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eeyoreish

Carb Balancing Woes

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I originally posted this in the Stevens forum but one of the guys there suggested that you G car owners might have more experience with carbs...

Having read lots about it here on the forum and got myself a copy of the Lotus service manual instructions on the subject I set about balancing my carbs today. Carbs have been recently rebuilt with genuine Dellorto kits.

Having previously balanced the airflow using a borrowed 'snail' manometer in each trumpet I thought I'd spend half an hour checking and rebalancing them properly via the air ports using my new 4 way carb balancer before checking the mixtures with a colourtune.

Having actually spent all afternoon on it I'm at my wits end as no matter what I do I can't seem to get a consistent balance between all 4 barrels. I followed the manual instructions and balanced lowest reading across both carbs first, then adjusted the remaining 2 barrels to match, adjusting the throttle linkage a touch where necessary.

The problem is, each time I get the readings close enough to think 'that'll do' after a blip of the throttle they settle back down into different positions! I've tried several times from scratch and keep getting the same problem.

The other issue I have is that each time on the back carb I seem to end up with one air screw fully in and one fully out just to get them the same.

All vac hoses are new and there are no air leaks that I can hear.

Am I doing something wrong???


Not worth starting anything now...🍺

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I have done it a few times now with no issues. When I first did it I stumbled across the following site wich has nice easy step by step instructions. Have a look and see if there is anything different in there to what you have done.

 

http://theapplebyclan.com/Richard/Tiger%20Super6/Useful%20info/Dellorto/

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I have done it a few times now with no issues. When I first did it I stumbled across the following site wich has nice easy step by step instructions. Have a look and see if there is anything different in there to what you have done.

 

Thanks for this Glyn, looks nice and clear but...

 

This is a different method to the Lotus manual , which doesn't have the 'use mixture screws to get strongest vacuum' step. It just says to open all the mixture screws equally then balance the airflow carb to carb (lowest barrel) and then between the barrels on each carb.

 

Only once air flow is balanced do you tweak the mixture screws to set the air fuel mixture up correctly for each barrel/cylinder (in my case with a Colortune).

 

Using the procedure in your link there's no mention of checking the mixture for rich/lean?

 

 

I've seen a similar method mentioned before but it kind of adds to my confusion a little since I will be adjusting fuel before balancing airflow (I think).

 

I also read that synchronizing the physical butterfly opening carb to carb is essential (which I did by checking that they pass the progression holes at exactly the same time). I'm not sure if adjusting the airflow carb to carb using the throttle linkage screw then messes this up or am I effectively doing the same thing there anyway? 


Not worth starting anything now...🍺

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Neil,

 

There is a small difference between the two methods but they'd give the same results when followed, but balancing isn't your problem, it's the inconsistencies you're getting after blipping the throttle.

 

This says to me you have worn components in the linkages.


Cheers,

John W

http://jonwatkins.co.uk

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it's the inconsistencies you're getting after blipping the throttle.

 

This says to me you have worn components in the linkages.

 

Yes, I'll be having a look at the linkages and spindle bearings at the next opportunity.

 

When the carbs were off the car everything seemed fine but a closer inspection now will hopefully tell me what's going on...


Not worth starting anything now...🍺

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I have done it a few times now with no issues. When I first did it I stumbled across the following site wich has nice easy step by step instructions. Have a look and see if there is anything different in there to what you have done.

 

http://theapplebyclan.com/Richard/Tiger%20Super6/Useful%20info/Dellorto/

 

Just re-reading this now and I don't understand step 15:

  1. After making the adjustments, your starting point may have been thrown out of whack. Go back and repeat the procedure from the start. Re-set the idle speed, adjust the idle mixture screws for strongest vacuum, balance the weakest barrels on the front and rear carbs, balance the strong barrel to the weak barrel on each carb. If nothing changes, you are done. If you need to make any significant changes during the second trial, you may want to repeat the procedure yet again until it stabilises.

What does it mean by 'your starting point may have been thrown out of whack'.

 

This part of the procedure just seems to suggest doing the whole thing again. Not sure I understand why...?


Not worth starting anything now...🍺

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Everything on a Lotus affects everything else..it's the "interconnectedness of all things" personified. All adjustments are iterative processes...you start out, and when you've finished, the first adjustment you made will probably have changed...so you go through the loop again... You have to sneak up on the optimum.


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Everything on a Lotus affects everything else..it's the "interconnectedness of all things" personified. All adjustments are iterative processes...you start out, and when you've finished, the first adjustment you made will probably have changed...so you go through the loop again... You have to sneak up on the optimum.

Agreed! But in this instance surely at the end of the process everything is balanced. What would be 'out of whack' that would mean I'd need to start again?

Am I missing something..?


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Yes....the effect that the adjustments you have made might have had on the original conditions. You set one parameter, then change another, and that is what affects the original setting...you're chasing your own tail, refining the solution with each iteration... You may find it's all fine after one cycle; but you won't KNOW this unless you go back and check!!


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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I spent ages trying to set mine up and get it through the MOT emissions test. With the mixture screws set to 3.5 turns it failed miserably and when I eventually got it through the screws were around 6 turns each which everybody tells me is way too much. Seems like a black art to me and unless you have a machine to measure CO and HC how do you know if it's correct?

 

I found the colour tune to be a bit hit and miss as there was no real yellow/blue/orange type effect that I could see. I could wind the mixture screw out or in a full turn and not be able to notice the difference.

 

My carbs had been stripped and had new bearings and rebuild kit.

Edited by Paul Coleman

Lotus Esprit [meaning] a 1:1 scale Airfix kit with a propensity to catch fire

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My carbs had been stripped and had new bearings and rebuild kit.

Paul - out of interest, did you replace the carb bearings yourself or have it done by a specialist?

If you did it yourself, how did you find it? If you used a specialist, who was it?

Just weighing the options in case my bearings need doing...


Not worth starting anything now...🍺

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Its in the des hamil? (cant remember his name) dellorto carb  book.   I don't think its too hard just care time needed.

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Its in the des hamil? (cant remember his name) dellorto carb  book.   I don't think its too hard just care time needed.

I've read some stuff online about replacing t he bearings, which apparently involves removing the butterflies, peening over new screws etc.. Sounded like a bit of a nightmare job!

I'm hoping mine don't need it doing.


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That's pretty much what it says in the book,  

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Paul - out of interest, did you replace the carb bearings yourself or have it done by a specialist?

If you did it yourself, how did you find it? If you used a specialist, who was it?

Just weighing the options in case my bearings need doing...

Yes I did them myself. My bearings were totally shot and there was no choice but to replace all 4 of them. I have a small press which made the job easier - I wouldn't want to do it without.

 

Realigning the butterflies was a pain and the first time I did it they weren't good enough so I then spent another 4 hours getting them better. Peening over the butterfly screws is not a difficult job you just have to make sure it's supported correctly.

 

Paul.


Lotus Esprit [meaning] a 1:1 scale Airfix kit with a propensity to catch fire

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Yes I did them myself. My bearings were totally shot and there was no choice but to replace all 4 of them. I have a small press which made the job easier - I wouldn't want to do it without.

 

Realigning the butterflies was a pain and the first time I did it they weren't good enough so I then spent another 4 hours getting them better. Peening over the butterfly screws is not a difficult job you just have to make sure it's supported correctly.

 

Paul.

Thanks Paul. So you'd say it's doable for a confident DIYer? I've already had the carbs in bits once for the refurb.

Do you reuse the spindles and butterflies with just new bearings and butterfly screws?

That's pretty much what it says in the book,

Is there a step by step in the book for doing this?


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Yes I would say it's doable. I got a quote tohave mine cleaned and rebuilt and it was £500 which is why I decided to do it myself.

 

As long as the throttle spindle shafts are not bent or worn you should be okay reusing them. The DHLA45 shafts have been discontinued according to... http://www.dellorto.co.uk/product-category/dellorto-car-carburettors-parts/parts/dhla-parts/

 

But make sure you take the pump rod linkage bracket off the middle of the shaft before you try to press it out or you will bend the shaft. If you make a note of the butterfly positions and which shaft is which and just put it back how it came out you should find it easier to realign them. Try not to adjust the pump rods as setting them up is a bit of a pain.

 

It's all in the Hammil book.

Edited by Paul Coleman

Lotus Esprit [meaning] a 1:1 scale Airfix kit with a propensity to catch fire

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Finally got some more tinkering time on this over the past couple of weekends.

I removed the carbs again and checked the spindles and butterflies. Everything was fine, no stickiness or graunching in the bearings and both butterflies in each carb closing together. I re greased the bearings anyway, just to be sure and re fitted the carbs.

I also removed and cleaned the air adjustment screws and idle mixture screws, which we didn't touch last time when we did the carb refurb for fear of screwing up the settings to the point we couldn't get it running. In hindsight a dumb move as the air screw o rings were shot, probably why the flows were fluctuating after adjustment.

I also replaced the o rings in the mixture screws. They were fine, if a little flat, and were missing from the Dellorto kits I bought, hence I hadn't realised they were even in there. Thanks to Eurocarb for sending our 'replacements' with no quibble.

Based on some advice on another forum I also checked the actual fuel bowl levels. I'd already set the float levels to the book 15mm but apparently it's also necessary to check the actual fuel level in the bowl as this can vary and is critical to proper progression.

With the carb tops off, the target is to have the fuel level 27mm down from the top of the jet stack. Mine (especially the front) were more like 30mm so a little low. A bit of trial and error now has them both at 27.5mm - close enough!

Finally I've rebalanced the air flows using the Carbtune 4 branch manometer. This time they balanced up much more easily and stayed put ?

The overall result is that it feels much better now. Ticks over smoothly, my off idle stumble is (I think) almost gone and if you give it the gas it goes like stink.

What I do still find is that it does not like to pull at low revs (say 1500-2000 ish) in high gear (5th). If I drop into 4th to pick up the revs it's fine.

I'm so used to driving a modern turbo diesel that I don't know if this is just driving style or there's something still amiss. Am I trying to tune out something that's just characteristic of this car?

Id appreciate if anyone with an N/A of similar vintage experiences the same problem..?

 

 

Edited by eeyoreish

Not worth starting anything now...🍺

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I found the same switching  from driving a tdi as a normal car for years.  As long as your rev limiter is connected and your  cambelt is good  then give it some beans.   Sounds great once its wound up!!!!  Check the rev limiter on an n/a as some people disconnect them as they can cause problems. Its in the box in the boot with the coil.  The car will still work with it unplugged JUST TAKE CARE!!!!

You will get used to it, just take a little re adjustment.

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I found the same switching  from driving a tdi as a normal car for years.  As long as your rev limiter is connected and your  cambelt is good  then give it some beans.   Sounds great once its wound up!!!!  Check the rev limiter on an n/a as some people disconnect them as they can cause problems. Its in the box in the boot with the coil.  The car will still work with it unplugged JUST TAKE CARE!!!!

You will get used to it, just take a little re adjustment.

Thanks Malcolm. So yours is just the same then?

When I say 'doesn't like to pull' I mean that it feels like it misses or stutters but it's a 'regular' stutter rather than something that feels fuelly. Kind of like it's starting to stall.

As long as I know it's 'normal' I'll learn to drive around it and keep the revs up. Pretty sure my rev limiter is plugged in but will double check. Haven't hit it yet as (again due to the TDi car) I'm more used to driving at more relaxed revs...


Not worth starting anything now...🍺

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I was fairly nervous re piling on the revs when I 1st started using it but after watching it being set up on a rolling road im not so cautious now. 

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On Sonntag, 11. Oktober 2015, eeyoreish said:

What I do still find is that it does not like to pull at low revs (say 1500-2000 ish) in high gear (5th). If I drop into 4th to pick up the revs it's fine

 

Have a look on the push rod adjustment of the accelerator pump.
If it has to much slack, fuel squirting starts to late on the low revs.
There is some measure in the Hammil book what the gap of the plunger to the push rod shall be, just check with a feeler gauge.
Readjusted mine as they had a too big gap, now pulling fine on lower revs.

It is the same as with not taking out the air balance screws, lot of talking not to mess around with it, but if you don´t check yourself you never know if the O-rings are still allright.


Website with information and pictures PBB St Tropez convertible Esprit:

http://www.lotusespritconvertible.de/

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