dma251

Over the winter I swapped cams in my 1990 350 from stock to a Comp cams xe-262-h with the matching springs, and swapped in the GM cast iron high rise from the 350 Magnum. Before I was hitting about 4600 rpm with a 21P Laser 2 and about 58 mph (GPS)

Now I can't get the engine to rev past 4000 rpm with a Mirage Plus 24p, or a Laser 2 23P. There was absolutely NO difference in max rpm between these two props (both lab finished by Houston Prop) This leads me to believe that there is an engine issue limiting the max rpm. I set the cam timing to stock position per the timing set instructions. I notice a little play in the valve guides, but other then that the engine seems tight.

What could be happening here? Is the cam to big? I have through-hull 4" exhaust, and a quadrajet that I installed CK secondary rods in. That's about it for modifications. HELP!!!!

mcollinstn

1) did you check the cam installation with a degree wheel?
2) what is your ignition timing set to?
3) what is your fuel pressure at WOT?
4) did you check your float level when the carb was apart (or did you just drop the rode in without splitting the carb?)
5) have you tested with your old prop?
6) do your plugs show rich or lean condition at WOT?
7) any changes to hull, drivetrain, or weight?
8) are you testing with the same amount of fuel that was used in your previous baselines?
9) are you using "old" fuel from last year?
10) have you checked/replaced your water separator?
11) do you have a mud wasp nest in your fuel tank vent?


I could go on, but the fuel pressure reading will actually rule out further fuel system issues unless it comes back low..

robyw1

advance that cam 4-deg. I think that will work for you.

Roby

SmallPP

Careful. If you advance the cam, it will improve low end power and reduce high end power. As backwards as it sounds, that is how the harmonics work out. Sounds like a fuel or ignition problem. Maybe your spark arrestor is restrictive. Try a short run without it as long as there are no fuel vapors.

blue thunder

Try the 21p prop you got 4600rpm with and see what rpm you now achieve. Sounds to me you will be right back at about 4600rpm. Generally changing only cam and intake will yield little improvement if they are not the bottleneck to start with. What was done to the heads and exhaust?

BT

robyw1

True, I brought it up because I think his cam may be a little on the larger side for his RPM. However I can not find the specs on that grind. You are supposed to pick up a little bit on the big end by backing it off, but on the other hand this only works if other mods are made to coincide with it. Since were only talking 4000 to 4500 RPM I figure there will be no gain by doing so.

Roby

dma251

I've got an 11"x2" oval K/N air cleaner on it right now. The prop I had is unfortunately gone, so I can't put it back on unless I buy another one. I am thinking this is an engine issue because the engine could only turn 4000 with the big 24" Mirage Plus, and then when I went to the smaller laser 2 23P the rpm changed not one bit. My thinking is that with a 21P back on it it would do about.... 4000rpm.

The heads are stock other then the Comp Cams springs, and the exhaust is stock unmuffled 4" dual out the transom. I haven't checked my fuel pressure, what should it be? I had a friend rebuild the carb before I ran the boat, and then I swapped in the richer secondary rods a couple weeks later.
How do I check my fuel pressure? How do I verify the secondaries are opening?

robyw1

Don't worry about your fuel pressure just yet. Do you have a Holley carb? can you tell me which one it is? (model# on choke shroud)

You swapped in an intake that I believe is for a square bore carb. (someone correct me if I'm wrong) You can not go back with the quadra-jet carb on this intake even though some were dual drilled. (for who knows what reason)

Give me carb and intake details. take pictures if you have to.

Roby

IRONMAN

You need to prop it so that it turns 4900-5100. Then dial in the rest of the engine parameters good luck

jaroot

that camshaft is gonna make peak hp at about 5000 - 5100 in the "straight up" position with your cylinder heads but mostly because comp has 2 to 4 degrees ADVANCED ground into their cams as delivered instead of TRUE straight up. i degree all of my camshafts and the last 2comp rollers i did were ground +2... i installed a 262 in a sbc last year and it was ground +4...

if you install advanced you'll build low end torqued and kill top end HP and bring the hp peak back in the rpm range.. retarding does the opposite. i generally go -2 from straight up to keep my hp peak in a reasonable range with larger bumpsticks in marine sbc's...

i would check your ignition timing at full advance... your cylinder heads (NON VORTEC) and that camshaft are gonna want to see 34 - 36* full advance... bring it in as soon as possible (3000 rpm or so) if oyu are running an ignition system you can tweak such as an MSD. do not worry bout the intial advance... the total is what you want to concentrate on unless the motor will not idle in gear then you need more initial...

lastly.. MAKE SURE you don't have 2 plug wires crossed or one that isn't snapped on... pull every plug and look at them all...

make sure you are actually getting WOT out of the carb... and since it is a q-jet check the filter screen at teh carb for debris... takes a 1 inch and a 5/8 wrench to remove...