3800 rpm wall
 

Gentlemen, I am new to this site. I hate to beat a dead horse about an RPM range but I could use some help. I have a 24' 1999 Baja Boss H2X with a 7.4 MPI and a Bravo one drive (don't know ratio because it's not on the drive ANYWHERE) running a 23P prop. Engine starts and runs great, no hesitation, no missing, throttle response is very crisp. We have done a compression test and everything is great. We are currently doing a leakdown test to verify no other problem exists. The issue I am having is the engine RPM will not go over 3800. I have ran a 21P prop and no difference. I have even used the trailer switch while running down the water and raised the prop out until it would throw an 80' rooster tail and the engine RPM never went above 3800. Never did it even try to hit the rev limiter. We have hooked it up to a computer and went for a ride, verified that my tach was reading correctly and it matched the computer to the tee, no codes, no warning buzzers. This has me stumped, and I'm not one for throwing money at something and getting the same results. Do any of you have any suggestions on what this could be?

I would check for a fuel restriction. Verify the pick up tube in the tank is clear, nothing in the anti siphon valve. The best test would be a vacuum gauge connected to a tee fitting placed in the fuel feed line to the engine. You are looking for little to no vacuum.

Is the throttle opening all the way?

I pulled the anti-siphon valve out and it was as clean as a new one. I did not do a suction test though. I will try that. Now if it was fuel restriction would the plugs not show a lean condition or would the ECM correct for that? The plugs look like a perfect running engine.

Throttle buterfly is deff opening all the way with the throttle lever

Have you checked running fuel pressure? Or fuel volume? You could spray "carefully" carb cleaner or brake clean running wide open and see if it changes engine rpm.

Is it maybe in "limp mode"?

Fuel pressure is a little on the low side but stays constant through out the RPM range

According to the computer, everything is within specs while running under a load. No codes are being kicked out and none stored

try cleaning the filter on the hi pressure pump in the VST tank.

OK I'll look at that when I do the vaccum test on the fuel line

Not aware of a VST tank on a 7.4L MPI.

maybe its a diesel! :)

Then I'll just look for ANY and all filters and ck or replace them

Just got my leak down results back and something is going on. 80% on #1 and 60% on #8. we stopped with that. I may be looking at a refresh and seeing what will happen after that.

You have a cylinder down, maybe more than one.

Pull the heads. Check plugs first.

Wannabe

Already pulled the plugs, they looked really good all the way to the bottom of the ceramic. Going to pull engine and go through it or drop in a 502 so it will be tip top for next spring. Our season has ended here in KY anyway.

80% leak down!!!!!!!!! 60% leak down!!!!!!!!!!!!!!!!!:eekdrop::eekdrop::eekdrop: Didn't you smell raw gas with the captains call on? No way #1 could even fire.

Im surprised you even got 3800 rpm. Sounds like a engine rebuild to me. Maybe you can get lucky and just have a couple stuck/burnt valves and just do a top end.

That's what I'm hoping for is valve train, but I am prepared for the worst

limp mode.

this may sound stupid but did you change the fuel filter,it does not take much water to impede the fuel flow and pressure.

mine has one, it also houses the 650 dollar fuel pump.......

[ATTACH=CONFIG]510282[/ATTACH]

I just ran this thing on the trailer and it runs up to 4800 smooth as silk. Hits the rev limiter at 4900. Runs fine not under a load. I'm going to put a fuel pressure gauge on it and run it in the water to see exactly what I'm getting under load at WOT, also taking some spray to squirt in the intake to see if RPM's go up to try and narrow it down to fuel pump problem.

how did you get enough cooling water to the motor on the trailer? Curious because my engines pump more than the hose will give at anything over 1000 rpms.

Hose wide open with deep water pickup! Temp runs around 170. Plus just bounced the rev limiter, didn't hold it at a high RPM very long

My 350 mag did that when timing was off, under load it cough and sputtered?? On hose ran fine.

Youu said you had 80% and 60% leakdown on 2 cylinders and stopped after that......DING!!!! DING!!!!

No fuel issue is going to cause that. The engine needs to come apart.

Griff I agree about the rebuild!! I'm going to do my own leak down test to confirm results from service. If this is the case then I think that I'm just running out of HP because of loosing power from the week cylinders. 57 mph at 3800 with the equivalent of a 5.5L in a 4200 lb boat running a 23P prop isn't to bad. She should come up and be happy with all cylinders working again

If that motor doesn't run like a complete piece of $hit at all rpm then those results are wrong.

Yours is the older LX with the Lingenfelter intake. The L29's like his will not have that.

Engine runs smooth as silk, thats why I want to do my own test and verify. My shop is next door to an engine shop and he was out when I was running it and said he couldn't hear any miss. Throttle response is as crisp as a whip, starts and idles at 600 rpm's, no backfiring, plugs look excelent, and no raw gas smell with captains call ON.

I found the fuel pump finally!!! I think if they tried a little harder they could have found a better place to put it. I may have to pull the engine if it ever needs replacing.

If I end up pulling this engine then is getting gone through from the top to the bottom so it will be ready for next season. What do you guys think about the Imco or the Dana Marine exhaust manifolds? The reason for asking is my port side is rusting thru

Its impossible to run 57mph at 3800rpms with a 23"prop and 1.5 ratio drive. Get a GPS speed and be ready to be severely disappointed.

I had a gps app on my phone and it matched the speed-o within 1 mph. The drive has no serial # on it and I haven't taken it apart to verify ratio, so I really don't know what ratio is in it. Plus I was running with my buddy beside me the whole weekend.

Then you need to verify your tach readings.

if it is not running horrible at all other rpm ranges those leak down results are incorrect. you said the fuel pressure is low what is it reading??? i believe you have a bad high pressure fuel pump. also the imco or eddie are all good exhaust and much lighter.

I believe the techs computer was showing 35 psi but was holding at all rpm ranges. I feel it needs to be up in the 40's. That's why I was going to run it again with a pressure gauge on it to get a second reading. I'm also going to do my own leak down test before pulling the engine. Tach was also checked using the computer while running on the water and it was dead on (I was actually surprised how close it was).
The tech said it had a mechanical and an electric pump. I looked where a mechanical pump "should" be but I can't find one, just the electric.

Agree with Griff, your tachs off by 6-800 rpms on the low side. 3800 at 57 would be -3.3% slip. Using 11% would be running 4400ish. Note: 1.5 and 23p used in calc.

I'll "somewhat" agree, the drive is probably a 1.26 ratio.

Here is a screenshot of my radar verified GPS speed on a 1.26 XR , 37 pitch 5 blade, slipping 21%

http://www.altfuelsgroup.org/picture.../Top-Speed.JPG

Yours would be slipping 13% if that tach and my 1.26 calculations are correct.