Rochester Q-Jet 17080561 bogs only at top end?

[vandentr]

OK, so I've pretty much been through this boat from top to bottom. The only issue I've got left is some bogging at the top end of the throttle. The 5.7l 260 MCM starts on a dime, purrs at idle, and jumps out of the hole, but above 80% throttle she falters - no backfire, and doesn't die, just kinda fizzles until I drop it back?

Any thoughts on this? It's got a brand new fuel separator with less than 2 hours of use...

Thanks,
TVB

 

[HT32BSX115]

Re: Rochester Q-Jet 17080561 bogs only at top end?

OK, so I've pretty much been through this boat from top to bottom. The only issue I've got left is some bogging at the top end of the throttle. The 5.7l 260 MCM starts on a dime, purrs at idle, and jumps out of the hole, but above 80% throttle she falters - no backfire, and doesn't die, just kinda fizzles until I drop it back?

Any thoughts on this? It's got a brand new fuel separator with less than 2 hours of use...

Thanks,
TVB

Howdy,


"Bogging" at top end is frequently related to fuel delivery (Carb, filter, tank screen/vent, etc) but can also be related to (centrifugal) timing advance.

"Bogging" can also be termed as not being able to reach full rated RPM. Check the "Adults-Only" sticky above in the main list on this forum.

I'll add another to it's list that happened to my previously installed engine............ 1 or more FLAT camshaft lobes will absolutely cause an engine to be unable to reach full rated RPM.......

Cheers,


Rick

 

[John_S]

Re: Rochester Q-Jet 17080561 bogs only at top end?

Can you tie the bog to when the Q-jet air door is opening? Q-jets are famous for it. While it could be an issue in the carb/fuel delivery, it may also just be an adjustment, such as the air door spring windup tension. It sounds like you have the stock carb, and not a "generic" replacement. Do you have the Merc SM? If so, it has detailed tear-down and adjustment procedures.

 

[joncrisler]

Re: Rochester Q-Jet 17080561 bogs only at top end?

Q-jets also have a small filter in the carb itself. For whatever reason I have had to replace those a few times- you would think the primary filter would not let stuff past, but..... Also, water contamination can cause this- I have had cases where up to a gallon of water got into the tank- after replacing 3-4 of the regular WSF I bought a Racor with a drain bowl.

 

[vandentr]

Re: Rochester Q-Jet 17080561 bogs only at top end?

Interesting.

Forgive me as I know VERY little about properly adjusting a carb. To tear one down might be a touch intimidating for me.

At any rate, as I think about it, fuel starvation of some kind (adjustment or delivery) makes the most sense just because of the sound the engine makes. It's as if the "air door" John mentioned is opening but there's no extra fuel left to mix with it? Up until that point, it's strong and smooth as silk?

John, can you elaborate a little on the function of the "air door" and the symptoms of it being out of adjustment?

Note that it is indeed the stock carb, with the little Merc aluminum tab still on it.

Thanks!

 

[joenapier]

Re: Rochester Q-Jet 17080561 bogs only at top end?

Don't forget to check here:

http://forums.iboats.com/showthread.php?t=490503

 

[John_S]

Re: Rochester Q-Jet 17080561 bogs only at top end?

Interesting.

Forgive me as I know VERY little about properly adjusting a carb. To tear one down might be a touch intimidating for me.

At any rate, as I think about it, fuel starvation of some kind (adjustment or delivery) makes the most sense just because of the sound the engine makes. It's as if the "air door" John mentioned is opening but there's no extra fuel left to mix with it? Up until that point, it's strong and smooth as silk?

John, can you elaborate a little on the function of the "air door" and the symptoms of it being out of adjustment?

Note that it is indeed the stock carb, with the little Merc aluminum tab still on it.

Thanks!

The air door will start to open when the the throttle has been advanced far enough to open the secondaries and there is enough engine vacuum to open the air door. The amount of vacuum that is required is controlled by the spring wideup. More turns means tighter spring, means door opens latter. Less, which is more common, and it opens too early, and can create a bog, as you described. The air door is linked to a set of rods, that adjust the secondary mixture.

The spring windup can be done on the engine, but can be hard to access screw. Maybe not something you want to mess with. If you have a gram pressure scale and merc service manual you can varify if the wind-up is set correctly. I didn't have the scale, so used the appropriate number of nickels taped to a extended paper clip.