OMC Chevy 350 Rough Idle

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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

I completely agree with you on that.....I'm just on a budget right now. I'm trying to decide if I want to


Buy:

a.)Timing light, condenser, points, and a dwell meter or....

b.)Timing light and the conversion package.

Both are roughly the same price and I think I would probably be better off in the long run with the conversion.
 
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Cheapthrill

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Re: OMC Chevy 350 Rough Idle

Hey Kevin, I would rule out fuel problems completely before you start playing with ignition. ONE problem can become many.I still say its fuel problem, and that pump sounds like its weak. Any electric pump will work. The points conversions are KICK ***! I have had a mallory conversion in my bike for 12 YEARS! and have not touched it. However,if you put it in your boat you have to add a little interface board so the ESC will work which momentarily cuts the spark when you put it in gear.There is a very good webpage on OMC stuff as soon as I can I will post it.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

So if its a fuel pump issue then I would obviously need to replace it. I've seen a lot of pumps out there. Would I need a low pressure pump? Especially since there is no return line?
 
bruceb58

bruceb58

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Re: OMC Chevy 350 Rough Idle

Cheapthrill said:
However,if you put it in your boat you have to add a little interface board so the ESC will work which momentarily cuts the spark when you put it in gear.There is a very good webpage on OMC stuff as soon as I can I will post it.
Click to expand...
He has an electric shift...not needed.
 
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Cheapthrill

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Re: OMC Chevy 350 Rough Idle

bruceb58 said:
He has an electric shift...not needed.
Click to expand...

Duh, I KNEW that..heh heh. You need an electric pump for a carbureter fed motor,so technicaly its a low pressure pump-4 to 8 psi. I would run a regulator on the carb just to be safe. Another thought- have you taken the fuel line off at the carb inlet & checked to see if there is a sintered bronze filter in there? If its clogged that could be some of your trouble too.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

I have replaced the metal fuel line from the pump to the carb and replaced the filter when I rebuilt the carb. I have been talking to some friends and just about everyone agrees that the next step is ignition so I plan on doing an electric conversion and timing the engine. If that doesn't work I will replace the fuel pump. I suppose before all that I plan to do a quick vacuum leak test at the intake manifold. Was told that could be a possibility.

It is also worth mentioning that I tested the compression before I started this whole mess with trying to solve the idle problem. Compression was 123-132 psi across all cylinders which isn't great but also not too bad. I didn't have the throttle open 100% either during the test which would have proven to be a bit higher compression so yeah.....Anyways I suppose it's on to the the ignition and thanks for confirming the low pressure fuel pump in question. I hope it doesn't come to that so I can save some money. Just want to run it a couple times this summer and then I will do a rebuild in the winter.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

Plan on replacing the intake manifold gasket today. Got to thinking about it last night and that would explain a lot. Rough idle, hard to start....etc. I'll post results.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

Got everything back together and waiting 24 hours for sealant to cure before initial start. I have turned the engine over by hand to 4deg. BTDC and the rotor is pointing towards #1 cylinder and I am running 87 OCT fuel. Is this where the timing should be set? My book doesn't say surprisingly or I just can't find it. I also purchased a timing light.

I noticed that my distributor (Mallory YL624 JV) doesn't have a vacuum advance. Just a single wire going to it which I assume is the mechanical advance and two vents on the side with screens. How should I go about timing the engine?
 
bruceb58

bruceb58

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Re: OMC Chevy 350 Rough Idle

You need to use your timing light....period! You can't even get close to the timing by what you have done. Should be 8? BTDC.

Mechanical advance is accomplished with weights and springs inside the distributor. The wire just goes to the points.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

Sounds good. I will go ahead with the timing light then. Thanks for the advice. I'm really new to this timing stuff.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

Okay I can't get it to start.

1.) Getting spark to number #1 cylinder because timing light will flash.
2.) Getting a puff of air backfiring thru the carb. (The engine is not firing...this is just air shooting backwards.)
3.) Checked and double checked firing order.
4.) Battery is at 12.01 volts and engine turns over at normal speed.
5.) Rotor is pointing to #1 cylinder when engine is at 8deg BTDC.
6.) Throttle is 1/4 open but I have tried completely open.
7.) Squirted some fuel down in carb.


Don's really know what to do now.
 
bruceb58

bruceb58

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Re: OMC Chevy 350 Rough Idle

I am assuming you pulled your distributor to change the intake manifold gasket.

You probably have the distributor 180? out. Make sure the #1 piston is on the compression stroke and not on the exhaust stroke when you have the rotor pointed at #1 spark plug wire. You can probably just yank it and turn it 180? to correct your problem.

You have a 4 stroke engine so the crankshaft is rotating twice for every one distributor revolution.

Next time when you need to pull your distributor, either mark where your rotor is pointing or rotate the engine to TDC and the rotor pointing at #1 and then pull it.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

Distributor was 180. Got it too at least run now but can't time it with the light cause it's too bright out and unfortunate I work nights. I suppose I will try and get it tuned as best I can for now and double check timing with light Wednesday night. Still running rough though. Wont know till I dial in the carb. and get the timing light on it. It just doesn't make sense. All symptoms point towards a vacuum leak and I even sprayed some liquid wrench all around the carb and intake manifold and no change. I suppose it could still be the ignition or the fuel pump but I guess we will see. Any new ideas would be greatly appreciated. The port side exhaust is restricted just a hair but I doubt it could make this much of a difference. (Had to block off riser coolant passage with a plate so I could run Half coolant/half seawater)

Also worth mentioning is a sucking noise coming from carb but that's normal right since it has to suck air and fuel in?
 
bruceb58

bruceb58

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Re: OMC Chevy 350 Rough Idle

The plates that block the cooling water typically don't extend into the exhaust area so that shouldn't be an issue.

Did you change the points yet? You need to do all the basics first like points, dwell, timing, fuel filters, spark plugs...basic tune up stuff before you start going beyond that.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

I had to make my own plate so yeah it extends a bit. If I can't get this tuned up tomorrow I'm gonna have to bite the bullet and do all that. Was just hoping it was a simple fix. Gonna get the conversion and fuel pump.
 
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Cheapthrill

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Re: OMC Chevy 350 Rough Idle

Check & make sure that you have or dont have a DUAL point distributer.if its dual one set could be bad. I still think you should replace that fuel pump with a new one. just my humble opinion. Sometimes problems like this will make you jump around to different things instead of eliminating one thing at a time.
 
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kevinomc1989

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Re: OMC Chevy 350 Rough Idle

Thanks man could definitely save some money. I went ahead and ordered it from marinedepot.com

Thanks and I will post when I get everything installed.
 
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