15 horse hard warm start and poor performance

[tomjoh11]

The story begins:

I had a 91 merc 15 hp that was perfect. but it was stolen from my 14 ft alumacraft. It could push the boat up to 24 mph (gps) and started every time.

I recently purchased a 96 Johnson 15 hp that was well cared for but stored for the last year and a half. By all appearances it was very well maintained.

The biggest problem is that the boat starts very difficult after being warmed up. It starts immediately when cold.

Once warm, if shut down it needs to be immediately choked and pulled twice to get a cough and then 3 pulls later it starts without the choke. I've tinkered with the fuel idle mix and the performance with the cups is great. However, in the water it is now worse.

Occasionally at idle the motor will cough and die (in gear or out).

Secondly, the merc would push up to 24 mph and this one tops out at 19. It seems like it should have a lot more left. Any suggestions on this? it runs very smooth at WOT.

Thank you for the advice!

I'm working on the carb rebuild right now.

 

[tashasdaddy]

Re: 15 horse hard warm start and poor performance

personally, i would follow this, you know absolutely nothing about the history of this motor.

http://forums.iboats.com/showthread.php?t=158086

when properly tuned, motor and boat set up correctly, correct prop for boat. it should out do the merc.

 

[jmendoza]

Re: 15 horse hard warm start and poor performance

It could be a lean cough, that is typical and would correspond with needing a choke to start when warm.

The one thing you mentioned is setting the low speed mixture with ear muffs on (cups is a non-conventional term that is confusing, loose that terminology), and if you try to set it up that way, it will always be lean. The low speed needs to be set with the engine running in forward gear, on the boat, in the water, at low throttle.

The other thing it could be is some debris and/or dried varnish in the carb's low speed circuit, and this would make the low speed mixture adjustment whacky. To cure this, the carb has to come off and be cleaned, and if it has a freeze plug over the low speed orfice, it need to be removed to clean the orfices and a new plug swaged in place, they come with the carb rebuild kit.

 

[tomjoh11]

Re: 15 horse hard warm start and poor performance

I thoroughly cleaned the top side of the carb at the passages for the idle screw when I did the carb rebuild with no change. Could a drop in compression explain this?

It overheated and shut off the first time I used it because of some weeds wrapping around the inlet.

 

[bktheking]

Re: 15 horse hard warm start and poor performance

The only way to answer that is to do a compression test.

 

[tomjoh11]

Re: 15 horse hard warm start and poor performance

I did the compression test while thoroughly warm. Both cylinders were below 30 which is the bottom side of the measurements on the gauge.

I used the lend-a-gauge from O'Reilly's and it didn't have an O-ring so I'm not sure how valid it is. I yanked like a banshee and it was holding pressure when I pushed in the relief valve.

My first reaction is that the head gasket is blown. Is it common to warp the head at the same time?

 

[Daviet]

Re: 15 horse hard warm start and poor performance

30 lbs compression is way low. Remove the cylinder head and check the head gasket, make sure the cylinder head is flat. Look at the pistons and cylinder walls for damage, if all is ok, install a new head gasket and recheck compression.

 

[Rick.]

Re: 15 horse hard warm start and poor performance

I am amazed you can start it at all with only 30lbs. compression. I would first get a gauge you trust and re-do the test. If still bad then resurface the head and put in a new gasket and torque it down. Best of luck. Let us know what you find. Rick.

 

[bktheking]

Re: 15 horse hard warm start and poor performance

No o-ring? i'd run another gauge/test on it.

 

[tomjoh11]

Re: 15 horse hard warm start and poor performance

New test shows 125 psi when cold in both cylinders.

After a warm up the compression dropped to 105 in one and 110 in the other. I was under the impression that compression should go up after warming up.

Any new suggestions? I'm trying a new hose and new tank today.

I've got a trip to Canada D Musky!!!) in two weeks and would like some confidence in this motor.

***UPDATE***

I ran it at idle with the ear muffs on for 10 laps of today's NASCAR race. Came out and there was quite a bit of white milky paste on the inside of the prop hub and around the prop where it seals to the lower unit. Again this was out of the water just at idle. There is an odor of gas coming from paste.

Back to the headgasket?

BTW, I'm very impressed with the level of activity on the board! Thank you so much for the help.

 

[Rick.]

Re: 15 horse hard warm start and poor performance

I wish I would have made you an offer on it when you were getting 30 !! A word of caution, don't leave it unattended when running on muffs. My buddy cooked an engine while having lunch. I can't remember the details but it didn't get water for some reason. Rick.

 

[samo_ott]

Re: 15 horse hard warm start and poor performance

White goo in the barrel is normal. It's the unburned fuel/oil mixed with exhaust and water.

Watch out for those Canadians when you go up there. They're a wild bunch!

 

[tomjoh11]

Re: 15 horse hard warm start and poor performance

Well, the headgasket was burned, but not through. There were scuffs on both cylinders and the impeller only had two fins left with nubs everywhere else.

We replaced the pistons/rings and rebuilt the whole block. We also used emery paper to flatten out the head. Replaced the thermostat and fuel pump.

The cylinder head is still getting very hot while running on the earmuffs (attended for 5 minutes). The starting is also still semi-difficult on restart.

Any new suggestions?

 

[samo_ott]

Re: 15 horse hard warm start and poor performance

Did you recover all the nubs of the impeller or is it possible some had lodged in the water tube?

Did you test/check the thermostat since the overheat?

Have you rechecked compression? what is it if so?

Another common problem with the 9.9/15's is the top grommet on the water tube collapses I hear. Under the powerhead I believe.

 

[tomjoh11]

Re: 15 horse hard warm start and poor performance

Did you recover all the nubs of the impeller or is it possible some had lodged in the water tube?

Did you test/check the thermostat since the overheat?

Have you rechecked compression? what is it if so?

Another common problem with the 9.9/15's is the top grommet on the water tube collapses I hear. Under the powerhead I believe.

One nub was missing. The water stream is much stronger now than before. I assume that a nub lodged in the system would prevent this from happening. The water stream is also very cold and we reinstalled the thermostat (it was backwards, thank God we didn't run it longer).

It is having the exact same starting problems as before. There is also a lot of white buildup still coming out of the exhaust more than what could be attributed to unburnt fuel. I can actually see it forming on the outside of the hub and growing out.

My assumption is that the head is warping again already. Does this sound correct?

I can't tell you how frustrated I am to go through that whole tear down and rebuild only to have the same problems as before.

 

[bktheking]

Re: 15 horse hard warm start and poor performance

Do another compression test, I don't think you had compression issue before you did the rebuild, the numbers looked good to me. What do the plugs look like?

 

[tomjoh11]

Re: 15 horse hard warm start and poor performance

I did not have the compression gauge with me today.

We reinstalled the headgasket and the starting issue is resolved. It starts on the first pull hot or cold The motor is definitely pulling better and went up to 20 mph.

There is still some water passing through the combustion chamber. I see this in the white slime coming out in the exhaust as well as in white balls floating in the water. I pulled a spark plug and I could see a little bit of foam/white fluid on the piston/cylinder walls. This seems dangerous to me.

I don't think it is leaking at the head gasket, where else could water be getting into the cylinder or into the gas mix?

The only parts we haven't replaced on the block is the exhaust jacket gasket, and the water grommets.

 

[tomjoh11]

Re: 15 horse hard warm start and poor performance

Do another compression test, I don't think you had compression issue before you did the rebuild, the numbers looked good to me. What do the plugs look like?

The plugs are wet (see post above regarding water in the chamber), a little black, but normal...

 

[bktheking]

Re: 15 horse hard warm start and poor performance

Water into cylinders could be a bad lower seal on the crankshaft head or a bypass cover gasket/hole in the cover.

 

[Cofe]

Re: 15 horse hard warm start and poor performance

Getting water into the cylinders can be caused by a warped, leaky,or hole in exhaust cover parts caused by overheat.
The water tube grommet on the underside of the power head is a problem also. I believe they make a new modified version of that grommet.