Hello:

After 4 1/2 years of not having the boat out, I decided it was time. I will provide a bit of info of what I have done and where I am struggling at this point.

- Old Gas removed from tank.
- failed fuel pump; replaced. It was a mechanical and replaced with same.
- Total overhaul on ignition - spark plugs, wires, cap, rotor, coil, and ignition sensor.
- Rebuilt carburetor - Rochester Quadrajet with divorced choke. All internal parts with exception to the idle needles and float - all rebuilt within specs.
- Replaced fuel hose and anti-siphon valve.
- Timed at 8* BTDC - according to specs for a V8 (350 Mag)

When starting cold, engine idles (set at 700 RPM) just fine, when engine is warmed up (5 -10 min), engage full throttle and all is good. Smooth engine operation and can get full throttle to about 4200 RPM. After running at full throttle for about 5-10 min, pull back to idle. Upon throttle up, I then get backfire through the secondary on the carb. Only get a few pops and a bit of hesitation. Once I get through that, by throttling more, it runs normal again at full throttle.

At that point, I tried to make adjustments to the idle needles to lean out the air/fuel. No change. Made minor adjustments from 1/2 turn out and tested running each time I made an adjustment. I did this until I was 4 1/2 turns out on each idle needle. No effect on backfire.

Went out for a longer run, 20 min at 3200 RPM; when I got close to ramps, went back to idle, then wanted to test a theory about engine hot condition. As suspected, backfire was worse almost continuous, but also could not get up to regular operation; it bogged down as if too much fuel/flooding.

I am at a loss before I decide to take it in the shop or even more, spend $300 on a new carburetor to find that it is totally unrelated.

So here are some ideas, but want to see if I might be off base.

1. Should I have changed the float??? Float level seems to be within specs.
2. Is this timing related? I am thinking of advancing the timing 1* to see if this stops the backfire. Also I did take distributor out - set engine to TDC and marked distributor. When reinstalled, I thought it was tight and slipped off gears, but when I looked at the crankshaft it was slightly off TDC, so set again and reinstalled distributor. I am pretty sure it wasn't set on a different cylinder when I re-installed, but did not dive into that.
3. Could the metering rods be the cause? Too much gas?

I think I got everything covered, but questions may spawn memory of something I may have missed in mentioning.

Any ideas would be appreciated.

Thanks.

take it to a good boat mechanic and see what he [or she]thinks.

Reversion? Flappers? Risers?... check those

A pop through carb is a lean condition. It sounds like it's in the secondaries.....but, if this happens when hot check the engine grounds. If they are corroded, the resistance will be higher when hot. Also, check the fuel vent on the tank for debris, along with the fuel lines for rotting from ethanol based fuels.

John I Agree, mostly. Popping thru intake=lean/popping thru exhaust=rich. I am thinking an accelerator pump. When cooler the choke could be masking the problem and when warm with choke fully open problem shows. Just my thought I am by no mean a carb expert.

Ummm, your timing light would ultimately know. What did you set initial at ?

You definitely should have replaced the needle and seat.

Did you replace/check the fuel filter in the carb ? I ask because many don't know it exists.

I would take carb back apart again, recheck things, and give it a good cleaning out, followed by blowing thru passages with an air chuck. I've had to do this a few times with carbs that where gummed up by E10 jello schit.

I had a similar problem when I rebuilt the quadrant carbs on my old Scarab that sat for a few years.

I bought carb re-build kits that contained plastic floats. They kept absorbing fuel and would get heavier causing fuel to flood the engine. I ended up buying brass floats and that solved the problem. Also had to clean the carbs a few times to get rid of all the varnish and crud from the fuel sitting there for years. You may have to change the carb filters out also a few times.

quadrajet carbs have a secondary air flap that is adjusted by spring pressure,if adjusted to light it will bog and sometimes lean pop.when these carbs are working properly they are great.

I had a Quad issue while I was driving a Chevy Box van, pulling an F1 Tunnelboat in Missouri. I called up a drag race buddy and he directed me to a shop in Fenton area. This shop built NHRA Prostock carbs. he didn't just rebuild the Quad, he blueprinted it while I waited.

I swear..... after installing the thing, I thought I had a supercharger added. What a difference when you blueprint a Quadrajet!!

Isn't that a vacuum secondary? Check the little vac line that opens the secondary. 10 degrees initial works well on the 350 mag. Use your timing light to make sure you are getting advance out of you distributor.