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Old 09-12-2011, 08:37 AM
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rjracin240 rjracin240 is offline
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Boat: Mastercraft ProStar 190 1988 351 Wheezer
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Wont stop detonating

Look for some advice to try tonight.

Here is the scenario:

The boat had been running great all summer, ran the boat several weekends ago and then took off for Iowa to drop my daughter off in school the following weekend. Came back and ran the boat the following weekend (just about two weeks of not running the boat). When running the boat, the boat would run up to WOT and then fall flat on its face. Diagnosed it as a fuel flow problem, so pulled the filter bowl and found some water in the bowl. drained water and then put it back together with the result of boat running up to WOT and maintaining it. New problem is it detonates/preignition when dumpping the throttle. If you slowly accelarate up to 2,000RPM there is no issue, however if you pass 2,000RPM then you have quite a bit of detonation. Might be my imagination, but seems more predominate on the STBD side.

Checked timing and have 10BTDC @ 600RPM in gear. Drained fuel tank completely and put in fresh 89 octane fuel. Checked all plug wires to insure they were seated in distributor and on plugs.

Was going to run boat tonight and run engine up past 2,000rpm and try and see what total advance is, as it seems to me that I have some sort of issue with distributor advance.

One friend said that he thinks for some reason I am going lean on the top end and this is causing the detonation. I know earlier in the year I was having some issues with detonation when I had the automotive carburettor from the previous owner. Once I put the new Holley on that problem went away. if there is a problem where something on the carb got plugged up, where on the carb would I look?

Any other suggestions would be appreciated.


1988 Prostar 190 with 351 and electronic ignition conversion
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Old 09-12-2011, 08:51 AM
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1redTA 1redTA is offline
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put some 93 in it and see if the detonation goes away. I would also run some seafoam thru it
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Old 09-12-2011, 09:45 AM
tommcat tommcat is offline
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do you know for sure it's deto that you're hearing? are there any signs of it on the plugs, how do they look? do they look lean?
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Old 09-12-2011, 10:43 AM
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rjracin240 rjracin240 is offline
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Quote:
Originally Posted by tommcat View Post
do you know for sure it's deto that you're hearing? are there any signs of it on the plugs, how do they look? do they look lean?
Pretty sure it is detonation, admit I grasped at some straws and replaced the exhaust manifold gaskets as they looked blown out at the bottoms of the exhaust ports. Thought I was mistaking exhaust leak for detonation. Not the case! Will pull plugs and see if they are lean, hate to think I need to go into a brand new carb.
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Old 09-12-2011, 10:52 AM
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JimN JimN is offline
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Quote:
Originally Posted by rjracin240 View Post
Pretty sure it is detonation, admit I grasped at some straws and replaced the exhaust manifold gaskets as they looked blown out at the bottoms of the exhaust ports. Thought I was mistaking exhaust leak for detonation. Not the case! Will pull plugs and see if they are lean, hate to think I need to go into a brand new carb.
Check your timing at higher speeds. If it's advanced too much, you'll see it. If you don't, all you can do is guess. You didn't mention checking the centrifugal advance weights & springs- do that, too. You might have a broken spring, or two.
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Old 09-12-2011, 10:55 AM
tommcat tommcat is offline
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Quote:
Originally Posted by rjracin240 View Post
Pretty sure it is detonation, admit I grasped at some straws and replaced the exhaust manifold gaskets as they looked blown out at the bottoms of the exhaust ports. Thought I was mistaking exhaust leak for detonation. Not the case! Will pull plugs and see if they are lean, hate to think I need to go into a brand new carb.
rejetting a carb is no big deal and you really cant expect one to be correct out of the box. jetting realistically could be adjusted several times from spring to fall just due to the temp changes. altitude and temp have quite an effect on jetting.

only a few causes for deto if that's actually the case. lean fuel mixture, too much timing at any given rpm or some hot spot in the combustion chamber such as a sharp valve edge or carbon build up. check fuel first since you did just do carb work.
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Last edited by tommcat; 09-12-2011 at 10:59 AM.
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Old 09-12-2011, 10:57 AM
tommcat tommcat is offline
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Quote:
Originally Posted by JimN View Post
Check your timing at higher speeds. If it's advanced too much, you'll see it. If you don't, all you can do is guess. You didn't mention checking the centrifugal advance weights & springs- do that, too. You might have a broken spring, or two.
question for you on that part jim, in a boat do you usually set the springs to have advance come in real early? what rpm do you try to get full advance in by?
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Old 09-12-2011, 11:04 AM
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Are all of your plug boots tight?
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Old 09-12-2011, 02:48 PM
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Carburetors don't like water sitting in them. Where did the water come from in the fuel and did you get all of it out? Check timing first. Like listed above check all of the plugs. Look at or replace wires. Check weights and springs in distributor. Look at cap and rotor. If all that looks good then pull carb and go through it. You don't need a new carb---you may need to rebuild or clean your carb.
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Old 10-17-2011, 12:58 PM
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rjracin240 rjracin240 is offline
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Finally got a chance to do some extensive troubleshooting while having a friend drive the boat. Drove the boat at about 2,500 while pouring some water down the carb (I can hear the groans now!) to decarbonize the engine, no luck still detonating. Double checked the timing again, set the mixture and and adjusted idle, still detonating. Then started looking and listening to engine to see if the detonating could be isolated. Found water spurting out of #1 cylinder head lower bolt and a little out between the head and block. The water spurting out of the head bolt was hitting the back of the alternator and was making a sound like detonating. My friend helping me out had his son with him so had him torque the head. in retrospect should have done it myself. Dont have a Indmar manual so use the PCM manual dowloaded from this site. Torque value for the cylinder head bolts was listed as being step torqued @ 90-100-112 So when he went to 100FTLBs the bolt snapped. Thinking if it was me would have felt bolt stretch and would have known to back it off. Valuable lesson for both him to know what stretch feels like before letting loose and for me not to have done this crucial step myself. Also has me curious as to wether the torque I got out of the manual is correct. Standard torque table for a 1/2-13 bolt was no more than 60ftlbs, with a note saying that if other secific torque was specified to use that!

Hoping that I can easy out the bolt left in the block by using a left hand bit and lots of penetrating oil and prayers!

Not sure if this is a common problem for the head bolt on #1 to loosen like that. Engine had steady vacuum at 15 with no oscillation so thinking the head gasket sealing #1 cylinder is intact.

Any suggestions or comments welcome
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