What rpm should secondaries kick in at?

When I got my boat last year I don't remember it being very noticable when the secondaries kicked in on the carb.

This year after the engine rebuild it is very noticable. They kick in around 3800-4000rpm and there is a noticable difference in sound and increased acceleration.

What rpm should the secondaries kick in at? Is the 3800-4000rpm range normal or should it be lower?

Thanks

That's about where mine kick in and it just takes off and screams.

Now it's not really a function of rpm but manifold vacuum and the power valve you have that opened the secondary's and depending upon the load on the boat the RPM will vary above and below the 3800-4000 range.

What mod's did you make to your engine? verses stock config. I upgraded the cam, heads, ign, intake and balanced the motor and on mine, it just comes to life at 4k, it's like you down shifted to passing gear and it takes off, so you have to be prepared for it. Do you see an increase in water temp after running at WOT for a little bit?

Reid,
Yes, the secondaries work by load, ie. if you change a prop or add more power the secondaries can and in most cases will change the pattern when they kick in.. The power valve does not control the secondary throttle plates, only fuel when the vacumn reaches the power valve selected.
A spring kit will change when the back throttle plates open. To see this work hook up a vacumn guage while running the boat and see how and where it all happens.

AWhite70 - sounds like our engines are real close. I'm in the process of nailing down the right jets and have gone up eight sizes so far. Did you modify/change your carb or re-jet?

I had the 299 jets as well but I have a 6.5 power valve. You can use a vacuum gauge to set the idle mixture screws so that you get max vacuum at idle. Then to check the jets you need to read the plugs, get the engine hot run wide open for a while then while still under power kill the engine(turn off key) and coast to a stop, pull a plug on a short and long runner and if you have a nice lite chocolate brown color to the porcelin your good to go. If they are white it's lean, black or sutty then your rich. Just be carefull because if you are lean you will burn the valves.

SS 201
my carb is a single fuel inlet holley (i.e. non-double pump) with a primary metering block only. Is there a way to richen up the secondary's? Will changing the jets effect the secondary's on this type of carb? I have replaced the 66's jets with 69's and it was still lean on the top end and I have since replaced the 69's with 74's it still seems to be running lean and I'm begining to wonder if it is due to the secondary fuel passages limiting the amount of gas flowing throught them?

Bill,
Thanks for the great info on the carbs. And this unsolicited accelaration scenario did/does correspond with a higher pitch prop. Next question, is a surge at cold idle most likely a vacumn leak or could it be anything else. This rebuilt engine has done this for several years and it goes away once warmed up. Can I look for anything besides a carb base or intake leak?

Last I did one it cost about $50.00 about 12 years ago. You the throw away the plate and add block, then go jetting.
Again with all the additives in gas it takes a long time to get color sometimes never. Again I would add octane booster to get color on the plugs to see where you are at. With the cam change you say you have, at least 69 front and 77 rear, power valve not less than 6.5 but again you should put a vacumn guage on to see what your vacumn is where you want it to open.

Thanks for the advice on the carb SS 201.

Depending on the carburator, I believe that they are vacuum/linkage drive. Because of this, it depends on how hard you put the throttle down. Mine will open at a much lower RPM if you nail it out of the hole. You should hear a throughty sound when they pen and get quite a punch. If you think that they may not be opening, you can check the vacuum line and diaphram. If you suck on the hose, it should activate the linkage cam. If it slowly moves back into position, then you might have a leak. You can also check all of the linkages/cams on the carb for binding. If it is binding you can usually use some carb cleaner to clear it up. One the Rochester 4Bbl carbs, the secondary jets hang down from a cam on the top of the unit. You can see thing when you remove the flame arrestor/air filter. If it is sticking, just clean it and maybe give it a small dash of light lube.

Otherwise, there is probably something else more complicated going on, which you might want to have a good CC mechanic look at.

Hope that helps.