Th350 Idle Way Too Low, And Low Power In 1St Gear

Sean Owner Arizona Posted   Latest  
Updated
Driveability
Drivetrain
Idles At 450 Rpm In Gear 1250 In Park

I have this 1978 K10 that has been sitting for many years, I just got done rebuilding the engine, new Edelbrock carburetor, new Summit Racing hei distributor, total advance set at 35 degrees. Set the idle speed to 850, and it idles at 350 in gear. Set it at 1250, and it idles at 450 in gear. Take it for a short test drive, because the tires are shot, and it is very sluggish until you get up to 10mph, then it wakes up, and accelerates like you would expect it to. 

Looks to me like something inside the transmission might be applying when it shouldn't be, and it could be a direct result of this vehicle sitting for many years.

What tests should I perform, or what area should I look at. I normally don't do transmission repairs, but this customer is pissed, because he is moving to California, and expected it to be perfect after we were done, even though I thoroughly warned him that it was going to be a money pit before we started on it.

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Mark Engineer
Colorado
Mark
 

Sean Let’s see where the restriction/drag is. Do we know if it was ok when it was parked several years ago? Does it have the same low idle in reverse or L1 or L2?

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Sean Owner
Arizona
Sean
   

It happens in d, r, l1, and l2. No history at all. It was his dad's truck, and supposedly had 10k on a rebuild, then he parked it because of a head gasket.

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Vincent Technician
Georgia
Vincent
 

If you have a camshaft that is even slightly bigger then stock,you will need a stall converter​.​Maybe 2000 or 2200 stall​.​converter has to be loose enough to let it idle and get it into power range

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Sean Owner
Arizona
Sean
 

It's just a mild rv cam. I've installed many of these with stock converters.

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Randy Curriculum Developer
North Carolina
Randy
 

Google “Testing stall speed”. You may have a stator issue in the torque converter.

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Bryan Engineer
Florida
Bryan
   

Do you know what work has been performed on this car? What modifications have been done? Has the cam been replaced? If so do you know the cam specifications? Sounds like it has low engine vacuum which is quite common for people to put too large of a camshaft in a stock engine and expect to drive it on the street. I had the same issue on a small block Chevy that’s a 23 Tbucket. Somebody put a…

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Sean Owner
Arizona
Sean
 

I assembled this engine. It is a mild rv cam, with stock compression.

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Zeb Diagnostician
California
Zeb
 

Do a stall test flashing the converter. Apply the brakes and give it some gas, using a tachometer see where the RPM holds. Stock converters are somewhere around 1400. Broken sprag will lower stall.

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Gary Owner/Technician
Colorado
Gary
   

Hello,Sean. If it has a THM 350 transmission, it starts in 1st gear which engages against a sprag clutch, then adds the intermediate clutch for 2nd gear, then adds the direct clutch for 3rd gear. Reverse engages the low/revers clutch and the direct clutch. So how does it run in reverse? If the trans is original and has not been serviced, it could be that a brittle neoprene clutch seal has…

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Obie Technician
Washington
Obie
 

A quick test of the advance at and just off idle could help. Those old style 350’s idled with full vacuum advance which dropped off on take off. Then the mechanical came in. At cruise they used full vacuum and mechanical. The 8-12 degree initial advance was set with the vacuum disconnected. That gave quite a bit of advance at idle with the vacuum hooked up. Helped with idle and drive off a stop..

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Rolando Diagnostician
Texas
Rolando
 

Old car problems are bad, worst when you've got a customer expecting you to fix something in a short amount of time that has been sitting for a LONG time. I wouldn't be surprised if you find more issues. Hopefully you informed him that this would likely become a money AND time pit. Usually for vehicles like this, we give exceptionally long lead times and the higher spectrum of costs and gauge…

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Sean Owner
Arizona
Sean
 

No, it was a solid no crank, due to the engine being rusted solid from a blown head gasket. Same engine bored .060 over, new distributor, new carburetor. Advance was siezed on original distributor. Intake manifold got bead blasted, so no issues there.

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Chris Owner/Technician
Montana
Chris
 

Sean, shift the transmission to manual low and see if it bogs down as you said and then up shift manually. Make sure that you have good vacuum at the modulator with truck idling remove hose and engine rpm should increase. Also make sure there is no restriction in the exhaust pipe. Set your timing to 10 to 12 degrees advance with the vacuum advance unplug and see if that helps.

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Chris Owner/Technician
Montana
Chris
 

Sean, one other thing that could be happing is the governor is sticking so when you take off if that governor is sticking it will feel like its starting out in 2nd gear until line pressure over come that governor sticking.

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Bill Owner
Maryland
Bill
 

Does the distributor have vacuum advance and where do you have it hooked up to ? I always use full manifold vacuum on anything other than a completely stock vehicle.

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Chris Owner/Technician
Montana
Chris
 

Bill, Yes it should have vacuum advance usually hook to a port on the carb depending what carb you have. If it is a quadrate carb hooks to the front of the carb.

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Frank Manager
Tennessee
Frank
 

Different small blocks, different balancers, different pointer locations. Maybe shelve the timing light and do it old skool. advance till its rattling and work back from there.

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Sean Owner
Arizona
Sean
 

That's exactly what I did, except I already know about the different pointers, and applied my paint mark while I was assembling the engine. Turns out the Summit Racing distributor has way too much centrifugal advance. The advance weights were identical to the original ones, but the center piece was completely different, so I swapped that one in, and that fixed most of the issue.

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Sean Owner
Arizona
Sean Update
 

Thanks guys for your help. This vehicle had to be done Friday last week, because he was moving, so I rushed the completion, and only set the total timing without paying attention to anything else. Turns out that the Brand new Summit Racing distributor uses chinese parts, and has way too much centrifugal advance. I set the timing by ear, and took it for a drive. Sure enough, it was pinging, I…

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Rolando Diagnostician
Texas
Rolando
 

Nice that you got that straightened out. Now the trans, does it look like anyone has ever taken it out. Maybe someone at some point installed a different converter with a low stall speed? Dunno why anyone would do this but anything is possible. Chinesium parts are becoming more and more a pain. I have been where you have been before. Good luck getting the rest fixed up.

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Tom Owner/Technician
Tennessee
Tom
 

IIRC correctly, there were tighter than stock torque converters being sold back in the day with the claim of “improved MPG”. Somebody could have had one installed earlier in its life. Compression was often in the high 7's on a late 70's production motor. Add an “RV cam” of almost any additional duration into the mix and you end up with a horrible combination. Tom

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Glenn Owner/Technician
Texas
Glenn
 

Hi Sean, Poor planning on the customers parts does not constitute and emergency on your part. You have it running, but the tranny is creating too much stall. It likely needs a full tranny rebuild with a quality reman of the convertor. An RV cam is not that dramatic of a change. As for the carburetor, it does not have too big of a carburetor, does it? Typically, an Edelbrock around 525- 600 CFM…

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Sean Owner
Arizona
Sean
 

Edelbrock 1406. He is going to drive it, and I warned him that that would be a terrible idea, and that it should be trailered. He is going against my advise, but he did say that he is going to take backroads, because I warned him that sustained rpm is not good for a fresh rebuild, that he should vary the rpms as much as possible.

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Mark Engineer
Colorado
Mark
 

Sean: You said: “He is going to drive it, and I warned him that that would be a terrible idea” It looks like you have advised him and he's got his mind made up. Time to let it go and move on to other challenges where the customer recognizes/appreciates you as an expert with knowledge. I hope you documented what you advised to do and not to do. Sometimes customers like this have a mechanic call…

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Sean Owner
Arizona
Sean
 

Yes, very well documented. Also documented that if anything goes wrong with the engine, it must be towed back here for warranty. Had I known he was moving, I would have declined working on it, but I tore the engine down 3 months ago, and just found out about the move 3 weeks ago.

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