Rebuilt carb, same problem

[nola mike]

my boat was bogging on acceleration. bought an ebay rebuilt carb so that i could take my time rebuilding the old. new carb was leaking fuel @ idle, so i finally rebuilt the old one and put it in. same problem now. if i hit the gas hard, it sputters and dies. on muffs i had an impressive fireball through the carb when it did this. if i'm slow on the throttle it hesitates somewhat, then catches and takes off. otherwise it runs great--in fact, i'm surprised how quick the warm up is now that i don't have to burn off a ton of fuel. anyways, what did i do wrong? i was by myself today, so couldn,t check acc pump, but throttle plates were wet after i pumped it with the engine off. what else aside from the accel pump would do this? mercarb 2 bbl btw.

 

[dubs283]

Re: Rebuilt carb, same problem

tune-up??

dwell/timing??

fuel pump pressure??

just a few things to check

 

[robjen1019]

Re: Rebuilt carb, same problem

you might need to adjust your secondary spring

 

[nola mike]

Re: Rebuilt carb, same problem

you might need to adjust your secondary spring

if there are secondaries on this 2 bbl, i'm in more trouble than i thought.

@ dubs: no, definitely prob with the carb. symptoms present with this carb, gone with the other, reappeared when i put the old one on after the rebuild. all tune up stuff done anyway. just for fun, i checked my compression as well: 150, 150, 145, 140. not too bad for a 24 year old.

 

[rodbolt]

Re: Rebuilt carb, same problem

you missed something in the accerelerator pump circuit or used the wrong ball under the venturi cluster or left out the check ball spring from under the cluster.

 

[jtmarten]

Re: Rebuilt carb, same problem

Either the accelerator pump is not pumping, or has diminished flow, but a fireball indicates your timing is way off. Sounds more like low/no flow from the AP.

 

[rodbolt]

Re: Rebuilt carb, same problem

fire ball from the carb on rapid hrottle opening does not indicate ign timing issues, it simply indicates the air/fuel ratio in the intake got very lean and ignited in the intake manifold.
thats the SOLE function of the acelerator pump.
it prevents lean intake ratios at throttle opening.

 

[Aloysius]

Re: Rebuilt carb, same problem

listen to rodbolt.......

 

[nola mike]

Re: Rebuilt carb, same problem

you missed something in the accerelerator pump circuit

i'll pay special attention to the passages when i take it apart again--but if i see 2 streams when i pump the gas, that means all is good there, right?

or used the wrong ball under the venturi cluster or left out the check ball spring from under the cluster.

not sure what we're talking about here--looks like there was an aluminum "T" in the carb in that general area over a hole. i didn't see it in the parts diagram though, and it wouldn't come out easily, so didn't mess with it. i assume that's where that little leftover spring goes that was in the kit...so maybe that's my problem. what's the function of this?

 

[EddiePetty]

Re: Rebuilt carb, same problem

....."what's the function of this? "


...acts as a check valve when the pump is on stroke. Forces the fuel through the squirters instead of back into the bowl.

 

[nola mike]

Re: Rebuilt carb, same problem

interesting, thanks. since that's the only part i didn't redo, maybe that's the issue. i could see if that didn't seat right, i'd at least have diminished flow. i'm curious as to what the stream looks like when the throttle is pumped...and do you just pull that "T" piece out with some pliers? seems like it was in there pretty good.

 

[westportfish]

Re: Rebuilt carb, same problem

You can pull the check ball spring retainer using needle nose pliers. There is also an adjustment on the pump rod linkage. If this isn't correct the pump shot will come in late and also cause your problems. If you can't find this spec pm me the carb number and I will look it up for you.

 

[Danny Mc]

Re: Rebuilt carb, same problem

The T will pull out with needle nose pliers, there should be a spring and ball under it.

 

[Jet Wrench]

Re: Rebuilt carb, same problem

NM,

I just finished re-building my quadrajet and had a similar problem with fuel leaking from the bowl to the intake. Fuel will leak through the aluminum caps/plugs that are peened in the middle half of the bowl. There are actually five places where fuel can leak, see attachment: http://ramchargercentral.com/index.php?action=articles;sa=view;article=97
Epoxy these plugs during assembly of the carb. Remember, these carbs are made out of cheap pot metal and over time develop issues. Perform the Mercury Service Bulletin # 93-15 "Rough idle or can't adjust mixture screws" as well. You seal the bottom attach screws that hold the bottom plenum to the middle plenum after torqueing the screws. This SB can be found in this very useful link for quadrajet carb information: http://www.florida4x4.com/tech/quadrajet/index.php. Really good information and tips to follow, even if it's not a quadrajet.

My engine runs great now. Good idle at 750 RPM at idle, no hesitation when I throttle up. Remember, final adjustments should be made with the boat int the water as opposed to running it on the muffs. You have some back pressure on the exhaust and the idle jets may need to be adusted a bit more rich for that reason. I found that an initial idle jet adjusment of 2 to 2.5 turns back from seat is a good starting point at first. Any other issues will need to be taken care of first.

JW

 

[nola mike]

Re: Rebuilt carb, same problem

You can pull the check ball spring retainer using needle nose pliers. There is also an adjustment on the pump rod linkage. If this isn't correct the pump shot will come in late and also cause your problems. If you can't find this spec pm me the carb number and I will look it up for you.

cool. i was going to leave well enough alone till tjhe winter, but now i'm too curious. i'll check things out this week. and yeah, i did adjust the pump rod linkage.

 

[rodbolt]

Re: Rebuilt carb, same problem

wrong on the ball and spring under the T.
that check ball seals when the pump cup is on the UP stroke, forces fuel to be drawn into the accelerator pump bore from the fuel bowl rather than draining the accelerator pump circuits.
some 2bbl rochesters used a check ball in the accel bore to force fuel into the venturi cluster rather than back into the fuel bowl.
if the check ball under the T fails to seal its quite possible the rig will bog on acceleration while the circuits are refilling, if its stuck in place fuel cannot pass to the cluster and it will backfire.

 

[nola mike]

Re: Rebuilt carb, same problem

that check ball seals when the pump cup is on the UP stroke, forces fuel to be drawn into the accelerator pump bore from the fuel bowl rather than draining the accelerator pump circuits.

so it basically allows the AP to prime and ready itself for the next acceleration.

some 2bbl rochesters used a check ball in the accel bore to force fuel into the venturi cluster rather than back into the fuel bowl.

but not the mercarb?

if the check ball under the T fails to seal its quite possible the rig will bog on acceleration while the circuits are refilling, if its stuck in place fuel cannot pass to the cluster and it will backfire.

so then this check ball is the only inlet to the venturi cluster? if so, i would expect more then acceleration problems if it were stuck in place. and if it fails to seal, it means that i have a dry accel bore, so even if the AP is functioning normally, there's no gas to inject, right?

 

[rodbolt]

Re: Rebuilt carb, same problem

if the ball under the T fails to seal then the accel circuits can be drained, then on the next throttle opening the A/F mix goes lean due to the circuits refilling and not spraying.
if its sticks shut no fuel will be sprayed causing a very very lean mix.
on the check ball in the accel bore some mercarbs had it,some did not, some rochesters had it, some did not.
depends on the design of the bore and where it intakes fuel from the bowl.

 

[nola mike]

Re: Rebuilt carb, same problem

So upon further review...
I'm getting 2 good streams from the venturis when the throttle is pumped. Does this rule out the AP circuit?

A couple of things about the boat operation that may help.

I noticed that the boat won't start now if the throttle is too far open--it backfires. It will start no prob with the throttle barely cracked.

This condition happens from a standstill, but also when I open the throttle gradually. If I don't open it far enough where it stalls, it will hesitate before "catching" and speeding up normally.

It is MUCH more pronounced at low speeds--if I'm over 3k rpms and punch it, no hesitation whatsoever.

I'm wondering if I may have an air leak--it might let in the same volume of air regardless of speed, which would explain the greater effects at low speed. Or am I way off?

What's my next step?

 

[Aloysius]

Re: Rebuilt carb, same problem

An air leak affects the IDLE speed/mixture more than anything else.

The higher the manifold vacuum (idle/cruise) the more effect the leak will have. A leak is essentially imperceptible at open throttle.