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Weber tuning for new cams

8K views 22 replies 7 participants last post by  howndog 
#1 ·
I'm putting this request in this forum - but possibly it belongs in the ignition forum.

I have a 1967 GTV with a 100% rebuilt 2000 engine. I have RJ 221/785 cams with 40DCOE carbs and and a Centerline ID405 distributor - I have a full megajolt/EDIS setup alongside the ID405 ready to turn on but I want the engine to run solidly with the distributor first.

The problem is a nasty off-idle hesitation when cold - developing into a full blown surging and bogging-down when warm. I live at the top of a really steep hill, and getting home around the 7 uphill switchbacks in the last 1/4 mile is an adventure in clutch slipping and revving the nuts off the motor in first gear as the engine bucks and lurches. (Note I have about 40 miles on this motor to date and would much rather be out on the freeway breaking in the new rings etc - in fact maybe I'll do that tomorrow)

So - basic DCOE setup is
34mm venturi, (stock aux)
140 mains
190 air corrector
F16 emulsion
50F9 idle
can't remember accel jet but I think it's a 35

I've tuned for max idle speed with jets from 45F9 to 55F9 (about 5 increments total) and the hesitation when coming off idle is always there. I've tried to do the "is it a weak or lean progression" test. That is, increase idle speed to about 1300rpm with the idle-speed screw, then open (richen) the idle adjusters to see if idle speed increases or decreases, then close them (weaken) a little etc. The speed change is pretty small - but overall I'd say it's a slightly rich progression (which is the way I'd guess would be preferable). I've also tried dropping to 130 mains - didn't notice any change while tuning for good progression in the garage. I also tried some F11 tubes I happen to have but didn't notice improved progression.

Then, I have a gas analyzer - that says I'm fairly rich everywhere - but when the engine is hesitating it goes down off-scale (weak-end) and jiggles around - very inconclusive and I guess indicative of non-combustion, which is what it feels like. Trying to see what direction it heads in before bogging down I'd say rich, but not off scale.

The timing is set to about 10deg BTDC at idle for about 32 max. Tonight I tried 15 deg at idle to see if that was the problem -and it may have been slightly, slightly better for preogression, but no revelation. Does this sound timing related?

So what next? - today I ordered some 55F21 idle jets and F9 emulsion tubes to try, because I've seen alfaparticle's reports of success with that (partial) recipe. I tried to buy some F34 emulsion tubes, but Pierce Manifolds actually removed them form their website yesterday and say they are now officially NLA - hmm. (btw, what size are the air-bleeds on a F21 idle jet?)

One other symptom - the idle is very unstable. rev it up and it bogs down to below where it was previously set. Blip it one more time and it picks back up, then maybe slips slowly down - very annoying when so many tuning functions are tweaking for max idle. What does that mean? plug fouling maybe?

Grateful for all suggestions. Thanks,

-Richard
 
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#2 ·
Richard,
Stop buying expensive Weber parts. First, contact Gron Parry on his cell phone 408-781-3770. He is a Weber pro tuner. Does this for a living. Offer to PAY him for his help, a $25 consultation fee will allow you more information than you need. You may need to provide considerable engine build information, but Gronway will get you pointed in the right direction. Tell him I suggested you contact him from your note on the Alfa BB.
Depending on where you are in California, Gron sometimes makes house-calls.
 
#4 ·
Richard,
Stop buying expensive Weber parts. First, contact Gron Parry
Thanks Gordon, I'm aware of Gron and his skills at this - but my goal here is to learn what is wrong and why so that I'll know next time. If I just pay someone to tell me the magic recipe without properly understanding why it works...I'll never learn.

I may have to call him eventually of course - but not just yet. As for the parts being expensive...yes, true, I was looking in my jet box and mentally calculating how much I've spent accumulating it. A few more pieces won't move the needle much there. I actually have the impression that prices have come down a little, at least at Pierce. And buying F9 emulsion tubes seems like a good idea since they were the original 2000 fitment in some cases.

Alfaparticle..Very interesting that you use 16 degrees static - not sure I actually got that far advanced while adjusting last night. I will try the leaner mixture-screw idea as well - that tuning for max idle instruction is so universal in every tuning guide that it never occurred to me to deviate from it.

Thanks guys

-Richard
 
#3 ·
The Weber settings that I posted are for emissions type carbs.
I am using the JK advance curve with about 16 degrees static and 38 max. But I have used other curves and had no problems like yours.
One general impression is that the RJ cams in my motor need a leaner mixture than the previous CB47's and 10548's.
I have my idle mixture screws set a bit leaner than for max idle rpm. This gives a cleaner pickup.
 
#6 · (Edited)
So - basic DCOE setup is
34mm venturi, (stock aux)
140 mains
190 air corrector
F16 emulsion
50F9 idle
Have you given any thought to the venturi size? Might 32's work better than the 34's you have in there? My understanding is that the ratio of carburetor bore / venturi diameter should be in the 1.25 range - your's is 1.18 (40/34) . Is it possible that this results in too low an air velocity through the venturi at low rpms?

I understand where Gordon is coming from - buying a whole new set of venturis, and finding it does nothing to change the symptoms would be foolish. And, admittedly, I'm no expert. But, you asked for ideas....
 
#7 ·
Ed, they're both 40DCOE27

Jay, I've run this motor with 34s for several years - now it has different cams, but identical porting and valving - so I wanted to carry on with 34s to make the most of the power these RJ cams should unleash. If I have to go to 32s then I will of course.

-Richard
 
#8 · (Edited)
I've run this motor with 34s for several years - now it has different cams, but identical porting and valving - so I wanted to carry on with 34s to make the most of the power these RJ cams should unleash.
Well, again, I'm no expert, and would like to hear the comments of those who know more than I do (practically everyone).

You're right, larger carb bores and larger venturis have the potential to deliver more flow, and as such, more power at the top end. But, below the top end, the throttle plates are reducing the carb's effective diameter - the difference between 32mm and 34mm probably only matters for the last few hundred rpm.

My concern is that with hotter cams, which probably have more overlap, the low rpm airflow through your 34mm venturis is dropping to near zero, preventing fuel from getting drawn in, and producing the lean readings on your A/F meter.

Sure, racecars run big venturis, because they don't care about low end performance. But to make your engine streetable, you may have to sacrifice some of the top end.
 
#9 ·
Jay, my 1600 GTA engine runs 34's in 45DCOE14's for street. For track, I can go to 36's. On the other hand, a single plug Ausca test engine I just restored, has raised ports on a 105 1600 head that Ausca modified from a raw casting in about 1967. It has a GTA manifold and was run on a dyno with both 45DCOE14's and 40DCOE2's, both with 33 mm chokes. This engine used GTA 10.6 cams, and now has Richard Jemison #242 with 10.55 lift and less low end duration. Both back in the day, and today, it develops more low end torque with the 40DCOE's and is only off about 4 Hp at 7500 from the 45DCOE's. This is an unusual case, with an odd engine, but both the 45DCOE14's and old 40DCOE2's are far from Richard's DCOE27's in that the low end and progression holes on botyh style Weber are pretty rich to cover the transitions, which seem to be Richard's problem.
When I get into the low speed transition problems, and richening the low end mixture does not help, for me, it's time for a call to Gron Perry. With his help, I was able to get a pair of 40DCOE138's and 139's (a neat variation!) to work very well on a performance 2 L. As I had never done a restoration of this variation before, and they are not seen that often here in the U.S., I failed to write down the changes that made them work.
 
#11 ·
Your idle jets may be a bit rich. Do you have 50F8 or 50F11 that you could try? The other settings look about right. I think that 34 venturi's are fine for road work. Mine work well with Richard's cams.
The F34 and 55F21 that I use are specific to emissions Webers. They are not for yours.
 
#15 ·
I believe you are right Ed. It seems the history of the DCOE series started with 40DCOE2's and 45DCOE9's which both shared pretty rich running low speed circuits. They also had 3 progression holes, positioned in such a way and size that made them very easy to tune to any application. Over time, Weber tried to improve on the overly rich low end and transitions, and they became a little harder to set up. The "classic" jetting from the older DCOE32's was carried over to the attempt at "one-size-fits-all" 151's.
My appreciation of the variations comes from my work restoring them for customers, and finding what is involved in making them work for the various Alfa applications. The design is extremely flexible, but generally, it seems, the more current variations take a little more effort, and often more Weber knowledge, than I have. In these situations, Gron Perry has saved me from spending a fortune with Pierce and the other good suppliers trying to find a working combination! As Jay mentions, if you run 30 mm chokes in your 2L, many problems seem to vanish, when in reality you may have just hidden them with greater air velocity! The hard part is to get them to behave with chokes the engine displacement and cams can utilize at idle, transition, AND full throttle. This is where the different variations need the knowledge of someone that has been there and figured it out.
 
#16 ·
Tuning Webers

As you describe initial open throttle issues I would suggest you are lean. Ignore the WBO2 info.
Put the 55 f9 idles jets back in it. At idle (which is 800 RPM (not 1300 RPM) the adjusting screws should not be out more than 1.5 to 2 turns. If screwed in or out past 1.5 to 2 turns drops idle speed then you are about right on idle jets.
If there is still a bog, increase mains to 150 or 155s .
If the cams are correctly timed at 102/104LC it should pull fine.
If not suspect other carb issue. Float levels, etc. Without the correct fuel level the E Tubes won`t mix correctly.
 
#17 ·
As you describe initial open throttle issues I would suggest you are lean. Ignore the WBO2 info.
Put the 55 f9 idles jets back in it. At idle (which is 800 RPM (not 1300 RPM) the adjusting screws should not be out more than 1.5 to 2 turns. If screwed in or out past 1.5 to 2 turns drops idle speed then you are about right on idle jets.
If there is still a bog, increase mains to 150 or 155s .
If the cams are correctly timed at 102/104LC it should pull fine.
If not suspect other carb issue. Float levels, etc. Without the correct fuel level the E Tubes won`t mix correctly.
Thanks Richard.

yesterday I advanced the timing to about 16degrees BTDC at idle (~900rpm) and now the progression off idle is much better - but definitely still there. It's very drivable now but still with tendency to bog at around 1300-1800rpm especially under heavy load (up a steep hill). I then changed from F16 to F9 emulsion tube - pick up seemed slightly crisper, but still with tendency to bog.

so then the set up was 34mm choke, 50F9 idle, 140 main, 190 air, F9 e-tube

then, today trying Richard's suggestions:
with the 50F9 idle jet the idle screws are turned out less that 1 turn - I tried the 55F9 idle jet again as you suggest and so the screws are turned out even less for max idle. No difference noted in the progression off idle. This really felt like too big a jet with 0.5 to 0.75 turns out on idle screw - so went back to 50F9. Tempted to go smaller still but...45F9 seems pretty small for 2000/34mm chokes.

I moved to 145 main jet (largest I have) and again no difference noted.

I opened the carbs and carefully re-measured float height - set it to 8.5mm - one had uneven float height side to side which I fixed - one was more like 9.5 mm which I fixed. Took it for a test drive and - no real difference noted.

(by the way could someone remind me how to measure fuel height by removing a main jet and dipping into the fuel with depth gauge? - I like that method better as a real check but can't remember the correct measurement)

So I agree - with all of the things I've tried, this bog is still there to some extent and so it's hard to believe it's only jetting, could well be something else wrong, but I'm not sure what.

Thanks everybody for the ideas - thanks especially to Jim for the conversation on Saturday.

-Richard
 
#18 · (Edited)
Weber Tuning

Since your description indicates a very rich idle circuit, you should address that first. Idle jeting has nothing to do with choke tube size.
The F9 designation on the idle jets refers to the air bleed & internal diameter of the jet . Are you sure the fuel orifrice in the 50 F9s is really .020 diameter and have not been drilled?
The correct jet will tune at about 1.5 to 2 turns out at idle. Less is too rich, more is too lean. But the motor runs primarily on the idle circuit until about 2000 RPM, so adequate fuel delivery (fuel inlet size) is critical. Perhaps a more typical idle jet designation to a leaner design is appropriate. (f8/F11/F15 etc)
(FYI, the 50 F9 can be made into a F15 by drilling the airbleed from .040(1.0mm) to ..043(1.05mm), or F8 by drilling to .047(1.2mm) as internal diameters are the same. By drilling 2 air bleeds of .043 you can make F7s of them. ) However I think you have other carb issues....

Your enrichment squirters (acceleration circuit) may be overwhelming the system.
Check the pump bypass jet to insure it is in fact a "bypass" jet and not a closed jet. A 55 or 60 should work. The ball in them freely working.
The actual pump jet that "squirts" the fuel should be a 35.

I can`t immagine that you would use a main jet smaller than 140/145 with these cams. Although they are high lift/duration, the initial valve events decrease overlap for less wasted fuel and higher combustion pressures at lower RPMs. They will still demand more fuel off idle, & above 2000 RPMs or so...

Have you driven these carbs on another motor? What is your intake system (airbox) design. How is the carbs internal vent matched to the air box??
 
#20 ·
getting closer

Since your description indicates a very rich idle circuit, you should address that first. Idle jeting has nothing to do with choke tube size.
The F9 designation on the idle jets refers to the air bleed & internal diameter of the jet . Are you sure the fuel orifrice in the 50 F9s is really .020 diameter and have not been drilled?
Yes these are jets that I bought new and are stamped - I can't measure the hole size but can certainly see that the F9 jets have smaller air holes than the F8s.

So, since I agree it's likely a rich progression problem tonight I went down to 45F9 again - I had tried this jet before but not with so much advance or with the F9 e-tubes - This change to 45F9 made a definite improvement to the bog tendency, climbing the hill to the house was quite enjoyable! Definitely going in the right direction. I'd like to try a 45F8 but don't have a set- I'll get some tomorrow.
The correct jet will tune at about 1.5 to 2 turns out at idle. Less is too rich, more is too lean. But the motor runs primarily on the idle circuit until about 2000 RPM, so adequate fuel delivery (fuel inlet size) is critical. Perhaps a more typical idle jet designation to a leaner design is appropriate. (f8/F11/F15 etc)
So this is still strange - the 45F9 barely needed the idle-mixture screws any further open...so still less than 1 turn each. Aren't there two shapes of idle screw, one with a faster ramp than the other? Maybe I have the wrong shape screws?
Your enrichment squirters (acceleration circuit) may be overwhelming the system. Check the pump bypass jet to insure it is in fact a "bypass" jet and not a closed jet. A 55 or 60 should work. The ball in them freely working.
The actual pump jet that "squirts" the fuel should be a 35.
Yes they are 35 pump jets - and I can't remember the size of the pump bypass jet but I know it's not closed and I know the ball worked when I rebuilt these carbs. One thing that makes me think it's not the accelerator circuit is that I've tried slowly increasing revs by turning the idle-stop screw and watching the rpm climb. The hesitation is still there. in other words it picks up from 800 to 1100 well enough and then more turning of the idle-stop screw has no effect for a while except that the engine is hunting and puffing/sputtering - and then after more idle-stop turning it suddenly bursts to much higher revs - I haven't tried that again with the 45F9s
Have you driven these carbs on another motor? What is your intake system (airbox) design. How is the carbs internal vent matched to the air box??
I've only driven them on this motor before the rebuild - so same porting but different cams. I think it's probably not tooo different from how it was then (always had a slight hesitation problem that I came to accept) - not quite back there yet though.

I'm also wondering how well the advance mechanism is working - if that bounced around during the progression zone then this would happen I think? It's a bit inconclusive watching the timing mark with the strobe because I don't know whether I'm observing the cause or the effect as the engine hunts.

Getting closer though - nearly there - I reckon the 45F8s might just do it. Thanks again to Richard, Ed and Jim and others.

-Richard
 
#19 ·
Hi Richard,
I have 135 main jets in my 40DCOE116/117's with 34mm venturis. I think that I might need only 130's in classic Webers as the emissions carbs typically take slightly larger mains. I think that your cams require a leaner mixture than Colombo & Bariani or factory cams, in my motor anyway.
 
#21 ·
Your idle mixture screws should be the ones with the long thin tip. The fast-ramp, or the more blunt ones usually run at about 3/4 turn, but were not used in the emission Webers.
 
#22 · (Edited)
50f11

Following some sound advice from Ed I think I now have this problem licked.

The 45F8s were definitely the best yet on the trip to work on Wednesday. That evening I stuck in the 50F11 set I have - it felt good in the garage, but couldn't drive it until this evening. Tonight's short run was great - good idle, off-idle progression and transition to mains all seems smooth. Pick up from low revs is very nice and there was very little sign of the bogging down - so going leaner on the progression was the ticket for me (RJ's advice to ignore the WB02 was spot on for sure) - some good lessons learned with this one.

My Weber books seem to group F8 idle jets together with F11s as one air size - but obviously that's all wrong - must start making notes and not put myself through this ever again.

Proper test tomorrow with the drive down to Monterey for Concorso Italiano. See you there maybe?

-Richard (Lane)
1967 GTV (red)
 
#23 ·
Following some sound advice from Ed I think I now have this problem licked.

The 45F8s were definitely the best yet on the trip to work on Wednesday. That evening I stuck in the 50F11 set I have - it felt good in the garage, but couldn't drive it until this evening. Tonight's short run was great - good idle, off-idle progression and transition to mains all seems smooth. Pick up from low revs is very nice and there was very little sign of the bogging down - so going leaner on the progression was the ticket for me (RJ's advice to ignore the WB02 was spot on for sure) - some good lessons learned with this one.

My Weber books seem to group F8 idle jets together with F11s as one air size - but obviously that's all wrong - must start making notes and not put myself through this ever again.

Proper test tomorrow with the drive down to Monterey for Concorso Italiano. See you there maybe?

-Richard (Lane)
1967 GTV (red)
hey all,
I am having the same problem, although my carbs are Del'Orto's. Would anyone know if the progression tubes be the same nomination? I am not sure what I've got in there now, but are the jets etc thesame? Also were to buy.
Any suggestions will help...
 
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