Slightly lean weber 32/36

[tweety]

Ok, many of you have been following my other weber threads.

 

Been on a 300 miles run with the 32/36, ea81 auto trans. Economy is good (25mpg US gallons), performance is magnificent. Spark plugs upon returning home are white i.e no brown at all meaning lean mixture. Carbie was tuned with Gunson colortune device that I found exceptional.

 

Idle mixture screw is turned out 1 2/3 turns out which is ideal meaning idle jet is correct (60)

 

My options are-

 

turn the mixture screw out say 1/4 turn to richen up the mixture all round at light throttle

 

Go the larger idle jet (65) which I've tried once before and it results in the mixture screw turned out 1 turn only meaning its a bit rich that jet.

 

One size larger on the main jet (145) to 150

 

Leave it as it is.

 

 

Your thoughts form your experience?

[coxy]

Hi tweety do not change the idle jets it sounds as if they are as close to perfect as you will get, If as I suspect it is slightly lean on the prinary 32mm throat then do not change either pilot jets.

 

What you need to do is play with the main jet, Air corrector jet combination this is where everybody stuffs up with carburettor jetting note the following explanation.

 

Idle jet works at idle and up to roughly 1/8 throttle on a weber carb.

 

Main jets this is where the mistakes are made by most people because all the majority of people change are idle jets and main jets.

 

There are also emulsion tubes the ones the mains fit into at the bottom and the air correctors fit into at the top, there are options for these even with the 32 36 carbs though many more with DCOE side draught carbs, Generally with a aftermarket downdraught they will be somewhere close ex factory.

 

Where you will find a difference is when you learn how main jets and air correctors work inj real mlife and it is not like most people assume.

 

Main jets are not all about top end rpm tuning far from it the Top end is a combination of main jets and air correctors and the main jets actually start working from around a quarter throttle or sometimes less the Air correctors tend to work from higher up ijn the rpm range and they work in concert.

 

So when you find a main jet that seems perfect at high rom but is say too rich lower down in the rpm range then what you do is go to a smaller main jet till the lower midrange mixture is correct and that now means the top end will be too lean, That is then fixed by going to a smaller air corrector jet which allows the smaller main jet to supply the correct top end mixture due to the smaller air corrector restricting the air flow into the emulsion tune thus making the top end richer by the restriction of a smaller air corrector.

 

Conversely if too lean then the opposite applies, go to a slightly bigger primary main jet in the 32mm throat that will richen up the main circuit but with more emphasis lower down in the rpm range remembering trhe Air corrector on top of the emulsion tube is there to dial in the top end in concert with the main jet.

 

So when it comes to the main circuit main jets are more effective when changes are needed lower in the rpm range and air correctors are more effective at the top end of the rpm range, Basically the air correction is more effective when more air is moving through the system and that occurrs at higher rpm so the main jet while it works across roughly three quarters of the rpm range is more effective from about one quarter throttle to three quarter throttle than it is at top end the top end is a combination of both air corrector and main jet but the air corrector has a greater effect at higher rpm than the actual main jet.

 

For ideal results they must always be considered as a pair working together with the main jet doing the early heavy lifting and the air corrector doing the heavy lifting higher up the rpm range.

 

In your case I would go up on the primary main jet say 5 at a time to test so if you are running say a 105 main jet in thye primary go to a 110 that will fix the lean issue on lighter to slight acceleration loads in the mid range but probably then end up too rich up top requiring a smaller air corrector,If the air corrector is say a 165 then going down to say a 145 will balance out the change in main jet at higher rpm, A good starting point is one size change in the main and then two sizes in the air corrector.

 

This combination as an example would richen up the engine around one third throttle and the change in air corrector would mean the top end mixture if correct previously will end up the same  as it was with the older combination of main jet and air corrector.

 

A change of one size with weber mains is considered as five numerically so 105 to 110 is a change of one jet size, With Air correctors a change of one jet size is considered to be 20 so going from say a 180 down to a 160 is considered a change of one jet size.

 

The only area where you can be caught out with the air correctors is in a very rare situation where the Air jets become too small and that means below fifty numerically because that can then mean there is not enough air flow to properly mix the fuel and air otherwise known as emilsify  the mixture which is why we have a tune with holes in it called an emulsion tube.

 

This is how emulsion tunes work they can be the hardest thing to get right if they need changing but will make a massive difference especially onj the DCOE dual barrel side draught carbs they have a series of holes for the air to enter they can and do vary in bothy size and number as well as placement along the tube all of which controls the emulsification of the air and fuel, Then on top of that they can and do also vary in diameter with a smaller diameter emulsion tube meaning it has abigger reservouir of mixed fuel to draw from which can stop the engine suddenly going lean in some situations when placed under sudden hard acceleration and heavy load at the same time, The accelerator pump alone cannot cope in such situations hence the different diameters in emulsion tubes.

 

But as I said generally with a downdraught such as the 32 36 the factory supplied emulsion tunes are close enough to make it work Ok so the focus is more on actual jets and air correctors.

 

If however after getting it running correctly when you open only the primary carb throat and it suddenly bogs down as the secondary throat opens you then need to go through the same process for the seondary throat as for the primary throat jetting wise.

 

So now I have hopefully explained how the system of jets and air correctors interoperate with each other it should allow you to decide exactly at what rpm and load something needs changing and as you have already found out it is lean now you should have a better idea on which combination should give you the result you are looking for.

 

Considerably more detail here, Including a chart showing how the different circuits overlap and work together

http://www.datsun2000.com/tech/weber_tuning_methodology.htm

[tweety]

Thankyou. I will read that a few times and the link. Much appreciated

Good reply for many other owners tot