Erratic timing and Vacuum

[Century Arabian]

Hey guys
Earlier this summer I installed a freshly rebuilt 350 in my 21 foot bow rider. I had a little trouble with a surge at around 1200 RPM. I replaced the distributor cap, wires and rotor button and the plugs. This did not solve the problem. More troubleshooting revealed a broken vacuum line on the Quadrajet vacuum brake. Replacing this vacuum line solved the surging problem, but at WOT the motor would start of strong and rev to 4200 RPM, but would only hold it for maybe 30 seconds or so and then the RPM would slowly drop off to 3000 RPM where it would remain. I changed the fuel pump, fuel filter, checked the filter in the carb and rubber fuel line from the tank, but the problem persist. I decided to recheck the timing and connected a vacuum gauge to the vacuum brake port on the carb as the manual states that the break operates on manifold vacuum not ported vacuum. The timing is all over the place, bouncing between 0* to 12* and the vacuum gauge jumps between 13 and 18. I'm pretty sure I have a thunderbolt IV ignition as there is only 4 wires coming from the plug on the module. Planning to get back at it again tomorrow, so any advice would be great. I have spent more time under the cowling than behind the wheel this summer and would really like to get this figured out before I get it ready for winter storage.
TIA Clyde

 

[Bondo]

Hey guys
Earlier this summer I installed a freshly rebuilt 350 in my 21 foot bow rider. I had a little trouble with a surge at around 1200 RPM. but the problem persist. I decided to recheck the timing and connected a vacuum gauge to the vacuum brake port on the carb as the manual states that the break operates on manifold vacuum not ported vacuum. The timing is all over the place, bouncing between 0* to 12* and the vacuum gauge jumps between 13 and 18.
TIA Clyde

Ayuh,..... Who adjusted the valves,..?? 'n How were they adjusted,..??

 

[Century Arabian]

Ayuh,..... Who adjusted the valves,..?? 'n How were they adjusted,..??

I had the engine rebuilt at a local well respected machine shop, they do a lot of work on Mercruiser motors

 

[tank1949]

I never heard of any vacuum advance on TB4. Learns something new...

 

[Century Arabian]

I had the engine rebuilt at a local well respected machine shop, they do a lot of work on Mercruiser motors

Did some investigating this morning. I pulled the plugs and tested the compression test on all cylinders:
Findings: # 1 cylinder, the center electrode was missing in the plug and there was no compression on that cylinder.
# 6 cylinder, the center electrode was missing in the plug and there was no compression on that cylinder.
All other cylinders had 150 psi and the plugs looked pretty good.
Going to remove the valve covers next to see what's going on there.

 

[Century Arabian]

I never heard of any vacuum advance on TB4. Learns something new...

Tank, The vacuum brake is on the Quadrajet Carb, When the engine is making vacuum, the brake prevents the secondaies from opening, once you go WOT, the engine does not create enough vacuum, and the brake releases. That's my understanding anyway. Sorry if I confused you on that point.
Clyde