Carb Issue/backfire - Merc 350 Mag (1990)

[cwburkeva]

Carb issues

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.

As of yesterday, I played with timing - advanced to 13*. No real change in symptoms. I did notice this time that I had a tapping from starboard side valve cover - by the time I got to the ramp at virtually idle, tapping has stopped. I had not heard this before.

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?
4. Valve issue?

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.

 

[FreeBeeTony]

My $0.02 on this issue:

1) idle adjustment has no affect above idle RPM's
2) backfire through carb is usually a lean condition
3) the noise you heard when you increased the timing was probably detonation.....NOT a good thing, go back to 8*BTDC and leave it there.

 

[cwburkeva]

Thanks FreeBeeTony:

1. I have idle set at 700RPM - idle screws I am referring to are the idle needle screws.
2. Adjustments are being made on the two idle needle/air mixture screws at base of qjet. I have adjusted in 1/2 turn increments from setting needles lightly and out - each time testing throttle in drive - carb backfire does not cease at any adjustment.

3. Noise was definitely not detonation - I could feel the tapping on rear section of valve cover, didn't have my stethoscope to hear exactly which valve was tapping. Timing was set back to specs at that point, which I didn't mention.

Any other ideas?

 

[FreeBeeTony]

The idle/needle/air adjustment screws are what I was referring to....they have no effect above idle RPM's.
Do you have a water separator/fuel filter? If so, I would replace it and don't just empty the old one and re-use it.....put a new one on.
Not familiar w/ the Rochester carb......is there a screen or filter at the fuel inlet?

 

[cwburkeva]

Replaced fuel separator and the filter cartridge when I rebuilt the carb, emptied fuel tank and replaced fuel pump along with anti-siphon valve.

Everything I was diagnosing was based on lean condition.

I'm going to pull plugs to inspect, perhaps I have a bad plug - even though new, perhaps I have a ground electrode that bent when I put them in. Also going to test plug wire resistance to be sure, again, new plug wires.

I also thought about pulling distributor and starting from scratch, reset engine to TDC and reinstall. It's pretty hard to get distributor in wrong gear since it has a notch at the end of the shaft that seats in the receiving end in the engine. Unless it's completely 180 off.

 

[Rick Stephens]

Tony is correct. Idle mixture screws are inactive once the throttle plates open. Timing is typically the cause of backfires - means a plug is firing while the intake valve(s) is open. Can also happen when plug wires are incorrect firing order.

1) Check firing order.

2) Timing set exactly according to the book

 

[harleyman1975]

only other remote possibility would be metering rods but not sure they are adjustable in a Q-Jet. Look for plug wires crossed (2 & 5 are easy to mix up)...engine will still seem to run "normal".

 

[cwburkeva]

When I get to the boat tomorrow I will double check plug wire order. THAT WOULD BE SO NICE!! Its like that almost too good to be true feeling. If not will delve into other stuff.

 

[alldodge]

My guess is you have some tulip valves from to much timing and a lean condition. I would suggest a compression test

 

[79_banshee]

Could be TDC on the exhaust stroke but it probably wouldn't run very smooth even at an idle. I had a 98 Chevy with the 350 and the distributor gear had worn out so much that have the teeth where missing. When I replaced the distributor I ended up with it TDC on the exhaust stroke it did run but backfired and popped as soon as I gave it an gas. If you run out of options it may be worth taking the valve cover off and making sure your on the intake side of TDC. Good luck

 

[cwburkeva]

Hello Everyone!

First and foremost, I really appreciate all the tips and support on this issue.

PROBLEM SOLVED (special thanks to harleyman1975)!

It was such a stupid fix I feel embarrassed about it, perhaps that's why many people don't post how they solved a problem. Anyway, here goes:

First thing I did was track all wires from plugs to distributor cap. Found #4 and #7 crossed on cap. Reversed and voila! Runs like a super champ! Even though it did run 100% better before changing ignition parts, this was a game changer! Runs even better. No backfire through carb, runs smoother than a kitten purring. Also was able to achieve another 600 RPM, went from WOT @4200 to WOT @4800, and that wasn't even with the throttle all the way down (need to figure how to put a throttle stop or adjust so you can't open it up even more.)

I did notice, and this is probably just adjustment, tapping of valves after running it for a while at upper RPM's. The other day I heard what sounded like a single tap, but now it sounds like multiple coming from both heads. Hoping it just tightening down the rocker arms with appropriate lash.

Thanks again for the tips! Wish I had a staples "that was easy" button!

 

[79_banshee]

Right on. The easy no cost fixes are the best

 

[cwburkeva]

I should also note, that what got me turned around on this (probably) is that I replaced my cap with a Sierra Marine cap. It has the cylinder number embossed above the post, however they also have the opposite cylinder number embossed above the lower number.

The lower numbers are for a clockwise rotation, the upper numbers for a counter clockwise rotation. I suppose their are those out their??? Anyway, the post for #4 also has the #7 embossed, and conversely for #7. So basically I followed all the lower number except when my brain farted when it hit #4 and/or #7 I didn't process and switched.

 

[Bondo]

and that wasn't even with the throttle all the way down (need to figure how to put a throttle stop or adjust so you can't open it up even more.)

Ayuh,..... Welcome Aboard,..... Ya put a bigger pitch prop on the drive to lower the Wot rpms,.....

The lower numbers are for a clockwise rotation, the upper numbers for a counter clockwise rotation. I suppose their are those out their???

All of our motors are counter-clockwise, left-handed motors,......

Right-handed clockwise motors are gettin' rarer than hen's teeth,....

 

[cwburkeva]

Ayuh,..... Welcome Aboard,..... Ya put a bigger pitch prop on the drive to lower the Wot rpms,.....

I'm running a 19* pitch prop. Was told many years ago that was appropriate size for this 350Mag-AlphaOne; I don't ever recall exceeding 4800RPM @ WOT and I've had this for 14 yrs.

 

[Bondo]

I don't ever recall exceeding 4800RPM @ WOT and I've had this for 14 yrs.

Ayuh,.... Hard to believe it's been runnin' so poorly for 14 years, 'n now suddenly has more horsepower,..??....

How beat up is yer prop,..??

Boat motor top rpms at Wot are limited by the prop ya put on it,....

 

[cwburkeva]

Well, I wouldn't say it ran poorly - perhaps less than optimal.

Prop is in really good shape, some scratches. It a PowerTech Prop, can't remember the model, but I remember paying about $300 for it.

 

[NHGuy]

A couple hundred rpm over 4800 won't really hurt anything. But with the design of the rest of the engine it will start to run out of breath up there. Get you a 21 pitch prop or maybe a 4 blade 19.
How big, heavy, long is your boat?

 

[Baylinerchuck]

I laughed a little when I read your fix. I did the same exact thing on my 4.3L LX with a new distributor cap. I accidentally swapped 2 & 6. Clockwise and counterclockwise number stamped like yours. Had me ready to sink my boat....felt like an idiot when I confessed my sin to this forum. My buddy still rags on my about that!! Lol.

 

[Pusher]

I don't suposse the throttle linkage has a stop knob you turn out on it does it? May not be an inboard thing.