Was reading “timing chain symptoms” thread and saw guy had same problem.
Starts ok, but sputters under any load until warm, then fine for rest of day.
I HAD that problem, now won’t go away when fully hot, however very cold here right now.
350, original motor, 120K miles. LOL
Ignition - plugs, wires, cap, coil,pick-up, all ok. Timing at 8,BTDC. Vacuum can ok. Carb gas filter OK.
It is very cold here right now. If I let it warm all the way up (so oil is HOT) will go away (I think) and run like a top.
Lifters? Slack in timing chain? Dying fuel pump? I would think the motor had cratered if it didn’t completely go away now and then.
Was reading “timing chain symptoms” thread and saw guy had same problem.
More than likely a carburetor problem and it’s time for a carb overhaul or replacement.
It’s not likely at all the timing chain is behind this problem. You can also verify if a chain is worn by putting a timing light on the car. If the chain is worn the timing mark will be bouncing around against the timing plate instead of remaining steady.
I am thinking something to do with the choke pull-off. What type of carb? if you say Q-Jet we will need a real old timer to sort this out.
Thanks for the answers guys. Heh heh, yep. Q-jet. I’m sure it’s the original, and the choke is a little hinkey as usual. I’ll double check again by setting the choke wide open, but I don’t think that is the culprit.
Sticking valves due to the cold? Anyone have experience with this?
(I’m gettin’ up there, but I ain’t a REAL OLDtimer yet!LOL.)
Setting the choke wide open during warm up won’t work.
I have a shop manual for Chev & GMC pickups 1967 thru 1982 and it shows me 2 Quadrajet carbs.
One is a typical Rochester 4MV and the other is a M4MC series carb.
Is yours a manual or automatic choke? I don’t have a scanner so I can’t post pix or text out of the manual.
I’m not a tech like OK4450 but I grew up around these old cars.
OK mentioned it might be time to overhaul/rebuild the carb but you say you don’t think the choke is the culprit.
Perhaps the choke isn’t, but do you know how many parts there are in this carb?
I’m looking at an exploded view of the M and MV carbs and the list of parts adds up to 59.
Does this engine have a (spring loaded) heat stove on the exhaust manifold hooked to the choke rod?
I had trouble with this spring (rusted then broke) on a '78 Jimmy (350) I used to have.
Thanks for mentioning the right terminology, Caddyman.
Heat riser valve. (we used to call it a heat stove)
Q-jets have a list (not really so long) of items not regualry taken care of with just a “kit”. You need to find someone who has Q-Jet overhaul experience.
I am curious, are the chemicals that we used to soak carbs in when a overhaul was being done still avialable? as I remember they were some real nasty products.
You may want to find another oldtimer this one will have a operating Sun engine analyzer (with ocilloscope) this would give you a good look at the secondary ignition system performance,maybe a power balance test and a two gas analyzer,all really ancient stuff.
I would not concentrate on the sticking valve idea,are you getting any backfires (either through the carb or out the tailpipe?)
Your warms up and runs fine description points to some feature that is designed to improve cold engine performance that is not working properly.
Buy this time, they used an electric choke heater to heat and open the choke quickly. Also, a vacuum choke pull-off to open the choke a little almost instantly.
Remove the air cleaner and observe the carburetor before you attempt a cold start. Open and close the throttle ONCE. The choke should snap shut. Without touching the gas pedal, crank the engine. It’s should start and run at fast idle and the choke should open part way. If the engine seems to be idling TOO fast blip the throttle once and the choke fast idle cam should drop down allowing the idle speed to drop. After 2 minutes, the choke should be almost wide open and after 3 minutes it should certainly be wide open and the engine idling normally. That’s what you should observe with a properly functioning and adjusted Q-jet.
On V8 engines, there is a heat riser. On the Right-side exhaust pipe, right where it bolts to the exhaust manifold, is the heat riser valve. On a cold start, it is closed, forcing exhaust gas through a passage in the intake manifold under the carb. Most of the exhaust gas is directed through this passage and out the left side exhaust pipe. These hot gasses heat the intake manifold up quickly, warming the entire engine up quickly. Then the heat riser opens (some use a simple spring, later models use a vacuum controlled valve) to allow the right bank to exhaust directly into the headpipe. If this heat riser valve is stuck open, your Chevy V8 will be VERY slow to warm up and run smoothly. If it’s stuck closed, performance will suffer and overheating is guaranteed.
The most common problems with 37 year old Quadrajets, they develope internal leaks, allowing the float bowl to drain empty overnight. Then, on a cold start, the engine refuses to start until it’s cranked long enough to refill the float chamber. “Pumping it” hastens this process. Another common problem is the plastic covered foam float becomes porous, absorbs gasoline, gets heavy and no longer controls fuel level properly. The engine gets hard to start when warm, floods easily, and gas mileage suffers.
I hope this information helps you…
“I am curious, are the chemicals that we used to soak carbs in when a overhaul was being done still avialable? as I remember they were some real nasty products.”
“Cold Parts Cleaner” is not seen much any more. Mostly trichlorethylene and carbon tetrachloride, REAL nasty stuff.
Since the problem goes away when the engine is hot (near as I can tell) odds are there is a choke related problem; sticking choke flap, faulty choke pulloff diaphragm, etc.
Sputtering under a load could point to a weak accelerator pump circuit.
The reason I suggest an overhaul is that odds are the carb is about due for it considering the age and I’ve never been a fan of piecemealing something.
The Q-Jet is a great carburetor but along with a kit they generally require a few other modifications to be “right”. These include sealing the jet well plugs, replacing the float (prone to gas logging at times), verifying the throttle shafts are tight in their bores, etc.
It’s not a bad idea to check for vacuum leaks also as this era of vehicles were generally vacuum hosed and thermovalved to death. The best method to determine if you have a vacuum leak is with the use of a vacuum gauge. An air leak anywhere is more detrimental to a cold engine than a warm one.
Wow, Ok lots of information there. By the way, this isn’t a show car. I bought it for pocket change ten years ago and have been putting miles on it every winter since.
Firstly, I’ve been around these engines for a long time, and I’ve rebuilt a few 350’s and a few Q-Jets… although I’ve never heard of “sealing the jet well plugs”. What you state about the float is setting off a few bells though. Learn learn learn. I may have been around them for a while, but I haven’t had experience with every problem because they are so reliable.
This one has a thermostatic choke(Bleh) and has to be reset EVERY spring and fall or it doesn’t open all the way or never closes. You may know what I mean… If I can get past this bug, I have a manual choke cable I will install… somehow. I gutted the heat riser (removed the flapper) years ago and haven’t thought about it since.
I know what you mean about “thermovalved to death” LOL and I checked for leaks as well. Even around the base of the carb. What I meant about setting the choke wide open was that I would do that after it was warm to be sure it wasn’t the culprit. It has warmed up a little today so I can do some testing without freezing my nards. I’ll run her on the highway for a while and report back what I find out.
Well, I have to conclude it is some sort of metering problem. She started ok, I let it warm for 10 min and set the choke wide open. Black “water” from tail pipe. Engine not fully warm: stalls just putting it into gear. Give it gas just to get rolling and with very light throttle I can accelerate. Once rolling I can step on it but still sputters a bit. Warming up a bit now, will idle in gear at light. Part throttle improves. WOT ok.
Had her up to 100 MPH a few times to stretch her legs. Fully warm, problem completely gone. Idles at lights, doesn’t hesitate on take off, cruises without surging. Only 14 F today.
It does seem to take an unusually long time to warm up. Could be my disemboweling the heat riser…
Maybe at -4 F it just couldn’t warm all the way up at all.
So, does that fit with your thinking?
The sputtering on acceleration is likely a weak accelerator pump circuit and this means a carb overhaul. With the air cleaner off operate the throttle by hand and you should see an instant gasoline discharge from the discharge tubes in the front carburetor bores. If there’s a delay in the discharge or it appears weak the accelerator pump diaphragm has probably dried out or the acc. pump discharge ball is sticking or gummed up.
The old heat riser operated chokes often failed due to clogging of a port in the carburetor and this prevents the flow of exhaust gas through the passageways.
There is a tiny port inside the choke housing and the carburetor. When running, manifold vacuum draws the heat through the housing and operates the choke. When the port becomes clogged heat does not circulate through it and simply dead ends at the housing. The choke will then only become lukewarm.
Removal of the choke housing and cleaning the port could help that problem. My preference would be to adapt an electric choke to it. I’ve adapted electric chokes from Subaru and Nissan carburetors to Q-Jets probably half a dozen times in the past with good results.
Check the pipe from the top of the exhaust manifold to the air cleaner. If it is missing, you will see about an inch and a half long pipe coming down from the inlet tube. Usually you get stalling when you start out from a complete stop after the car has been running long enough for the choke to start to open up. Your plug wires could be shot.
The plugging of that port is a distinct possibility. I never feel heat on the tube leading to the choke and this used to be hot enough to burn. I’ll look into that. Thanks.
Nahh. Plug wires are fine. That heat pipe is another problem piece I discarded long ago. LOL I have a 350 in my truck I run with an open face K&N year round, no problem. Minus 10 or whatever. Of course, it is still tight.
Update: Reset the choke after letting it cool over night and voila! I had it set to begin opening a little too soon, so what was happening was flooding, especially when it was that freakin’ cold! Combine that with a known sticky valve and you have all my symptoms. I am not willing to replace all the thermovalves and plumbing that a simple manual choke will fix. What was so hard about them in the first place to replace them with all this gadgetry?
Another thing is that I tossed a quart of tranny fluid into the oil (after draining some) and ran the pants off her yesterday. I read that transmission fluid is a highly detergent oil, and seemed to work, now she purrs again. I’ll let this oil work for a little while, then change it and the filter.
Thanks everybody for your suggestions! I’d still be checking spark plugs if I hadn’t come here, hat in hand.
Glad to see you found the problem and posted back.
Adding/replacing a quart of tranny fluid is an old, old trick, but don’t keep it there for more than 300 miles. It will break down fairly quickly.
I once did that to clean/lube and quiet down sticky lifters.
Yeah, I don’t like people who don’t post when they find the fix. I leaves a reader hanging. I’ll definitely be changing that oil soon, thanks, and I’m impressed with how well it has worked. I dont’ think I’d recommend it for newer vehicles…
I had it out again tonight without any problems whatsoever. So, hats off to OK4450 for having it right on the first crack, but thanks to all again for taking the time.
OK4450, if you come by this way again could you explain what you meant by “sealing the jet well plugs”? I’m always curious to learn something new.
So 4450 beat me by 1 minute,what am I “chopped liver” Just joking. I happens a lot that we are typing at the same time. I did call it as a choke problem and 4450 called it as a “carb” problem…hea
Oh oh, walking on egg shells here. He did only beat you by a minute. LOL I didn’t look at that until now. Everyone in this thread but me was looking at the carb or choke. I’m just happy to have the old girl running properly again. But seriously, if anyone knows what he meant by that, could you fill me in?
The jet well plug is an area underneath the main jets that is sealed by a plug. Over time these plugs can leak and gasoline will dribble into the intake manifold both when the engine is running and when it’s not. This can cause a flooded, hard to start condition and a rough idle with black smoke along with decreased fuel mileage.
I’ve only overhauled one Q-Jet in the last millenium it seems but the last kit I got actually had a rubber jet well plug cover inside the box. Back in the old days the kits did not come with this rubber plug and it had to be purchased separately.
This of course led to many people not even knowing what it was.
The new plug is epoxied in place and in theory it should be good for a long time.
I don’t think the jet well plug is your problem; only mentioning it for future reference it you ever go into the carburetor since it has to be disassembled to do this.
I’ve got a '79 Z-28 here but it doesn’t have this problem since it was purchased with an Edelbrock Performer carb on it. If you ever do a carb change the Edelbrock is one I would highly recommend. They’re dirt simple to take apart with fewer parts, work very well, and they’re also easy to tune.