'77 70hp trouble at the lake

[testmoron]

Can't get it to run at the lake.In the water, I can pull off two plugs and run it on one cylinder.It will idle rough,but it will idle.Advance the throttle and it will rev over 5000rpm, and hold there.To me, that proves my high speed jets are clear and working properly. It also proves my high speed ignition on each cylinder.I must be missing something. I have done my link and synch.Any thoughts?

 

[Mntom]

Re: '77 70hp trouble at the lake

Have you taken all three carbs off and cleaned them? Sounds like the low speed jets may be dirty.

 

[testmoron]

Re: '77 70hp trouble at the lake

Yes I have, and it will idle for an hour without a miss.It's a little tiny 11 foot boat but it won't even plane out.

 

[jtexas]

Re: '77 70hp trouble at the lake

a tiny 11 foot boat, that's not a lot of planing surface... jus kiddin, how many RPMs will it make, pushing the boat?

Motor revs in neutral, doesn't prove anything. unloaded, it'll idle on one cylinder. It's how it performs under load that matters. Put a load on a motor, more heat, compressed air in the cylinder, higher resistance, takes more to produce a spark.

Back to basics. Whats the compression on this motor?
Spark test (not a "spark plug" test). get an 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's not firing in the hole under heat & compression.

If you got good spark & compression then the carbs need rebuilding with new kits. If you already just installed new kits, go back & clean 'em again; punch out the core plugs, soak all the metal parts overnight in carb cleaner, remove the jets & clean 'em. Follow the "carb cleaning & rebuilding" link from the "top secret thread" at the top of this forum.

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

btw, here's some information about "runaway", you should know before revving it up in neutral:
http://forums.iboats.com/showthread.php?t=430070

 

[testmoron]

Re: '77 70hp trouble at the lake

It runs but falls on it's face.Complete carb rebuild including the plugs.Had them apart twice after that because I don't know where to go.Compression is 110,110,112.That's not an issue.If a single cylinder will run it to 5000 rpm I would bet we can rule out fuel obstruction in the carb.I can do that with all three cylinders.To me, it also means I've got good spark throughout the rpm range on each individual cylinder.I was surprised it would run on one cylinder let alone turn 5000 rpm.I'm still scratching my head.

 

[jtexas]

Re: '77 70hp trouble at the lake

"falls on its face". you mean dies when you throttle up, instead of just bogging along? sounds familiar -- 2 things I know of make that happen when everthing else is good...idle speed set too low, and throttle cam hitting the roller too soon -- throttles start opening, RPMs increase before you get enough spark advance, it's like the flame is chasing a retreating piston.

And a broken fitting on the spark advance rod (that little plastic ball & socket joint holds the linkage in place). Sorry, that's three things.

I have a 1979 vintage, it runs on 2, but won't even idle on one cylinder. Compression is 121-120-120, wonder if that makes the difference?

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[btw, here's another recent "runaway" -- http://forums.iboats.com/showthread.php?t=430437 ]