Rebuilt engine - low compression on cylinder no.2

[rgh0]

19 bar / 275 psi compression is higher than expected. it should be around 180 - 200 psi maximum. Is the compression gauge working properly ?

cheers
Rohan

[Charles73]

19 bar / 275 psi compression is higher than expected. it should be around 180 - 200 psi maximum. Is the compression gauge working properly ?

cheers
Rohan

I guess I can't be certain it is, but I have no reason to believe it isn't working properly. I think I have another so I try that too.

If the gauge is working, what would explain high compression? The engine has not yet been run in or driven, if that makes a difference.

Many thanks

[rgh0]

Very hard to get a compression that high in a twin cam without major engine modifications done wrong which is why i would suspect the guage

cheers
Rohan

[Charles73]

Very hard to get a compression that high in a twin cam without major engine modifications done wrong which is why i would suspect the guage

cheers
Rohan

I've retested with a different gauge. Getting a reading of 225 on every cylinder. I think I can be confident valves are okay.


I decided to start again with distributor.
I pulled it out, set no.1 to TDC, refitted with lead direction at around 8 oclock, fitted cap with no.1 lead on plug where rotar arm points (bottom right), then going counter-clockwise 1-3-4-2.

Can't get it to fire up. Plugs are sparking when out the engine. Tried the set of Super 4s with where in the old stromberg head and a set of NGK. Nothing. It was firing so I've now made it worse!

Any ideas?

[alanr]

I think due to the distributor skew gear drive affecting how you are positioning the distrubutor as you fit it you may have the distributor either one or two teeth out?
With TDC set, back two valves rocking, clearance on front two valves confirming that you have it at TDC on No1 cylinder and not TDC on No 4 (!) and assuming the plug leads are in their original position in the cap, the rotor would normally be around the 10'oclock position with the cam/rotor arm positioned so the points are just about to open and you are just about to fire No1 cylinder. Direction anti-clockwise.

Alan

[nmauduit]

I've retested with a different gauge. Getting a reading of 225 on every cylinder. I think I can be confident valves are okay.

sounds quite promising... now back to basics : fuel + timed spark = bang ...

Do you use a timing light ? if you had it more or less running it should run again and you can progress from there, one can check if a chamber is running or not by the exhaust temperature variations among outlets (esp. if you have tubular header - that can help pinpoint a failing wire or spark) - caution, it gets real hot quick. Also.., a twink in decent condition can even run on 2 cylinders, so don't assume running "better" is running on all 4.

good luck !

[promotor]

Very hard to get a compression that high in a twin cam without major engine modifications done wrong which is why i would suspect the guage

cheers
Rohan

I've retested with a different gauge. Getting a reading of 225 on every cylinder. I think I can be confident valves are okay.


I decided to start again with distributor.
I pulled it out, set no.1 to TDC, refitted with lead direction at around 8 oclock, fitted cap with no.1 lead on plug where rotar arm points (bottom right), then going counter-clockwise 1-3-4-2.

Can't get it to fire up. Plugs are sparking when out the engine. Tried the set of Super 4s with where in the old stromberg head and a set of NGK. Nothing. It was firing so I've now made it worse!

Any ideas?

Have you definitely got it set with no.1 firing and not no.4 firing? A picture of inside the distributor and where the rotor arm sits and the points cam when @ TDC might help us help you (I know its difficult to get in under the carbs on an elan, but just in case you can...).

[Charles73]

I decided to take carbs off and swap them round. Now the issue with no.2 is now on no.4. So there is clearly something up with the carb on that intake, not necessarily ignition or valves.

It's basically running way too lean on that intake (also the one showing low compression when tested with closed throttle). Any ideas on what to look for whilst stripping the carb? The butterfly's are all in the same position when checking through the progression holes.

Also the other carb, which was originally fitted to no.3 and 4, is way too rich and is flooding. This issue also followed the carb once fitted to no.1 and 2. So any ideas on what the issue there is would be most appreciated.


Many thanks

Charles.

[SENC]

Before you fully strip, remove the idle mix screw and progression hole covers as well as the idle jets. You'll see the channel for fuel inside the idle jet chambers, just down and towards the outside of the carbs. The channel is just large enough for carb cleaner straw - shoot a jet of carb cleaner through the straw into the orifice and watch through the progression hole chamber and IMS chamber. Free flow will confirm whether you have a blockage in either channel. A minor blockage from fuel residue may well get cleared by this process.

While the progression hole covers are off, twist the throttle rod and watch for the butterfly location, to ensure they are all at the same location relative to progression hole when fully closed and at various points in the rotation. I find sticking a small flashlight in the chokes aids with identifying differences.

Inspect the IMS screws side-by-side to make sure you don't have a mixture of the old (blunter) and new (sharper taper) screws. The older screws, in particular, tend to allow air flow around then. I use EZ-Flow fuel lube on these screws to prevent unwanted air flow. Also check the orings on the progression covers.

I'd be hunting for either a fuel blockage or for an air leak on the lean carb as initial starting points.

Additionally, with the engine running, with a good light peer into the chokes and look for fuel leaks in the richer carb... either a dripping accelerator pump or a drip from the main jet into the aux venturi. The latter might indicate fuel level in that carb is too high. The former might be that the pump jet isn't seated well. Could be a bit up buildup in that well, or that the bottom of the well is slightly deformed. The latter can sometimes be resolved by dropping a small ball bearing in the well and lightly peening it.

[Craven]

Bit of a long shot but check the throttle plate has been refitted correctly, Not back to front.
Edge of plate is tapered not square. Deg printed 79 30 all on same side.

[vxah]

You did say that the suspected dodgy carb barrel gave that cylinder a low compression compared to the others when tested with closed throttle?
That being the case I would suspect something amiss with that throttle plate?
What I do when rebuilding them is connect my workshop Henry vacuum cleaner to the engine side of the carb and check the flow at the other end with a balancing flow meter. You can compare both barrels at fully closed and various small openings and make adjustments to the throttle plates before finally tightening the screws.
I was surprised at the changes it makes just fiddling with the plates even when they look the same!
Don’t do this unless there is no fuel anywhere in the carburettor or your Henry might end up in orbit!!

[mbell]

If there is a throttle plate that is at a noticable different position to the others then that is incorrect and likely your problem. The question is why? Incorrectly fitted? Bent shaft?

It certainly won't run evenly until the issue is addressed.

[Billmack]

Recheck valve pash on low cylinder if clearance exists or is a little more than normal its off with the head

[2cams70]

When you had the carburettors off the engine did you check the seating of the plates in the barrels with the the idle speed screws fully backed off? You do this by holding the carburettor up to a light and observing how much light you can see around the edges of each throttle plate when it’s fully closed. You should only see a very slim sliver and all plates should be approximately the same. If they aren’t you need to back of the screws securing the offending plate/plates and realign them accordingly. As Craven mentioned previously it is also possible a plate has been installed upside down or back to front as the edges of the plates are angled. If you just observe the plates through the progression holes you may not get the full story.
With regards to the ignition I set the ignition up initially by aligning the pulley timing marks in the correct static timing position with number one piston on compression stroke. I then fit the distributor so the rotor is pointing roughly at the correct pole in the distributor cap. Once that’s done I loosen the distributor clamp bolt and turn the distributor anticlockwise a fraction. With a points system the points need to be closed. At that stage I turn the the ignition on and hold the number one spark plug lead so I am in a position to see a spark jump from the lead to ground. I then very slowly rotate the distributor clockwise until I see a spark jump from the lead to ground. At that point the timing is correct and I tighten the clamp bolt and start the engine. Once the engine is running I check and fine tune using a timing light. Usually though I find that fine tuning is not required and the timing is spot on. If you google the Lotus Marques website there’s quite a good technical article on fitting the distributor so that the rotor arm and the spark plug leads are in the correct position relative to each other.

[Charles73]

Before you fully strip, remove the idle mix screw and progression hole covers as well as the idle jets. You'll see the channel for fuel inside the idle jet chambers, just down and towards the outside of the carbs. The channel is just large enough for carb cleaner straw - shoot a jet of carb cleaner through the straw into the orifice and watch through the progression hole chamber and IMS chamber. Free flow will confirm whether you have a blockage in either channel. A minor blockage from fuel residue may well get cleared by this process.

While the progression hole covers are off, twist the throttle rod and watch for the butterfly location, to ensure they are all at the same location relative to progression hole when fully closed and at various points in the rotation. I find sticking a small flashlight in the chokes aids with identifying differences.

Inspect the IMS screws side-by-side to make sure you don't have a mixture of the old (blunter) and new (sharper taper) screws. The older screws, in particular, tend to allow air flow around then. I use EZ-Flow fuel lube on these screws to prevent unwanted air flow. Also check the orings on the progression covers.

I'd be hunting for either a fuel blockage or for an air leak on the lean carb as initial starting points.

Additionally, with the engine running, with a good light peer into the chokes and look for fuel leaks in the richer carb... either a dripping accelerator pump or a drip from the main jet into the aux venturi. The latter might indicate fuel level in that carb is too high. The former might be that the pump jet isn't seated well. Could be a bit up buildup in that well, or that the bottom of the well is slightly deformed. The latter can sometimes be resolved by dropping a small ball bearing in the well and lightly peening it.

Thanks for your suggestions. Very helpful.

I shot some cleaner through the idle jet as per your suggestion, and plenty of flow showing through progression holes.

Viewing the butterfly's through the progression holes showed that they were all in the same position on all 4 inlets.

I ran the engine and this showed excessive fuel coming from the venturis on idle (rich carb). I checked float levels, and both measure 8.5mm in-between gasket and top of float when held vertically, just touching valve ball. I also removed floats and cleaned valve and it seems to functioning correctly. Ran the engine again and it was very much the same.

I then removed the two carbs to check the butterfly's. When held up to the light, the butterfly's on the rich carb are showing they're not sitting correctly. There's quite a bit of light showing through all the way round each butterfly - including the sides. All four butterfly's are 79 30, installed with numbers showing on correct face, but the two on the rich carb have a bigger hole than the two in the other carb (which are seated correctly) ...surely the holes should be the same size? And what is the function on these holes? Based on the fact that they do not sit correctly and they also have a gap either side - not just top and bottom - I think I'll replace them as I think I'll be chasing my tale trying to get them to sit correctly (I did manage to improve them, but still not enough). It seems whoever refurbed the carbs replaced one pair and not the other.

I'm presuming that having the butterfly's sit right once replaced won't necessarily solve the fuel flowing in the choke/venturi area? What is likely to be the cause of this? As I say, float levels and valves seem okay.


Many thanks

Charles.