Weber carbs out of control - follow-up

[porschepalle]

Hello again ....

By setting the idle speed between 850-900 rpm with the motor warm solved the problem re very high idle rpm (>3000). Thanks

However, when the motor runs warm it sounds and feels as if one cylinder drops out: less power and more vibrations When the motor is started in cold condition later on, it runs relatively well, i.e. the car accelerates swiftly to 70 mph. - and then the problem starts over again. It may happen that the motor suddenly gets its power back and then again lose it - an on-off symptom.

I am aware of the fact that one symptom may have different causes. Does the weber deliver too little fuel to the engine after running a while, or...? What do you suggest?

Regards Palle

[WFBowen]

Pale
Do your carbs have heat isolators on the manifolds?

[Peter Graham]

First thing to do is check the float levels, but the issue could be jetting. Without being able to track CO2 levels/AFR % driving or on a dyno, changing jets would be a guessing game.



(BTW, I believe the picture should be annotated '40 & 44 IDA and IDF Series)

Peter

[Jaems]

Did you ever check your valves adjustment?

[porschepalle]

Yes - valves have just been adjusted.....
Palle

[porschepalle]

No heat isolators on the manifolds - so.....(?)
Palle

[porschepalle]

Jetting.....do you suggest the fuel mixture is too lean - or?
Palle

[Peter Graham]

An engine will run lean (and hot) but will eventually destroy itself. On the other hand you can drown an engine with a rich mixture and it will continue to be pretty happy (as long as gasoline does not make it into the oil). Without knowing the particulars of the engine build, the altitudes at which you drive and gasoline formulation(s), there's no way to even guess at jetting.

That said operating at up to 6800 feet in elevation with 10% ethanol gasoline and a big-bore motor (no other mods), I was running F7 emulsion tubes with 195 Main Air Correction jets, 130 Main Gas jets through 32mm venturis. (356/912 Weber 'kits' usually have 28mm venturis/F11 emulsion tubes installed.) I converted to OE Solex 40PII's several years ago for better performance and gas mileage.

Recent thought: 'dropping a cylinder' may be a fuel supply issue. Check your pump output (should be ~2 psi) and for crap (rust, debris, deteriorating rubber hose) in the fuel system. Check for rust in the gas tank/clogging of tank pickup filter screen.

Peter

[porschepalle]

Thanks Peter for these useful informations - I'll go on and solve this problem - hopefully.
Palle

[NICE 69 S]

Don't remember if you have taken the carbs apart and cleaned them yet. If you haven't then that would be a good idea. Also check the idle jets and make sure they are clean. Most of the time you are driving, the engine is running on the idle jets. Mains only kick in when you open the throttle. A clogged idle jet makes the engine run like you have explained above, like it is on three cylinders. If it runs better when you step on the gas, probably a clogged idle jet.
Bob B

[porschepalle]

Thanks Bob. You are right. Last weekend I began to check the needles for idle mixture. One was rather rusty! - The rest OK. Now I will take the jets.
Palle