Weber Main Jetting - swapped 1.15 to 1.3 and back

[mikelars]

Ok, so my Weber’s were working fine but had 1.15 main jets and 28 venturis for a 1904 with stock DP heads. I was told it may be too lean and I should try 1.3 mains with the 28 vents, so I did.

I swapped the 4 main jets, but am not 100% sure I put the same jet tubes back in the same position. I then drove it with the 1.3s for 2 days. It felt fine, even perhaps a but livelier up the rpms, but then it started popping on decel at 3000-2500 rpm.

I figured maybe the 1.3s were too big, and swapped back in the 1.15s. Again not sure I kept the positions the same on the jets. But it continued popping with 1.15s.

So I thought goddamit if it wasn’t broke why did I “fix it”. But then I synched up the Weber’s and recalibrated the idle jet screws, and the popping went away.

So my question is, since I didn’t recalibrate anything after installing the 1.3s, maybe the Weber’s were knocked out of balance with the new main jets, and I never gave the 1.3s a fair shake because I never readjusted - figuring it would be fine to just swap in the four main jets without rebalancing…

Carb newbie error? I read that the popping shouldn’t come from overjetting, but rather from running too lean, so it doesn’t match that the larger 1.3 main jets caused popping and backfiring….
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[67rustavenger]

Read through this thread. Even if you don't have a wideband, it will help you understand carb setup and tuning.
https://www.thesamba.com/vw/forum/viewtopic.php?t=293837
The Gold is in the first 5-10 pages.

Always adjust the mixture screws after making jetting, timing, really any adjustments.
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[Clatter]

Beat me to it!

https://www.thesamba.com/vw/forum/viewtopic.php?t=293837&highlight=wideband
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What’s That Noise?!?

[volksworld]

different areas of the country get different fuel blends and different altitudes require different jetting...combos that needed 140 mains at sea level on long island NY ran fine with 115's in central north carolina ...so most carb setups usually are too lean with the weber factory jetting but people that rejet them for you prior to sale usually err on the side of too rich (sluggish) as it will drive better than too lean( hesitation and backfiring on acceleration)...in addition many carb suppliers will re-drill jets rather than spending the extra 25 bucks on new ones...so your "115 "s are actually 130s and you wonder why the car runs leaner when you put in bigger ones...so if you're going to play around with this stuff get a set of jet gauges so you can see where you actually are to start with...the idle jets usually come drilled out bigger than they are marked also...all in the interest of a carb setup that will "run right out of the box" but doing it as cheaply as possible

[Ohio Tom]

Changing main jets should have no affect on lower rpm running.

115 main is almost always too small for 28mm vents.
I like 125's in them usually. 130 should work well and add power up top.

No reason to think they had anything to do with your popping on Decel.

I have found that popping on decel is an idle mixture screw set too fat.

I would go over them one by one, finding the "lean best" setting for each. That should clear up any popping.

[oprn]

Volksworld is correct about not trusting the markings on the jets to be correct. Ohio Tom is right on the money about the mains having nothing to do with your deceleration problem.

Just for comparison my 1911cc engine with 40mm Webers was running at 14.2 AFR on the mains with 31mm vents and 1.27 mains. With 1.32 mains (yes I drilled them) I am now at 13.2 AFR. I would expect smaller vents to take a smaller main but yours do sound too small to me too. In a light Buggy like I am running you might get away with it as it would never be at full song for more than a few seconds but in a Bus? I would be concerned. You need to get a wide band on that engine to see where you are at!
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[modok]

Going from 115 to 130 is too great a jump,
over 25% increase, 122 would have been a more appropriate step up

Main jets size is usually 4.2 times venturi size as a rough rule.