SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: el mojado on December 16, 2015, 03:17:27 PM
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I have a 21 ft cruiser. 454 BBC. 10.2:1 calculated compression ratio. 750 vac. Sec. Holley worked over by Performance Carb Shop in Ontario (Ron), Edelbrock Air Gap, MSD 6A and Distributor w/ vac adv locked out all in by 2500 at 32°, 049 heads SS valves next size larger than stock, Aluminum center rise exhaust manifolds, i cant remember which cam at the moment. 91 octane.
I had a previous issue where blew the head gasket and head cuz I was running too lean. So thats when I had Ron work my carb. Well, I still here that ugly detonation sound around 4500 rpms so i just let off the throttle. But heres where it gets interesting... my last trip (Halloween) i went from Havasu to the Gorge and made a Uturn when i got to the end of the no wake zone. So by now the engine is pretty cool. Oil pressure is up. So i get on the throttle. Just about to 5000 rpms and I hear nothing. Runs great! After a few minutes of going from 4000 to almost 5000 i hear the detonation so I keep it at 4000 the rest of the trip. What gives? Spoke to Carb Shop cuz i thought maybe Im jyst one jet off. He says NO WAY is it lean at this point. What should I check over the winter? Sorry so winded. Tried to give as much info as I could. Thanks for any input.
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Have you checked your plugs? Hard to say it's not lean without at least seeing plugs.
What's your timing set at?
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No. I never checked plugs.
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What octane fuel?
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No. I never checked plugs.
Pull a plug and post a picture.
What's your water temp run at?
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I still think you are building a lot of cylinder pressure at torque peak. Did you end up installing your cam advanced or straight up?
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I will have to ask the machine shop. Is one preffered over the other Joe?
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Lifters might not be keeping up? Nice cool oil pumped up just fine? Getting a little hot/thinner might not be keeping up? Just a thought?
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Hmmm... ^^^
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I had pretty much same problem found out my cam was going flat. Something els to check.
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I will have to ask the machine shop. Is one preffered over the other Joe?
Not necessarily. Only testing will tell for sure.
Most cams are recommended to be installed 4-degrees advanced. Some cams are ground with advance built in, not necessitating an adjustable timing set.
Advancing your cam increases cylinder pressure because more air/fuel mixture is trapped in the cylinder when the intake valve closes earlier while the piston is lower in its stroke. A cam with shorter duration has the same affect.
I seem to recall that you selected a relatively short duration cam, which would build good cylinder pressure. I think it was a good choice for your style of boating, but I think your dynamic compression ended up a bit high. I also think your heads and intake should produce good flow velocity enhancing your effective compression ratio, building a lot of cylinder pressure at the peak torque RPM.
Do you know if your block was zero-decked? If the quench distance is too wide, it can cause the engine to be detonation prone. If your block is not decked, your pistons will be ~.020" (deck height) in the hole. Your quench distance will be your head gasket thickness plus the deck height. Anything over .055" is bad news in a NA deal.
Just some thoughts,
Joe
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There are three things that really need to be checked before a good guess could be made.
The first thing that came to my mind was wide open throttle fuel pressure and manifold vacuum.
It sounds like you are running it out of fuel. A vacuum secondary carburetor is very sensitive when it comes to bowl fuel level.
Another thing you need is to verify a stable timing advance. This will be a little tough to verify, but throw a timing light on it and see what the timing does.
Lastly, there is no way a carburetor guy can tell you if or will run lean/fat without running the carburetor on the engine at the load you are under. In other words, if he was not in the boat when it detonated, he is simply guessing. Your fuel pressure should be about 6.5 psi and never drop below 5. It's not unheard of to have a fuel system that can't keep up with demands of the motor.
Even a questionable tachometer can cause a timing issue and do what you describe.
It's not a Rev limiting tach. correct? I have had those kick my ass.
Good luck.
GT
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One last thing. What spark plugs and where are they gapped?
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I dont recall which spark plug, AC 44s or 45s, but they are gapped at .035. Fuel pressure was as high as 9 psi. I am running 3/8 hose from tanks to T fitting to pump to filter (cheap metal 3/8 universal style) which I change every 2 trips, to carb. I have no rev limiter. I have not chevked timing with a light while running it WOT. but my balancer is used. So its possible timing marks are off. I will be getting a new balancer to eliminate that possibility. Wires are MSD Street Fire and brand new. Floats were set at idle.
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It is no zero decked. I dont recall what it is. But when i measured it and used the number to calculate my CR with .051 thick gasket and 117 CCs, I came up with 10.2:1
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I dont recall which spark plug, AC 44s or 45s, but they are gapped at .035. Fuel pressure was as high as 9 psi. I am running 3/8 hose from tanks to T fitting to pump to filter (cheap metal 3/8 universal style) which I change every 2 trips, to carb. I have no rev limiter. I have not chevked timing with a light while running it WOT. but my balancer is used. So its possible timing marks are off. I will be getting a new balancer to eliminate that possibility. Wires are MSD Street Fire and brand new. Floats were set at idle.
9 psi?
Most carburetor needle/seats are only capable of holding back 7psi.
What carburetor?
Also, what is your cooling water pressure at WOT?
Has the exhaust been pressure checked?
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Carb guy says thats not so high. He tests them at 15 PSI
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Carb is vac sec Holley 750 dual feed bowls. I have my valve from the pump wide open. I have no water pressure gauge. Jacuzzi WJ.
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Cam is installed straight up
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10.2:1 with a short cam and poor quench will cause detonation, especially in a high load situation like a 21' boat slinging a large impeller. Not saying that is your problem... But that is certainly a good recipe for a detonation problem. Typically, I won't go past 9.75-10:1 on a large boat with a short "log friendly" type cam even with being zero decked and quench landing between .040 and .047.
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No disrespect to anyone but if we dont know if the balancer is even accurate, or what a spark plug looks like its really hard to diagnose the problem without ruling out the basics... it could be lean as fuck!!! Imho...it shouldnt have a vac sec 750 on it to begin with... but we've been thru that already on another one of your threads about carbs n tuning..it seems you want everyones advise but you never take it and you keep having problems... just my. 02.. best of luck sir sorting this out...
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Iv had carbs work bitchin in the driveway!! Then hit them with some load on the water and have it all fall apart and get smacked in the back of the head with vacuum caps!!! Lol..
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I intend to get a new balancer, check tdc, re time it, go out and run it and pull the plugs and take some pics.
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Just dead stop it and see if the mark is anywhere close. Even with a "new" balancer, I'd still check it... Might as well check the one you already have installed and paid for.
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Update... so I gave my boat a tune up; MSD cap and rotor, replaced the fuel pump with a new Clay Smith mech unit (it leaked), r42 ac plugs and a new Summit balancer. The cap and rotor was used but looked good. The fuel pump was a Pro Comp deal which always made me nervous. I ran r44 plugs but wanted a colder plug. And the balancer was a used unit which looked questionable. Took the boat out with all those changes to Havasu this weekend. IT RAN GREAT!! I was having what i believed to be a detonation issue before these changes. I had a good feeling when i checked timing at 2500 RPM and saw that it was now way off. I now have timing set at 33° and i'm confident it is accurate. It was such a great feeling running that boat all the way to 5k with no funny noises and running like a beast (to me)! Just want to say thanks to all who chimed in whenever I had a question. I really appreciate it!!!