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Rough running

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captain_peter

Contributing Member
"For you experts out there to

"For you experts out there to scratch your heads as I been doing for the last while.
Engine starts, idles, accelerates, at 2200 runs rough, at higher speeds smooths out again.
Where do I start looking."
 
"Capt. I will give it a try, F

"Capt. I will give it a try, First we need a little info, the age,motor size, type carb,etc.At a steady 2200 rpm does it continue to run rough or will it smooth out. Could be distributor problems, springs for counter weights are weak causing advance to come in to early causing that trouble spot. Or carb, weak vaccuum signal at that rpm causing a stumble at that particular rpm."
 
"Hi Harry. Engine is 89 GMC&#4

"Hi Harry. Engine is 89 GMC( OMC )4.3 ltr. 4bbl rochester carb recently rebuilt (2 yrs )no vacuum attachments except for choke, Distributor,checked counter weights seem to respond,doesn't mean They are 100% I have no way of checking. At steady 2200 rpm it continues to run rough. Where could I purchase springs for the distributor ?
When you say weak vacuum signal that would be in the ventury is in it?
Thanks Harry for your feed back."
 
"springs are PN 984399 and you

"springs are PN 984399 and you can order from this site $7.50 (that can't be right, nothing on a boat costs under $50 right???)
happy.gif


http://www.marineengine.com/newparts/part_details.php?pnum=OMC0984399&ptype=&Eng ine=&Model=

Wish I had something else to say about this one, other than you have the option to swap parts between such as, coil, plugs, wires even the carb fairly easily. Not the most scientific method to troubleshoot though."
 
"Hi Bob Thanks for the feed ba

"Hi Bob Thanks for the feed back, swapping parts is probably the simplest way of finding out. with out buying and replacing lots of parts.
Is there also kits available to rebuild the distributor ?
Would a pressure test help in any way locate the problem?
Thanks"
 
"rough like coughing, sputteri

"rough like coughing, sputtering?
or rough like shuddering?
If the latter, I'd look at the prop and for a bent prop shaft, loose u-joints, marine growth on hull etc.

If the former,
does it do it in neutral?
or just under load?
"
 
"Hello Hy Stat,
Its rough li

"Hello Hy Stat,
Its rough like if you took one plug out of a spark plug and it does it in neutral and on gear"
 
"Hello ones again, I got the d

"Hello ones again, I got the distributor out but I am having difficulty changing the springs. There is a piece of plastic that is in the way. The pins that hold the springs slide inside the slots of this plastic piece. Any clues ? It's a Prestolite for six cylinders 1989
Thanks"
 
"Hello Capt. sorry about slow

"Hello Capt. sorry about slow response,been very busy. I have not worked with the prestolite yet (noticed i said yet) so not sure about the plastic piece. When did this problem start, first of the this season or after some other changes were made,and yes the weak signal would be in through the venturi due to a clogged air bleed or internal issue with carb.If the springs do not work and you have the option of trying another carb i would try that.Curious to know!"
 
"Hey Capt. Check your adjustme

"Hey Capt. Check your adjustment also on that carb using a vacuum gauge on a vacuum port that is a straight source of vacuum,(base of carb or intake) not on a ported vacuum and adjust it while in gear tied off or on the trailer in water under a load until you reach the highest vacuum point while turning in or out."
 
"Hi Harry thanks for taking ti

"Hi Harry thanks for taking time out on your busy schedule, I tried to find a vacuum source on the base of the carb and intake manifold and hard to believe there is non. The only vacuum that there is from the carb is the one going to the coke. Would this work as well?
Going back to your first question regarding when I noticed this problem ? actually this started a while ago and is just got worse over the years. Since then I did complete tune ups, put electronic ignition, rebuilt carb,and not ting made any difference so now I am hoping the springs in the distributor would make a difference I would keep you informed.
If this won't work I will try the carburetor exchange with the other engine.
Thanks"
 
Finally I got the springs for

Finally I got the springs for the distributor counter weights for both engines and replaced both of them. But no improvement. I fiddled with the timing up and down and the only change I noticed instead of running rough at 2200 rpm was doing it at different Rpm. So if I retarded the timing it ran rough lower rpm. so what's the next plan of attack ? Any Ideas ? Thanks
 
"Thanks chiefalen for your qui

"Thanks chiefalen for your quick response. I just discovered on my Seloc manual that there is a vacuum outlet right behind the carburetor on the intake manifold.Farley large set screw, next time Iam at the boat I will take it out and match a fitting. Thanks for the website. Answer to your question on a warm motor and the throttle fully down, runs smooth with out checking the vacuum."
 
"You can also use the vacuum p

"You can also use the vacuum port that drives the choke pulloff (vacuum break). If you are doing the idle mixture check, just insert the vac gauge hose and you can leave the vacuum break hose off. If running in gear, you'll need to put a "tee" in the line. Much faster than pulling the pipe plug.

Did the issue exist before the carb rebuild two years ago?"
 
"Thanks Makomark for your inpu

"Thanks Makomark for your input.
I was at the boat yesterday and tried to take the plug out with no success. The plug has a square hole and I could not fit a 3/8 or 1/4 inch ratchet, the 3/8 is too large and the 1/4 is too small obviously is some thing in between.
This engine always ran rough at this speed 2000 rpm and I been dickering with it for a long time. changed points to electronic conversion rebuilt the carb, total tune up. Notting have made any difference so far.I have to do a compression test dry and wet and see if the devil will show up.
Thanks"
 
"Hello ones again, I was at my

"Hello ones again, I was at my boat this weekend trying to do some tests but with no success.
Tried to do compression test for the first time. And boy was crazy trying to press the gauge against the plug hole streched to the back side of the engine. Could'nt get an accurate reading, hard to fight the forses coming out of the plug hole. Is there a better gage perhaps that screws in the hole or what else is available out there ?
I managed to take the plug out from the intake manifold and replace it with proper adopters to do a vacuum test ( horray ) but when it came to adjust the idle screws on the carburators I had notting to match this weard flat blade aprox 1/8 x 1/4 besides there is another bunch of hoses and stuff in the way. Any clues ?"
 
when i took the plugs out they

when i took the plugs out they all looked the same not fully burning the fuel sounds like a choke problem but the choke seem to open fully when warm. what you guys think ?
Thanks for any feed back.
 
"the 'press on' gauges

"the 'press on' gauges are, as you found out, awkward at best. You can buy a good thread in gauge fairly cheaply and the accuracy and ease make themm money well spent.

The screws are referred to as D shaped-two parallel flats with curved (D) ends. Yoou may find it easier to pull the carb, remove the screws, and slot them just deep enough for a screwdriver to fit. There are also flexible shaft adjusting tools for a few more bucks.

I doubt the mixture screws will fix your 2200 rpm issue though it can't hurt to get them right.

when they rebuilt the carb, did they change the float as well as the needle and seat?

As suggested earlier, get a good gauge and do the compression test. Then run somme of the vacuum gauge exercises noted in the link. take a pencil and paper and write down the results - you'll be glad you did. Also, look down into the carb throat, when running, and note and differences between the primary bores."
 
"Hi Mark thanks for taking tim

"Hi Mark thanks for taking time to help out.
Back to your question when they rebuild the carb was suppose to be complete but than again who knows. I was reading a chapter on Rochester 4 bbl carbs.(Power system) enriches the fuel mixture in the primery bores. This system is made up of a vacuum-opearated power piston and a spring located in a cylinder connected by a passage to the intake manifold vacuum. The spring under the power piston operates against manifold vacuum and pushes the power piston upward. I am wondering if its all related to this power cylinder, since its only doing that at a specific rpms? when I checked the plugs they were all the same on both engines fuel is not burned hundred per cent. You think is better to just put a new carb. Instead playing around with the old carb. And if a replacement is required what do you suggest to stick to the same brand or use a substitute?"
 
"very familar with the meterin

"very familar with the metering in a q-jet. The only way the power system would be causing you a problem was if it wasn't reassembled correctly.

I think Bob's suggestion is fastest. If it turns out to follow the carb, you know what the problem is caused by. My preference is to fix what you have but that may not be the best choice for you. You will NOT find an original or service replacement NEW carb. Best case would be a remanufactured unit of the original model. I know many people that have swapped their Rochesters for edelbrocks and been very happy with the end results.

Before spending money on new parts, I'd still get the gauges out and make some measurements. if you do swap carbs, I'd suggest getting a few base gaskets BEFORE getting out the wrenches."
 
Thanks guys for getting back t

Thanks guys for getting back to me so quick.The reason I haven't swapped the carbs is because the other engine is doing the same but more mildly and the other one the carb was not rebuilt. One of my fellow boaters was suggesting to replace with edelbrocks he claims that on his engines work marvelous and the gas consumption have imnproved by quite a margine.
 
"Thanks guys for all the effor

"Thanks guys for all the effort in trying to help and by no means Iam not ignoring your suggestions, it's just at times get very frustrating. in the mean time I have winterized my boat because after this weekend they will be raised out of the water,and the water became too shallow to run the engines since the out-drives were sinking in the bottom muck. Running the engines means siphoning silt into the engines cooling system."
 
"Hey Peter...where abouts is y

"Hey Peter...where abouts is your boat, ours start coming out after Thanksgiving too plus the water in our channel is very low this year. Ojibway Bay Marina Lake Couchiching."
 
"Hey Bob, is good to know you

"Hey Bob, is good to know you are in my neighbourhood.We dock there quite often I am in lake Simco the lower end of Cooks bay in Holland river at Albert's marina."
 
"OK...I know where you are, ac

"OK...I know where you are, actually I live in Keswick on the east shore of Cooks Bay but we keep the boat up on Couch...it just didn't work for us to be within 5 mins of the boat, might sound funny but we weren't using it enough!

ps....I've been down to Albert's a few times many years ago...couldn't handle that long slow trip in the river!"
 
Yes I know what you mean about

Yes I know what you mean about the slow drive in the river but I hate driving in the traffic down the 400 hwy since we live in Thornhill. Only 40 mins from my home to Albert's. I consider that's pretty close. Yes I agree hundred per cent your location is great cause you don't have to cross Simcoe each time you want to go north.
 
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