IS it Time to rebuild Rochester Quadrajet?

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patvital

Member
Joined
May 29, 2007
RO Number
26526
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Ok guys,

Looking for recommendations on whether to make the drastic decision to remove and rebuild that Roschester Quadrajet carb.

Symptons: cold start works great, take boat for a 20+ minute cruise around the sound, then when I slow down or back to idle, that's where the trouble starts: engine stalls when give it more gas from idle. it restarts ok, after a few pump, but switching to gear will stall it again.

Let the engine sit for half hour or so, the same cycle of events would repeat (i.e. works fine at the begining then stall later on ).
Based on my research and analysis, I Think it is a symptom of the carburator flooding and may need rebuild, however I am not sure if there any other troubleshooting steps you guys would recommend before I take this route.

Here are what I have done so far for troubleshooting this:

1. Setup is twin 305s with Divorced choke with single tank (This eliminate fuel issue since other engine is fine)
2. At startup, verified that choke plate is closed, then opens up when engine is warming ( This eliminates possible stuck choke or defective choke pull-off)
3. retrieve and inspect both idle screws and both are in sharp and pointy state. Even replaced with new ones that yielded
same results. ( switched back to the old ones )
4. Inspected fuel canister and no presence of water and/or dirt.

The only task that I haven't checked is the fuel return line, and I am not if a clogged line would show this behaviour.
If you think it's worth testing the fuel return line, how would I go on testing it?

Any pointers would be appreciated, as I am trying to cover all corners with this drastic decision to rebuilt.

Thanks all!
 
God I'm old....

The last Q'jet I had was on a '67 Chevelle SS 396. There was an issue with the float flooding.

A friend rebuilt his Rochester's a couple of years ago. Took him forever to get it right and he's a pretty decent wrench.

Holley makes a retrofit that some with ($) went to in the early 70's. They made a marine version as well. Check National Carb in Jax,FL.
 
I wonder why you have a return line with the carb setup? If electric fuel pump do you know the PSI rating?

When it stalls, if you open the deck fill cap is there a lot of sucking air from tank vacuum condition? After stalling , if you remove the flame arrestor , can you smell a strong gassy odor to indicate your flooded suspicion?

What is your present idle RPM in N and in gear? What is idle spec.? If set too low , it is normal to stumble or stall.

What is the condition of the plugs and dist cap inside electrodes? Base timing to spec.? Flame arrestor clean?

If you have no trouble at higher rpms, and since you ruled out fuel quality and choke operation, I would suggest 1st filling the remote fuel filter 1/2 full of Sea Foam /1/2 fresh gas and running the engine just long enough to get that mix into and part of it through the whole carb, then shut down and let it work for an hour or maybe longer or whatever the instructions say to do.

Then , run that out until smoke goes away and you know you are running on just gas again.

Then , with idle-stop speed screw back off as much as possible to keep it running , a little at a time adjust the idle mix screws in or out one at a time using a vac gauge to achieve highest vacuum readings.

Then repeat between the screws until you have the best readings. The vac gauge will also tell you if there are any of a myriad of other unsuspected factors affecting your engines, including, say, carb base gasket leak .

Then reset the idle stop speed screw to engine mfgr's spec. (650 -700+?RPM idle in gear?)

If you are still inclined to rebuild the Q-jet , these helpful videos may be handy:
http://www.boatered.com/forum/topic.asp?TOPIC_ID=133180
 
To do a test for 'too lean'. Next time it stalls, give it at least half tp 3/4 throttle. Trying to get it to backfire thru carb. May indicate sticking float or needle...not filling the float bowl fast enuff.

Also test accelerator pump. Look down the carb to see a stream of gas. Doesn't need to be running but we need to know if the engine was 'in running condition'.(Not stalled)

What is age of carb?

I bought the Holley replacement for the Q-jet. Love it. BUT, doesn't sound like you need to replace yours.
 
why would carb cause stalling on shift to gear?????
 
quote:
Originally posted by pdecat

why would carb cause stalling on shift to gear?????

Click to expand...


If it's a Merc, there is an electric interrupt switch that literally shorts out the engine coil for a moment killing it as the boat goes in and out of gear. If there is not enough "oomph" in the engine to get it restarted, it dies.

https://www.youtube.com/watch?v=cKX8tQd3gqA

Around 4:35 of the video. Classic boat as well.
 
The interrupt switch is made to 'kill' engine when coming 'OUT' of gear. It releases pressure on the 'engaging' surfaces of the gears which, incidentally, has about a 3*-7* negative bevel that requires the Interrupt.

And as dave says, if the idle is too low, the interrupt will kill the engine....Also, the interrupt switch can be adjusted so the 'INTERRUPT' is shorter.
 
Originally posted by Sandy

I wonder why you have a return line with the carb setup?
it's a mechanical fuel If electric fuel pump do you know the PSI rating?

When it stalls, if you open the deck fill cap is there a lot of sucking air from tank vacuum condition?
Don't know, but I'll find out...
After stalling , if you remove the flame arrestor , can you smell a strong gassy odor to indicate your flooded suspicion?
Yes, indeed I do smell the fuel, however I couldn't visually locate any actual spill.

What is your present idle RPM in N and in gear?
it's been 650 for that last 5 seasons with no issues.
What is idle spec.? If set too low , it is normal to stumble or stall.

What is the condition of the plugs and dist cap inside electrodes? Base timing to spec.? Flame arrestor clean?
Plugs and rotor are replaced every season, along with cleaning flame arrestor with berryman.

If you have no trouble at higher rpms, and since you ruled out fuel quality and choke operation, I would suggest 1st filling the remote fuel filter 1/2 full of Sea Foam /1/2 fresh gas and running the engine just long enough to get that mix into and part of it through the whole carb, then shut down and let it work for an hour or maybe longer or whatever the instructions say to do.

Then , run that out until smoke goes away and you know you are running on just gas again.

Then , with idle-stop speed screw back off as much as possible to keep it running , a little at a time adjust the idle mix screws in or out one at a time using a vac gauge to achieve highest vacuum readings.

Then repeat between the screws until you have the best readings. The vac gauge will also tell you if there are any of a myriad of other unsuspected factors affecting your engines, including, say, carb base gasket leak .

Then reset the idle stop speed screw to engine mfgr's spec. (650 -700+?RPM idle in gear?)

If you are still inclined to rebuild the Q-jet , these helpful videos may be handy:
http://www.boatered.com/forum/topic.asp?TOPIC_ID=133180
Click to expand...
 
quote:
Originally posted by jtybt15

To do a test for 'too lean'. Next time it stalls, give it at least half tp 3/4 throttle. Trying to get it to backfire thru carb. May indicate sticking float or needle...not filling the float bowl fast enuff.

Also test accelerator pump. Look down the carb to see a stream of gas. Doesn't need to be running but we need to know if the engine was 'in running condition'.(Not stalled)

What is age of carb?

I bought the Holley replacement for the Q-jet. Love it. BUT, doesn't sound like you need to replace yours.

Click to expand...

I'll try the lean test next time.
I will also try to cruise at low rpm ( ~1500 ) with the same amount of time ( ~ 20+ minutes ) and I experience the same outcome...
Will update then...
Carb is a 17080565 which an early 80s one.
Thanks!
 
quote:
Originally posted by jtybt15

The interrupt switch is made to 'kill' engine when coming 'OUT' of gear. It releases pressure on the 'engaging' surfaces of the gears which, incidentally, has about a 3*-7* negative bevel that requires the Interrupt.

And as dave says, if the idle is too low, the interrupt will kill the engine....Also, the interrupt switch can be adjusted so the 'INTERRUPT' is shorter.

Click to expand...

I am pretty sure it's not an interrupt issue I switch in and out of gear ( maneuvering around the busy LI sound ) with no issue until we stop cruising half hour later.
This sure smells like a rebuild is needed.
I just want to make sure I eliminate all the possibilities of potential other misdiagnosed issues. As I read in previous threads, some folks have rebuilt their carbs to then find out the original issue to be still persisting.
 
quote:
Originally posted by KiDa

quote:
Originally posted by pdecat

why would carb cause stalling on shift to gear?????

Click to expand...


If it's a Merc, there is an electric interrupt switch that literally shorts out the engine coil for a moment killing it as the boat goes in and out of gear. If there is not enough "oomph" in the engine to get it restarted, it dies.

great info! Thanks!
Looks like I might have a nice little winter project... :)

https://www.youtube.com/watch?v=cKX8tQd3gqA

Around 4:35 of the video. Classic boat as well.

Click to expand...
 
quote:
Originally posted by patvital

Originally posted by Sandy

I wonder why you have a return line with the carb setup?
it's a mechanical fuel If electric fuel pump do you know the PSI rating?

When it stalls, if you open the deck fill cap is there a lot of sucking air from tank vacuum condition?
Don't know, but I'll find out...
After stalling , if you remove the flame arrestor , can you smell a strong gassy odor to indicate your flooded suspicion?
Yes, indeed I do smell the fuel, however I couldn't visually locate any actual spill.

What is your present idle RPM in N and in gear?
it's been 650 for that last 5 seasons with no issues.
What is idle spec.? If set too low , it is normal to stumble or stall.

What is the condition of the plugs and dist cap inside electrodes? Base timing to spec.? Flame arrestor clean?
Plugs and rotor are replaced every season, along with cleaning flame arrestor with berryman.

If you have no trouble at higher rpms, and since you ruled out fuel quality and choke operation, I would suggest 1st filling the remote fuel filter 1/2 full of Sea Foam /1/2 fresh gas and running the engine just long enough to get that mix into and part of it through the whole carb, then shut down and let it work for an hour or maybe longer or whatever the instructions say to do.

Then , run that out until smoke goes away and you know you are running on just gas again.

Then , with idle-stop speed screw back off as much as possible to keep it running , a little at a time adjust the idle mix screws in or out one at a time using a vac gauge to achieve highest vacuum readings.

Then repeat between the screws until you have the best readings. The vac gauge will also tell you if there are any of a myriad of other unsuspected factors affecting your engines, including, say, carb base gasket leak .

Then reset the idle stop speed screw to engine mfgr's spec. (650 -700+?RPM idle in gear?)

If you are still inclined to rebuild the Q-jet , these helpful videos may be handy:
http://www.boatered.com/forum/topic.asp?TOPIC_ID=133180

Click to expand...


great info! Thanks!
Looks like I might have a nice little winter project... :)
Click to expand...
 
Sounds like it could be an old vacuum line that might be leaking? You could switch carbs on the motors to see if the problem follows. Edelbrock makes the new Q jets now 1409's. Good luck.
 
More info brings more possibilities and temporarily eliminate others.

Some Q-jets will develope a leak in the well tubes. They can be identified by turning the carb upside down. They will have the soldered ends of these tubes. The easiest fix for this condition is a dab of JB weld at the end of the tubes.

What this leak does is drain the float bowls. It will also cause the back-fire out of the carb cuz of empty float bowl. Read no fuel for the accelerator pump to work.

The other possibility from gas smell is stuck float valve that will OVER flood the float bowl. A sharp jar or knock will often unstick the needle valve. I advise to hit at the fuel inlet cuz it's the thickest part of the carb with the most structure to withstand a hit. The best thing to hit it with is the plastic handle of a screw driver. Do this test before removing the carb to check the well tubes.

There are a few more possibilities but since I'm a two finger typer, we'll leave those for later.
 
quote:
Originally posted by jtybt15

More info brings more possibilities and temporarily eliminate others.

Some Q-jets will develope a leak in the well tubes. They can be identified by turning the carb upside down. They will have the soldered ends of these tubes. The easiest fix for this condition is a dab of JB weld at the end of the tubes.

What this leak does is drain the float bowls. It will also cause the back-fire out of the carb cuz of empty float bowl. Read no fuel for the accelerator pump to work.

The other possibility from gas smell is stuck float valve that will OVER flood the float bowl. A sharp jar or knock will often unstick the needle valve. I advise to hit at the fuel inlet cuz it's the thickest part of the carb with the most structure to withstand a hit. The best thing to hit it with is the plastic handle of a screw driver. Do this test before removing the carb to check the well tubes.

There are a few more possibilities but since I'm a two finger typer, we'll leave those for later.

Click to expand...


Thanks Charlie.
I'll give it try this weekend and post results then...
 
I agree with Charlie. Early Q-jet carbs had this problem. The casting plugs on the underside of the float bowl would leak. You could remove a carb, hold it up and actually watch it drip. Good ole JB Weld was a permanent fix. I worked at a GM store back in the day and we fixed a bunch this way.
 
I don't get it. It starts well cold so How could that be a leaking bowl? the bowl would be empty. The problemis after running when the bowl should be at its fullest.

Do you see fuel vapors drifting up from the warm carb? Flooding should be obvious, gas smell dark exhaust etc.

IMO there is an electrical heat related issue.
 
Since you have a carb and low pressure mechanical fuel pump, by the confusing mention of the "return line" are you by any chance referring just to the clear plastic (Zygon?)sight tube from the fuel pump to the side of the carb leading to the j-tube? That just relatively-safely redirects fuel from the pump into the carb throat in the event of a ruptured pump diaphragm.

Otherwise, I can't imagine why you would have a fuel return line to the tank like some injected engines. Even my high fuel pressure MPI I/B engines are "returnless" (by preferred personal option for FloScan use.)

Since you have a single tank, when the balky engine stalls, for the heck of it have you tried restarting with the reliable engine turned off and kept off to see if the balky one will keep running when placed in gear ?

The Q-jets are certainly known for leaky Welch plug issues ( do a search), but, like Bruce, I'd imagine cold starting problems would manifest rather than warm as in this case.

I don't know much about the Alpha drive ignition interrupt switch ( though I had one 40 long yrs ago :) ), but even though I do know as Charlie says that it is designed to do its thing only when shifting OUT of gear to N, anybody care to comment on whether the cable could possibly be just far enough out of correct adjustment it works normally as designed until the cable stretches slightly with unvented , accumulated, stopped-engine heat? Then, could it move the micro-switch lever when going INTO gear?
That still seems awfully unlikely , but ..more info:
http://www.sterndrives.com/shiftinteruptorswitchkits.html
 
This comes under the heading as brucie says of the leaking plugs, "Why does it happen only some of the time'?

Sandy hit on one of the other possibilities. The shift cable can get corroded or frayed and hang up in the cable housing, which will activate the shift interrupt.

It's been years since I've tried to explain the workings of the shift interrupt system. Most people don't care as long as it works. Some people are curious...so I'll try to explain one last time.

The shift interrupt switch is activated by feed back from the cable pushing or pulling on the cable housing. First, the cable housing is anchored only on the aft end within the drive. The other end is anchored on the pivoting shift plate. The pivoting activates the interrupt switch.

The feed back comes from the reverse bevel or undercut gears. They are undercut to keep the gears locked in place. Trying to pull them apart during shifting puts pulling pressure on the shift inner cable that transfers the pressure to the cable housing. The only release for this pressure gets transferred to the pivoting shift plate...which pivots to release pressure. The pivoting plate causes the interrupt switch to activate, momentarily killing engine and relieving pressure on the gears which allows the gears to disengage. I know it's confusing...and hard to explain without tearing everyting apart to see how it functions.
 
I would recommend a client of mine that just does rebuilding of carburetors, he is located in Brentwood NY.

Great guy and has been doing this since Muscle Cars. If your intrested I will pass his info on.

Walter
 
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