Rejetting carbs

Thanks

 

You bet. I'm hoping you did a lot of the leg work for me already One question though, do you have a stock exhaust? If so I may need bigger idle jets but am not sure what that will do to the rest of it. I should finish taking it all apart tomorrow.

 

the exhaust is NOT standard. whilst i don't know what brand it is (but i've been told ANSA), i do know that it flows a lot better than a standard one.

 

Both Birdman and i have 77s with 55 stock idles. When we put on low restriction replacement mufflers, we had to go to 60 idles to get away from backfirnig through the carbs and exhaust. It is stinko rich and bum mileage but works. I am going to try an overhaul, sync, then going smaller in search of better mpg and less smell. My preliminary LM-1 check shows cruise in the high 11s - no wonder the mileage is stinko.

best,
chris

 

Aircon there is one other little trick to do to relieve the lean spot that is just off the idle jets and that is to use a small file and chamfer the bottom side of the throttle plates in the vicinity of the progression ports thus drawing from them sooner. If you had it running perfectly in all aspect but that, this may complete your package.

 

Well, I finished getting the airbox off and found that the idles were 55's. I'm thinking I need to start from the beginning and check float levels and then jet it. I'm going to start with what Aircon finished with and hope it's close although I'm not sure about smaller idles than I have now. I also reset the idle adjust screws and will work those in conjunction with the jets.

For all you experts out there....if I change from 200AC/125main/55idle to 170AC/135main/50idle does it seem possible that this would alleviate what seems to be a lean condition on the idles now? This seems counterintuitive to me but I'm not sure how all the changes would interact with each other. This is all with F36 tubes

Thanks,
John

 

I may be wrong, but I really think you can tune it better than 60 idles. I do not know what ETs or mains you are running but I would look at going back to 55 idles (even 53s) and perhaps up on your main jet size. I had a similar deal with exactly the LM-1 numbers you are seeing and this worked for me to improve cruise A/F and cover the transition by reaching down with the main.
best
rt

 

Always a good idea.

Kind of like I was telling Chris - at least for me, my tuning philosophy and approach has been to get as small an idle jet as possible and cover transition with a larger main. Webers cruise mostly on the idle circuit as everyone here knows, and there is a HUGE cushion for leaner idle jets if you can reach down and cover the transition with the main. To wit: The engine will be fine with leaner low rpm mixtures as it is in an off-power condition during cruise and daily ops. And, if you need power down low, when you punch it the accel pumps are going to bring it down to around 10 and cover the A/F until the mains kick in. If you think about it, when you're on the mains, it **mostly** is a power-on situation, and when you are trolling to the Deli on idles, you are off power. This also gives the best (but not great) fuel economy. Look at Aircon's example, through judicious leaning he achieves by report FI levels of fuel economy -- it can be done.

Yes, this is the general idea. Even with 50 idles, you are probably cruising in the A/F 13/14 range with the downside being a transition hole - here solved by a larger main (although personally I think 170 a/c may be a bit rich, but it's all variable car to car).

Good luck and have fun!

 

yes....ive been wondering why mine isn't rich up at high revs. the dyno guy seemed to think the a/c doesn't make THAT big a difference, though. *shrug*

 

the best way that I have heard in thinking of the A/C is from Mike Kuhn at Norwood's: it acts as a "brake" ( by 'shorting out' the intake vacuum signal to the main jet) on the amount of gas sucked through the main jet, and is more apparant at high rpms (makes sense - higher vacuum signal). I think it does have an effect range wide, though, and it's potency varies with different ETs and float settings.

 

Hi Russ,

Just after I got the car, i dumped the thermal reactor, as above and did a shotgun approach with the one step richer jetting 55- to 60s, 220 to 200 ACs and 135 to 140 mains.

The car has about 28k on the clock and I have no idea what the po or the "used car dealer-jerk" that sold me the car have really done to it, So, I thought I'd start from ground up to get it running smoothly, economically and powerfully.

I have been reading many posts on jetting, and in particular, syncronization, so here is my plan, (nothing original but a lot of good stuff gleaned from many posts:

1) Remove carbs and overhaul, new needle and seat, (stock height)
2) Make sure carb and intake manifold surfaces are flat.
3) Install new canister purge nipples to facilitate syncronization with 2 four tube manometers.
4) Make sure all linkage pivots are snug.
5) Tighten all idleair bypass screws down.
6) Start with all throttle plates just covering the first progression hole, (one of Pierce's ideas?).
7) Use the Manometers to make sure each carb throat is matched to its sibling only.
8) Adjust the throttle linkage cable out for a 3000rpm setting and use the manometers and linkage to get the barrels as close as possible.
9) Let the cable back in to 900-100 idle and just set the idle speed screws to just touching with the playing card trick.
10) Set idle mixtures with Infra red gun for max idle temp at header pipe by cylinder head, (This approach needs to be verified, it was suggested by an old mechanic of considerable experience).


For a baseline jetting, I would go with the stock 55 idles, 140 mains and 200 ACS. If i recall correctly, your feeling was that the F24s were for power and the F36s were more "environmentally friendly", so the F24 sound like a better starting place, if we are going to try to avoid the dreaded "hole".

I've got two bungs, (I know some of you miscreants out there are grinning, but just let me finish) - anyway, one bung welded into each header pipe before it enters the flowmaster, so i can check both banks.

The final jetting would be worked out with the LM-1 and a lot of jet swapping.

How hard, ($$) is it to get the rpm plotted with the AF ratio?

I am going to try installing limiting bushings in the distributor advance so i can start with 10 initial and max out at 34. The exhaust is opened up with a big flowmaster+two double resonator tips and the ignition is Norwoods single distributor with mag pick up and a MSD6Al.

thanks,
chris

 

Wow, I'm a miscreant! OBTW, here a stock carb advance curve.

Sounds great.
As far as RPM plotting on the LM-1, Pelican Parts sells an LM-1 'RPM Kit" with the pick-ups and adaptors to allow you to do that. I just got mine, but have not opened mine yet.

I think your plan is good, and I will be similarly occupied as I am putting new carbs (44 DCNF) on and need to do the same drill in a couple of weeks. Although totally non-comparable to your situation ( engine here is a 3.2 l qv with large intakes and ported head and higher lift cam), just for entertainment, I am starting with 55 idles, a 38mm venturi, 140/F24/180 and see where we go. While you'd think I'd start with a larger main, I'm told by the head guy that the flow bench shows a much increased velocity, which means a stronger vacuum signal at the carbs. I fully expect I may have to back down to 36mm venturis for drivability, but might as well start big!
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I have really enjoyed your detailed threads, wish i had a bit more time and $$ to persue it.

best,
chris

 

Yes -
I think the 2v benefit from a bit more advance than the 4v, perhaps as much as even 40 max at 3500. I would look hard at your advance curves as you dial your car in - starting with 10 degrees initial is NOT uncommon in many high performance Italian cars.
This may shackle you to premium fuel, but you're probably already using it.
best

 

Hey Russ,

did you go with both the lm-1 rpm kit and the inductive pick up??

I will go with more initial advance and yah, the 77 just loves premium :-0

best,
chris

 

Russ, is it possible that the fuels today have just enough alcohol content that the cars run too lean with the original stock jetting? I see old charts showing 45 to 50 idles 220 AC jets and 125 mains, which by the sounds of things would be almost impossible to run with today. Or is it just that the F36 ET with the later version DCNF (78 and up emissions cars) require richer jetting? There is also an air correction jet (air bleed) for the idle circuit on the top of the carb body, its actually a brass bushing pressed into the carb body. In all these books not one shows what size it should be, or what range would help.

Playing with carbs has always required heavy reading. When I was 17 I had to figure out how to tune my MG by myself because my world was filled with incompetent morons. The Haynes manual article was two or three pages of fine print and it took me a good day of study before it started to gel. Weber carbs are many times more complex. My whole life all I have heard is how troublesome and problematic they are. Yet if you read through enough you start hearing this one or that who says no, not if you know how they work. Ive heard people argue heatedly, that a DCOE on a 944 Porsche or Alfa will always have a flat spot and nobody can tune out. Then someone else says they have seen it and the arguing starts. Thats why FI is so popular, it just works. Set and forget it. The beauty of the carb comes in its simplicity, that you need nothing more than barely a screwdriver to monkey with it. Which is also the danger if your brother in law starts "helping" you.

Here are some "rules" I have found that seem to cross through the various books.

For a given AC jet, a larger main will increase mixture across the rpm range.

For a given main jet, a larger AC jet reduce mixture in the higher rpm range.

Larger venturis push the power band up higher and reduce botom end power.

One would assume that what works on the main circuit should work on the idle circuit. Hence a smaller airbleed may possibly richen up the top end of the transition area.

The muffler and aircleaner, camshaft style, valve clearance, idle jet, idle jet air bleed, progression holes (both in number and in size), throttle plate setting (and throttle plate chamfer) and idle mixture screw setting will all effect transition from idle circuit to mains. Change one thing and it has effects something elsewhere. But there really needs to be much greater discussion on the ET (emulsion tube). Simply discussing jet changes with no regard to what ET or carb numbers you have tells us nothing of any real value. Do you have a F36 or an F24 ET, and what "series" DCNF carb. The early series carbs have different progression holes that the later models, and effects the transition area. In comparison to the DCNF, the DCOE has a full dozen different ET's you can choose from. The numbers on them offer no "range" to thier performance, IOW they range from F1 to F13, but in no particular order. One manual shows them as such:

(rich) F6-F12-F9-F8-F11-F13-F2-F4-F5-F7-F1-F3 (lean)

Almost seems like you need a crystal ball to know what to use. But in choosing them, you can correct for lean top end, or bottom end, or rich top or bottom, without effecting the other end. I dont know if DCOE ET's will work in a DCNF, but at present we only have two available, the F24 and the F36, and possible an F26 as a third choice though it may be a typo.

 

You guys talked me into it, I'm just going to rebuild the carbs while I have them half apart already. Paul you are right, it does take a lot of reading. This is really my first time getting this far into a carb. I've been reading everything here and a bunch of stuff on line too.

"Might as well, while you're in there" starts again! Gotta love these cars.

 

OK, I've got a US '78 GTS with 55's, F36, 135 mains & 200 AC. When adjusting the idle mixture screws for highest rpm, they are all out about 5-6 turns. This seemed like a lot so I was thinking about trying 60 idle jets...hoping that I could then bring the idle screws in some. I am runnning catalytic converters, but no muffler, so there shouldn't be too much back pressure from the exhaust.

I think I'm running a bit lean on the top end, so I'm probably going to try smaller AC's.

I'm hoping to dyno the car within a few weeks, so I'll have some hp & a/f numbers then. I just want to try and get as good of a set up prior to the test.

Henry

 

I just saw this question - apologies for the delay.
Yes, definitely. I think Paul is spot-on. Fuels were formulated much much different when these carbs were initially jetted than today. In fact, if a car has not been rejetted in the last THIRTY years, then I can about guarantee you that the jetting, while it may work, is sub-optimal. I have not heard of many of the late carb cars that still have their 125 mains that did not have a flat spot (unless it is covered up by the effect of a clogged air filter ).

Just my humble opinion based on observation, of course.
YMMV

 

Hi Russ,

I took your advice and called Mike at Pierce Manifolds. Great Guy, took 15 minutes with me discussing carbs and one of the things he said was that today's gas is not what it used to be and that proper jetting would most likely require going richer than stock.

He is out of F24 tubes but will send some up soon with new sealed throttle shaft bearings.

Stock float height is the way to go and watch out for floats with hard,dark grey, (almost unscratchable) solder), prone to leak.

Did you ever get the RPM kit hooked up to your LM-1? It apparently comes with a bit more software. Can't wait for work to slow down a bit so i can try mine.

best,
chris

 

.. lots of information here, I am tuning my 75 US GT4 and the carbs are "all over the place"..

- out of synch
- very rich idle.. stinks and too dark plugs
- squirts from at least two acc. pumps..
- no cats

spent hours rebuilding the dizzy's today and later on went to carbs. I did not write the jetting numbers, but from my memory 55, 120, 220, F36

This discussion gave me some ideas for a starter, 50, 135, 200 with the same emulsion tubes.

Anyone have a link to someone selling weber jets with "fiat/alfa jet" pricing?

Would it be worth it to get a LM1/LM2 or could I just go by "smell", performance and plug colors?

 

This guy has everything you need regarding jets and he also has rebuild kits.I have used him and have had great success
http://stores.ebay.com.au/ALFA1750S-CARBURETORS-PARTS-STORE_W0QQssPageNameZstrkQ3amefsQ3amesstQQtZkm
The LM1/2 is very useful as well
http://cgi.ebay.com/ebaymotors/Innovate-Wideband-o2-Sensor-LM1-AF-Gauge-Kit-LM-1_W0QQcmdZViewItemQQ_trkparmsZ72Q3a543Q7c39Q3a1Q7c66Q3a2Q7c65Q3a12Q7c240Q3a1308QQ_trksidZp3286Q2ec0Q2em14QQhashZitem170270793684QQitemZ170270793684

 

Some 308s came with 125 mains from the factory, but with modern gas your 135/F36/200 + 50 or 55 idle is a good starting point.
Have fun!

 

I have 50 idle jets, 120 & 220 plus F36 tubes.

Based on the knowledge here I think 135, 200 and possibly F24 tubes.. Any input?

I will at least get 135 main and 200 ac, most of the spluttering seem to be due to 2 acc pumps leaking so I will fix this and set float heights as well.