Only Backfires Through The Carb Regardless Of What I Do (1450)
 

I got my engine back from the machine shop. Turns out my binding issue was two-fold. The crank needed to be polished .0005", and the aftermarket rod was doing some goofy binding.

Now that I have every single thing back together I can't get it to run. The official Kohler manual says the points should be set to .020". I read in the archive that .015" - .020" might work. Someone even said as much as .030". I found in the library where it said to go off of the S on the flywheel. I tried every technique and I still can get it to do more than backfire through the carb! Every once in a while it will run for a few seconds, but it's fighting the piston, and runs rough! Everyone's probably thinking that the timing is a tooth off. I checked that before I put everything back together. The "." on the cam lines up with the "-" on the crank.

I'm really at ends wit here. I tried setting the points from .012" - .030 in .oo2" increments, and it still wouldn't start! Everyone was right. I would have been a lot of time and money ahead if I would have bought something new with a warranty...

I'm really stumped on this one, and I'd appreciate some input...

If I can't figure it out can someone suggest where to get something like a K341? I'd also like to know if there is a conversion kit available for this for pointless ignition. I know for sure it will at least need a new flywheel...

Thanks!

Mike

EDIT:

I forgot to mention the carb is a brand new Walbro unit....

Try this to get the timing correct. Backfiring out of the carb means either the timing is too far advanced, or something is up with the valves or valve lash. If you've verified the crank/cam timing is correct, and you've verified the timing is correct with the above link I provided, then either the valve lash on the intake valve is too tight, or you didn't lube it enough and it's sticking open.

That brand new Walbro carb is non-adjustable and a POS, although that probably doesn't have much to do with this.

First off, (you're about to get a scolding) I'm not sure who "everyone" is but the people who have told you to buy "new" don't know know "something" from apple butter. New will get you -$2000.00 and some junk to throw away in four or five years. Rebuilding a quality product (imho) is much better than buying tomorrows trash today. (scolding over)

The last time you had this engine apart did you check to see if the ACR was working? The exhaust valve should open a little each time the engine is turned over slowly. Once the engine starts then the exhaust valve opens and closes just as any other valve would.

The next question is: Did you replace either the Flywheel or Crank shaft in this engine and did you put in a new push rod in for the points and is there a groove worn in the cam shaft where that push rod "rides"?

Just to rule out possible causes, if it were mine I would probably pull the head and watch the parts as they move when you turn the engine over (by hand) paying close attention to the valves to make sure they open and close as they should.

Good luck and let us know what you find out. If you do take it the head off, pictures wouldn't hurt either.

One other thing you could try first is to take the plug wire off and see it it turns over as it should or if it still stops/pauses. If it does the same thing, then the problem isn't electrical.

Unfortunately I saw that link yesterday, and I still had no luck.

The valves I didn't touch. The machine shop did that. I was fearing that it would be the valves mis-adjusted...

Getting too late to mess with it today. I guess I'll check the valves tomorrow... :/

I didn't start this thread to argue with anyone... but I know for a fact that a new Kohler 7/8 hp engine with a new carb is $185, and comes with a 1 year warranty... I have more money than that in machine work alone.


Yes, the ACR is working. The tang was broken, but I replaced that part.

I replaced neither the flywheel or crank. The pushrod for the points is a new steel unit.

Did that yesterday. I didn't get out a dial indicator, but it seems to be functioning properly...

You are getting a true "backfire". As matt already stated, there are only two causes....the intake valve is sticking, or timing...and if the intake valve seats nice, then your timing is 180 deg off.
If the timing is all correct, and the valve seats good.....then the cam is a tooth or alot of teeth off.
And you may not be able to actually see it seating with your eye.....do a compression test first...then try lapping compound on the valves (intake is the problem, but do both) and look at how it's really seating.

The machine shop put in new valves, and should have cut new seats. I didn't pull it apart and double check tho. I don't understand how it could be 180* tho. Is there more than one timing mark on there? Plus the last time it was at the machine shop the machinist timed it. I know he's built a few of these engines, so I'm sure he did it right (plus I double checked it before I put the pan on).

According to the Kohler manual, you can't do a compression test due to the ACR.

Eastonct124 didn't phrase parts of that very well...I think he means the intake valve may be sticking, which would definitely be a possibility if the guy assembling it didn't properly lubricate the valve stem and guide before assembly. I'm still thinking there's practically no valve lash on the intake valve. Thankfully, it's easy to check and correct if necessary.

There are two timing marks on the flywheel: "S" and "T". "T" is top dead center, and "S" is 20 degrees before top dead center, and that's the one that should be centered in the timing sight hole as the points begin to break.

Oh, I meant the timing marks on the crank and cam gear itself.

Thanks matt, I'm alittle illiterate when typing. :)
To rephrase things:
It pops back through the carb because the intake valve isn't seating completely, and when the fuel mix ignites....it naturally blows out through carb.
But, if the valve does seat well, then the (intake) valve must be open at the wrong time...therefore, the cam is off.....or the timing is too far advanced.
And, you're right, with the crv, a comp test can't be done....sometimes I think out loud and forget.
what 'm saying is, it can only be one of those two things....it's not rocket science.....just take your time, and double check the valves, and the timing (including the cam to crank timing).

Yesterday when I went to mess with it here's what I found...

After trying several times to set the points yet again I noticed that the trigger for the coil had somehow completely broke! The little fork that goes under the nut broke completely off! I have no clue how this happened. I did get the engine to run, but it ran violent. It must have broke at that point. See that machinist put the wires that to to the stator on the wrong freaking side. In noticed this after I had everything including the PTO and clutch installed so I just extended the wires 6-8". I ran them along the points wire and then made a loom with electrical tape so it would at least look like it belonged there. The wires are about 2" too long, but they're not in the way of anything so I left them. So really I don't know how it broke, but whatever...

At this point the battery was dead, so i decided to check the valves. Since the "t" mark is either non-existent, or too rusted to read, I put my finger over the spark plug hole to find the compression stroke. I then pulled the head off and used a dial indicator to find true TDC. The intake was set right at .010", but the exhaust valve was set way too tight. At this point the battery was still not charged, so this is where I left off.

I did manage to get the engine to start and run, but it only ran for 2-3 seconds then died. After that it wouldn't even attempt to start. I thought that maybe it was flooded so I took the plug out, cranked it a few times, and blew it out with compressed air... Still nothing. I probably should have left the points alone at that point but I didn't thinking I could tweak it a bit and get it to run.

At this point I'm 99.99999999% sure it has to be something other than timing. The plug is new and set at .035", the points are new Kholer points. The shaft is a new shaft. I thought maybe it wasn't staying set, but it is. Turning it over by hand I have compression, and the valves are set correct (well now they are). The carb is brand new and the only adjustment it has is for the idle set screw. It's getting fuel to the chamber, too... What can possibly be preventing this engine from running? Is it possible that the govenor is preventing it from starting? This is the only thing I can think of...

You still have not told us if the ignition timing is correct or not. If it is off far enough, it will not start.

Possibly the coil? After replacing my stator and putting my engine back into the tractor, my 1250 ran like crap for a few seconds and would backfire loud enough to make the neighbors think I shot someone. After following the suggestion to check the coil specs I put the coil from my 128 on and it fired and ran great until it would get hot say maybe 10 to 15 minutes. This is when I found out the PO had put on a Nissan spark plug wire which had a bunch of resistance built into it causing the coil to overheat and die. Now I doubt you have the plug wire issue, but the coil could be bad. Mine even showed spark but the coil trouble shooting sequence showed my coil was very weak.

Coil Specs:
http://www.onlycubcadets.net/forum/showthread.php?t=11

I'll be bob darned! I had a bad coil!!! All this time... Believe it or not, but when I pulled out the plug to check for spark it had spark every time! I have seen coils fail under load, so I decided to go that route (the only way I caught them is with a scope). I decided to check the resistance of the coil. From + to the tower I had 8K ohms, from the case to any terminal it was OL. Now when I checked from + to - I had 3.6 ohms. This is still technically within spec since the spec is 4-8 +/- 10%. I would have scoped it, but my batteries were dead... At this point I didn't care if it needed a coil or not. It was getting one along with a new plug wire. When I called the small engine shop, she said that they probably had one. Well, "probably" isn't enough for me to justify driving to get it. She then asked if I had a part number. Even if their was one, it wore off along time ago. I was then on hold because she didn't know how to look up the part. When I got there the guy tried telling me that it was the condenser. I might agree with him if it wasn't brand new, and I tested it before I went to get the coil. He told me that "they either work or they don't". Horse hockey! He's still arguing with me, so I told him that I have seen coils start and run a vehicle, but fail under load. Then he asked if I saw that type of coil fail like that. I then told him that it was out of spec so he'd just STFU. He was like "well if it's out of spec...". If I was a small business owner, I wouldn't try and downsell a $55 part for a $5 part.

$70 and 30 minutes later I went to crank the engine over. It wanted to start. I reset the points and that baby fired right up on the third try. After it was running I seated the rings just watched that little beauty idle. You guys have no idea how happy I am that this darn thing runs! Even on a cold start it fires right up without even touching the choke. :biggrin2.gif: I started this project 5 years ago off and on and I was always convinced that it was a timing issue.

Now more fun stuff! The clutch stays engaged and the thingy (technical term) that raises and lowers the snow blower doesn't work (but the tractor will move back and forth). Can someone guide me with links on how to fix and repair these two things?

Other than that I just need to put everything back together. This tractor is just going to be used with the snowblower, and I plan on making a cab for it as well as getting regular V tires, along with some wheel weights. In the winter this could be a nice little money maker @ $10 a driveway.

Thanks everyone for the help! :beerchug:

Oh and I forgot to mention that the plug wire they sold me is the wrong one! I need to bring that back as well as the old coil so I can tell him were to put it. lol

Glad you got your cub going! :beerchug:

Persistence, gotta love it. Glad you got that problem figured out. I am sure with the great guys we have in here, you other problems will be fixed soon.

LOL.....soon or later, a lot of us have been down this road or will be. One goes round and round endlessly, then all of a sudden, you discover the problem. It is a learning experience for you as well as others here. Glad you shared you experience here.

:ThumbsUp:

You didn't have a "Backfire". You had a "Pop". I didn't think of getting specific.
A pop always sprays cool wet gas back out of the carb.....not a flame (backfire).
That happened to me on the old Broadmoor I have. It drove crazy....turned out it was the points needed filing and gapping.
Ran fine afterwards.
Good to hear it's running.
The clutch is simple fix...probably not adjusted right.

Glad ya got it fixed!