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  #1  
Old 08-29-2009, 05:43 PM
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JBB JBB is offline
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Boat: 1988 Mastercraft Prostar 190 351 Indmar Powerslot
Location: Southeast
Posts: 27
What else can i do?

Posted a thread a couple weeks ago about my '88 prostar with 351 Indmar. The engine starts and idles great but starts shuttering/losing power around 1500 rpm wether in the lake or under the carport. I've done the following:
1. Rebuilt carb
2. Replaced filters
3. New fuel line
3. Checked all tank fittings
4. Checked fuel pump pressure
5. New points, cap, rotor, wires, plugs
6. Checked/lubed distributor weights/springs. Rotor shaft was seized up and i fixed that.
7. Baffles in muffler were broke and obstructing so i replaced with straits
All of this and still the same. I am going to verify the timing but know that this hasn't been screwed with since it last ran right. I feel like it is something associated with the distributer. Also, I noticed some oil residue coming from the valve covers at the low end (back). Obviously I need new gaskets but could this be causing a vacuum leak. Please help anyone!!
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  #2  
Old 08-29-2009, 06:29 PM
FrankSchwab FrankSchwab is offline
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Boat: 1998 Maristar 200VRS
Location: Phoenix, AZ
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I got no answer for you, but I'll add a comment to see if it brings in a few more readers.

Seriously, good luck. I hate those kinds of problems. It sounds electrical to me, as you're getting the same results under load or no load, based on RPM. The spark system really doesn't care much about load most of the time, but is heavily RPM dependent. However, this problem needs an experienced mechanic, and my best shade-tree mechanic days are long behind me.

Have you checked compression? It seems the only thing you haven't mentioned. Perhaps a bad coil? Does the stuttering happen on the lake at all throttle settings as you go through 1500 RPM (can you whack the throttle wide open and have it work better)? Any black smoke or heavy gas smell out the exhausts at this point (flooding)? Have you tried pouring a tiny bit of gas down the carburetor when this happens (starving)? Hook up a timing light and watch the timing as you throttle up - if it's erratic, that's a good hint.

Hopefully one of the engine masters on here will be by to help out; those questions are just to keep you busy until someone who knows what they're talking about shows up.

/frank
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  #3  
Old 08-29-2009, 07:31 PM
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JimN JimN is offline
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Quote:
Originally Posted by JBB View Post
Posted a thread a couple weeks ago about my '88 prostar with 351 Indmar. The engine starts and idles great but starts shuttering/losing power around 1500 rpm wether in the lake or under the carport. I've done the following:
1. Rebuilt carb
2. Replaced filters
3. New fuel line
3. Checked all tank fittings
4. Checked fuel pump pressure
5. New points, cap, rotor, wires, plugs
6. Checked/lubed distributor weights/springs. Rotor shaft was seized up and i fixed that.
7. Baffles in muffler were broke and obstructing so i replaced with straits
All of this and still the same. I am going to verify the timing but know that this hasn't been screwed with since it last ran right. I feel like it is something associated with the distributer. Also, I noticed some oil residue coming from the valve covers at the low end (back). Obviously I need new gaskets but could this be causing a vacuum leak. Please help anyone!!
If you changed the points, you HAVE TO check the timing. It's never the same after changing any ignition parts that may require adjustment. Look at the timing mark- if it jumps around, the bushing(s) have worn. Valve covers have nothing to do with vacuum.

You need only a few things for a motor to run properly. Compression, proper air/fuel mix and spark at the right time in relation to the valves and pistons.

Check the timing and set it at idle. Take it out on the water for any other performance related adjustments. None of them will be correct if you do them on the trailer. The motor must be under load and you can't provide that unless it's in the water. Did you check the compression? (needs to be done with the motor at normal operating temperature, all plugs removed)

If you can, do a vacuum test. That and the compression will indicate the general health of the motor.
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  #4  
Old 08-29-2009, 09:27 PM
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JBB JBB is offline
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Boat: 1988 Mastercraft Prostar 190 351 Indmar Powerslot
Location: Southeast
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Ok. Sounds like i have three major things left to do. Check timing, compression, and vacuum. I sprayed some carb cleaner around intake while running but no reaction. Is there something more to testing vacuum. Also, JimN, I think I saw a thread you posted on how to test compression. I'll try to find that. Thanks.
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  #5  
Old 08-29-2009, 09:42 PM
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Boat: 1988 Mastercraft Prostar 190 351 Indmar Powerslot
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I had an extra coil and it did the same with it. Engine accelerates fine until it gets around 1500rpm and start shuttering and if I keep throttling it will just bog down. If i just shower down on the throttle is just wants to choke out. No black smoke or excess fuel odor
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  #6  
Old 08-29-2009, 09:59 PM
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JimN JimN is offline
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Quote:
Originally Posted by JBB View Post
Ok. Sounds like i have three major things left to do. Check timing, compression, and vacuum. I sprayed some carb cleaner around intake while running but no reaction. Is there something more to testing vacuum. Also, JimN, I think I saw a thread you posted on how to test compression. I'll try to find that. Thanks.
Vacuum gauge for testing that. Carb cleaner just shows where a leak is if it's around the intake manifold but there are other places it could be leaking. If getting it down to 650RPM for idle takes a lot of messing with the carb, a leak is a possibility. How clean is the flame arrestor? If it's clean and you have carb cleaner that makes the RPM increase, give it a shot of that at 1500 RPM when it lags to see if it wants to increase or decrease the RPM. If it increases, it's a lack of fuel and if it decreases, you may already be running rich.
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  #7  
Old 08-29-2009, 10:46 PM
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Boat: 1988 Mastercraft Prostar 190 351 Indmar Powerslot
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Where do i connect the gauge and what is the acceptable range?
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  #8  
Old 08-30-2009, 11:05 AM
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JimN JimN is offline
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Quote:
Originally Posted by JBB View Post
Where do i connect the gauge and what is the acceptable range?
I would try to find a service manual for this motor.
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  #9  
Old 08-30-2009, 03:22 PM
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Boat: 1988 Mastercraft Prostar 190 351 Indmar Powerslot
Location: Southeast
Posts: 27
Ok, I checked compression and I'm getting:
1-151
2-149.5
3-149.5
4-149.5
5-149
6-149
7-148
8-149.5
Checked vacuum and I'm getting 16-17 inches at idle and 20" at 1500rpm before it starts to shutter/bog. I also shot carb cleaner through the flame arrestor and the rpm decreased. Also when I pulled the new plugs they were fouled up.
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  #10  
Old 08-30-2009, 03:25 PM
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Boat: 1988 Mastercraft Prostar 190 351 Indmar Powerslot
Location: Southeast
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Forgot to mention that I have not checked timing yet. Waiting on a buddy to do it for me. I've got his timing light but haven't found any step-by-step instructions on here to help me do it.
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