Back in September, we took the 311 out and it ran great. Solidly into the upper 70s. Strong acceleration. Scared the Mrs. Perfect (not the part about scaring her).

Two weekends later, the boat was uncooperative and the port engine started sporting some strange AFR readings. AFRs in the upper 13s and getting into the 14s. Put her on the trailer and went after the carbs.

To note I’m running O2s on the even bank (I will pop the tail pipes off this winter and add bungs so that I can swap sides).

Carbs are Demon 850s (1000+ cfm) and now use aftermarket custom metering blocks. Current jetting is 84/90. Pv circuit is .063 (up .006 from .057). Stock jetting was 86/93 but we lowered based on changes we made to emulsion.

Today we ran the boat. We never took it beyond 3500 because we were worried about burning it down. Port side even bank was going into the 15s for AFR. Was hoping the original metering blocks had shaving issues. Guess not.

84/90 with 8.5 pv

Here’s what we gathered for data:

Starboard DLG-1 A

RPM / Vac / A/F-R

1000 11 12.6

1500 10 11.5 - Power valve beginning to open

2000 8 11.5 - Power valve should be wide open.

2500 7 12.2

3000 7 12.9

3500 5 13.5 (lean secondaries)



Port DLG-1 B



RPM Vac A/F-R

1000 11 12.1

1500 10 12.7 (where’s the power valve?)

2000 9.5 13 (no power valve?)

2500 7 14 (really no power valve)

3000 7 14.8 (umm)

3500 4.5 15.7 (ouch)


We didn’t run her harder. Interesting finding was that the port engine was running twenty degrees warmer (190 vs 170) so it seems that running leaner is affecting temperature by a good amount.

Based on the starboard numbers and speaking with my carb expert here in VB, we’re going to run jets two numbers higher and change high speed air bleeds to tone down the emulsion speed some.

Pulled plugs on 6 and 8 looking for a misfire (burnt plug wire grounding out) and they looked great. I didn’t pull the other side.

Intake leak with smaller jets making worse? Anything stand out? The engines were running pretty close numbers before it skewed.

Any thoughts on troubleshooting on the trailer? We won’t be back on the water until later March or April depending on weather.

If it was okay before and new issue, clean out every passage both directions with carb clean then same with compressed air.

Exhaust leak ? Ck and fix.

Swap 02’s see if condition follows 02 ?

Anithervthing, Misfires will typically make 02’s read lean.

You may have volume issue too, so change your needle and seats , check your fuel psi, and since your float bowls have glass windows for fuel level, put a go pro or telephone pointed at it to video your fuel level while your running into this issue. If volume still issue, ck fuel pump and fuel pick up area.

would have to agree with SB. if it ran fine one week and not the next the jetting didnt change itself so it is somewhere else. what ignition is on it? possible that the timing is not advancing for some reason which will make it run hotter. or one of the air bleeds has something in it. or as said fuel pressure is dropping off. bad hose or pump or even filter.

Yep, ran it Labor Day weekend and it was perfect. And then took it out for the Blue Angels the 17th and it was jacked.

I did a jet change on the primaries prior to going out on the 17th and ended up with this (on the old BG metering blocks - one jet size, 86 down to 85 just to lean it a tiny bit). The starboard side was okay (still rich) but the port was way off.

It’s going to be a bit before I can mess with it again. I’ll winterize the plumbing and change the oil and oil/fuel filters this week, fog it. I’ll yank the carb and go through it again and hope to be back at it in late March or so.

Side note… I swapped the O2s from one to the other and recalibrated. There was no change. Swapping carbs would have been a good test, too, but I didn’t want to get into that yesterday - was getting late and we were three miles offshore. As it was we had to putt-putt back in (load on plane was making her heat up pretty good).

Could be as simple as a piece of trash in one of the needle valves. I had it happen to me when I made a new fuel line.

I know at idle it’s showing good fuel pressure and the float levels are all good. Doesn’t mean squat though if it’s not filling fast enough. I’ll go out tomorrow and yank off to see if anything is in it. I’ve got a bunch of new needle and seat assemblies. Easy swap.

Coming back around to this now that spring isn’t too far away. I had surgery not a week after I posted this so carbs remained where they were. I did get to winterize (and when I did, both engines started like they were efi).

Plan is to yank the port carb and go through the whole thing to make sure there’s nothing causing an issue. I’m going to change the fuel filters.

Hoping to get here straight early so that spring is an easy transition to summer. It’s time to enjoy this thing!

Hope everyone is being successful with getting winter projects done.

Do you have fuel press gauges at the carbs?

I ran into low fuel press issues with my current boat when I purchased it. Found tiny shards of teflon tape inside the mechanical fuel pump check valves preventing them from sealing on both engs. Had to go thru the entire fuel delivery sys removing all remnants of teflon tape. Went back with liquid thread sealer.

I do have fuel pressure gauges at the carbs - trouble is that I can’t see them while running. With that said, everything was great until I screwed around with them so I’m thinking that something became dislodged somewhere and there’s now a blockage that I haven’t discovered.

No teflon tape anywhere - I don’t use it. Instead use
liquid Teflon where needed and those connections are few and far between.

Possible too that there’s a blockage at the tank or something similar. It is 34 years old after-all. Disconnect everything from the tank to the engine and purge/flush.

Had a well respected marine mech tell me at the time, never use teflon tape on fuel or hyd sys (P/S, trim, etc). Some get away with it if applied right but some don't..

Maybe a Go-Pro mounted in bilge to watch the fuel press gauges once you're back on the water? My old Go-Pro has a wifi broadcast function so phone can be used to monitor Go-Pro in almost real time (abt 1-2 sec delay).