1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

[Too Fast]

Sorry, this is the second post on this issue. The first time I though we fixed the problem but it didnt.

PROBLEM: With muffs or on lake symptoms are the same. Starts and idles GREAT. Once we take off and the engine has been running for a bit it starts missing around 2800-2900 rpms and up. It makes it impossible to get up to speed. Its very tempermental, the severity of the problems changes from minute to minute.

WHAT I HAVE DONE:
Replaced Plugs (ensured proper GAP of .035)
Replaced Distributor Cap
Replaced Rotor
Replaced condensor
Replaced points
Replaced Anti-Siphon valve
Adjusted dwell in accordance with the manual (tolerance is 36-41, adjusted to ~39)
Adjusted timing in accordance with the manual (manual says 1BTDC, I assume that means one hash mark past zero on the BEFORE side?) There wasnt a ONE hash mark so I split the difference between ZERO and the TWO hash mark.

I just dont understand this AT ALL, two weeks ago it ran PERFECT!!! I have exhausted all my theories and am ready to call it quits. I hope its nothing serious with the engine. It seems to do it once the engine warms up. Does anyone have any idea for what direction I need to go in next.

Josh

 

[Randy J]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

question your anti siphon valve new it have the check ball in or not ? i bought a new one today just finished install with new fuel line also but local OMC dealer dont carry check ball type any more.. my original was aluminum and was corrioded and broke at base of hose and barb when i took it off. and made of Brass $6.95 proud of it they was huh? i could of got same thing at Lowes probably lolo cheaper. i run AC R43 T plugs. did you replace fuel filter?? i use same one 2 seasons then replace with new but when store for 1st winter i just take off and dump out fuel. then refill with fresh gas start of next season use. also this so called gas we now have to use only good at max 2 months. i empty out my gas tank as much as i can then refill with fresh.. if i need to run it at home before i winterize i just take gas line and put it in 5 gal jug.. other is top rear by distributor big plug has glass 30 amp fuse is similar to old style 12 v light bulbs. the solder ends after time wear flat from vibrations and can make poor connection in its holder. theyre proud of those also at OMC dealer. well good luck just a few random ideas i share with ya.. i have a 89 corsica 175 sunbird had about 10 yrs now and 4.3 cobra nice motor have a good one toofast, and some times its the little simple things that cause headaches.. gl Randy... ps lil check box said post quick reply well er uh ???? lolo

 

[1970 Aristocraft]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

Also make sure you are not using gas with ethenol in it. Ethenol draws moisture. Very bad for boats. Once the fuel line heats up the fuel will evaporate, making the boat sputter and even quit. Check fuel/water filter. Also i read you are using a old style ignition. consider switching to electronic ignition. So much better and not all that constant adjusting points and stuff.

 

[Too Fast]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

Seriously NO ONE has a clue?

 

[Joshua Nichols]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

consider switching to electronic ignition. So much better and not all that constant adjusting points and stuff.

Only if you have the cash to dump... Remember this is a boat.. Do you use it for transit?
Constant adjusting? What are we calling constant? Once a year? I checked the dwell on mine.. Still the same as lastime I changed the points a year ago.. I wouldn't go electronic unless you have money to throw away and are too lazy to replace some simple points.. You can install a electronic unit and have the same problems or more.. Stick with what you have.. You seem to have setting points figured out and you can install a set a year for the next 12 years or so for the price of the cheap electronic kit..

Now.. Since you have done all the ignition stuff(other than wires and a coil).. I would start looking at fuel related stuff.. Does it have the little 2 barrel carb? If so.. do a carb rebuild on it(bet it has the funky in it).. Cheap and easy to do.. The carb is harder to remove from the engine that the thing is to overhaul.. Change all the filters.. Is the choke staying open?

 

[Too Fast]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

Only if you have the cash to dump... Remember this is a boat.. Do you use it for transit?
Constant adjusting? What are we calling constant? Once a year? I checked the dwell on mine.. Still the same as lastime I changed the points a year ago.. I wouldn't go electronic unless you have money to throw away and are too lazy to replace some simple points.. You can install a electronic unit and have the same problems or more.. Stick with what you have.. You seem to have setting points figured out and you can install a set a year for the next 12 years or so for the price of the cheap electronic kit..

Now.. Since you have done all the ignition stuff(other than wires and a coil).. I would start looking at fuel related stuff.. Does it have the little 2 barrel carb? If so.. do a carb rebuild on it(bet it has the funky in it).. Cheap and easy to do.. The carb is harder to remove from the engine that the thing is to overhaul.. Change all the filters.. Is the choke staying open?

OK, I purchased the carb rebuild kit for it. It will be in tomorrow at 8am. It is a 2 barrel carb. The fuel filter is new and the one in the carb is good too. I have never messed with carbs before, my only concern with rebuilding it is not getting it adjusted right.How do you tell if the choke is open? It starts and idles fine up to 2900 rpms.

 

[Joshua Nichols]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

Take the spark arestor off and see if it's open or closed.. You do have a repair manual don't ya?

I just cranked mine wide open... It was giving me grief and causing me to have running issues.. I think mine is just wore out..

 

[Too Fast]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

Yah I bought a new manual but its worthless. Is the choke the metal flap in the carb? If so its always straight up and down (meaning I can see straight down in the carb)

 

[Joshua Nichols]

Re: 1987 OMC 4.3 ltr Stumbles during acceleration around 2900 Rpms

Worthless? Which one you have?? Even the crappy one has the carb rebuild correct.. It's real simple.. I think you can do it..