PROP or ENGINE SHAKE???? above 4,000rpm

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HuntOnTheWater
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PROP or ENGINE SHAKE???? above 4,000rpm

Postby HuntOnTheWater » Fri May 24, 2019 3:40 pm

I have a 15x8 diamondback with a 2:1 belt reduction, 383 stroker, and a 78” prop. Boat cruises great at 3600 to 3800 rpm. When I get into it above 4,000 it feels like the prop starts or engine start to shake and wobble. Thought it could be a valve sticking as well. Was curious if anyone had any input on this? Thanks

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby Deano » Fri May 24, 2019 4:37 pm

If you don't find something more accessible and/or obvious, I would suspect your flexplate is cracked/cracking.
Exactly the symptom(s) you might expect to result from such an issue.

Bearings in the drive going bad could possibly lead to similar symptoms.
It is worthy to note running a bad flexplate would likely take out the bearings with it over time.

I'm sure Searching that issue here, will afford you more info about those possibilities.
I'd also recommend calling Century Drive Systems and get their take. Robert is always very helpful.

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby Keeth1123 » Fri May 24, 2019 11:26 pm

I had an issue where at times I also thought it was a wobble or something to do with the prop. Turns out, coolant was getting past my head gasket on one side. Replaced both head gaskets high performance ones and no issues.

Might be worth checking the spark plugs after the symptoms appear to see if any have coolant. When I had the issue, it would be random and felt during certain rpm ranges as you stated. This would be an easy check before taking down to the flex plate.
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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby unforgiven11B » Sat May 25, 2019 7:49 am

Is this symptom something that’s been happening or just started out of the blue?

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby HuntOnTheWater » Mon May 27, 2019 1:27 pm

Keeth1123 wrote:I had an issue where at times I also thought it was a wobble or something to do with the prop. Turns out, coolant was getting past my head gasket on one side. Replaced both head gaskets high performance ones and no issues.

Might be worth checking the spark plugs after the symptoms appear to see if any have coolant. When I had the issue, it would be random and felt during certain rpm ranges as you stated. This would be an easy check before taking down to the flex plate.


It is random but typically around 4000 rpm. I just replaced the flex plate a month or so ago and I know this is a different issue than that. At low rpm my engine stays around 160 degrees but when i run it on up around 3800rpm for a while it will get on up toward 220-225 degrees and I will sit it down and let it cool back down for making long runs. May be something to do with coolant leaking since it wont keep the engine cool.

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby HuntOnTheWater » Mon May 27, 2019 1:43 pm

unforgiven11B wrote:Is this symptom something that’s been happening or just started out of the blue?



It has been going on for quite some time and I figured it was part of my cracked flex plate issue. I replaced the flexplate which was bad and also the intake gasket I was told it could be something with my ignition system maybe in my distributor cap going bad but I replaced that 150 hours ago.

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby Keeth1123 » Mon May 27, 2019 2:44 pm

The spark plugs will tell you. Mine were wet on one cylinder which was the one where the coolant was getting by. It slowly got worse until it was smoking. At first, it would not smoke only sputter every now and then.

I first stopped to see if the prop was ok because I thought it was wobbling or shaking like you said. Then I thought it was my fuel because it started intermittently or was only felt during a specific rpm range
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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby One Eyed Gator » Tue May 28, 2019 11:27 am

Are the secondary's working properly or fuel pump issue, could be going lean at higher rpm, would get hot as well.

Do you run a fuel pressure gauge?

Just a thought

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby HuntOnTheWater » Tue May 28, 2019 12:55 pm

One Eyed Gator wrote:Are the secondary's working properly or fuel pump issue, could be going lean at higher rpm, would get hot as well.

Do you run a fuel pressure gauge?

Just a thought



I do not have a gauge to see pressure. I was going to look at that this afternoon and t into the line with a gauge from the pump to the carbi reckon. I was told 3.5 to 5 psi minimum is what i am looking for.

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby Rich Andrews » Tue May 28, 2019 2:37 pm

Make sure the bolts that hold the gear box to the engine mount are tight and if the bushings are toast that doesnt make things any easier but a wobble is serious
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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby Deano » Tue May 28, 2019 9:24 pm

HuntOnTheWater wrote:... I do not have a gauge to see pressure. I was going to look at that this afternoon and t into the line with a gauge from the pump to the carb i reckon. I was told 3.5 to 5 psi minimum is what i am looking

If you are running a Holley, that 3.5 - 5 psi is likely contributing to (if not creating) your problem if that's all you have.
Years ago, had a Carter pump get tired on a mild 350 w/650cfm that was leaning out and acting stupid with 5 psi.
Replacing that pump and supplying with it with 7 solved the problem.

That incident taught me that having an inexpensive pressure gauge can save a bunch of time and grief. :oops:
No need to even mount it if you're just using it diagnostically. Mine lives in the toolbox by the timing light and vacuum gauge.

Holleys are designed for 7 psi max so you do not want more than that or you may well have a different problem.
Naturally, pressure is only half the battle . . . you need that pressure while supplying prerequisite volume.

As One Eyed Gator pointed out, running leaner will run hotter.
Running lean will also guarantee a decrease in longevity, whatever that may be.
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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby keys2pines » Wed May 29, 2019 8:02 am

Keeth1123 wrote:The spark plugs will tell you. Mine were wet on one cylinder which was the one where the coolant was getting by. It slowly got worse until it was smoking. At first, it would not smoke only sputter every now and then.

I first stopped to see if the prop was ok because I thought it was wobbling or shaking like you said. Then I thought it was my fuel because it started intermittently or was only felt during a specific rpm range

Definitely could be a misfire, check your plugs as stated above. Do a compression check while you have them out. Check for arching plug wires at night (look for sparks), didn't read what engine, but check distributor cap/rotor also.
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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby One Eyed Gator » Wed May 29, 2019 2:30 pm

Deano, I was the same about the fuel pressure gauge, set it and forget about it. But having it on the motor is being able to better determine the problem on the water before you screw something up. Just like I run an extra manual oil pressure gauge screwed directly into the block, no manual oil line to break.

Hunt on the water - Put the gauge as close to the carb as possible.

Does the shake goes away after your above 4,000?

The cammed 6.0 I have been messing with holley 750 DP had a nasty lean spot just before the secondary's started to opened. Put an wide band afr on it and it did had a spot where it spiked lean.

Wide band will tell you what is really happening fuel wise.
What Carb and was it tuned for the motor or just put on out of the box, rarely seen one correct out of the box.

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby HuntOnTheWater » Wed May 29, 2019 5:37 pm

One Eyed Gator wrote:Deano, I was the same about the fuel pressure gauge, set it and forget about it. But having it on the motor is being able to better determine the problem on the water before you screw something up. Just like I run an extra manual oil pressure gauge screwed directly into the block, no manual oil line to break.

Hunt on the water - Put the gauge as close to the carb as possible.

Does the shake goes away after your above 4,000?

The cammed 6.0 I have been messing with holley 750 DP had a nasty lean spot just before the secondary's started to opened. Put an wide band afr on it and it did had a spot where it spiked lean.

Wide band will tell you what is really happening fuel wise.
What Carb and was it tuned for the motor or just put on out of the box, rarely seen one correct out of the box.



I have a holley 750 with vacumn secondaries it was tuned for the 383 it is on top of but i have put atleast 500 hours on it since. I took it off last night and cleaned it up some and am going to clean some more on it here shortly and I bought a complete rebuild kit for it. It doesn’t clear up past 4000 only worse and loss of power. From my pick up tube in the tank the fuel line goes to a fuel water separator and then into the electric fuel pump. Then I would say a 5 foot run to carb. And a foot out of the pump toward carb I put an inline shutoff valve t fitting. That stays open constant to carb and sometimes when I need to fill up my generator instead of carrying a gas can I have a 2 foot piece of hose off of the other end of the t I can open for that. So i put the pressure gauge in there and was getting around 4 1/2 psi. With valve closed to carb i was getting 10 psi. So im assuming with valve closed to where i was putting the gauge in at i will have more than the 4 1/2 psi to carb. I bought new plugs to put in as well.

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby HuntOnTheWater » Fri May 31, 2019 8:24 am

Update: Rebuilt the carb and changed spark plugs. Went an ran the boat and it got rid of the shake I was having at high rpms, but engine still creeps up in temp. So I did a compression test yesterday and have 180 to 200 psi on all cylinders except number 5 cylinder there is about none. Took valve cover off and dont see anything out of the ordinary really springs look good. I am going to pull heads now. I guess it’s a bad head gasket.

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Re: PROP or ENGINE SHAKE???? above 4,000rpm

Postby unforgiven11B » Fri May 31, 2019 8:36 pm

I would suggest a cylinder leak down test. Will tell you more than Standard compression test.


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