Backfire while throttling up

[Star01]

hello folks. I bought a 1970 chieftain with 2.5 mercruser. At first would only run if I put gas directly in carb. Took Rochester 2 barrel apart cleaned and blew out everything with air. Re installed. Float bowl empty inlet plugged. It started right up and ran smoothly. Put in water to try it out. Haven’t had in water yet. Started right up runs smoothly eased it up to 2500 rpms no problem. Then 3000. Then 4000. That was wot. Twice while increasing rpms especially if I went up fast it would backfire maybe couple times then run smoothly. What would make it backfire as how to fix it. I ran off a clean new 6 gal gas tank with filter in line. Also is that the right rpm for wot on this 120 hp. 21’ Aluminium chieftain. GPS speed at wot was 30 mph At 3000 was 26 mph

 

[Star01]

Thank you in advance for any help or info on this.

 

[alldodge]

Backfire at higher rpms is classic motor is running lean. Need to get back into the carb because the main jet circuits are not clear

 

[Bondo]

What would make it backfire as how to fix it.

Ayuh,..... Welcome Aboard,...... A Lean condition,........

Go back to the carb, the accelerator pump circuit is lean,....

I think you should be seein' 4600 rpms or so at Wot,.....

 

[Star01]

Thank you both for the info. I’m an HVAC guy so I had never taken a carb apart. I did do a very complete cleaning took ever item apart took pictures assembled back the same way. I was thinking though with so much junk in the fuel feed fitting and under the screw where the float needle seated. That more could get into carb any chance there’s still junk in the fuel line from fuel pump to carb. Or in fuel pump for that matter? I will take apart again. But I’ll get a rebuild kit for it. Should I install a filter between fuel pump and carb ? Also I have no idea what the proper setting is for the needle screws. So I marked them and counted turns when I backed them out to clean. Screwed them back in with the same turns. Should I bottom them out and count turns backing them out instead? Really appreciate all your knowledge. Thank you again.

 

[Star01]

Hello. One other question. Would the lower wot be a cause of the carb running lean thing ? Or possibly a wrong prop thing ? As I bought this from a fella who took it in trade. I don’t believe he knows much about these issues. I own a 353 javelin bass boat with 70 hp evinrude. The prop effects my wot rpms a lot. And a Starcraft Jupiter. 40 hp electric shift. Have rebuilt a few boats 5 actually. But all smaller and outboards. Thanks again.

 

[Scott Danforth]

the inlet of the carb has a filter

its either running lean or the points need serious loving

the engine issue is what is causing your low WOT

go back to the carb. clean each passage. verify by passing a piece of fishing string thru each one

service your points. at a minimum, file them (wifes nail file works well), gap them (0.018") then verify dwell (28-32 degrees), then set timing (6 BTDC)

then with any new to you boat - assume that the prior owner did nothing for maintenance. service everything else.
raw water pump
bellows
gimbal bearing
alignment
engine oil
trailer bearings
etc.

 

[alldodge]

Would the lower wot be a cause of the carb running lean thing ?

No, its the other way around. Your currently running lean, so the motor does not have enough fuel to reach WOT rpm. If you were not backfiring, then the incorrect prop pitch would reduce your WOT.

The backfiring is your motor starting to damage itself and needs to be addressed

 

[Star01]

Wow!! Thanks to all. I will do all mentioned. It does have new bellows and gimble bearing. When I gave it a test run and heard the backfire twice I throttled back and took my time back to the ramp. No over heating but I knew something was not right. Will do a complete service on the engine. I have a good friend as a car mechanic. He has the tools and knowledge to check dwell, and get timing set at 6degrees before top dead center. Didn’t realize how much this marine engine has in common with a vehicle engine. I will take some pics of this project as I get along. But want the mechanical things addressed and made right. First things first. It does have a shifting issue also. Forward perfect. Reverse not so perfect. But have enough to do right now with all the info you folks have givin me. Brick by brick as they say. Can’t thank you all enough for taking the time and sharing your knowledge with me. What a great site and forum.