I am going to be ordering a triple weber setup for my L28. 

First, my engine basic stats:

L28 F54/Flattops, .040 bore,  E31 Head which is ported/etc, 500/300 cam, 6-1 header, pertronix igntition, etc.  My whole engine install thread here:

 

I've been running the round top SU's with SM needles.  They go lean up top despite the needles and repeatedly checking the floats/fuel delivery/etc. 

Looking to go triples. The good news is my engine was built as a backup for a race car and it came in the crate with jet sizing written on the valve cover. So I have a good starting point. Just hoping you guys can help fill in the blanks. I am learning here. 

image2 (1) by blodi, on Flickr

So what else is missing from those settings?  Based on those and what I've learned in some helpful searching it seems I need to go for the 45's and use a 38mm choke? 

Do the rest of those settings make sense? 

Thanks!

 

It is not that bad.  There are a few things you can try:

 

Overall you are in the ball park and it should be nice to drive. There are just two items to consider for tweaking further:

1. The low end richness is either the E-Tube being richer in the low end or the acceleration jet kicking in too much.   If you punched the pedal then maybe we are seeing the effect of the squirt of too much liquid fuel.

2. The tapering from middle to the end is minor. You go from 12.7 to 11.7.  It could be due to the air tube being a tick too small or the E-Tube again.

 

Here are a few first steps to try:

1. Repeat the test a few times to make sure the shape we see above is consistent and real.  Testing has to be repeatable.

2. To address the start of the main richness, first  repeat the test but try not to stomp and see what happens.

3. If there is a difference from "squeezing" the accelerator rather than stomping, then remove the accelerator jets and do a few runs to see how the mains engage without the acceleration jets. This will give us their leanest limit.

Report back to us with the data.

 

Here are some other tuning items but please hold off for now as changing too many parameters will cloudy the waters:

1. If you look at the shape of the mains during the pull,  there is a peak in the middle of the pull.  You can flatten it by smashing down the middle or bringing up the ends.  The only way to attack the middle would be with E-tubes so it is too expensive and too exploratory.

2. The high end looks simple to play with.  A larger air corrector will lean out more as the RPM's increase.  This will  flatten things  nicely.

3. The whole shape will rise if you reduce the main fuel jet.

4. You may be able to lean the low end by using smaller acceleration jets.

 

 

Awesome feedback as usual!  Thanks.  Yes, I did stomp on it at the start of the run. It did stumble for a split second when I did that too.  Would I need smaller accel jets or exhaust jets that actually have an opening?

Larger corrector at the top makes sense. How much larger is the question? I have 185's in now...do I go 195? 205? 

Also, The results have been repeatable...I did 5 logs and they are all similar. 

 

Edited June 2, 20177 yr by blodi

Great stuff for the repeatable measurements. You are testing like a pro!

Put bolts in the bottom of the fuel bowls to kill the acceleration jets.  Replace those 0 bleed back spill jets  with the bolts. Quick and efficient.

Once you do this and measure, you will see what that squirt of liquid fuel is doing. I think it is significant, especially with the bleed-back blocked in those 00 spill jets you have at the bottom of the bowls now.

I would continue to tune with the bolts in place then do your final tune with the accelerations jets back in place.

 

To attack the curve, a change from 155main to 150main will lean out the whole WOT run. Hopefully it will go from you current average ~12 towards 12.5-13.0. You can then go to 145 if needed.

To lean out the high end, a change in the air corrector from 185 to 195 may do it.

Edited June 2, 20177 yr by 240260280

OK, ordered 195 correctors and I have 150 mains at home already that I can swap in.  I'm lost on the bolts in the bowls part. Replace the exhaust jets with bolts? Aren't the 00 jets basically a block already? 

  I'm lost on the bolts in the bowls part. Replace the exhaust jets with bolts? Aren't the 00 jets basically a block already? 

 

I believe the 00 means there is no bleed black hole so the accel pump chamber (under the piston) can only empty into the throat of the carb. It can not "bleed back" into the fuel bowl.

The trick to disable the accel. jets is to replace this component at the bottom of each fuel bowl with a small bolt. By doing this, no fuel can flow into accel jet passages.

Note: at high RPM's the strong venturi effect in the throat can pull raw fuel out of the accel circuit. This scavenging effect can enrich the higher RPM part of the curve so it is good to tune w/o the accel circuit first then tune it last to see what impact it has on transitions and also at high RPM's

Edited June 6, 20177 yr by 240260280

Here is a log (one of several) with the 150 mains and 195 air correctors.....

AFR-6817 by blodi, on Flickr

Much leaner throughout but the top end is pretty good. So, I'm thinking put back in the 155 mains and leave the 195 correctors. I didn't mess with the accel jets this go around. 

I wish I was there with my jet drills! They make tuning faster and cheaper.

 

Below is the 150 & 195 run graph (black) fitted over the 155 & 185 run graph (purple). This overlay confirms the overall shape did not change so the air corrector was not enough to lean the high end.

 

 

Below is the 150 & 195 run graph (black) with the 155 & 185 run graph (purple).  They are matched to the A/F scale

The change in air and fuel simply leaned the overall curve by 1 point.

 

So to flatten the top end you need to go larger with the air corrector.

Try the 155 main with the 195 corrector to give us another data set then we may be able to estimate air corrector size.

 

To summarize: You may have to keep the 155 and go larger on the air corrector or simply drill the 150 to 152/153 and live with the slight lack of flatness.  I recommend trying to  attack the shape with the air corrector first.

 

 

Edited June 9, 20177 yr by 240260280

If you can get a 3/32" drill bit, you can drill out your 185's to "238" as an experiment.

 

Home Depot has them: http://www.homedepot.com/p/DEWALT-3-32-in-Titanium-Split-Point-Drill-Bit-2-Pack-DW1372/202248725

Edited June 9, 20177 yr by 240260280

Imperial Drill size to Weber  Jet Size

3/64" = 119
1/16" = 159
5/64" = 198
3/32" = 238
7/64" = 278

Got the 155 mains and 195 correctors in now. But haven't had time to do any testing yet. (Having a 2 year old and 4 month old severely limits my time  )

I'll report back with the results and we will go from there. 

 

Update:

I ordered and installed 35mm chokes just to spilt the dif and get a bit more top end back from the 34s. No flat spot and pulls nice up top. So great there. Still have the 155 mains and 195 airs in.  

Result is lean: 

 

Put in the "richest" combo I have just to see... 155 mains 170 air. Put it a full point richer. Same curve though. 

I've read a general rule of thumb is +50 over the main for the corrector...so maybe I jump to ~165 main and 215 corrector? That would richen and flatten it, right?