Hi all,
I was out at the lake this weekend and I could only pull 4300 RPM and normally it will pull 4700-4800. Also, as you can imagine top speed dropped considerably. I changed jetting last week so my question goes right to that, can over rich cause this significant of an RPM loss? Any help appreciated.
Yes, jetting can affect RPM. Also, air temp, humidity, altitude also have significant effect.
Not sure if drive or jet but figure at least 50-80 HP loss for your 400 RPM. When was the last time you ran it at 4700?
Other issues, advance weights in dist stuck or not advancing.
It's unlikely to lose that much RPM with jets. Are you sure the secondaries are working properly? Which carb is on the motor? If a DP or vac holley, make sure they are opening fully and adding fuel.
Any hesitations when going to WOT? Looking for a lean or rich stumble here...
It's unlikely to lose that much RPM with jets. Are you sure the secondaries are working properly? Which carb is on the motor? If a DP or vac holley, make sure they are opening fully and adding fuel.
Any hesitations when going to WOT? Looking for a lean or rich stumble here...
The drive is alpha one Mercruiser. The holley carb is a 600 CFM Vac secondaries. Everything thing seems normal, idles fine transistion from primary to secondaries is solid, secondaries are opening up all the way, just wont pull past 4300 RPM. I ran it two weeks ago ran 60.8 MPH (gps) now can get past mid 50's. I did a compression test just before going to the lake and checked out fine. The boat still jumps out of the hole strong, mid range is real strong, just this high RPM ting is bugging the hell out of me. I verified my timing again, 10 degrees initial, 36 total. I am at a loss here.
Fuel filter? Things that limit upper band power are air, fuel, and then ignition. Too rich on the jets / along with a high density day could cause it to nose-over, but probably not THAT much.
If secs are opening, as you said, then I would look at fuel delivery. Maybe plugged filter or pump going away?
Other than that, could be the cam went flat, valvesprings are weak?
Fuel filter? Things that limit upper band power are air, fuel, and then ignition. Too rich on the jets / along with a high density day could cause it to nose-over, but probably not THAT much.
If secs are opening, as you said, then I would look at fuel delivery. Maybe plugged filter or pump going away?
Other than that, could be the cam went flat, valvesprings are weak?
"Other than that, could be the cam went flat"
Now why'ed you have to go and say that, it was already on my mind
I installed a Comp 268H about a couple months back. I wiped a lobe along time ago on my big block jet but the lobe went really fast and was obvious. Is it possible that the lobes are slowy going away? I have a feeling I am going to be pulling it apart to have a look at a lifter to see whats going on. I have heard some neg info on Comp Cams any truth to it? I know alot of people use them
In the mean time I am going to start checking the easy things over tonight, fuel filter etc.
"Other than that, could be the cam went flat"
Now why'ed you have to go and say that, it was already on my mind
I installed a Comp 268H about a couple months back. I wiped a lobe along time ago on my big block jet but the lobe went really fast and was obvious. Is it possible that the lobes are slowy going away? I have a feeling I am going to be pulling it apart to have a look at a lifter to see whats going on. I have heard some neg info on Comp Cams any truth to it? I know alot of people use them
In the mean time I am going to start checking the easy things over tonight, fuel filter etc.
I doubt there is anything wrong going on with CompCams as a Company. They have always been top-notch. If you didn't use diesel oil (Delo, Rotella) or any cam-break-in type of oil supplement, there is a good chance your new cam didn't make it. There is lots of literature out now from both Comp and Crane about oils for break in.
I doubt there is anything wrong going on with CompCams as a Company. They have always been top-notch. If you didn't use diesel oil (Delo, Rotella) or any cam-break-in type of oil supplement, there is a good chance your new cam didn't make it. There is lots of literature out now from both Comp and Crane about oils for break in.
I'm not saying anything bad about Comp, I have had nothing but positive results from them in my last boat. The cam that went south in my jet boat was another company. Anyway, I broke the cam in by the book, lube, run in time, etc. This has just got me stumped though, the boat pulls really hard, I mean like put you in your seat hard all the way through to it's new found RPM limit. There is no popping clanking banging..no hesitation, no black crap on the transom....nothing.
What you need to do is take it to the lake , have a friend who knows how to drive , or work on a motor, and while you are going about 25mph or so, just up on plane, have him, or you open the air fuel screw on the carb, since you rejetted, you may need to open or close more, and thats a good way to do it, you will feel the boat either slow down and pop, or as you open it more you will feel the boat pick up rpms, I would bet you are to lean , or to rich, and that way of doing it will tell you right where you need to be!
just push harder on the damn gas pedal!
Posting Permissions
Reply With Quote