1969 Merc cruiser Rochester Two Barrel 120hp backfires

jrsrugs

Recruit
Joined
Feb 19, 2009
Messages
2
Mercruiser 120 carb/throttle problem?????
I rebuilt the Carb on the engine, 2 bbl soaked it, took out everything cleaned and replaced used a rebuild kit and replaced old parts with new.


Engine starts and idles fine
when in the water I push the throttle up and the boat stalls out. If it baby the throttle up carefully (sometimes I have to drop it back to idle then baby it up again and it back fires , When I get it running fast then came down to slow it backfires ,not all the time , once it catches I can throttle up and it goes fine unless I drop it to idle then I start over again.

When it is cold I can sometimes push the throttle and away it goes. I took the carb apart checked the accelerator pump the rubber looks good. With the linkage disconnected I throttle up and see gas go into the throat but not everytime. I start the engine and manually work the throttle and the engine revs up. I connect linkage and push the throttle and it dies out.

Spark plugs are dry when examined. I don't smell fuel as I would if it was flooded.

I am at a loss???

Any Ideas?
 

thumpar

Admiral
Joined
Jun 21, 2007
Messages
6,138
Re: 1969 Merc cruiser Rochester Two Barrel 120hp backfires

Check the timing and the weights in the distributor.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: 1969 Merc cruiser Rochester Two Barrel 120hp backfires

Ayuh,.... Shes runnin' Lean,... No gas....

Could be the carb is junk, or the fuel pump is dyin', or some other restriction, Somewhere....
 

stratcat88

Petty Officer 2nd Class
Joined
Nov 16, 2009
Messages
174
Re: 1969 Merc cruiser Rochester Two Barrel 120hp backfires

Check the timing and the weights in the distributor.
i just rebuilt my rothester 2 barrel carb myself. engine fired up and idled but as soon as i gave it gas it would bog down. took it to butch the carb man and it came back brand new. i mean new. slapped it on my car engine on boat, pushed on the throttle 3 times, turned the key and it fired right up and ideled sweet. layed in on the throttle and it purred like a kitten. next step.im going to do what you said. ignition system.jrsrugs take your carb to a carb man.
 

cr2k

Captain
Joined
Mar 19, 2009
Messages
3,730
Re: 1969 Merc cruiser Rochester Two Barrel 120hp backfires

2 things...You need to use compressed air to blow out the carb passages not just carb spray. Sometimes you need to go digging around in the accelerator pump passages too. 2nd you MUST use the original STIFFER spring from the carb NOT the weaker one in the kit on the accelerator pump.
 

artificialreef

Chief Petty Officer
Joined
Feb 21, 2009
Messages
504
Re: 1969 Merc cruiser Rochester Two Barrel 120hp backfires

This is suspect
"With the linkage disconnected I throttle up and see gas go into the throat but not everytime. I start the engine and manually work the throttle and the engine revs up. I connect linkage and push the throttle and it dies out."
You should see gas go into the throttle every time you work the linkage with the fuel pump running. Regardless of whether you have the linkage connected or not. If it is sporradic you have a fuel delivery issue.
 

FreeBeeTony

Captain
Joined
May 15, 2002
Messages
3,991
Re: 1969 Merc cruiser Rochester Two Barrel 120hp backfires

Could be water in the fuel.............
 
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