Guest 1948Pontiac Posted October 17, 2016 Share Posted October 17, 2016 I have a 1948 Pontiac silver streak with a flat head straight 6. It seams to have a rough idle and the exhaust sounds rough. it seams to clear up when i rev up the RPMs a little. I have new spark plus in it and the points look like they are in great shape. I did a compression check on it and it ranges from 110 to 100 psi. does anyone know if it is normal for a little rough idle on these older flat head 6 engines? Link to comment Share on other sites More sharing options...
Grimy Posted October 17, 2016 Share Posted October 17, 2016 Verify point gap first, THEN check dwell. As I've mentioned before, I had a 1950 Pontiac 6 whose distributor cam lobes were worn, and I had to set points at 0.014 to get correct dwell and factory setting was 0.020 or more. Get dwell right, then set timing. Link to comment Share on other sites More sharing options...
Xander Wildeisen Posted October 17, 2016 Share Posted October 17, 2016 With the car running, place a timing light on the spark plug wires one at a time. Shoot the light in your hand, and watch to see if on one or more of the spark plug wires, you get a miss in the flash of the light. Link to comment Share on other sites More sharing options...
Rusty_OToole Posted October 17, 2016 Share Posted October 17, 2016 Should idle down to 400 RPM and be very smooth.With good even compression which you have, it must be something in the carburetor or ignition that is not quite right. 2 Link to comment Share on other sites More sharing options...
JACK M Posted October 17, 2016 Share Posted October 17, 2016 Try looking at it idling on a dark night. If you see a light show replace the plug wires. 1 Link to comment Share on other sites More sharing options...
GARY F Posted October 17, 2016 Share Posted October 17, 2016 this might sound stupid, but when you take the plugs out of the package always check the gap. ask me how i know. 1 Link to comment Share on other sites More sharing options...
Xander Wildeisen Posted October 18, 2016 Share Posted October 18, 2016 4 hours ago, GARY F said: this might sound stupid, but when you take the plugs out of the package always check the gap. ask me how i know. I would ask you how you know. But I know how you know. Link to comment Share on other sites More sharing options...
padgett Posted October 18, 2016 Share Posted October 18, 2016 (edited) Took this out of a engine that was running. Never saw a gap that small before. Spec is .045-.060" Edited October 18, 2016 by padgett (see edit history) Link to comment Share on other sites More sharing options...
John348 Posted October 18, 2016 Share Posted October 18, 2016 On 10/16/2016 at 9:45 PM, 1948Pontiac said: I have a 1948 Pontiac silver streak with a flat head straight 6. It seams to have a rough idle and the exhaust sounds rough. it seams to clear up when i rev up the RPMs a little. I have new spark plus in it and the points look like they are in great shape. I did a compression check on it and it ranges from 110 to 100 psi. does anyone know if it is normal for a little rough idle on these older flat head 6 engines? Every time I had a problem similar to this it either the idle circuit in the carb had some dirt in it, or a vacuum leak somewhere. Considering it operates fine under throttle I would look close at the idle circuit. I would hook up a vacuum gauge and see what that tells you first. least invasive test to do 1 Link to comment Share on other sites More sharing options...
Guest 1948Pontiac Posted November 27, 2016 Share Posted November 27, 2016 Thanks all for the assistance. After rebuilding the carb 5 times and resetting the points 5 times i found the problem. I had a small hole in the sleeve on the intake manifold. It was allowing the exhaust to blow back into the intake manifold. for some reason i never looked in the manifold but when i did i found the hole. Link to comment Share on other sites More sharing options...
padgett Posted November 27, 2016 Share Posted November 27, 2016 Free EGR ? Link to comment Share on other sites More sharing options...
auburnseeker Posted November 27, 2016 Share Posted November 27, 2016 I've had those manifold tubes before new in the GM packaging and didn't quite understand how they went or worked. Now I know. I might even have one if you can't find one. 1 Link to comment Share on other sites More sharing options...
roadking Posted September 18, 2023 Share Posted September 18, 2023 (edited) auburnseeker I am looking for one of the Gm manifold tubes, wondering if you have one to spare. Thanks , or if anybody else can help out with information to locate one that would be great. Edited September 18, 2023 by roadking More information given to locate one (see edit history) Link to comment Share on other sites More sharing options...
auburnseeker Posted September 18, 2023 Share Posted September 18, 2023 I'll have to look. I got out of parts a while ago, but a few things still seem to linger around. 1 Link to comment Share on other sites More sharing options...
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