Tuning 40 DCOE Weber 151 on an L24

[Lazeum]

your curves are very interesting, I wouldn't have guessed such a difference between the 2 cases. The change seems to have translated the whole curve up by 1 to 2 AFR points, that's a lot!

If it is a WOT trial, isn't AFR level too high?

I'm also wondering what could be the reason for the rich spot at 20s on the leaking trial. It does not exist on the second run so I would imagine it could come from accel circuit.

I can see as well that despite your mains coming early you have a lean spot in between 2500 & 3000 rpm, would it be the progression circuit being out too early? hence the importance of progression port back to throttle plate position you're talking about.

If you have read the post on webers on HBZ, maybe you've seen my AFR results (p13-15 if I recall), I'd got a rich spot at 4000/4500 rpm for some reason I cannot really explain besides engine dynamic... it appears this rich spot has disappeared with my new head.

how do you measure AFR also? with a copper pipe used as a sniffer or with a bung welded around the transmission area?

[240260280z]

The plots are from a nice guy on the yahoo side draft forum.

I have a theory about the rich spot at ~2,200 rpm too. It is because the top of the throttle plate pinches off with a thick choke next to the accel. nozzle promoting more flow. The accel pump will always open and push as the throttle is pressed and vacuum at this pinched spot will pull more fuel for a moment of transitition.

I was banned from HybridZ so I will never join...even if I wasn't banned... some useful info there but the admin are "weenies"

I am sniffing at the tail pipe like you. The copper pipe I made is 600mm long to get a better sniff at idle as it goes deep into the exhaust system.

[240260280z]

snip I always setup idle at best lean condition then play with idle screw to set the idle speed correctly.

snip

THANKS! I have been thinking more about your words and it suddenly occurred to me that the idle and the progression do interact however tuning at idle seems illogical as the car must be tuned to run, not to idle AND because the idle enrichment screw can fudge things.

I think tuning by addressing the progression and cruise first is the best way to start. Fudging air/fuel mix with the enrichment screw at idle and throttle plate is sort of backwards because at cruise, the idle jet should not be significantly feeding the motor. In theory at cruise, the jet and the progression holes AND the throttle plate are the only significant components and the throttle plate will fall where it naturally does to provide x amount of air at cruise.... so by cruising, one can alter the jets to get the right a/f mix then go after the idle.

In fact it would be interesting to try to tune with the enrichment screws closed completely and just use the throttle plate opening to pull from the first prog hole. If the car will idle like this then a quick cruise with A/F monitoring will tell whether the idle jet is too rich or lean.

Edited July 29, 2014 by Blue

[Lazeum]

just to make sure, when I was referring to "idle screw", I was talking about the screws used to balance the carbs & RPM at idle, not the ones used to play with AFR.

[240260280z]

Yes I understand. In fact just last night I read a section from "How to Build & Power Tune Weber & Dellorto DCOE & DHLA Carburettors" (SpeedPro Series) by Des Hammill. The section on choosing and tuning idle jets is exactly what I concluded above... convergence is a good thing. Basically he says to choose jets for cruise at progression and for power transitions through progression then address the idle.

I'll post some of my internet vacuuming finds in a new post.

Edited July 29, 2014 by Blue

[Zedyone_kenobi]

Today I changed out my 30mm chokes for 32mm chokes. As you would expect not much has changed as far as idle quality, but having said that. For the first time, I was able to get the carbs perfectly balanced at 1000 rpm dead on. All 6 cylinders were pulling identical amounts of air, and also, my AFR was as close to steady as it has ever been. I could get a decent idle before, but it would fluctuate from 12.3 to 13.4, and would cycle seemingly randomly around there, but average about 12.7. Now I do not vary more than .4 on my AFR. I stay about 12.9 to 13.4. My part throttle seems smoother, and it picks up revs very quickly. I cannot wait to drive it to see how she feels.

But for now, I have the car on jack stands with the front spook off, I am waiting for my ducted spook..

[240260280z]

try 20 degree initial timing and see if you idle smooths out you may have to retard more at heavy load/wot but it made a big difference for my tuning yesterday

[Zedyone_kenobi]

negative. If run more than 13 I get some pinging at heavy load in 3rd and 4th gear. With 10.5:1 compression that was going to be a risk I was taking. AT 10 degrees advance, I get none. Car seems to run fine. I will try to bump up the octane and run about 14-17 to see how she likes it. I know webers like lots of initial advance. If I can ever get my mallory unilte to work without blowing the ignition module I will order the curve kit for it so I can run more advance initially.

[Lazeum]

To tune timing at idle, I start the engine (while already hot) with distributor loose so I can turn it by hand. Turn it and hear the difference from the engine, the rpm will increase or decrease. Best timing will be when the idle will be the highest. This is where you should setup your timing.

Check with a light where you stand and record the value.

Do a quick math to determine your dizzy advance based on targeted total advance wanted. With a Mallory Unilite, you can set up this advance. You don't need specific tools (even if it would be nice to get). To offset properly the dizzy, I measure the arc length vs. the radius from where I do the measurement. then I use drill bits with corrrect diameter as a gage for proper adjustment of the 2 plates inside.

[Zedyone_kenobi]

That's a good idea

Never even thought about modifying it myself. That definitely easy to do

[Zedyone_kenobi]

Well I took the Z out for a drive with its new 32mm chokes. I made all the appropriate jet changes to come to this configuration:

130 main Up from 120 main

180 air corr Up from 170 Air corr

55F9 idle Up from 50F9

My first impression was that there was no change. It still idled as smooth as it ever had (no small part to finally getting the air flow through them perfectly balanced). With the 55F9 idle jets, I get 13.9 to 14.7 AFR on steady state cruise at about 60 mph. Even better when I tip into the throttle the AFR drops to only about 12.8 or so. Very nice. At wide open throttle the power improvement is not trivial. It just seems to have more legs up top. It pulls with more authority.

So all would seem to be well. But is it? The answer is mostly, yes, and a little no. As one might have expect the throttle response is NOT nearly as sharp as with the 30mm chokes. 32mm is definitely on the ragged edge of what I would want to have in a street car. Not that it feels sluggish, or laggy. It does not. It response to all the inputs I give it in the gas pedal. But not nearly as sharply as before.

The engine with the 30mm chokes seemed to want more air all the time. So any opening of the throttle plate made the engine just respond like getting a breath of fresh air after surfacing from the water after a long dive.

The engine seems more relaxed with the 32mm, it idles better (not sure chokes should make a hill of beans at idle though), and the ferocity of the upper rpm is notable.

So long story, for a street car 30mm chokes will work fine on a L28, and give you immediate throttle response that makes a car VERY fun to drive on the back roads. But for a car with deeper lungs, the 32mm are the ticket. If you track your car and live in the higher rpm, I can completely recommend 32mm chokes maybe even bigger.

My cliff notes is that going to 32mm chokes is not a holy grail of modifications to webers. There is a downside. The magnitude of that down side depends on your personal preference and what you value.

[240260280z]

Great report! What was your AFR at 4k and at 6k?

Last weekend when I measured and recorded A/F as my brother drove, his driving commentary matched the A/F ratios I recorded: Going with bigger idle jets and lower AFR made the car "sluggish"...similar to your commentary above wrt 32 vs 30mm.

I am wondering if you can play with mains and air with 32mm chokes to see what happens on each side of 13AFR between 4k and 6k. (120f & 180a vs 130f and 170a)