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Weber tuning for non-sports-car applications

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tomj View Drop Down
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Post Options Post Options   Thanks (2) Thanks(2)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Topic: Weber tuning for non-sports-car applications
    Posted: Mar/07/2019 at 3:27pm

i have to work this stuff out from basics since pretty much all of the recommended information is flawed, as it pertains to engines that have utterly nothing to do with ours. i can back most of it up with measurement and documentation.

it's amazing how much lore is just plain wrong (here webers, but all through car culture...). i suspect it starts out well-meaning, shorthand/shortcuts/simplified reductions for some particular set of cases, but then it drifts into dogma and nonsense. (for weber carbs, there's this inane "if your idle screw is out more than 1 turn you have the wrong low speed jets". THIS IS WRONG AND I CAN PROVE IT IN METAL AND NUMBERS.

so there.


1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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dbltrbl View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dbltrbl Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 6:24am
You are using 44Idf carb, 44mm is the throttle blade size, whats the venturi size?
You mention "only later did i discover the nomograph below on page 24 of the Weber factory manual, which indicates 240 (2.4mm) as starting jet size. i know now that is too rich, but it would be driveable/tuneable, a solid starting point."
 
If the venturi size is 38mm for example then the jet size should start from about 190 (to 230) according to chart, but there is multiplication factor depending of the how many cyls one venturi is feeding.
 
I would still put smaller main jets than 180, maybe 170 to 175.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 9:54pm
oops, you're absolutely right. venturi is 36mm. i'll go back and correct that whole section.

by the chart (USED CORRECTLY, lol) it suggests 200. i'm at 180.


> I would still put smaller main jets than 180, maybe 170 to 175.

i think you are right, and in fact i suspect that there will be a number of combinations of main/air jets that will achieve the desired AFR -- i have this open question: assuming that is true, do smaller air jets cause the emulsion to rise further in the emulsion tube?

for a given draw down the carb, the pull on the booster venturi pulls fuel+air up the tube; as the hole in the air bleed gets smaller (to pick just one variable) more air enters, lowering the pressure gradient at the top, and the fuel won't rise up the emulsion tube as far.

i plan on jetting down (both). i suspect i could go smaller than 170, maybe 150.

1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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Post Options Post Options   Thanks (0) Thanks(0)   Quote dbltrbl Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2019 at 11:23am
One can achieve correct afr with many air/fuel jet combinations but fuel jets affect more on low/mid rpms and air jets on hight rpms.

My limited understanding about these carbs tells that if mixture is rich near rev limiter @ wot one needs to put bigger air jets, if lean then smaller air jets.

I think you are not too far from optimal with your jets, idle/progression looks good and economical if it drives ok, then you get into main jet area which are a bit rich for economy, you could try to lean that a bit if it doesnt create flat spot when accelerating. When you get that sorted then adjust your air jets for correct afr @ wot. 

If fuel/air jet change will not give you nice afr curve, then you need to change the emulsion tubes but that area is too complicated for me, we have local tuner here who makes his own emulsion tubes if the ones available from weber/dellorto wont give good results.



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tomj View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2019 at 11:31pm
Originally posted by dbltrbl dbltrbl wrote:

My limited understanding about these carbs tells that if mixture is rich near rev limiter @ wot one needs to put bigger air jets, if lean then smaller air jets.

yes, i'm able to do that now. with this engine, i'm going to err on the side of rich at higher flows/rpms, as cylinder filling issues essentially makes it lean and detonation sensitive. long stroke small bore and awful wedge chamber. and revisit spark timing, since they all go together.

Quote If fuel/air jet change will not give you nice afr curve, then you need to change the emulsion tubes but that area is too complicated for me, we have local tuner here who makes his own emulsion tubes if the ones available from weber/dellorto wont give good results.

yeah, emulsion tubes are a black art! making your own i've never even heard of. i think i'll be able to do what i need without messing with them.

the business about small main/small air vs large main/large air is to "modulate" the emulsion tube action.


i opened the youtube video on another tab i will watch it in the morning. thanks!

1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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