I am starting to play around with tuning and plan to start getting some small increments in jetting. Just to see if I can improve drivability for now. I have twin 44s

Simple question for now: I need 4 turns of idle mixture to now stumble off the line. Should I consider jetting richer?

thanks

Idle speed screws or mixture screws? There's no "idle mixture screws".s

If it's the mixture screws, 4 turns would indicate you need to go up a size.

Thanks, yep idle mixture screws.

2 full turns on the idle mixture screws sounds like more than I recall using when I had Webers on a modified 2.0L years ago, but I'm not sure that's necessarily an issue. Is the engine running rough? Or is there another issue you're trying to solve?

Scott

I am initially working for initial throttle tip in power delivery and performance. Runs well with 4 complete turns but not very linear.

The root of my question is based on a concern I have seeing the flow diagrams> it seems that excessive idle mixture would be a less consistent charge to changing idle jets.

You should post the entirety of your setup. Engine size? cam? exhaust? Jet sizes? Venturi size?

Also what linkage are you using? If it's the cross-bar hex linkage toss that immediately before you waste a bunch of time chasing you tail.

4 sounds like a lot just to get it stumbling off the line, the default starting point recommended by weber is 1.5 turns I think. Like McMark said though, there's a ton of other variables that would be good to know.

Thanks for all your input. Sorry, got delayed by another issue.

Getting back to the set up. This is the hard part. I bought the car with no build documentation out of Billings Montana. Sounds like it has a cam in it and is not a stock 1.7l but besides that I have very basic info.

a 1.7l block / car
b race header w/ supertrap muffler although I do try the megaphone once and a while.
c MSD 6AL with MSD coil
d twin Weber Carbs seems relatively mild

Carb Spec
44mm
Main Jet F-11 / 175 / 135
Venturi 36-4.5
Pump Jet 50EW
Idle Jet 50

Issue I am working on:
a. only drivability issue is backfiring off throttle.
b. looking for improved performance
c. preparing for a track day in Sept.

I know its limited info, appreciate any guidance.

36 venturis in 44s for a 1.7 liter is HUGE. I'd hate to say start over with a track day so close, but if you're looking for better drivability, that large of a venturi is really hampering the intake "signal" running through the carb. For reference, I have a 1.8 and run 40 IDFs with 28 venturis. I would also imagine that with those venturis, you're not getting enough of an effect on the idle circuit to pull the fuel through them, which is why you need to open them so much. But that's just a SWAG (scientific wild ass guess).

For a track day, you're going to be full throttle most of the time anyway, and at high rpms, so you might want to stick with what you've got till you have more time to play with it.

I think george nailed it. The venturies are VERY large for your displacement. Invest in a Weber manual from CBperformance if you don't have one.

I think I know something here...lets see if I can help.
My car has had a drivability issue around 2500 to 3500rpm. It sucks for around town driving, but on the track it doesn't matter at all to me because I try to keep it in the power the whole time. If its drivable and the A/F ratio is good at WOT then run it.
Now, I also agree that those chokes are to big. I had 32 on a 2.0 liter and the car behaved nicely. I hate panicking before a track day. I just did it and I missed my ignition problem. (a story for another day) But if you have the time, i'd choke those carbs down.
On a side note, my idle jets were not playing nice so i removed them a found what I would describe as varnish in all four. One was really bad. I bought a hobby drill set and lightly polished them up. All better now. (this is using only 98 octane fuel too, but the car sits a lot)
Not sure I helped.
Have fun at the track! Its where 914s belong.

I should have been clearer that engine block is a 1.7l block. I am wondering if the engine was bored out to 1.8 or more. No evidence at this point. This is why tuning from a book or reference at this point is hard.

I appreciate all the input and I think I understand you setup input. To be clear, drivability is not a real problem because it is mostly a track car. Just the off throttle back firing.

Should mention that car pull smoothly from 3k plus.

You're going to have a really hard time without knowing something about the engine size. If it were me, I would shoot for running a little small and then working up from there. Or to be fully honest, I would pull the engine and inspect a few things to find out what's actually in there.

This can be a tricky feeling. It makes you think you're close to the correct setup and you only need a little tweaking. I'm sorry to say, you're not close. If you ignore the repeated advice we're giving here because the changes we're suggesting seem drastic, then you're dragging this process out or you'll never actually get it right.

First, I'll agree with the other posters, you're carb setup is WAY too big. You don't need 44mm carbs until you reach or pass 2.3liters. You can choke them down with smaller venturi, but if it really is a 1.7/1.8/1911 the 44mm carbs will always be too big to tune well. If you really want to get it set up right, sell the 44 carbs and get some 40s.

Carbs need airflow speed to work correctly (except for 0% throttle idling). If you have a small engine it naturally only sucks in a small amount of air for each cycle. The smaller the intake piping (and I mean the carb throat size, in your case) the faster the air moves. Too small and it can restrict wide open throttle where you need as much air as possible. But as you go larger you reduce the air speed across the board for all throttle positions. If you go too large, you reduce the air speed so much that the carb stops working, especially at the lowest air speeds (just off idle and low RPM acceleration). There isn't enough air flow to create the vacuum that literally sucks fuel out of the float bowl.

It's also worth touching on that you can't create horsepower from carburetors. At all. None. Zero. Never. What you can do with carbs is get the correct fuel mixture and air flow characteristics for your particular engine setup. And only at the correct air fuel mixture (and the correct ignition timing) will you optimize the horsepower capability of your engine. It's not about throwing MORE MORE MORE, it's about getting air-fuel-ratios and ignition timing correct to make peak HP at all RPM.

This is also why you need to pay attention to your carb linkage and carb synchronization. If all four throttle butterflies aren't opening equally, the air flow and mixture will be different on all four throats. Think of it like this-- only the highest flowing cylinder(s) do the work of accelerating the car. If all four cylinders are equal, they all work together optimally. If two cylinders are opening more (which is almost always the case with cross/hex bar linkages) then you've got yourself a nice 2-cylinder engine with an extra 2 cylinders to make extra heat and use extra fuel unnecessarily. Okay, one final example/analogy-- If you were picking up a heavy couch to move it, would you want four guys of equal strength? Or two strong guys and two children? In the latter case, who's doing all the work? The two strong guys.

Spend time getting your carbs right, and you'll be amazed at how smooth it can run. Try and ask for a quick fix and you're missing out, even if you solve the problem you're focusing on. Invest the time and money to do it right.

Boom. Mic drop. Mark knows his stuff. It sounds like over simplification, but its actually basic physics. For now, ride it out before you screw with them and make your track day a headache. Then, move to 40s and work your way up the jet sizes. I highly doubt it's a 2.4 L motor, probably a warmed up stock unit.

Look I bought the car with this set up and I am trying to learn my way thru it. It runs well and pulls reasonably hard a 2500-6800rpm. After fixing the fuel tank, fuel supply, rebuilding(gaskets) the carbs, fixing the carb weeping, syncing carbs, clutch cables, accelerator cables, installing race seats and belts, I am finally on to engine tuning. I really appreciate the time taken and technical input from all. I am taking it all in.

However the tone I do not appreciate. Thanks for making this a beat on. Quote " If you ignore the repeated advice we're giving here because the changes we're suggesting seem drastic, then you're dragging this process out or you'll never actually get it right." If it continues I'm out.

If this is going to be a technical forum then lets talk data. Weber flow data is limited. What I've found seems wrong for 40mm and 44mm. Calc show about 180-216 cfm: assuming 1800@6800rpm. Does anyone have reliable flow data for Weber IDF's 40 or 44's?

First principles and feedback answered my concern early on. See if i am about right? To overcome my oversized carburators, my overly rich idle adjust may be working on throttle up. However, since there isn't enough air velocity in the carb, it then runs lean once backing off throttle(especially at throttle idle-position) and gives me roll backfire. upsizing the idle jet will not impact backfiring.

BTW, If I were doing it right, I would completely dis –assemble, inspect and build the engine back up. I would compare it to the 5 last race engines I’ve built. I would have build plan to improve on the weaknesses of the past 5 engines. After that I would have my race shop flow bench the carbs, intake, heads and exhaust. Then I would spend a few weeks on the chassis dyno at my race shop with continuous emissions sniffing. None of which I have or have the budget for. Street tuning sucks.

But I'm in the market for a used wideband o2 sensor....

I didn't intent for you to feel beat on. I'm sorry you felt that way. I was just trying to help.

LOL, I just realized your only 1h away from me.

If you want to come meet me in person and see that I'm not a 'bully', swing on by anytime. I can show you around the shop and the projects I'm working on and we can talk carbs. I'll even buy you lunch.

Or if you can drive the car here I'll put my O2 meter on it and I can help you tune sometime. Or come grab the misc jets and venturis I have if you want to play with sizes.

Thank you for your note.

Again, I really appreciate the guidance and offer.

Forget that 6.8k rpm number for your calculations. It doesn't really apply in the world of type 4 engines. If you do just 1 thing at this point, down size the venturis. Esp with an engine of unknown displacement and cam. You're not going to spend mutch time at max rpm. Short bursts. Sorry if you took offense at the replies. BUT, you seem unwilling to consider the overwhelming concencus about Venturi size.

The redline on my 4 cyl car is 5.6K. Can you really wind a T4 motor to 6.8K...more than once??

I find thats the best way to remove that pesky alternator belt.

Quit brow-beating the new guys, Mark




Innovate LM-2s aren't that expensive if you shop around ... http://www.ebay.com/itm/Innovate-LM-2-Digi...8oT&vxp=mtr

I understand the consensus ( for overall sizing is 40mm max). In order to do the road tuning thing, I will need an O2 sensor even if I had 40mm. Budget and time limited for the time being.

As for the technical feedback, Bernouli's equation defining air flow although a first principles estimation, is not invalidated because we are talking about a VW engine. I am only interested in tuning my car for track use.

Yes, 6800 rpm is per tach(i know this is not entirely accurate). This is just a sizing calc point. And yes I threw off the alternator belt that was on the car when I bought it. Not having that problem since I put a new one on.

eye candy.....

Keep the 44s. Don't expect good performance below 4500 rpm. Anything you do to make the engine work better down low will only hurt the top end.
I run 40s because of SCCA rules but I use 36mm venturis and made a lot of mods to the carbs to improve flow at WOT.

Depending on your specific engine build you may even want to increase the venturis to 38mm. I've used 37.5mm vents in my 40 IDFs but that's nearly a straight tube in a 40mm carb body.

I was going to help as I have A LOT of experience (not NEARLY as much as McMark) in this department since I have a 2.0L race car with dual webers and ~30 years experience with carbed 914s

But based on that comment I would not want to hurt your overly sensitive disposition.
Figure it out yourself.