Author Topic: Carbs Melting  (Read 4238 times)

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Offline mowerracer313

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Carbs Melting
« on: September 03, 2007, 11:12:43 AM »
Im runnin a 691 twin and after 15 laps in the feature it quit
Come to find out the blue emulsion tubes had melted over
I cant seem to find the cause  Valves, compression, spark everything checks out ok  also just running premium gas
Any ideas???


Brian Singsime
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Offline BriggsandStratton1218

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Re: Carbs Melting
« Reply #1 on: September 03, 2007, 11:38:53 AM »
Does the engine seem like its running hotter than usual?  And im guessing these are the plastic tubes?
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Offline berthyd

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Re: Carbs Melting
« Reply #2 on: September 03, 2007, 12:56:58 PM »
Brian, I'm going to guess you have some valve bounce or float. The intake valves, which are too big and heavy in stock form, may be bouncing a little at closure, letting hot combustion chamber gasses creep through. When's the last time you changed springs? And what are you valve spring seat pressures?

Bert
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Offline mowerracer313

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Re: Carbs Melting
« Reply #3 on: September 03, 2007, 09:22:52 PM »
Bert
I put new double springs in when I built it last year
It doesnt seem to float any sooner or any different rpm
As for running hotter when it happened it was runnin 100 degrees cooler than usual
How long do valve springs last????
Thanks
Brian Singsime
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Offline George Herrin

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Re: Carbs Melting
« Reply #4 on: September 04, 2007, 05:16:29 AM »
I have run springs for two years (duals with no noticable difference. I have never seen that happen to a carb that was running. Sure sounds like exhaust backing up which could be a combination of the valves not seating andor valve float. The large valves do require much more maintenance. Buy some newer heads (VTX) they come with the smaller valves. Put the dual springs and billet retainers and a heck of a port job and if you can afford it a larger intake valve and you got a good setup that will stay with you.
George Herrin #6
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Offline mowerracer313

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Re: Carbs Melting
« Reply #5 on: September 04, 2007, 07:39:39 AM »
Do you guys put different valves in or modify the stock ones to help with float and bounce???  Im runnin the stock valves just that I polished them.   Since I got a #3 cam do I need lightened valves or would it help in all around performance.
Thanks guys.
Brian Singsime
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Offline George Herrin

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Re: Carbs Melting
« Reply #6 on: September 04, 2007, 07:55:02 AM »
A #3 cam you are done at 5200. Regardless of rpm and cam lighter valves helps period. easier on the valve train/rockers and less likely to flaot since not as heavy, and doesn't need as much spring pressure because they are lighter. I have one motor with big valves one with small valves and its a noticable difference.
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Offline berthyd

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Re: Carbs Melting
« Reply #7 on: September 04, 2007, 05:51:18 PM »
Brian, if you're at Medford, we can talk then. You're not running anything funny in the fuel, are you? Probably best not to answer here that you've got 90% nitro, but what the heck, I haven't seen any tech done in years.

Like G said, you've got a valve seating or a valve bouncing problem. Those big stock valves are heavy and will wear out e set of doublr springs quicker than the stainless valves George and I use. Also, set your lash at .004 COLD. No closer than that.

Bert
Gary Busey and Rick James both used to live in my shop. Redline stole them.

Offline mowerracer313

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Re: Carbs Melting
« Reply #8 on: September 04, 2007, 09:36:22 PM »
Ive kept the lash at .004 really only needed adjusting twice once last year once two races ago.  As for fuel 92 octane gas >:D
Thanks for the help  I need it.
Brian Singsime
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Offline George Herrin

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Re: Carbs Melting
« Reply #9 on: September 05, 2007, 05:35:30 AM »
Do a leak down it will tell you things you cannot see otherwise.
George Herrin #6
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Offline mountan man

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Re: Carbs Melting
« Reply #10 on: October 08, 2007, 07:52:55 AM »
Well i put a walboro carb on my enduro 17.5 and it is a beast i suggest you jet out a waly and fab up some brackets and flanges - no plastic to melt. Plus you would do away with that stupid carb solanoid. 8)
Oh god, a redneck with a computer!

Offline George Herrin

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Re: Carbs Melting
« Reply #11 on: October 08, 2007, 11:21:05 AM »
Quote
Well i put a walboro carb on my enduro 17.5 and it is a beast i suggest you jet out a waly and fab up some brackets and flanges - no plastic to melt. Plus you would do away with that stupid carb solanoid.



While that may be legasl with your group understand the persons rules he runs under before you make recomendations. He must run those carbs. AND you ain't run a real built Tecumseh carb either or you would dump the walbro. And if he is melting the tubes which he should not be the reason should be figured out before it results in more than melting plastic instead of ignoring the problem and changing carbs.
George Herrin #6
Indian Power.. You gotta Go big to beat em or Go HOME
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Offline mowerracer313

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Re: Carbs Melting
« Reply #12 on: October 08, 2007, 10:02:39 PM »
Did leakdown with no leakage in the valves just some bad blowby.
Needs bored and rings.   As for valves anybody got some numbers for some lighter ones??
Since it needs rebuilding might as well make it better.
Thanks
Brian Singsime
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Offline George Herrin

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Re: Carbs Melting
« Reply #13 on: October 09, 2007, 04:39:19 AM »
VTX Valves are lighter 1/4 stem but you will have to bushing down your guides. Or get some 5/16 titanium valves from a nascar team I here they ebay em. Cut the head to the size ya need and the stem. I converted to the VTX heads I like them better come ready for 1/4 inch valves. About 94 ea. plus valves. We have good valves to fit those will need a machine shop to make ya a new seat and install it for the intake its bigger.
George Herrin #6
Indian Power.. You gotta Go big to beat em or Go HOME
8 time ARMA National Champion
4 time USLMRA National Champion
Retired from BP
www.herrinmowersports.yolasite.com
www.allamericanmowerchampionships.yolasite.com
My other racer is a 1/5th scale Dirt Late Model!!!!