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| | | | Joined: Jan 2018 Posts: 298 'Bolter | | 'Bolter Joined: Jan 2018 Posts: 298 | Trying to dial in my 1957 rebuilt 235 installed in my 37 1/2 ton. Checked timing and is on the timing mark. Compression test is around 140 on all cylinders. No vacume leaks and holds around 12 lbs until back fire in carb. Have 2 carbs and both seem to do the same thing. If you hold hand over intake on carb with air cleaner removed seems to run better and less back fire in carb.
Going to remove valve cover and check clearance on valves tomorrow. Looking for any thoughts about my issue. Thanks for any help you can give me. Bill
1937 GC
| | | | | Joined: Mar 2010 Posts: 10,275 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,275 | It could be a sticky intake valve.
It could be one of your carburetors is dumping raw fuel into the intake and then into a cylinder or two, causing a delay in combustion, whereas combustion is occurring while an intake valve is open.
Disable one of the carburetors and see how it runs. If it runs the same or worse, disable the other carburetor (enabling the first carburetor, of course) and see how it runs.
You could make a carburetor base gasket without a center hole in it for this test.
Carl
Last edited by 52Carl; 10/20/2019 11:34 PM.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Dec 2017 Posts: 1,652 'Bolter | | 'Bolter Joined: Dec 2017 Posts: 1,652 | Start here. Rochester’s can be a bit of a pain to set up. http://www.inliners.org/tech/tech4.htmlCarter YF carbs are a better choice if you want dual 1bbls. A pair of 1974 Ford pinto Weber progressive 2 bbls are even better.
Last edited by Dragsix; 10/20/2019 11:44 PM.
Mike
| | | | | Joined: Jan 2018 Posts: 298 'Bolter | | 'Bolter Joined: Jan 2018 Posts: 298 | Maybe I stated that wrong. I only have 1 carb on engine but have a extra carb that I tried. Both have some back fire so that is why I think it is not the carb. Bill
1937 GC
| | | | | Joined: Dec 2017 Posts: 1,652 'Bolter | | 'Bolter Joined: Dec 2017 Posts: 1,652 |
Mike
| | | | | Joined: Jan 2013 Posts: 1,518 'Bolter | | 'Bolter Joined: Jan 2013 Posts: 1,518 | Improper valve adjustment (valves too tight) is a cause of pop back through the carb. If it is a solid lifter cam set intakes to no tighter than .006 and exhausts .016. Hydraulic lifters go 1 turn past zero lash.
Last edited by sstock; 10/21/2019 2:06 PM.
1953 Chevrolet 3100261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY GalleryVideo of the 261 running1964 GMC 1000305 Big Block V6, sm420, the next cab off restoration
| | | | | Joined: Jan 2018 Posts: 298 'Bolter | | 'Bolter Joined: Jan 2018 Posts: 298 | It has hydraulic lifters
Last edited by Bill Trotter; 10/21/2019 2:03 AM.
1937 GC
| | | | | Joined: Dec 2017 Posts: 1,652 'Bolter | | 'Bolter Joined: Dec 2017 Posts: 1,652 | How were they adjusted initially?
Mike
| | | | | Joined: Jan 2013 Posts: 1,518 'Bolter | | 'Bolter Joined: Jan 2013 Posts: 1,518 | Bill, with hydraulics, I take up all the clearance by spinning the push rod between the thumb and forefinger, then when the push rod ceases to spin go only 1 turn further. Assuming you have the crank in the proper position for adjusting said valves and the lifters pumped up.. Point is, intake valves that fail to close all the way will cause the popping back through the carb. It would be considered to be one of the first things to check after assembling one's engine.
Last edited by sstock; 10/21/2019 2:11 PM.
1953 Chevrolet 3100261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY GalleryVideo of the 261 running1964 GMC 1000305 Big Block V6, sm420, the next cab off restoration
| | | | | Joined: Jan 2018 Posts: 298 'Bolter | | 'Bolter Joined: Jan 2018 Posts: 298 | Drag6, were those pics for me? As per your Pryor question, the valves were done after assembly and Pryor to running the engine.
1937 GC
| | | | | Joined: Dec 2017 Posts: 1,652 'Bolter | | 'Bolter Joined: Dec 2017 Posts: 1,652 | No. Wrong string. I think I am with the others. Readjust the valves first. Then double check your timing, then readjust the carb. Doing those three things costs nothing more then some time and eliminates a bit of low hanging fruit. Next, check for vacuum leaks, including at the vacuum advance. Make sure the vacuum advance is actually working. If not, it may have a ruptured diaphragm and will need replacing.
Mike
| | | | | Joined: Mar 2010 Posts: 10,275 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,275 | OK, now its clear that you only have one carburetor (my mistake, not yours).
If we adopt my mantra of "Its never the carburetor", then its time check everything else besides the carburetor.
It could be as simple as one lifter is set too tight, preventing a valve to close.
I would start by removing one spark plug at a time while the engine is running. You will find the offending cylinder when the removal of its spark plug causes the backfire to stop.
If you introduce air at the spark plug hole of the offending cylinder while both valves are closed, you will be able to hear air escaping through the carburetor if the intake valve is set too tight.
Remove the valve cover and see if backing off the rocker arm screw stops the air leak through the carburetor.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Jan 2018 Posts: 298 'Bolter | | 'Bolter Joined: Jan 2018 Posts: 298 | Will be checking valves one day this week and will post what I find. As per vacuum advance it is working and have checked for leaks and found none. Thanks for your help. Bill
1937 GC
| | | | | Joined: Jan 2018 Posts: 298 'Bolter | | 'Bolter Joined: Jan 2018 Posts: 298 | Checking valves tomorrow night and have been thinking about my back fire in carb issue. It is a 57 235 with a 54 flywheel so it matched up to my stomp starter. Could I have install the flywheel wrong? I seem to remember that it would only fit one way. Is that correct? Maybe I am over thinking this!!! Thanks for any help
Bill
Last edited by Bill Trotter; 10/24/2019 1:24 AM.
1937 GC
| | | | | Joined: Jan 2013 Posts: 1,518 'Bolter | | 'Bolter Joined: Jan 2013 Posts: 1,518 | One way only on the flywheel
1953 Chevrolet 3100261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY GalleryVideo of the 261 running1964 GMC 1000305 Big Block V6, sm420, the next cab off restoration
| | | | | Joined: Jan 2018 Posts: 298 'Bolter | | 'Bolter Joined: Jan 2018 Posts: 298 | It's been 1 1/2 year since I put it together so could not remember for sure. Thanks for putting my mind at ease. Bill
1937 GC
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