My dad and I are working on a 98 450 Foreman S that we bought used knowing it had issues. Originally we got it running it had low compression and was popping through the carb so we thought the exhaust was plugged. Took the exhaust off and it still did the same thing. So we begin to tear it down and sure enough its worn clear out. It had pretty much bored itself .020 over and it was amazing it even ran as loose as it was. So we get it bored, new piston rings, the works, lapped the valves good and put it back together me knowing it would do the same thing, my dad knowing this would fix it. So now we have compression and still popping back through the carb. We checked for slack in the cam chain when we had the topend off by rotating the crank back and fourth and feeling for any slack while holding pressure down on the lifters and there was none. The valves adjust up fine even with the crank set in time both valves are closed and like I said the valve adjustment process works fine. But why would it pop back through the carb? The only thing I can think of is valve timing like the cam jumped a tooth or the chain is stretched that much but if its that far off how would it even run? Any ideas? Also its got 9000 miles on the engine if that makes a difference.

 

When does it pop? Going on the throttle? Or just sitting there idleing? Is the stock air box back on, and complete? Lid on? Air filter freshly oiled?
If it pops going on the throttle, then its a sign of lean carburation in the idle circiut. Clean out the pilot jet, and if its clean, and still pops....turn the mixture screw richer.

 

It does it all the time. Its more of a constant back pressure in the intake with every cycle of the engine. It is actually blowing large amount of gas back out of the carburetor if you rev it up and let off. Doesn't matter if the air box is on it, the filter, exhaust, no exhaust, no air cleaner, doesn't matter.

 

if the carb. seems ok, i would look toward valve adjustment or perhaps ignition timing.

 

Sounds like the intake valve is too tight, and its not closing completely. Hows the compression?

 

We just rebuilt the topend. The valves are adjusted right, it hasn't even been ran a minute on the new bore job, piston and rings. Rolling the engine over to T takes all pressure off the valves and they adjust up exactly like they should just like if the cam was timed properly. Compression is good, especially for the rings not even being seated in, and the valves aren't tight, they are set to specs.

 

did you check the ignition timing?

 

if the cam timing or ignition timing is off at all it can cause this. Its definitly not a carb issue. Check flywheel key if sheered at all ignition time will be off.

 

oldred95 there are several thing's that could be wrong , to start with who did the machine work ; and how far did they have to rebore the cylinder ? And do you know if the person you bought it from tried to do any work to the engine before you and your dad got it from them , like replacing the timing chain ; or anyother work to the engine that you are aware of ? I ask this because if someone re[laced the timing chain , they could've gotten the timing off in the process ! It could be the CDI box going bad , or it could be a bad carburator ; or even the valve timing could be off ! When adjusting the valve's , you must be on the compression stroke with both valve's in the closed position ; with the piston at TDC (top dead center) of the compression stroke ! Please don't get the idea that i'm suggesting that you or your dad don't know what you're doing when it come's to machanical skill's , with over 20ys. in this service and repair i've caught myself doing this when someone was around distracting me from what i was doing ; that's why i alway's wait untill i'm alone when working on somehing this important ! When this happen's i call it " to many cook's in the kitchen " , and happen's more than i like to admit when someone is watching ; and they're asking question's while i'm doing a job ! But the best way to ensure you're on the compression stroke is to remove the sparkplug and put your finger in the sparkplug hole , then turn the engine over untill you feel pressure pushing again't your finger ; you know for sure you're on the compression stroke when you can feel the pressure again't your finger ! Then all that's left is line up the timing mark , and adjust the valves and you're done ! And there's a couple other thing's you should check , one is intake tube from the carb to the engine could be leaking from a crack or a warped condition ; you can check this with the engine running spray some carb cleaner between the carb intake to engine if there's a leak the engine speed should increase ! And you should set the sparkplug gap at 30" to 32" i had a 86 Honda TRX 350 Foreman 4x4 that wouldn't run at the factory setting , it would only run right at 30" to 32" inch ; and i had a friend who had an 88' Honda Foreman and he told me his was the way it wouldn't run right at the factory setting either only at 32" inch gap also ! If none of these thing's workout before tearing into the engine again , if you can't barrow a CDI from a friend to change them out to see if that might be the problem ; i would remove the CDI and take it to a dealer you can trust and have them check the CDI to see if that's your problem ! I've heard of other member's on the forum say that they had to replace their CDI because when the CDI goes bad it will effect the timing as well , and do make sure your carb is clean of dirt/trash or water ; it only take's a drop of water to foul - up a single cylinder engine ! I hope you get it going the Honda Foreman is a very good machine , my old 86' Honda TRX 350 Foreman Fulltime 4x4 was a tough quad i got it in Feb 1989 from the original owner when it only had a little over 20 hours on it ; and it was still going strong when i sold it to my brother in Nov. 2005 ! He blew the engine about a week ago racing our nefew's 98' Yamaha 350 Wolferine , and from what they told me the old Honda Foreman was beating the Yamaha Wolferine by a longshot before the Foreman blew ; it didn't sling a rod it just jumped a tooth on the timing chain and was still runing ! It was running the same way you were describing the way your's is doing now , that's one reason i ask you about the timing chain ; my brother didn't keep up the maintence on the 86' Foreman he only changed the oil 2 time's after he got it from me so that was a facter in him blowing the engine on the 86' Foreman ! I hope you get your quad going and that it doesn't cost you to much to get it going right , i know from having owned one how tough they really are ; so good luck getiing it going ! .......

 

Well we pulled the engine today and popped the front cover off and the timing chain was all but piled up. It looks to have a good half inch too much slack in it and we rolled the engine around to T at TDC on the compression stroke and the cam is roughly 20-30 degrees advanced so it did jump time. Still not sure how it was even able to run. When we got the chain off its all kinked and binding up in the links and its one of those weak little multilink quiet running chaings that IMO suck. At least we figured out what it is and will know for future reference.