Air leak

[Trevorbm]

Chasing an air leak plugged the exhaust and used a pop off gauge/tool to pressurize the crankcase. I can hear and feel air passing through the reeds. I made sure the intake and reed cages were sealed with a little yamabond. Is it time to replace the reeds?

 

[Trevorbm]

62t 701

 

[cpolaris502]

Following.

I don't think the reeds are expected to be completely air tight. When I pressure tested, the kit that I borrowed had a block off plate for the entire intake manifold.

 

[Trevorbm]

Following.

I don't think the reeds are expected to be completely air tight. When I pressure tested, the kit that I borrowed had a block off plate for the entire intake manifold.

I would think that they would at least hold 10psi. I would also think that on the power stroke on the engine they would seal and not let anything back out through the carbs.

 

[Jyzmbe]

When you pressure test you should use a piece of wire or a small zip tie to hold the reeds open so the pressure equalize is in the manifold and isn’t on the reeds.


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[Trevorbm]

When you pressure test you should use a piece of wire or a small zip tie to hold the reeds open so the pressure equalize is in the manifold and isn’t on the reeds.


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Hold the reeds open and block off the manifold?

 

[Jyzmbe]

Hold the reeds open and block off the manifold?

Yes


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[Jyzmbe]

I have these Jett lab block off plates but he no longer makes them. If you have a speed plate you can make one by marking the bolt pattern on another plate and drill your holes and use something like that with either the speed plate orings to seal it or cut a piece of rubber. If you use rubber spray both sides with wd40 first. For the exhaust I just got a block off cap from the plumbing section at Home Depot. I’d spray it with wd40 as well. Make sure your spark plugs are tight and if you have power valves and it leaks at the gasket take them out and check your gasket. If the gasket is good apply some grease to it and re-install.

 

[Jyzmbe]

Check around your crank seals (front and rear) , intake and exhaust manifold, pulse nipples, base gasket, power valves, crank case seem, and any epoxy work on your cases. Sometimes it will leak at the front if they cut the cases and epoxy for a big billet cylinder. I think it’s recommended to test at 8psi for 15 minutes. Some say it’s acceptable to loose 1 or 2 psi over 15 minutes but I do not consider any leak acceptable

 

[Trevorbm]

Update
Air leak is solved engine is holding pressure, but it still ran away from me…. Can the carb be the culprit? To much air not enough fuel? Rebuilt and cleaned the carbs before install. Not ruling out crap in the carbs. Otherwise I’m lost lol

 

[keith t]

Update
Air leak is solved engine is holding pressure, but it still ran away from me…. Can the carb be the culprit? To much air not enough fuel? Rebuilt and cleaned the carbs before install. Not ruling out crap in the carbs. Otherwise I’m lost lol

the carbs could be, richer them up and retest

 

[Pro-pulsion]

Check your throttle shafts for any play, they should be pretty much a clean solid fit. If you can lift them and wiggle them around or when you open the throttle fully and can get the throttle valves to slide back and forth, you need shaft rebuild kits. If those are good, pressure test the carbs and pay attention to the fuel pump body. I have seen so many leaking pump bodies lately from cheap aftermarket rebuild kits it makes tuning impossible.

 

[RiverRides]

Update
Air leak is solved engine is holding pressure, but it still ran away from me…. Can the carb be the culprit? To much air not enough fuel? Rebuilt and cleaned the carbs before install. Not ruling out crap in the carbs. Otherwise I’m lost lol

Sounds like it may be "dieseling". Just hot spots in the cylinder causing the fuel to ignite and cause run away.

 

[Sparkplug]

A runaway. Dieseling. And high revving engine are 3 totally different things.

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[Trevorbm]

A runaway. Dieseling. And high revving engine are 3 totally different things.

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It idles, blip the throttle it revs up full speed, pull the lanyard doesn’t kill it and I have to choke the carb out to kill it

 

[Trevorbm]

Carbs look clean no shaft play. They were not synchronized! Hopefully that’s the problem. Also throwing a little Yamahabond between the carb and intake manifold when reinstalling. Rebuilt with oem mikuni rebuild parts.

 

[Trevorbm]

One Butterfly Plate may be open too much. Check that Throttle Plates are synchronized, meaning they both have identical positions at all Throttle Pulls.

Throttle Cable too tight causing the Butterflies to be open too much at Rest/Idle. Give it slack or remove at Carb for Testing.

Idle Screw pushing on Throttle Shaft too much causing Butterflies to be open too far at Rest/Idle. Completely back out the Idle Screw to allow Butterflies to fully close.

Carb Base Gasket may be leaking. Dress both sides of Base Gasket with Sealant like 3 Bond 1211 to ensure leak proof seal.

The Air Leak could also be coming from the Fuel Lines, Fuel Filter or the Fuel Selector Switch.

And for God's Sake, always use Genuine Authentic Made in Japan Mikuni Brand Carb Rebuild Kits.

Mikuni Knock-Offs will cause way more problems than the 20 bucks you save on a fake Kit.

Post a Photo of your Engine Bay to check for anything unusual...

One Butterfly Plate may be open too much. Check that Throttle Plates are synchronized, meaning they both have identical positions at all Throttle Pulls.

Throttle Cable too tight causing the Butterflies to be open too much at Rest/Idle. Give it slack or remove at Carb for Testing.

Idle Screw pushing on Throttle Shaft too much causing Butterflies to be open too far at Rest/Idle. Completely back out the Idle Screw to allow Butterflies to fully close.

Carb Base Gasket may be leaking. Dress both sides of Base Gasket with Sealant like 3 Bond 1211 to ensure leak proof seal.

The Air Leak could also be coming from the Fuel Lines, Fuel Filter or the Fuel Selector Switch.

And for God's Sake, always use Genuine Authentic Made in Japan Mikuni Brand Carb Rebuild Kits.

Mikuni Knock-Offs will cause way more problems than the 20 bucks you save on a fake Kit.

Post a Photo of your Engine Bay to check for anything unusual...

Line is ran from the tank to the carbs nothing is securing them. Should I zip tie to be safe?

 

[Sparkplug]

It idles, blip the throttle it revs up full speed, pull the lanyard doesn’t kill it and I have to choke the carb out to kill it

That is an air leak initially, causing the engine to Rev out of control. And then dieseling, or pre ignition is causing it to run on its own.

I'd pressure test the entire setup and scrutinize it very hard. I'm not understanding how an air leak before or at the venturi would cause a runaway engine. Base gasket of carb and below is where I've seen air leaks.

I pressure test engines to 15-18 psi for 1 hour. With a bottle of soapy water. No drop at all is allowed. Stock engines don't really see the crazy signal that stroked engines do in my experience.

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[kevbo]

Just open the bottom adjustment screw, it controls low idle mixture.

 

[Trevorbm]

That is an air leak initially, causing the engine to Rev out of control. And then dieseling, or pre ignition is causing it to run on its own.

I'd pressure test the entire setup and scrutinize it very hard. I'm not understanding how an air leak before or at the venturi would cause a runaway engine. Base gasket of carb and below is where I've seen air leaks.

I pressure test engines to 15-18 psi for 1 hour. With a bottle of soapy water. No drop at all is allowed. Stock engines don't really see the crazy signal that stroked engines do in my experience.

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Pressure tested it. Put 10 psi in it lasted for an hour no drop.