help needed: 502BBC engine problem
 

Hi,

I rebuild over the winter my 502 EFI engine. after refit in the boat it starts very good idle perfect and run up to 2800rpm or 40% throttle position, then it suddenly struggle, sound as a few cylinders are missing, no chnace to overrev this. if I take back the throttle the engine run fine again. it happens all the time under same condition. I checked
-fuel pressure (35psi)
-ignition timing (10 in idling)
-air fuel ratio (perfect)

the problem is the same if I rev the engine without gear in.
Had somebody a problem with the ignition module in the Deöco HEI distributor? If the problem happen the ignition timing jumps from 32° suddenly to 11°.

Hoping for help, no more idea what I could check :-(

Daniel

Check valve adjustment. If lashed down too much, lifters will pump up and hold valves off the seats, causing the miss you described.

thanks for your reply. I torqued the bolts of the rocker arms with 60nm as described in the Merc manual, did you think a bit less torque helps?

The rocker arms are non-adjustable, you have installed them correctly. I sounds like you have a scan tool, look to see if this is caused by knock retard to the ECM timing. Might try replacing the knocksensor, but they rarely go bad. I have seen worn shaft bushings in the distributor do bizzarre things to the timing, but I think your on the right track with the ignition module. Good luck.

check the firing order....had a small block that a friend redid and ran like you described...asked him a few times on the plug wires...after messing with it for a long time found 2 wires crossed..idled fine, no backfire....but would not turn RPM

I've seen it once also, I remember the wires were swapped on cylinders 2 and 4. It was barely noticable at idle.