Rough cold running
Posted: Fri Nov 05, 2021 20:44
My Excel was running OK and driven on a weekly basis. However, she had been a bit neglected over the last two years as I worked on my M100. So, I thought it was time to give the Excel some attention - a good service and tune up.
One problem that has been there for some time is it doesn’t run great when cold. This was correctly pointed out by lotus-e-clan in this other thread. viewtopic.php?f=7&t=11022&p=84415&hilit ... uge#p84415
So, I changed the oil and filter, fitted new plugs, cleaned the ‘for life’ K&N filter and re-oiled using the K&N kit. Removed the distributer for a clean and oiled the weights, checked the vacuum advance was working by applying a vacuum to it and observing the plate moving. Fitted a new rota arm and cap. Old one was fine but had one in stock so fitted it anyway.
Started the car and brought it up to normal operating temperature. Checked the timing with a strobe and found it was at 30 degrees because of removing the distributor so re-set it at 10 degrees. Balanced the carbs using my usual method that I have done for years. Took the car for a spin and it was going better than ever – ticked over nice and steady, accelerated well with no flat spots. Good days work I thought. Parked the car up.
Next morning – started the car. It started but it ran very roughly and required me to keep blipping the throttle. Turned the engine off. Advanced the ignition slightly and the car started and ticked over well. Let the engine warm up and checked the timing with a strobe – 30 degrees BTDC. Reset the timing to 10 degrees. Took the car for a drive and it drove perfectly. Next day with a cold engine the whole sequence started again. Ran extremely badly, advance the ignition, engine ticked over well, reset to 10 degrees, car drove well.
My suspicions fell on the green thermal valve. This valve should be open when cold so vacuum can be applied to the vacuum advance unit on the distributor and advance the ignition during cold starting. As the engine warms to 60 degrees the valve closes and cuts off the vacuum and the timing returns to 10 degrees. Removed the green thermal valve attached a pipe to No1 position and tried to blow though it but the valve was closed – no air came out. Purchased a new one and did the same and air did come out. Great I thought. That must be the problem. Defective thermal ignition valve.
Fitted the new green valve and also fitted a new thermal enrichment valve for good measure. Checked and double checked all the connections against the service notes and Vacuum system table.
Still the same. Made no difference. Removed the distributor again, made sure the weights were free to move and replaced the vacuum unit even though I didn’t think it was faulty. Still no change.
With a cold engine, attached the strobe timing light. Got the engine running very roughly. The timing mark is about 10 degrees, rev the engine slightly and the timing mark moves backwards to zero!! Then as it warms up, engine runs better and timing mark moves to 10.
To summarise – the engine runs perfectly when warm and ignition is at 10 degrees. Problem is when the engine is cold – starts just about, runs rough, can only just about keep it running. Advance the ignition and engine starts and idles well.
Not sure what to do next – any idea’s welcome. Carb problem?
One problem that has been there for some time is it doesn’t run great when cold. This was correctly pointed out by lotus-e-clan in this other thread. viewtopic.php?f=7&t=11022&p=84415&hilit ... uge#p84415
So, I changed the oil and filter, fitted new plugs, cleaned the ‘for life’ K&N filter and re-oiled using the K&N kit. Removed the distributer for a clean and oiled the weights, checked the vacuum advance was working by applying a vacuum to it and observing the plate moving. Fitted a new rota arm and cap. Old one was fine but had one in stock so fitted it anyway.
Started the car and brought it up to normal operating temperature. Checked the timing with a strobe and found it was at 30 degrees because of removing the distributor so re-set it at 10 degrees. Balanced the carbs using my usual method that I have done for years. Took the car for a spin and it was going better than ever – ticked over nice and steady, accelerated well with no flat spots. Good days work I thought. Parked the car up.
Next morning – started the car. It started but it ran very roughly and required me to keep blipping the throttle. Turned the engine off. Advanced the ignition slightly and the car started and ticked over well. Let the engine warm up and checked the timing with a strobe – 30 degrees BTDC. Reset the timing to 10 degrees. Took the car for a drive and it drove perfectly. Next day with a cold engine the whole sequence started again. Ran extremely badly, advance the ignition, engine ticked over well, reset to 10 degrees, car drove well.
My suspicions fell on the green thermal valve. This valve should be open when cold so vacuum can be applied to the vacuum advance unit on the distributor and advance the ignition during cold starting. As the engine warms to 60 degrees the valve closes and cuts off the vacuum and the timing returns to 10 degrees. Removed the green thermal valve attached a pipe to No1 position and tried to blow though it but the valve was closed – no air came out. Purchased a new one and did the same and air did come out. Great I thought. That must be the problem. Defective thermal ignition valve.
Fitted the new green valve and also fitted a new thermal enrichment valve for good measure. Checked and double checked all the connections against the service notes and Vacuum system table.
Still the same. Made no difference. Removed the distributor again, made sure the weights were free to move and replaced the vacuum unit even though I didn’t think it was faulty. Still no change.
With a cold engine, attached the strobe timing light. Got the engine running very roughly. The timing mark is about 10 degrees, rev the engine slightly and the timing mark moves backwards to zero!! Then as it warms up, engine runs better and timing mark moves to 10.
To summarise – the engine runs perfectly when warm and ignition is at 10 degrees. Problem is when the engine is cold – starts just about, runs rough, can only just about keep it running. Advance the ignition and engine starts and idles well.
Not sure what to do next – any idea’s welcome. Carb problem?