Sudden leaning of mixture after short storage (40DCOE’s)

[sam1904]

All,

I have a 76’ 2002 that I did a 40DCOE install on around a year ago and installed an A/F gauge. The car has been running really well and other than a very minor ‘transition’ stutter sounds and runs fantastically all the way to red line. OK, it soundED, and rAN well until recently….

 

• The car was parked for around three weeks recently and prior to that was running great, A/F ratio from low 13/1 climbing to 15/1 ish at higher RPM
• After storage (or whatever happened) the situation has had a step change.
• The car now runs far leaner and although the change is not as significant at idle it now leans out to 17-18-19/1 at higher RPM’s (E.g. 4000) and especially in 3^rd / 4^th gear and associated speeds.
• As expected with this A/F ration the car stumbles badly above 3-4000, especially under load.
• Initially I had wondered if the change from 70F temps previously to 30F the first few times I had the issue created the problem given denser intake air. Subsequently I have had the chance to drive the car back in the 60/70F range and the issue persists
• I pulled the plugs and while they all look lean no single plug, or pair seem to be worse
• I pulled the jets and blew them out with no luck.

 

Before I dig further, and blindly does anyone have any suggestions?

Thanks as always,

Sam

[Son of Marty]

First I would soak the jets overnight in carb cleaner or denatured alcohol then blow them out I would also make sure you needle valves can move freely, the crap they sell as gasoline these days tends to leave varnish when it evaporates.

[2002iii]

Might also try getting some new gas. When the temperature changes they change the additives in the gas.

[Chris_B]

I would think zebras (fuel), not horses (ignition or jets). Things sound a bit lean. Check fuel supply (pump/filter). Hard to imagine both carbs going off at the same time, but maybe fuel level?

[Stevenc22]

Also check your float level. That has alot to do with the lean transition

[Hans]

Hard to believe gas would evaporate and clog things in three weeks. But nothing else has changed. So do jets,  spray carb cleaner into passages. As above, make fuel supply is good. How old is the gas in the tank? Maybe clogged pickup? 

[TobyB]

Did the plugs on the right side of the picture come from the same carb?

 

Because that cracking of the deposits is one indication of significant detonation.

 

Check your float levels, needle valves and check the little filters in the inlets of the carbs,

if you have them.  Also confirm that you're getting fuel flow-

a plugged inlet screen can deliver enough to idle, but not enough

to accelerate hard.  You'd see that as the a/f trending lean after

reading ok on initial launch- the car will run a LONG time on just the

fuel in the bowls.   And if junk gets into the bottoms of the bowls,

it'll cause problems for ages, since-

-it's hard to see

-there can be a lot of it.

And yes, older fuel (etha-suck) can formulate junk out of thin air, given time.

 

Related only by 'carb'- we have a carbureted lawn tractor, and it's 

had an intermittent fuel delivery since we got it (well used)

I finally got it to die repeatably, and found a small chunk of broken old hose

wedged in the fuel inlet, 

partially blocking the needle valve.  Blew that out and it's running better than

it ever did.  But that hose chunk had been in there for years...

 

t

[AustrianVespaGuy]

On 12/10/2021 at 7:43 AM, Stevenc22 said:

Also check your float level. That has alot to do with the lean transition

This, my bet is your float(s) got some fuel on the inside and is/are sinking a bit!