Question on gap between 3500 and 4000 RPMs any ideas?

[dgie]

One more thing I forgot to ask. I have a single point Mallory and a holley 750. I just put a quick change kit in the vacuume secondary. I changed the original spring to the lightest one in the kit. The reason is because I had a gap between 3500 and 4000 rpms going with the lighter spring seems to help it but there is still a little popping between the rpm ranges. Could it be a carb adjustment, or?

[dgie]

I am going to post a response for my own question because I a pretty sure I found the problem when I went to the lake yesterday. Also just in case anyone else runs into the same problem in the future. I was running with a Mr Gasket bug grabber (scoop) and inside there is a filter about 8" in diameter. The engine would cut out when I stomped on it and there would be a gap just before the secondaries kicked in. When I removed all this cool looking crap and ran with open intake and readjusted the idler screws for the new air to fuel ratio. The problem seem to go away.

[Hal]

readjusted the idler screws for the new air to fuel ratio. The problem seem to go away.
:D Thats good that it worked for you. You may want to buy a book on carb tuning to help you out a little. Good luck... :)

[dgie]

that I do need, as I am new to the world of boats and motors as we all were at one time :p

[Hal]

OK, if you mean you had a bog (gap) between 3500 and 4000 rpm when your secondaries opened you should use a heavier spring to slow down the opening of secondaries to keep from having a lean condition when they open. I'm sure there are ways to cure your problem. I'm not a carb guy I only know what works for me. I have always used mechanical linkage (which seem to work best for boats) You can get some good info here.. (http://www.chevytalk.org/threads/postlist.php?Cat=&Board=UBB64)
Sign up and ask your question. These guys are pretty good.

[dgie]

I actually used a lighter spring, the lightest in the kit to be exact. It was a bog, the problem seems to have went away, but i am going to put all the diferent springs in and see which works best. I bought a quick change kit so it will make it better. I rescently learned that mechanical secondaries are better. Too late for me now. I am not sure what size air cleaner should be on there, i am going to replace it with the right one, and it will be a flame arester type. Thanks for the link, I enjoy learning all I can. Any more sources on carb tunning throw them my way.

[Hal]

K&N has a good air cleaner that is coast guard approved. Cost about 65 to 80 bucks.

[dgie]

awsome I will order it tomorrow, where do I get it from? direct or? what size is it ? also i am curious if a flame arestor is nothing more then a screen (i think thats all it is) how do you keep water out of the carb?

[Hal]

K&N (http://www.knfilters.com/marine/default.htm)
[ June 24, 2003, 08:37 PM: Message edited by: Hal ]

[dgie]

I just looked at cp perfomances filters and looks like a lot of them are 8"OD what size should it be on a 455 with a 750 cfm holly carb??? anyone?

[victorfb]

the US coast guard approved flame arrester that is on my 455 with a 750 vacume secondary is just a bit larger in diameter than the mounting surface on the carb. havnt measured as of yet so i dont want to give numbers. originaly it was about 2 inches tall and when i ran higher rpms it seemed to whistle a bit. what i did was grind the rivets off and took out the 1.5 or maybe 2 inch screen, (yes only a screen) and made my own by cutting a length of stainless screen (just like the original) about 4' by 4.5" and wrapped it around till it fit right in the base, then spot welded the screen together to hold it from unravelling. then installed the lid and bolted it back together. i now have a 5 inch tall flame arrester. and it still "SAY'S" US coast guard approved. replacement flame arresters that i priced were very expensive because of the coast guard approval and with my mods i saved alot of cash. i am getting ready to make a stainless scoop aswell that the flame arrester will still fit inside. this weekends project. (well one of em).
im not sure what i have in the stock 750 vacume secondary. i got it for $75 from a guy who had it on his pontiac trans am for about a month and he crashed the car. the carb looked very new and i took a chance, replaced the 600 vacume secondary with the 750 and didnt have to do much tuning at all. the thing ran and still runs great for me. ( just checked the float levels and adjusted the idle) the air/fuel scews were just about right. (amazing huh?)
and as far as i can tell it is the same way it came from holley and out the box. so whatever stuff they come from the factory with is what im running and it does work well for me. hope that helps. maybe you can call holley and ask what it came with originally.
by the way... its a 455 olds, (which has a plate that reads "built by harmin marine 390 HP") that i rebuilt. i didnt check all the specs, deck hieght, and such, and dont know what cam. it had the "K" heads that were distroyed, (never loan your boat out) and all i could find was "G" heads that i had redone, and i did some mild porting, added a torker aluminium manifold and the 750 holley. i run harmon marine logs. (yes i know) but headers were to dangerous around the little ones. i wish i would have done more research on 455 olds motors before i rebuilt it as i have no idea if it has the oil restrictors. i just didnt know at the time. i did however install a 10 qt dooley pan. so far so good. its mated to a berkely JC "A" impeller all redone at the time of engine rebuild. before the head swap it ran 4300 rpm, after the head swap with mild porting it runs 4700rpm. i just dont like to get the 455 that high of rpm. like i said, this set up has been working great for me and i hope this info helps in any way.
the place diverter is going on this weekend and will be trying it out next weekend. not for more speed, i just want to see that VERY COOL rooster tail behind MY OWN boat. way cool........ :D

[dgie]

vic what is your speed at 4300 RPMs? and I think I have the same size filter that yours used to be 2 inches high, mine however has paper which could be the source of my restriction. I wish to do the same thing, where did you get the screen material and what type did you use? Also how do you keep the water out of the carb? I have that Mr Gasket scoop and would like to keep it, that would require a custom fit. Out here in Texas we really do not get hassled by anyone on the lakes, hell we can carry guns out here. Anyways let me know, Also the reason that I am asking what kind of speed you are running at that RPM is because I am trying to still figure out why I am only running 43 mph. I have nothing but questions I know :D

[victorfb]

hey, dont worry about asking questions. thats why i come on here. to ask and if i can, asnswer as many as possible.
i cant give you an honest answer on the speed as of yet. i will GPS it on the 4th of july weekend. but so far ive had 2 boats along side and they both said just above 60MPH while i was running 4300RPM. im not going to believe their speedos yet till i get the GPS reading. ill let you know on that one.
i dont seem to have a problem with water entering the carb that i can tell, but i am making a stainless scoop this weekend, just to hide the large flame arrester. that will help keep water out even more.
the paper filter is quite possibly a sourse of some of the trouble you are having. i wouldnt run it but thats just me. i used some screen that i found at a local hardware store for mine. it is basically a very small diamond screen. and it was difficult but i found it in stainless, only after making one first out of the same size zinc or galvanized coated screen. (i had another flame arrester from an old boat).
ive been lucky so far and not hassled or even checked about the US coast guard approval on it, but i can tell you this, if i had a gun on me id be sure to need a bail bondsman that evening. they dont like that sort of thing here. unless your fellow officer of course.
43 mph seems pretty slow for running that high of rpm, but that doesnt mean your not running at optimum efficiency. your hull desighn, size, weight, hook on the bottom, and even the placement of the jet pump on the hull can all contribute to lower speeds. ive learned that some hulls werent desighned for jet pumps but installed in them anyhow just to please the demand of the puplic. (boat buyers) also the type of pump you have can be a factor. i would highly recomend you have your pump rebuilt with up to date products such as collared wear rings and such. and of course impeller size is critcal. another thing to look at is the coupler. its condition and alighnment. the u joints may be old and robbing you of alot of HP. a well built pump thats working efficiently is a huge advantage.