OMC Chevy 350 Rough Idle

Hi,

I am about to go bananas over this problem and I am hoping someone can put in some great input.

OMC Electric Shift 800
Chevy 350

Problem: After I bought my boat about a year ago I am just now getting it cranked and trying to tune it up. When I would crank it, it would start just fine and the choke seems to work correctly only it idle at 1500 rpms after it warmed up. I took the throttle cable off pulled the throttle lever down and it would idle around 1200 RPM's then die if I went any lower. I adjusted the idle mixture screws on the Rochester Quadrajet 4MV and was able to get it down to fluctuate around 750-900 RPM's. Manual says to idle around 600-650. If i pulled the throttle down that far it would just die. I figured I would start out with the most common and replaced the plugs (.035) and plug wires...No change. I just recently rebuilt the Quadrajet and the only change is now when I pull the throttle down as far as it will go it is at 800 RPM's and fluctuates +/- 50 inconsistently. I am assuming the idle mixture screws are just out too far and that's the only reason it isn't dieing when the throttle is down all the way. Also I mixed sea foam in the gas tank and have changed the oil twice to flush out all the moisture in the oil form sitting so long. Any input would be greatly appreciated. I thought that it might be the fuel pump but it runs fine if i rev it up to 4000 + RPM. So that rules that out. I know on some configurations there is a vacuum line that runs from the carb to the fuel pump but mine is capped off at the carb and there isn't a vacuum port on the fuel pump. Should there be?

Re: OMC Chevy 350 Rough Idle

Hey Kevin welcome aboard!

Just so you know, the OMC 800 isn't electric shift. It's a mechanical shift. And the reason that I'm mentioning it at all is to ask if you've got the drive on or off? I'll suggest taking the drive off if it isn't to rule out a stiff gear case choking out the engine. At such low RPM, I would think that it was weird to be the case, but I'd try it anyway just to rule it out.

Re: OMC Chevy 350 Rough Idle

kevinomc1989 said:

Hi,

I am about to go bananas over this problem and I am hoping someone can put in some great input.

OMC Electric Shift 800
Chevy 350

Sounds to me like a vacuum leak,either the carb base or intake gaskets could be leaking.put your hand over the choke plate while its running,if it keeps running you have a vacuum leak. I have to same problem in my 86 OMC Chris Craft I havent got to fixing it yet.

Click to expand...

Re: OMC Chevy 350 Rough Idle

Hey and thank you for your post. I was under the impression that a 70's omc v6 or v8 wad considered an OMC 800 series. What i do know is that it is a 235 hp electric shift with a omc GM 350 v8. Ive had the drive off replacing the water pump and have expirenced the joys playing with the shift wire anyways im going to check for a vacuum leak but i think i figured it out. I didn't realize that the fuel water seperator bowl held the water and when its full it looses its effectiveness to seperate the water. I know... i know....pretty dumb huh. Well anyways i spent most of the day running the engine for 5-10s at a time filling up that bowl and then draining it and then said the hell with it and siphoned out the fuel tanks and poured some heat in. Plan on filling them up tomorrow and trying to get the remaining water out but im pretty sure this is going to fix it if there is that much water in the fuel for the bowl to fill up that quickly. Let me know wjat you guys think about my therory. Thanks again.

Re: OMC Chevy 350 Rough Idle

In the 70's, you had three different generations of Stringers - Electric shift (up until about 77 or so), hydro-mechanical (for just a few years) and then mechanical (last generation into the 80's). I know the most about the electric shifts ('cause that's the hand that Fate dealt me ), but only enough about the others to tell you the 400 and 800 series are after the electric shift (and I think are the hydro-mech and mechanical shift respectively).

In '77 the electric shift 235 looks to have been a 351 Ford, but in '76 could have been a 351 Ford or 307 Chevy. I don't see anywhere where they put a 350 Chevy into one, so you might have a bit of a mod going. Do you have a manual for it?

Re: OMC Chevy 350 Rough Idle

Good to know. Not much information out there on these beast anymore. Yes I do have a Seloc Manual and I completely know what you mean by there not being a 350 for those years. After pulling my hair out doing research I eventually ended up calling a company in Canada that still works on these drives and sells parts, (sterndrive.info) and informed them that I have a model number that doesn't match up to any records. They said it was actually pretty normal but yeah you could definitely be right about it originally being a 351 Ford. Any who... I decided to cut right to the chase today and ran the fuel line going to the electric fuel pump straight from a fresh can of fuel to rule out fuel completely. Now when I start it, it has to have fuel manually pumped into the carb( via accelerator pump) to start and throttle wide open. After it starts I pull the throttle down and then the engine speed slowly starts to die off and until the engine sputters to a stop. This sounds like a clogged line or bad fuel pump to me. I also noticed a small vacuum leak at the very base of the carb but I don't think that would cause the engine to slow down like that and eventually stop. I may have put that gasket on backwards so I will check that first and rule the leak out. The fuel pump has a very loud clicking noise when it is running...not sure if that helps.

Thanks again,

Kevin

Re: OMC Chevy 350 Rough Idle

Make sure you get as much as that bad fuel out as you can. Don't rely on the gas drier.

Re: OMC Chevy 350 Rough Idle

Thanks Bruce58 and I definitely will. Okay I just finished solving the vacuum leak at the carb base with a different gasket that came with the rebuild kit. Turns out I used the wrong one. Now the engine will start and run without dieing.....as long as I don't dip below 800 rpm's. I'm at normal operating temperature and have pulled the throttle down to idle and when I do it will get to 800 and start the fluctuating again. I am able to get it down to 600 which is what the manual says it should be but it will bounce around and eventually cut out. I am running the engine out of a fresh 5 gal. fuel can so fuel isn't the issue. This is my first carb but I am fairly sure I'm tuning it properly. Manual says to bring engine up to normal operating temp, then adjust idle mixture screws until you get the highest smoothest running idle. I started by screwing them in until the engine begins to choke and then back out to the smoothest run I can get. Then bring the throttle down to 600ish and it still fluctuates and or dies. The good thing is it starts right back up. Let me know what you guys think but I've done everything I can think of and the only thing left that I can think of is the timing could be off. I don't have a timing light or ANY experience with timing.

Thanks,

Kevin

Re: OMC Chevy 350 Rough Idle

Yes, you need to check dwell and timing. I would put in a new set of points as well. Use a dwell meter to set the dwell, not feeler gauges.

Adjusting timing is very easy but you may want to enlist the help of a friend the first time.

I was up in Soldatna last September to visit friends who have a house on the Kenai river. Beautiful area.

Re: OMC Chevy 350 Rough Idle

Thanks,

I'm going to go ahead and tackle the timing first and then dwell and points. Just read up on timing in the manual and talked to a friend so it sounds pretty easy. Just have to do it by ear cause I don't have a timing light. Thanks

PS Yeah I love it down here.. Especially now that the snow is gone

Re: OMC Chevy 350 Rough Idle

Ok,

So I just loosened the securing bolt on the distributor and slightly adjusted the timing to the best of my ability to run the smoothest possible but I am still in the same boat as before. I starting to think that this is less of a timing issue as it still is with fuel. I've established that the fuel is clean so it can't be that. I'm thinking that is must be that fuel pump. I've noticed a couple times that when the engine died at low RPM's that the fuel pump would start to get louder. Most of the time when it's running half way descent it seems to be quiet. Would an electric fuel pump fail this way? Runs great at high RPM's and bad at the lows. I really hate to have to try and replace it because it is electric which means it's aftermarket and I haven't been able to find one yet that looks the same as the one I have. I also started to jiggle some of the wires around the fuel pump area to see if I could get a response but nothing. I just figure that if this was a timing issue that the engine would consistently run poorly. Instead it seems to surge.

Kevin

Re: OMC Chevy 350 Rough Idle

First of all, don't time your engine without a timing light unless you want to detonate a hole in your piston.

Second, why do you have an electric fuel pump? Is it wired through an oil pressure switch so it doen't run with the key on and the engine not running?

Re: OMC Chevy 350 Rough Idle

That's the way I bought the boat. It's wired so it kicks on while the key is on and then turns off. Just took the pump off and rewired it so that doesn't seem to be the problem. Sounds like it may be time to take it to a professional if I can find one, unless there are anymore ideas. One more thing worth noting is there is an oil pressure gauge mounted on the engine. When the engine surges it seems to fluctuate the oil pressure a lot. Between 38psi and 50-60psi. No surprise there I suppose since the oil pump is directly linked to engine speed. Just thought it was worth mentioning.

Re: OMC Chevy 350 Rough Idle

You definitely need to get a timing light and a dwell meter! You can get by to start by gaping the points with a flat feeler gauge, but you need a timing light for sure! You have to set the point gap (dwell) FIRST as it affects your timing. Definitely start with new points and condenser. I found on my motor ('75 OMC 175, 302 ford) that using a dwell meter to set it right at the spec 31* it ran much smoother than gaping the points with a feeler. I found I was usually getting about 27-28* dwell by feeler when checked with a meter. That extra couple of degrees does make a difference. You may STILL have fuel pump issues, but at least you won't be fighting multiple issues at once. Look for my thread titled 'Stringer info' for some basic info from my factory owners/shop manuals.

Re: OMC Chevy 350 Rough Idle

Thanks,

I definitely will take a look and I'll go buy a dwell meter and check that as well as a new set of points and condenser. I think I know where to find a cheap timing light. I know there are different kinds and all. Is there a specific timing light that you suggest? Thanks again...There still might be some hope just yet

Re: OMC Chevy 350 Rough Idle

If you want a really nice one that will measure your spark advance digitally:
http://www.amazon.com/Equus-5568-Pr...U8J8/ref=sr_1_3?ie=UTF8&qid=1338877656&sr=8-3

If you want cheap:
http://www.amazon.com/Tooluxe-Xenon-Automotive-Timing-Light/dp/B000TE46SY/ref=pd_sbs_auto_8

And there are others in between. Search Amazon for many choices.

EDIT: The top one also measures dwell.

Re: OMC Chevy 350 Rough Idle

Was just looking at a conversion kit from point to electronic. It's Mallory's "E-spark" conversion kit. Think it would be easier and worth it to just go ahead and convert? I don't know a lot about points anyways and it will save money in the long run.

Re: OMC Chevy 350 Rough Idle

Personally, I would just stick with points.

If I was going the electronic route, I would just spend a little more money and buy a Delco EST distributor.

Re: OMC Chevy 350 Rough Idle

I would but that is a lot of extra money. Conversion kit is 100 with shipping

Re: OMC Chevy 350 Rough Idle

Yes but also comes with a full set of spark plug wires, cap, coil and rotor so the actual price of the distributor alone is somewhat less if you need to buy those other things anyway.

Conversion kits are not the most reliable in the world. The Delco EST is what OEMs put on boats so you know they are somewhat reliable.