Erratic Engine
- Thread starter esray
- Start date
raylish said:
Yes. I've just realised that......
In which case if the vac unit moves the baseplate that will be in the opposite direction to the direction of the rotor arm, which makes all that you previously said correct. I don't know where I got the thought that it was a V8 from.
So I'd check with the strobe again, set the timing to 8BTDC as a starting point, and then rev the engine while looking at the timing marks with the strobe, and see what happens.
I've done a little spring cleaning to my previous posts in the hope that it will be a little less confusing if it's referred back to in the future.
JVY
Active Member
Just to be clear, when you did the test, was the vacuum hose to the distributor connected? If it was NOT connected, the problem seems likely to be with the distributor. E.g. centrifugal advance mechanism has a problem which means that it is advancing the timing at idle speed rather than at high rev's.
I am trying to remember how much advance to expect from the centrifugal mechanism when you rev the engine up to a few thousand rpm and seem to recall I measured mine somewhere between 10-20 degrees(??). At less than 1000rpm idle speed it should not be advancing ignition timing and it should remain at 8 degrees BTDC. However, if there is another problem causing the engine to rev, the distributor advance could be working as it is supposed to and that's why your seeing the timing advance??
When you say the rev's rise, can you measure or estimate to what speed? For example 1000-2000rpm, 2000-3000 or more?
Do you have a spare distributor to try? Or spare advance mechanism at least?
Do you know if your distributor is a Lucas 25D4? Some good pics and advice on Lucas 25D4 distributor here:
http://www.simonlacey.com/pages_new/content/distributor rebuild.htm
I am trying to remember how much advance to expect from the centrifugal mechanism when you rev the engine up to a few thousand rpm and seem to recall I measured mine somewhere between 10-20 degrees(??). At less than 1000rpm idle speed it should not be advancing ignition timing and it should remain at 8 degrees BTDC. However, if there is another problem causing the engine to rev, the distributor advance could be working as it is supposed to and that's why your seeing the timing advance??
When you say the rev's rise, can you measure or estimate to what speed? For example 1000-2000rpm, 2000-3000 or more?
Do you have a spare distributor to try? Or spare advance mechanism at least?
Do you know if your distributor is a Lucas 25D4? Some good pics and advice on Lucas 25D4 distributor here:
http://www.simonlacey.com/pages_new/content/distributor rebuild.htm
raylish said:
If it's initially set at idle to 8BTDC, the engine speed would have to increase considerably to get the mechanical advance to 20BTDC by that alone, and for it to slow and alter the timing back through it's initial setting and as far as TDC I think I'd be inclined to check for slack in the bottom timing chain, and remove the distributor baseplate, and check the mechanical advance weights and springs underneath. Two things that could cause the problem, and are easy to eliminate.
The bit I really don't get there is the timing going back from the 8 BTDC set point to TDC. That's very odd indeed.
Just in grandmother mode here a minute. When you say you've set the ignition at 8 BTDC, how exactly did you do that? Was that static by having a light bulb across the points? Or dynamic with the strobe on a running engine? If the latter, did you make sure that the vacuum advance was disconnected when you set the timing? And then reconnected it before running the car to get this result? (by removing and re-attaching the vacuum pipe)
I think I might want to set the timing static here, because of the unknowns that are floating about. If you've set it static there might be a small error but you know exactly where you stand.
I agree with Harvey about wanting to have a look at the springs and bob weights under the distributor base plate. But timing chain tension? To give that amount of wander its got to be really very loose indeed? Surely it'll have the mother of all timing chain rattles if it's that loose. I'd doubt you'd even be prepared to run it!
What do you think Harvey?
Chris
Just in grandmother mode here a minute. When you say you've set the ignition at 8 BTDC, how exactly did you do that? Was that static by having a light bulb across the points? Or dynamic with the strobe on a running engine? If the latter, did you make sure that the vacuum advance was disconnected when you set the timing? And then reconnected it before running the car to get this result? (by removing and re-attaching the vacuum pipe)
I think I might want to set the timing static here, because of the unknowns that are floating about. If you've set it static there might be a small error but you know exactly where you stand.
I agree with Harvey about wanting to have a look at the springs and bob weights under the distributor base plate. But timing chain tension? To give that amount of wander its got to be really very loose indeed? Surely it'll have the mother of all timing chain rattles if it's that loose. I'd doubt you'd even be prepared to run it!
What do you think Harvey?
Chris
esray
Member
Harvey, Chris - Thanks.
Chris - Strobe with vacuum disconnected - However, could still be distributor weights/springs faulty and moving timing incorrectly? Would it help us if (this evening when it cools down some), I remove the rocker cover and check that I can peg the flywheel and the cam-chain sprocket at number one exhaust fully open?
Ray
Chris - Strobe with vacuum disconnected - However, could still be distributor weights/springs faulty and moving timing incorrectly? Would it help us if (this evening when it cools down some), I remove the rocker cover and check that I can peg the flywheel and the cam-chain sprocket at number one exhaust fully open?
Ray
Mmmm. You wouldn't be able to identify any slack in the bottom chain from moving the top chain if both crank and cam are locked. BUT, if you just locked the cam, you could see how much crank movement each way you could get. That would identify total slack in the system. Then from identifying how slack the top chain is, you could infer the state of the bottom chain. It's only the bottom chain that really matters here, because the distributor is driven direct of the intermediate pulleys / jackshaft, and it's the relationship between distributor and crank position that interests us.
I suppose it could be sticking bob weights. When you initially set the timing, the bob weights being stuck with 8 advance, then a quick rev frees them off and they return to nil advance, giving you this strange effect of a retarding advance system...
So let's set the timing static. That way you can be certain that the centrifugal advance is fully home at zero advance. Then see what the strobe tells us during running.
Does feel like we might be homing in on the problem
Chris
I suppose it could be sticking bob weights. When you initially set the timing, the bob weights being stuck with 8 advance, then a quick rev frees them off and they return to nil advance, giving you this strange effect of a retarding advance system...
So let's set the timing static. That way you can be certain that the centrifugal advance is fully home at zero advance. Then see what the strobe tells us during running.
Does feel like we might be homing in on the problem
Chris
esray
Member
chrisyork said:Mmmm. You wouldn't be able to identify any slack in the bottom chain from moving the top chain if both crank and cam are locked. BUT, if you just locked the cam, you could see how much crank movement each way you could get. That would identify total slack in the system. Then from identifying how slack the top chain is, you could infer the state of the bottom chain. It's only the bottom chain that really matters here, because the distributor is driven direct of the intermediate pulleys / jackshaft, and it's the relationship between distributor and crank position that interests us.
I suppose it could be sticking bob weights. When you initially set the timing, the bob weights being stuck with 8 advance, then a quick rev frees them off and they return to nil advance, giving you this strange effect of a retarding advance system...
So let's set the timing static. That way you can be certain that the centrifugal advance is fully home at zero advance. Then see what the strobe tells us during running.
Does feel like we might be homing in on the problem
Chris
Thanks Chris,
I have already tried static timing - Cannot seem to use a bulb because of magnetic timing instead of points? - So I set the distributor at magnetic rotor pointing at dead center of the appropriate magnet sensor when at 8 degrees BTDC? Not sure if this is correct?
symptoms remained unchanged.
My thinking in my previous post was that if the relative positions of the crank, the cam and the distributor ARE correct and I set the crankshaft to TDC and remove the rocker cover, should the pot towards which the rotor arm points be at TDC/both valves closed/cam 90 degrees/directly opposite the valve stem?
Incidentally, I notice that there are bubbles in the pump to carb fuel line when running and although I understand that the carb works on an 'oversupply' from the pump basis - Could the rising and lowering revs be caused by a fault in the pump/carb? Could Increasing/decreasing revs through varying fuel supply advance/retard the timing via the weights, even if the vacuum is disconnected, or would he 'oversupply and the surplus held in the float chamber overcome this?
Ray
That sounds a good procedure for static timing with an electronic.
With the timing set by this method, was the timing still going "backwards" to TDC?
You are correct about the closed valves on the cylinder it's pointing to.
Some bubbles in the fuel line is normal. All bubbles and only a little fuel isn't.
Did you do the cam locking test for the bottom chain yet?
I think when that is done it'll be time to dismantle the distributor (or the lower timing chain!).
Chris
With the timing set by this method, was the timing still going "backwards" to TDC?
You are correct about the closed valves on the cylinder it's pointing to.
Some bubbles in the fuel line is normal. All bubbles and only a little fuel isn't.
Did you do the cam locking test for the bottom chain yet?
I think when that is done it'll be time to dismantle the distributor (or the lower timing chain!).
Chris
OrganDoctor
New Member
Since you mentioned magnetic pickup, a thought might be that something is catching near the distributor cam and advance plate, causing the plate to max -advance temporarily.You might want to look closely at the electronic ignition installation to see if it is clear of obstructions.
raylish said:
They don't move far, and although I've not measured it in that way I'd say that was about right, but I'd still remove the baseplate and have a look because you will still get the same amount of movement, and return, even if you've only got one spring fitted instead of two.
esray
Member
Harvey, Chris and All,
I THINK I have now traced the timing problem to the distributor fly weights & springs - They must have been disturbed when the vacuum unit became disengaged from the shaft with the knurled adjustment nut on its other end and the whole unit was being pulled out when revving - wrenching the base plate after it!
I stripped the dizzy down and rebuilt it and It starts and runs at 8 degrees BTDC (vacuum disconnected) and it advances when revved both with and without vacuum.
I have double-checked that the cam and crank shafts are correct relative to each other by successfully pegging the cam and flywheel correctly at number one exhaust fully open and flywheel at EP.
It starts and runs, but it is lumpy unless over-enriched and there is a very bad flat spot when you first throttle-up - If I hold it in this flat spot it stalls - get past it and it revs fine. Blue flame with colour tune and it runs even lumpier (with the exhaust even blowier) and flat spot worsens; very occasionally I get a backfire through the carburetor?
I assume I now need to start looking more closely at the carb?
How about Demetris's suggestion - The o rings at the choke?
ATB Ray
I THINK I have now traced the timing problem to the distributor fly weights & springs - They must have been disturbed when the vacuum unit became disengaged from the shaft with the knurled adjustment nut on its other end and the whole unit was being pulled out when revving - wrenching the base plate after it!
I stripped the dizzy down and rebuilt it and It starts and runs at 8 degrees BTDC (vacuum disconnected) and it advances when revved both with and without vacuum.
I have double-checked that the cam and crank shafts are correct relative to each other by successfully pegging the cam and flywheel correctly at number one exhaust fully open and flywheel at EP.
It starts and runs, but it is lumpy unless over-enriched and there is a very bad flat spot when you first throttle-up - If I hold it in this flat spot it stalls - get past it and it revs fine. Blue flame with colour tune and it runs even lumpier (with the exhaust even blowier) and flat spot worsens; very occasionally I get a backfire through the carburetor?
I assume I now need to start looking more closely at the carb?
How about Demetris's suggestion - The o rings at the choke?
ATB Ray