Rough cold running

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Tanz
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Rough cold running

Post by Tanz »

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?
Cheers, Phil
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Lotus-e-Clan
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Re: Rough cold running

Post by Lotus-e-Clan »

First thought: a little clarification.

:?: When cold, are you measuring the 10 degs with the dizzy vac capsule disconnected or connected?

Because with the dizzy vac capsule disconnected (both cold and hot at tickover) it should show 10 degs (or whatever static you set it to) .

At cold, with the capsule connected, tickover should show around 30 degs on the pulley with the strobe then it slips back to 10 degs when hot. I can't remember the vac capsule max advance number but it is somewhere around 20 degs IIRC - so when you add in the static number it should show around 30 degs BTDC on the pulley using the strobe.

Is this the case?
Peter K

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Tanz
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Re: Rough cold running

Post by Tanz »

Lotus-e-Clan wrote:
Sun Nov 07, 2021 18:23
First thought: a little clarification.

:?: When cold, are you measuring the 10 degs with the dizzy vac capsule disconnected or connected?

Because with the dizzy vac capsule disconnected (both cold and hot at tickover) it should show 10 degs (or whatever static you set it to) .

At cold, with the capsule connected, tickover should show around 30 degs on the pulley with the strobe then it slips back to 10 degs when hot. I can't remember the vac capsule max advance number but it is somewhere around 20 degs IIRC - so when you add in the static number it should show around 30 degs BTDC on the pulley using the strobe.

Is this the case?
Hi Peter - What you describe as should happen I totally agree with - that was my understanding. What is happening is with the engine hot I can set the timing to 10 degrees (Vac capsule connected) - car runs perfectly. Come back to it the next day car barely runs and timing mark is less than 10 degrees (Vac capsule connected).
I think the best thing I can do is shoot a video to demonstrate what is happening - it really makes no sense at all :? Give me a few days to video it as a bit tied up with NEC stuff for Club Lotus at the moment.
Cheers, Phil
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Re: Rough cold running

Post by Lotus-e-Clan »

OK, but in order to get a baseline, I'd also try setting the dizzy to 10 degs static with the vac capsule disconnected (hot or cold - it doesn't matter). Then check the MAX mech advance with the strobe (capsule still disconnected) to see if you get the value stated in the Service Notes for max mechanical advance (at the stated rpm) for your particular spec (? Spec 10), - and that it consistently returns to static (10 degs) at tickover.

Only then, would I re-connect the vac to see if: a) you get 30 or so degs when cold (below ~50C) and that b) it returns to 10 degs when warm (above ~60C).

This might narrow down the cause of the fault to somewhere within the vacuum advance system or, specifically to the mech advance (or dizzy clamp slipping etc).

:?: Hopefully you have the correct spec dizzy fitted. Eg Is it possible that someone unbeknown to you, may have fitted an old spec dizzy from an earlier model (inc. Elite)? - Some older specs had vacuum RETARD capsules IIRC.
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Re: Rough cold running

Post by Alan_M »

A quick thing to check - do you have the restrictor (carb idle jet) fitted in the vac hose? This is needed to smooth the pulsed vacuum so the distributor sees a constant vacuum when cold.

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Re: Rough cold running

Post by Tanz »

Alan_M wrote:
Mon Nov 08, 2021 21:59
A quick thing to check - do you have the restrictor (carb idle jet) fitted in the vac hose? This is needed to smooth the pulsed vacuum so the distributor sees a constant vacuum when cold.
The restrictor is in place. However, because I replaced the vacuum advance capsule, I had the old one on the bench which I think is in good condition and didn't actually need replacing. So I got the engine running, engine temp was below 50 degrees, pulled the pipe off the new vac capsule and connected it to the old one. As I connected it I could see the lever move as it should but it did pulse quite a bit.
Cheers, Phil
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Re: Rough cold running

Post by Tanz »

Lotus-e-Clan wrote:
Mon Nov 08, 2021 19:53
OK, but in order to get a baseline, I'd also try setting the dizzy to 10 degs static with the vac capsule disconnected (hot or cold - it doesn't matter). Then check the MAX mech advance with the strobe (capsule still disconnected) to see if you get the value stated in the Service Notes for max mechanical advance (at the stated rpm) for your particular spec (? Spec 10), - and that it consistently returns to static (10 degs) at tickover.

Only then, would I re-connect the vac to see if: a) you get 30 or so degs when cold (below ~50C) and that b) it returns to 10 degs when warm (above ~60C).

This might narrow down the cause of the fault to somewhere within the vacuum advance system or, specifically to the mech advance (or dizzy clamp slipping etc).

:?: Hopefully you have the correct spec dizzy fitted. Eg Is it possible that someone unbeknown to you, may have fitted an old spec dizzy from an earlier model (inc. Elite)? - Some older specs had vacuum RETARD capsules IIRC.
Some great things to try there Pete. Will get back onto it after the NEC.
Cheers, Phil
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Re: Rough cold running

Post by Tanz »

OK - I'm back on this now
Here is a video of what I am seeing. The first part of the video shows the car with a hot engine, timing set at 10 degrees - running perfectly. Then 24 hours latter try to start the car . As I was shooting the video I noticed that when I turned the engine off there was a hissing sound from the brake servo. Not sure if that is normal but maybe I simply have a leak in the vac system so the distributor vac unit is not receiving sufficient vacuum??
Sorry about the poor video - don't think I'm ready to be a YouTube influencer yet :D

https://youtu.be/A4hGUuTkPvc
Cheers, Phil
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Re: Rough cold running

Post by richardw »

That’s a perfectly acceptable video Phil. Very strange behaviour - it’ll probably keep me awake tonight!

Cheers, Richard
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Re: Rough cold running

Post by Lotus-e-Clan »

I strongly suspect an air leak!

It won't start when cold because air is getting into the induction tract thus SIGNIFICANTY increasing the air-fuel ratio (too weak). But you should whip out a plug to see what's what at this stage to confirm it is running weak and not rich.

Weak mixtures need more advance but the air leak is preventing the vacuum advance working properly. So you have to manually advance the mechanical advance which is the only part of the advance system working properly when STONE cold.

Once warm, the air leak reduces (something is closing-up with heat to reduce the air leak).

There is 'some' vacuum advance when cold - but hardly anything!

When it warms up the vacuum is increasing (because the air leak is sealing itself) and so you see the dizzy vac advance working again to the point that it's now too far adavanced and you have to reset it back to 10 static again.

So find the air leak:

Cold be the DHLA manifold O rings for example or something else at the induction manifold or the manifold gasket. Can't think of anything else at the moment ...

Great video btw ..haven't watched the last couple of minutes because I was tooo excited :D ...I just needed to post this before anyone else did because I'm massively competitive ... going back to watch it now :lol:
Peter K

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Re: Rough cold running

Post by Lotus-e-Clan »

Just went back to watch the end at the point you said it's up to 40 what the hell's going on. (Love the brummy accent btw - I was born in Coventry!!).

The end of the video supports my previous post in that, as the mystery air-leak seals itself, the green thermal valve gets to work FULLY - then the manifold coolant temp goes above 60C and the green valve closes as it should.

So the air leak is at an interface that gets hot enough to seal itself. Not going to be a rubber tube or anything plastic ..it will be a metal to seal interface OR a crack in metal (unlikely). So yes , DHLA O rings and things that plug into the manifold and carbs.

Also maybe an air leak that is common to both carbs rather than one because it sounds balanced once you advance the ignition when cold -if it were just one carb going weak it would sound unbalanced when warming up ..which removes the suspicion away from DHLA O rings as it's unlikely that ALL O rings would leak simultaneously.

Check the brown cold enrichment valve too ...

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Re: Rough cold running

Post by Lotus-e-Clan »

In summary following the video review (I suggest):
  • 1) The air leak causes very weak running when cold (AFR > 15:1 which need lots of ign advance to run)

    AND simultaneously

    2) The same air leak prevents full vacuum to the dizzy even though the green valve is working as it should (ie likely the leak is systemically somewhere before the green valve)
3) the air leak will be a smallish one

Also try searching around your servo vacuum pipe unions as you said you heard some hissing I think??? Or could be an internal servo leak.
Peter K

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Re: Rough cold running

Post by Hawaiis0 »

How does the crank case breather fit in? Is it still secure and together?

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Re: Rough cold running

Post by Tanz »

WOW! Thanks for the reply Peter. What you say makes perfect sense. Maybe that's why it passed the MOT a couple of weeks ago - usually fails on emissions :D . A weak mixture has its advantages! I will check the manifold bolts are at the correct torque and not loose. If I put some washing up liquid around the joint it might show a leak if it get sucked in. My money is on the brake servo. I could disconnect the vacuum pipe from the servo and block it (clamp it) when the engine is cold and see if it starts properly. There is definitely a hissing sound from the servo when I turn the engine off.
Cheers, Phil
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Re: Rough cold running

Post by Lotus-e-Clan »

Hawaiis0 wrote:
Fri Nov 19, 2021 15:32
How does the crank case breather fit in? Is it still secure and together?
Good point - also check that the hoses fitted to the brown valve are the right way around. Mind you, the top one is always open so wouldn't explain the air leak sealing when greater than 60C.

Also check connections to the orange airbox diverter valve. There should be vacuum in the F-pipe to the orange valve (opens the diverter flap in the airbox when cold). If fitted the wrong way around the F pipe could be sucking in extra air when cold (so could be be the cause of air leak / cold weakness).
Peter K

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