1988 marathon 2PG backfiring

[Troy2PG]

So I have put a lot of time and a little money into this cart and I am dedicated to making it dependable, it is my first cart, so far I have had the crank reworked, new piston, rings, wrist pin and bearing, crank bearings, and seals. Ran great for about a day then backfired a few times and popped the seal out, please help, I have had this motor out three times already.

[newtjl]

The backfire is typically caused by gas going into the cylinder without spark, then when you give it ignition spark, it explodes causing the backfire. If the cart is backfiring when you let off the pedal, then press on the pedal, this is the problem. The fix is to adjust the throttle cable and carb so that it closes completely before the ignition spark stops. With me so far? With the key off and seat removed, lightly press the accelerator and make sure the ignition micro switch engages before the throttle begins to open. Adjust accordingly to ensure that happens. If it is randomly backfiring while you are driving it with no changes in throttle, you likely have the piston backwards or the reed valve is shot. The first scenario is the most common though, especially on two strokes.

[Troy2PG]

Thanks for the info, it is only backfiring when letting off the throttle, will get back to work on it and post results, thanks again, I appreciate the help.

[newtjl]

Glad to help. You may want to make sure the idle screw is backed all the way out as well. It may be holding the throttle slightly open when there is no spark. I had an old parcar in a few weeks ago with a cheap aftermarket carb on it that was backfiring pretty badly. I checked the usual adjustments with no luck and finally discovered the mixture screw was nearly falling out. I guess it was running a bit rich! I set it and adjusted it a couple of times and got it straightened out. I saw the old feller who owns it a couple of weeks ago and he told me its running fine. The default is open on those old carbs and it seems to cause a fair amount of problems. Its hard to get the ignition and the throttle set to optimum with no backfires. It also seems the two strokes are a bit more prone to backfiring than 4 strokes. I read the reason why somewhere but cant recall exactly what it is.

[mark.miklaucic]

Also as CartAround has mentioned in many threads, the spring that helps close the carb gets worn or lost . You can replace it with a ball point pen spring. It helps close the throttle plate quickly which inhibits backfire, but making sure you adjust the pedal and microswitch timing would also be important to verify.

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[Dabble Inn]

I worked on an 88 a few years ago, it had a factory spring-loaded throttle cable. It was at the cable sheave mount on the engine, it's a pretty heavy duty spring too. Wish my 85 had the same set up

[Troy2PG]

Thanks for all the support, I pulled the engine, both seals popped out. had to do a little fabrication on the clutch, reset the seals, put it all back together and ended up using the pen spring to get the throttle set up. Of course the battery was dead so it is charging, will fire it up tomorrow and post results. What is a good spark plug for this engine?

[Troy2PG]

Its up and running, no backfiring as of yet but my youngest daughter has not got a hold of it yet, lol. Have learned a lot from BGW, you folks are great. Gonna keep on making improvements as long as it keeps running.

[Dabble Inn]

Great job, I guess the factory spring was gone at the mount?

[Troy2PG]

You guess correct, no springs at all on it, running good for two days now, my persistence and yalls help paid off.