48IDA drawing differently

jonalley · #1 (**permalink**) 01-20-2014, 12:46 PM

48idas with all internal parts matching. Floats set and trying to balance the carbs. Linkage disconnected and banks are running on a rear idle screw on each side. O.K. I need some ideas about why two of my carbs are drawing at a different rate front to back- i.e. the front most stack is drawing ,say, 3 on my gauge while the second stack(on the same carb) is drawing 5. I've run through this several times blipping the throttles after each reading.Shouldn't both stack be near or exactly match each other? Is the main shaft bent? So the question is what's causing this and how can I fix it? BTW I'm in the early stages of this and I've never really gotten this set up to run right. I'm about to try swapping carb positions but thought I'd ask for some ideas first.Regards, Jon

1985 CCX · #2 (**permalink**) 01-20-2014, 12:50 PM

Sounds like a bent shaft if all the associated parts have been matched...

saltshaker · #3 (**permalink**) 01-20-2014, 03:07 PM

Are you reading the 4 center barrels? That you should be able to even out.
The four outer barrels are usually off a little.
Second question, are your slave carbs adjusted to open with the main? That can throw the slave carbs off on each side.
Who's linkage are you using?
Jon

jonalley · #4 (**permalink**) 01-20-2014, 03:36 PM

Saltshaker
I can get the front and back-i.e center stacks to read fairly close. But when I test the other stack on the front carb the number is grossly different 5.5 to 2 or 3. That just doesn't sound right.
Linkage- my build BUT I have it disconnected and am running the motor off the screw throttle stops on the end of the back two carbs. Linkage shouldn't be an issue at this point as far as I can determine.Am I wrong in that? That's what is so confusing. Jon

cab · #5 (**permalink**) 01-20-2014, 03:48 PM

with my center shaft connected and my pass. Side idle backed out and driver side idle screw turned in to keep rpm up a little say 1000 to 1500 I have my air screws 1turn out ,I know every one will tell you 3/4 but this works for me.I start in the drivers side and turn the screw between the back and front carb, the numbers should start to come together it my take a few turns on the 2center carbs then I do the other side ,now you might have different no" side to side at this point, but then you can pull them together with the center shaft ,idle will change once the no# are the same set idle. Turn off and tighten center shaft.mine as done the same as yours ...

jhirasak · #6 (**permalink**) 01-20-2014, 05:40 PM

jonalley,

I had a similar situation when I first installed a brand new set of 48 IDAs on my car. With the linkages disconnected, and the idle stop screw not touching, a similar range of air flow varience was noted between the two throats of a single carb and from carb to carb. Finally, I took all of the carbs off the car and made a temporary flow bench for a single carb using some PVC tubing to make a single carb mounting plate and a shop vac. This was to confirm that the flow variation was due to the carb instead of engine problems (leaky valves, worn rings, manifold leaks, etc). Using the shop vac test fixture, I equalized the air flow on all four carbs by drilling small holes in the butterflys on the opposite side from the transition holes so all of the carbs would read nearly the same air flow value with the butterflys completely shut. Upon later query to the Weber tuning forum, it was suggested that I should have slightly twisted the shaft to equalize the airflow on a single carb so both throats would flow the same value. In hindsight, I think that this would have been preferable to drilling the butterflys.

I would recommend that you take all of the Webers off the car and build you a test fixture so you can initialize the air flow on a single carb by using the method of slightly twisting the throttle shaft. After you have done that, then you can remount the Webers on your car and equalize the idle air flow by using the idle stop screw and the balance screw on the linkage between a pair of carbs on a single bank. Then, you can repeat the process on the opposite bank. This should bring you close to having the same air flow on all throats at idle.

Let me know if this helps your situation.

John

jonalley · #7 (**permalink**) 01-20-2014, 07:15 PM

I communicated with Jim Ingelese and think I will send the carbs off to him. I probably should have gotten them from him in the beginning. The "cheap Jon" route is biting me in the rear at this point. Jon

saltshaker · #8 (**permalink**) 01-20-2014, 07:38 PM

Good move,
Jon

zrayr · #9 (**permalink**) 01-21-2014, 06:51 AM

Quote:

Originally Posted by jhirasak

jonalley,

"........... Upon later query to the Weber tuning forum, it was suggested that I should have slightly twisted the shaft to equalize the airflow on a single carb so both throats would flow the same value. In hindsight, I think that this would have been preferable to drilling the butterflys.

I would recommend that you take all of the Webers off the car and build you a test fixture........"

A slight twisting of the throttle shaft is the preferred method of fixing your issue. This problem is not uncommon and will reoccur . When it does, the drilled holes in the butterfly are going to have to be re-filled and start over. Not a good thing.

I don't see the need to remove the carbs and build a test fixture. Just lightly bend the shafts and be done with it.

Z

Gaz64 · #10 (**permalink**) 01-23-2014, 01:36 AM

Most of the problem with IDAs, IDFs, DCOEs etc is from tightening the throttle shaft levers. All long shafts will twist. Once the levers are fitted, tightened and the lock tabs knocked over, the shafts should be twisted back and check that both butterflies are seating in the bores with the speed screw backed out. The butterflies could also need some shifting around if necessary.