48 IDA Weber tuning

[Mark]

Symptom...runs great 700-1800rpm and 3100 to 7000rpm...1900-3000rpm popping and spitting gas thru carbs...appears its right at idle to main circuit changeover

Motor: 69 Boss 302
Cam: isky 381380 250 @ .050 // LC108

Weber setup:
- Fuel system setup: Carter 4070 fuel pump (no pressure regulator per Carter recommendation)...pressure varies from 3.5 to 6
- Mixture all set at 3/4 turn
- Main: 145
- Emulsion tube: F5
- Air corrector : 195
- Idle: 60
- Idle Holder: 100

Best I can tell the only thing out of whack is the fuel pressure.

Diagnosis would be appreciated...

Thanx
Mark

 

[Jac Mac]

You dont mention what size chokes/venturi's you have fitted...

FWIW here is a setup that worked on a Race Boss 302 from the early 70's, never drove it in 'street mode'

45mm chokes
170 mains
130 air
F7 Emulsion
50 pumps
70 F16 idle
Timing was 19° initial & 42° total......... This would need a complete rethink with pump gas & curent street driving conditions

I think your problem is in the transition area..might need third hole & large richen up. Boss with large ports & small Cu In can be a bit of a pig at those low RPM's. Some work might also be needed in the advance curve/timing etc

 

[Randy V]

Mark -

I would definitely get the fuel pressure under control... 3.5 PSI "steady" is what I understand works the best with IDAs.. That's what mine runs at (Dyno time only so far) and the A/F ratio was very consistent..

Jac has some really good tech here as well...

 

[Mike Christian]

Basic Jetting Theory and Procedure - Aircooled.Net Tech Article

This article might help...covers jetting webers. Sounds like your mains need to be bumped up (if you have already confirmed the rest of the engine is tuned and the fuel pressure is right).

Mike

 

[Frank Catt]

As JacMac said, first stop is to drill the third hole, then go from there. Frank

 

[Andrew Robertson]

Symptom...runs great 700-1800rpm and 3100 to 7000rpm...1900-3000rpm popping and spitting gas thru carbs...appears its right at idle to main circuit changeover Diagnosis would be appreciated..

Obviously nail down any issues with fuel pressure, then double check float levels. A low float level will delay the onset of the main circuit, and a higher float level can bring it in earlier - this makes a really big difference in my experience.

I had an issue like this with a DCOE cross-ram on a SBC - once I got float level right, I found that by going slightly fat on idle jet, and using the richest possible emulsion tube, I cured the problem, but at the cost of a considerable degradation in cruise fuel economy.

Hope this helps.

Cheers, Andrew

 

[Cliff Beer]

Just my personal experience, so take for what it's worth (probably not much):

1. bigger main jet, maybe 155,
2. chokes, shouldn't be too big, 45's are OK, but perhaps a bit smaller for initial dial in, particularly with a smaller displacement SBF.
3. e-tube, I'd try an F6. Then try an F7, but F6 will probably be better.

Take a look at the configuration of progression holes drilled near the butterfly. These can vary across IDAs and will dramatically affect the off-idle progression - look for number and proximity to the closed butterfly position, and also diameter of each. If you have more, and greater diameter, progression holes than typical then you'll likely need less gas flow for a smaller SBF (like you have there).

Yup, get that fuel pressure under control for sure. I aim for 4.0lbs constant for IDAs.

You've checked for consistent volume from/across the accel pumps probably.

A smaller choke will usually mask or moderate lean/rich spots in the rpm spectrum along with low flow spots too in the lower rev ranges due to aggressive cams. I like to start with smaller chokes to generally get things dialed in with jetting and such then try increasingly larger chokes to max flow before losing power due to poor atomization.

Don't get too hung up on it all. Carbs/IDAs are never going to run as smooth as EFI throughout the whole rev range because the transitions from one circuit to another (and the inherent limitations of using a choke to atomize) just don't work as well as high pressure through a controlled spray valve. But you can get it relatively close. All you're trying to do is get the right AFR throughout the rev range as you progress from one circuit/jet/needle to another (I know, not easy). Think of the analogy of dripping gasoline into a wad of steel wool upstream of a butterfly to atomize it, you're just varying (through a range of circuits and rpms) the rate at which you drip/pour the fuel.

Good luck!

 

[(C)arlos]

Hi
What is the diameter of the fuelpipe and what material are you using, steal braided ?
Reason for the questions that normal rubber pipes can vibrate or pulse during certai pressure conditions.
Cheers
Skeleton

 

[Cliff Beer]

Just my personal experience, so take for what it's worth (probably not much):

1. bigger main jet, maybe 155,
2. chokes, shouldn't be too big, 45's are OK, but perhaps a bit smaller for initial dial in, particularly with a smaller displacement SBF.
3. e-tube, I'd try an F6. Then try an F7, but F6 will probably be better.

Take a look at the configuration of progression holes drilled near the butterfly. These can vary across IDAs and will dramatically affect the off-idle progression - look for number and proximity to the closed butterfly position, and also diameter of each. If you have more, and greater diameter, progression holes than typical then you'll likely need less gas flow for a smaller SBF (like you have there).

Yup, get that fuel pressure under control for sure. I aim for 4.0lbs constant for IDAs.

You've checked for consistent volume from/across the accel pumps probably.

A smaller choke will usually mask or moderate lean/rich spots in the rpm spectrum along with low flow spots too in the lower rev ranges due to aggressive cams. I like to start with smaller chokes to generally get things dialed in with jetting and such then try increasingly larger chokes to max flow before losing power due to poor atomization.

Don't get too hung up on it all. Carbs/IDAs are never going to run as smooth as EFI throughout the whole rev range because the transitions from one circuit to another (and the inherent limitations of using a choke to atomize) just don't work as well as high pressure through a controlled spray valve. But you can get it relatively close. All you're trying to do is get the right AFR throughout the rev range as you progress from one circuit/jet/needle to another (I know, not easy). Think of the analogy of dripping gasoline into a wad of steel wool upstream of a butterfly to atomize it, you're just varying (through a range of circuits and rpms) the rate at which you drip/pour the fuel.

Good luck!

Forgot to mention, remember, weber e-tubes aren't progressive in their markings and sizing....they're somewhat random. So you might have to try a few but don't assume that the next one in sequence is just progressively more or less restrictive.

Here's a book that should be mandatory reading before ever touching a weber carb (if you don't mind me saying):

Best $15 bucks you ever spent.

 

[Mark]

Got fuel pressure under control. Steady at 3psi. All mixtures about half a turn to 3/4 of a turn out. Same problems. Popping thur the carbs from 2100-3000. Nothing noticeable thru the exhaust. Sluggish response in that RPM range until above 3000 rpm then great response. Solid perfromance on either side of the 2100-3000 range. Clearly its the the idle-main crossover region.

Went thru all your comments and reviewed my Braden Weber Carb book. Since main circuit seems ok, looks like if need to fatten up the idle and the e-tube.

Thoughts on 70 idles and F8 e-tube (braden book shows thay rich)?

Mark

 

[Jac Mac]

You still have not told us/me what choke size your running????

Your idle size-60- is in the ballpark for 302 cu in, I still feel that your problem is in the transition area, ..but until you give the choke size I cant reccomend a main jet & air corr size, my crystal ball is a bit cloudy today , but I feel your main circuit is well out of whack, prob needs much smaller air corr & larger main.

 

[Jac Mac]

Your choke size is 40mm, therefore your main jet size should be around 160 ( 4 x 40mm).

You mention that your idle is acceptable, so increase the idle speed to the point just below where you can see fuel just start to flow from the secondary booster venturi( ie, increase the RPM to the point where fuel starts to flow then slow it down a bit)...now turn the mixture screw(s) in or out, if engine rpm drops with either change the idle jet settings are OK, If it increases rpm when screwed in its too rich, & too lean if it increases with the mixture screwed out.. in doing this we are checking the operation of the transition section

 

[Mark]

Well, changing mixture didn't do it so here's where I am.

Have two sets (8 carbs) of Webers, one with Boss 302 manifold on Boss 302 motor, and one 427 manifold on 427 side oiler.

Based on Weber choke size chart I found on line for V8, looks like for BHP/choke size: 300/33, 318/34, 338/35, 358/36, 378/37, 398/38, 420/39, 440/40, 464/41, 486/42, 510/43, 534/44, 558/45, 582/46.

The sizes of different Weber jets and chokes I have avaialble are:
choke: 37 and 40
aux venturi: both 45
main: 150 and 145
emulsion: both F5
air corr: 210 and 195
idle: 70 and 60
idle hold: 110 and 100

Right or wrong, the boss 302 setup came with 40/45/145/F5/195/60/100. The 427 setup came with 37/45/150/210/70/110.

The Boss 302 setup is in a running motor. The 427 setup is going on a side oiler being built.

Based on your inputs, the HP choke chart, and the fact neither car will be used for racing, I am thinking (with what I have available now) the Boss 302 setup should be (choke/aux ven/main/emul/air corr/idle/idle hld): 37/45/150/195/70/100. The 427 setup will probably req components I don't have: 44/45/175/225/70/110.

What do you all think? And thanx for the help, especially the entire choke discussion.

Mark

 

[Andrew Robertson]

Hi, is float level right on all carbs? - I has hideous transition issues and it turned out to be almost entirely due to float levels set significantly too low. Cheers, Andrew

 

[Mike Christian]

If the engine runs fine at idle, then it is most likely not the idle circuit. It seems to me that you have not addressed what seems like a main jet that is undersized. Not sure why you think the main circuit is okay - that is where you issue is...you describe that the engine runs fine below 2100 rpm (the idle circuit range) and above 3000 rpm (towards the air correction circuit range). If I were you i would try larger main jets like the other setups have shown - throw your 150s in there and see if it acts better...if it does, then you are on the right track -heading the right direction. You may need to be more rich than that, but you might feel a difference with the 150s.

1900-3000rpm is the main circuit and popping is indication of a lean circuit condition.

Mike

 

[Mark]

SUCCESS!!! Thanx much. Absoluely no hestiation between idle and 6100RPM with:

choke: 37
Aux venturi: 45
Main: 150
Air Corr: 195
Emul: F5
Idle: 70
Idle Hold: 100

At 6100 (should be able to get 7000+ out of Boss 302), noticed it start to stall out. Will probably put the 40 chokes back in with 160 mains and 210 Air Corr.

Happy holidays
Mark

 

[Jack]

40 chokes, the 160 mains and 210 ac sounds about right. Try first with the 195's.