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WOT less than 3k

GAS engine, transmission and generator repair and maintenance discussion forum.
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Capn Crunch
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Re: WOT less than 3k

Postby Capn Crunch » September 13th, 2018, 12:44 am

tomschauer wrote:Source of the post all of the above plus, with the right prop (and drive ratio if the drive was changed) it should plane at 3400, even if it takes awhile to flatten out. Did you leave it at 3400 for a minute or two? and did it start to lay down?


We did stay at WOT for a few minutes and no speed increase.

km1125 wrote:Source of the post Find out the drive ratio for the trans. Then you can do some simple calcs to see where that prop should be in term of speed, with a bit of slip. If it is only a 13 pitch, then you should be jumping up on plane even if you're not maxing out on speed.

I think something is still holding the engine back. The total timing thing could be that, or you have some restriction in the fuel delivery.


OEM says 1.50 to 1, I'm assuming current drive is the same. Forgive my noob-ness, how do I confirm drive ratio, and once I do what simple calc?

Thanks for any edd-u-mah-k-shun you send my way!

Mark

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Re: WOT less than 3k

Postby km1125 » September 13th, 2018, 10:00 am

With 3400 RPM, a 1.5:1 gear and 13 pitch prop, at 14MPH that means a 50% slip in the prop. That's pretty severe.

If the engine made it to 4400 RPM with the 1.5:1 gear and a 13 pitch prop, that would result in 27MPH with 25% slip. That's probably in the range of normal.

You can check the gear ratio on the trans by finding the part numbers on the tag somewhere on the trans or reduction gear.
It may state the reduction directly, or you might have to look up the model number to see what it is.

You can use this site to check slip:
http://www.mercuryracing.com/prop-slip-calculator/
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Topic author United States of America
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Re: WOT less than 3k

Postby Capn Crunch » September 13th, 2018, 11:54 pm

Okay, today's events. Hauled boat to verify prop - confirmed, 16 x 13. Mechanic's plan, approved by shop owner, is to come early tomorrow AM and first do a compression check, then replace fuel filter if previous checks out. If sea trial nets same results (3400 rpm and no speed) he then plans to pull Rochester 4bbl and rebuild. If that fails it's anyone's guess on where to look further, so I'm reaching a bit... following are my questions:

1) Isn't there a check valve on the fuel tank vent line? If so, how do I find it? Is it difficult to replace, and would it create issues like this?

2) If the coil were going bad and the spark were diminished, would that cause issues similar?

3) Any chance the spark arrestor/air cleaner being dirty would also create these issues?

Thanks again all, learning much from those wiser than me!! greatly appreciated.

Mark
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Re: WOT less than 3k

Postby bud37 » September 14th, 2018, 8:23 am

Howdy......there is an anti-siphon valve ( check valve ) or should be where the fuel line ( not the vent line ) attaches to the tank, looks like a barb fitting ....that would be the first thing to check and clean ( its free ).....next remove the flame arrestor and clean ( again free )........please before the carb gets rebuilt etc get the mechanic to check the ignition timing curve and total timing, if for any reason that is not correct you can go in circles for ever.....compression check is good idea and will give you an idea of the state of health of your engine.

Glad you verified the prop as that is a bit telling.
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Topic author United States of America
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Re: WOT less than 3k

Postby Capn Crunch » September 14th, 2018, 10:10 am

bud37 wrote:Source of the post Howdy......there is an anti-siphon valve ( check valve ) or should be where the fuel line ( not the vent line ) attaches to the tank, looks like a barb fitting ....that would be the first thing to check and clean ( its free ).....next remove the flame arrestor and clean ( again free )........please before the carb gets rebuilt etc get the mechanic to check the ignition timing curve and total timing, if for any reason that is not correct you can go in circles for ever.....compression check is good idea and will give you an idea of the state of health of your engine.

Glad you verified the prop as that is a bit telling.


Again, thanks for the input. To check timing curve and total timing is that done under way? Never had to perform in depth timing like that, but seems to me running @ 3600 rpm with no load could damage engine? Thanks!
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Re: WOT less than 3k

Postby km1125 » September 14th, 2018, 10:50 am

DEFINITELY remove and clean the flame arrestor if there is any doubt that it is clean. It will DEFINITELY affect performance if it is seriously clogged up. It will also cause you to use a lot more fuel at any RPM or load.

You can check the timing advance at the dock. You do need to run the engine up to speed, but not necessarily top speed, just to where the max advance is on the curve. It should be obvious and you raise and lower the speed if the advance is happening smoothly and consistently as the RPM is going up and down.

No valves on the vent line, but you could have something clogging it up. You can pull it off at the tank and blow through it to the outside to see if there are any restrictions.
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Re: WOT less than 3k

Postby bud37 » September 14th, 2018, 12:05 pm

Capn Crunch wrote:Source of the post Again, thanks for the input. To check timing curve and total timing is that done under way? Never had to perform in depth timing like that, but seems to me running @ 3600 rpm with no load could damage engine? Thanks!

If your mechanic is good he will know how to do it correctly, all at the dock, and it cant be just seeing the timing mark move somewhere with the light, it has to be fairly accurate ( dial back light or what I like is timing tape ) or the engine won't have the power to pull the skin off a grape.

Timing is in my opinion very critical on a boat, the curve is more important than on a car especially a single engine boat.....again good luck man..
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Re: WOT less than 3k

Postby waybomb » September 14th, 2018, 12:11 pm

Get a timing light with an advance adjustment. Find the line on the damper and paint it white. Fine ZERO on the indicator and paint it white.

Once you determine what your base timing is and at what rpm, with engine running, bring the two white lines to match each other, and then look at the timing light to see what your base timing actually is. If it's off, adjust.

Then, determine what rpm max timing is in. Have somebody bring the engine up to that speed, and bring the two white lines to match each other with the adjustable timing light. If base is right and all-in is off, the module must have a glitch. There is
no mechanical advance in a TBIV system, it's all in the module.

My TBIV book only shows the curves for 6/14, and 8/24. I looked through some others and I thought I had a book that showed them all, but I can't find it. Viper???

If you have the actual curver, you could also check advance anywhere in between idle and all-in.

I would guess base timing is at 8 degrees at 700 rpm, and total timing would be 30 degrees at 3600. BUT, find out for sure before you adjust anything!

And make sure your engine actually was spec'd with that 8v-22 module.
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Topic author United States of America
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Re: WOT less than 3k

Postby Capn Crunch » September 14th, 2018, 12:23 pm

So compression check proved interesting:
1) 160
2) 130 and too clean, plug looked new
3) 150 clean also
4) 150
5) 150
6) 150
7) 175 and WET with fuel
8) 160

Mech says is carb is going to rebuild, I asked about valves and intake... when he polled carb was standing fuel in top of intake. I guess I'm going to just leave it up to them... the question in my mind, are these numbers off enough to suggest motor is not rebuilt but a tired motor instead?
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Re: WOT less than 3k

Postby bud37 » September 14th, 2018, 12:35 pm

#2 needs further looking at, a little over 15% low....plug clean and looking new sounds like it is not firing.......#7 could be a bit high as a result of a wet cylinder and not firing. Did you say the heads were rebuilt ?....well they are on their way, lets see what happens.
I have seen some interesting things come back from a machine shop so nothing would surprise me.

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