Setting up 40DCOE151

[holywood3645]

I posted 2 the following questions on sideraft centeral that may be answered in simplier terms here, and hopefully someone with 40dcoes151 on a twin cam has had experience setting these up.

Engine Spec'
Newly rebuilt engine 1700cc lotus twincam (145 ish bhp)
the motor was built to QED 420S spec then I fitted
Chokes 34
Main air correction 155
Emulsion tube and F16
Idle jet 45F8

All jetting is per QED recommendations

I don't appear to have any Vac leaks. (tested with starter fluid)

Q1 How/when should the air bypass screw be adjusted?

Q2 The throttle return spring is working fine, however on deceleration the revs
stay high, (as if throttle stuck) I need to use motor load to bring rpm down

later note:
I downloaded and read the Keith Frank white paper, from sidedraft centeral and it helped.

Link to whitepaper
http://autos.groups.yahoo.com/group/sid ... e%20Paper/

The balance adjustment i have been performing may be contributing to my problems. I have been adjusting the
idle useing the slow running speed adjustment screw then balancing the second
carb using Carb sync adjustment screw on the shaft to get a matching readings on
an air flow meter. I'm thinking im adjusting the butterflys to be partaially
open causing the Lazy Idle Syndrome discribed in Paragraph 7.

If im reading the white paper '5 airflow balance' paragraph correctly that
balance should be done at 15% power. How is this done ??? (at 1/4 throttle or
?) And should be doing the low rpm Sync with the Air By-Pass Screws.

Im thinking with the butterflys closed. Is this the correct butterfly position
to start? Can some give me baseline is a good starting opening turns out for
the Idle bypass screw.


Also what is a good starting point for the Idle mixture Screw?


Thanks for your input

James

[types26/36]

James I believe the air by pass screws should be all closed for initial setting and you open the indervidual screws to match the choke with the highest airflow, also the mixture screws should be open about 1-1.5 turns to begin with.
Have a read of the article:http://www.redlineweber.com/html/Tech/dcoe_adjustment_layout_typical_i.htm

[holywood3645]

Brian i had not read the paper, thanks for the info. I will try the procedure and see how i do.


I see tha redline are about 70 mikes away. They may have someshop services.

Thanks

James

[twincamman]

Nonono use a butain torch unlit to check for leaks around the carbs ....ether wil set the car on fire at the first hint of a back fire ...I just set fire to a jeep. starting it with quick start . Nothing I couldn't handle though.... Ed

[rgh0]

James I believe the air by pass screws should be all closed for initial setting and you open the indervidual screws to match the choke with the highest airflow, also the mixture screws should be open about 1-1.5 turns to begin with.
Have a read of the article:http://www.redlineweber.com/html/Tech/dcoe_adjustment_layout_typical_i.htm

The bypass screws are used to achieve 2 things

1. To balance out individual difference in airflow between the two barrels in the same carb. The carb balance screw can then adjust between the 2 carbs
2. To help position the butterflies correctly versus the slow running jet transition holes. Opening up the bypass enables the butterflies to close a little at idle so the first transition hole is covered, this eliminates the lazy idle syndrome and if positioned right also stops the off idle stumble.

Getting it right takes a little practice as multiple items affect the butterfly position at normal idle speed eg bypass screws, balance screw, idle stop screws, idle mixture screws. The end aim is to have all four butterflies just closing off the first progression hole with each carb barrel flowing the same, the idle speed right and each bypass set as required to achieve the above and the idle mixture correct and not to lean. The procedures in Keiths white paper will help you achieve this point.

Cheers
Rohan

[jk952]

James,
re question 2; I had the return spring foul on the extra threaded boss on the side of the carb. body of the 151's, a blip to the throttle sometimes cleared when I wasn't sure of cause, but once not - had to kill with key and quickly pull over on the multilane highway !! - later back at the house I discovered the (intermittent) cause. This boss is not on earlier carbs.
(if I am understanding correctly your question, may not be your cause though from description)

others can advise better than I on settings...

Jack

[miked]

James, just saw what Jack put about the return spring catching on the 151's. See my thread about over rev. That was the cause but the sticking point was much more than a slight one. . Have sent you a PM. Mike from my phone.

[simonknee]

If the mechanical side of the linkages in fine with your carbs then your problems are down to your method of balancing and idling which is wrong. You have what Keith calls a lazy idle probably from fiddling with your air bleed bypass things too much.

First get yourself a decent synchrometer like an STE (Demon Tweaks or Aldon here in the UK).

Now screw in those air bypass bleed things so they don't affect things. Rohan points the way but here is how to actually do his number 1

Forget your idle and get a proper balance first...

Set all your idle mixtures to the same thing - there are two types of thread out there so one mans 3/4 turn is a couple of turns for another - anyway get your engine running at some kind of rough idle, can be above 1000rpm.

Now on a single carb body measure the airflow on the two inlets.
Open up the air bypass on the side that has less flow until both are equal.
Do this to both carbs.

Now balance the two carbs with each other using the linkage between the two.
Ensure that this airflow balance is good over a range of rpm.
What you now have is balance with the minimum opening of the air bleed bypass screw.

Now adjust the idle

Get your revs down under 1000rpm and adjust the mixtures as per the method in the weber doc that was linked to.
Ensure that you if you close each idle mix screw (not too tight please) the engine revs fall.
If not your idle speed needs to be lowered.

Now:
Does it stumble coming off idle and/or is it still reluctant to get back down to idle?

Post back what happens

Simon

[simonknee]

Oh and to respond to a later bit of your first post the butterflies should never be actually closed. If you want the engine to run at all it's going to need some air! The idle speed setting is what holds the butterflies open a crack. Without the air that flows over the top of the butterfly you would never draw fuel from the idle mixture hole. Look at some of the drawings on page 5 of my Hypojet thread to visualise the butterfly/hole interaction

[holywood3645]

I balanced and synced the carb?s yesterday evening, and adjusted the idle mixture per the recommendations from Keith on sidedraft central. The motor ran a lot better, however I noticed the idle mixture screw are pretty far out, much more the 1 ? turns (but it behaved better on the test drive) I pulled the plugs after and they were pretty sooty. I didn?t do anything else until today.

Thinking its rich, I adjusted the idle mixture screws again, but this time I did not try to find the mid point (between lean and rich stumbling). I adjusted coming from a lean stumble until I got it running smooth then went a ? turn richer. Took it for a drive, and was not as good as previous RICH setup. It was spitting a bit, but the plugs look better. I?m going to go an another Half turn richer and go try it again. It is idleing pretty smoothly

Later note
The lazey idle is no longer an issue since i adjusted the idle mixture. Rev's drop when off the throttle. However, It is stumbleing and spitting a little on gentle accelleration, but will pick up. It's also spitting intermittantly at higher rpm. Almost like it losing spark (maybe on No4). No. 4 is sooting up more than rest as if it running rich. I have leaned it but no change.

I'm ordering a new set of HT leads and will replace the plugs an see if that helps.

I don't have any vac' leaks

Weber 40DCOE151
Choke 34
Main 140
Air Correction 155
Emulsion Jets F16
Idle Jet 45F8

Thanks
James

[simonknee]

Glad to hear a proper balancing sorted out many of your issues. How close do you think you got the airflows and did you have to open any of the airbypass bleed thingies to do it? There is more work we can now do with the airbypass thingies as per item 2 posted by Rohan but where did you get to so far first.

Idle mix screws:
You probably have many more turns because you have finer threads on your idle screws. This is a desirable thing as there is less likely hood of a leak through the threads and finer control of the mix. It does cause confusion when reading a set up where the author of said procedure only had coarse threads to twiddle!

Rich Idle:
Setting a rich idle is generally a good thing when it comes to drive-ablility. See my other thread. This is not because you want a rich idle but the side effect of a rich idle is the proper air/fuel ratio at some of the other stages of throttle. Especially on those gradual accelerations.

Stumble and Spit:
The stumbling and spitting is is most likely caused at points where the mixture is going lean. The reason you get a better drive when setting those idle mixtures richer is because this promoting the correct mixture at those gentle throttle openings.

That dirty number 4:
It is unlikely that a sooty plug will cause a misfire. I would hazard that you are confusing a lean air/fuel condition with a misfire. My plug number 4 is as black as the night (more details in other thread). The thing still works. You can always put a little less mix on number 4 to see if it helps (EDIT -oh you did, ah well doesn't help me much either). Also do you have the breather pipe going from block into the back of the airbox. If so the perceived wisdom is that this causes more gunk on number 4 (one day I will re-route this to see if it is true). Changing plugs is no bad thing as the spark is generated on a the sharp edges of the contacts and these start to get rounded off fairly quickly. New NGK BPR7ES once a year (there are threads about hotter plugs but these are really not needed IMHO). Doubt your leads have an issue either.

Performance:
If you can give a list of particular throttle position/movement and rpm at the time that cause issue. For instance if the higher rpm stumble is in the 2-3K range (a classic issue) then is your 25mm float level set correctly as per Keiths paper - I will assume yes but I don't think you mentioned it yet.

[holywood3645]

Simon, I'm wondering how you are making out on your KF hypojets? I changed the HT leads and now don't think it was contributing to the issues I was having. I found my static timing out by about 7deg (advanced) reset to 10 BTDC and it is behaving better.
My engine runs well at low rpm with steady idle then smooth up you around 1800 rpm, (then what appears to be missing and spluttering, before it clears) It accelerates well on wide open throttle (WOT) again with a stumble around 1600 to 2200 rpm with some occasional spitting. It is a lot more noticeable on slow smooth acceleration and is less evident and pulls well at low rpm on wide open throttle.
I do however still has the stumble spit at the 1600 to 2200 rpm range and believe all other contributing factors may be eliminated. Plugs look good and maybe a little lean if anything.
I would like to rule out float height and made the rod tool and attempted to check height. It?s fiddly and haven't mastered identification the light to dark transition. I would like rule out all other issues before using the KF parts. However depending on how good your experiences are with the jets, I may just go for it.

Looking forward to you reply and update on you project

James