Demon 750 Idle

[pacman350]

i had the same problem dat was caused by a leaking intake gasket which was sucking in air from the outside. get a spray bottle with soapy water in it and spray it around the carb base and the where the intake and head meet if the intake gasket is leaking it should idle down substantually then idle back uponce it sucks the water in becausewater inst combustable

 

[Geed_up_442]

Got in there yesterday and found some random intake bolts were loose so i'm sure its a vacuum leak from the intake. i tighten them up and it was noticable when i cranked it up. Alot less shaking and i got it to idle down a little and throttle response was better.

My gaskets looked okay when i pulled the intake though. I use rtv on the gasket and just added more when i dropped the intake back on. i of course cleaned the head/intake surfaces and peeled as much rtv off the gasket as possible. Should i just rule out trying to reuse a gasket or is there anyway to salvage this. $40 gaskets are getting expensive especially since I'm about to be $160 injust intake gaskets

 

[mebe007]

I would personally use new gaskets. $40 is cheap in the scheme of things

 

[565bbchevy]

I put intake gaskets in same category as head gaskets as being non reusable. And if you had a leak at the intake gasket before most likely will be worse attempting to use an old gasket.
So you have bought new intake gaskets three times already, why so many?

 

[DRIVEN]

1500 idle is very high for your application it sounds more like a timing issue it is probably extremely advanced but I would physically check all your hoses and make sure none are causing a vacuum leak and make sure all your plug wires on correctly due to the excessive shaking. You definitely need a timing light.

Yes, yes, yes, yes and yes.

If it was running fine before you took it apart you can probably rule out a carb problem. Replace the intake gaskets with a NEW set first. After that, set the timing with a light. You'll probably find that it runs pretty well after that. Most gaskets aren't intended to be reusable.

 

[Geed_up_442]

565bbchevy you asked why so many. Keep in mind this is my first build first time, i broke a bolt off in the egr , i got it back together and had a very small leak out the back corner where the head, block and intake meet. Then there was this issue and the innitial gasket i bought when i had the motor built. Alot of stupid mistakes but learning as i go.

All the other gaskets tore appart so it was never an option before to reuse the gasket. This time the gasket came out whole no tears just rtv stuck on it so i figured what the hell. And in my mind i'd put everything back together exactly as it was before so i did end up messing with everything carb and distributor related throughing off all the carb tuning i'd done before.

Picking up my gaskets tomorrow and Going to get the timing light. I'll be out there saturday

 

[mebe007]

Make sure you use a quality felpro gasket. Such as a 1205 for non race port heads or a 1206 for large race port heads. Just depends on what your heads require. I do not use the front and rear rubber seals. A simple thick bead of RTV (Permatex Ultra Copper) front and back will seal the intake up without the rubber seals. I use a little bit of permatex forma (white bottle with brush applicator similar to indian head shelac) gasket on my intake gaskets but do not use rtv on the gasket surface IMO

 

[Geed_up_442]

mebe007 whats the reason not to use rtv on the gasket itself. And the difference between the rtv and the permatex forma you mentioned. I myself always used the grey intake rtv.

 

[565bbchevy]

I know everyone has there own proceedure for doing intake gaskets but what I do is use a very thin coat of Permatex Ultra Black RTV and put it around just the water ports on the head side only, this will hold the gasket in place then I lay down a nice consistent bead of Ultra Black across the ends of the block and after the intake is tightened down I clean off the excess on the front so it looks nice.
I never use the end seals in gasket kits the cork ones can squish out and the rubber ones will sometimes split, using Permatex instead eliminates the issues with fitment variations from decked blocks and milled heads, etc.

 

[mebe007]

basically i like the aviation forma gasket best for doing skim coats on gaskets. it seems to seal amazingly well to be honest. it was designed to create a gasket like seal in ones absence essentially. if youve ever noticed rtv seems to come off of gaskets really easy. Now the downside of the forma gasket is it takes a lot of work getting old gaskets off. also when dealing with RTV i have always love ultra copper because its high temp properties. we use both of these products all the time in high performance marine applications and dont tend to have any issues and these motor spend 90% of their time at top end.