Flat top end

[L-Robby]

I picked up my boat at the end of last summer. It needed a TON of work. My tach wasn't working, so I wasn't really sure of the RPMs I was running. I fixed the tach issue and was out on the lake over the weekend. My boat won't pull over about 3600 RPM. I'm going about 40-42 mph. I'm running a 13x13, 4 blade prop. Looking for a little help. I feel like the motor should pull more on the top end. I just rebuilt the carb. Have a slight flat spot when the secondarys open up. I think I need to adjust the accelerator pump a touch. Any help would be appreciated.

[csuggs]

Need to know year and model of boat, motor and transmission . . .

[L-Robby]

1986 comp. Stock 351. Borg warner 1:1

[csuggs]

Are you sure your tach is correct? With the set up you've got I don't think you can go that fast at only 3600rpm. Low to mid 40's is gonna be your top speed, but not at 3600.

[OUI]

My Saltare had a slight bend in the rear skeg. Because of the turbulence generated from that, I couldn't run anything other than a 14 X 14 without it over reving. Too much cavitations. I removed the bent skeg, put on a 13 X 13 and it delivers 4,200 to 4,400 rpm at a little over 42 mph. I’m happy.

[docdrs]

I picked up my boat at the end of last summer. It needed a TON of work. My tach wasn't working, so I wasn't really sure of the RPMs I was running. I fixed the tach issue and was out on the lake over the weekend. My boat won't pull over about 3600 RPM. I'm going about 40-42 mph. I'm running a 13x13, 4 blade prop. Looking for a little help. I feel like the motor should pull more on the top end. I just rebuilt the carb. Have a slight flat spot when the secondarys open up. I think I need to adjust the accelerator pump a touch. Any help would be appreciated.

First glance if rpm's are correct is you are over propped, but I don't know what your max wot rpm are suppose to be. But the speed sounds ok as above posts, but with a better prop you may increase that.

[cadunkle]

How are you measuring the speed? Gotta be sure that's accurate too? When I got my Saltare the speedos read over 10 MPH slow before I calibrated at slack tide on GPS.

If you're only getting 3600 RPM there's definitely a problem. We can assume your float level is set correct on primary and secondary side? What carb is this? 4150 or 4160? Double pumper of vacuum secondary?

What jets are you running primary and what jets or plate secondary? What is your initial and full timing? What RPM do you get full advance?

[csuggs]

I've driven two Supras with 351W, 1:1 trans and13x13 prop. Both run the same - 2000rpm = about 19-21mph; 2500rpm = about 24-26mph and so on. One of these boats is mine, one is a friend I run with regularly, and now it sounds like OUI has the same experience. That's why I find it hard to believe that you are running 42mph at 3600rpm. I gotta believe something's not indicating correctly.

[L-Robby]

The speed is fine. I'm not to worried about a lot of top speed. Since my lake I live on isn't that big. Both speedos read the same. I'll check my timing this weekend when I get back on the lake. The carb is a vac secondary. If the RPMs are normal with the prop I'm running I'll be happy. I'm going to try another tach also. Thanks guys.

[cadunkle]

I'd be more inclined to believe the speedos are off than the tach. Does it sound like it's turning 3600 RPM or 4400 RPM? Should be quite noticeable difference.

Given what we have, I'd assume you're actually running mid-high 30s for MPH and the engine just isn't revving as high as it should. If you have a GPS for your car or whatever, bring it on the boat and calibrate your speedos.

I assume when you rebuilt the carb you're running the same metering plate on the rear? Did you happen to catch the number on the plate? Or if the carb is a 4150, what jets and do you run a power valve in the rear? Your stumble when the secondaries open is not the accelerator pump since it's a vacuum secondary. Usually a stumble like that is caused by secondaries opening too quickly... Too light of a spring in the diaphragm. What spring are you running? Do the secondaries open smoothly by hand? We want to be sure they're opening fully and that is not what's limiting your RPM.

Do you feel any surging when WOT at 3600 RPM? If so it's likely running lean and needs more jet on the secondary side. If it doesn't surge, but you get some dark smoke out the exhaust it could be rich, and needs to go leaner.

So, basically confirm your speed and RPM, go over the carb to verify that's all good, check timing, go from there.