Weber runs fine on main jets but is tough to start and bogs off idle

[twobone]

I recently went to a richer air jet in my Webers. I think my old one was 200 and the new one is 180. I believe the smaller number means less air passes thru.

 

My car sometimes stumbles off idle (for example taking off at a stop light...very embarrassing). She is also tougher to start when hot.

 

Does this sound like enough to cause the problem?

 

I didn't think that the main jet assembly had a major role to play in off-idle or starting

 

I will switch back when I get a chance, just curious folks thoughts

 

I have a long road trip (Ontario to Watkins Glen) and don't want to discover I have a bigger issue

 

Thanks

[m wirth]

Hello, in the middle '70's I put 40 dcoe's on my toyota 20r, and am still driving everyday with 597,000 miles, same carbs several rebuilds on the engines, and remember having teething problems about the same as yours. I got to experimenting with the emulsion tubes and looking at the position of the orifices and just thinking of how the fuel flowed, be one with the flow, and tried the F2 variety, even though everyone said they were for alcohol they worked perfect, no stumble you can mash the throttle in 4th at 1200 rpm and it will pull clean. just my experience

mike

[TheDingo8MyBaby]

It sounds to me like a problem with the transition circuit.

 

Try here:

 

http://autos.groups.yahoo.com/group/sidedraft_central/files/Weber_Tuning_White_Paper/

 

"Transition Circuit

The cornerstone of the Weber DCOE carburetor is the transition circuit. It

operates over a fixed rpm range designed into the carburetor, defined by the placement of

the progressive holes relative to the throttle plate. Essentially, this range is nonadjustablewe

must tune around it. This has two practical consequences for the tuner. One, it

defines a required idling rpm so as not to create an off idle stumble. Two, it defines a

transition rpm at which the main jet system must be activated to eliminate a flat spot.

To measure the true air-fuel ratio (AFR) of the transition circuit, we must disable

the main jet circuit by temporarily removing the emulsion tubes. If the floats are set

correctly (see Section 2) the fuel level will remain 2mm below the passageway leading to

the auxiliary venturi. Consequently, no fuel should flood into the carburetor via the

auxiliary venturi, so this shouldn’t be a safety issue. With the emulsion tubes removed,

fuel will continue to flow and the engine will run normally on the idle and transition

circuits as long as the throttle plates are not opened past the last progressive hole. This

occurs at approximately 10% of the pedal’s full travel. Driving the car with the emulsion

tubes removed provides a baseline for how the transition circuit was designed to perform.

Be careful when doing this test because you will be lacking about 90% of normal engine

power to get out of harms’ way.

Drive the car gently at a steady speed on level ground and note the AFR measured

by the air-fuel meter. Don’t move the throttle while taking a reading- the accelerator

pumps will shoot fuel and make the AFR reading inaccurate. Moving the throttle plates

past the last progressive hole will kill the engine so just don’t do it. Swap the idle jets

until the AFR is approximately 12.5:1.

The next test measures the transition rpm at which the main jet system must be

activated. Shift the car into high gear and slowly increase the engine rpm until the engine

dies. The maximum rpm at which the transition circuit keeps the engine running while in

high gear is the transition rpm to which you MUST tune the main jet circuit to begin

providing fuel. Typically, this target rpm is about 1400 rpm in high gear. The main jet

circuit must be contributing fuel at this transition point without the AFR deviating from

the desired 12.5:1 value. Any gap in fuel delivery between these circuits produces the

classic and much dreaded flat spot! At part throttle cruise in the higher gears the main jet

circuit is actually providing all the fuel so concentrating on fine-tuning its low rpm

performance is crucial to achieving the best tractability possible."

 

I suggest reading the entire document.

 

This yahoo group has a ton of excellent information on weber tuning. Keith Franck is a weber guru.

 

Also - if you have an aldon distributor, it could be the advance on the distributor as well. Caterham didn't appropriately spec out the advance. If you send it in to aldon in the UK, they can fix the problem for a modest fee.