Will you damage anything running a bike with a vacuum leak?

Yesterday I went out for a ride and I noticed a couple new issues. I started the bike choked, let it run for a minute and took off on half choke. As the bike warmed up I let it out and didn't think anything of it as I wasn't stopping at all.

As I was driving around I noticed lots of popping as I let off the throttle. Then after a half hour or so of driving at 80 km/h (so the bike should be plenty warmed up), I got to a stop light and the bike was idling under 1k rpm.

From my reading this seems like a vacuum leak to me. I thought I had a can of carb cleaner so this morning I went to diagnose it a bit, but it turns out I don't. I started the bike up to listen to it and inspect a bit anyway and it needs to be fully choked or half choked to run, even after a few minutes. Blipping the throttle, the bike responds like it should but it holds the rpm high for a few seconds and lowers slowly.

If I have time today I'll go get some carb leaner to spray around my boots but I'm pretty sure thats the cause.

If they are the problem and I need to wait a couple weeks to get new boots in, can I still drive the bike like this? Is the biggest concern fuel economy and power, or would driving around a bit like this cause bigger problems? I live pretty close to work but the bike is my only vehicle.

I wouldnt ride it simply because I wouldnt want to take any increased chances of getting stuck. If you are able to ride wearing a short sleeve shirt, you shouldnt need a choke after its started up.

Could be a spec of dirt stuck in a carb.. It happens even with a filter on the line.

NewHavenMike said:

I wouldnt ride it simply because I wouldnt want to take any increased chances of getting stuck. If you are able to ride wearing a short sleeve shirt, you shouldnt need a choke after its started up.

Could be a spec of dirt stuck in a carb.. It happens even with a filter on the line.

Click to expand...

I'm not overly concerned about getting stuck, my main thing is just getting to and from work. If I got stuck at work I could leave my bike in the garage, walk home and deal with it later. So if thats really the only thing and I won't damage the bike, then I'll diagnose, order parts if needed, and take it to and from work until I fix it but not go anywhere with it where getting stranded would cause me problems.

Its 26 c (79 f) here right now. Even when its 30 (86) I still need to start it with choke and let it run for a minute or so before its good to go.

It could be something in the carbs, but hopefully not... I dread pulling those again.

Could be a vacuum leak...but feels like it could be carb tuning...cleanliness.
You won't hurt the bike short-term...really SHORT term.
BUT...if you are running rich you will eventually fowl the plugs and get stuck . If running lean you might end-up causing the engine to heat-up more than it should and warp the head.

Grabbed some carb cleaner and sprayed around my boots - definitely a vacuum leak on one carb. The clamp had come a bit loose so I tightened it up, which greatly helped. Theres still a small leak though, show by spraying the boot, but I'm not seeing any cracks or anything. I'll have to give a good look and see what I can find.

After tightening it, the popping in my exhaust is gone and the bike starts up and idles without choking it. It seems to me like it still holds revs high for too long and maybe hesitates a bit when I blip the throttle but it was pouring rain yesterday so I was just in the garage. I've never just sat and reved the bike in neutral before so I don't really have something to compare it to. I'll take it for a quick run around the block to see what I think as soon as its late enough for me to not feel like a jerk causing a bunch of noise on my road on a long weekend.

A major part of the carbs that is overlooked are the throttle shaft seals. They are kind of a pain to replace but its worth it. New reproduction boots can also be found very easily too.

Without replacing the shaft seals, Id say that getting the carbs "dialed in" is worthless, since the seals mentioned are an unknown factor for you at this time.. Its easy enough to check if they are synced and the timing is good though. Do all your final adjustments with the bike fully warmed up. That includes the idle set screw.

How do you have the carbs set up? Main jets? slow jet? idle mix screw?

I had the same hanging idle you mention, I just cant remember what I did to fix it... Honestly, I think it was the throttle shaft bushings

My mechanics did a full job on my carbs a couple months ago, no idea about the shaft seals though. I couldn't tell you anything about how the carbs have been set up, but the mechanics I use are awesome. The bike was running fantastically for the last couple months right up until the other day.

Do you know where you have seen new boots? I did a quick search yesterday and came up blank. I was going to check with my Yamaha dealer to see if they can get them in but they are closed on Sundays and todays a long weekend day off so I'll have to wait until tomorrow to call around. Its for a Maxim, not a SOHC.

Not familiar with DOHC bikes but Yamaha dealers can still get shaft seals for sohc bikes. Might be the same part, if not they're still the best bet.
Dealers probably can't get you carb boots.

DOHC carb holders are much harder to find.
http://www.xs400.com/threads/dohc-12e-intake-manifold-part-number.16702/#post-170348

If it just started happening, Id bet on it being a piece of dirt. Id keep an eye out on getting new carb boots anyways. I did a search and they seem to be going for around $50 a piece, and thats in the USA...

Fixed it! Turned out to *not* be my carb boot... the vacuum line from my petcock had a small crack on the back of it that I couldn't see unless I was at just the right angle.

I had extra vacuum line so I popped it off, put a new one on and the bike sounds fantastic in under five minutes. I now feel dumb for how much time I spent on this.