[lt1silverhawk]

This sounds like an odd question, at least to myself, but I have been wondering if a bad or failing ignition component such as the distributor, ICM, or the coil can act up or show symptoms of failure during certain temperature conditions.


I have been running the truck without a thermostat while doing the cooling system flush. I don't drive it and it rarely gets over 160 degrees. Should be completing the suggested 3-6 hour "flush run"this weekend. I will be replacing all hoses and putting in a new thermostat at that time.

[lt1silverhawk]

Today, I repeated what I did yesterday. I started the truck, let it warm up for a few minutes, then revved the engine while in Park. The engine bogged and didn't want go beyond 2000 rpm. I moved it back and forth slowly while near the curb, put the transmission back into Park, and revved it again. Same result.


I then let the engine idle while I looked under the hood. I noticed that the glass-cased fuel filter right before the carburetor seemed a bit dirty. The engine was idling just fine. The picture of the filter is below. Interestingly, the filter before this one, also a see-through, didn't appear dirty. the only picture I took of it came out unclear so I will post another one next time.


I then revved the engine again, all the way up to 4000 rpm, and it did just fine. I then drove it around and again, no issues. What is it that seems to go away after some idling and high-revving?


I am planning to complete the cooling system flush this weekend, at which point I will install a new thermostat and check and adjust the timing from there (thank you Pugsy).


On a side note, I am still wondering about the fast idle cam and how sticky it seems. Could that be the cause? Perhaps it "moves out of the way" after the engine has been revved hard enough. I will try do this again tomorrow, and video the carb to see what happens. First, clean out the filter, of course.

[crussell85]

how much is this truck driven? How fresh/old is the fuel in the tank? Maybe dump in some dry gas or something. Did you ever do a compression check to try and track down what caused the oil fouled spark plugs?

[lt1silverhawk]

Originally Posted by crussell85

how much is this truck driven? How fresh/old is the fuel in the tank? Maybe dump in some dry gas or something. Did you ever do a compression check to try and track down what caused the oil fouled spark plugs?

It usually wasn't driven much. Just warm it up once or twice a week and move it for the street sweeper. I've been using it a lot more frequently the past few weeks because of the change in job location. The gas in the tank shouldn't be more than a month and a half old, but I could be wrong.


I will be removing the filter and inspecting the contents some time this weekend. At the friendly nudging of another member, I will also do the timing as soon as I can set some time aside for it (no pun intended), hopefully this weekend.


Also, another thing I've come to notice about the truck recently is that after it is started and/or driven, it doesn't always start back up right away. The lights and everything come on but the engine doesn't turn over. What I end up doing is shifting the transmission all the way up and down the column and then trying again. This always seems to fix the problem. This does not happen on cold starting.


Thank you for the continued help!

[Augusto]

cooling system has nothing to do with this problem

the symptoms lead to a clogged exhaust or a clogged carburator

also a bad fuel pump can do the same

if there are no backfires or the engine keeps running after the ignition is cut, I would say the ignition sustem is fine, but I'll look anyway under the distribuitor rotor for a frozen timing advance mechanism.

does it smoke black when the problem happens? this will lead you to an overly rich clue, like a choke plate that shuts or a bad needle valve.

[lt1silverhawk]

Originally Posted by Augusto

cooling system has nothing to do with this problem

the symptoms lead to a clogged exhaust or a clogged carburator

also a bad fuel pump can do the same

if there are no backfires or the engine keeps running after the ignition is cut, I would say the ignition sustem is fine, but I'll look anyway under the distribuitor rotor for a frozen timing advance mechanism.

does it smoke black when the problem happens? this will lead you to an overly rich clue, like a choke plate that shuts or a bad needle valve.

I've always wondered about the fuel pump. Any way to test it? It is a mechanical. Over a year ago, the truck began having problems after someone dumped water and debris into the gas tank. I eventual had to rebuild the carb. I imagine the junk may have caused some damage to the fuel pump.


I can't imagine what could possible be clogging the exhaust, especially since the problem appears to clear up after a few minutes of idling and revving the engine several times.


There have been no back fires, but the engine does not keep running after the ignition is cut (I hope I understood that correctly).


Aside from a bit of white smoke after starting (condensation?), there is no smoke that I've noticed. I will pay attention to that next time.

[Augusto]

you test the pump for pressure and volume

for pressure you need a tee and a gauge with a piece of hose, you connect the tee between the pump and carb and read the gauge around 5 psi will be ok

now for volume, be careful and don't set your ride in fire, shut off the engine, disconect the line from the carb, attach a piece of hose to it's end and run it to a container of at least one gallon, have someone start the engine and run it at idle, and at the same time start a stopwatch and count one minute, shut off the engine, measure how much fuel was pumped in the container, there should be more than one quart.

[75gmck25]

I have a '75 GMC and when I had similar problems it turned out to be a sliver of RTV that had been sucked into the carburetor when i had the air cleaner off. The sliver of RTV was plugging the venturi on one side, so the fuel mixture was way off. Once I took it apart (its an Edelbrock carb) and cleaned out the debris, the truck ran properly under load.

You may not have exactly the same issue, but I am almost sure your problem is also fuel related. Your idle circuit is working fairly well, which is why it runs at idle, but once you transition to normal cruise you don't have any power.

When I was troubleshooting my truck the Edelbrock tech suggested holding a mirror at an angle and using a flashlight to look down into the carb while the engine was running. When I did that there was an obvious dribble of raw fuel on one side, which was due to the plugged venturi.

Bruce

[cobalt327]

Have you had a chance to get the truck out on the road and drive it? It'll need to have a thermostat installed at some point before you begin using it, I'm supposing after you flush the cooling system. After the thermostat's installed, I'd run the truck around the neighborhood for 15 minutes to give it a chance to get up to temp and see how it behaves then.

I seem to recall the fuel was drained/replaced?

That vacuum line to the modulator valve- bend the hard line so it's pointing directly at the valve so the rubber hose connection is straight. It might be the photo, but the hose looks like it's becoming delaminated where it's bent. If there is any question, replace that length of rubber hose w/a new piece.

[lt1silverhawk]

@Augusto: I was going to do these tests this morning before going to work, but it just so happened that the fuel pump tester kit didn't have the tee connector. I'll stop by Harbor Freight and get a replacement if I can, or I'll pick one up at Autozone.


@Bruce: I do have a strong suspicion that the fast idle cam may be a part of the problem. As I mentioned in an earlier post, it was very sticky and took some effort to get it to move. I'll try spraying some carb cleaner on it to see if helps. If there is a better method, please do share. Btw, good tip on using the mirror. I'll give that a try as well if needed.


@Cobalt: Ever since this problem began, I have only driven it up and down the street a few times. Before that, I did drive up to five miles for work a couple of times per week. Another member has suggested that I check timing before doing anything else and the thermostat isn't necessary to do that. A 15-minute spin should get the temperature should do the trick for the getting the temperature up there and testing the timing (I never properly set it after the carb rebuild and the distributor install. Just did it by ear.)

The gas tank that was contaminated was drained. But I have been using the other tank instead this whole time. I know I did flip the switch between the two tanks here and there a few times for a couple of seconds, but nothing more. I still need to drop the contaminated tank and clean it.

The metal line is not lined up with the modulator (thanks for providing the correct term)[/i], which is why the rubber hose looks "S" shaped. I will try bending it and replacing the rubber hoses on both ends. I am tempted to yank the whole thing out and inspect it for any damage.




So, here is the game plan for now:
- Replace/buy the the-fitting for the fuel pump tester.
- Clean out the fuel filter right before the carburetor.
- Do the fuel pressure and volume tests while keeping an eye on the fast idle cam.
- Check/ replace the rubber hose ends on the vacuum line to the transmission modulator.
- If all's well, take the truck for a spin and get it warmed up.
- Check and adjust the timing.
- Continue with flush.

Looks good?

[T-bucket23]

From the looks of the plugs and the symptoms you have explained and things you found. I would be willing to bet your choke is not working properly. A bad choke will foul plugs in short order and will limit rpm as the mixture will be real rich and air will not be able to flow freely.

Try this, when the truck is cold like in the am after sitting all night.
Open the hood and take off the air cleaner top.

Push the gas pedal to the floor and release then check the choke plate, it should be shut tight.

Start the truck with the air filter top off and look at the choke plate, it should pop open 1/4" or so, If not the pull off is not working.

Let it run for about 5 minutes and look at the choke again once the temp gauge gets up to about 1/4 this may take longer if you have no thermostat. The choke should be fully open. If not then it is binding or junked up.

If you tap the gas at this point the fast idle cam should drop out.

If the choke is binding do not attempt to oil it, clean it with carb cleaner.

Until you are sure the choke is working properly don't waste your time on anything else.

[Augusto]

about the modulator's piece of hose, it's a good idea replacing it for a new one even though I don't believe that's is your problem, but let me tell you that I have seen modulators that fail, they get the internal diaphragm riped and let the engine suck ATF, that makes them smoke and run terrible. easy check, disconect the lline at the carb end and stick a piece of white paper, a napkin or something, it should come out clean, no traces of red ATF.

[lt1silverhawk]

Hey Chet,

Originally Posted by T-bucket23

Try this, when the truck is cold like in the am after sitting all night.
Open the hood and take off the air cleaner top.

Push the gas pedal to the floor and release then check the choke plate, it should be shut tight.

I tried that this morning and below is a quick 14 second video of it. The choke plate closed the first time I floored the accelerator pedal. I did it a total of three times.

Originally Posted by T-bucket23

Start the truck with the air filter top off and look at the choke plate, it should pop open 1/4" or so, If not the pull off is not working.

Let it run for about 5 minutes and look at the choke again once the temp gauge gets up to about 1/4 this may take longer if you have no thermostat. The choke should be fully open. If not then it is binding or junked up.

If you tap the gas at this point the fast idle cam should drop out.

I was able to run it for about five minutes this morning, but not long enough the to get the temperature up there. Outside temperature was approximately 45 degrees F this morning when the video below was shot.



In Case You Can't See the Video:

- Truck is started with no issues. Choke pull off operates immediately.

- Truck is allowed to idle/warm up until 2:25 mark, at which point the engine is revved to about 2,000 rpm, and can be heard bogging (a hissing sound can be heard). The fast idle cam is seen dropping. Fuel flow can be seen in the glass-cased fuel filter right before the carb as remaining steady.

- The engine is revved again about three more time up until 3:00 minute mark. The engine revs slightly higher, but the hissing and bogging can still be heard.

- Engine is revved again at the 3:35 mark, slightly past the 3,000 rpm mark. Hesitation and hissing still evident, but less so than before.

- Engine revved at the 4:25 mark to about 4,000 rpm. No hesitation or hissing heard.

- Transmission is slowly shift through every gear starting at 4:55 mark. Shifted back into Park at 5:05 mark.

- Engine revved again at 5:10 and 5:17 marks to 4,000 rpm. No hesitation or hissing is heard.




I did take the truck for a spin twice and it was running crappy again, despite me pulling over and revving it. I would have liked to have gone for a longer spin but, it was morning rush hour and I really didn't wana deal with all the middle fingers while barely pushing 30 mph.




I'll try to get another test run in before going to work, hopefully long enough to get the temperature up.




-----




@ Augusto: Thank you for the heads up on testing the modulator. I will do that when I also end up replacing the rubber hose ends on the vacuum line.

[Augusto]

been thinking of your problem, did you know that q-jets have this infamous welch plugs under the fuel bowl that always leak? they are under the main circuit fuel wells, two adjacent plugs that are some sort of pinned/riveted/hammered down or someting, they need to be seal covered with epoxy, when they leak your mixture becomes very rich, and is likely that when the carb gets hot the gaps grow bigger and the leak increases upsetting your mixture, they'll dump lots of fuel directly above the throttle plates.

one sure way to know you have this problem is when you let the car sit for two or three days, it takes long to start because the fuel bowl has drained empty.

some carb rebuild kits have this piece of dense foam that is inserted between the main body and the throttle plates body gasket, where the big hole in the center is, to help sealing this leaks but I have found it does nothing, only epoxy cures this problem.

[lt1silverhawk]

Hey Augusto,

The carb was rebuilt about a year ago. The previous rebuilder had used some sort of silicone-ish material to cover the wells. I used some five-minute epoxy. You can see pictures of it here on on post #21 on page 2.


The truck is usually started only once a week, and I always pump the pedal three times and then floor it when starting. I'm pretty sure I've had to do the same even when starting it the following day. I'll test it out over the next couple of days to see what happens.


I know I am not checking things and coming back with results very quickly, and it is a time issue. The tentative plan now is to do what I listed in post #25. Got a 4 day weekend coming up so I'm hoping to go through all this in a day.


EDIT: If need be, I can remove the air horn and check the wells. Most likely a last resort deal.