Jetting recommendations for Weber 40 IDF's

seahag · Post by **seahag** » Wed Mar 29, 2006 4:12 pm

He also does a great job on cut to length HD aluminum pushrods. Quality work,timely delivery.

Gabe · Post by **Gabe** » Sun Apr 16, 2006 7:24 pm

I installed the carbs this weekend:

The Good: Very, very nice increase in acceleration/hp. I never knew a stock 1600dp could produce so much power! In addition, I averaged over 30mpg on a long trip back from my parents' home. I thought I would lose fuel economy, but it seems this setup is very efficient. It took me a little while to dial in and balance the carbs, but once I got the hang of it, it went smoothly. The car now starts up right away, and comes to a smooth idle in less than half a minute. No more carb icing from poor manifold heat either (my main reason for going to a dual setup, besides the power increase).

The Bad: The nuts towards the front of the car that secure the manifolds to the heads are a bitch to tighten. I'm not looking forward to changing spark plugs; I really should have put in a new set when I removed the old intake manifold. I also had to cut down the air filters about 3/4" so that my hood springs wouldn't hit the air cleaners.

I'm having a problem getting used to small changes in throttle. It seems like when I give it a little throttle, the engine accelerates smoothly to about 2k-3k rpm, and then wants to shoot off like a "bat out of hell" with just a little more throttle. I think this is because of the jets, but I'm not sure. Idles are 50, Mains are 130. CB never sent me the 115's they said they would, so I ended up ordering a set (from another vendor) which should arrive soon. Hopefully these will allow a smoother throttle progression.

I'm also getting an occasional backfire when decelerating. I made sure my exhuast connections were tight in case of an exhaust leak, but I still get the occasional backfire (only on deceleration; idle and acceleration are great). May be too lean - I may try moving up to a 52 or 55 idle jet to get a more rich mixture if the 115's don't have an effect on the backfire.

Other than that, I am very pleased overall!

500LbGorilla · Post by **500LbGorilla** » Sun Apr 16, 2006 7:58 pm

Yeah, rich jets will cause it to be a little more "responsive" at early throttle, in other words, a little jumpy. Lean it out to the correct jets and it will smooth out and make good power.

Bruce2 · Post by **Bruce2** » Sun Apr 16, 2006 8:11 pm

Gabe wrote: I'm also getting an occasional backfire when decelerating. I made sure my exhuast connections were tight in case of an exhaust leak, but I still get the occasional backfire (only on deceleration; idle and acceleration are great). May be too lean - I may try moving up to a 52 or 55 idle jet to get a more rich mixture if the 115's don't have an effect on the backfire.

Do not mess with the idle jet. You've already proven the idle jet is right whey you said "idle and acceleration are great"

The backfiring in the exhaust are from 2 sources. You have an exhaust leak. Air is getting into the exhaust. Second, your mixture at higher rpms is too rich, feeding the extra unburned fuel into the exhaust. Fuel plus air makes for a combustible mixture = backfiring. Put the 115 mains in it and your backfiring will reduce.

500LbGorilla · Post by **500LbGorilla** » Sun Apr 16, 2006 8:34 pm

Depends. If decel is while no throttle, mixture screws will prevail. That doesn't mean idle jets are wrong, just mixture screws might benefit from a little more leanness. It's a toss up lots of the time.. you can get it so it idles smooth in 30 seconds, or runs an agreeable AFR at decel and hot idle. Big exhausts make it worse too, but exhaust leaks are very common on these engines, especially if you don't have flanged joints everwhere. Slip joints don't help either.

Gabe · Post by **Gabe** » Mon Apr 17, 2006 6:56 am

Thanks for the comments guys.

The backfire during deceleration is with no throttle. I usually only notice it if I rev up a littler higher than normal in 2nd gear when transitioning into 3rd. If I keep a little throttle during the transition, no backfire. If I let off the throttle completely during the gear change, then I get a small backfire as the engine decelerates. Nothing too big to worry about, and I'll probably adjust the mixture screws a little to see if I can tweak it. Muffler is not flanged, and has the stock metal brush ring/clamp setup where it attaches to the heat exchangers.

I'm going to wait on making any adjustments though until I install the 115's. Other than the occasional backfire and "jumpy" throttle with the 130's, I'm very happy. I'm so pleased that my gas mileage didn't suffer, especially with gas prices shooting up again. With my 34pict3/DVDA setup I could get almost 30mpg on the highway. Last night, with the Webers and my DVDA timed as an SVDA with the retard unplugged I swear I got at least 32mpg, maybe more, on a 90 mile trip (nearly all highway). Car ran very cool, and had no problems passing traffic. I guess you CAN have your cake and eat it too!

500LbGorilla · Post by **500LbGorilla** » Mon Apr 17, 2006 8:43 am

Oh, you don't mean deceleration then.. I think of deceleration as being the situation where you are slowing down, letting the gears do the work to slow you down.. What you're getting may very well be related to main jets like Bruce said. Check for exhaust leaks too.

Gabe · Post by **Gabe** » Wed Apr 19, 2006 4:45 pm

The 115's came in the mail today, so I installed them and took the car for a quick spin. Happy to note that the backfiring is gone and the throttle progression is much, much smoother. Except . . .

Now I have a flat spot on acceleration, right around 2500 rpm. I'm going to rebalance the carbs this weekend and tweak everything some more and see if I can solve this new issue. Always something . . .

500LbGorilla · Post by **500LbGorilla** » Wed Apr 19, 2006 5:51 pm

Good deal Gabe.

What's your current float level, and what timing do you have your distributor set for? 10-11mm on the floats is good, and timing with SVDA should be in the 28-30 degree ballpark with the hose disconnected.

Also you need to T the line from both vacuum ports (one on each carb at #1 and #4 cylinders, I believe) to get enough signal.

Go with that and see what's up. If not quite right, pull the advance up a couple degrees at a time until you have ping and then back it off 2 or 3 degrees. Play with that and drive it a little and see what you get.

Post by **Marc** » Wed Apr 19, 2006 6:08 pm

Do you know anyone with a set of number drills? Or you could take the jets to a hardware store and check them on some, should be close enough to tell a 115 from a 130.
A #55 is 132
#56 is 118
#57 is 109

Gabe · Post by **Gabe** » Wed Apr 19, 2006 6:13 pm

I set the floats myself at 11 mm before installing the carbs. Timing is set to ~8 degrees BTDC (no retard line), with vacuum line disconnected. I set it last week, while I still had the 34pict3 on, and haven't touched it since then - running cool and strong. Vacuum ports are "T'd" to the distributor.

Strange, because I didn't notice any flat spot with the 130's two days ago. I simply swapped the 130's for the 115's; didn't make any other adjustments. I've read that clogged idle jets can cause a flat spot, so I'll pull those and blow them out to make sure. Hopefully this weekend I'll get it set right.

500LbGorilla · Post by **500LbGorilla** » Wed Apr 19, 2006 6:38 pm

The timing at idle is no indication of what you're seeing at the flat spot. Set the timing for MAX advance. I use between 28-32 full mechanical advance to start, on an SVDA.

It's not strange, your carbs were pig rich with the 130s, and couldn't possibly develop a flat spot.

Gabe · Post by **Gabe** » Wed Apr 19, 2006 8:30 pm

Gotcha. I might play with it tomorrow after work since it seems like it's going to rain this weekend here. Thanks again for the input.

Gabe · Post by **Gabe** » Thu Apr 20, 2006 4:22 pm

I set the timing today to ~32 degrees max @ 3k RPM, vacuum line off. It definitely helped out with the flat spot, but it is still noticable. Rather than a dead stop in acceleration, there's a slight hesitation. A lot more drivable, but I'd like to get rid of the flat spot all together. Getting better each day though . . .

I tried advancing the idle a little more, but it didn't seem to do much for the flat spot so I put it back to ~32 Max. My idle now falls ~12 degrees at 900 RPM. Will this be ok, or will it run hot? (this is a DVDA running as a SVDA without the retard hooked up).

I played with the accelerator pumps briefly, but didn't notice an affect on the flat spot. The flat spot seems to me to be right around the transition from idle jets to main jets. I wonder if going to a slightler richer idle jet (52) would solve the problem?

500LbGorilla · Post by **500LbGorilla** » Thu Apr 20, 2006 4:53 pm

Might. If you have some 55s try throwing those on first and see what you get. How's the top end? Powerful or weak? Smaller air jets might change it too, if your top end is weak.

Basically it's a matter of when the main jets are coming into play versus when the idle jets are going out of the picture. Both taper in their function and if the overlap is not enough, it will lean. Upping floats, going to a larger main, a smaller air are all options depending on what other parts of the RPM range are doing.

This is where an AIR FUEL meter does wonders. They're expensive though, so seat of the pants is what most people end up using.

I might have a set of 120s I can mail you to try, and if they work it out, you can buy some. Let me know if you want to try them. I'll see if I have some. I do wan them back.