I'm hesitating!!!

[billwill]

You can replace the spark plugs in the process of doing a colortune.

1. take out old plug
2 insert colortune
3 set mixture
4 take out colortune
5 insert new sparkplug

[Elanconvert]

bill....I can tell that you've done this before!!!!!
got the colortune and plugs, but air balancing tool hasn't arrived yet...I was just going to change the plugs and see if that was the problem..........however, I suppose balancing should be done first, as doing it after colortune would upset the settings, then have to do colortune again......
fred

[billwill]

You may have to do everything more than once anyway.

Some Dellortos have an "idle Bypass" screw (page 40 in Hammill's book) which can be used to balance the two throats of a twin carb.

If you are sure that you want to do this, since you said it had been setup on a Rolling Road this is how to do it, well at least this is how I did it.

Actually Rolling Road was probably more to do with higher speed stuff, jet sizes etc rather than idling.

This is for after your air-flow meter arrives.

Take the front of the airbox off so that you can get at the trumpets. Get pencil & paper to make notes.

Find a piece of plastic pipe you can use to listen to the flow in the trumpet, i.e. one end at your ear, the other end at the trumpet.

Get engine idling as exactly as possible at 1000 rpm or 900 RPM. When you make adjustments the idling speed may change, so you will need frequently to use the idle speed screw on front edge of the rear carb to reset it to your chosen RPM. If it is not at one defined speed your measurements will not be consistent. All airflow measurements for balancing should be at the RPM speed that you chose as exactly as possible.

~~~~

First you are going to record the existing state of the carbs as accurately as possible so that you can get back to your starting conditions if you make a right cockup.

Listen to each trumpet in turn and note down a description of the sound. A burbling sound probably means the idle bypass screw is closed and a slight hissing sound is added when the idling screw is partly opened. If one throat of a carb is burbling and the other is hissing slightly, the bypass screws have probably already been set correctly. One bypass screw should be closed (in my opinion) and the other used to balance the throats.

Use the meter to measure the air flow rate in each trumpet and write it down.

Stop the engine then
for each idle bypass screw, gently screw tit down in turn untill it 'bottoms' , while counting the turns accurate to a quarter turn. Write it down, then screw it back up that number of turns, back to where it was.

Repeat this turns-measuring process for each of the 4 idle-MIXTURE screws. WRITE THEM DOWN..

There is no convenient reference point for the screw with opposing springs that sets the relative position of the throttle spindles of front & rear carb, so just leave that alone for the moment. It was, for me, the most difficult thing to get right, but with the flow meter it is dead easy and it is the last adjustment you make.

~~~~~

OK. Now if the flow rate between two throats of a carb differred by more than one mark on the scale of the meter you probably need to set the bypass screws for that carb.

It's worth a quick try with the existing mixture settings at first.


If the carbs are already self-balanced (both throats showing the same airflow) skip this and go set the two-spring central screw as described below..

Just do one carb at a time, don't worry about balancing front carb to back carb yet.

If one bypass screw was closed, leave it closed and adjust with the other one. If both were partly open close one. At the chosen RPM, measure the flow in the throat with the closed bypass screw, then transfer the meter to the other throat and adjust the idle-bypass screw until the flow is the same. Re-adust RPM if necessary. If you cannot get them the same, swap throats, close that idle-bypass screw and remeasure it and try to set the other throat to match instead.

Then do the same to the other carb.

Now each carb is self balanced and those idle-bypass screws will probably not need to be adjusted ever again.

~~~~~~~

Then use the central two-spring screw to get the same air-flow on trumpet 3 and trumpet 2. The spindle stays fixed for the rear carb and moving the screw rotates (slightly) the throttle spindle of the front carb. However you will need to adjust the IDLE RPM screw on the front edge of the rear carb to get back to your set speed .

You will need to keep swapping the air-flow meter between trumpet 3 and trumpet 2 as you make this adjustment but you should fairly quickly get them both the same. Do a final check of trumpet 1 and trumpet 4 they should now be the same figure.

~~~~

OK, now use the colortune in each cylinder in turn and use the relevant idle-mixture screw (not the idle-bypass screw) to set the idle mixture at your chosen RPM. You want a nice blue colour with pretty sparks of yellow and a flash of all yellow when you dab the throttle lever on the carbs. You will probably need a cardboard tube from an empty toilet roll surrounding the colortune plug so that you can peer down in to see the colours.

When you have done that put the airbox on and go for a brief run to see if all is improved. Then come back and do the balancing all over again because changing the mixture may have slighty changed the required perfect balance settings.

~~~~

Fun ennit ?

[Elanconvert]

wow! ....thanks bill!

I could have saved the purchase price of des' book...!!!!

what would be useful would be an illustration of which screw is which..[haven't received the book yet - maybe it's in there?] knowing me, I'm sure to get them wrong or mixed up.....

fred

[billwill]

The illustrations of which screw is which are in Des Hammill's book, shout if the book hasn't arrived before your air flow meter and we can probably find an illustration elsewhere for you without breaking his copyright.

That tuning sequence of mine is not in Des's book, but something similar might be in Brian Buckland's book.
(I must get a copy of that sometime ).

[Elanconvert]

des' book arrived this morning.....after a quick flick through, it seems to be the business, with lots of pretty pictures and explanatory arrows......just the thing for dummies like me!
just need the air alancer now and I'm away!

[Elanconvert]

update time...
air balancer came this am [bit late], so had a quick go....[.btw, since I cleaned the plugs, the idle/running is much smoother!!]
anyway, at idle, I'm getting 2.5 on three cylinders and 3 on no.4.........how closely should they match? [can't find a ref in des' book]
am I going to have to twist a spindle???

how much do readings vary with engine under load?
fred

edit...aha...just found the bit on ide by-pass..still doesn't give a range..I suppose I shuld do a colortune check as well first...

[holywood3645]

Sounds like you?re honing in on the problem(s)

I?m sure you having fun, doing this work. It?s all pretty interesting. You will start to play with different settings to get the optimum running performance. I?m not sure if you said or it has been mentioned, Electronic ignition is a must! Then I would say, perfect cam timing (adjustable cam sprockets, or offset dowels)

Before long you will be playing with chokes and jetting.
Maybe not, either way it's one of the quirks that brings us all to this site to share information

Good luck and keep us informed on your progress.
James

[richardcox_lotus]

The air balancer meter I use says in its instructions that they should be within 0.5 of each other. From memory I think it said the range at idle for webers is 1.8 to 2.3.

And I got the same readings as you !!

Still haven't managed to lower the value on #4 to match the others so have left it. I'm wondering if I have a tiny tiny air leak or infitesimal twist on the butterfly spindle, but I'm struggling to tune it out.

However.....to me it's no big deal. The car runs pretty smoothly, off-idle stumble is pretty much gone.

I made sure all my tuning was undertaken with a warm engine a new(ish) fuel.

I runs like crap after 1st startup after standing for more than a week but with 5 mins of warming up its as good as gold.

[billwill]

The vacuuum take off of No4 leaking?


~~~~~
However I think, but I'm not at all certain, that you get the higher flow reading on the one that is NOT leaking, because ALL the air required is coming through the trumpet & therefore through the meter.

~~~
Actually I think 2.5 and 3 is probably near enough.

[Elanconvert]

thanks for the comments guys....

bill - where exactly is the vacuum take off? btw, I'm not ignoring your excellent tuning instructions...it's just that I was thinking - given that the carbs were reportedly set up on a rolling road not too long ago, and also given my unrivalled ability in the 'cocking things up' department, it would be an idea to take some readings with things as they are i.e. before I start messing about ...[maybe I won't need to!!] ........will be doing a colortune check later.....

when I took the airbox off, I noticed some gungey oil [or was it oily gunge?] in the bottom, and some brown stains on the trumpets which was easily wiped off. is this normal? could be result of spitting back? [crankcase breather tube was removed by PO and hole plugged]
PO used tetraboost additive.......I have bought some millers octane booster, but not added any yet...I've noted the difference of opinion on additives on here
fred

[Galwaylotus]

[crankcase breather tube was removed by PO and hole plugged]

Not a good idea to plug the crankcase breather. It should at least vent to atmospheric pressure. Mine runs to the filtered breather canister in the nose of the car.

[45bvtc]

Classic WEAK fuel mixture. Make sure the idle mixture screws are set correctly first before you go looking elsewhere: for DHLA40E Dellorto carburettors set the idle mixture screw(s) 2.1/2 turns out for initial setting, earlier DHLA40 Dellorto carburettors were typically 5.1/2 turns out.

If those are ok but the problem persists then it'll get exciting...

[Elanconvert]

[crankcase breather tube was removed by PO and hole plugged]

Not a good idea to plug the crankcase breather. It should at least vent to atmospheric pressure. Mine runs to the filtered breather canister in the nose of the car.

ah! sorry! the hole in the airbox is plugged, not the one in the crankcase which is vented to atmosphere....however, seems a good [cleaner!] idea to take it to the air filter housing.....except that the filter will need renewing more often!

just heard from PO who had a leaking vacuum pipe most probably for some considerable time, i.e. when the carbs were set up, so maybe bill is on to something??
fred

edit; also thinking about it, would the rolling road tuners take account of oily fumes from breather pipe entering no. 4 choke and compensate by setting air-flow slightly higher for that one only?

[billwill]

Somewhere did you say whether it is a 2-seater or is it a Plus 2.

The front side of inlet manifold 1 and the rear side of inlet manifold 4 have a screwed hole for vacuum take off.

On the two seater Elan the rear one is plugged and the front one serves both the headlight lifting and the brake servo (if fitted).

On the plus 2, the front vacuum feeds the headlight lifter and the rear one feed the brake servo(s).

The rear one could perhaps leak air into cylinder 4 making the mixture weak.