Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

[glengold]

I've noticed posts with a similar low power/rpms problem I'm having suggest diagnosing fuel delivery by directly spraying fuel/oil mix down the carbs. Can I do this on the hose or do I need to be on plane WOT? When a cylinder is found to not be getting fuel what's next to check? could spray carb cleaner really clean the carbs enough to solve the fuel delivery problem? If the lines are leaking air wouldn't both cylinders be affected? I'm not getting surging, stalling or hesitation. full throttle seems like 3/4 throttle.

 

[jtexas]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

Thing about carb cleaner is, if you spray it into the carb, it just goes right past the jets and other passages that need cleaning straight into the crankcase where it will do what it's designed to do: hold the oil in suspension, and strip the lubricating oil right off the cylinder walls. If you wouldn't run your motor with no oil, don't run it on carb cleaner.

If you have good compression and good spark, and if the fuel line holds pressure, the tank vent is open and clear, the fuel is fresh and free from water and debris, then you need to rebuild the carbs.

Read the carb rebuild thread over on the Engine FAQ section of iboats. Don't be intimidated, these carbs are really simple devices. You can do it, and you'll be glad you did.

 

[glengold]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

Thats why I am also worried about running it with a starved cylinder. Plus I didn't want to trailer it all the way to the lake to discover I'm not getting fuel/oil to a cylinder. So can I test by spraying fuel/oil directly into the carbs at idle on the hose. Instinct says I need to be WOT with the butterfly's 90 degrees. I just purchased this motor and when he ran it on the hose we removed the plug wires one at a time and the difference was obvious. I'm concerned that test wasn't sufficient for detecting fuel delivery issues. (obviously it gets fuel at idle but WOT under load.....) Is the spray mix down the carbs, on plane, WOT the best way to check this?

 

[Solittle]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

Have you run a compression check? Have you tested the plugs with a plug testor - - not by grounding a plug? Running an engine on a hose dosen't tell you a lot. What do the plugs look like? My point is don't chase solutions until you have an idea of what the problem is.

 

[jtexas]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

to quote myself:

If you have good compression and good spark, and if the fuel line holds pressure, the tank vent is open and clear, the fuel is fresh and free from water and debris, then you need to rebuild the carbs.

Read the carb rebuild thread over on the Engine FAQ section of iboats. Don't be intimidated, these carbs are really simple devices. You can do it, and you'll be glad you did.

compression, spark, fuel. if she gets all three, she runs. if she don't, she don't.

compression tester, less than $40. need help with that, just ask.

Inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it won't fire in the hole under heat & pressure.

Your theory is correct. Spraying fuel into the carb throat at idle tells you nothing about the high speed jets.

rule out compression. rule out spark. rule out gasoline quality, tank and fuel line issues. What's left when everything else is ruled out? Carburetors. Stop fretting over it. Just do it.

 

[glengold]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

Fair enough, thanks for the nudge. I have a compression tester but not a spark tester. Will get one tomorrow and report my findings. Guess I was hoping for a stupid easy solution to my problem. Compression, spark and carbs are all supposed to be perfect according to my mechanic. Compression was reported at 155 in both cylinders (recent rebuild). Spark and fuel delivery was confirmed by previous explained test (removal of plug wires while running). I understand all of this could be BS reported by my mechanic. I did need to replace my starter solinoid after a few runs, he didn't charge me for a the new solinoid, i provided the labor. He thinks my boat is too much for the 55. I had a 97 50hp evinrude on it that turned close to 6 grand. I went from worried I was gonna over rev to worried I won't get on plane!!! He said my old motor was mounted too high, allowing cavitation to occur. the boat used to scoot along great, now it plows to maintain a plane. I traded the 97 in because it had a bad lower unit that's hard to find/repair. I thought this would be a good investment because i found many lower units for the older models while looking for a (rare) 17 spline one for mine. I will check all of the above (compression, spark) before I run it again. If those all check out then i know its a fuel delivery issue and will precede with a carb rebuild.

 

[jtexas]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

no sweat. seriously, carb rebuilding is not that bad. I can think of way worse ways to spend a Saturday morning.

The cylinder drop test really just shows that both are firing at idle, on the muffs. Negative result can quickly narrow a problem down to a cylinder, but a positive result doesn't really rule anything out. As RPMs rise, things heat up, resistance increases, gets harder to fire a spark across the gap.

I didn't know about your mechanics report...he's probably right, but the spark test is so quick & easy, I usually just do it first whenever I have symptoms without obvious cause.

Your 1977 55 horse motor is rated at the crankshaft; conventional wisdom says it would deliver about 45 to 50 horses to the propshaft. By 1997, outboard makers were rating their motors at the propshaft, so you have about the same or a little less power now.

Did your mechanic voice an opinion about propeller pitch?

 

[glengold]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

haha, thats a funny story I'll make it quick. The old motor had dolphin fins on it which i kept, as well as a 13 pitch prop. the new "77" motor came with a 15 pitch. first time out we couldn't even get on a plane with the 15, no fins. changed to the 13 and got on plane, barely. next weekend I added the fins and it hops on a plane quick now but doesn't rev out like the old "97". thats interesting about the hp rating, i didn't know that, explains it possibly, because he mounted this motor about an inch lower than the other. The other motor would cavitate if you turned sharply at full throttle. Do you think i might try mounting the motor higher? the holes are there....

 

[glengold]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

I might add that the 13 pitch is recommended for water sports and 15 pitch for top speed w/ my boat. I also have not tried the 15 pitch with the dolphin fin. The "97" 50 had a 17 on it which was garbage when i bought it. My tach doesn't work with the new "77" motor so I cant give an exact RPM reading but it doesn't have the "full power" sound the "97" 50 had.
thanks for the responses

 

[jtexas]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

it's a trade-off...raising the motor means less hydrodynamic drag, should buy you some RPMs and MPHs. If you can live with an enlarged full throttle turning radius.

water pressure is a concern, but if it wasn't a problem before..........

come to think of it, if your hydrofoil is still in the water at WOT, raise the motor for sure.

 

[glengold]

Re: Diagnose Fuel Delivery 77 'rude 55hp 2 cyl. low power

Ok, thanks. I will test it this weekend and let you know what happens. I'm hoping to get another 500-1000 rpms out of it