1986 Chevy 350 Rochester problem

[jddenham]

Re: 1986 Chevy 350 Rochester problem

Please also note, that if I set the choke, pour a little gas directly into the carb, and crank it, it will fire immediately.

 

[jddenham]

Re: 1986 Chevy 350 Rochester problem

I will help you but you must not answer these question from memory. Please go and retry these tests again with a cold engine. When you do, also check the choke valve when it closes. Is it tight against the housing when you attempt to open it by hand or is there a slight amount of freeplay in the valve?

Don't crank the engine more than 5 seconds, as I have another test we'll perform pending your results...

Fair enough. The truck is at home and I am at work. I will get home and perform the steps as you described by about 6pm PST.

I never crank it for more than 5 seconds at one time.

I will remove the air cleaner and inspect the position of the choke butterfly. I will then pump it twice, and inspect the position of the choke butterfly. I will then inspect how tightly closed it is by hand, and look for any freeplay in the butterfly mechanism.

At this point I will post my findings here, not having cranked the engine at all.

 

[SuperNova]

Re: 1986 Chevy 350 Rochester problem

Theres a fuel filter diaphragm that works like a check valve? I didn't know that .I thought the fuel pump diaphragm was just responsible for the fuel pressure staying up and not bleeding back to the tank. Glad I learned that. Might come in handy some day. um... but, how could the fuel get past the pump back to the tank unless the pump was bad too?

This is all wrong.
The rubber thing up at the fuel filter is a bypass valve..it allows fuel to flow if the filter gets clogged...the inlet needle and seat of the carb are above fuel level...fuel CANNOT flow back down out of the carb..period.
The fuel pump is lower than the top of the fuel tank and therefore fuel will not flow backwards from the pump to the tank because of gravity. If the pump diaphragm is ruptured the pump just won't pump very well and you may end up with fuel in the engine oil past the actuator pushrod.
--
Stan

 

[schematic]

Re: 1986 Chevy 350 Rochester problem

Lets kill 2 birds with one stone.
After checking the choke valve position, hold it wide open with a finger. With the other hand, open the throttle 2-3 times by grabbing the throttle linkage. Observe while doing this if there is a good steady strong dual stream of fuel being dumped into the throttle bores below the choke valve. This will give us an indication of the fuel level in the bowl...

 

[jddenham]

Re: 1986 Chevy 350 Rochester problem

Lets kill 2 birds with one stone.
After checking the choke valve position, hold it wide open with a finger. With the other hand, open the throttle 2-3 times by grabbing the throttle linkage. Observe while doing this if there is a good steady strong dual stream of fuel being dumped into the throttle bores below the choke valve. This will give us an indication of the fuel level in the bowl...

The problem is that fuel ISN'T squirting - or so I'm assuming by the fact that it will fire if gas is manually squirted from a can. I will verify this tonight though.

 

[jddenham]

Re: 1986 Chevy 350 Rochester problem

Lets kill 2 birds with one stone.
After checking the choke valve position, hold it wide open with a finger. With the other hand, open the throttle 2-3 times by grabbing the throttle linkage. Observe while doing this if there is a good steady strong dual stream of fuel being dumped into the throttle bores below the choke valve. This will give us an indication of the fuel level in the bowl...

OK attached is a pic of the carb before I touched anything showing the choke butterfly open.

I then turned the key on, the choke idiot light came on, and the carb remained in the same position.

I then pumped the gas to the floor once and set the choke. The choke butterfly then closed firmly.

I then grasped the choke butterfly and it was firmly seated in the closed position. I felt a little bit of free play when opening it by hand, maybe 1/16 to 1/8 of an inch.

I then fully opened the butterfly so that I could visually see the squirters. I manually gave throttle and heard the carb trying to squirt fuel, but none was there. Nothing came out of the squirters(venturi's?) but I could hear a sound like when a squirt bottle has ran out of liquid.

What now? Pull the carb and look for a leaky welch plug?

Any input is greatly appreciated.

 

[thefairlaneman]

Re: 1986 Chevy 350 Rochester problem

I havent messed with that carb alot but do know that they are prone to have leaks. From what you say about cranking it sounds like the acelerator pump diafram. easy cheap fix. That is what squirts gas into the carb one the first pedal pump. After it cranks engine vac sucks it in .

 

[schematic]

Re: 1986 Chevy 350 Rochester problem

It appears by your picture that the air cleaner assemly is covered with soot. Is that correct? That may suggest a lean operating condition resulting in numerous backfires. Is that correct? If you can hear a hissing/spitting when actuating the throttle, and a good fuel stream is not seen, your float level may have been maladjusted giving you the above symptoms. Also, what is the temperature where the vehicle is parked. There should be no free play when ambient temp is below 75-80 deg.
Even 1/8 is too much.

If you have leaking welch plugs, a gasoline leak and smell should be evident on the intake manifold. Look for fuel stains and washed spots below the carb on the manifold.

There is always a bit of fuel loss from evaporation when a hot engine is shut down. This normally is not a problem if your float level is set properly....There is a rochester tool which is designed to check float level from the outside of the carb when still on vehicle. This may be a easy way to check this theory, although pulling the top off to check is not a terrible job either....

 

[jddenham]

Re: 1986 Chevy 350 Rochester problem

The truck has never backfired through the carb or exhaust.

The temperature was around 30 degrees F.

When I said free play, I should clarify. I could move the butterfly a very small amount before heavy resistance, but the linkage always moved with the butterfly, even during the slight amount of low resistance.

I will inspect the manifold tomorrow in the daylight. If the carb is in fact leaking until empty, would it be leaking on the outside of the manifold, or into the intake ports?

More tomorrow . . . .

 

[dolluper]

Re: 1986 Chevy 350 Rochester problem

No fuel squirting .....float bowl or bad fuel pump

 

[schematic]

Re: 1986 Chevy 350 Rochester problem

Sorry, I jumped the gun....I should have read all your posts.....

Since it runs fine when warm, the float level, accelerator pump and fuel pump must be fine. If no external leaks are found, you best remove the carb and check the welch at the bottom of the main body. You'll have to remove the throttle plate assembly from the main body. If you are not familiar with this carb, be sure to get a carb kit before you start. The parts breakdown with assist you with disassembly.

It still sounds like the choke linkage needs some attention, as there should be no freeplay at that temperature. This still will not acccount for the lack of fuel...

 

[jddenham]

Re: 1986 Chevy 350 Rochester problem

I'm going to see what I can figure out tomorrow as far as leaking gas.

I'm not sure if the gas would leak externally from the carb onto the intake manifold, or internally into the intake ports ??

I guess I will pull the carb, fill the bowls full of gas via the fuel inlet, and look for leaks around some sort of plug, or through the junction of a throttle plate assembly.

I'm sure I will have some questions, so I will post as I go and include pics if needed.

 

[Pascal]

Re: 1986 Chevy 350 Rochester problem

Hey jddenham

The best thing to do would be to disassemble the carb, clean it with carb cleaner and blow it dry and clean with compressed air. Lay all the parts out in the order they were removed.

see a generic exploded view here

http://www.carburetion.com/diags/4mv.jpg

There is a manual available on line here

http://www.tocmp.com/manuals/Carbs/Rochester/QJet/index.htm

you can read it from either HTML or PDF linkes provided.

When the vehicle is left sitting, the float bowl can drain through the lead plugs in the primary and secondary metering jet wells into the intake manifold. You can locate the wells when you remove the carburator and turn it over. The primary metering wells will be visible through a pair of irregular shaped holes between the primary throttle holes. The secondary well plugs are further back, between the secondaries. All are exposed to engine vacuum and if leaking, affect fuel mileage when the engine is running.

You will need to remove the base plate where the throttle plates are located. Use some spray carb cleaner and medium emery cloth to clean them. I've had good success coating them with epoxy or JBweld in the short term. They will eventually leak again.

A view of the bottom side of the carb is in figure at this link

http://www.tocmp.com/manuals/Carbs/Rochester/QJet/HTML/MCarbRoch1973_4M_0026.htm


If the accelerator pump is not giving a full shot when the float bowl is full, the likely cause,outside of a bad cup seal, is the pump discharge ball located under the pump plunger. It is held in place under a threaded plug retainer. It's main job is to prevent the syphon of fuel out of the pump well when at cruising speeds and keep air out of the pump discharge passage while the plunger is lifted in the well.

Dirt and erosion are the enemies of keeping the ball seated. Remove the ball and clean the seat with carb cleaner and blow out any liquid and dirt. install the old, smooth, clean ball and give it a light tap into the seat using a pin punch and hammer. install a new check ball, the retainer and put the rest back together. with any luck it will be as good as new.

The little check valve in the filter inlet is to prevent the fuel tank from syphoning in the event of vehicle rollover. The float bowl will drain through the carb bowl vent.

good luck 30 years ago I was fixing them in my sleep after a long day in the shop.

 

[jddenham]

Re: 1986 Chevy 350 Rochester problem

UPDATE: I took the carb in to a reputable carb shop. The owner told me that he was familiar with the problem and that there were O-rings in the welch plugs that needed replaced.

$50 later I put the carb back on and started it. Naturally it had no gas in it at this point so I couldn't test.

I got the engine started and it seemed to run fine but the idle seemed a bit low. I bumped it up 200 RPM or so. I drove it around and it seemed to drive fine at all throttle positions.

The next morning I expected to pump the throttle twice and have it start right up on high idle.

I cranked it for 3-4 seconds and it fired a little but died. I then had to crank for a total(not continuously) of maybe 10 seconds to get it to fire. It would act like it was choking and going to idle up and then would die. It seemed like the choke wasn't staying on. I finally pressed on the gas just barely and it kept running but not at high idle.

I think that I might have solved one of the problems of many. I now need to check out the operation of the choke.

There are two locations where there is an electrical connection on the carn. One is a two conductor watertight plug, and the other is a slide on factory style connector that attaches to the choke ***'y. The choke connector plastic is broken and it does not fasten securely. I'm wondering if a bad connection there could be the culprit.

Any ideas at this point??

 

[SuperNova]

Re: 1986 Chevy 350 Rochester problem

Sounds like the choke just needs to be adjusted properly including the high idle speed. Maybe the pull-off isn't working right.

 

[dolluper]

Re: 1986 Chevy 350 Rochester problem

You probably only have bench spec's on it now,so it will need some final tweaks to get it right it will be close with bench spec's...one pump on the gas will put it on high idle choke ,two might slip it to second stage idle choke,check you have power coming to choke connector assuming it's an electric choke when cold

 

[Coors]

Re: 1986 Chevy 350 Rochester problem

Doesn't the voltage pull off the choke? Maybe why have to touch the gas a little-now too much choke?