flat head question

[flattiefreak]

Its been a while since I posted here, but have another question My 1958 plymouths flathead runs OK, but there are a couple of small problems( if you can call them that). First, sometimes when the car is running, usually when idling after a stop or start up, the engine will begin to slow down..sorta like it makes a revolution...stop...revolution ...stop..then itll start to speed up back to the normal rpm...Second, when first starting up, the high idle circuit will stay on unless I press the accelerator.Is this normal? Please bear in mind I was born just as everything was going to fuel injection so I dont have too much experience with carberated engines.....Third, While driving, there is a small rumple or popping noise coming out of the tail pipe. It is not like a backfire, but is noticable.

Thank you

[aero3113]

Sounds like your carb may need to be cleaned out. Your high idle is working correctly.

[flattiefreak]

Thank you for the reply.I will probably get the carb professionally rebuilt soon since it hasnt been COMPLETELY redone. Thanks for letting me know about the high idle system. Glad to know that its working propperly.

[randroid]

flattie,

Rebuilding your own carb isn't very difficult and certainly less expensive than having a "pro" do it for you. May I suggest you scan the "Search" files here for what's involved and then consider doing it yourself?

-Randy

[flattiefreak]

I sure will consider rebuilding it myself. I will search for info on this site on how to rebuild it. I know this sounds dumb, but the thing that intimidates me the most about the carb rebuild is setting the float level..the rest of the internals look simple..

[greg g]

You likely will not need to reset as nothing should be knocked out of whack by the dissasembly process. Just a clip and a pivot pin to pull straight up.

[Greenbomb]

Good luck with the carb. You'll do OK with it. As far as that exhaust noise goes: there are many here who have paid big bucks trying to get their exhaust to sound like that!!!

[flattiefreak]

Thanks guys.. like I said, it doesnt run bad, it just could run a little better.. Ill search around fore a carb kit and if I have any questions I will post them.

thank you

[Joe Flanagan]

Roberts sells them. Everything you need in one box. Also, I used one of those cans of compressed air that they clean computer equipment with to blow out the passageways. They have those skinny red nozzles on them that are very handy.

Edited March 22, 2011 by Joe Flanagan

[Don Coatney]

Hold a piece of paper directly behind the tail pipe when the engine is running at idle speed. The paper should be pushed away from the tailpipe. If the paper is sucked into the tailpipe every couple of seconds you may have a valve hanging up. I suspect you do as this will also cause the popping sound you hear. Do this paper test when you first start the cold engine then do it again after you have driven 20 or so miles. Report the results.

[Jim Saraceno]

I bought my carburetor rebuild kit at NAPA.

[flattiefreak]

I did the paper trick...it didnt suck the paper at all...So thats good. I do have a question on how to set the float level. I think I have a high float level because I have tostep on the gas to get the car to start when hot. I opened the carb up and everything is clean. so, I think I could just try to set the float level a tad lower and see what happens.. Anyone know how to do this?

thank you

[Niel Hoback]

Bend the little brass finger on the float that pushes against the needle. Don't get carried away, just a 32nd of an inch or so and try it. You will be able to see the difference in the float height if you do it with the bowl full of gas. . Measuring from across the bowl top edge to the top of the float should be 5/64". Set it a touch lower, maybe 1/8", and see how it goes. You can do this with the carb on the manifold, just remove the top.

[greg g]

bending it toward the needle valve lowers it. Basically making it come into contact withthe valve with less fuel in the bowl.

[flattiefreak]

Thank you for the replies. I just cleaned and regapped the plugs. They were covered with black soot. I know that means it was running rich, so I pulled out the vacuum gauge and set it til I got the highest reading which is a steady 15. Is that good? I will set the float a little lower after the engine cools down a bit.

[Joe Flanagan]

I read that 20 on the vacuum gauge is ideal. I think that comes from the info that came with my gauge but others can chime in.

[greg g]

A reading from 18 to 21 inches for a stock engnie is where it should be. Depending on wear. Your's suggests worn rings or a vacuum leak between the the carb and engine. Was he needle steady? Have you done a compression reading lately??? Should be in the 100 to 140.

You can check for vacuum leaks using an unlit but opened propae torche, running it around suspected places like the carb to manifold meeting, or the intake to block area. If the idel smoothes or increases, it suggests an air leak there.

However an itermittant thin like you originally describe sound like a ignition shut down rather than a fuel problem.

[flattiefreak]

I havent done a compression test, but the rings should not be worn. They were replaced in summer 2008 with minimal driving..The hand is steady. It jumps from 14 1/2 to 15.. I do not have a propane torch, but I do have a can of WD40. Would that work? keep in mind that I have a cheap vacuum gauge so that could be causing the low reading.. Its running smoothly, but It cant hurt to double check and make sure everything is tight.

[flattiefreak]

I just searched online about using the WD40, went outside and sprayed all over and around the carb, vacuum advance.. Nothing happened so thats good. I got everything tuned and seems to run better, not that it run bad before. Thanks for all the help. I will take her for a spin later on and see how she acts.

thank you

[greg g]

when you finish the run take a look at the plugs again.

[flattiefreak]

I checked the plugs again today and they are just as black as they were yesterday. Now maybe i am confused, but dont you turn the screw in to lean the carb out?

thanks

[TodFitch]

I checked the plugs again today and they are just as black as they were yesterday. Now maybe i am confused, but dont you turn the screw in to lean the carb out?

thanks

The screw adjusts the idle mixture. If you have an issue with the mixture at speeds other than idle, then you need to look elsewhere, perhaps changing out the main jet, etc.

[flattiefreak]

How exactly do I change out the main jet?

thanks

[Tom Skinner]

My book says Low Steady Vacumm at @15 or so is Late Ignition Timing.

You don't mention a Timing Light.

Set Points at .20, then set RPM's to @450-475 then Check Dwell @32-38then set Ignition Timing, then use a Vacumm Gauge in that order.

Then everything should be as close as its going to get.

Seek the highest setting with the Vacumm Gauge.

Bring us back some results from these tests and we can look into other stuff.Remember loose wires are the bane to an ignition system.

Clean and tighten all connections.Good Luck

Tom Skinner

Huntersville, NC

PS. Just using a Vacumm Gauge to diagnose isn't going to isolate trouble unless the Hand is Fluctuating and you have a good Vacumm Gauge Interpretation Chart.

[greg g]

http://www.secondchancegarage.com/public/186.cfm

Use the search button for Vacuum gauge or setting timing with vacuum gauge