I Need The Carb Whisperer

I'd like some carb advice from the gear-head experts. Engine is a 1994 351W that has been recently retro-fit back to carb & distributor. Carb is a Holley 4160 marine (600 cfm). (Pro Boss model with GT40 heads)

The engine runs very strong and I'm quite pleased with the performance after the retro-fit. It makes me realize just how poorly the boat was running with the failing Pro-Tech TBI and ECM.

I believe that my initial set-up is "spot-on" with proper timing, new plugs, wires, etc. I believe that the Carb is MOSTLY dialed-in. However, I'm getting a 1/2 to 1 second "bog" or "stumble" or hesitation when I go from idle to wide-open-throttle. After the initial hesitation, the boat takes off like a scolded dog. No problems with bogging or hesitating during RPM changes once the engine gets past the initial hesitation. Throttle response excellent and with current prop I can reach 4,500+ RPMs.

My initial trouble-shooting thoughts to fix the "hesitation" are: "Accelerator Pump" circuit". The linkage is properly adjusted and I ran a 0.015" feeler gauge to ensure proper fitment. I pulled the squirter and it is a 31 standard by Holley. The accelerator pump cam on the linkage is orange with the screw in hole #2.

I'm tempted to see what happens with the next size up squirter (35?) and see what happens. Perhaps even bump the screw from position 2 to position 1.

Am I chasing the wrong solution?

Thoughts? Advice?

JQ

If you adjusted the lever like the below video, you should be in good shape.

Accelerator pump arm adjusting

Have you replaced the pump diaphragm recently? If not, it wouldn't hurt to replace it and see if your hesitation (lean spot) goes away.

The squirters are not that expensive ($9.00), it wouldn't hurt to have a 35 or 31 in your back pocket if you still can't get this lean spot to go away. Is your current squirter a 25?

What do you mean by position 1 and 2?

mamigacz wrote: Have you replaced the pump diaphragm recently? If not, it wouldn't hurt to replace it and see if your hesitation (lean spot) goes away. What do you mean by position 1 and 2?

1. Pump is new and appears to be working normally.

2. Accelerator Cam "inserts". Holley has different cams with different profiles (lift, duration, etc.). The various colors represent the different profiles. The cams have 1, 2 or 3 holes for the mounting screw to connect to the linkage assembly. The different mounting holes (labeled 1, 2 or 3) allow for additional tweaking of the cam profile. If memory serves, hole 1 is for low RPM idles (600-700) and hole 2 is for higher RPM idles (900-1,0000). It is my recollection that hole 1 gives the fuel squirt sooner and shorter. I may need to do a bit of refresher research.

JQ

Jonny Quest wrote: JoeinNY wrote: Just how did you use that feeler gauge to check the linkage? You need make sure it doesn't bottom out at full stroke - but there should be no play at idle. You shouldn't be able to perceptibly move the throttle before it starts squirting. If that doesn't work you should make sure your idle circuit is properly tuned. I disconnected the throttle linkage and opened the carb up to full throttle, then I slid the feeler gauge between the linkage and the pump lever arm. The feeler gauge had a snug but not too tight fit. I believe that the idle circuit is properly tuned -- I used a vacuum gauge per Holley's recommendations. JQ

Just want to highlight Joe's response here. They key to tuning out accelerator pump off idle hesitation, is when the throttle is in the idle position. If there is any kind of up and down play in the acc pump lever when the carb is at idle, then the pump squirt will be just the tiniest bit delayed on the squirt, and you'll get a hesitation.

Half turn increments are huge - the fact that it would be best in the ball park of 1.5 is not surprising based on my experience, but it might be better at 1.375 or 1.625. Get them equal and exactly where you get the highest reading in gear and go skiing.

To find the "ball park" I used the 1/2 turn increments. I did utilize 1/4 turn increments at 1, 1 1/4, 1 1/2, 1 3/4 and 2 turns to find the "sweet spot". Per the vacuum gauge, 1 1/2 was indeed the spot. I was just a little surprised to find my initial starting point to end up as the dialed-in point.

JQ