Best way to fix an air leak

[Endee]

Hi all, couple quick questions hopefully.
I found an air leak on my right side cylinder where the rubber boot attaches to the engine. I took the boot off to inspect and it is in good shape. I noticed what looked like some dried rtv or sealant on the gasket. I did not remove the gasket between the engine and the boot.
What's the best way to fix the air leak? New gasket? Or is there a hi temp rtv I can use to help from a good seal?
Also, when checking for air leaks is it better to use starter fluid or wd40? Will the starter fluid damage rubber or seals?

 

[Mailman]

I have also spent a fair amount of time chasing intake air leaks, I know a lot of guys swear by spraying products like WD-40 or starter fluid. But I recently tried another approach. Using the following items.

An ear plug for one ear to block out ambient noise, a length of 1/4" vinyl tubing and a flashlight. Sticking one end of the tubing in my ear , I probed around looking for the leak and as I got closer to it, the sound of the leak got louder, it allowed me to zero right in on it. It worked really well for me.

 

[Endee]

Haha like a stethoscope. I have heard of mechanics being able to use a long wooden dowel to tell which cylinder isn't firing correctly on car motors.
What's the best way to fix it though? I picked up some some stuff called motoseal (I think) from advance Auto parts. The guy there says he uses it all the time for dirt bikes. Real thin layer, let get tacky for an hour, then torque to spec.

 

[Signal]

I have used Yamabond no 4 (the same as used on the engine cases) along with new gaskets before to seal inlet manifolds to the head.
Clean up the excess with acetone on the inside and out.
I was reluctant to use it as I considered it a bit of a bodge but it fixed the leak.
The only downside was the extra clean up when the rubbers were removed at a later time.

 

[5twins]

I use a small amount of sealer only on the manifold side, not the engine side. I want the gasket to come off with the manifold and not stick to the motor. Make sure the gasket is an original or original style. By that I mean thick, about 1/8". Some of the aftermarket replacement manifolds come with thin paper gaskets. I won't use those.

 

[Downeaster]

Blue silicone snot applied with a trowel seems to be the fix of choice by previous owners...

 

[mrtwowheel]

I've got nothing against using high temp silicone there, yeah it's messy but a toothbrush and rag does a good clean up. I walk away from a bike sale if it has silicone showing at engine gaskets. I've used black silicone to seal cracks in manifolds after cleaning with lacquer thinner.

Scott

 

[Endee]

I used this stuff called motoseal that was recommended. The guy at the store said he used it on dirt bikes for the same problem all the time.
It worked, it fixed the leak at the boot with no mess, but I am still getting a terrible at idle. I'll have to keep tracking it down.

 

[grizld1]

Endee, some vacuum leaks can't be fixed with sealant. I once had the mount off the right cylinder of an XS650 motor half a dozen times before I discovered that the PO had cranked down hard enough on the mounting screws to extrude the edges of the holes. I still don't know how the threads could have survived that, but they did. A few strokes with a fine flat file fixed the problem without use of gumpucky. It's also common for overtightening to deform the flange on the mount, so that only the points under the screws are making firm contact with the head. Again, all the slather you can buy won't seal that defect; sometimes a few strokes with a file under the screw holes on the mount will let it seat evenly, sometimes the mount has to be replaced.

 

[Endee]

I'll double check that. There was 0 backfire and popping before I took the carbs off and apart, so I feel like it's something I did.

 

[peanut]

I shouldn't give too much heed to warnings about how difficult it is to remove gasket cement from the cylinder and inlet rubbers its one of the biggest bits of bullsh*t bandied about.
Unless you plan on stripping the engine every few months the likelihood of you ever needing to remove the inlet rubbers in the next 5 years are probably nil.

I'd rub the engine side of both rubbers on some sandpaper and wet n dry until there are no high or low spots then apply Threebond 1184 or Hondabond to both sides of a fresh gasket and fit tighten and forget. Be aware that over-tightening the inlet rubbers will distort the metal holders and cause distortion and leaks

 

[gggGary]

spray carb cleaner while chasing leaks. My go-to sealer is hylomar a bit hard to find but I make sure I have a tube on hand. It's a sealer not an adhesive so dis-assembly clean up at a later date is easy. That said I seldom use any sealer on the intakes.... Best to shove some paper towel in the intakes before scraping off old sealers.
Confession: I watched an entire "keep the intake clean" rag get spit out the exhaust of a running 4 cyl MC motor in tiny little bits and threads, AFAICT no harm done!

 

[grizld1]

Permatex Perma Seal is good too, very resistant to fuel fumes and ethanol gas. It's the best stuff I've found for sealing adapter plates for aftermarket petcocks. NAPA stocks it.

Edit: Right, Scott, it's PermaShield--thanks for the correction.

 

[mrtwowheel]

Permatex Perma Seal is good too, very resistant to fuel fumes and ethanol gas. It's the best stuff I've found for sealing adapter plates for aftermarket petcocks. NAPA stocks it.

Permatex PermaShield, this stuff? Says, for intake manifolds.https://www.permatex.com/products/g...uel-resistant-gasket-dressing-flange-sealant/

 

[Endee]

I used the grey stuff. Took the manifold off, dressed it with the gunk, let it sit for an hour, put it on. Next day went and fired up the bike. It is still popping, however not as bad. Check for air leaks with starter fluid, everywhere I spray speeds up the damn bike now.
I'll track it down. I'm just annoyed that it is running worse then before I tore the carb down.

 

[peanut]

it really only needs the minutest of air leaks on the inlet side to screw your idle .
You could try covering half your carb venturi with your hand and then test for air leaks around your carbs with an accelerant . By shutting off some of the normal idle air supply you'll increase the need for the engine to pull air through any unwanted leak sources and it should make it easier to detect the leaks.

I found a significant air leak around my choke assemblies where they bolt to the carb body so I dressed the choke housings flat on some fine wet n dry then applied a smear of Threebond to both sides of those gaskets which cured the leaks I had.

Instead of using an accelerant which is difficult to direct to a small area you could try using a soapy water spray instead so that the soap bubbles show up any leaks

 

[Paul Sutton]

Gumpucky - What an amazing word!!

 

[Endee]

I didn't know you could use soapy water. I had planned on killing one of the cylinders to try and help track it down. Would that be the same as covering the Venturi?
I didn't pull and clean the choke plunger or the mix screws, so I'm not sure if that could be it. And I'm still a little lean. I want to replace the vacuum lines with some smaller tube. Not sure what is one there now but it pulls off very easy.

 

[peanut]

I didn't know you could use soapy water. I had planned on killing one of the cylinders to try and help track it down. Would that be the same as covering the Venturi?
Soapy water is routinely used to test pipes and fittings for gas leaks,...( although professionals buy a really expensive, brand name, gas test solution but its effectively the same thing ) A little spot of washing up liquid to a pint of water is sufficient as long as there are plenty of little bubbles . I make a solution in an empty kitchen /bathroom spray bottle (shake it hard), so I can direct the spray.
be aware that you are looking for the soapy bubbles to be sucked into the joints not blown out

I didn't pull and clean the choke plunger or the mix screws, so I'm not sure if that could be it.

Nor did I for the first few times I cleaned my carbs ...big mistake !... when I finally decided to check everything I found that the gaskets were fubar on both choke bodies and were a source of air leaks.
If you haven't removed them and checked how do you know they are air tight ?


And I'm still a little lean. You will be if you have an air leak somewhere
I want to replace the vacuum lines with some smaller tube. Not sure what is one there now but it pulls off very easy.
Vacuum fuel lines are a right pita imho and just add an unecessary complication to the fuel delivery system . I would strip them out and fit 2 x simple 2$ inline fuel taps and inline filters if you are worried about originality,.... otherwise there is a good tech guide on converting petcocks somewhere in the tech section

 

[weekendrider]

How would you peek down through there to see the bubbles? They won't show on the outside cause the solution is being sucked in? Or does a vacuum leak push air not suck it?
Now on a pressure test I can see have seen this working, it just never occurred to me to try it on a vacuum test.