Help identifying a hot engine vacuum leak

[Rented Mule]

I just completed restoring a 1978 XS650SE. I learned a lot from the project, sometimes more than I wanted, but there is one area I’m having a very difficult time with, that is finding a carburetor vacuum leak. The carburetors were professional rebuilt, new carb boots installed, and fuel tank cleaned. The engine starts and runs extremely well, however, when the engine gets hot, usually around 50 miles or stop and go traffic the RPM, will climb to 3000. To get the RPM down, I have to drag the engine down with the transmission engaged in order to get a normal 1200 RPM idle speed. These symptom I believe, are indicators of a carburetor vacuum leak of/or faulty components.
I have watched numerous YouTube videos demonstrating various methods used to find vacuum leaks. Such as, starter fluid, carb cleaners, or propane sprayed in and around engine components. All of which are extremely flammable and in my case with a hot engine, I’m sure, will ignite the vapors.
Any thoughts or ideas on which areas I should concentrate on? And, which aerosol application method would you recommend using to identify a vacuum leak on a hot engine?

 

[member28833]

Check that the flat blade screw that covers the mixture screw is tight.(both sides) Those could be easily forgotten to tighten securely after making mixture screw adjustments.

 

[Rented Mule]

Thanks, I’ll check. I just remembered, I used an aftermarket foam air filter square I cut to size, oiled, and installed in the air boxes. Could this possibly contribute to the problem? It shouldn’t but I’m out of ideas and options.
I see you have capped off you vacuum port from the carb to the petcock. Any problems there?
I’ve done the same to my bike not wanting to fork over $80 + $10 for shipping to Mike’s for a rebuild kit.

 

[Jim]

All of which are extremely flammable and in my case with a hot engine, I’m sure, will ignite the vapors.

Highly unlikely a hot engine will ignite flammable vapors. I've had gas pour all over a hot engine before without causing a fire.

I use CRC contact cleaner. It evaporates pretty quickly without leaving any residue or oily mess. Use the straw it comes with and just short squirts directly on areas where a leak is possible/suspect. Being heat related, I'd suspect the intake boots.

It's a long shot, but a sticking advance will cause a high idle.

 

[member28833]

I see you have capped off you vacuum port from the carb to the petcock. Any problems there?

Temporarily capped off, tank is dry still.

 

[Rented Mule]

Great idea Jim. I think I have some contact cleaner on hand. As for the boots they’re new with fresh gaskets. However, they are in an area that heat expansion could be a problem.

 

[Jan_P]

I had a leak at the intake manifold to the cylinder ..
2 Allen bolts a piece at 3 o clock on Mr Machines picture
Those on the inside can be awkward to get to
And also one on the outside I cut one allen wrench down to half that length
To get it in .

 

[GLJ]

It's a long shot, but a sticking advance will cause a high idle.

Very good thing to check.

 

[Rented Mule]

This is a real head scratcher. As far as I know, everything between the carb and cylinders has been replaced to include rubber boots, bolts, and clamps. My blind mans search will continue.

 

[Rented Mule]

I had a leak at the intake manifold to the cylinder ..
2 Allen bolts a piece at 3 o clock on Mr Machines picture
Those on the inside can be awkward to get to
And also one on the outside I cut one allen wrench down to half that length
To get it in .

View attachment 195434

Excellent suggestion. It’s tips like this that you will never find in repair manuals. I think it would be a great idea to see a stand alone forum thread where homemade or improved tools can be displayed and discussed.

 

[Mailman]

I had a persistent air leak on my ‘77 that I was having a hard time locating. I finally did this. Cut a piece of tubing about 15” long, put an ear plug in one ear to block out ambient sound, stuck the tubing in my other ear and began probing all around the carburetors while the bike was running. You wouldn’t believe how well it worked, an intake air leak makes a pretty good sucking sound and when you get close to it with the hose , you can zero right in on it.
Mine turned out to be the choke housing on the left carburetor had a bad seal.

cheap tools

 

[bosco659]

I had a persistent air leak on my ‘77 that I was having a hard time locating. I finally did this. Cut a piece of tubing about 15” long, put an ear plug in one ear to block out ambient sound, stuck the tubing in my other ear and began probing all around the carburetors while the bike was running. You wouldn’t believe how well it worked, an intake air leak makes a pretty good sucking sound and when you get close to it with the hose , you can zero right in on it.
Mine turned out to be the choke housing on the left carburetor had a bad seal.
cheap tools
View attachment 195455

I tried that, but when I stick the tube in my ear it comes outa the other one. Lol.

Good tip… I’ll actually try that, thanks.

 

[Rented Mule]

I had a persistent air leak on my ‘77 that I was having a hard time locating. I finally did this. Cut a piece of tubing about 15” long, put an ear plug in one ear to block out ambient sound, stuck the tubing in my other ear and began probing all around the carburetors while the bike was running. You wouldn’t believe how well it worked, an intake air leak makes a pretty good sucking sound and when you get close to it with the hose , you can zero right in on it.
Mine turned out to be the choke housing on the left carburetor had a bad seal.

cheap tools
View attachment 195461

Great tip. I plan on putting it to work.

 

[Rented Mule]

Update, I took the bike on a 25 mile warmup ride yesterday. When I got home, I continued to idle the engine while I retrieved my tools, aerosols, and a box fan. After repeatedly revving the engine above 3000 RPM the engine fell smoothly, each time, back to 1200 RPM and continued to idle normally. I don’t know what happened but it seems to be working normally for now. So it fixed itself but I’ll always have that nagging feeling I’ll see this problem again at the most embarrassing or inconvenient time possible. Like Yogi Berra said, “It ain’t over till it’s over.”

 

[member28833]

I'll betcha the pilot circuit of a carb was partially blocked.
Continuing to ride it, think, and test it without a panic carbureator disassembly was smart. I've had a very similar experience with BS38's and continued to test jetting with lots of throttle. That time my hanging idle went away just as yours has.

 

[5twins]

When you assembled the advance rod, did you grease it and the bushings in the ends of the cam it turns in? The bushings at each end of the cam have grooves cut into their I.D. .....

You want to pack them full of grease .....

This will provide a reservoir of lube for the advance rod and keep it lubed for years to come. But still consider this a service item. You'll want to re-do it every 5 years or so. The shop manual recommends a grease with moly in it.

 

[ram_may]

Hi.
Don’t know if something of this could be useful. Both seems bizarre or unconventional solutions, but could work for search unwanted air flows.

• Colored smoke https://www.aliexpress.com/item/10000305728521.html

• Smoke a large cigar and blow the smoke in the vicinity of the suspicious location.

Hope help.

 

[HOTEL71]

I just completed restoring a 1978 XS650SE. I learned a lot from the project, sometimes more than I wanted, but there is one area I’m having a very difficult time with, that is finding a carburetor vacuum leak. The carburetors were professional rebuilt, new carb boots installed, and fuel tank cleaned. The engine starts and runs extremely well, however, when the engine gets hot, usually around 50 miles or stop and go traffic the RPM, will climb to 3000. To get the RPM down, I have to drag the engine down with the transmission engaged in order to get a normal 1200 RPM idle speed. These symptom I believe, are indicators of a carburetor vacuum leak of/or faulty components.
I have watched numerous YouTube videos demonstrating various methods used to find vacuum leaks. Such as, starter fluid, carb cleaners, or propane sprayed in and around engine components. All of which are extremely flammable and in my case with a hot engine, I’m sure, will ignite the vapors.
Any thoughts or ideas on which areas I should concentrate on? And, which aerosol application method would you recommend using to identify a vacuum leak on a hot engine?

I noticed you replaced the stock air boxed with some type of PODS that clip on the back of the carbs.
The bike originally had stock air boxes that had an air tube that connected them by a T,
then another tube went from the T
to the top of the motor. is the top of the motor capped?

since this tube set up is not being used anymore?

This will def cause the overheating, massive hot air you are feeling on your legs while riding.
let me also take a stab..when hot and running .....ya cant find neutral?

 

[Tully]

When you assembled the advance rod, did you grease it and the bushings in the ends of the cam it turns in? The bushings at each end of the cam have grooves cut into their I.D. .....

You want to pack them full of grease .....

This will provide a reservoir of lube for the advance rod and keep it lubed for years to come. But still consider this a service item. You'll want to re-do it every 5 years or so. The shop manual recommends a grease with moly in it.

quick question if these are not lubed will these cause a lot of heat? enough to cause the new electronic ignition I have installed to fail? can it cause a lean condition?

 

[5twins]

No, I don't think it would cause much extra heat, the advance rod doesn't move much. It will cause it to stick though, so it won't smoothly go from retarded to advanced and back again. This causes all sorts of running problems because the timing isn't matched to the engine speed as it should be.