SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: el mojado on August 06, 2015, 10:29:12 PM
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I want to add the secondary metering block to my 750 vac sec. it is currently running too lean for my application. What jets should I also order along with the Holley kit? I plan to get the block with the power valve. Its going on a 454; 10.5:1 comp, comp cam 270 AH, 049 heads, slightly larger SS valves, Air Gap manifold, MSD dist and box. What other carb mods should I be considering? Thanks guys, for your advice.
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I say ditch the vac secondary 750. Put a 850 double pumper on it and call it a day.. just my .02..O0
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What he said (Aquaholic72). An be done wit it. My 2 cents
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If 100% of people said it wont work , I would ditch it. But I have heard enough say it will work. Just gotta set it up right. Its not a racer. Its a 20.5 cruiser.
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If 100% of people said it wont work , I would ditch it. But I have heard enough say it will work. Just gotta set it up right. Its not a racer. Its a 20.5 cruiser.
Working and working without drama are two different things.
It's this a spread bore?
If a square bore, you should be in the neighborhood of about a 76.
GT
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Square...
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Are you sure about a power valve in the secondary?
Just a question.
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You tell me. Been talking to an old jet boater. He knows more than I do bout these boats. He says i can make it work. Carb Shop in Ontario says the same.
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Here's my stand on it..take it or leave it.. if it worked, u wouldnt be trying to add a feature to the carb it never came with.. if u really wanna run it.. step up ur main jet and read the plugs again.. power valve on the secondary side is pointless imo...especially on a vac secondary carb..again just my .02..
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You tell me. Been talking to an old jet boater. He knows more than I do bout these boats. He says i can make it work. Carb Shop in Ontario says the same.
The thing about a power valve is that it can get convoluted when selecting jets. A power valve can make a 6 to 8 jet size size difference.
A jet boat engine makes exactly the same load at any given rpm, pretty much no matter what. If this was a 454 in a motor home, I would say your going down a good road. But on a jet boat engine, there is no need. It's a water dyno. I say no power valve, probably 76 jets, what bowls do you have? Is it s dual feed? Center hung floats?
I want to see you succeed, but this is deep tuning 101 territory.
GT
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Center hung floats...
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And are you saying 76 all around?
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If 100% of people said it wont work , I would ditch it. But I have heard enough say it will work. Just gotta set it up right. Its not a racer. Its a 20.5 cruiser.
it really seams that u want the tunability of a double pumper, hence the secondary metering block instead of the plate,but u dont want the simplicity of the mechanical linkage.?.. O0 i myself love the k.i.s.s method..
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77 frt 87 rr
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Dropping $500 is not simple right now. Changing the plate to the block and installing jets: very simple. And cheap. It might not be the best. But I think it will be good. As long as I dont melt any parts I'm having a good day.
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I got the carb cheap and brand new. I still will have less money into it had I bought a 4150 vac secondary. Which is all I'm doing.
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K..i understand what ur trying to do.. my next question is.. when is it lean? After u make a full throttle run, shut it down and read the plugs? Or is it lean after u putt back to the launch ramp load the boat and go home? Have u tried just steppin ur primary jets up and see if it changes? Does it spit pop and act stupid when the secondarys come in?? How do you know ur secondary side is the problem? As u said its a cruiser not a racer.. just tryin to give u some cheap advise that may help..
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Blew the head gasket between 2 cylinders. Jets are original to carb: 70s. It would reach 4800 rpm then would drop to just above 4000. Would pop a little. More like a crackle. I would let off and bring it down to 4000. Would cruise between 3500 and 4000 nicely. New fuel filter. Fresh gas. 91 octane. 36 degrees timing. Good fuel pressure throughout: 7psi.
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The new motor just puked the head gasket???
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Blew the head gasket between 2 cylinders. Jets are original to carb: 70s. It would reach 4800 rpm then would drop to just above 4000. Would pop a little. More like a crackle. I would let off and bring it down to 4000. Would cruise between 3500 and 4000 nicely. New fuel filter. Fresh gas. 91 octane. 36 degrees timing. Good fuel pressure throughout: 7psi.
I would have gone up two jet sizes on the primary side and used the lightest yellow secondary spring and let her buck.
GT
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Yup. 2nd trip out. But probably started at the end of my first weekend out.
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But now with a secondary metering block what is a good place to start regarding jets for primary and secondary?
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I 100% agree with gt...except id hit the primarys with 76's...just freshend up a 750 for a stock 460 ford..put the jets back to factory settings. 70 primary side.and it wanted pop crackle and act lean.. hit it with 76's on the primary side runs great now.. alil primary jet goes along way..
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Yup. 2nd trip out. But probably started at the end of my first weekend out.
Your thinking the carburetor caused this?
Do you have a picture of the piston tops?
GT
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No i do not have a pic. No damage to pistons. Chipped porcelain in spark plug near electrode.
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Wow..i didnt see the gasket pic
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Block looks ok. Lots of carbon though. Cleaned up ok.
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77 frt 87 rr
good start.. better fat than lean but damn!!! Smoked that head gasket pretty quick
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Hot!!!
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I'll start with those jets.
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Thanks for everyones thoughts, advice, and time. I'll keep u posted.
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Do u have a pic of a spark plug? Maybe u have a water problem..
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No. There was zero signs of water in cylinders, or oil
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Wow!! Id have the head checked just because.. did ur machine shop check the deck and surface the heads when they put it together? U have head bolts or studs? Clearly there was a problem.. not convinced it was a carb problem..
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Block decked. Head resurfaced.
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Your head gaskets, acting like a fuse, saved your engine from detonating itself to death.
It would really help to see pictures of the sooting on your pistons and combustion chambers. Reading the soot pattern can provide insight into what is actually happening inside the engine.
I'm thinking along the same lines as Aquaholic in that it is not entirely a carb lean condition. You are building a lot of cylinder pressure at that RPM and your combination.
You need to spend some time creeping up on your final tune. What you have now is not plug-n-play.
Sorry to put it bluntly. I know what it is like to be on a tight budget and being excited for some seat time. Throwing super wazoo carb tuning capabilities at it is cool and all,but you still need to be able to tune it or you are heading for a nuclear meltdown.
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Someone duggested MLS head gaskets but now I'D be afraid of it holding it togeger and blowing a piston. Should I shim the gasket?
My understanding is I was way too lean and too much timing. I want to be closer to where I should be and not blow this thing up.
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Post up pictures of pistons, combustion chambers, and spark plugs.
Was the cam installed advanced?
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Someone duggested MLS head gaskets but now I'D be afraid of it holding it togeger and blowing a piston. Should I shim the gasket?
The block and heads need to have the correct surface finish or MLS gaskets will not seal.
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Someone duggested MLS head gaskets but now I'D be afraid of it holding it togeger and blowing a piston. Should I shim the gasket?
My understanding is I was way too lean and too much timing. I want to be closer to where I should be and not blow this thing up.
. Correct me if I'm wrong please. I see some big problems that needs the attention of some knowledgeable people to help you. If I remember right everyone said that it was a bad combo from the get go. At this point you need help not a guess. If your not sure about your timing or your lean condition find someone that is. Sorry to put it this way but your builder that new everything needs to help out before it's too late. I hope the best and hate to see this happen to anyone good luck.
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My builder says that this combo will work. Cam, pistons, heads, etc. He said bring down timing a little and richen up the carb. Thats it. He didnt agree with MLS gaskets. He seems very confident.
The cam was installed straight up.
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He feels that 10.5:1 compression was going to be fine
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Builder, old jet boat guy, and Ron Blanchard (carb guy) said the 750 vac sec could be made to work.
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Builder, old jet boat guy, and Ron Blanchard (carb guy) said the 750 vac sec could be made to work.
Indeed it can. But I think it is not the root cause of your problem. It may be lean, easily corrected. I think too much timing, also easily corrected. I think way too much cylinder pressure.
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What u think about a head gasket shim? Or thicker head gasket?
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Is this at your house? Offer still stands to come calc out your compression ratio.
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I am going to put my 2 cents in here well just cause. I think jimpen is right!! You should hand the boat to someone that knows what's going on!! No offense, but this time it cost you a head gasket next time it could be the the whole motor!! You have a issue that is not going to fix itself! New motors don't burn head caskets like that!!!
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Yes. It is at my house.
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As far as calculating my EXACT compression... i have not. This is what i understsnd needs to be done. Measure CC of heads. CC of piston head. How far down is piston in cylinder. gasket compressed thickness. Punch all those numbers into compression calculator.
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When we build a new engine, the timing starts in the low 30's and typically fat as hell. It is not unheard of to start two steps fat and we run 2% Marvel oil in the gas.
From there we start playing with timing and then working on the fuel. Fuel is always last, then reevaluate the timing.
FYI
GT
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As far as calculating my EXACT compression... i have not. This is what i understsnd needs to be done. Measure CC of heads. CC of piston head. How far down is piston in cylinder. gasket compressed thickness. Punch all those numbers into compression calculator.
. Take Joe up on his offer you will have answers and places to go.
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Is this at your house? Offer still stands to come calc out your compression ratio.
honestly, i never saw the post where u offered. I will take your offer. I live in Ontario though. Not sure where u live. I really hope its not too much trouble.
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honestly, i never saw the post where u offered. I will take your offer. I live in Ontario though. Not sure where u live. I really hope its not too much trouble.
Colton
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I am available Sunday if you are. I can take u my heads if thats all you need. Thank you!
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Sunday works
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Sunday works
Hey Flusher. I really aporeciate the offer but I'm gonna hold off for now on the project and go back to the drawing board. I'm gonna weigh all my options and then make a decision. Change pistons, change heads, change carb??? I'll update later. Everyones right... enough with the guess work.
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So.... I measured the CCs in my heads. Measured piston to deck clearance etc, and punched all the numbers in to the compression ratio calculater. With a thicker head gasket (not mls) i will be at 10:1. The Carb is being tuned at Performance carb in Ontario by Ron Blanchard for my application. He says the vac sec will be fine. I will be at AVI next weekend running it. I really appreciate everyones thoughts and opinions. I'm gonna start at 32 degrees timing and go up from there.