Rochester Quadrajet Issue

[Dtrom]

1987 Sea Ray w/ Mercruiser 260 and Rochester Q-jet. Rebuilt carb to spec but encountered issue regarding full throttle operation. Discovered that when I open throttle while underway to 3/4 or better, the engine bogs down. However, if I disconnect the choke linkage it runs great. I’ve received a new divorced choke but after looking at the existing one I don’t believe this is the problem. I’ve opened the 2 air mixture screws at base of carb to 5 turns out. I am stumped. If not for the fact that I need choke for the initial cold start of the day I would not worry about it as I operate in both in Phoenix and San Diego where it is warm. I have no problem starting engine once it’s warmed up. Replace water/fuel separator. Where should I be looking?

 

[wellcraft-classic210]

To be clear

Are you saying your choke is not coming off once the engine warms up ? If so does it move at all ( as in how far is it open after running for 10 minutes with the engine reaching 140 degrees ).



Typically the rods can be bent / set for the cold setting and the bi-metalic strip will move it to the wide open position as engine heat is introduced. This can take a few minutes but I would expect the choke to be off in about 7 minutes or less assuming the manifold is heating up as it should.

The secondaries ( back barrels ) will not be able to open until the choke is off.

The 2 front screws are for idle mixture only -- Once the RPMS pick up the fuel is delivered through the venturi passages and the idle passages are not in play.

 

[Grub54891]

Agree with the above. Five turns out is a bit much.

 

[QBhoy]

Disconnect it and lock it in off position and be done with it. Done it to all my 4 barrel Rochester. 8-10 pumps of throttle when cold. Crank and catch it. Let it warm at high idle for a few mins. Off you go. Very common issue.

 

[Dtrom]

You are correct Master Chief, the choke is not opening once the engine rises to temp. When I manually push the choke open it will only open a minimal amount. It seems to be restricted due to the linkage arm that it is connected to. The divorced thermostat coil is relaxed and uncoiled allowing me to manipulate it freely. It appears as though the linkage arm running from the choke shutter down thru the body of the carb is too short to allow for further extension and therefore opening. That arm is straight and has not been bent or manipulated. This is why when I disconnect that arm and the choke is free and open the engine performs at full throttle. At this point I am convinced I have overlooked something obvious and simple, yet so simple I am not seeing it. Any ideas from you or others will cause me to look harder. Instead, I am NOT haze grey and underway as we used to say.

 

[Lou C]

Head over to cliff's high performance Quadrajets, (website) and take pix of what you got and post it up on his forum. There are real experts there in the set up of Quadrajets there. Mine works just as it should, I will say that if you just let it sit and idle the choke will take a long time to open up due to the fact that the divorced choke spring gets heated by the exhaust crossover in the intake manifold and at idle with a low temp marine thermostat, the engine just does not build enough heat to get it to open fast. If you run it on the water, even at no-wake speeds it will open reasonably fast.
We had the same set up on our cars back in the '70s and with the 195* stats they warmed up faster. They do make an electric choke conversion kit that will eliminate the slow warm up issue.

Mine starts instantly from cold as long as it has not sat more than a week at a time. The other problem we have more or less chronically here in salt water is that with raw water cooling, the thermostats tend to get stuck open which slows the warm up even more. When I re-power this boat I am for sure getting a closed cooling system for my new engine.

 

[Dtrom]

Great advise. I will check out that website and see what they have to say. Thank you for your input.

 

[wellcraft-classic210]

Can you post a pic of your linkage Wondering if its been altered in some way from the original setup.

 

[Lou C]

Here's a pic with the carb not installed. Just took it apart to rebuild it about a year ago. The only thing missing is the rod that comes up from the thermostatic spring

 

[Lou C]

This one shows the choke spring rod...

 

[Dtrom]

Lou, your photo shows the choke much further open than mine is at the same position. I will check to see if there was some way the choke was adjusted where it swivels. Could be the problem. Will advise. And thank you all.

 

[Lou C]

Keep in mind that when the engine is cold and the choke rod is hooked up, the choke valve should be nearly closed. The idea with this linkage is that the secondary air doors (vacuum operated) will not open if the choke is still closed or mostly closed. The other thing going on with the linkage is the vacuum break. That is the round plastic thing hooked up to the choke linkage and the vacuum hose.

What happens: Before you start a cold engine, you push the throttle all the way forward, this allows the choke plate to close as it should. Then you advance the throttle fwd about 1/4th way to set the fast idle. When you crank over the engine, the vacuum break pulls the linkage to open the choke plate about 1/8th inch, this is to give it enough air to keep running with that rich mixture. Then as the engine warms up the exhaust heats the exhaust cross over inside the intake manifold. This causes the thermo spring to unwind and open up the choke plate, which also moves the linkage and will allow the air doors to open when engine vacuum drops enough to call for it. On the Q-Jet the actual secondary throttle plates are mechanically operated but the air doors above them are vacuum operated.

You may have your idle mixture screws set too rich, the standard setting is 3 3/4ths turns out.

 

[Dtrom]

So the problem isn’t that it won’t start as it should with the choke working like it should. The problem is that the choke will not open after warming up to allow for full throttle operation. It runs great from mid-range till just before the secondary kicks in. As soon as I try to go full throttle it bogs down. When I remove the choke linkage (after initial start) it runs perfectly. It will even start easily for the rest of the day. The divorced choke coil is unwound to allow the linkage to open.

 

[jimmbo]

Q-jets are designed to prevent Airflow thru the Secondaries as long as the Choke is not Fully Open. It was done it in a couple of ways. The first way, done only for a few years of Early Q-Jet production was to have Tang prevent the Secondary Air Valve from opening. The later way was to prevent the Secondary Throttle shaft from rotating. The later method requires a Spring Loaded Linkage as part of the Secondary Throttle shaft that still permitted the linkage to open the Primaries fully even if the Secondaries were locked closed. The Locking Tang was part of the Choke Assembly, that kept the shaft from rotating.

Which kind of choke does your engine have, Divorced, Integral, or Electric? All are Adjustable, but there might be a problem that enough Heat to unwind the bi-metal is not getting where it needs to be

 

[Lou C]

If you see that the engine warms up to normal temp and the intake gets hot where the exhaust cross over is then you may need to bend the choke rod so the choke is open when the engine is warmed up. Sometimes the choke coil does not move far enough when they get old. They can be replaced and the choke may work better.

 

[Dtrom]

[No message]

 

[Dtrom]

I believe I found MY error. Comparing photos I see where I mistakenly place the vacuum tang on the back side of the vacuum break rod. Too late tonight to rectify but will do so in the AM and see if that is it. Will advise. If so, it’s true; A picture is worth a thousand words.

 

[Lou C]

correct, that tang goes in front of the linkage rod not behind it...

 

[Lou C]

The way it is, the choke will not open up even if the spring unwinds and the secondary air doors will not open either. Once you get it right you should have a nice performing engine again!

 

[Dtrom]

Success! That was the problem the entire time. Thanks to all who gave their input.