How to diagnose carburetor vacuum leaks on your motorcycle

This is a simple video demonstrating the best method to detect vacuum leaks around your carb boots. Vacuum leaks are a fairly common problem in older bikes, and luckily they can usually be remedied quickly and easily, the first step however is diagnosis.

If your bike is running rough, won't rev up or has an irregular idle, before you take off the carbs and open them up, check for vacuum leaks. A vacuum leak occurs when too much air is introduced into the combustion chamber via a failure in one or more of the carburetor boots or other rubber mating surface. When too much air is sucked in, then the bike cannot combust the gasoline efficiently; there must be an optimal ratio. Therefore it is up to you to detect these leaks, and either plug them up or replace the carb boots.

In this particular bike, the engine was running VERY lean. Not only did we have vacuum leaks, but the baffles were taken out of the engine, and the previous owner had removed the air filter and neglected to install a new one. This is not only dangerous by allowing foreign material to enter the engine, but it also makes the engine run much hotter, as lean explosions create more heat than regular ones (think about blowing on a fire in your fire place: more air, hotter fire). This can be detrimental to a variety of parts in the engine, and it is a good thing we didn't ride this bike around too much before finding the problem.

If anyone has any questions as to why this is important or how to find/repair these leaks, leave a comment and I will respond.

Thank you

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[jquickj]

Great videos man! you're helping me out a bunch. I'm curious, what would the redneck repair method be to cheaply patch your boots? 

Thanks!

[mrmaxstorey]

@jquickj Spread some rtv over the cracks, and then run the engine. The vacuum should suck it into the cracks, sealing them. It doesnt last forever tho, the rtv breaks down after a while

[OldFordsRun]

I have a 90" Honda vfr 750f. I cannot get the carbs to come out, even though ive loosened the screws for the carb boots, and everything else it still wont come right out. How do i remove these carbs?

[mrmaxstorey]

@OldFordsRun Squirt some pb blaster or wd40 around the carb boots, then heat them up with a hair dryer. The rubber should get more pliable as the lubricant expands, freeing them

[Tulippimp1]

I have a yzf 600r 2007. It will only start on full choke. I have 4 carbs on my bike. doesnt starter fluid ruin rubber? let me know. it just started

[mrmaxstorey]

@Tulippimp1 No it shouldn't ruin the rubber

[Tulippimp1]

it dies when i back it off choke. i think i should try your method cus that was simple and right to the point

[55jpk]

cool brov!!! very nice piece of info, happy dayz sees ya 321gone.

[mrmaxstorey]

@neutronjet It could be the same problem, the boots may expand when they warm up and leak. Try the test with the bike warm and see what happens.

[neutronjet]

hey there, informative vid. my bike also has this surging, but it happens after riding for quite sometime(15km). the rpm which supposed to be at 2, surges to 3+. what is the cause of this?