Help with 383 stroker in formula 21.0ft
 

New 383 marine stroker motor. 365hp

1: All new parts and drive.
2: New 650cfm holley double pumper.
3: 17" stainless quicksilver prop.
4: Timing at 8 after on marks rh motor.
5: Carter rotary vane electric fuel pump.

Runs good until you get around 4000rpm backfires and misses through carb.. I have tried alot wondering now if carb is wrong or timing or some other issue. Fastest I saw was 48.6mph on gps at 4000rpm.. I also tried a stock 19" aluminum black max mercruiser it was horrible my 17" stainless quicksiler was alot better. I was told boat should be around 5000 to 5500rpm and around 55 or more.. Please Help... Thank You..

What distributer and box are you running? What is final timing?
650cfm carb was too small for our 383, went up to a 750 and ran much better.

I had a leak in my fuel pick up tube. acked a lot like that. or a bad ball valve on the pick up. Hope that helps.

What is you total timing advance at 3000~3200 RPM? Should total at around 32 degrees

Thunderbolt ignition . 8 degrees after on idle. Holley 650cfm carb marine double pumper. Heading down to boat now to see what else I can come up with..

I would check out the carb floats to see if they are bleeding over or set to high or low at that rpm the secondaries are opening up. vac or mechanical secondaries? is it a 4 corner idle carb?

The carb is a 650cfm Holley marine carb. Also I noticed while looking around the inside of the carb is all black and always wet like someone misted gas inside it. I am running the stock flame arrestor from my old 260 motor and now when I think about it when I'm at high rpm it sounds like a vacuum cleaner is on under there or I have a super charger I am wondering now if its restricting the air flow and causing me to run rich and have all these problems. That would be great if that is the issue if so I am a real dumb ass I am gonna try it with flame arrestor off and hatch open and run it and see what happens. I think I should see at least 4800 to 5000 and 55mph but I might be wrong..

Initial timing should be about 8 deg before TDC. That noise you are hearing sounds like a vacuum leak to me...and that will throw the whole metering circuit for front and rear in the toilet. Spray some WD-40 around the base of the carb with it running at idle. If the engine changes speed you have a leak...leplace the carb base gasket. Tighten these things down symetricaly and in stages...take at least 3~4 trips in a crossing pattern to get it uniformly snug. That is a good carb...if the floats are set up right you should be ok in that department. Make sure the linkage is allowing the secondaries to open all the way....I have seen this before.

Seriously...get a timing light with a dial on it...static timing doesn't mean squat if the distributor isn't advancing.

I wish we were closer...I would love to get that sweet rig focused and see it fly.:evilb:

Hello and thank you to everyone for all the advice and suggestions. Here is what I found first off you were right about the throttle nit opening all the way I fixed that and now when I push the throttle wot it starts that small backfiring and missing. I will try the wd-40 thing and check that. Here is were I'm confused when you look at the timing plate you have the zero and then before and after well here is the weird thing when I hook to number 1 cylinder wire and check the light at idle it's set at 8 after that's the only way it will run to 4k rpm. If input it to 8 before it won't get out of it's own way. U have tried everything pulled distributor lined up top dead center everything but it will run were it is just wot stumbles and acts crazy . I am at a loss for everything I don't know what else to try ...

Have you checked to see what the total advance is? Again...the initial timing is meaningless with out knowing what the total advance is. You need 30~32 deg total advance at 2800~3000 RPM or that engine will not perform as it should. I had this exact scenario with my starboard engine and ended up changing distributors after I discovered it.

As mundane as it sounds...double check your firing order. It is very easy to get this off by one or two cylinders. A SBC can fool you into thinking that you have No 1 at TDC when in fact it is another cylinder that is up instead. I am curious about the 8 deg BTDC vs ATDC issue...BTDC is advanced 8 deg...ATDC is retarded. This prompts me to question the fireing order.