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Dual 40 jetting setup - HPMX's on a 1.8 T4
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MikeyM73
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Joined: December 06, 2016
Posts: 506
Location: Napa Valley, CA
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PostPosted: Wed Dec 01, 2021 4:51 pm    Post subject: Dual 40 jetting setup - HPMX's on a 1.8 T4 Reply with quote

Hi all,

I know this has been covered a billion times and I've done a lot of Samba reading but haven't really found much close to my specific scenario so thought I'd post up to see what you all might think.

So over the past year I've completed the build of a 1.8 T4, specs below in my sig. Dual HPMX 40s that have been running very well and without an LM2 / LM1 I've been slowly dialing them in (or attempting to) going by plug readings and very carefully observing engine behavior during all driving conditions and CHTs at various speeds after different idle, main, air and pump jet combos. Ideally, at some point in the near future, I'll have access to an LM2 (Thanks Clatter) to see for sure but in the mean time, after literally a couple months of tweaking, my current jet combo is 55 idles (going to try 52s), 135 mains, 210 airs, 50 pump jets.

I've had an issue with a small stumble on what I think is the progression from the idle ckt to main as the throttle plates pass the progression holes. At this time, the engine runs very smooth, has no stumble, and seems crisp and not "boggy" if that makes sense. I've tried almost every combo of idle / main / air jet from 52 / 130 /170 air and up but the current combo is the only one that now has no stumble.

I realize that an LM2 is the way to go so I guess my main question is does my current combo seem overly rich / lean from a ballpark point of view? I've read quite a few posts where those experienced with Webers / dual 40s say that the most jet a 28mm venturi would need is a 125 with 200 airs, etc. But I'm worried that if I down-jet or increase the air jet to 215 or more that that would lean out the mains too much and cause CHTs to rise.

As it stands now, CHTs on the highway at 68 mph sustained for about 5 miles level / 72* ambient air temp are 340-350. They were a bit lower with the 130 mains and 180/190 airs but the stumble was there. I was concerned going with 210 airs on 130 mains would be too lean. My distributor is a Pertronix but no vacuum advance due to crappy vacuum signal so it's all mechanical advance - timing is 10* @ idle, 30*@ full advance and my advance starts right around 1500 RPM.

I realize this is all subjective until I get it on the air/fuel monitor so I really appreciate any input.

Thanks,
Mikey
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'73 Pop-top Westy, found sitting in a field for 10+ years, cleaned up, rebuilt furniture, reloved. Original 1.7 block/fully polished crank, 93mm 1.8L balanced AA pistons & cylinders, new 1.8L balanced rods, HAM 42/36 heads, Scat C25 cam & lubalobe lifters, Dual HPMX40s, R2C filters w/ Outerwears pre-filters, functioning thermostat & flaps, Pertronix Flamethrower III, 4-1 exhaust w/ Cherry Bomb 2" turbo muffler & OEM heater boxes, averages about 19-22ish on the highway.
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Lingwendil
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Joined: February 25, 2009
Posts: 3987
Location: Antioch, California, a block from the hood
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PostPosted: Thu Dec 02, 2021 12:30 pm    Post subject: Re: Dual 40 jetting setup - HPMX's on a 1.8 T4 Reply with quote

What size intake valves? Stock 1.8 ran 41 intakes, 34 exhaust if I'm not mistaken.

What size vents? 28mm?those mains sound huge unless you're running big vents,like 32mm or something. One would expect something like 115-125 or so with 28-30mm vents.

Falling flat on it's face after progression is usually from too fat of a main jet.
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MikeyM73
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Joined: December 06, 2016
Posts: 506
Location: Napa Valley, CA
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PostPosted: Thu Dec 02, 2021 3:02 pm    Post subject: Re: Dual 40 jetting setup - HPMX's on a 1.8 T4 Reply with quote

Thanks. 42mm intake, 36mm exhaus, 1.5" tubular 4-1 header, 28mm vents.

The smallest jet I have is 130, at the moment, so maybe I could try that with a smaller air jet? Currently running 210s. It seems to be pretty smooth during transition if I push the throttle normally and the power is good up top but if I push it fast, it seems to have a small flat spot right after that.

If I need to down jet, that's fine, I just don't want to run it lean but I'll have to check plugs for that until I can get a hold of a meter.

Thanks,
Mikey.
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'73 Pop-top Westy, found sitting in a field for 10+ years, cleaned up, rebuilt furniture, reloved. Original 1.7 block/fully polished crank, 93mm 1.8L balanced AA pistons & cylinders, new 1.8L balanced rods, HAM 42/36 heads, Scat C25 cam & lubalobe lifters, Dual HPMX40s, R2C filters w/ Outerwears pre-filters, functioning thermostat & flaps, Pertronix Flamethrower III, 4-1 exhaust w/ Cherry Bomb 2" turbo muffler & OEM heater boxes, averages about 19-22ish on the highway.
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