Bubble / off idle stumble

[jstew]

Working on a marine 488 (3.7 4 cyl) with q/jet

Just rebuilt with Cliff's marine kit, having issue off idle. I seem to have an inconsistent pump shot. The pump bore is very smooth, and I used a punch and an old check ball to develop a new seat. I'm thinking the issue may be with the check ball / seat.  Should I have used the same ball that I used the punch to bed in?

Separately, how do you tell if an of idle stumble is lean or rich?

Thanks,

John

[73 Z28]

If you used a brass drift on the steel checkball your probably ok, but if you used a steel punch on it, you may have dimpled the check ball. Don't ask me how I know this..........and you'll probably need to get a new check ball.

Paul

[jstew]

Thanks 73,

I used the new checkball included with Cliffs kit.  I used the old one as a "tool" to make the new seat.  I shouldnt have any issues but I'm getting some bubbles in each pump shot.  Should I attempt to reseat with the new checkball (with brass drift this time)?  I wouldnt think the pump system would need so much attention in this area, but I have not seen all the problems that you guys have.  I cleaned up the accel pump bore as described in Cliff's book.

I am not convinced the off idle stumble is related to the lousy pump shot.  Could the power piston spring be too weak?

This marine carb is very strange.  It's off an '85 Century but from what I can tell, its a pre '68 casting. (Its got an old style needle/seat assembly) No part# (nothing stamped, and the foil tag is long gone).  The installation is on a 3.7 4cyl, and you would suspect it to have a lean set up, but it has .037 idle tubes!  The 67 jets and 44B rods make sense but it has DG secondary rods, which I find suspect/overly rich. WOT, however, was great when I test drove the boat.   It has an adjustable airbleed at the top of the airhorn and when I crack it open, the engine immediately drops in rpm.  Mixtures set to 3 turns, seems fully responsive to mixture adjustments.


Thanks,

John

[73 Z28]

Did you drill out the accelerator pump discharge holes in the airhorn to approx. .028?  May want to double check the airhorn screws to make sure they are tight also.

Good Luck
Paul

[jstew]

Thanks 73,

I have checked the air horn. It appears tight and there isn't any warpage to speak of. New gasket, etc.  As for the accel pum discharge holes, I am hesitant to enlarge anything on this Quadrajet for feeding a 4 cyl. I did set the float level up to .250 and that seemed to help the pump shot some what. I'm not sure why it helped , but it helped. It's got a stumble even if I advance the throttle slowly, so I'm not sure if maybe I have more than one issue. I am going to perform Cliff's "tip in" procedure on Tuesday when we water test it. Cliff, any suggestions?

Thanks,

John

[Cliff Ruggles]

Sounds lean on the primary side, and most likely is with that jet rod combination.  Might want to try smaller primary metering rods, and even go up on the jet size one number....Cliff

[jstew]

Cliff,

 SUCESS!!  We spoke about the marine carb last week with 67 jets and 44B rods - way too lean indeed.  Installed the 39 rods you sent and water tested today.  The boat runs perfect!

 Thanks for your help, your book (which after reading 3 times and working on these things for a few years I am finally reaching a novice level), and most of all thanks for your commitment to your work.

 Back in the day (late 70's), my father told me about the trips he was making from Anderson, IN to Rochester to work on the E4M project. He was a sales engineer for Delco Remy. It was their solenoid and they were pretty heavily involved.  I was 6 years old - probably should have paid more attention to what he was saying!
 

John

[Cliff Ruggles]

Good news!....Cliff