advance curve applies (with the 12 degrees static timing added already):
1000 RPM: 12 degrees
2000 RPM: 12 degrees <-- after this point, the centrifugal advance mechanism should kick in
2500 RPM: 14.5 degrees
3000 RPM 16.5 degrees
3500 RPM 19.0 degrees
4000 RPM 21.3 degrees
4500 RPM 23.5 degrees
5000 RPM 26.0 degrees
Hope I got that right.. comments?