Hello, I've just finished a complete maintenance of my engine, carb are fully rebuilt, now it runs but the cylinders 5 and 6 (one carb for both) is doing like a "blop blop blop" when idling. I've check the compressions : near 11 bars, the fact is my flowmeter is giving between 2 and 4 kg/h for all other cylinders but near 10 for n°5&6 :-( As the sync is not made but the value too much high for idling at low speed and the flap seems a very little opened (less that 0.5 mm) whereas the throttle stop screw is off, the mixture screw are 3 turns off, I've not idea of what can be the main cause ? And if the flap are the cause, what to do (please not to remove all the carbs :-( to check flap position). Does anyone have a good idea ? Thanks.
This must be corrected/balanced by adjusting the various linkage adjustments and the air bypass screws (and should be expected to need to be done whenever the carbs are removed/reinstalled -- even if the carbs themselves weren't touched at all as slight shifts in their physical positions affect the linkage adjustments). Have you read Birdman's write-up on the procedure? http://www.birdman308.com/service/sync/carb_synch_tutorial.htm On a carbed euro 308 like yours (some BTDC timing at idle), each barrel should be in the ~3.5 Kg/hr ballpark at 1000 RPM warm idle, and probably no greater difference than ~0.25 Kg/hr from highest to lowest barrel flow. The worst case would be, if with the air bypass screws fully closed, the linkages disconnected, and the throttle stop screw fully backed out, you still had too high an airflow into a barrel, or pair of barrels, that would indicate something mechanically wrong with the throttle plates -to- throttle shaft -to- throttle bore fit(s). PS Interesting to see Weber carbs that have the provisions for adding the snap-on plastic anti-tamper cap to the mixture screws and the throttle plate adjustment screw -- can you give the "series" marked on these carbs (would be something like 72/73/74/75 - with one carb marked 72, another 73, and so on.) -- might be stamped into the large outer face on the side of the float area.
Thanks for your answer ! Yes, I've read the Birdman's tutorial, a very good material ;-) On natural light, it appears that the throttle plates are not fully closed on this carb (on the table it was ok, maybe not so much to adjust), I think it is the problem, so I fix it by unscrewing this carb, not an easy job !!!! My car is an euro one, build in December 1978, on the rear side of the carb you have : 40 DCNF 60 150 7L Regards.
Are all four carbs marked the same? Your 155/78 OM indicates the 57/58/59/60 series in the specifications.
I don't remenber if all are the same but I think so. I've unscrew the involved carb to check the throttle plates that seems to be ok but I re-adjust to be sure and then I put it on the engine and adjust very precisely with the other carb. Engine started : it is ok, flowmeter agrees with my ears ;-) So it was probably a very little cause in the mechanical adjustment that was the problem ! Regards.
When you say blop is it like a popping noise as I have same issue. Almost like a intake valve coughing back
