Help with warm starting difficulties

Mon Apr 08, 2013 9:06 am

My 1958 FCub was gone over in the past year. It needed attention everywhere. The last portion of the restoration involved replacing the piston rings, cleaning the head, resetting all the valves (parts in good condition), replacing the spark advance springs and setting up the timing to specifications. These changes eliminated the exhaust oil smoke and the engine has good power.

To cold start the engine, I hold the choke closed while cranking the starter. The engine catches in five seconds or so. I continue to hold the choke for another thirty seconds, during warm up. The tractor runs well and responds to throttle and load changes fine. However, if I kill the engine and try to restart it, I have difficulty. When the engine does not catch, I apply some choke which appears to flood the carb/intake. Once flooded, the engine will only re-start with towing the tractor and starting by compression. The compression starting takes a bit of towing...to clear the flood? Once re-started, the engine operates fine. What's going on??

I checked various engine components and found the following:
the Zenith carb idle (air) adjustment needle valve is optimum at only about a half turn open
there are no fuel delivery issues from the tank to the carburetor
the air cleaner is clean and fully serviced and functional
the spark plugs and ignition system is clean and set up to specifications; no power problems at any speed
the intake manifold and connections to head and carb are not leaking

Have any of you had this trouble? What should I be looking for and how do I fix warm engine re-start problems?

Re: Help with warm starting difficulties

Mon Apr 08, 2013 9:51 am

After it does start and runs a good while does the coil get real hot.

Re: Help with warm starting difficulties

Mon Apr 08, 2013 12:30 pm

I agree with LA Mike's train of thought - coil breaking down. Either that or condensor breaking down.

Re: Help with warm starting difficulties

Mon Apr 08, 2013 12:47 pm

You did not say what kind of ignition system you have, but I went through this a while back, and it drove me nuts. I had 2 tractors both with magnetos. 1 magneto would work good on either tractor, and one magneto would not start hot on the recently rebuilt tractor, but would on the older more worn one. took a while, but i finally realized the coil was weak in the second magneto, and when it was on the tractor with the higher compression it would not fire well. It does take a hotter spark to jump the plug gap as compression goes up.

Re: Help with warm starting difficulties

Mon Apr 08, 2013 10:05 pm

I agree with coil issues, especially if you are running a magneto system. The spark MUST be hot and blue-white, not yellow.

Re: Help with warm starting difficulties

Tue Apr 09, 2013 6:49 am

John said that he replaced the advance springs, so I'm pretty sure he has battery ignition. I would check spark quality, especially when hot. A little more info would help, 6v or 12v, coil type, does it have a resister, etc. If all components are proper, I would suspect either the condenser or coil and I'd try a condenser first, cause they are much less expensive.

Re: Help with warm starting difficulties

Tue Apr 09, 2013 9:17 am

Gary Dotson wrote:I would suspect either the condenser or coil and I'd try a condenser first.
To check for condenser fault. Engine warm, in no start situation. Remove distributor cap, rotor and dust cover. Ground out coil wire to engine block.

Tractor out of gear. Ignition on, crank over engine. Observe the color of the arc at ignition points. Yellow arc = condenser fault. Sight blue arc = condenser OK.

Re: Help with warm starting difficulties

Tue Apr 09, 2013 12:32 pm

one other thing to check if it still wont start after all ignition components are found to be good. I have had sevrial cub distributers that the bushing in the distributer shaft was worn to the point that the points quit opening after engine was shut down after using. Usually would fire right up after they set for a few hours. I have had some worn as much as .040. New bushing fixed the problem.

Re: Help with warm starting difficulties

Wed Apr 10, 2013 8:54 am

Thanks, fellows, for the timely and thoughtful replies. I've got an update...I changed-out the coil with a spare one that know is good (rechecked the replacement coil using the procedure appearing in the CBok resources before installing). Same problem...my Cub starts fine when cold but won't restart when hot. I reinstalled the original coil and replaced the points condenser with a used spare that worked fine. Same hot restart problem...I had some back-firing during compression starting the Cub. I recognize that I flooded the carb trying to battery start the unit hot. The hot Cub floods very easily.

The fuel delivery system is fine...gas is getting to the carb (no tank vacuum, line or strainer plugs, etc). My Cub is a distributor ignition type. The carb is a Zenith which was gone over but not changed from when I got the tractor in March 2012. I guess my next step is to check the float level in the carb, remove the carb and disassemble it for inspection.

During the condenser switch/check, I examined the distributor bushing and found it to be okay...slight wear only.

I have difficulty accepting that the timing is off. The Cub starts and runs fine from a cold start and has good power and thottle response. The carb air needle valve is set to about a half turn from fully closed to give smoothest idle speed mixture which seems too little (may have a boogered seat?).

I will give feed back to what I find with the carb...please share any experiences or ideas.

Re: Help with warm starting difficulties

Wed Apr 10, 2013 9:11 am

since you mentioned backfire I would check tappet clearnce.. If you already done this then ok. another thing without going trough reading all your post, did you do any engine work? if so did you seat the valves in by hand? If so this might be the problem. when valves wear in they do so to fit that particualar hole they came out of. 9 times out of 10 hand seating causes problems. I have yet to find valves that wear in cemetrical. to do aproper valve job it has to be done pn a valve machine. also the valve seats wear inconsistant and need to be touched up with seat grinder. You mentioned that the distributer shaft had minor wear. was that cold or at oporating temp? at oporating temp is when the most wear can be detected. More than .010 war in the shaft bushing will cause irratic firing.

Re: Help with warm starting difficulties

Wed Apr 10, 2013 9:51 am

Don't pull the choke when hot starting. Flooding the tractor is throwing off your troubleshooting.

It could be a carburetor problem, running far too rich. When the tractor is cold it needs a rich fuel/air mixture to fire and run, and it will continue to run with the heat of combustion compensating for the rich mixture. Once you shut it down the tractor cools a little, the mixture is too rich to fire.

Only thing that could be making the mixture too rich is having the float level set too high in the bowl AFAIK, unless you have a weird zenith like I did, that had a high speed mixture needle.

Re: Help with warm starting difficulties

Thu Apr 11, 2013 6:47 am

Throwing out a couple of thoughts.

Carburetor doesn't flood during cold start, should not flood during hot engine start.

Timing light, check timing on cylinder #1 and #4 cylinder at engine cranking speed. Do the previous test with cold engine and again with hot engine in no start situation. Check both cylinders at fast idle. If you are getting a back fire while cranking engine, something wrong with ignition or ignition timing. Have you checked the distributor cap condition? Still thinking - ignition advance sticking.

Vacuum test engine. Also compression tests.

Once flooded, the engine will only re-start with towing the tractor and starting by compression. The compression starting takes a bit of towing...to clear the flood?
With updraft carburetor, it shouldn't take more than a couple of engine revolutions to clear an excess fuel situation.

Cam shaft removed during engine rebuild?

Re: Help with warm starting difficulties

Thu Apr 11, 2013 8:02 am

Sometimes it's the simplest things. I noticed you put in new rings. For what it's worth, I would check the spark plugs. Often the plugs won't fire well under increased compression and when the engine is cold the compression may not be as high as when warmed up. If the some of the plugs are black and damp (wet looking) they're not firing consistently and may have become glazed. Some believe that the plugs that are used to fire the engine after a rebuild get ruined because of the excessive oil in the cylinders from the rebuild.

Re: Help with warm starting difficulties

Fri Apr 12, 2013 8:27 am

PROBLEM FOUND!
First of all, a big THANKS to those providing ideas and guidance. I hope I can help others here in the future by giving you all a post-mortem on what the problem was and how it was fixed.

After the checks I mentioned previously, I removed, cleaned and inspected the carburetor. I found some varnish and applied some minor adjustments to the float (1-5/32 offset) but did not see any causes for my problems. With the carb removed, I next took off the side panel to the valve tappet gallery and checked the tappet clearances. The tightest one measured about 0.009 and the loosest one measured about 0.012. Apparantly, I did not get the valves fully seated when I had the head off for piston ring replacements. The valves and seats appeared to have been serviced and were in good condition so I only cleaned away the minor carbon buid-ups. I returned each valve to its previous location but neglected to seat them completely. Clark (see above) was right on the target.

When the engine would heat up, the valves (especially the exhaust valves) warm more than the head and, therefore, grow in length more than the cylinder casting grows in depth. This expansion closes (some of) the valve tappet clearance which results in the valves being open too long.

I reset the valve tappet clearances to 0.015, cold condition. I'm aware that the literature gives several setting from 0.013 to 0.015. The engine was run about 20 to 30 hours after the piston ring/head work, which was enough to re-seat the valves (lower) and cause the lack of compression and subsequest hot start problems.

I checked the spark and found it to be hot and blue...no condenser or coil problems (this time). You can find a photo of my (Daniel's) cub on my post "1958 FCub Engine Cylinder and Head Restoration". What a great bunch of guys...give yourselfs a gold star.