'81 johnson, 70hp no power?

[jedimaster]

I sold my 50 after I amanger to get it running right, then I bought a 70hp.
It idles perfect, starts up great. No missing or anything.
I put it in the water today and when I gave it it gas the boat would not plane out. Again I had it pinned and I couldn't get more than 5 mph. It sounded ok, no misses just nothing.
Where should I start?
I haven't checked anything yet.

Compresion is even on all three at 110psi.

I checked all the plugs for spark with a spark plug sitting on the block and they all seem fine.
Tach is not hooked up right now so I am not sure of the RPM's that I was getting. I couldn't get the tach to work. I have a backup clip on tach I am gonna hook up this afternoon and run the motor in a bucket.
Any ideas on where I should start would be greatly appreciated.

 

[jedimaster]

Re: '81 johnson, 70hp no power?

Do you think its just too much prop? I have a 3 blade 17 pitch alluminum on it. My cousin said with his bass boat if he has too much prop it won't get out of the hole and I should try a 15???

 

[jedimaster]

Re: '81 johnson, 70hp no power?

Took it to the marina to have em take a poke at it, the guy said he can't see anything wrong, spark compresion timing all seems fine. He said to toss in some new plugs and give it a try. if that doesn't work take ot the high speed jets and check for blockage. I took the jets out and they were fine, also changed the plugs. I'll take it back to the lake after lunch and give her another go see if the plugs fixed it.

 

[jtexas]

Re: '81 johnson, 70hp no power?

classic symptoms of clogged carbs, happens all the time.

carb rebuild will fix ya up.

kits, about $12 apiece on iboats, it's a pretty easy job, highly recommend you get a manual, save yourself $300 or so on this job alone.

 

[jedimaster]

Re: '81 johnson, 70hp no power?

I have a manual. The spark plugs did nothing. The marina only had one carb kit, so I'll do the top one this afternoon and see if that does anything it seems to be flooding a bit. I'll have to do the others another day.

 

[cmchesse]

Re: '81 johnson, 70hp no power?

I was having the same problem last month. Change the plugs, good compression, nothing worked until we removed and cleaned the carbs. Runs like a bat out of ?

worth the time and effort. In a pinch clean the others without the carb kit.

 

[jedimaster]

Re: '81 johnson, 70hp no power?

Well I rebuilt the carb, and cleaned the other two, changed the plugs and took it out....And it was still the same. Ready to just unbolt it and turn it into a reef, I moved my new trolling motor batteries to take a look at the gas lines make sure it wasn't pinched,Gas line was fine, I took off and boom up she went. I guess the batteries I just bought are too heavy. It wouldn't plane with them in the back. I'll have to move them forward, or get a lower pitch prop. maybe go to a 4 blade 17 pitch SS prop over the 3 blade alluminum 17.

GRRR all that for nothing.

 

[jtexas]

Re: '81 johnson, 70hp no power?

Wouldn't plane with batteries in the stern?

What boat is that motor on?

 

[jedimaster]

Re: '81 johnson, 70hp no power?

'16 foot 1993 stratos bass boat. Bass boat hulls are notorius for not getting on plane. Do you think the motor is up too high or too low? I have it setup so that the ventilation plate is even with the pad.

Has a 13 3/4 17 Pitch prop.

Didn't have a tach setup when I was out yesterday but the tach is setup and the gps is mounted so I should be able to get some performance numbers later on after I go for a run.

I was thinking of going to a 12 3/4 4 blade 17 pitch prop, but I am gonna wait to order something til I get some rpm and gps numbers on the boat.
Top speed on plane felt good, cornered well, would start to ventilate on heavy cornering unless I trimmed down a bit, so I think the motor height is good, water preasure is good during corners and up on plane.

 

[jedimaster]

Re: '81 johnson, 70hp no power?

http://forums.iboats.com/showthread.php?p=1749435#post1749435

I made a post in the prop section with the numbers,
5400rpm's 33.6mph gps

 

[Benny1963]

Re: '81 johnson, 70hp no power?

were is your cav plate at in relation to your boat bottom
are they both the same size mids had one the other day that did a swap and his new motor was a20 and his old was a 15 is it pushing water back over transom when running full out his was a combination of clogged hih speed
jet improper link and sink timeing off and motor cav was 5 inches below hull
clened carbs set timeing sinked it and picked motor up with mini jacker off iboats little boat know screams just some stuff to think about,

 

[Benny1963]

Re: '81 johnson, 70hp no power?

are you gettig 5400 ot of it you might be looking for 5800 but still
boat may be heavy i thought you were haveing probs gettig rpm .disregar my previous post check you engine hight at cav plate though if ya drop in pith you may gain some rpm but its probly not going to help much