New longblock with ~15hrs low speed miss .

[mikeneal]

I installed anew long block about 15 hrs ago ( stock 350'to replace 5.0GL ) and last night took boat out ant in no wake zones would develop a miss after a bit . Seemed to idle fairly smooth and if I advanced throttle it pull clean and ran fine . Its stock 350 with thru prop exhust and (a Holley 4175 vac secondary carb now). When I started it that morning I pumped throttle a couple times and it backfired once but then started. What are symptoms of hurt power valve? Can backfires still hurt those? One terrifying thing is at ramp when I tied boat up and went to get truck when I retsrateted it turned hard for a split second then started and I thought maybe water in cylinders. But started fine at house to flush it and I started it 3 more times since and no issues and cranks fine. I had underwater lights and stereo running so may e just battery was drawn down ). Exhust is about 1.5yr old and tested it with alchol when I did motor. I just restarted again on nose and cranked fine, but it seemed to load up and start to run Rough at idle, if I advance it to say 1200-1500 Roma for a bit it clears up. I will pull plugs tomorrow. Carb is about 8 yrs old, ( put it on old 5.0 for little extra power) maybe rebuild it and replace PV? Sorry for long post.

 

[Scott Danforth]

if your carb is newer than 1991, there is a backfire protection check valve in there to prevent rupturing the power valve.

backfiring when starting is a sign of incorrect timing.

if the power valve is gone, it will run really rich below 2000 RPM

I would look at your distributor cap and rotor if you have a slight miss.

 

[mikeneal]

It only backfired that once but I thought I heard all newer carbs had the PV protection. I set timing when I dressed the longblock but can't hurt to double check it. I think cap and rotor are about a year and 100hrs old but can look at that too.

if your carb is newer than 1991, there is a backfire protection check valve in there to prevent rupturing the power valve.

backfiring when starting is a sign of incorrect timing.

if the power valve is gone, it will run really rich below 2000 RPM

I would look at your distributor cap and rotor if you have a slight miss.

 

[Scott Danforth]

Holley started using the check valve in 1991, so anything newer would have it

 

[mikeneal]

Carb acts weird, I can turn the air mixture screws fully seated and it actly idles slightly better. They typically like about 3/4 turn from seated and are very sensitive.
I ordered a rebuild kit for carb from local speed shop and pick it up tomorrow. When I got home from work pulled spark plus out and they were fairly black sooty and smelled like fuel. No water on them at least best I could tell. PV is currently a 5 and plan on replacing it with that but may buy a couple others when I am picking up rebuild gasket kit.

 

[mikeneal]

So I dropped it in the water this afternoon and carb rebuild didn't fix it. But had more chance to troubleshoot and brought more tools. While in a wide open part of ICW I set throttle at the setting where it chugged the worst and removed plug wires one at a time and when I got to # 7 no change. Had a timing light along and when I checked it the spark was intermittent on that cylinder at best. Pulled cap off and it looked like death. Wire brushed it as a bandaid and it ran smooth and had good spark according to the light. Cap and rotor are about a year old and have 160 hrs on them and don't know why they look so bad but I bet anew one fixes it.