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q jet bog

2K views 17 replies 10 participants last post by  adantessr 
#1 ·
I rebuilt the q jet on my 262 after bogging issues, but bogging persists. It will run smooth then suddenly bog, then run smooth, then bog. Sometimes it will bog, release, bog release in quick succession. Sometimes it runs for quite a while before bogging, and it is much worse around town than out on the highway where it might run smooth. Now, I will say that I never touched the idle mixture screws coz I was advised not to for the carb rebuild, but I'm considering taking it off again and doing those, as well. Could it be something other than the carb?
 
#5 ·
This is a stock engine. It will bog any time, but always around town. When it first fires up, it runs smooth for 30sec, then bogs for a few then releases again. It can go back and forth like this around town, but often clears up on the open road, but not always. Wide open, it can still be bogging on the highway. The heat riser works, and I replaced the fuel cap. This vehicle only has 100k on it; most of it putting around the ranch by an old dude. I haven't checked out the fuel pump or tank strainer yet.
Thanks for any and all replies. I don't have a clue what is causing this.
 
#7 ·
Perform a basic fuel pressure check. Hook a gauge o it verify correct pressure. Then while driving watch the gauge when the situation occurs.
I tie the gauge to a windshield wiper arm so it easy to see. You will need to "T" into the pressure line and run a long hose. Check for leaks then drive.
Could also be an EGR valve snapping wide open when not needed ,or opening too far. You said it is a 262. 262 what? how bout year make model, so we have a bit more to go on.
 
#9 · (Edited)
Thanks again for all your advice.This vehicle is an 86 chevy half-ton, and the carb is an R4-M4ME. No tach, so I don't know idle speed, but the timing is bang on. Strange thing that makes me think it is more than fuel pump or carbon build up: it was just sitting at idle, then bog, release, bog release several times in quick succession. I replaced the EGR valve earlier, but maybe the new one is faulty, and that's the culprit. When I rebuilt the carb, I soaked, and totally sprayed out everything; it was spotless. I didn't touch or fool with the idle mixture screws, though. It will take time to go through the list of possibilities you've suggested coz I live in the Canadian Rockies, and no garage.
 
#10 ·
So it is a Canadian 86 truck?
does the distributor have just the one big wire to it ,or is there an extra 4 wire connector or some extra connector?
Sounds like a fuel problem to me , jut to be clear. You need to start somewhere. Its a bit hard to know which direction to go without experiencing the symptom myself.
But, it sounds like a fuel starvation problem
 
#11 ·
Sounds to me like a cylinder(s) that is intermittently misfiring. When was the last time a tune-up was done? How do the plugs, wires, cap, & rotor look? Have you ohmed the wires for resistance? Look inside the HEI cap and if there is corrosion on the terminals, or the rotor, replace them before you do anything else.
 
#12 ·
check all your vacuum lines I just had the same problem with my q jet I replaced the fuel pump gas tank fuel sending unit cap rotor wires plugs pcv valve and rebuilt the carb still had the same issue ended up finding a hole in one of my vacuum lines replaced all of them to play it safe and amazing the bog was gone. I learned always start with the most simplest stuff first and then get into the bigger stuff.
 
#13 ·
Can you be a bit more descriptive about the "bog"?
To me a bog is the engine trying to die as you accelerate.
If it is trying to die at idle while in par or neutral that sounds like a bad miss or an intermittent electrical problem.
One thing that is pretty common with a Q-jet is the black Nitrophyl float absorb fuel over time and loose their buoyance and allow the carb to flood.
While running at low cruzing speeds it can be fine, but if you hit a bump it can cause a momentary flooding that can cause a stumble.
Just a thought.
Good luck with this problem.
 
#15 ·
Thanks to all, yet again. So much to think about. One thing at a time. I'm sending a pic of the distributor. Besides the 6 plug wires, there are several smaller ones on the driver's side of the distributor. I hope the pic shows those. When I use the term "bog" I mean the rpm's drop, and it goes from a smooth hum to a "thrap, thrap, thrap," with almost a slapping noise. At idle, you can smell unburned fuel, too. I fired it up cold today, and after smooth running for a couple of minutes, it bogged twice for a few seconds, then ran nice and smooth. I didn't take it out for spin, but I ran it for a while, and it didn't bog again. I changed all the plugs and wires recently, but I haven't checked the cap, yet. I'll do that now.
 

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#16 ·
The smell of unburned fuel could be a carb flooding issue, or it could be a cylinder, or cylinders, not firing and pushing the raw fuel out the exhaust.

Also, have you checked to be sure that your choke is opening after engine warm up and staying open? If the choke is staying closed too long, the engine will definitely run horribly and raw gas will be running out the exhaust. If you don't know the over-all condition of the engine, I would recommend going back to basics at this point. Do a compression test on all 6 cylinders to verify that you don't have valves sticking open, and that the rings & pistons are still capable of producing strong compression. If the compression is not where it needs to be, anything else you do is just wasted time & energy.
 
#17 ·
The fuel smell makes me think you have intermittent spark or some other ignition issue.
Try fussing with the supply wires to your ignition while the engine is running. Maybe wiggle them with a stick or something else non conductive. If that causes the stumble chase it down there.
You could also have a leaking vacuum choke pull off. It partially opens the choke during warmup. It also damps the opening of the secondary air doors. It's on the rear left of your carb. When it leaks it allows too much choke and robs other parts of the system of their vacuum. That can affect advance and fuel atomization. To check it put a vacuum pump on the back and see if it holds. If you have no vacuum pump you can take off the line, push in the link, and put your finger over the vacuum inlet. If the link springs out, the diaphragm in the device is bad. If it stays down it's good.

If you find nothing there, perhaps you could have some glunk in the carburetor idle tubes or passages. Not as likely though, since there's a fuel smell when it stumbles.
If it's not ignition you could take these steps on your carb. Get the top off and pass a very thin copper wire strand through each idle tube to physically knock free any stuff in there. I'm a proponent of removing them when doing an overhaul. Not everyone does though.
And as far as the idle mixture. You really ought to set it. First find out how many turns from seated your mix screws are at by counting the turns to the stops. Gently turn each screw to it's seat and back to where it was. Gently, gently, no crushing against the stops. Now you can go back to where your screws started if needed.
To set your mix, start your engine, let it warm. When the motor is warm and off choke get it to your idle rpm. Nice and slow, if it's an auto trans make sure there's enough rpm so it won't stall in drive. Then, lower each mix screw (clockwise) til the engine stumbles, then add fuel (counterclockwise) til it acts up, then split the difference. I tend to go a bit less than the mid point. On my stock marine 350 with 64cc heads, it's about 1.25 to 1.5 turns off the seats.

Give that a shot, could help.
 
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