Power drops at WOT and starts rattling!
 

re: Rebuilt 383 in Boat, newly rebuilt Quadrajet
This may be 2 separate problems, but not sure.
Problem 1: At WOT (4500-4800RPM) it will run for half a mile or so, then start losing RPM's till it dies down to about 3800RPM.
Problem 2: At WOT I start to hear a rattle from the engine. It sounds like, valve train/Pinging/exhaust flappers. It's a tinny noise, not a knocking.

They seem to start happening at the same time, and if the rattle is causing my RPM loss, then it would have to be lifters colapsing, or predetonation I would think. I don't ever hear any pinging or valve noise at any other time. Best case scenario, is that my exhaust flappers are rattling, and I am just starving for fuel. Both easy fixes, but collapsed lifters scare me! What would the causes of that be? Lack of sufficient oil at high RPM? Too stiff of springs? I just used Z springs which aren't that stiff. The lifters are brand new. What should I check? HELP!!!

Re: Power drops at WOT and starts rattling!
 

Detonation due to a lean condition.

If it is, it will probably quit doing it soon, right after it burns a hole in the piston :eek: :eek:

Re: Power drops at WOT and starts rattling!
 

That makes sense. Starving for fuel could be due to a couple things. I just rebuilt my Q-jet myself, and could have screwed something up, or maybe the qjet/stock manifold need to be upgraded to and Edelbrock or Holley setup. This 383 pulls hard and probably wants some fuel! I'm running 10:1 with Comp Cams Extreme Marine cam (don't have the specs on me) and World Product Sportsman 2 heads which are 72cc chambers 2.02 valves and 200CC runners. I replaced my fuel pump already, switched my fuel hose over to the other pickup in my tank (it's a twin engine tank), and replaced the check ball fitting with a straight thru fitting. This seemed to help, but not solve the problem. Any other ideas?

Re: Power drops at WOT and starts rattling!
 

It sounds like a fuel starvation problem. What did the carb come off of? A highly modified engine like yours will generally want richer jetting than a stock Mercruiser. Because it is hard to find a good assortment of jets and metering rods for a Q-jet, most people just buy a Holley. Do plug checks. Maybe the carb bowls are emptying after a hard run. Q-jets have a smaller needle and seat than a Holley and may not flow enough gas. Don't run it hard before you fix the problem. Sportsman 2 heads are iron aren't they? Better get the jetting right, always use 93 octane, keep ignition timing conservative, keep your engine temp under control ( use 140 thermostat, make sure it stays under 160 ) or you will detonate the engine to death.

Re: Power drops at WOT and starts rattling!
 

I think you are all right about the lean condition. It is the stock carb off of the boat, which was a 260. when I rebuilt it the only thing I changed from stock, was the secondary metering rods. I went with the next size richer. Dont remember the letter now, but I kept the same jets. I also epoxyed the well plugs while I had it apart. I have a new edelbrock performer intake for it sitting in my garage, so maybe I'll pop for a carb too. I know edelbrock makes a marine carb thats only a few bucks more than the non-marine. $300 or so I believe. I know the Edelbrock carbs are not the greatest, but they are pretty easy to tune as long as you buy the tuning kit. I am almost positive I have no restrictions to or from the pump, and the pump is new, filter in the carb is new, so it sounds like jet time/carb replacement time/set the timing back a bit, and/or start running premium gas. My marina doesn't sell it, but the one up the river 8 miles does.

Re: Power drops at WOT and starts rattling!
 

One more question. This is in an 84 boat, so it has a 3" wishbone exhaust pipe. Do I need to worry about exhaust restrictions at 4500-5000RPM's? I have heard people say not to run hot motors at high RPM's through these old wishbones, but I'm not convinced this would be an issue. Mercruiser ran 454's through these wishbones back in the day..........

Re: Power drops at WOT and starts rattling!
 

I wouldn't worry too much about the exhaust however it is not the best set-up for high perf. With that engine, aftermarket exhaust would help a lot. I would put a Holley 650 dbl pump on it. Make sure you always use premium gas, 10 to 1 is a lot of comp. for a boat engine. If you want to use 89 octane, you'd have to drop it to 9 to 1. with aluminum heads you can add a half a point.

Re: Power drops at WOT and starts rattling!
 

That fuel system is way to restrictive. The inlet filter in the carb combined with the small float bowl is more than likely your problem. Holley 650 sounds about right. Plug the power valves and start around 86 jets front and rear. Max timing I would run with that combo would be 32 degrees at 2500 to WOT

Re: Power drops at WOT and starts rattling!
 

The qjet should be a good performer for your application. They were stock equipment on the 365hp 454 magnum engines so I think with the correct metering rods you would be fine. DA was the designation on the metering rods for the stock big block engine. I would try those along with a plug reading after a WOT blast. I would also take fuel pressure readings during this run to eliminate other possiblilities like clogged fuel pickup and/or anti syphon valve. If the bowl is going dry, you can purchase larger needle and seat to flow more fuel. Just need to lower the float level a little during this change.

BT :cool:

Re: Power drops at WOT and starts rattling!
 

Where does a guy get non-stock parts for a Q-jet. It's easy to get parts for Holleys and E-brocks, but Q-Jets? Is there a web site anyone knows about?
Thanks for all your help so far guys! Reallly appreciate it!