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Weber Tuning Woes


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I have a weber 38/38, although if my problems go much further, i may not for long!


engine is a fresh rebuild with a bmw 300 sport cam and 9.5:1 pistons.  rebuilt carb and electric fuel pump giving me roughly 3.5psi. 


i have used a range of gets, 50, 55, 60 on the idle, 130 thru 150 on the main, and 175 and 185 air corrections and pretty much every combination of them.  i have read every thread the faq has to offer on weber downdraft tuning a well as a number of other sites.


no matter what i do, i have 3 problems:


1. the worst issue, is it leans out under a light load, typically this is doing 110kph on the highway, occasionally to the point of stumbling, almost like its getting stuck between the idle and main circuits. tried both larger idle main jets to try and boost that to no success.

2. it leans when i come to a stop, raising the float has stop it from stalling out when stopping, but hasnt fixed the lean out issue.  i have unplugged my brake booster, and drove around the block, no noticeable improvement.  no other vacuum ports being used, and all the plugs have been checked.  float is set at 37mm right now, i started at 41mm (brass float).

3. rich when im flat on the gas, you get the initial richness from the accelerate pump, but then when thats gone, its still rich (ie, do 25000rpm, floor it and let it stretch its legs, it will stay at roughly a 10-11 afr until you let off the gas)  getting size has almost no impact on this, from 130 to 140 main jet, maybe half a point.  it goes well, but im sure there is some more hp there if it get it closer to 12.5.



i would appreciate any suggestions, thanks!

Edited by Zorac
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Chech the power valve diaphraghm and the valve itself.


Recheck your float level (I know, I know...)


Check for air leaks.  These will lead you to close the throttle more for idle, leading to 

a 'transition gap' that you're experiencing.


You can file the edge of the butterflies to get the transition passages open sooner, at least, a bit.


Check the dimensions of the jet wells in the 2 barrels against each other.

I have a 45dcoe that has one well sunk 3mm too far, and the passages to the main

jet don't line up at all (a dcoe problem) due to really really piss- poor machining (a Maid In Spain problem)


Does your float valve seal positively?



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dumb question, but how do you check the float valve?  i have a fuel pressure gauge, but the system doesn't hold pressure for very long after i shut the car off, but the pump could be relieving the pressure.  i go from 3.5psi to about 1 psi in a minute or two.  i have two float valves, and i tried both today (double checking the float height each time) (one old with a brass tip, one newish with a rubber like tip).  power valve diaphragm is brand new as well.

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Ha, not a dumb question-


I blow on them, and if they don't hold air pressure, toss them.  It's not scientific, but it seems to work.


Losing pressure in a couple of minutes might be a problem if you're having trouble with

hot starts, but it shouldn't matter for the problems you're having...




Have you tried a lower fuel pressure?  2 psi is plenty to run a Weber....


I'd try sticking the 130 mains and the 60 transition jets into it and see what happens.

In fact, I'd go even lower on the mains,  just to see if it will respond at a certain point.



If not, I think I'd probably put it into a box in the attic...


Oh, how are you reading your air/fuel ratio?



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ill have to give it a blow, tomorrows project...  :)


running 130 main, 185 air, and 50 on the idle currently (this morning was completely different!).  mixture screws are roughly 3/4 of a turn out.  i did blow out the idle circuit with compressed air, rest the float to 40mm/50mm.  I did start the day with smaller air jets (175 and 170), and 60 idle jets at 1 full turn out.


i sprayed carb cleaner on all the vac connections, no leaks that i can find.


ill have to get a regulator, the electric fuel pump just goes at 3.5.  the reason i have an electric fuel pump though was becaues i though my mechanical pump was having issues which was causing my issue.


at idle i aim for afr of 13, but it floats usually between 12.5 and 13.5.  when you give it gas, the accel pump push the aft down to 10 or so, but if you hold your foot to the floor it come back to about 12.  the current jetting gives me around 15 when im just cruising at 110kph, and it will bump to 16 and give a very slight hesitation on rare occations.  when your driving around, youll see it spike anywhere from 8 to 16, but for very short periods. when stopping, the afr will spike to 16-18 range, but it doesn't seem to bother it now.


didnt check the jet wells yet.  though id do the easy stuff before i pulled the carb right off.


im at roughly a 3000ft elevation.

Edited by Zorac
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That sounds a LOT like what I saw-

I know everyone seems to like this carb, but I had

the transition problems you describe, too.


I'm not sure you can get it to be much better- I couldn't.


But do try lower fuel pressure- the 38 has a big needle valve,

and it seemed to be better lower.  I never had a problem emptying the

bowl, even on the track.



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given the minimal load, would the lean out when im cruising be an issue for the engine?  last thing i need is a hole in a piston.  plug are usually a little more grey/white than i would like when i pull them.


ill see if i can find a regulator that doesn't break the bank, might try smaller jets too (main, idle, and air).

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