Engine Surge

[catchee]

Hello all...I'm new to this forum so please forgive me if I'm not following protocol. I'll get right to the point. I own a 1990 115hp Evinrude mounted on a 1978 17" Mako. Was out this past saturday doing some fishing at a favorite spot and when I went to leave I started from an idle and went WOT to get on a plane, got between 5000-5500 rpms and traveled 50-75 yards and the engine immediately dropped back to around 2000 rpms and ran on the rough side....did this on and off the entire trip back. Took forever to come in. I'm thinking a fuel starvation problem such as a faulty fuel pump. The VRO has been disconnected some time ago and I now pre-mix. Went out this am and checked for spark at each cylinder with a spark gap tester and appears OK. The carbs were rebuilt about 2 months ago and I have had it out since then and everything seemed fine. It will start fine and idle OK. I just started it up a few minutes ago and while idling I pulled each plug wire off one at a time and found that cylinders 1 & 2 made a noticable difference in rpm drop while 3 & 4 only made a slight difference...changing the plugs did not seem to make a difference. Kind of at a loss here on what to try next and am hoping someone will be able to help, Thanks in advance.....

 

[tashasdaddy]

Re: Engine Surge

i would first make sure you have spark on all 4 (spark tester set a 7/16in). if you have spark, check compression. my gut feeling is a problem with the lower carb.

 

[JB]

Re: Engine Surge

Howdy catchee.

Welcome to iboats and Merry Christmas.

I think your next step is a compression test.

When side by side cylinders on an OMC V type give trouble I suspect the carb or fuel supply to that carb.

Let us know your compression numbers and what happens if you spray a bit of premix into the lower carb.

 

[JB]

Re: Engine Surge

Daddy types faster than I do, but I am glad he agrees with me on this one.

 

[ezeke]

Re: Engine Surge

Sounds like the bottom carburetor is obstructed or the float is sticking. The fuel pump is not selective, so it is probably not the problem.

 

[catchee]

Re: Engine Surge

OK went out and checked compression...my findings are:
#1--120psi
#2--Haven't been able to check as the save-a-thread that was in that cylinder backed out with the plug and I have to put a new one in. By the way can I use Loctite threadlock (red) on the new insert?
#3--115psi
#4--100psi

 

[tashasdaddy]

Re: Engine Surge

not terribly out of line, #4 could be higher. look into the carbs. threadlock, i don't know.

 

[flabum]

Re: Engine Surge

you can use threadlock, but just don't put the plug bak in for at least 24 hours and clean the excessive thread lock from the hole.

 

[catchee]

Re: Engine Surge

Thanks for the input from all...I did check compression on cylinder #2 and it also had 115psi. I too am leaning towards a carb problem. I'm assuming that the lower carb feeds cylinder 3 & 4 and the upper carb feeds cylinders 1 & 2.
Is this correct? I'll pull them off on Wednesday and take a peek. Is there anything else that I should be looking for while I have them off?

 

[JB]

Re: Engine Surge

Try squirting or spraying some fuel premix into that carb with her at idle before you start pulling them off, catchee. If the carb is at fault you should get an increase in idle speed.

 

[tashasdaddy]

Re: Engine Surge

after you get it running right a decarb, may bring the compression up in # 4.

Decarb, take a can of seafoam put 3/4 of it in the gas tank, with only 1 gallon of premixed gas. put the rest in a spray bottle. start the engine, and let it come up to temperature. then remove plugs, and them some real good shot of seafoam into the cylinders, replace plugs, let sit 15 minutes. restart, and spray the rest of the seafoam into the carbs, so the the motor almost stalls, wait and repeat until the seafoam is gone.then take for a wide open spin. then put in new plugs, ad premixed gas to the tank, and take it for a wide open throttle spin. it is going to smoke like a house on fire, during this process.

afterwards recheck compression.

 

[ezeke]

Re: Engine Surge

In my opinion, that is much too big a range to be just a likely carbon problem, so be prepared to pull the port cylinder head and bypass and check it out. Decarbing is usually effective if all the pots are reading relatively low.

If I had a 20 PSI difference on a V4, I would not run the motor at all before inspecting the cylinder.

 

[catchee]

Re: Engine Surge

ezeke, I would tend to agree with you except for the fact that I can go WOT from an idle..get on a plane but just can't maintain it....just drops back to say 2000 rpms. It just seems like a fuel starvation problem.

 

[ezeke]

Re: Engine Surge

Well, check the float and valve on the bottom carburetor and make sure that the fuel pump's pulse limiter and diaphragm are OK.

 

[HighTrim]

Re: Engine Surge

Just throwing out a thought, ur not overheating at all are you, going into SLOW? As far as compression goes, after inspection I would run a decarb as Tash said. Remember #@%& flows downhill, and that may be why the bottom cylinder is carboned up. Give her a good dose of seafoam then drive her like you stole her.

 

[Gary H NC]

Re: Engine Surge

Another thought to throw in the mix, Try a separate portable fuel tank to rule out bad fuel or a restriction from the original tank.

 

[ezeke]

Re: Engine Surge

I am fairly certain that the powerpack does not have S.L.O.W. or QuikStart. To check, look for a black and white wire running from the port temperature switch. If it is not there, you don't have those functions.

 

[catchee]

Re: Engine Surge

Again, thanks everyone for all your input...will take it all in to account when I start working on it tomorrow. I have the rest of the week off so should be able to narrow it down and hopefully have it fixed before the weekend. I bought the engine new back in 1990 and never really have had too much of a problem with it. I normally do all my own maintenance on it myself. Have turned a few wrenchs over the years...usually on autos though but am never afraid to try something new and have a vast assortment of tools (never enough though...more tools!)

 

[catchee]

Re: Engine Surge

After taking everyone's advice so far I'm still at square one. I pulled the carbs and found both of them to be squeaky clean. Blew out all passages with compressed air and made sure float level was correct. Disassembled the fuel pump and found it to be clean also with no tears or pinholes in the diaphragm. Put it all back together and de-carb as suggested with a remote fuel tank...with approx 3 gal of fuel and 1/2 can of seafoam...also shot seafoam into the throats of the carbs while engine was running....did notice that the upper carb would almost choke out but the lower carb would hardly make a difference. Also while engine is running I could remove the plug wire from cylinders 1 & 2 and see a drop in rpms and it would run erratic. If I did the same to 3 & 4 there was very little difference. I could also choke out the upper carb with a rag to the point that the engine would die but if I tried it with the lower carb it would continue to run. Still makes me believe I have a fuel delivery problem to the lower 2 cylinders (3 & 4). When I pulled the plugs from cylinders 3 & 4 they were completely dry. I have a strong spark at each cylinder. I looked into the intake manifold and could see the reed valves and they appeared to be OK...none were bent or broken. Did another compression check after the de-carb...my findings #1--120psi, #2--110psi, #3--120psi and #4--100-105psi. I'm really at a loss on what to do next and any more advice would be welcomed. Thanks....

 

[ondarvr]

Re: Engine Surge

Fill a spray bottle with some premix and with the motor running spray it into the carb. If the RPMs increase, then you know it's a fuel issue, if not, then there are more problems, but it doesn't rule out a fuel problem along with them.

With a compression difference like that between cylinders, you may have already done some damage.