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Old 01-19-2010, 08:01 PM   #1
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Default popping through carb when warm.........

Hey all. Though this is my first post I have spent countless hours on here researching my problem. I've read till my eyes are crossed but havn't found a solution yet.
Heres my deal. I bought a 1994 Marathon with the 295 Robbins recently. I have no idea what has been done to this thing in the past. The thing runs great for about 5 minutes then starts running crappy and pops back through the carb. Let it sit for a few minutes and it will take off again for a few minutes and repeats the cycle. It is spitting gas back through the carb. A tiny bit when running good but more when it starts popping back of course.
Here's what has been done.......
Replaced ignition coil due to no spark at one time. Ran again but only till warm. Changed again with known good coil but it ran worse?
Pulsar coil ohmed to 22 ohms, Replaced fuel pump due to leaking gas,
Multiple sets of spark plugs,
Set valves to .004, .005, and .006, checked all wiring and regrounded the grounds. Did find that the black pulsar coil wire was spliced together at one time and only had a few strands holding it together, spliced wire back but still did not fix it,
Removed muffler and drove it with no change,
Tryed a different ignitor, even figured out how to hook an older Ford ignition box to it. Still same thing, the Ford ignition box did work by the way.
Cleaned carb twice, and checked the fuel pressure to the carb, looks ok,
Checked compression, looked low, 135. Checked and found that the cam gear was one tooth retarded. Reset timing and still have the same problem.
Rechecked compression after fixing cam timing and came up with.........
150 and 165 dry........
180 and almost 200 wet........
I think I may have a burnt valve but not sure. this is why I posted here. I have ran out of ideas..................
Does it sound like maybe a burnt valve? Any and all help will be appreciated. Sorry the post is so long but I've done a lot of things to it. probably some that I failed to mention. Please help!!!!!!!!! Haha!
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Old 01-20-2010, 04:29 AM   #2
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Default Re: popping through carb when warm.........

I would say your right about the valve problem, burnt, bent, whatever looks like you covered everything else.
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Old 01-20-2010, 08:05 AM   #3
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Default Re: popping through carb when warm.........

Did you recheck your valve lash after fixing the timing? Jon, what's it supposed to be on that engine, .004?
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Old 01-20-2010, 08:07 AM   #4
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Default Re: popping through carb when warm.........

What about a leak down test on compression....
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Old 01-20-2010, 08:10 AM   #5
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Default Re: popping through carb when warm.........

Seems to me if the compression is good the engine is mechanical sound.The only thing I can think of is a valve sticking when hot.Maybe try some top end cleaner. Also just because the pulsar ohms ok doesn't mean it good.The the only way to really check one is with a lab scope. Thats how I check all pulse generators. Good luck, Craig
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Old 01-20-2010, 08:45 AM   #6
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Default Re: popping through carb when warm.........

I always back off a 1/4 turn and lock them down. I think that ends up being around .006. I find it hard to believe anyone that works on these engines for a living takes the time to monkey a feeler gage in there to do the adjustment. most don't.
I would lean toward it being a slightly bent valve and when it heats up it distorts more. He said the cam timing was off..... who knows what was done before they got it that close. regardless there's only a couple things that'll let pressure blow back through the intake on a 4 stroke and thats the valve not sealing or the cam timing is off. If the timing is right, pulling the head to figure out what's wrong is the only option I see
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Old 01-20-2010, 11:27 AM   #7
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Default Re: popping through carb when warm.........

I had a yamaha gas (95ish) come in doing the same thing and figured it out as a worn valve guide. The valve was easy to push down while cold, but when warm the valve would stick in the head. replaced head and valves and it ran like new. How easy are the valves to push down by hand?????
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Old 01-20-2010, 06:13 PM   #8
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Default Re: popping through carb when warm.........

Flintrock........Yes I did reset the valve lash after fixing the cam timing. Again tried .004, .005, and .006.

Scottyb......I don't have the proper compression leak tester. Although I did shoot compressed air in the cylindes with rockers backed off. I could hear some air going past the rings but could not hear air from the carb or exhaust.

Chardonite........I also don't have a lab scope. It is sparking when it starts running crappy. But, is there any way possible that a bad pulsar could actually send the spark signal at the wrong time? I've been wondering about that?

Ruredy.......I am thinking the same thing on not knowing what has been done to this thing before I got it. Pulling the head was going to be my next step but I just wanted to here what everyone thought about it first. Valve sounds likely being that it seems to be heat related.

Courtesycart.........I know they are easy to push when cold but I havn't tried when it was warm. I will check when I get the chance to look at it again.

One thing I forgot to mentionis that the flywheel side crank seal is leaking. I know 2 cycles don't like that but what aout these 4 cycle robbins motors? I am new to these things. Possible that a bit of oil is scewing the pulsar signal maybe?

Thank you to all that have replied. I will get this figured out especially with all your good advise. And I'll keep you posted on my findings.
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Old 01-20-2010, 06:28 PM   #9
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Default Re: popping through carb when warm.........

Sounds like you have lots of good leads ........let us know what turns up next?
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Old 01-20-2010, 06:37 PM   #10
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Default Re: popping through carb when warm.........

I will let you know what I find Scottyb. Thanks.......
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