Hello, I re-built my carbs recently and now started to synch them and have some issues. Before the carb re-build I had issues with slight popping at light throttle around 2500-3000 rpm and a slowing idle when warm (this was the reason for the carb rebuild). Now after the carb rebuild with a cold engine it keeps an idle of 900 nice and steady but when warm it climbs to 1800 and starts popping through the intake. The floats have been set correctly are I've also double checked them, and the carbs have sealed bearings. Could this be a vacuum leak, or could this be caused by a sticky idle advance mechanism? When I installed my Pertronix ignition last year and adjusted the ignition I saw that sometimes the timing jumped a few degrees for a split second. This leads me to think that is something ignition related. If it is vacuum leak related, any ideas on how to diagnose? Any ideas on where I should look?
Check for vacuum leaks with a smoke machine https://www.matcotools.com/catalog/product/MDSMOKE/diagnostic-smoke-machine/ Carb synch with synchrometers, need one for each carb/throttle body https://www.amazon.com/Carburetor-Synchrometer-Weber-Dellorto-Mukuni/dp/B00SLH03R4
The fast idle mechanism is disconnected. That smoke machine was quite expensive. Aren't there other ways to detect vacuum leaks?
I use carb cleaner with tube and spray near carb base gaskets and vacuum hoses. Shoot, wait 10 seconds while listening or looking at remote tach. I even pull vacuum hoses, plug one end and use handheld vacuum pump to see if it holds vacuum. If you see cracked hose ends, replace with new. If timing is changing....Check your ignition System. Are you using dashpot?
When I rebuilt my carbs, I replaced the complete needle valve assembly. I also verified all the jets.
Needle valves have been replaced and I also checked that the close and no fuel is dripping into the throats.
You haven't mentioned anything about airflow rates, set these to around 4.5 - 4 kg/hr when the engine is at normal operating temperature and reset tickover to around 900 rpm when hot. My synchronizer is rated in kg/hr and i normally set mine around 4 to 4.5 kgh/hr If you just concentrate on purely air flow balance without keeping an eye on rpm (aim for 850rpm) then you have a tendency to open the throttle plate/s to far and thus bring in the 1st progression port. Under low operating conditions this will then advance the accelerator pump circuit /s and possible (will) flood the intakes with fuel (more than needed ) ie fuel dripping from the accelerator jets. This will give you snapping out of the barrels. I preset my throttle plates using a feeler gauge between the diffuser bore and butterfly. A .003 gap will get you in the right ballpark. Popping at tickover can be an indication of lean condition, at 2-3K rpm the idle circuit is just about dropping out so check idle jets. Make sure the bypass screws are not open when balancing initially as this can cause lean condition in 1 or more cylinders. If you need to adjust then do so, but in the first instance they should be shut.
Your symptom sound like it could be a vacuum leak. Areas to check would be the base of the carbs (can warp or loosen) base of the manifold (same issues), throttle shafts (wear) and any vacuum connections (power brakes, air cleaner flap, etc). Vacuum leaks like you are describing happen *after* warmup as metal surfaces can expand under heat and separate, causing a leak. My preferred detection method is a propane torch with the tip removed, flowing gas towards suspected leaks. The gas will make the engine rev higher if a leak is found, and not make as much of a mess as carb cleaner spray. Good luck!
Why would that be? And why would anyone be concerned with synch at idle? One synchs carbs/throttle bodies to get all the cylinders working together. This achieves more power. Are you concerned with how much power you make at idle? I never was. I always synched at open throttle positions. In theory it should be the same, but in practice it is not. There is always some play in linkages.
Yes its true that idle mixture is not applicable once throttle valves are open. But in order to properly synch carbs, the idle mixtures for each barrel must be synched. Mikias seemed to have idle issues so my guess was that he may have preset idle mixtures to lean.
Synch is most important at the lowest rpm and the lowest flow rates. A small air leak, makes a relatively bigger percentage increase in air available for combustion. Idle is were small leakages, make a big difference. Once above about 1800 rpm, carb leakage is no longer an issue, because it is small compared to the massive at larger openings. Synchronization is best carried out using an 8 bank manometer system, reading inlet manifold vacuum for each cylinder, simultaneously. Misfires are identified by a momentary drop, of the mercury column in my case. I think a point often overlooked, is that the idle mixture affects the beginning of the transition region. If one cylinder starts off too lean at idle, that cylinder will likely exhibit a misfire. Initial condition is important for every cylinder. Bill
I've got a Weber flow meter, carb rebuild kit, and alll kinds of jets (mains, AC, idle) if anyone is interested.
Short update on the very elusive problem: Once the car warms up the idle climbs to 1800 rpms still with some popping through the carbs. After revving the engine the idle does not go back down. However, if I shut off the engine and then directly re-start it while warm the idle stays normal. This is quite perplexing since if it was a vacuum leak the high idle should still be there when I restart the warm car, no? My needle valves are not leaking when there is normal fuel pressure on a cold engine that is shut off but could they start leaking once the engine warms up? Perhaps it is a sticky advance mechanism in the distributor that only manages to push the weights back once the car stops and doesn't manage to do that while rotating. However, the timing seems OK when using a timing light. I'm really really perplexed. I will try to recheck the timing advance mechanism and perhaps also build a home made smoke machine (as per youtube instructions) to verify that there are no vacuum leaks.
Further investigation shows that the ignition timing is stable when cold but starts to jump between 5-15 degrees at idle when warm. What could be causing this when I have a Pertronix unit? Can a bad coil (they are original) cause this type of behaviour?