Choke Problems on SBC

[MrMarty51]

Not a Square Body. Sorry.
This is on My step sons 1970 Chevrolet C20 with a TH400 and a 350 SBC.
I rebuilt the carb and it is functioning very nicely.
The portion that fails to function is the automatic choke.
The choke mechanism is one just like this one in the picture.

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The problem I believe lies in the exhaust cross over through the intake manifold.
I dont believ that the choke stove is getting warm enough for the bimetal spring to fully function. Thinking that the cross over within the intake manifold might be plugged I got out the IR thermometer and the temp on the choke bimetal mechanism surface of the intake manifold was only 115 degrees.
I removed the bimetal mechanism and that mounting surface was barely over skin temperature which would be slightly over 100 degrees.
Can someone do a temperature check on the choke stove surface of their intake manifold and let Me know what it is, or, what it should be if You already know.
I dread the thought of having the step son buy an intake manifold gasket set, go through the pains of draining the coolant and popping off that intake manifold to only find that it is not plugged.
We will be starting on this sometime fairly early AM so the sooner a reply the better, and, it will be very much appreciated.
Thank You all in advance.

 

[SirRobyn0]

@MrMarty51 I don't have a this style of choke on my square but I have worked on them. Could it be that the heat riser is stuck open?

As you know when the heat riser is shut copious amounts of hot exhaust is running though the port in the intake manifold and that will really speed the process of heating and opening the bimetallic choke spring.

Another option would be to convert it to electric choke.

 

[Trucksareforwork]

@SirRobyn0 beat me to it but I have something to add: a 1970-is heat riser valve should be fairly floppy- easy to move with your hand by just flicking the weight on the valve. I have had a few late 60’s and early 70’s engines where the heat riser has seized and just stayed in one position.

You *can* bring one back to life with cleaning spray and some lube (which all burns off posthaste). I believe there are even cleaning sprays out there specific for heat risers. My experience has been poor at the cleaning scenario. I have replaced a few.

Another possibility is the heat riser has lost its bimetallic coil. I can’t remember if they default to open or closed, but that could cause problems.

If you are just looking to get some heat to it, you could consider wiring up a piece of aluminum heat duct (used for heat stove from manifold to air filter housing, probably $8-10 in the Dorman help section) to just route heat to that choke coil from the exhaust manifold. It would be pretty country looking but would passively route heat to that coil.

I *guess* the exhaust crossover could be clogged but would start with the heat riser first.

 

[PrairieDrifter]

If the engine hasn't been open for a long while. The passage will be mostly blocked. Every chev I've ever opened up that passage is very choked up with carbon around the intake and head mating surface.

Mine gets real hot. I don't have that style choke, but my plate gets hot hot. I also don't have any heat riser valves and good flowing manifolds and exhaust, even in the cold.

 

[MrMarty51]

@MrMarty51 I don't have a this style of choke on my square but I have worked on them. Could it be that the heat riser is stuck open?

As you know when the heat riser is shut copious amounts of hot exhaust is running though the port in the intake manifold and that will really speed the process of heating and opening the bimetallic choke spring.

Another option would be to convert it to electric choke.

@SirRobyn0 beat me to it but I have something to add: a 1970-is heat riser valve should be fairly floppy- easy to move with your hand by just flicking the weight on the valve. I have had a few late 60’s and early 70’s engines where the heat riser has seized and just stayed in one position.

You *can* bring one back to life with cleaning spray and some lube (which all burns off posthaste). I believe there are even cleaning sprays out there specific for heat risers. My experience has been poor at the cleaning scenario. I have replaced a few.

Another possibility is the heat riser has lost its bimetallic coil. I can’t remember if they default to open or closed, but that could cause problems.

If you are just looking to get some heat to it, you could consider wiring up a piece of aluminum heat duct (used for heat stove from manifold to air filter housing, probably $8-10 in the Dorman help section) to just route heat to that choke coil from the exhaust manifold. It would be pretty country looking but would passively route heat to that coil.

I *guess* the exhaust crossover could be clogged but would start with the heat riser first.

Yup. Had the left side manifold removed to install spark plug wire heat shields, for the two front cylinders.
Cleaned everything real good and the heat riser functions smooth and easily.
Buckhorn manifolds I believe should have these shields installed.
They are made for the 55-57 SBC with the forward engine mounts.
That smaller notch is one I cut so the shield would fit around the engine block mounting boss.
This is all for the step son. He bought two of these trucks. The other one is a 1972 K30 and someone did a real nice conversion to a 454 in it. Step son has cut out all the rust and is in the poorsess of replacing sheet metal.
Poorsess because these trucks is making him poor. :laff

 

[MrMarty51]

Oh yes.
Thank everyone for the replies.
With this information I can now honestly and sincerely tell the stepson that the manifold must come off to get it right.

 

[MrMarty51]

I just got started hammering on the butt end of the handle of that Snap On screw driver to chisel out the carbon, then the thought, pics, I need pics to make everyone happy.
And so here they are.
Plugged solidly shut.
Oh yes, too, snap on screw drivers are good to use for chisels too.

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[MrMarty51]

Got all the components scrubbed and ready to reassemble.
Step son is a mighty tough task master. He dont slow down.
He is a fast learner too. I hand tools to him like He has never before used, I explain their function and then get out of the way. I mostly just sort out for the next step and allow Him to learn, simply and plainly explain how items must go back together so it will last and not fail nor leak.
He is a real go-getter.
He and His BIL purchased an electrical shop. Installs wiring and repairs components.
He always wants to help me with my home/shop wiring solutions, I tell Him I do not want You on My place putting in time unless You are on the clock. If You want to help with vehicular or other items off the clock then fine, but, anything electrical related You will be on the clock. He reluctantly agreed.

 

[MrMarty51]

He also has a 1972 K30 GMC that has a 454 planted within.
A lot of rust.
Before I knew about it he had pulled the flatbed off. Found a box and got it home.
He proceeded into rust repair. Before long he has the cab lifted off, floor cut out, rockers, cab corners and ant thing else that needs replaced. LOL
Now he is prepping it all for paint. I warned him about what it will be like if he moves too fast through those steps of prep work.

 

[PrairieDrifter]

I just got started hammering on the butt end of the handle of that Snap On screw driver to chisel out the carbon, then the thought, pics, I need pics to make everyone happy.
And so here they are.
Plugged solidly shut.
Oh yes, too, snap on screw drivers are good to use for chisels too.

You must be registered for see images attach

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Just as I expected. Another one for the list lol

 

[Snoots]

I just got started hammering on the butt end of the handle of that Snap On screw driver to chisel out the carbon, then the thought, pics, I need pics to make everyone happy.
And so here they are.
Plugged solidly shut.
Oh yes, too, snap on screw drivers are good to use for chisels too.

You must be registered for see images attach

You must be registered for see images attach

Hot tank it. I mean, dayum!

 

[MrMarty51]

Hot tank it. I mean, dayum!

There is no more hot tanks in town.
I would have loved to do dat to the intake maneeefold, rocker covers, bolts and etc.
Yeah, hot tank is the answer for sure.

 

[MrMarty51]

It is all back together.
Before dismantling, brought the timing marks to 8* BTDC, marked the distributor how it all sets.
Got it back together. Set the choke, Nick twists the switch to start, boom, not even one revolution and it is running.
Turned on the timing light, he Nick, look at this. Timing was exactly and perfectly on 8*BTDC. LOL
Choke stove mounting was at 275* F and I am sure it was going to get hotter. The choke now functions as it should and the repairs we accomplished will serve the step son for many years to come.
Thank You everyone for Your wisdom, help and support.
Oh Yeah.
MERRY CHRISTMAS TO ALL.
Reminiscing back, it was 28 years ago, right at this time, that I took my last drink of any sort of an alcoholic beverage.
I was in a total black out drunk state and My son had witnessed My demise to get this far down.
A real miracle and the greatest gift I ever did receive, granted to Me from My Heavenly Father. AMEN

 

[PrairieDrifter]

It's nice when blind, internet diagnosis works out perfectly!

 

[Craig Nedrow]

It is all back together.
Before dismantling, brought the timing marks to 8* BTDC, marked the distributor how it all sets.
Got it back together. Set the choke, Nick twists the switch to start, boom, not even one revolution and it is running.
Turned on the timing light, he Nick, look at this. Timing was exactly and perfectly on 8*BTDC. LOL
Choke stove mounting was at 275* F and I am sure it was going to get hotter. The choke now functions as it should and the repairs we accomplished will serve the step son for many years to come.
Thank You everyone for Your wisdom, help and support.
Oh Yeah.
MERRY CHRISTMAS TO ALL.
Reminiscing back, it was 28 years ago, right at this time, that I took my last drink of any sort of an alcoholic beverage.
I was in a total black out drunk state and My son had witnessed My demise to get this far down.
A real miracle and the greatest gift I ever did receive, granted to Me from My Heavenly Father. AMEN

Right on Martin!