79 GS750L Popping Noise

[Lucius]

I recently purchased this 79 GS750L. I was running very poorly and the seller had set the idle over 2k RPM to keep it running. After a short ride, it bogged down and would not accelerate. I found out later that there was fire in the air box melting the intake tubes. I have since had the carbs professionally rebuilt and purchased new intake tubes and air filter. It starts up again and idles better than it did before, but there is a popping noise that sounds like a vacuum leak near the carbs, but I'm not sure. Also the bike still is prone to stalling, but that may just be due to carb adjustments needed. I posted a video on youtube where you can hear the sound.

http://www.youtube.com/watch?v=aaDNLWG-Pq0

Any suggestions?

[Dave8338]

Welcome to the fourm. Your 750 looks to be suffering from running lean. There are several causes of this condition but it being an 8-valve head, leads me to believe that the O-rings between the head and the rubber intake tubes are bad. Unless you changed them out, I can all but promise you, they need to be replaced. Also... the popping is through the intake and is likely the reason the bike caught fire in the first place.

Have you adjusted the valves yet?

Again... welcome to the fourm. Cliff will be along at some point to give you the full treatment but in the mean time, do a little digging here through the search feature (be specific when entering in the words/terms) and enjoy the best fourm on the web for the GS bikes.

[Lucius]

intake tubes

I did some looking at I think there might be a crack in the tubes from the head to the carbs. Guess I should've replaced them at the same time as the carb rebuild.

I have not done any sort of valve adjustment. I'm not sure I know how really. I'm trying to get the bike in good enough running condition where it can be ridden to a shop instead of towed. I've already put more money in the bike than I originally paid for it though, so if there's anything I can do myself, I will try.

Thanks for the assist. Been a member for a month or so now and just been reading a bunch and soaking it all in.

[slyone]

I just got a 79' 850 from my brother last week w/12k (not ridden much?) 2 of the 4 intake boots had rotted completely apart!? So I replaced those and the o-rings which are sold seperate-what a rip! anyways, mine does that on warmup, not as bad though. I believe that is just a regular misfire going on. I plan to clean carbs this winter and adjust valves while replacing leaky camcover gasket. I was thinking if not air/fuel then look at replacing points & condensors anyways as a PM job since recommended every 6k IIRC..? After that..maybe a coil issue or sticky/leaking valve? not really sure?

[Dave8338]

Cracked intake tubes are a certain cause for what you are hearing. If they are creacked or hard from years of use, they will leak air and the popping on the intake side of the engine is "usually" a lean condition. Air leaks and poor storage, is the reason most of the GS bikes get parked to begin with.

Also... you mentioned that the boots from the air box to the carbs were "melted". This leads me to believe that things got rather hot and may, as a result, have warped your air box to a point that it no longer seals correctly. This too, is an additional source of air leaking past the filter and into the combustion cycle (read even more lean) AND add to that, the fact that Suzuki set these bikes up to run lean to meat emissions restrictions on imported bikes, leads to a rather small window for your air/fuel ratio.

[spyug]

Welcome. First thing to know is if you own these old bikes you need to learn to work on them yourself or you're going on a straight road to the poor house. None of the basic stuff you will have to do to get her running right is that difficult and all the information and help you will ever need is here for the asking.

Popping, a mentioned, is a result of a lean condition usually caused by an air leak and intake boots and or o-rings ar often the culprit. You should change the intake boots and o-rings but for a quick test to confirm your suspicions about the boots you can paint them with liquid electrical tape which you should be able to get at the automotive store. Clean the boot with naptha or paint thinners then brush the liquid on the outside. Let it sit and dry out for a day or so. If you can get o-rings locally change them too but in the meanwhile you can smear on some silicone over the old rings before you put the boots back on. That should tide you over until you can order up some new replacements.

You definitely should do the valve adjustment too and details will be in your manual. If you don't have a manual check out Cliff's mega site. There will be one there along with a tutorial on the subject, I believe. Here is Cliff's site: http://members.dslextreme.com/users/bikecliff/

If you haven't wrenched before it may seem overwhelming but it really isn't. To gain some confidence have a look at some of the vids on Youtube. Just about anything you can do to a bike is profiled there.

Give it a go, you'll save a bunch of money and you'll gain a lot of satisfaction.

Good luck,
spyug

[Lucius]

Thanks

Thank you all for the great replies and information. I found the intake tubes with o-rings and clamps on ebay for $170, but I'm going to check the price at my local shop before ordering. I know I can put them on myself. When re-installing the carbs after getting them rebuilt, I only had two issues.

1. Where do all these lines go? Seem that there are a LOT of run-off/overflow lines from the carbs and such. I did manage to figure out which ones needed to be attached to something. Looks like there was a fuel overflow hose at the bottom of each carb as well as two hoses on either side of the vacuum hose. I would love to see pictures of someone else's gs750 so I can see where you routed the hoses.
2. It was very difficult getting the vacuum hose from the carb to the fuel petcock re-attached. Any helpful tricks for this?

I did purchase a service manual for the bike and it is very useful, although sporting very poor black and white pictures. I will look up how to adjust the valves. I am to take the bike back to the shop to get the carbs sync'd and adjusted once it's able to ride.

You mentioned a warped air box. I switched out the burnt k&n filter for a factory one, and it seems there is a lot of gap between the filter and the hole it sort of covers. Maybe a picture of a proper one would let me know if mine is unusable?

Thanks again everyone.

[slyone]

when replacing my intake manifold boots (head to carb) I found a few of the screws seized and in order to get a solid direct hit on the impact screwdriver thingy I had to remove the battery box cause I had a long but high quality bit I used.. Locally I paid $44.08 ea. and the o-rings were $2.28 ea. for a total with tax of $200.28

[BigD_83]

Thank you all for the great replies and information. I found the intake tubes with o-rings and clamps on ebay for $170, but I'm going to check the price at my local shop before ordering. I know I can put them on myself. When re-installing the carbs after getting them rebuilt, I only had two issues.

1. Where do all these lines go? Seem that there are a LOT of run-off/overflow lines from the carbs and such. I did manage to figure out which ones needed to be attached to something. Looks like there was a fuel overflow hose at the bottom of each carb as well as two hoses on either side of the vacuum hose. I would love to see pictures of someone else's gs750 so I can see where you routed the hoses.
2. It was very difficult getting the vacuum hose from the carb to the fuel petcock re-attached. Any helpful tricks for this?

I did purchase a service manual for the bike and it is very useful, although sporting very poor black and white pictures. I will look up how to adjust the valves. I am to take the bike back to the shop to get the carbs sync'd and adjusted once it's able to ride.

You mentioned a warped air box. I switched out the burnt k&n filter for a factory one, and it seems there is a lot of gap between the filter and the hole it sort of covers. Maybe a picture of a proper one would let me know if mine is unusable?

Thanks again everyone.

http://www.boulevardsuzuki.com/fiche_section_detail.asp

$31/ea intake and clamp

[RobNeal4]

Intake Tubes

The prices you quote seem high. I bought some for my GS750 a while back and although I don't remeber the exact cost, it certainly wasn't that much. Have you tried Robinsons in the UK? Great bunch of people who know their stuff. Shipping costs won't be too much. try www.robinsonsfoundry.co.uk or call +44 1227 454366. Rob