260 -Quadrajet rebuild

[joenapier]

I have a 1985 boat that I have completely restored. Because a project like this is never done, I have been working on the carb.

I rebuilt the Quadrajet 4MV (divorce choke) carb and it would not idle at first. I did a bonehead thing and used the paper gasket in the rebuild kit instead of one similar to the one I took off - thik rubber with a mettal sandwhich inside. I think it had a vacuum leak because a squirt of sarter fluid at the base made it rev a little. Once I got a good gasket - problem solved. It idles around 6-700 which is good for a boat shifting. BTW - I replaced the jets, primary metering rods and spring, mixture screws, and float.

Now I have another problem and I think I need to ask you guys. I can cruise at 3000 RPM fine, but if I wind it up to 4000 RPM, it will run for 30-45 seconds and then gag and sputter. I can't tell if it is getting starved for fuel or starved for air, it just won't hold that RPM. If I immediately power down and run at idle for 30 seconds, I can rev right back up to 4000. Then bog down again like before.

It's either rich or lean...

If the thing is starving to fuel (going lean), it may be that the bowl runs dry. I just replaced the mechanical fuel pump, and none of the fuel lines are replaced. Hmmm. Boats have a water / fuel separator that looks like a oil filter - I wonder if that is slowing th flow down. I replaced that about a year ago. There is a little fuel filter "basket" type that is inside the fuel inlet on the carb, but it is clean - I just did it.

I don't think the float has much to do with this, as it seems like an inability to fill the bowl at the same rate the engine demands fuel. A fuller bowl would let it fun a few seconds longer at 4000.

If the thing is getting flooded rich, maybe my secondary choke plates are not opening. So it is getting "choked" when it should be doing the heavy breathing. I saw on another post someone suggested putting a paper clip on the linkage and a paper clip that is moved would indicate that the secondaries opened. (revving the boat in my driveway, I could not get them to open. I could not rev much passed 3000 because I'm running on water muffs and afraid that the hose will not supply enough water to keep me cool). BTW - anyone know what RPM the secondary chosk plates should open?

Back to my original question - I run at 3000 RPM, no prob. I run it at 4000 RPMs and it consistantly bogs down after 30-45 seconds? Sound like a problem you all have seen?

THANKS

 

[joenapier]

Re: 260 -Quadrajet rebuild

here's a pic

 

[concentric]

Re: 260 -Quadrajet rebuild

Same happend to me, my problem was water in the fueltank, and when the water gets to the small filter in the carb, the filter (if you have the paper type) is swelling. Cant deliver enough fuel for high speeds.
Take off the filter and try to blow trough it, in my case almost impossible, but when dried out you could easily blow trough it.

 

[Bondo]

Re: 260 -Quadrajet rebuild

Ayuh,... My guess is the fuel ain't gettin' To the carb...
Definitely sounds like it's goin' Lean,...

 

[John_S]

Re: 260 -Quadrajet rebuild

The fuel water sep filters should be checked and replaced yearly, especially in this alcohol added era. Dump the contents into a clear jar and see if there is water. If yes, going to have to get it all out of carb, and possibly pump out gas tank.

 

[PitchFork]

Re: 260 -Quadrajet rebuild

I was fighting the same problem a few weeks ago. I replaced the water / fuel sep and had no water in it. So I went to WOT and went to 4200 and dropped down to 3900. So I cut the engine as it was popping sputtering then pulled the flame arrestor off and pumped the accelerator pump 4 times and it pumped out fuel all four times. I then concluded not a fuel delivery problem not sure this was the right to concluded but I did anyway. So I then left the flame arrestor off and was able to hold 4200 RPM which is still a little low. So on the next run I had my son push the sec butterflies open completely and BAM I was able to get to 4600 RPM.

I ended up troubleshooting it out to the windup spring was too tight, but your case may be different. I just takes testing on the water to find this problem. I never rev it past 2K on muffs as the lake is less than 3 mins away.

 

[joenapier]

Re: 260 -Quadrajet rebuild

Hey, thanks for the pointers. I should have checked that fuel-water separator because that is an easy thing to do but I didn't know it could clog like that.

I'll runn it in the harbor this weekend and have a look at the carb under real operaional load. Checking the accelerator pump is agood idea if I do it immediatley after it bogs down. We'll see.

Again, thanks for the help!

 

[dan t.]

Re: 260 -Quadrajet rebuild

Definetly starving for fuel, these are classic symptoms of a q jet not getting sufficent fuel volume. First thing to check is the main fuel filter/ water seperator, good luck

 

[Alpheus]

Re: 260 -Quadrajet rebuild

these are classic symptoms of a q jet not getting sufficent fuel volume

Has nothing to do with the carb being a Q-Jet. It could be any carb with lack of fuel that gives off those symptoms...

 

[6meter]

Re: 260 -Quadrajet rebuild

Did you put in wrong jets and/or metering rods? For 15 years I worked for a Chevy dealer and rebuilt 100's of Q-jets and never replaced jets or metering rods.

 

[joenapier]

Re: 260 -Quadrajet rebuild

It was kinda tough getting the jets out and I curled a little burr on the slot that might have contacted the metering rod. I filed it off, but then said - what the heck - I'll just rpelace the jets. They were size 70. The metering rods were size 42, stamped very tiny on the side, and I was not sure if the single taper tip was showing signs of wear or not. They looked a tiny bit "hourglassed" to me, but, like I said, I had nothing to compare them to. My inexperience is showing here - I don't even know what a "worn out" metering would look like, of even if they do that.

I ordered new jets, new metering rods, and new idle mixture screws (one of "D" ends was broken off) from Carburetors Unlimited in Auburn, WA. TWICE they sent me the wrong metering rods - the first time they sent me size 41. I mailed them back. Then they sent me size 42B, which is double taper. Now I'm like 3 weeks behind schedule on this project.

I still have the new 42B rods but I am running with the old ones, size 42. The double taper on the 42B is quite a bit fatter at the top and looks like it would make the engine run leaner and leaner as I accelerated and the vacuum pulled the primary metering rods deeper into the jets.

BTW - I am not sure why I found size 70 jets and size 42 rods. I have seen other "standard" setups listed for a 1985 Mercruiser 260.

 

[6meter]

Re: 260 -Quadrajet rebuild

Are these the metering rods that are put in after the air horn is on, go in just behind the primary venturi's and around the wing nut stud? If so, the faster you go the more they come OUT of the jets.

 

[dan t.]

Re: 260 -Quadrajet rebuild

primary metering rods are controled by manifold vacuume over spring pressure they are out of the jet at low vacuume,pull down into the jet at high vacuume, rpm has nothing to do with it

 

[cr2k]

Re: 260 -Quadrajet rebuild

Yep from that picture I would say you have a quadrajet alrighy.

 

[hydraulic lock]

Re: 260 -Quadrajet rebuild

Hi all, first post here.
Been building Q-jets for a long time. A fine carb. with lots af adjustments. Can do anything a Holley can.
First, you should not have to change any metering parts to fix a hesitation or bog.
Sounds like you do have a secondary problem. Not a fuel supply one. You can ck. fuel pump volume, but that should show up before getting to 3400.

The secondary air valves, the big ones on top,(not choke plates), are held closed by engine vacuum, connected by linkage from the the choke pull-off, which is high when the primary throttle plates are closed.

As you open the throttle, vacuum starts to drop and starts to overcome spring pressure, and begins opening the secondaries, lifting the metering rods, more fuel.
Make sure the choke pull-off diaphram is not blown, should be pulled back at idle, you shoulnt be able to open the secondaries. There is a spring adjustment for the sec. air valves, but that should be ok. You can check it on the rebuild spec sheet for that carb. #.

 

[joenapier]

Re: 260 -Quadrajet rebuild

OK - bad news for the fuel / water separator advise I got.

I replaced the filter (it was due to be replaced anyway) but a test run Saturday shows the problem still exists.

"Hydraulic Lock" has an interesting suggestion that makes a lot of sense:

Make sure the choke pull-off diaphram is not blown, should be pulled back at idle, you shoulnt be able to open the secondaries. There is a spring adjustment for the sec. air valves, but that should be ok. You can check it on the rebuild spec sheet for that carb. #.

I did buy a new choke pull off but did not install it because I compared it to the old one and the location of the hole on the tab for the linkage rod was about 3mm off of the old one. Wary of making extra bends in the linkage to adjust for this (you gota remember that comments about this carb have got me a bit timid about making willy-nilly adjustments), I went with the old one.

(You know, I did test the old choke pull off by licking my thumb, placing it over the vacuum hose connector, and pulling out the tab. The tab snapped back in like it should have). Nevertheless, the thing could be operating poorly.

Hey Hydro-Lock, I found a funny thing when I took the carb off the engine last month. The vacuum line running from the fuel pump to the carb was full of green waxy stuff and was nearly plugged up. What would this have done to the performance of the engine? I replaced this vacuum line. Maybe I should have used the new choke pull off and bent the linkage to account for the shorter tab?

I still have not done what I need to do - ride in the back of the boat and observe the carb doing its thing under a real open-water run. Doing this would show me right away that the secondary barrels may not opening up. YES - I agreee that this would give me the same issue with an engine bogging-down starving to fuel (actually starving for AIR, which is the way it gets more fuel).

High tide is around 5:00 pm today so maybe I can get it out there this afternoon...

The saga continues....

 

[joenapier]

Re: 260 -Quadrajet rebuild

Took her out and let the 13 y.o drive while I stood over the carb with the flame arrestor off.

The secondary choke valves only open up about 1/4 of the way, even under full throttle. The rush of air around them is so fast and the dynamic pressure so low that water condenses on the plates, forming droplets.

I think both the old and new chocke pulloffs are working fine. I unhooked the linkage from the choke pulloff to the secondary choke valves and they still did not open all the way - only about 1/4 of the way.

I don't have a device to measure spring pressure on the plates, but I have not adjusted it during the rebuild. Nevertheless, maybe it needs to be adjusted lighter?

One more thing - the bogging down problem seemed to get worse and worse as I ran up and down the creek. Is there a buildup of wetness in the intake manifold because of the condensation? (my intake manifold is pretty rusty inside).

Whaddabout that, Hydraulic Lock?

 

[hydraulic lock]

Re: 260 -Quadrajet rebuild

Hey Joe,
Good idea watching the secondaries while ur son drives.
A couple things:
u wont get the sec. valves to open w/o enough engine load, that wont really happen out of the water, with a car u could power brake it!
The condensation is normal, u could even pour small amts. of water down the throttle, and it wont matter. Called a water treatment, actually done to remove carbon in the combustion chamber and valves. One of the best things for any engine.
Just to clear it up-
The sec. air valves will not open unless the (choke plate) is just about fully open. The choke is the upper plate on the primary. Thats so they dont open til its warmed up. (Throttle plates) are the ones at the bottom of the carb. that create the venturi effect, pushing fuel from the bowl from atmospheric pressure out the jets (that didnt need to be replaced). (Acellerator pump) is just for initial squirt of fuel, dont even need it if u open throttle slowly.

First mechanical ck.
Disconnect the throttle cable at carb., block the choke open, and push open the sec. valves, they will open b/c no vacuum, should feel some spring pressure. With air valves open, slowly open the throttle and watch the primary throttle plates opening, when they get to full open the sec. throttle plates start opening by linkage on the left side. When u get to full throttle the 4 plates should be at WOT, wide open throttle. If that happens the throttle linkage is good.
gotta go but ck. that.

 

[hydraulic lock]

Re: 260 -Quadrajet rebuild

Joe,

The secondary air valve adjustment is on the rebuild sheet for ur carb.#. The # of turns is listed, the allen lock nut and adjustment screw are located at the right side upper rear of carb. facing down. No special tools needed.

Loosen the allen lock nut and loosen the adjustment screw so the air valves have no spring tension, then tighten to the # of turns listed and tighten lock nut.

If ur carb. # isnt listed, just (increase the tension) in 1/4-1/2 turn steps from the current setting, then see if perf. improves. Yes increase.
Most secondary bogs are caused by the valves opening too soon!!! The sec. valves open only enough for what the engine demands.

Its a good idea to remove the fuel filter at the inlet, and install one inline right after the fuel pump, for higher fuel delivery when needed.
Also, did u epoxy the main fuel well plugs? They are cronic for leaking.

 

[kujawat]

Re: 260 -Quadrajet rebuild

Joe,

The secondary air valve adjustment is on the rebuild sheet for ur carb.#. The # of turns is listed, the allen lock nut and adjustment screw are located at the right side upper rear of carb. facing down. No special tools needed.

Loosen the allen lock nut and loosen the adjustment screw so the air valves have no spring tension, then tighten to the # of turns listed and tighten lock nut.

If ur carb. # isnt listed, just (increase the tension) in 1/4-1/2 turn steps from the current setting, then see if perf. improves. Yes increase.
Most secondary bogs are caused by the valves opening too soon!!! The sec. valves open only enough for what the engine demands.

Its a good idea to remove the fuel filter at the inlet, and install one inline right after the fuel pump, for higher fuel delivery when needed.
Also, did u epoxy the main fuel well plugs? They are cronic for leaking.

hydraulic lock is correct about the bogs... but make your spring adjustments in smaller steps.. 1/8 turn from the current max then test.. most boat carb specs call for 3/8-3/4 total.. so a little movement can mean a big difference... also.. the plugs leaking thing was mostly in the pre 1969 carbs, they improved the design/materials in 1969 then again in the late 70s... i haven't seen a leaking plug in a modern (manufacture date 1979 or later) in years... but, even knowing that I epoxy them anyway when i rebuild... why take a chance.. don't forget to clean and scuff that area before applying the epoxy or the heat cycles will loosen it and it will end up in the lake right through your engine and exhaust... only reason i mention it is that most likely that's not going to be it if this is an 1980s carb.. so before you go down that road too far check other things.. the q-jet is an awesome carb.. next best thing to fuel injection... when dialed in properly they cant be beat by Holley, Edelbrock/weber, or any other carb you might run across.. 99% of the hatred for q-jets is a direct result of not understanding them... once you get up close and personal with the Quadrajet you just cant deny its the best carb ever designed for sub 600HP applications.. PERIOD... dyno HP and Torque curves dont lie..

good luck