Weber 44 IDF's - rough idea on Idle and Main Jets

NickD

Supporter
Hi,

Looking for some advice. I have just changed over from my Holley 600cfm to 4 x 44 IDF's. The Jetting on the carbs I believe was initially set for a VW. (seems these carbs are a popular upgrade on VW Beetles etc). In running a 300hp Ford windsor 302. Any suggestions on a jetting combination that would be a good start. I believe I have 50 idle jets and 135 main jets.

I'm getting a little exhaust backfire as she warms up and then some when I'm a little off the throttle. No exhaust backfires when under full acceleration.

No exhaust or vacuum leaks that I can see and the carbs all seems balanced with an air flow meter on tick over.

Think I may be under fueling with my current jets and not getting a good combustion burn leaving un-burnt fuel ?

I'm also running with no air filters with the velocity stacks fully open so no air restriction at all.

Thanks.


Nick.
 

Mike Pass

Supporter
A bit of info. You can look to see what emulsion tubes, main jets and air correctors you have by unscrewing one of the jet assemblies. I have attached an exploded diagram, a pic of the jet assembly and some settings that I have seen for comparison. I assume your float levels have been set and your fuel pressure is around 2.5 - 3 psi. As heads, cams etc vary a lot the only way to get it spot on is a rolling road session.
Cheers
Mike
 

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Just my 50 cnts..
Don't fool around!
Get yourself an AFR gauge kit (Air Fuel ratio gauge kit like Innovate MTX or similar) and monitor what's happening and change jets according to needs..
Or spent some bucks and go to the rolling road & get is all sorted to your engine.

No engine is the same. Just the tiniest mistake in jet size can burn holes in a piston..
 
Just my 50 cnts..
Don't fool around!
Get yourself an AFR gauge kit (Air Fuel ratio gauge kit like Innovate MTX or similar) and monitor what's happening and change jets according to needs..
Or spent some bucks and go to the rolling road & get is all sorted to your engine.

No engine is the same. Just the tiniest mistake in jet size can burn holes in a piston..
JP is right , use AFR gauge to clean out and rolling road to fine tune.... the only way to do it right
just one observation.....inflation & high prices are now also on this forum..... before we said : just my 2 cents..... 2400% increase... wow...
 

NickD

Supporter
Quick update. I've just stripped out the mains to check the parts were as I expected and fully stripped. Low and behold, two out of the 8 135 main jets had what appears to be manufacturing brass still inside, not fully blocked but certainly screwing the jet up . I've attached pics but even if it doesn't cure, it certainly wasn't helping.
 

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Mike Pass

Supporter
Sometimes the holes in the jets are not correct. This is common on old carbs where someone in the past has drilled them out rather than buying new ones. However you can buy sets of lots of tiny drills from Ebay ( very cheap) and use these as "feeler gauges" or for cleaning up roughly drilled holes.
I have cleaned up and checked a few sets of Webers and some of them are pretty rough. Check everything.
Cheers
Mike
 

NickD

Supporter
Sometimes the holes in the jets are not correct. This is common on old carbs where someone in the past has drilled them out rather than buying new ones. However you can buy sets of lots of tiny drills from Ebay ( very cheap) and use these as "feeler gauges" or for cleaning up roughly drilled holes.
I have cleaned up and checked a few sets of Webers and some of them are pretty rough. Check everything.
Cheers
Mike
Thanks Mike.
These were actually new carbs but I guess it is what it is.

Nick.
 

Mike Pass

Supporter
I have done a new set of Spanish 44IDFs recently and they were pretty rough in places. Some of the castings inside needed a fair bit of work. Ordered some new ones but they were just as bad. Also the float levels on some needed setting which is much easier on IDFs than IDAs. Just take the top off and set with I think a 10mm drill bit.
The sets of micro drills
eg. https://www.gt40s.com/threads/our-plus-4-stretch-model-is-ready.55940/#post-583483
The air fuel meters work well. I have used the AEM one (which has digital and analogue displays but there are several available. You need to weld a bung in the exhaust for the sender and run two wires up the the gauge mounted under the dash and feed ther gauge with pos and neg wires. Very interesting to watch the gauge as you drive along.
Cheers
Mike
 
The first step would be to ensure you have four matching 44 IDF's.... This doesn't mean you have "four 44 IDF's"....this means you have four "matching 44 IDF's."

To be specific, IDF have a wide range of progression holes drilled in the throat in the area of the butterfly, particularly near the closed or slight part throttle position. The arrangement and size of these progression holes dramatically affects the AFR in the transition from idle to WOT. So, ensure all four of your IDFs have the same size and arrangement of progression holes.

There are other, more subtle differences, between different series of 44 IDF's. So, again, make sure you have matching 44 IDF. In other words, a 44 IDF 50/51 is not the same as a 44 IDF 32 (different progression holes) and different standard jetting.

A common 44 IDF is the 50/51, often used on bored out air cooled VW. The standard jetting (straight from Weber) for the 50/51 is:

Main venturi: 36
Aux venturi: 4.5
Main jet: 135
Em tube: F11
Air corr jet: 175
Idle jet: 50
Pump jet: 50
Float valve: 1.75

Secondly, I highly recommend buying and reading Pat Braden's Weber Carburetors book. It contains all the specifications and settings....everything you need to know to understand, fix, tune and run Weber carbs.

Good luck!
 
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