Walbro carburetor with cold start enhance

A friend asked me to look at his mower for him. It's a 1986 model 5254 19 inch aluminum deck with F series engine. He said it would never restart after it was warmed up. I thought it might be the coil going bad. I went to pick it up and he started it up for a minute and then shut if off. It surged a little bit at high RPM.
I smelled gas as soon as he shut it off and noticed it was dripping gas from the bottom or the carb. I had never seen a carb like that one. It had the primer tube
going into the bottom of the bowl nut. I told him I thought is was flooding and that was the carb and not the coil pack. So I took it home and took the carb off,
cleaned it good and took the seat out. Its a walbro LMR 1. The new needle and seat won't be here till Friday. After some web searching I figured out that the primer tube going into the bowl nut was something called cold start enhance. It is a discontinued item. What I was wondering about was if the carburetor still leaks out the bottom would a regular Walbro carburetor bowl nut work oK? I thought some of you guys might have worked on these before and might have some suggestions. Another thing, I thought most F engines around that time period used the same coils, but this one has two different leads and wires that run to the kill or shut off. According to the Lawn Boy parts viewer, it calls for the same coil as my F engines but its a completely different one. I thought that was kind of weird. Another strange thing is that I have a L21ZPN with serial no. starting with a G but Lawn Bow doesn't list it at all and I have 1997 model 10302 and the drawings and parts list says its a 4 stoke Tecumseh but its really V series engine. Very strange. Thanks for listening.
Jerry

Hi Jerry,

I had an 8255 with the same problem (d-400 engine, I think). I sent a letter to Lawn-Boy describing the problem (via snail mail,,,lol). A tech did reply and said the crankcase seals may need replacing. It would start cold no problem, but refused to restart after it was hot.

Check the seals.

Regards

Jeff

There are a few things to look for with a lawn boy engine with this symptom. I would check the compression, low cylinder/crankcase compression will give these symptoms of not starting when the engine is hot, could be seals or piston ring issue. I would also check to make sure the engine is breathing good by checking for excess carbon in the ports and the muffler. These 2 cycles need to breath well, especially if the engine has some age and compression is not quite what it used to be. Something else to check is the choke return spring, those delicate springs on the Walbro carbs have a tendency to get weak and not very good at returning the choke plate to the run position (open) if the slightest bit of grime builds up on the spring, so what you end up with is a choke that is stuck partially on, or fully closed, and this will work fine when the engine is cold obviously, but not good at all when the engine is warm, or hot.

Hi Jerry,

The minimum compression on a F engine is 90 p.s.i. BTW.

Regards

Jeff

I started to take the muffler off to check the exhaust ports and to look up at the bottom seal but could not got the tapered blade holder off. I've had a rough time getting them off lately. I've ruined 2. The first got bent up so bad that I couldn't straighten it out good enough to use and the welds broke on the other one. I read on a forum that one guy puts a puller on them under a little pressure and heats them with a propane torch and they pop off every time. Well it
didn't work for me. My 4 pound mini sledge wouldn't budge it ether. Those blade holders fall right off on my mowers but some of the mowers I get to work on
are like they are welded on there. I guess I could cut it off but will wait to see if the carburetor rebuild kit will make it run better without flooding.
Thanks for the replies.
Jerry

Hi Jerry,

Yep, the blade driver is tough to remove without damaging them. Try spraying liquid wrench, let it soak overnight, heat the driver with a torch and strike it in a counter clockwise direction. Give it a really hard whack and put the blade nut on the shaft in case you miss the driver and hit the threads. Corrosion doesn't help matters out.

Regards

Jeff

Well after 2 days of soaking in PB Blaster and ten hours setting with a puller on it with pressure, the blade driver finally popped off. I cleaned up the inside of the muffler and the exhaust ports looked real clean. I couldn't tell if the bottom seal was leaking or if it was PB Blaster so I change it anyway. Put needle & seat
in and a new bowl gasket and the put the cold start enhance plug in and the turned on gas. Gas started running out the bottom right away, so I decided to put a regular bowl screw retainer on it and leave off the primmer tube. It would not start. Then I remembered to reconnect the spark plug. Duh! It started right up
so I let it run for a couple minutes. I shut it off and tried to restart it but it wasn't having anything to do with that. I was a little disappointed. Next I tried out my new spark tester and there was none. So I went to lunch. About a half hour to 45 minutes later it had faint spark. So next I went to my donor mowers and got a coil. While I had the top off I noted there was a lot of caked on dirt and oil around the top of the engine. I had never seen a metal clad crank seal before. It took some effort to pick it out but did get it out and replaced it also. Put the new coil in and a new plug and it started on the first pull even without the use of the primer, (which isn't there anymore). After running for awhile I shut it off for a few seconds and it started right up again. It does have s slight surge to it though. I don't know but maybe it will come out of it after it warms up. Does anyone think by turning the little adjustment screw on the carb that it would help it run smoother? It sure made me feel better that I got my buddies mower going again. I have learned so much by reading this forum about other's mower problems and all the advice that you guys give. Thanks a lot.
Jerry

Hi Jerry,

The F engine came with the 'new style' and 'old style' carbs. The adjustment for the New style is, pre-set the needle 2 turns from seat (closed). Start the engine and allow to run for 3 to 5 minutes. If equipped with a speed control level, move to high speed. Slowly turn the needle clockwise an 1/8th of a turn at a time until the engine begins to hunt or slow down, back out until it runs smooth. Shut off engine, immediately attempt to restart.

Old style carb, preset altitude needle 1/2 turn from seat. Start engine and allow to run 3 to 5 minutes. Place speed control in low speed position. Slowly turn needle in 1/8th turn until the engine begins to hunt, hunt or slow down. Turn needle out until engine is running smoothly.

In either case, do not prime a hot engine. If there is difficulty restarting hot, open the needle an 1/8th to 1/4 turn.

Hope this helps

Jeff

Thanks Jeff,
This must be one of the old style carbs. The screw was only about 3/4 of a turn out. I backed it out about another 1/2 a turn and it smoothed right out, on low and high throttle. The good thing is that it starts back up when hot with one pull.
My next little job is to service my church's Hustler zero turn mower including changing the hydro pumps oil and filters. We'll be mowing in a week or so here, but today was only in the 40's and dreary out.
I really appreciate your help! Take care.
Jerry

I have the same thing on my 1987 Lawn Boy 7351. Its the metal carb with the throttle lever and choke and it also has a primer next to the carb. Does anyone know where I can get a OEM Lawn Boy carb rebuild kit? Also where do I get a spare primer bulb?