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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old June 29th, 2020, 07:12 PM
Dano64 Dano64 is offline
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Default Where's the smoke coming from??

Hi.

I've been stymied long-term on time availability and carburetor mysteries.
The front pointed out vacuum problems and after replacing the intake and exhaust manifold gasket and plumbing vacuum lines properly, the 250 sounds excellent and idles beautifully.

But... smoke.

At first I thought it was oil on the exhaust manifold but it's not burning off.
I think this might be fuel leaking from the carburetor and smoking, clearly worse than oil.

Best I can tell as a source is that maybe the floats are set wrong and it is overflowing and pushing through the marginal gasket. I have a rebuild kit on the way from Mike's carburetors. They have been helpful, though I'm not sure I have the right kit for my (probably) Rochester B.

Any tips on finding source of smoke? I've tried looking all around and even using my son's awesome inspection camera (Ridgid from Home Depot) but can't tell other than the wet carb gasket (between top and bowl).

Thanks. The truck has been slowly tinkered with and sitting lonely in the driveway and I really really really want to get past this. I'm ok with mechanical and electrical fixtures (what I don't know, I learn), but carburetor stuff blows my mind somehow.
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  #2  
Old June 30th, 2020, 10:04 AM
James James is offline
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Default Re: Where's the smoke coming from??

Does it smell like fuel or oil? Do you have a video to show us?

If the float level is too high the engine would run rich and might not run smoothly. Gas mileage would also suffer. With the engine running at idle, look down the carburetor. Do you see fuel dripping off of the discharge nozzles? If you do the the float level too high, if not then it is OK.

I also was looking at one of your video and noticed a HEI distributor had replaced a point distributor. The point distributor has a resistor wire to help prevent burning up the point a low engine speed. You will need to bypass that wire so the HEI distributor will get full power. Also the original wire gauge is too small. The easiest way to update this is with a relay and a fuse. Also make sure you engine idle is set to spec because the centrifugal advance will start to kick in around 1000 rpm.
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Old June 30th, 2020, 06:03 PM
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Default Re: Where's the smoke coming from??

1. Gas does not smoke. You have either an oil leak that is dripping down on exhaust or you have blow by or exhaust leak. You will need to visually figure out where smoke is coming from. Which side of motor or something to give us a clue to help you. I assume it is blue smoke.

2. The vacuum advance hose needs to be on the carburetor, NOT on the PCV like the video shows. Get a single port PCV and hose it to the same place you show on the manifold. Hose the distributor vacuum advance to the carburetor throttle body port, next to the air/fuel adjustment screw.

3. The distributor gets one wire directly from ignition switch "on" terminal to Distributor Red. The other distributor wire (yellow) is not used unless you have a tachometer. You seem to show two wires attached to the red distributor wire. This is incorrect. Take off any wires on the starter solenoid "R" terminal and remove or tape them up. This may or may not be the two wires you speak of in the video. Those to wires need to be taped up or removed. As I said, Install a wire from ignition "ON" to the distributor Red. That is the only wire involved.

4. You don't need a relay on anything but the horn.

5. You don't rebuild a carb if motor is running good. identify where carb is leaking exactly. Tighten the screws a little. If that doesn't do it, replace the leaking gasket. Those carbs are known to warp and gaskets won't seal anymore. There really is no fix for that unless you are very handy or a machinist. Hope you do not have that case. Mikes carburetors speaks of that and how to possibly fix. Try a new gasket and tight screws. Don't strip the screws.

6. On the video you keep mentioning TDC. If you are timing a motor, you are looking for TDC on the compression stroke. You mention that #1 and #6 are at TDC at the same time. That is correct BUT one is on the compression stroke and the other is on the exhaust stroke. BIG DIFFERENCE.

7. You are asking this in the V6 and V12 engine forum, this is a straight 6 motor? Every time you start a post tell us the year of the truck, the year of motor, the type of motor. the size of motor. Video says 1970 250 Chevy motor.

If you are unsure of anything, ask before acting.

Last edited by AZKen; July 1st, 2020 at 05:57 AM.
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Old July 2nd, 2020, 03:27 AM
Dano64 Dano64 is offline
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Default Re: Where's the smoke coming from??

Quote:
Originally Posted by James View Post
Does it smell like fuel or oil? Do you have a video to show us?

If the float level is too high the engine would run rich and might not run smoothly. Gas mileage would also suffer. With the engine running at idle, look down the carburetor. Do you see fuel dripping off of the discharge nozzles? If you do the the float level too high, if not then it is OK.

I also was looking at one of your video and noticed a HEI distributor had replaced a point distributor. The point distributor has a resistor wire to help prevent burning up the point a low engine speed. You will need to bypass that wire so the HEI distributor will get full power. Also the original wire gauge is too small. The easiest way to update this is with a relay and a fuse. Also make sure you engine idle is set to spec because the centrifugal advance will start to kick in around 1000 rpm.
@James it looks mostly like oil burning off, and maybe it is and I just didn't give it enough time. Not quite as white of smoke as I was expecting and can't tell for sure. I haven't driven it since getting the gasket fixed - because the smoke is too much and I was fearful it was fuel. Will try to get video.
Distributor resistor wire - bypass that to get full power to HEI. Relay and a fuse. Why a relay? Seems relatively low current.
Noted on idle; that was one of the main problems before the gasket (vacuum leak) fix; easy to idle down now!
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Old July 2nd, 2020, 03:35 AM
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Default Re: Where's the smoke coming from??

Quote:
Originally Posted by AZKen View Post
1. Gas does not smoke. You have either an oil leak that is dripping down on exhaust or you have blow by or exhaust leak. You will need to visually figure out where smoke is coming from. Which side of motor or something to give us a clue to help you. I assume it is blue smoke.

2. The vacuum advance hose needs to be on the carburetor, NOT on the PCV like the video shows. Get a single port PCV and hose it to the same place you show on the manifold. Hose the distributor vacuum advance to the carburetor throttle body port, next to the air/fuel adjustment screw.

3. The distributor gets one wire directly from ignition switch "on" terminal to Distributor Red. The other distributor wire (yellow) is not used unless you have a tachometer. You seem to show two wires attached to the red distributor wire. This is incorrect. Take off any wires on the starter solenoid "R" terminal and remove or tape them up. This may or may not be the two wires you speak of in the video. Those to wires need to be taped up or removed. As I said, Install a wire from ignition "ON" to the distributor Red. That is the only wire involved.

4. You don't need a relay on anything but the horn.

5. You don't rebuild a carb if motor is running good. identify where carb is leaking exactly. Tighten the screws a little. If that doesn't do it, replace the leaking gasket. Those carbs are known to warp and gaskets won't seal anymore. There really is no fix for that unless you are very handy or a machinist. Hope you do not have that case. Mikes carburetors speaks of that and how to possibly fix. Try a new gasket and tight screws. Don't strip the screws.

6. On the video you keep mentioning TDC. If you are timing a motor, you are looking for TDC on the compression stroke. You mention that #1 and #6 are at TDC at the same time. That is correct BUT one is on the compression stroke and the other is on the exhaust stroke. BIG DIFFERENCE.

7. You are asking this in the V6 and V12 engine forum, this is a straight 6 motor? Every time you start a post tell us the year of the truck, the year of motor, the type of motor. the size of motor. Video says 1970 250 Chevy motor.

If you are unsure of anything, ask before acting.
1. Well I feel a lot better if it's oil not fuel. I failed at finding it visually; it was coming from a lot of places I think. It's possible - please don't laugh - that my son rather coated the exh manifold with oil as part of 'cleaning' it because he'd heard me say that oil can prevent rust (in totally different context). Even so, I thought it would have burned off by now in maybe 10 minutes of smoking a lot in my driveway.

2. Will change this.

3. Ignition on to distributor Red. Tie off Yellow.

5. Just got the carb kit for the gaskets; wasn't planning a whole rebuild. It was running poorly before, but after fixing the vacuum leak; MUCH improved. Smooth. Will try gaskets and if that fails will look up on Mike's.

6. Yep. the video was a while back and I did get this figure out. thank you! I pulled the lifter covers to see which TDC it was.

7. I'm fried and just messed up. Will do, thanks. And not really sure the motor year.
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  #6  
Old July 2nd, 2020, 08:59 AM
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Default Re: Where's the smoke coming from??


If smoke is all over, it is probably the oiled manifold. Will take some time. When cool, scrub with Dawn platinum dish soap and rinse. Other than that you will just have burn it off by running motor.
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Old July 2nd, 2020, 09:44 AM
James James is offline
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Default Re: Where's the smoke coming from??

Quote:
Originally Posted by Dano64 View Post
Distributor resistor wire - bypass that to get full power to HEI. Relay and a fuse. Why a relay? Seems relatively low current.
The HEI require more power. That why the wire that is plugged into the distributor is bigger. Using the resistor wire will cut back the power. Using the same size wire with a relay and fuse will allow the distributor to operated at it full potential.
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Old July 2nd, 2020, 06:25 PM
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Default Re: Where's the smoke coming from??

Just run a new 12 ga wire from IGN to Dizzy. No fuse. No relay. Done.
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  #9  
Old July 7th, 2020, 03:38 PM
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Default Re: Where's the smoke coming from??

Update: Smoke DID burn off. Seemed to take forever, but done.

new gaskets in, smoothed off some of the pitted pot metal & added permatex

Leaked fuel at first (where the line connects, not the gasket); I as being shy with the wrench; tightened up and it's dry, so ... running pretty well!

Now that vacuum, smoke and leaks are not an issue, I think I need to adjust timing. I'd done that before but wasn't confident in it and it's pinging under load.

Spent a few hours cleaning junk from the bed where it had become the crap repository. Family is on notice it's a truck again.
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