Modern Vespa

Think I have an air leak.
I recently installed a DR 85 kit on my 50s. It seemed to be running OK at first, but now sounds like I have an air leak.

Symptoms are that after riding a mile or so, it wont rev down with throttle off, and I've had to use the kill switch to drop the revs down.

Now that the carb is clean, it starts first kick and idles well.

Yesterday, I dropped the motor again and basically checked every bolt I had touched during the install. A couple needed a tweak but generally it all seemed OK.

Couple of thoughts.

1. I used the supplied base gasket smeared in oil as per the instructions, [ original barrel had no paper gasket ] but in hindsight I may not have cleaned off the previous gasket sealer as well as I could have. I don't recall there being much of it left, but the surface wasn't dead smooth.

2. I noticed yesterday the inlet bushing was barely through the frame enough to get the clamp completely on to it. Almost as if the engine was sitting too low. I hadn't noticed that previously but may be something.

3. I had not removed a barrel before, so I am wondering how you torque the barrel nuts down when you need an open ended spanner and can't use a torque wrench. They seemed pretty tight but.....[ the 50s does not have studs that go through barrel and head, but separate barrel nuts and head bolts ]

I dropped the engine yet again and this time took off the inlet manifold hoping it would be faulty.....it looked perfectly normal, although the bellows did have some oily fuel sitting in it and there was a bit of moisture coating the manifold where it met the case.

So while I had it off, I cleaned it all up and grabbed the clamp and went to slide it over the bushing and it wouldn't go over, which was pretty strange since I had just loosened it off the bushing to take the carb off.

I figured I must have at some point not located the clamp fully on the bushing and it was clamping only using one side of the double band design.

It's late and wet outside so will have to wait until morning but if that wasn't the problem I'm lost for answers

Good find. I noticed the rubber bellows are always too long now a days. Once installed I trim it back some so that I can assure the manifold sleeve, felt ring, slits in the manifold sleeve and carb clamp can all be accessed. If you dont already, sit on the rear of the scooter to compress the rear suspension and force the manifold forward. That will help you get more of the clamp on and get it secure.

If you still have issues try coating both sides of your manifold gasket with some kind of semi liquid gasket goop (permatex motoseal, RTV) something besides the paper gasket. That and a trick Voodoo introduced of sealing the 4 slits in the manifold sleeve and carb with RTV help limit air leaks.

Yeah thanks. Hopefully that was the reason.....its the most obvious thing I could see. Ive removed the smallframe carb so many times I could do it blindfolded, so that was a bad miss on my part.

Good idea to sit on the seat, didn't think of that. i have seen the voodoo slot seal idea and always thought it was going to extreme...now I know I'd try anything to cure an air leak.....the time I've spent on this so far!!!

This is the first time I have removed an inlet manifold. I was surprised the bellows was so tight onto the manifold sleeve, and also surprised the manifold sleeve had slots which were captured by the bracket on the manifold. I wondered how the heck you got the whole thing apart as I know SIP sell the items separately.

Leak down test? I can't recommend this enough. Never would have found my air leak without pressurizing the engine. You're just guessing right now.

It was the poor carb installation job I did with the clamp ring.

Well, maybe it isn't quite fixed. It seems to rev out slightly after a WOT run...not crazy reving but higher than it should.

I need to upgrade the clutch spring so I think I'm gonna pull the motor out completely and go over the whole 85 DR kit install. I had some specific questions it would be good to get answers to.

1. The felt washer is behind the inlet sleeve slots and not in contact with the end of the carb. Does it need to be? I can pull washer out, regrease and place so that card butts up to it.

2. I know about sealing inlet sleeve slots with RTV, so will look at that.

3. Kit came with base gasket which I smeared with oil. Should I smear with RTV?

4. Barrel is secured to case studs with nuts. How do you torque correctly when you need an open ended spanner?

5. Head has no gasket. Should I smear with RTV?

I'm not set up to do a leak down test, but it wasn't leaking before I did the kit, so I'm pretty convinced it's the barrel to case or head to barrel.

cheers

If you are at all concerned about air leaks (and should be as routine on any build), then a leak down test is the way we check.

A pump with a gauge, an old inner tube, jubilee clips and some ingenuity is about all you need.

Thanks Jack221

I just read through the "serious about leaks" thread and had missed the posts with the inner tube method. Going to try that. I think even I can manage that kind of tech!

1. Yeah it needs to be tight against the stub on the carb to seal well. Try rotating the aluminum sleeve & make sure it's pushed all the way to the back w/ its notches on the tabs on the manifold.

2. Yep.

3. You could, or heavy grease.

4. You could use a hanging spring scale on the end of your wrench and do some math. I confess I just get them good & tight.

5. I use nothing but my friend swears by spray copper base hi temp RTV.

I don't think a leakdown test can test all the connections in this pesky carb/manifold system because you are gonna get some air thru the carb, even fully closed, so I guess you'd do the test w/ the rotary valve in a closed position?