780 bored to an 809 problem

Rowdy1132 · Oct 7, 2009

Have a motor i bought used off snowest. Keeps catching a ring on the exhaust port. Must be on the down stroke. Can i just chamfer off the bottom of the exhaust port so that it won't snag it? It's done it twice with 2 different styles of pistons. Any help would be appeciated...thanks

DWHalverson · Oct 7, 2009

Has the exhaust port been opened up? Cylinders ported? How much piston clearance is there?

Dave

Rowdy1132 · Oct 7, 2009

Motor has been ported. Built by Val simmons when i first got it. Can't tell if the exhaust ports have been opened up, kinda looks like it. I'm no expert. Mechanic that just rebuilt it last said piston clearance was fine. I'm not much help, sorry. Just tired of buying junk motors. This is number 2 from snowest, built by professionals. thanks

Tree Basher · Oct 10, 2009

exhaust port

sounds to me like the exhaust port is opend up too wide,not good.

modsledr · Oct 10, 2009

are you sure you're catching a ring, and not experiencing detonation?

What is your compression ratio?
What kind of fuel are you running?
Is it always the same cylinder? And if yes, which one?
Do you have a pic of the top of one of the bad pistons?

Give us some more info so we can help you.

Rowdy1132 · Oct 12, 2009

Not sure, on the pto piston it pulled the exhaust side top edge. Started too on the other 2 pistons. also. Haven't dealt with detonation too much to see what it does to pistons. But i ran 2 gallons of race gas in each tank. Don't think it is detonation.

If it is the exhaust ports opened too wide. Is that fixable? thanks for all your help...

DRD · Oct 12, 2009

Post pics of the damage and specifics.
If the port is opened up to big it might be a good excuse to go to a 889 or 925.

Rowdy1132 · Oct 12, 2009

I've heard mixed feelings about going bigger with these motors. Some say they don't run any better? But i'm kinda screwed the way it is. Anybody have experience doing it? Who would be a good person to send it too? thanks
will try and post some pictures...thanks

DWHalverson · Oct 12, 2009

If this is an engine truly built by Simmons it should be a STRONG engine.

What type of pistons are you running and what direction is the dot or arrow on the top of the piston pointing? I wonder if the pistons are in backwards?

Dave

Rowdy1132 · Oct 12, 2009

I can't say that it was built by him. I bought it from sledhead racing. He said Simmons built it. So who knows. It had a fresh top end in it when i got it. Using wiseco pistons both times. Wiseco makes 2 styles for this bore. We tried both. All 3 pistons are doing it also. The arrrow points towards the exhaust. Thanks....Scott

d&mraceengines · Oct 12, 2009

780

hi scott im the one who worked on this engine last my specialty is automotive circle track and drag engines not 2 strokes but i do have alot of knowledge of the rotax tripples since i have 3 and have built them many times and others.
this engine is a 780 bored .020 not .040 which would make it a 809 bore it has very large ports big portting on intake and exhaust has .065 clearance on mag and center cyl has .075 on the pto side becuase it was scored some and needed honed with weisco pistons which should be enough cleareance .
it has close to 15to1 compresion becuase i have some domes from union bay that were 15.8 to1 and this head was one cc more volume then the union bay chamber on the union bay chamber they told me i had to run 70 percent race gas and stay above 6000 feet to not have a deto problem so this engine should have atleast 50/50 but i dont port or design combustion chambers for 2 strokes becuase that i leave for the experts so it is only my 2cents the cylinders didnt have much chamfer on the ports so i chamfred them and i was scared of the heat the l rings hold in the piston so i put in a regular style ring piston i was a little scared the ports might be to large to not catch a ring agian but it sounds like it was a deto problem that caused this i would sure like to get some pics posted on here and get some info from some one that knows 2 strokes better than me . i will try to get to scotts in the next couple days and see if i can look this thing over and possiblly get some pics posted but might be a while since my wife is workin here 7th 12 hour shift in7 days and is supossed to work through next tuesday straight through with the hospital moving and she does the computer stuff i just hunt and peck thanks guys for you time and thoughts dan

DWHalverson · Oct 12, 2009

Typically on the 780s and 809s when ported the exhaust port doesn't get widened. It will get raised though. If it has been widened too far the cylinders can be welded but the expense for welding and porting is tough to swallow sometimes. I would get different cylinders and start over.

Dave

d&mraceengines · Oct 12, 2009

780

this 780 definatley had the ports widened i didnt measure them but alot wider than my 780 i did take some picks if i can get the wife to post then it looks like detination to me but if i can get the pics on here would sure likee some input thanks very much dan

modsledr · Oct 12, 2009

Without seeing the pistons, and based on D&M's comments, I would bet detonation.

I have an 809 with Union Bay "trail" heads, 13:1 and I run 25% race.

If you truly have 14.5 or 15:1, you need AT LEAST 50/50 above 6000, and straight race below 6000.

Is there any way you could post a pic of one of the bad pistons?

d&mraceengines · Oct 13, 2009

here are some pictures of a burnt piston

Thanks, Dan

DWHalverson · Oct 14, 2009

Still looks like it snagged a ring on the exhaust port being to big. Looking at the other cylinder still on the case the main exhaust port is wide and the sub exhaust ports have been opened up quite a bit too, leaving a narrow bridge between the exhaust ports that could lead to heat issues. I feel you have the main exhaust port too wide to support the rings and it snags the rings. This was somewhat common on the 780s and 809s if people weren't familiar with the porting requirements of those engines.

Dave

Turbo11T · Oct 14, 2009

809

I have a 809 for sale if interested. Ill make you a good deal.

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780 bored to an 809 problem

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