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#1467803 Fri Sep 23 2022 12:35 AM
Joined: Nov 2010
Posts: 58
T
'Bolter
My Chevy engine backfires and diesels when I try to turn it off. Here is the set up. The car is a restored
1956 Chevy Belair that I recently completed. It has an inline 6 cyl. 250 motor in it (not a 235). I’m using a conventional distributor with points and an external coil. The motor was professionally rebuilt both the block and head at a machine shop. It has original intake and exhaust manifolds with a new NOS Carter YF carburetor. I have only starter it 5 or 6 times so far. It is nearly new in that regard.
The car starts without any problem. However, when I give it gas, it hesitates and sputters. As it warms up, it runs very rough and wants to die. I feather the gas petal and it will increase RPMs but when I back off, it starts to die. At low RPM I can hear some knocking. When I try to shut off the engine, it will not stop and begins to diesel. Sometimes it backfires through the carburetor.
What is most likely wrong? Is the timing off? Valves not adjusted correctly? Suggestions appreciated.
Thanks, Glenn

Joined: Feb 2004
Posts: 25,333
H
Kettle Custodian (pot stirrer)
Dieseling is caused by too-high idle speed. Find out why the engine won't idle, fix it, and your dieseling problem will go away. If that "NOS" Carter carb is speaking Mandarin, it's probably the source of the problem. The "new" clones are made offshore, and they're mostly good for paperweights, at least until they get filled with gas the first time. Then they become a stinky paperweight. If you paid less than $200.00 for it, it's probably not an original equipment Carter YF.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
Joined: Nov 2010
Posts: 58
T
'Bolter
The motor idles at low RPM, not high. The Carter carb is USA made. I paid more than $200 and I'm sure it is NOS. It was also professionally rebuilt with new gaskets, etc. before I installed it. Could the problem be that the choke is not opening or closing properly? I'm going to pull the plugs today and see if they are fouled.

Joined: Feb 2000
Posts: 4,566
J
'Bolter
Check the intake / exhaust manifold seal at the head, 250's are a little hard to seal up and they can cause all kinds of running issues. While the engine is running ( idling ), use an open, non-burning, propane torch and wave it around all the ports at the head. Go around the top and up under the intake. If you have a leak, the engine will speed up due to extra fuel it pulled. For exhaust leaks, just use your hand and feel for hot air rushing out.
For testing, mark your distributor and head with tape and a marker, then turn it some and see how it acts, if it's worse, go the other way, if it's the same, put it back to the marks and move on.

At this point, you have checked air and spark and both working, so you are down to the carburetor. Be sure the vacuum advance is working, and be sure the mechanical advance is not stuck. Pull the distributor cap and give the rotor a twist, it should snap back into place each time. Be sure the PCV valve is new and work, and you have no other vacuum leaks.

99.9% of all 250's have hydraulic lifters, they are set like all hydraulic lifters, find zero lash and go 3/4 to 1 turn more of the lock nut. For us " racers " we go 1/4 to 1/2 turn to gain a few more rpm at high speeds { at least we think we do }. To find zero lash, rotate the push rod with one hand while tightening the nut with the other, when you have zero play between the rocker and push rod, you found zero lash. The lifter must be on the base circle of the camshaft, this is where the valve s are closed. There are many ways to set valves, but I like to set the first cylinder on the compression stroke, both valves are closed, and check lash. I then rotate the engine till the #2 cylinder is on compression and check it, then 3 and so on till all 6 are done.

To hear knocking at idle is not good!

Joined: Feb 2004
Posts: 25,333
H
Kettle Custodian (pot stirrer)
That engine will have hydraulic valve lifters. It's possible your builder has the valve adjustment a little too tight, and the lifters are keeping the valves slightly off their seats as the engine warms up. Try backing all the rocker arm nuts off 1/4 turn, and run the engine again. There's also a remote possibility of some voltage feedback from the alternator to the ignition system keeping the coil energized for a short time after the switch is turned off. Try disconnecting the big charging wire at the back of the alternator and running just on the battery to see if the problem goes away.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
Joined: Jan 2014
Posts: 2,345
J
'Bolter
Something quick and easy to check…

Verify your wires are routed to the proper firing order. 153624. I don’t know if incorrect firing order would explain the dieseling, but it would the crummy running.

All of HRL’s and Joe H’s are good. Not trying to detract, just want to make sure all the basics are covered.

Last edited by JW51; Fri Sep 23 2022 03:51 PM.
Joined: Nov 2010
Posts: 58
T
'Bolter
I have been working on my car, but it still is not running right. I can get it to start when it is cold, but within 1 minute as it warms up it begins to run rough and starts to backfire. I must constantly keep feathering the accelerator to keep it running and then it will just stop.
I have checked these things: The wiring firing order is correct, and wires are new. The plugs are not fouled, but I replaced all of them anyway and it still did not fix the problem. I found no vacuum leaks, and the compression is good. There is 10V at the coil. The plugs have spark. I put in a different known good MST hotter coil. I rest the pints gap a bit wider. I removed the YF single carb and opened it up and it is clean with no debris or gunk in it. I drained all the gas and put in fresh 93 octane fuel. The vacuum advance can is new and seems to be working. I have not been able to check the timing because the engine will not run long enough for me to be able to do that.
So, I am now wondering if either the fuel pump is bad and not pumping enough gas into the carb or the distributor is not creating a hot enough spark at the plugs. When I checked the plug spark on 3 different plugs, the spark was a “red” color and not the normal “blue” color. I’m wondering if changing to an HEI type distributor would be a better choice. That is my next step. If that fails, I’ll replace the fuel pump.
Any other suggestions? What’s odd is that when it’s cold, it seems to run okay for a short time. As soon as it heats up it runs very rough and then dies.

Joined: Dec 2008
Posts: 1,113
5
'Bolter
Bad condenser maybe?


Brian
1955.2 3100 Truck


The older I get the more dangerous I am!!!!!
Joined: Feb 2004
Posts: 25,333
H
Kettle Custodian (pot stirrer)
Did you check the valve adjustment? If not, I'm out of this discussion! "Blue" sparks quit happening in the 1960's when non-metallic spark plug wires became standard.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
Joined: Apr 2005
Posts: 11,310
Grease Monkey, Moderator General Truck Talk & Greasy Spoon and HiPo Forum
Classic symptom of a failing condenser. It’s extremely hard these days to get a good one. Contact Jon G on this forum and order one of his replacement condensers and never replace another one.


Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne 4dr 230 I-6 one owner (I’m #2) “Emily”
‘39 Dodge Businessmans Coupe “Clarence”



"I fought the law and the law won" now I are a retired one!
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