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Sun Jun 30, 2013 2:06 pm

Hello all,

I need some help jetting these carbs. I'm new to 48 IDA's but I sure like how the jets are installed compared to IDF's.

Anyway, I need some help getting my new engine to run right with these carbs. Here's the specifics...

2110 cc
10.5:1 compression
FK 8 cam with 1.4:1 rockers
CB 42x37.5 heads - no porting
009 with compufire set to 29 deg.
1-5/8 extractor into fatboy muffler

Carbs currently are - sorry I'm not yet up on all of the lingo...

42mm vents
55 idles
jack stack is: 210 on top, F2 tube, 180 on bottom (main jet?)
3 progression hole drilled
Summit rotary fuel pump (like a Holley Red), AN6 line to the engine. Holley pressure reg set at 3 lbs.

Symptoms are idles pretty well, rpms ok up to about 2500 - 3K then it starts to miss and spit. Over 3K it coughs and spits but I can power through it up to the point it acts like I've just emptied the fuel bowls.
Straight driving on level it runs ok, but recently has begun to miss every once in awhile. Kind of strange.

Any ideas on jetting this? I don't even know if this is in the ballpark. I have just about any jets or parts I'll need to do this just need to know what to try.

Thx for any info.
Tom

Sun Jun 30, 2013 3:03 pm

I think your idles are on the small side and u should bump them up to 60s , maybe even 65s. I think you should also drop your airs to 200s and mains to 175s. I have a.feeling your having the same problem as i am, emptying your bowls on the long haul in 3rd. I ordered my 3.0 needles and should be getting them tomorrow. I got mine from Darren at drd and he also gave me some ida modification hints.

Sun Jun 30, 2013 4:00 pm

First off I would not be running 42mm vents with that combo drop down to 37mm vents

My ballpark starting point for the given combo would be

37mm vents
55 idle
150 main (bottom)
190 air (top)
F11 E-tubes

Sun Jun 30, 2013 7:02 pm

Joe Bence wrote: First off I would not be running 42mm vents with that combo drop down to 37mm vents...

Unpack that statement. Is it because the heads are not ported?
A'little lost...

Are the stock Needle Vavles 2.0 on the IDA?

Thanks...

Sun Jun 30, 2013 7:34 pm

CUBA wrote: Joe Bence wrote: First off I would not be running 42mm vents with that combo drop down to 37mm vents...

Unpack that statement. Is it because the heads are not ported?
A'little lost...

The genral rule of thumb is 2mm smaller than the intake valve so 40mm, but on a unported street motor even 40mm is over kill, 37mm will give better drivability on the steeet.

Are the stock Needle Vavles 2.0 on the IDA?
Yes and will be fine in this combo
Thanks...

Sun Jun 30, 2013 7:46 pm

what good is a 42mm venturi when its flowing thru a 32mm intake port?

Sun Jun 30, 2013 7:55 pm

I agree that 37mm venturis would work better on those heads.

volksworld wrote: what good is a 42mm venturi when its flowing thru a 32mm intake port?

Sun Jun 30, 2013 8:42 pm

volksworld wrote: what good is a 42mm venturi when its flowing thru a 32mm intake port?

Got it!

Mon Jul 01, 2013 8:46 am

Thanks for the info guys. I'll make some adjustments this week as time allows and post back. Some background on the carbs. The way they are jetted, with the exception of the idle jets, is the way I got the carbs. They were on my race engine with I bought it. I pulled these off that engine so they were undoubtedly jetted big due to being on a race engine.

I did change the idle jets from 60 to 55 due to it loading up at idle. Would not idle for more than a couple minutes before almost dieing. That seemed to cure that issue.

I have a set of 40 vets, but no 37's so I'll try those. Anyone want to trade a set of 37's for a set of 42's?

Thx
Tom

Thu Jul 04, 2013 1:56 pm

Hey guys, working with your tips and the jets I have available I put together a main stack that is

185 air
F7 tube
155 main

and boy did it come alive over 3K rpms! Under 3K still coughs and spits until it gets a bit over 3K then it take off.

I have some 40mm vents I'd like to try but can't figure how they come out? The tube assembly that extends up through the stacks wiggles but won't come out. I can't see what's hold it in there. Am I missing something here? I know the accel pump nozzles come out first.

I'm going to leave the mains alone until I get the idle / low speed sorted out now. Going to try 60 idles and see if that helps or gets worse. If its better, great, if its worse I'll probably need 50's in there. Trial and error.

Happy 4th!

P.S. I have a bunch of what look like idle jets that are marked: 70F10
Anyone know what these are?

Thu Jul 04, 2013 2:37 pm

Unless your IDAs have aftermarket velocity stacks, there is a little spring clip that holds the auxiliary venturis in. Just pull them harder. If you have aftermarket stacks, the aux. vents may be held in by a set screw on the outside of the carb body.

70F10s are 70 idle jets for the IDAs.

Thu Jul 04, 2013 2:47 pm

All IDA idle jets are F10 XX numbered so they are correct. You can try going to F11 and that might fix your rich smell off idle. This exploded diagram should help you some with your disassembly issues. Id also try a bigger air to help your transition. Sucks but i think you will find that you'll have to run a fat idle to compensate for the hole you have. Do you have the third hole mod? Check #24, there should be either 2 or 3 holes there. 155s are pretty standard baseline for 37 vents. However, if your pulling fine post 3k with no hiccups or studders, you'll need to find a nice strip, then open her up in the upper circuit, and immediately shut the car off to pull over. Then you'll need to read plugs. Remember to bring tools! Jetting takes time. Im up to a 185 main now on mine and i need to take a test run. You cant believe how much a higher duration cam puts demands on your fuel supply and fuel air charge requirements to the engine. Good luck!

http://www.webercarbsdirect.com/v/vspfiles/weber_carburetor_schematics/48IDA.pdf

Thu Jul 04, 2013 7:10 pm

What helped my 2500-3000 stumble was switching out my idle air holders from 120s to 100s. Alfa1750 sells them on eBay.

Wed Jul 17, 2013 3:20 pm

Hey guys, update...

I rec'd the 37mm vents (thanks Lee!) and installed them.

Made a small difference on the low end but I still have the stumble (for lack of a better term) down a bit lower now around 2K to 2.5K rpms. Idle jets are still @ 55. If I am just crusing down the road then lay into the throttle, it will start to accellerate then it will cough and spit until about 2500 rpm then it smooths out. Too much fuel? Not enough fuel? Fuel pressure is set at 3psi.

Thinking about adding a bit more timing and see what that does. Any thoughts?

Someone above asked if I have the 3rd progression hole in my carb, and yes I do.

Over 3K rpms pulls really nice and steady.

Any other advice?
Thx
Tom

Wed Jul 17, 2013 3:24 pm

it's never ever going to work properly until you ditch the F7s for some F11s. The mains will come in sooner with F11s.

Wed Jul 17, 2013 3:41 pm

Will do John. I'll see if I can scare up a set of 4 somewhere around here...