twin 496 diffrent rpm's
 

2004 fountain twin 496's at 3000 to 4000 rpms = then wot one motor 5200 the other the other 4800 rpm.130 hours any ideas?

well.... first thing i would do is switch the tach signal on the back of both tachs and see if that 400 rpm follows the tach or stays with the motor. fixing the tach is easy if thats all it is. second is just as simple... make sure you actually HAVE full throttle on that side...

once those two things confirmed you can go from there.

ok swicthed tachs samething used a snapon external tack samething.checked the throttle linkage no problem.what am I missing?

so the tachs are ok ( i don't understand your answer) if so and motor really down 400 revs then i guess the next thing is to have the plugs out and compression test and leak it down... maybe simple like a broken valve spring... after that valve covers off and see if you wore a lobe off the cam... maybe have the injectors out and pulse them and see if one has stopped working...

a couple of threads here already about people killing pistons for that suspected cause.

my answer is I still have the same problem.before I started this thread I did to compression check and changed the plugs.How ever I did not do a leak down test due to the compressionwas right on the money.help

Stop driving in circles:rolleyes:
Sorry, couldn't resist

My guess is prop differences. I've got one set of props that are always off a couple 100 rpm's in the 3500 rpm range but perfectly even at wot. I've got another set that is perfect at 3500 but off by about 150 at wot. Then I have another set that are perfect throughout the range. Of course, these are the ones I spent the most money on.

thats a good idea. my oringinal bravo 24s were a couple or 300 revs better than the new ones. turned out prev owner had dinged them and had them repaired and turned them into
23 1/2 's so that could easily be the answer...

all ready did that and now running labbed.I am almost ready to give up.help

If you haven't already, switch props to the opposite sides and see if the rpm follows the props (unequal labbing)

Also, check the magnet in the bravo drive port to see if something is going on inside... I had upper gears going, and it wasn't noticable until higher rpms... lot's more drag on that side.
Carder

thank you I will try that this week end,when I rotate the props what can I expect the boat to do and can I run it wot?

same issue on my 2001 353 fastech
 

I just re-installed my motors after repowering the 496 ho with upgraded pistons 5 lb procharged etc,etc,etc

I am not sure if I still have the rpm difference like before doing the upgrade ...my feeling always was that the fuel pump or screen was getting clogged or blocked at higher rpm I had a 200-250 rpm difference mine was at wot or above 4600rpm I know in the past 496 motors had issues with debris clogging the fuel pump because the filter is after the pump..496 have 2 fuel pumps 1 (boost pump l/s of engine in front of fuel water seperaotor)that draws the fuel from the tank and 1 that is on the r/s of engine which sends fuel to the rails.

It will handle differently, spinning out vs. in... overall slower, but you should be able to see if the lower rpms follow the props, or remain the same.
Carder

PS: did you check the magnets in the upper drives for any bearing or gear material sticking to them?

are we sure you just don't have a cam lobe leaving town ? or as happened before here, that you didn't get the firing order wrong on that motor somehow before all this started...

the firing order is fine when I replaced the plug I do one at a time also use a scope to see how it was firing.

could be a bad throttle cable, they can get stretched over time