Air leak, but where?

[trueblue550]

Hello everyone,
I'm wrapping up a complete rebuild of my 1980 GS550E and am having some idle issues. I was hoping to find some insight here before I start taking everything apart again.
Because my petcock has no prime position, only on and reserve, I used a mity-vac on the vacuum line to prime the carbs and it started right up. However, the engine took off to 5.5 - 6k before I killed it, and probably would go higher. I was using a little bit of choke. I removed the throttle cable, backed the idle adjust screw way out, fiddled with the choke a little, and got it to maintain an idle of around 2k. When I give it any throttle, the rpms hang or start to take off. I blocked the airbox with a shop rag for a few seconds and it still ran. So I'm thinking I have an air leak, but I'm not sure where. The intake boots and o-rings are brand new and I checked that all the clamps were snug. The carbs were sent off to moto-vintage for cleaning and rebuild, and I am incredibly confident in his work. I sprayed wd-40 around the entire flow path with no difference in idle. I appreciate any tips, thanks.

[phydeauxmutt]

Because my petcock has no prime position, only on and reserve, ...

Not sure about your "air leak" issue, but you need to re-visit your petcock functions.

Your petcock has RUN and PRIme. It does NOT have a REServe.

If it were a REServe, how would you activate it in the fast lane on the freeway?
That screwdriver slot is so you can activate the PRIme function before starting the bike. However, you should know that it does take a 'puff' of vacuum to start the fuel flowing after rotating the selector. Once it starts flowing, it will continue to flow until you turn the selector back to RUN.

[trueblue550]

Not sure about your "air leak" issue, but you need to re-visit your petcock functions.

Your petcock has RUN and PRIme. It does NOT have a REServe.

If it were a REServe, how would you activate it in the fast lane on the freeway?
That screwdriver slot is so you can activate the PRIme function before starting the bike. However, you should know that it does take a 'puff' of vacuum to start the fuel flowing after rotating the selector. Once it starts flowing, it will continue to flow until you turn the selector back to RUN.

No, it has reserve. It does not have a prime function.
i-QrvvSPJ-XL.jpg

[trueblue550]

It occurred to me that I had a hard time fitting all the air intake boots at once. They were oriented correctly with the sync ports down and to the outside, but they did not line up perfectly with the carbs. Not even close, really. This is a problem I have seen on the forum before, here.
I cant remember where I got the boots, probably Z1 enterprises, so I went ahead and ordered the $150 Suzuki set as a hopeful fix. I also decided that since I have a few heat cycles on the rebuilt engine and need to check torque on the cylinder heads, I'll recheck the valve clearances as well.

[metalfab]

If you spray an intake air leak with WD40 the engine will speed up. It is great to use for this purpose, and it doesn't hurt the engine.

[Who Dat?]

If it is a 1980 GS, it would be one of the very few that did not have the "dreaded '80 petcock".

I looked at the parts fiches and could not even find a 1980 GS550E. Lots of Ts and Ls, but the last E was 1979.

On the other hand, your picture shows a BS-series carb, which appeared in 1980.
The VIN label would have information for the model year designation ('80 should be T) and the production date.

I have seen VIN labels that showed an E when the actual bike was something else.
How about showing a picture of the rest of the bike?

[trueblue550]

If it is a 1980 GS, it would be one of the very few that did not have the "dreaded '80 petcock".

I looked at the parts fiches and could not even find a 1980 GS550E. Lots of Ts and Ls, but the last E was 1979.

On the other hand, your picture shows a BS-series carb, which appeared in 1980.
The VIN label would have information for the model year designation ('80 should be T) and the production date.

I have seen VIN labels that showed an E when the actual bike was something else.
How about showing a picture of the rest of the bike?

Here is the bike when I got it, I don't have any post-rebuild pictures yet.


And here is the headtube sticker, which is probably not useful anymore


It might be worth noting that in 1980, Cycle World Magazine had the following to say about the new quicker, faster, easier to start Suzuki GS550:

"An incidental change to the Suzuki’s fuel-delivery system is the new, two-position, vacuum-operated (fuel only flows with the engine turning over) petcock. Earlier Suzuki petcocks had a prime position to use if the float bowls were dry. If, for example, the bike ran out of gas, the rider could push to a gas station, fill the tank, switch to prime, wait a moment for the float bowls to fill, then start the bike. But the electric starter will turn over the engine fast enough to create sufficient intake vacuum to open the > petcock diaphragm, something kickstarting couldn’t do. This year the GS series machines don’t have a kickstarter (to save weight) so Suzuki engineers simplified the petcock as well."

Anyways, I'm going to wait for some new intake boots to show up in the mail, then I'll take the carbs out and double check everything.

[phydeauxmutt]

Well, the pictures show that it's NOT a T or an L.
The VIN tag is rather inconclusive.
The magazine article is interesting.

[salty_monk]

Leaks in the airbox (lid not sealing, foam not sealing), airbox to carb boots or carb to head boots would be my bet... Also worth making sure there are no leaks on the other side & that the vacuum line is secure & not split.