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Randy

1998 Sportster LX

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Randy

I purchased a 1998 Sportster with a blown engine. Installed a new vortec 350. The boat will not rev over 4400 rpm and I am only getting about 44 mph out of it. Tried new carburator, fuel pump, fue filter, distributor, different props, etc. Boat starts and idles fine, and comes out of the hole fine. with a slalom skier wide open I only get about 41 mph. Anybody have any remedies. I am wondering if it has something to do with the camshaft, or if the heads aren't actually vortec heads. Cry.gif

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RiverRunner
I purchased a 1998 Sportster with a blown engine. Installed a new vortec 350. The boat will not rev over 4400 rpm and I am only getting about 44 mph out of it. Tried new carburator, fuel pump, fue filter, distributor, different props, etc. Boat starts and idles fine, and comes out of the hole fine. with a slalom skier wide open I only get about 41 mph. Anybody have any remedies. I am wondering if it has something to do with the camshaft, or if the heads aren't actually vortec heads. Cry.gif

Depending on the prop I thought 45mph was about top end on that boat

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Slider

I would start with the prop. Demo a smaller prop from your local marine shop. Sometimes you can get more top end out of a smaller prop due to the higher RPMs.

Good luck.

Joe

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WakeGirl

Mmm, actually slider means a bigger prop. :) What prop are you running now? Also, are you sure that the speed is accurate (GPS)?

Edited by WakeGirl

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MoonDawg

That sounds about right. It is pretty linear 4400 RPM is 44 miles an hour in my 96 RLX.

You need more RPM. I am running an ACME 13 Dia x 12 pitch. I can get to 5200 RPM with my engine, A 320HP Monsoon (350 Vortec with MPI). Carbed you should be at 310 HP for the standard 5.7L Vortec Marine Engine.

Your boat may have a 13xll prop and be causing it to loose some top end.

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Greg_S
Mmm, actually slider means a bigger prop. :)

married? Biggrin.gif

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WakeGirl

Yes Greg, how did you know? :) Actually, slider's right, I was just giving him crap. To push the rpms up a bit he needs a smaller prop.

One other thing that I thought of, are you running at a higher elevation than normal?

Edited by WakeGirl

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Scot

Make sure the throttle is actually opening all the way. I mean pulling the carb ALL THE WAY OPEN. Also look for air restrictions. No I am not calling you stupid, Just check all the simple stuff first.

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Randy
Make sure the throttle is actually opening all the way. I mean pulling the carb ALL THE WAY OPEN. Also look for air restrictions. No I am not calling you stupid, Just check all the simple stuff first.

Thanks for the replies so far everyone. I did run the boat checking the carb and it is open all the way including the secondaries at wot. I am currently running a 13 x 13.5 four blade stainless. Tried the stock 3 blade 13 x 13 stainless that came with the boat and it only raised the rpm by 100. Do not like the way the 3 blade performs at all though. I had a 96 reponse before this boat with the 320 monsoon. Ran the exact same blade 13 x 13.5 and it ran 5200 rpm and about 53 mph. Just wondering why there is such a difference with only 10 h.p. difference between boats.

That sounds about right. It is pretty linear 4400 RPM is 44 miles an hour in my 96 RLX.

You need more RPM. I am running an ACME 13 Dia x 12 pitch. I can get to 5200 RPM with my engine, A 320HP Monsoon (350 Vortec with MPI). Carbed you should be at 310 HP for the standard 5.7L Vortec Marine Engine.

Your boat may have a 13xll prop and be causing it to loose some top end.

I would run about 5200 rpm with my 96 Response as well, pushing about 53 mph with the 4 blade. The sportster seems like pretty much the same boat except for being carbed, 13 x 12, would that make that much difference from running a 13 x 13 that came with the boat?

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WakeGirl

If anything, the Sporty should be a little faster (it's on the faster hull, plus it's smaller & lighter). If you were running a 13x11, you'd be pushing a lot more rpms than that. For every inch that you drop in pitch, you'll pick up about 150-200 rpms. Once again, are you sure that your setup is calibrated? Speedos in boats are notorious for this kind of thing, plus if you replaced the engine then maybe the tach is off. Are you running at a different elevation than you did with the Response?

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aneal000

The sportster should easily run 49+mph. If you are only getting 44 something has changed. My first thought is the motor swap. I'm guessing you probably didn't get an Indmar replacement shortblock? I'd suspect your new motor might be a standard off the sheld Marine 350 that only puts out 240-270 HP range - not 310. Slider had a good idea, see if you can even get the motor to spin above 4400 rpm by putting a smaller prop on it. I'd also check with the place you bought the motor to see what the recommended WOT RPM range is and what the "actual" specs are. Sportsters are super light boats that run real fast. Your correct to believe you should be getting more out of it.

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88Skier

My 310 carb Sportster runs at 5100 with a 13x12 acme. I'm about 75 feet above sea level.

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Baddog
My 310 carb Sportster runs at 5100 with a 13x12 acme. I'm about 75 feet above sea level.

What speed at 5100?

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St. Ellis

I am questioning your set up as well. My previous Sportster ran @ 5100 and would hit 50+ on a strong day.

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88Skier

My 310 carb Sportster runs at 5100 with a 13x12 acme. I'm about 75 feet above sea level.

What speed at 5100?

The speedo says just under 51 but, I have never calibrated it. I would guess around 50-51 though based on the 1 mph to 100 rpm rule of thumb.

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Scot

My 1998 Sportster runs 4900 rpm @ indicated 48.5 mph. not calibrated but both spedos were close. Will run a good 46 even in rough water. Are you trying to barefoot or race. I did not go over 38 mph all year, as long as the damn thing starts and skis we go.

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MoonDawg

Is the engine broken-in yet? (about 30 hrs)

Are the rings seated? (if running super rich at break-in, they may never seat)

What does a compression check show? (9.4:1 ratio should = 138 psi in a perfect world)

Is the ignition advance retarded? Initial should be about 10 deg BTDC

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Randy
Is the engine broken-in yet? (about 30 hrs)

Are the rings seated? (if running super rich at break-in, they may never seat)

What does a compression check show? (9.4:1 ratio should = 138 psi in a perfect world)

Is the ignition advance retarded? Initial should be about 10 deg BTDC

Well, the engine has about 70 hours on it now, and I too thought it might be timing, it was reading 10 degrees but when it throttled up it seemed like there was no advance, it has the distributer with the ecm in it but not sure how it knows to advance, there is an extra plug in with no wires going to it. On my response with the same distributer there was the extra wires going to it, but that was also fuel injected. I tried an older distributer with vacuum advacne and it read 25 degrees when throttling up. but no difference in speed. I will do the compression check just for good messure. Also confirmed with the manufactuer that this is a vortec 310 motor. Still wondering if the right cam set up is installed though!

My 1998 Sportster runs 4900 rpm @ indicated 48.5 mph. not calibrated but both spedos were close. Will run a good 46 even in rough water. Are you trying to barefoot or race. I did not go over 38 mph all year, as long as the damn thing starts and skis we go.

I was also wondering about the speedo calibration. It doesn't look like there is any way to adjust them. There was an external dial on my 96, but nothing on my 98, any suggestions on how to go about this?

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aneal000
I was also wondering about the speedo calibration. It doesn't look like there is any way to adjust them. There was an external dial on my 96, but nothing on my 98, any suggestions on how to go about this?

You can not calibrate the speedo in a 98 Sportster. You could replace the stock ones with Airguides, which is what I did. Or you could simply use a GPS to let you know how far off yours are.

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MoonDawg
....Still wondering if the right cam set up is installed though!

GM is secretive about the cam specs so that would be hard to test.

Since you are running a carb you can easily add a "hot cam". That is not so easy with the EFI. I would like to add More Power to my Monsoon but I am faced with reprogramming the GM MEFI...

There are some others that have a carbed engine and used the ZZ4 cam kit, 385 Fast Burn Heads and 4 inch Exhaust with really good results (58 MPH and about 385HP). This was with a 13 x 12 prop. The Vortec heads are not quite as good, along with stock exhaust so perhaps you might see 50-52 MPH and 340HP.

Do you feel like throwing a few hundred dollars more at the problem?

Edited by MoonDawg

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uk_exile

might seem a dumb question, by why the concern over top speed ? It's a towboat, the max you ever need is 36mph. If it's starting fine, pulls out of hole well and doesn't scoff fuel, who cares !

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Randy
might seem a dumb question, by why the concern over top speed ? It's a towboat, the max you ever need is 36mph. If it's starting fine, pulls out of hole well and doesn't scoff fuel, who cares !

Just want to reply to to this last post: The reason I am concerned is because when you spend this kind of money on a product you want it to perform how it should. When I am being robbed of 6 to 8 mph and can't go above 40mph with a barfooter I am concerned and this tells me that something isn't right. And if something isn't right, what other problems might result down the road.

And for everyone else that has tried to help in this matter, I greatly appreciate it. I think we have eliminated the prop and pitch as being a problem. I am also thinking that possible the transmission may have been tampered with. I will be inspecting the heads and cam and see if anything comes of that. Thanks!

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uk_exile

ok, I understand your reasoning. Iit just seemed like you were prepared to spend a lot of money on something that wasn't really going to effect the way it's used.

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1FootDan

Check your firing orderd on your spark plugs! I had a similar probem and turned out the 2 and 4 plug were reversed... In your manual you will see that your firing order is 1-8-4-3-6-5-7-2. When you are in front of the water pump (facing rear), the cylinders are counted from the one that is the closest to the front going to the rear. There is an ofset between each side of the block. Check that and give me an update. I have a 1999 sportster with the 31ohp and I WOT at 48MPH. Surfing.gif

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1FootDan

So, have you solved your problem?

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