Rebuilt engine - low compression on cylinder no.2

[Charles73]

One from each carb, same numbers stamped on them...

[mbell]

There shouldn't be a hole in the plates. They are added as work around to allow air flow at idle without using the idle screws to open the plates and having them expose the transition holes, which can cause an off idle stumble.

I'd consider replacing the throttle plates with new ones without the holes.

On the fuel level I'd check the fuel level via the main jet stack hole method rather than by measuring the distance between the floats and body top. It maybe a hole in the floats or incorrect weight which could cause high fuel level even with gap set correctly. To do this you remove the main jet stack from the carbs (via jet cover) and run fuel pump (engine for mechanical fuel pump). You should be able to see if fuel level is high enough to run down into the throat and measure how far down the fuel level is with caliper, needs to be 25mm or greater.

[2cams70]

Some carbs have holes and some don't. Either way though you shouldn't have a big hole in one plate and a small hole in the other. The carbs I've seen with holes only have a very tiny hole in the throttle plates. The small hole mod seem to be a factory one.

Note there's different plates with different edge angle settings. What figure is stamped on yours and are they all the same? What does the peening look like on the back of the screws? This will provide evidence as to whether the shafts have been out and the throttle plates removed by someone not really knowing what they are doing previously.

My carbs had the small hole in the plates. No off-idle stumble driveability issues.

[rgh0]

Hi Charles
do you have holes in the butterflys in one of the carbs but not the other`or holes in both but different sizes ? People drill these holes to get the butterflys further closed at idle to get the progression right. It is generally a work around due to other problems. I would get 4 new butterflys and start again.

cheers
Rohan

[Charles73]

Hi Charles
do you have holes in the butterflys in one of the carbs but not the other`or holes in both but different sizes ? People drill these holes to get the butterflys further closed at idle to get the progression right. It is generally a work around due to other problems. I would get 4 new butterflys and start again.

cheers
Rohan

Both pairs have holes. The rich carbs' plates have larger holes that the other carbs' though. The richer carbs' plates are also not seated very well and seem worn as they have the same numbers stamped on them, but seem smaller due to the amount of light that passes around them, no matter how they are adjusted.

I will certainly replace the worn plates. Is it possible to solder the holes on the other pair? As they seem quite new and the holes are <1mm.

This doesn't explain the dripping venturi/choke though does it? I will try measuring fuel level via main jet assembly chamber as previously suggested...anything else to check whilst the carbs are off?

Thanks

[nmauduit]

It seems that the carbs will need a thorough check for perfecting the tune... the "balancing" holes drilled in the butterfly could be filled with a dab of lead/tin soldering, but the key issue is to get the butterflies (all of them) to seal well and identically (or almost) at the same time : then from idle the air flows by the slit next to the progression holes, and sucks the fuel from these progression holes. If there are leaks (butterly not sealing ... or extra hole) then the air speed at the progression hole is lower and less fuel is sucked.

To be sure, I would first try the best centering and sealing I can get from these butterflies (assessing the gaps when closed againt the light, measured with feeler gauges ... this takes a while and must be attempted with carbs off to be accurate), but I'm worried that these holes indicate some other problems addressed by this "quick fix" (bent shaft? swapped shaft that was not installed properly? then checking the bearings and leaks there also).

If by fettling a satisfactory visual seal can be obtained on all four butterflies (e.g. from your 2 photos, one look very good - possibly even too closed for idel which is to be set by the stop - while the other is slightly ajar, can it be improved or is it on the stop? the most important is homogeneity within a carb, then the ability to get the 2 carbs synchronized from the start ), I would plug the extra holes and check what idle flow can be obtained with these carbs (the engine needs to run sufficienly smooth, to be confirmed with a good tachometer would help) using a snail-type synchrometer. A few l/min discrepancy is not the end of the world, esp. if you can tune the take off so that all barrels flow equally when starting to accelerate (and uncover the next progression hole : the butterflies should be synched on the progression holes as well).
nb: after swapping out or repositionning the butterflies, make sure you tighten the screws back adequately, but not bending the shaft (getting some support from the other side?) - you may consider loctite upon reassembly (can be heated to take out later if need be).

You may also borrow a pair of known good carbs for a quick sanity check (and get the timing right etc) so that the next step would be only focused on the carbs.

good luck !

[2cams70]

There’s a very tiny aluminium sealing washer that goes under each accelerator pump jet - 2 per carb. Did you remember to install it? Very easy to miss.

The throttle valves normally don’t wear because they don’t contact the inside of the barrel. If they are worn you’ll see a corresponding groove in the barrel in which case it may not be worth rebuilding the carb.

From the evidence that I’ve seen the small holes in the throttle plates appear to be something Weber themselves did at the factory. They are too well done and consistently drilled to suggest otherwise. Something has happened with your carbs though if they are both the same type (31’s?) but have different throttle plate drilled hole sizes. Replacement throttle plates don’t come with holes drilled.

[Charles73]

There’s a very tiny aluminium sealing washer that goes under each accelerator pump jet - 2 per carb. Did you remember to install it? Very easy to miss.

The throttle valves normally don’t wear because they don’t contact the inside of the barrel. If they are worn you’ll see a corresponding groove in the barrel in which case it may not be worth rebuilding the carb.

From the evidence that I’ve seen the small holes in the throttle plates appear to be something Weber themselves did at the factory. They are too well done and consistently drilled to suggest otherwise. Something has happened with your carbs though if they are both the same type (31’s?) but have different throttle plate drilled hole sizes. Replacement throttle plates don’t come with holes drilled.

I'll check the washers are fitted. Thanks for that.

I have to agree that the holes in the plates to look to be factory done.

[rgh0]

Elan Carbs never came with factory drilled holes from what I have seen so these may be from some other car or cars originally

[Craven]

If your carb’s started life as a pair their serial No will be fairly close, be careful though as when I say close this can be a thousand or two out. Term matched pair is a misnomer, the smaller of the two holes in the throttle plate looks standard and a factory fit 31, I’ve made this point many times the hole is an attempt to overcome the over advance progression of the 31 when used with a 1558cc Twincam. Enlarged hole size and plate fit suggests to me that carb is well played with but regards your problems at the moment is not a significant factor.
Ensure your pressure regulator is set on the low side around 2 – 3 psi, and should be mounted on the body not the engine. Have you actually measured the pressure..

[Charles73]

If your carb’s started life as a pair their serial No will be fairly close, be careful though as when I say close this can be a thousand or two out. Term matched pair is a misnomer, the smaller of the two holes in the throttle plate looks standard and a factory fit 31, I’ve made this point many times the hole is an attempt to overcome the over advance progression of the 31 when used with a 1558cc Twincam. Enlarged hole size and plate fit suggests to me that carb is well played with but regards your problems at the moment is not a significant factor.
Ensure your pressure regulator is set on the low side around 2 – 3 psi, and should be mounted on the body not the engine. Have you actually measured the pressure..

I haven't yet measured the fuel pressure. The regulator is plumbed in, just not mounted. The regulator is dialed in low (adjustment screw a turn or two in).

[mark030358]

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.

Hi,
My rotor arm points to No1 cylinder when firing on No.1 (or very close to it) yours is pointing to No.3 I guess the HT leads in the cap can be connected differently. See my picture

Cheers
Mark

[Charles73]

Further investigations have shown that the float on the rich carb did have a hole in it! (I somehow didn't notice the first time I pulled it out). After ensuring all fuel was emptied out of the inside of the float, I soldered up the hole, checked it was air tight and reassembled.

Also, the other carbs' spindle was slightly twisted, preventing the butterflys seating correctly. I managed to straighten it by using a pair of grips on each end of the spindle and gently twisting so both butterflys seated correctly- checking via the progression holes and using a torch.

I soldered up the holes in the butterflys too.

I also realised that because I had way too much fuel going into the cylinders, I had fuel go down past the rings into the sump. The dip stick showed this. I've changed the oil and filter. Something to be mindful of if you ever have an overfueling issue!

Anyway, fired her up and it sounded much better straight away and no fuel flowing into the choke/venturi area as it was before. I didn't have the time to balance and tune, but definitely making progress.

Thanks for your suggestions so far everyone, much appreciated. Not done yet though...


Charles.

[2cams70]

Hi,
My rotor arm points to No1 cylinder when firing on No.1 (or very close to it) yours is pointing to No.3 I guess the HT leads in the cap can be connected differently. See my picture

Cheers
Mark

Hi Mark,

If you point the rotor in the position i described earlier the distributor cap will end up orientated in the best position, the leads will be arranged neatly with little or no cross over and they will be placed to have the shortest possible run to the correct spark plug. I believe this is how it was from the factory.
True though functionally the engine doesn't care about how things are orientated. It just needs to receive a high voltage pulse in the correct sequence and time to each spark plug. Picture with cap fitted attached for reference.