IDA's popping back through carbs

[type3]

Hi all, I need help setting up my IDA's. Just can't get it to idle happily. The spec is:

2332cc
48 IDA's (three progression holes)
40mm Venturi (was 42mm)
10:1 Compression
Cam: Unknown (maybe CB equiv. of FK87)
Heads: CB Wedgeport - 42 x 37.5
1.5 Ratio Rockers
Mallory Unilite body with Crane Cams XR700 (was previously a 009 with electronic ignition). The XR700 has made a massive difference.
Flamethrower Coil (was Bosch Blue Coil)

Jetting:
60 Idle (it did have 70 but it smelt too rich out of the exhaust)
110 Idle Jet Holder (was 120)
200 Air correction (was 170)
F11 Emulsion
165 main gas (was 200) going to go back up to 195

Mixture screws: Back at 1.5 turns out (still needs lean best idle set)
Idle/Throttle stop screws/Balancing: now 7 on snail balance meter (was too high previously)
Timing: 15 idle, 29 full advance (Mallory set to advance 14 degrees)

Float level correct
Fuel Pressure: 3.5psi

Symptoms:
[edit]:
Much better now! Running a bit hot maybe so might need richen it up a bit and the timing tweaked a bit more. The biggest difference seems to be the XR700. With that fitted I could see the mixture and idle screws making the difference that they should. The Mallory optical pickup and the Lumenition just seemed to be getting hot and cutting out or making the timing wander. Also got some good advice from a friend to set the throttle plates initially with a 1mm drill bit. Will post a video soon.

[buildabiggerboxer]

Hi Mate, your carb spec looks about right, tho the airflow meter reading seems high, but do it with the linkage dis connected yes? so nothing pulls the carbs when you snap it back on? assuming the valve gaps are o.k also? but popping is always going lean ,, so look for intake air leaks with the WD 40 or similar product trick, your 60 idle is bang on, but something is wrong if the screw is not 'tetchy' it should start to pop and bang as you screw in' lean , and then stall the cylinder with no fuel, maybe a clogged idle jet, that or the plugs have fouled during set up, whack in some clean ones and try again, you could try the 70 idle again, set to the stall point then back out so its smoothest, the screws should be about 1 to 1-1/2 turns out max, i use 60 idle on a 2.5 ltr just for your info. good luck.

[Chris V]

Might try some smaller vents and some F7's...

Set the timing @ max advance around 32* - and let the idle fall-in where it will - report back with this number.

[type3]

Hi Mate, your carb spec looks about right, tho the airflow meter reading seems high, but do it with the linkage dis connected yes? so nothing pulls the carbs when you snap it back on?

Yeah, I thought it was high too. The twin 40 IDF's that I had on a different (smaller) engine were happy at about 7 iirc. I've had to it leave connected for the moment just to keep it running.

assuming the valve gaps are o.k also?

Should be fine as it's done zero miles since I did them. I'll recheck though.

but popping is always going lean ,, so look for intake air leaks with the WD 40 or similar product trick,

I'll try this tomorrow.

your 60 idle is bang on, but something is wrong if the screw is not 'tetchy' it should start to pop and bang as you screw in' lean , and then stall the cylinder with no fuel, maybe a clogged idle jet,

The 60 jets were brand new and only fitted last weekend. I'll whip them out tomorrow just to be sure.

that or the plugs have fouled during set up, whack in some clean ones and try again, you could try the 70 idle again, set to the stall point then back out so its smoothest, the screws should be about 1 to 1-1/2 turns out max, i use 60 idle on a 2.5 ltr just for your info. good luck.

This is the second set of plugs that I've put in, but I've got another fresh set to go in if need be. I'll swap the 70's back in and see if it behaves any differently (apart from smelling rich).

Might try some smaller vents and some F7's...

Set the timing @ max advance around 32* - and let the idle fall-in where it will - report back with this number.

Cool, will do. Thanks for the help guys.

For reference, last year, very briefly I had it running like this:

http://www.youtube.com/watch?v=jUHwDX0DhqM

Since then I've changed the muffler, gone from 70 idles to 60's. Added foam air filters (although it still pops without them). And set the fuel pressure correctly (had pump issues before and it was too high). New Mallory pump and filter fitted. Plus I've messed with all the settings before I realised I had fuelling issues...!

[Bruce2]

Don't waste your time with 70 idle jets, they're too big for anything driven on the street.
Get some 65s. The advanced student will try some 63s (custom made).

[Ian Godfrey]

It would be nice to know the cam, but anyway a few suggestions.
timing, a lot of stumbles are helped by more initial timing say 15 to 18 deg idle and make sure the timing doesn't start moving until say 1100 rpm (above you idle speed) you can bend tabs in the 009 to reduce the total timing say 14 deg so then you could have 15 idle, 29 all in and be able to go up to 18 idle and 32 all in.
Unless it is a big cam 40 vents will make tuning easier and the engine more responsive. that said I have 42 vents and put up with a bit of a rough transition.
get some jet gauges so you can verify the size of the jets you are using
But anyway I'm with Bruce on the idle mix, my 2387 86B idles best on 110 idle airs with a 63 jet. It also works pretty well on 115 idle air and a 65 jet. I've never had much luck with 120 or 130 idle airs.
Then you need to sync the carbs and linkage, and set the idle mix screws.
Yes, you should be able to hear the cylinder cut out as you screw each one in (idle speed drops a bit and 'note' gets rough)
Take the main stack of the emulsion tube etc out of both carbs. Go for a good drive, you will feel where it pulls to on the idle circuits. get it warm and reset the mix screws, try again. Listen/feel for the lean pop if it is still there.
note if you have short velocity stax, this will effect the signal to the mains

Get the idle circuits right and then start on the mains, remember every time you change the idle jets you are also adding/subtracting air or fuel that will effect the overall mixture as well 'cos the mixture the engine sees is the idle jets plus the main jets combined on an IDA.

Most big motors prefer F2 though F11 should work reasonable well

[FJCamper]

Hi Ian, Type 3,

I'm with you Ian. First thing I'd change is the venturis. 42mm is on the too-big side except for serious drag racing and all-out dyno runs.

Just a drop to 40's should make an immediate difference.

FJC

[type3]

Okay, didn't get to time it today as one of the plug leads fell apart! These are new Bosch leads too! I'll have to order a new set. Number three was firing when I last checked, but that might explain why it didn't pop through the carb on Saturday. I did replace the plugs though.


I'm going to summarise your suggestions so that I can keep track of them:

Recheck valve gaps
Look for intake air leaks with the WD 40 - I did try this today but I didn't realise the plugs were fouled so it ran rough anyway.
Try some smaller vents and some F7's
Set the timing @ max advance around 32* - and let the idle fall-in where it will - report back with this number.
Get some 65s. The advanced student will try some 63s (custom made).
Try 115 idle air
Try 40 vents
Try F2's as big motors prefer them

I think the plugs were fouled from running the 70's previously. I'll get the new leads and go again with the other checks.

Thanks again.

[type3]

Don't waste your time with 70 idle jets, they're too big for anything driven on the street.
Get some 65s. The advanced student will try some 63s (custom made).

That's good to know! I did read in a few places that the 70's will run too rich for ACVW's. It certainly stank when I ran mine!

It would be nice to know the cam, but anyway a few suggestions.
timing, a lot of stumbles are helped by more initial timing say 15 to 18 deg idle and make sure the timing doesn't start moving until say 1100 rpm (above you idle speed) you can bend tabs in the 009 to reduce the total timing say 14 deg so then you could have 15 idle, 29 all in and be able to go up to 18 idle and 32 all in.
Unless it is a big cam 40 vents will make tuning easier and the engine more responsive. that said I have 42 vents and put up with a bit of a rough transition.
get some jet gauges so you can verify the size of the jets you are using
But anyway I'm with Bruce on the idle mix, my 2387 86B idles best on 110 idle airs with a 63 jet. It also works pretty well on 115 idle air and a 65 jet. I've never had much luck with 120 or 130 idle airs.
Then you need to sync the carbs and linkage, and set the idle mix screws.
Yes, you should be able to hear the cylinder cut out as you screw each one in (idle speed drops a bit and 'note' gets rough)
Take the main stack of the emulsion tube etc out of both carbs. Go for a good drive, you will feel where it pulls to on the idle circuits. get it warm and reset the mix screws, try again. Listen/feel for the lean pop if it is still there.
note if you have short velocity stax, this will effect the signal to the mains

Get the idle circuits right and then start on the mains, remember every time you change the idle jets you are also adding/subtracting air or fuel that will effect the overall mixture as well 'cos the mixture the engine sees is the idle jets plus the main jets combined on an IDA.

Most big motors prefer F2 though F11 should work reasonable well

Cool, good info. I'll work through it.

Hi Ian, Type 3,

I'm with you Ian. First thing I'd change is the venturis. 42mm is on the too-big side except for serious drag racing and all-out dyno runs.

Just a drop to 40's should make an immediate difference.

FJC

Are the 40's an off the shelf item? I'll look for a supplier.

[Ian Godfrey]

yes 40 vents are off the shelf from a major weber dealer or Jaycee or piersideparts

[volksbugly]

Just for reference here is the proper procedure for tuning dual carbs
http://www.youtube.com/watch?v=5tigMg-Fn94

I was doing it a little wrong, after I followed this procedure to the T It ran so much better.

[type3]

yes 40 vents are off the shelf from a major weber dealer or Jaycee or piersideparts

Thanks, now on order.

Just for reference here is the proper procedure for tuning dual carbs
http://www.youtube.com/watch?v=5tigMg-Fn94

I was doing it a little wrong, after I followed this procedure to the T It ran so much better.

That's a lotta chrome! That's pretty much how I do it. It's just that it's registering 20 on the snail for both carbs for me. Anything less and it's not happy. Maybe that's just the way it is. I just need to get it to idle reliably so I can do it without the linkage.

[type3]

New leads arrived yesterday and the vents arrived this morning. Very quick deliveries from Machine7 and Pierside Parts!

[type3]

Mini-update: had the manifolds off today. The gaskets had nicely compressed all round the ports so I'm confident they weren't leaking. Cut some new gaskets as the old ones had soaked up the WD-40. Also got the 40mm Venturi's fitted.

[type3]

I've swapped out the dizzy for another 009 (with points). Timed it at 32 degrees full advance. It's popping back through the carbs less now.

Oddly, it'll get to a point where either the revs keep climbing and I have to cut the power so that it doesn't over rev, or the revs start to reduce, but very slowly. Any ideas?