What rpm should secondaries kick in at?
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=594
Printed Date: November-22-2024 at 3:40am
Topic: What rpm should secondaries kick in at?
Posted By: Guests
Subject: What rpm should secondaries kick in at?
Date Posted: July-07-2004 at 5:12pm
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
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Replies:
Posted By: SS 201
Date Posted: July-07-2004 at 7:13pm
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If you have vacumn secondaries, get a spring kit and change the spring according to table enclosed by color. Everything changes by the amount of vacumn created at different RPM under load, causing the secondaries to open.
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Posted By: 79nautique
Date Posted: July-07-2004 at 7:14pm
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?
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: reidp
Date Posted: July-08-2004 at 1:06am
Bill Burgess or anyone,
Would a spring kit change fix a problem I have with a boat that once it starts to gradually get into the secondaries it will continue to open them completely without giving it any more throttle?. In essence you can't run the boat at a steady 80-90% throttle setting without it pulling the throttle nearly all the way open (the vacuum secondaries, not actual hand throttle setting). Is the spring too weak?
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ReidP
/diaries/details.asp?ID=231&yrstart=1971&yrend=1975 - 1973 Mustang
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Posted By: SS 201
Date Posted: July-08-2004 at 8:26am
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.
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Posted By: Guests
Date Posted: July-08-2004 at 11:10am
79Nautique-Mine seems to behave the same as yours, it almost feels as if you downshift. It wasn't as noticable of a change before the engine rebuild. I don't notice an increase in water temp. I run a 160deg. stat and my temp fluctuates from 160 to 180.
My engine has an upgraded cam & intake manifold, heads are ported with 1.94/1.6 valves and roller rockers, Compression is up a hair to 8.7:1, I also added an GM style HEI distributor. I'm guessing she's putting out around 300 hp.
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Posted By: 79nautique
Date Posted: July-08-2004 at 12:01pm
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?
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: Guests
Date Posted: July-08-2004 at 3:55pm
I rebuilt the carb with the original 66 jets (marked 299 for some reason) and the 2.5"Hg Power Valve. The boat runs great and doesn't seem to run lean so I've left it alone.
SkiDIM thought the stock carb should be sufficient.
How do you tune the carb? Hook up a vacuum gage to set power valve and "read" the plugs to determine the jets? I've thought about playing with jets and the powervalve but I'd rather ski.
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Posted By: 79nautique
Date Posted: July-08-2004 at 4:11pm
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.
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: SS 201
Date Posted: July-08-2004 at 5:22pm
ReidP,
Also be careful you put the right length rod on the secondary diaphram as there are several length's. Someone may have may have done this before you.This will mess up everything, even with spring changes.
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Posted By: 79nautique
Date Posted: July-08-2004 at 6:08pm
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?
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: SS 201
Date Posted: July-08-2004 at 9:25pm
Not really, That carb does run lean.carb does not have rear jets only a plate, to do this you have to buy a kit, that carb unfortunately was not meant to do what you want.With that said you mentioned a 750 or 650 carb.
There are two ways to go, vacumn, and mechanical secondaries as you well know. For what you are doing a 650 or 750 double pumper work well with vacumn secondaries. I love the 4110, it has annular discharges really a strong unit, however hard to find.
If you buy a new or rebuilt they will be jetted accordingly.
Remember this, with fuel being as it is plugs take a while to color, here is what I would do. Put some gas(octane) booster in and run, it will color the plugs porclain red, then you can see where you are.
When you put on a larger pitch prop the color will change because you will turn less RPM.
Hell I could go on forever as there so many variables.
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Posted By: reidp
Date Posted: July-09-2004 at 12:45am
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?
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ReidP
/diaries/details.asp?ID=231&yrstart=1971&yrend=1975 - 1973 Mustang
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Posted By: AWhite70
Date Posted: July-09-2004 at 2:27am
Couldn't you change out the secondary metering plate to richen the mixture on the top end? I haven't checked to see if they're available but I know they are numbered and I assumed the number had something to do with fuel delivery.
I'm sure a metering plate would cost more than primary jets but it should allow you to richen the secondaries without getting too rich on the primary side.
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Posted By: SS 201
Date Posted: July-09-2004 at 8:05am
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.
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Posted By: SS 201
Date Posted: July-09-2004 at 8:14am
ReidP,
I would spray aroud the carb base and intake manifold and check for leaks. Secondly be sure the throttle plates are not icing up or sweating. They tend to do that, then when warm run great. Also turn the air screws out a tab. Another thing spray the air bleeds with WD40 ,they sometimes plug and will do the same thing. Hope this helps.
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Posted By: 79nautique
Date Posted: July-12-2004 at 11:29am
Thanks for the advice on the carb SS 201.
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: SS 201
Date Posted: July-12-2004 at 5:37pm
Thank You,
Anytime I can be of some help or assistance.
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Posted By: CCClassic
Date Posted: July-26-2004 at 12:57am
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.
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Dave
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