Thermoquad idle issue

-
crewchief said:
This 1972 340 block and heads sat in the basement since the mid 80's. I built it from the bottom up with all new internals, crank, rods and pistons last year. Block was bored 40 over standard machine work was done. No deck cutting and heads were machined for new valves 2.02. Stock intake was not cut either just checked for cracks and squared.
Hydraulic Flat Tappet Comp Cam 260H, duration 260/260, Lift .440/.440 with 901-16 spring kit.
Cam break in when perfectly, high zinc, drained after 30 min run. Engine ran better during break in then now. Rings should be set with 500 mi on the motor.
Click to expand...
intake not cut, but squared? not familiar with term squared. what does that mean?
when I used to do heads, I checked each port with vacuum to make sure each valve makes perfect seal with seat. In the past, Ive found heads cut with the form cutters, ie Serdi, et al, can be cut off valve stem c/l or out of plane. I don't know why, could be inadequate setup, poor procedures, worn machine or tooling, whatever, but I've found poor sealing issues on several occasions with professionally done heads. If you were close, I'd remove the carb, and rockers and check vacuum at the carb mounting surface. If that proved bad, I'd remove the intake and check all the intake ports for vacuum, too.
By the way, have you checked intake manifold bolt torque, since break-in?
 
Machine shop work on the intake included checking mating surface was not warped and magnafluxed, I guess squared not the correct term.
I did lap the valves when changing the springs.
Bolt torque was confirmed as well.
 
crewchief said:
Machine shop work on the intake included checking mating surface was not warped and magnafluxed, I guess squared not the correct term.
I did lap the valves when changing the springs.
Bolt torque was confirmed as well.
Click to expand...
if you're 100% confident in the valve seat seal, then let's look at the lifter preload. how did you set that?
 
Lifters were pumped up prior to install and preload was between .030"-.040". Valve train is not noisy at all. Engine has non-adjustable rockers
 
crewchief said:
Lifters were pumped up prior to install and preload was between .030"-.040". Valve train is not noisy at all. Engine has non-adjustable rockers
Click to expand...
my concern was that there was too much preload on the lifters, and some intakes hanging slightly open. if that were so, there would be no noise, either. How do you know they are all .030 to .040? How did you measure them?
FWIW, not a fan of pumping up the lifters before assembly.
 
I applied dye to the pushrods starting with #1 @ TDC, loosened the both rockers and made scribe mark on the rods using a straight rule. Torqued rockers back down and made a second scribe and measured between the two marks. Did this for all the cylinders.
I've never put new lifters in without priming them first, especially with a flat tapet cam.
 
crewchief said:
I applied dye to the pushrods starting with #1 @ TDC, loosened the both rockers and made scribe mark on the rods using a straight rule. Torqued rockers back down and made a second scribe and measured between the two marks. Did this for all the cylinders.
I've never put new lifters in without priming them first, especially with a flat tapet cam.
Click to expand...
if the lifters are already pumped up, couldn't tightening the shafts down actually open the valve some?
 
No, because of the hydraulic action of the lifter. If they were over pumped up, that would cause the valves to be opened slightly.
 
If the pre pumped up lifter is installed and then the valve train on top of it, by the time he would finish up bolting the engine on and firing it up to dyno test even if the engine is on the stand the lifters should easily allow the oil out and bleed down enough to close the valves.
 
rumblefish360 said:
If the pre pumped up lifter is installed and then the valve train on top of it, by the time he would finish up bolting the engine on and firing it up to dyno test even if the engine is on the stand the lifters should easily allow the oil out and bleed down enough to close the valves.
Click to expand...
you'd think so, but not always. But what I was trying to say is that checking for preload like he described may not be as accurate as he thinks.
 
rustycowll69 said:
you'd think so, but not always. But what I was trying to say is that checking for preload like he described may not be as accurate as he thinks.
Click to expand...
Well, if I'm wrong... then I'm wrong.
If the engine is on the dyno and your quick on assembly l, could the lifters still be holding up the valve? I guess it is possible.
 
crewchief said:
340 built with mild cam (because of power brakes). The TQ was rebuilt by National Carb, Fla. Thick gasket is used, vac at idle is 11 in. and increases to 15-18 when primary is closed slightly and runs smooth.
Click to expand...

On re-reading this,and with subsequent information given,I think there is a major clue here.
15/18 would be normal with a 262 cam, 22* lead, and 800 idle.
So IMO there is nothing wrong below the butterflies.Nor is the float level off , nor is it a pressure issue.
What happens if you reduce the throttle-opening by backing out the curb idle screw?
Where on the the throttle arm, do you have the return spring anchored? It should be on the same pin as the where the cable anchors. I drop it in between the arms of the clevis.
Check to see if there is sideways motion of the butterflies, that is to say;can you push/pull the throttle shaft from the drivers side, towards the passenger side?
Then push/pull the throttle-shaft away from the throttle cable.
I realize that the bushings would have to be extremely worn to create a 7inch drop in vacuum. I'm thinking more that the butterflies are pivoting in the bores, and pulling in air around the backside which is upsetting the T-Port sync, which with 22* of lead, is probably very fragile.
I suspect with the 22* of lead, the butterflies are very nearly closed, and any throttle-shaft play may upset the sync. This is why I suggested to reset the timing to 16* which IMO is plenty of idle-timing, for "a bit of a cam" . At 16* you will have to open the butterflies a bit, and reduce the mixture setting a tad. And this may put the butterflies in a less-sensitive position.
 
Last edited:
AJ/FormS said:
On re-reading this,and with subsequent information given,I think there is a major clue here.
15/18 would be normal with a 262 cam, 22* lead, and 800 idle.
So IMO there is nothing wrong below the butterflies.Nor is the float level off , nor is it a pressure issue.
What happens if you reduce the throttle-opening by backing out the curb idle screw?
Where on the the throttle arm, do you have the return spring anchored? It should be on the same pin as the where the cable anchors. I drop it in between the arms of the clevis.
Check to see if there is sideways motion of the butterflies, that is to say;can you push/pull the throttle shaft from the drivers side, towards the passenger side?
Then push/pull the throttle-shaft away from the throttle cable.
I realize that the bushings would have to be extremely worn to create a 7inch drop in vacuum. I'm thinking more that the butterflies are pivoting in the bores, and pulling in air around the backside which is upsetting the T-Port sync, which with 22* of lead, is probably very fragile.
I suspect with the 22* of lead, the butterflies are very nearly closed, and any throttle-shaft play may upset the sync. This is why I suggested to reset the timing to 16* which IMO is plenty of idle-timing, for "a bit of a cam" . At 16* you will have to open the butterflies a bit, and reduce the mixture setting a tad. And this may put the butterflies in a less-sensitive position.
Click to expand...
When I back off the idle screw, it stalls. Return spring is located where it should be. There's no side to side play on the shaft, slight fore and aft on the passenger side. Getting harder to start when cold and during warm up it stumbles for a period until warm. Choke is working as needed. Manually opening the choke during warm up does not have any effect. Once warm and choke is open it just runs rough but applying throttle response is good. Something is getting worse.

IMG_0385.JPG


IMG_0386.JPG
 
AJ/FormS said:
On re-reading this,and with subsequent information given,I think there is a major clue here.
15/18 would be normal with a 262 cam, 22* lead, and 800 idle.
So IMO there is nothing wrong below the butterflies.Nor is the float level off , nor is it a pressure issue.
What happens if you reduce the throttle-opening by backing out the curb idle screw?
Where on the the throttle arm, do you have the return spring anchored? It should be on the same pin as the where the cable anchors. I drop it in between the arms of the clevis.
Check to see if there is sideways motion of the butterflies, that is to say;can you push/pull the throttle shaft from the drivers side, towards the passenger side?
Then push/pull the throttle-shaft away from the throttle cable.
I realize that the bushings would have to be extremely worn to create a 7inch drop in vacuum. I'm thinking more that the butterflies are pivoting in the bores, and pulling in air around the backside which is upsetting the T-Port sync, which with 22* of lead, is probably very fragile.
I suspect with the 22* of lead, the butterflies are very nearly closed, and any throttle-shaft play may upset the sync. This is why I suggested to reset the timing to 16* which IMO is plenty of idle-timing, for "a bit of a cam" . At 16* you will have to open the butterflies a bit, and reduce the mixture setting a tad. And this may put the butterflies in a less-sensitive position.
Click to expand...
AJ, update on the idle issue. The PCV began to chatter every so often (sounded like a belt squeal). After investigating PCV issues on FABO, I found that many of the new plastic ones have been known to have leaking problems. I replaced mine the an AC Delco CV761C, metal base with plastic nipple. The issued with rough idle went away and vacuum settled down. GM part on a Mopar...go figure a $6 fix to care of it. I could have spent way much more for an NOS Mopar one.
 
rustycowll69 said:
the bottom of the plastic bowl has a slight raised Ridge to ensure positive seal by biting into the gasket. By filing the plastic body bottom you're probably introducing a leak. I've rebuilt a hundred TQ's and have never seen a warped bowl. I think that's an urban legend.
I agree there's nowhere near enough info to diagnose.
Click to expand...
Hey Brother , I have a thermoquad 1971 6139s that has a leak in the primary circuit after
the engine is shut off
passenger side only drips badly at shut off . New build I did and new plastic floats and set to 1.00
float level . I suspect the o ring in the primary on the passenger side is not compressing enough to seal it , the plastic body has some weirdness at the o ring seat area on the passenger side .
When I got this car last week the TQ was flooding like nothing I have ever seen , .
now it is driveable and work well but at shut off if you look down at the cluster on the passenger side you can see liquid fuel and vapors . Take 10 seconds to re fire the 340 up .Want to get it to stop this ......thanks ...Joe
 
joseph lamb said:
Hey Brother , I have a thermoquad 1971 6139s that has a leak in the primary circuit after
the engine is shut off
passenger side only drips badly at shut off . New build I did and new plastic floats and set to 1.00
float level . I suspect the o ring in the primary on the passenger side is not compressing enough to seal it , the plastic body has some weirdness at the o ring seat area on the passenger side .
When I got this car last week the TQ was flooding like nothing I have ever seen , .
now it is driveable and work well but at shut off if you look down at the cluster on the passenger side you can see liquid fuel and vapors . Take 10 seconds to re fire the 340 up .Want to get it to stop this ......thanks ...Joe
Click to expand...
are you using the correct O-rings? the obsolete o-rings were just flat, "square" o-rings. The revised design o-rings are called "X" o-rings, because they are molded with ridges. If you can imagine cutting a cross section of the ring it looks a little like an "X".
Another thing to check is to make sure the seat is fully tightened, and when you are adjusting the float level, you have to be careful to not press on the float and the needle, because you can deform the conical rubber tip. Also, rarely, one of those plastic floats can absorb fuel, and become too heavy and lose buoyancy.
 
one more thing, it is mandatory that you use the thick base gasket. I've never measured it but it must be around a 1/4" thick, and it isolates the carb from conducted heat from the intake manifold. The best base gasket is reusable, and made from solid black plastic with a very thin paper gasket glued to it, but they may be hard to find now.
 
rustycowll69 said:
are you using the correct O-rings? the obsolete o-rings were just flat, "square" o-rings. The revised design o-rings are called "X" o-rings, because they are molded with ridges. If you can imagine cutting a cross section of the ring it looks a little like an "X".
Another thing to check is to make sure the seat is fully tightened, and when you are adjusting the float level, you have to be careful to not press on the float and the needle, because you can deform the conical rubber tip. Also, rarely, one of those plastic floats can absorb fuel, and become too heavy and lose buoyancy.
Click to expand...
I actually am using the x type o ring but I do not believe I am getting the seal compression I need at that seat and x ring , so I am gonna fiqure out a way to get this to seal . A thicker x seal would probably do the trick but who has that ???thanks for the input !!!!!
 
You must log in or register to reply here.
-

Back
Top