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  1988 Yamaha 150 Runs Roughly, Lacks Acceleration

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Author Topic:   1988 Yamaha 150 Runs Roughly, Lacks Acceleration
JMARTIN posted 07-31-2015 01:17 PM ET (US)   Profile for JMARTIN   Send Email to JMARTIN  
YAMAHA 150ETLG 1988

SYMPTOMS:

- Running rough. Not a great hole shot, or top end. Boat tops out at 4600rpm at 32mph(gps)
- When at Idle, on water hose, cylinders 4 and 6 do not seem to fire.
- At Idle (650) the engine belches white/blue [sporadically]
- After running for 5 min with new plugs, both plugs do not show any signs of burn, yet are definitely wet with fuel.

Tests and Fixes tried:
(When I say No change, I'm referring to plug condition mentioned above, and shorting spark plugs mentioned below)

- When shorting out spark plug on 4 or 6 there is no noticeable change.
- Shorting out both plugs at same time, no noticeable change.
- Reversing the 2 wires, no noticeable change.
- Checked spark at ½" gap(special tool), all 6 have good spark.
- Replaced coil packs on 4&6 - still good spark, no change
- Verified timing and firing order. All okay
- Checked compression 110 ea +/-3
- Retuned carbs - No change
- Pulled jets and plugs. Drained and blew out carbs. - no change
- Replaced carbs and tuned - No change
- Replaced CDI and Control unit assy - No change
- Rebuilt heads, cleaned water jackets, new thermostats.

jimh posted 08-06-2015 07:50 AM ET (US)     Profile for jimh  Send Email to jimh     
You seem to have replaced just about every component I can think of that might have been causing a problem. You have me stumped. We are going to need an old-time outboard mechanic to figure out this one.
BQUICK posted 08-06-2015 01:28 PM ET (US)     Profile for BQUICK  Send Email to BQUICK     
Sounds like fuel pump(s) leaking into crankcase to me.
Jeff posted 08-07-2015 12:45 PM ET (US)     Profile for Jeff  Send Email to Jeff     
I see you have reversed them, but I assume the plug wires themselves have been checked and are free of faults?
JMARTIN posted 08-17-2015 02:07 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
The problem turned out to be a torn diaphragm in the fuel pump that was allowing way too much fuel to be introduced to the carburetors.--John
BQUICK posted 08-20-2015 03:35 PM ET (US)     Profile for BQUICK  Send Email to BQUICK     
You're welcome. :)
jimh posted 08-21-2015 01:15 AM ET (US)     Profile for jimh  Send Email to jimh     
I don't think a problem in the fuel pump was putting too much fuel into the carburetors. The carburetors have bowls. The fuel pump feeds the fuel into the bowls, until the float valve in the bowl shuts off the flow of fuel. A carburetor meters fuel into the air stream going into the cylinder at a particular ratio of fuel-to-air, and it doesn't matter if the fuel bowl is full or partially full.

I think the problem is as 'QUICK suggested. The fuel pump is most likely a vacuum pump, operating from the engine crankcase vacuum. The crankcase vacuum is on one side of the diaphragm, which moved back and forth, pumping the fuel on the other side of the diaphragm. If there was a torn diaphragm in the fuel pump, fuel could flow through the tear in the diaphragm and get into the crankcase. Extra fuel in the crankcase of a two-cycle engine upsets the air-fuel ratio mix as determined by the carburetors.

The extra fuel was not getting in via the carburetors; it was getting in via the tear in the fuel pump diaphragm and into the crankcase.

JMARTIN posted 08-21-2015 12:15 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
I am getting the information 3rd hand, but BQUICK pointed the owner to the fuel pump problem, thanks.

John

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