Super Lopey Idle on Cold Start - Choke Problem?

[1975Jimmy]

Hi guys,

It seems that in colder weather my truck has developed a really lopey sounding idle when first started and it blows a nice gassy cloud of smoke until I rev it up a little. After a quick rev the idle stabilizes and the smoke stops. Any idea on what I can do to cure the issue. Don't get me wrong, the loping idle sounds awesome, but it never used to sound this way on a cold start. I think its a running rich issue at first start, but it doesn't run rich after that quick blip of the throttle, nor when its good and warmed up. Choke issue? See videos below.

At cold start:

After throttle blip:

 

[diesel4me]

Assuming its carbed,it is probably the choke pull off,it has a diaphram that can fail--its job is to open the choke butterfly slightly right after starting cold,otherwise it can cause it to flood,run rich, and foul the plugs,if it stays completely shut.....if its not adjusted right it can do the same thing too..

 

[1975Jimmy]

@diesel4me - I checked the choke pull off and its functioning as its supposed to. The vacuum actuates the diaphragm and butterfly pops slightly open after i start it and it holds it pretty strong, so it looks to be working right. I just tightened down the carb in case there was a vacuum leak, but with a quick test, its still pulling 19 - 20 inches of vacuum. Stumped.

 

[76zimmer]

how far is it opening the choke plate butterfly? It should be about 3/16" open at full vacuum. pull off the hose and attach a vacuum pump (or use a longer vacuum hose and suck on it) and check its operation. I see its opening, but make sure its opening enough. when you start it and it lopes, just pry it open with a screwdriver a little at a time and see if it smooths out.

 

[diesel4me]

If it is a Quadrajet ,it might have leaky well plugs...fuel drips out into the intake after it has sat awhile and it could make it run like a bad pull off,though it should burn off fairly quick and smooth out within a minute or so...if it takes longer than that its probably the pull off isn't adjusted to spec,or maybe the carb developed an internal issue--float soggy and sinking,etc..

...also make sure the air filter isn't restricted..

I once had a truck that started running that way in the dead of winter,and I found out a field mouse had filled the air cleaner housing full of jute padding and straw!..when I opened the hood to investigate why it was loping,I saw the little bastid run across the cowl and jump off and zoom into the woods...soon as I took the lid off the air cleaner it started to run smooth again!..it had also chewed thru the wire mesh on the air filter,some of it got sucked into the carb...

 

[1975Jimmy]

Also, do you guys hear that obnoxious random chirp in the exhaust at idle? The sound isn't noticeable at higher RPM. Compression test shows 150-155 psi at each cylinder, so not a burnt valve or head gasket... Maybe an intake manifold gasket or spark jumping wires? Does it sound like something I should be concerned about? The truck runs great and hauls when I mash the go pedal, but that sound drives me insane.

 

[diesel4me]

Sounds more like slight exhaust leaks to me,maybe at the collectors if it has headers,or the exhaust manifolds...sometimes original cast iron ones crack and squeak like that until they warm up and expand and the noise goes away or fades...it could just be the way whatever mufflers your using sounds "normal" too...

 

[1975Jimmy]

If it is a Quadrajet ,it might have leaky well plugs...fuel drips out into the intake after it has sat awhile and it could make it run like a bad pull off,though it should burn off fairly quick and smooth out within a minute or so...if it takes longer than that its probably the pull off isn't adjusted to spec,or maybe the carb developed an internal issue--float soggy and sinking,etc..

...also make sure the air filter isn't restricted..

I once had a truck that started running that way in the dead of winter,and I found out a field mouse had filled the air cleaner housing full of jute padding and straw!..when I opened the hood to investigate why it was loping,I saw the little bastid run across the cowl and jump off and zoom into the woods...soon as I took the lid off the air cleaner it started to run smooth again!..it had also chewed thru the wire mesh on the air filter,some of it got sucked into the carb...

I think you're on to something with the well plugs... that would explain the gassy smoke cloud i get at start up.

 

[1975Jimmy]

how far is it opening the choke plate butterfly? It should be about 3/16" open at full vacuum. pull off the hose and attach a vacuum pump (or use a longer vacuum hose and suck on it) and check its operation. I see its opening, but make sure its opening enough. when you start it and it lopes, just pry it open with a screwdriver a little at a time and see if it smooths out.

I know I posted this months ago, but with the temps dropping, I did some recon on the loping issue and you were right. The pull-off wasn't pulling the choke plate open far enough when it fired up. I popped the hose off the vacuum motor, hooked it up to a vacuum pump, then bent the choke linkage a bit to where i could fit a 3/16" drill bit in the opening and sure enough, starts smooth and runs strong even in 20-30 degree temps. Even though carburetors were pretty much phased out of the automotive industry by the time i was born, it's pretty cool to see just how sophisticated these old-school fuel delivery systems really are. When tuned properly, they really can be amazing!

 

[Fastereddie]

Thanks for revisiting with the results.

 

[diesel4me]

Carbs can be fussy bastids,but once you get one "dialed in" you can get some pretty impressive results out of one..

Not bragging,but I seem to be pretty good at fooling with carbs--many I got were given to me and considered "junk",I'd put a carb kit and float in them,and adjust everything to specs,then try it out,and if need be,tweak a few settings and adjustments till they ran to my satisfaction..many times other issues like vacuum leaks or too much fuel pressure was to blame,not the carb itself..(or someone "rebuilt" it and put the wrong top gasket in it,etc--)..

I had a few Carter AFB carbs on engines,one on a '66 Buick 401 Nailhead would give 15 mpg on long trips,and could still roast the tires when you booted it...the Q-jet on my '69 GTO was a cold blooded beast,it would run rich and flood after a cold start,worst on damp days,until it was warmed up completely..

I never could get that one to run good on those kinds of days,often I had to keep the idle speed up by holding my foot on the gas pedal--then suddenly when the engine hit a certain temparature,it would smooth right out and idle perfect..it was also the best one I had as far as power when you booted in the secondaries..

The "worst" carbs I dealt with were the E2SE Rochester 2 barrels used on two of the 80's straight sixes I had...just a piss poor design right from the get go..basically they were a Q-jet sawed in half,so you had one primary and one secondary barrel,and they were probably THE fussiest ones as far as settings for the choke,pull offs,and accelerator pumps..I ended up hating them so much I made a custom adapter to put a Edelbrock on one 250 six I had in a '79 Bonanza,I ran that one with the secondaries disconnected...it actually ran quite well too..

 

[1975Jimmy]

Carbs can be fussy bastids,but once you get one "dialed in" you can get some pretty impressive results out of one..

Not bragging,but I seem to be pretty good at fooling with carbs--many I got were given to me and considered "junk",I'd put a carb kit and float in them,and adjust everything to specs,then try it out,and if need be,tweak a few settings and adjustments till they ran to my satisfaction..many times other issues like vacuum leaks or too much fuel pressure was to blame,not the carb itself..(or someone "rebuilt" it and put the wrong top gasket in it,etc--)..

I had a few Carter AFB carbs on engines,one on a '66 Buick 401 Nailhead would give 15 mpg on long trips,and could still roast the tires when you booted it...the Q-jet on my '69 GTO was a cold blooded beast,it would run rich and flood after a cold start,worst on damp days,until it was warmed up completely..

I never could get that one to run good on those kinds of days,often I had to keep the idle speed up by holding my foot on the gas pedal--then suddenly when the engine hit a certain temparature,it would smooth right out and idle perfect..it was also the best one I had as far as power when you booted in the secondaries..

The "worst" carbs I dealt with were the E2SE Rochester 2 barrels used on two of the 80's straight sixes I had...just a piss poor design right from the get go..basically they were a Q-jet sawed in half,so you had one primary and one secondary barrel,and they were probably THE fussiest ones as far as settings for the choke,pull offs,and accelerator pumps..I ended up hating them so much I made a custom adapter to put a Edelbrock on one 250 six I had in a '79 Bonanza,I ran that one with the secondaries disconnected...it actually ran quite well too..

View attachment 221460 View attachment 221461

Love seeing the straight six photos! My first car had a Ford 200 inline 6 in it that ran great. But, i felt there was a lot more power hidden in that car, so I pulled the 1 barrel off of it and modified the intake rail to accept a Weber progressive 2 barrel from a BMW, swapped the cam out, added a dual outlet header, and put in the pertronix ignition upgrade and coil and that woke it up big time! That little six could keep up and smoke some of with V8 cars no problem, plus with its 7 Main bearings and great balance, it would scream up to 7000 rpm and still get 20 mpg.

 

[diesel4me]

I wish they never stopped making the straight sixes...they were pretty bullet proof,had good low end torque,and were easy to work on,if they ever needed anything more than a tune up..yeah,you cant haul horse trailers with one very well on interstates,or beat many vehicles ,and their gas mileage wasn't great,but they took a beating and rarely died..

Only "major" issues I had with a 235 six and a 250 was the fiber cam gear shedding its teeth,and it sucked getting the camshaft out with the engine in the truck--probably would be easier to pull the engine,looking back..(or if GM had the brains to make the cam gear bolt on like a V8 instead of a press fit..)..they sell aluminum gears ,some had them factory on heavy duty trucks..

Chevy dropped the ball when they went with the "integral" head design though--best sixes were the '63-74 ones with one barrel carbs..

The 1953-62 235 was a good engine too,but its in a world of its own--different bell housing bolt pattern,motor mount location,etc..

The early '75 to 77 sixes still had a one barrel carb,but the head was integral with the intake--78-84 engines had the crap 2 barrel E2SE Rochester,except the 292,those were always one barrel and had the "old" head with separate intake & exhaust manifolds ..

The 78-84 integral head engines had a bit more HP and torque than the earlier ones,but the carbs held them back,so did the air pump and check valves in the valve cover..
I "deleted" them as you can see in the picture..my '81 G-10 van had the same engine,I left the E2SE on that one,it wasn't as crappy as the C-10 one was.

That engine had 170,000 on it when I pulled it out to swap in a 307,and it ran great,no oil burning--and I kind of regret having done so--the '73 307 had only 115 HP vs 130 for the six,and not much more torque,I think it went better with the six in it..

I'd still like to get a straight six if I can find one at a good price and keep it on hand for a spare engine in case my diesels decide to expire..but the truck might be "done" by then too..not worth the effort to do a swap..