Engine Stops While on Idling Rev

[schwalbewurger]

Hi everyone,

So my friend and I managed to start a 1978 GS750 engine running, special thanks to TrialsRider from CafeRacer.net for helping us out! However, we ran to a problem when running the engine. So the engine runs whenever we turn the choke on and increase the throttle. However, when we release the throttle and put it at idling rev, the engine stops.

There are two things we suspect that may happen: 1) the engine isn't warm up enough and needs more time under choke and 2) the carburetors aren't set/tuned properly because the guy who had the bike before us, even though he cleaned up the carburetor, might not have tuned it properly.

Is there any fix to this?

Link to video of starting the engine:

 

[hillsy]

Sounds like the carbs are in need of cleaning / balancing. I'm guessing it's been sitting for a long time? Doesn't matter what the PO says he did - if it's been sitting for a long time the carbs will probably be gummed up.

Does it have an airbox? What sort of exhaust?

Some more pictures of the bike would help.

 

[Rich82GS750TZ]

See Top 10 Newbie Mistakes thread linked in my signature. Sounds like you have a lot of work to do.

 

[allojohn]

Clean the carbs (24 hr dip) follow the instructions closely - NO SHORT CUTS . Looks like you have pod air filters - up the main jet to 117.5, check the pilot (bleeder jet) is 15. Good luck.

 

[Steve]

A couple of things I noticed from your description and the video:

1. Using the "choke" and turning the throttle are not usually done at the same time. If the engine is cold enough to need the "choke", you don't move the throttle AT ALL.

2. Engine dies when throttle is released. Quite possible it's simply an idle screw adjustment, but it's more likely that the carbs need to be thoroughly cleaned and rebuilt. When the "choke" is applied, you adjust the engine speed with the "choke" control, not the throttle, and it should be revving quite nicely over 2000 RPM.

3. In the video, it sounds like you do not have an exhaust system bolted to the engine. I will be looking for your next post, which will have the title "I killed my exhaust valves, how do I change them" or possibly "Does anyone have a replacement engine for a '78 750?"


Have you checked the valve clearances?
Have you verified anything that the previous owner has claimed to have done?
Do you know how to do any of this yourself?

You have come to a great place for help, but we need to know a little background before casuually tossing suggestions in your general direction.

.

 

[schwalbewurger]

Sounds like the carbs are in need of cleaning / balancing. I'm guessing it's been sitting for a long time? Doesn't matter what the PO says he did - if it's been sitting for a long time the carbs will probably be gummed up.

Does it have an airbox? What sort of exhaust?

Some more pictures of the bike would help.

Thank you for the response and help!

@hillsly Planning to work on the bike today with my friend and will update with more pictures of the bike. It does have an exhaust that we have not installed yet. Will install it the next time we try to run the engine. No, it does not have an airbox. Will install an airbox the next time we try to run the engine.

 

[schwalbewurger]

See Top 10 Newbie Mistakes thread linked in my signature. Sounds like you have a lot of work to do.

Thank you for the response and help!

@Rich82GS750TZ Just read your Top 10 Newbie mistakes linked in your signature - definitely really helpful! Yes, we still have a lot of work to do - will need all the help from everyone so I'll try my best to update everyone with every details possible. Have a question regarding point #3 on your newbie mistakes - how do I adjust the valves and what does "checking shim" mean? We've never adjusted engine valves before. Founded tip #7 to be extremely helpful as well - didn't know that Suzuki botched themselves by wiring one of the wires to the left handlebar. Good thing that we connected the three R/R wires directly to the stator. Have a question regarding fuses - the original wiring diagram calls for 15A fuse, should I use 20A instead as recommended by your tips (assuming that we are using 14 AWG wires for ignition system)? On point #4, is there a calculator/formula for knowing what kind of rejetting is needed? How do we check if a carburetor has the right jet?

 

[schwalbewurger]

Clean the carbs (24 hr dip) follow the instructions closely - NO SHORT CUTS . Looks like you have pod air filters - up the main jet to 117.5, check the pilot (bleeder jet) is 15. Good luck.

@allojohn Thank you for the video! Will be useful when we try to clean the carbs. A question - what's the difference between having pod air filters and an airbox? Does it serve the same purpose?

 

[schwalbewurger]

A couple of things I noticed from your description and the video:

1. Using the "choke" and turning the throttle are not usually done at the same time. If the engine is cold enough to need the "choke", you don't move the throttle AT ALL.

2. Engine dies when throttle is released. Quite possible it's simply an idle screw adjustment, but it's more likely that the carbs need to be thoroughly cleaned and rebuilt. When the "choke" is applied, you adjust the engine speed with the "choke" control, not the throttle, and it should be revving quite nicely over 2000 RPM.

3. In the video, it sounds like you do not have an exhaust system bolted to the engine. I will be looking for your next post, which will have the title "I killed my exhaust valves, how do I change them" or possibly "Does anyone have a replacement engine for a '78 750?"


Have you checked the valve clearances?
Have you verified anything that the previous owner has claimed to have done?
Do you know how to do any of this yourself?

You have come to a great place for help, but we need to know a little background before casuually tossing suggestions in your general direction.

.

@Steve I see. We will try to turn on the choke and not the throttle the next time we start the engine. We don't have a tachometer because the person before us removed all the speedometer and tachometer. Is there a way to try test the RPM without it? How do I check the valve clearance? We have not confirmed everything the previous owner has done. The previous owner said that the carbs have been cleaned - but that does not mean that it is really cleaned out, right? The pod air filters are new but not the carbs. Will install an exhaust system before we try to run the engine again. Can you provide me a brief explanation on why an exhaust system is needed so that it doesn't damage the valves? A lot of the things we don't know how to do it by ourselves but we're willing to learn...

Is there a right way to adjust the idle screw adjustment? Like how do we test to tune it so that it's working properly?

 

[allojohn]

@allojohn Thank you for the video! Will be useful when we try to clean the carbs. A question - what's the difference between having pod air filters and an airbox? Does it serve the same purpose?

Pods allow a lot more air through the carbs than the relatively restrictive airbox, therefore you must increase the amount of fuel to bring the mixture back into harmony.

 

[Big T]

You need to go to the BassCliff site, download a factory service manual, and read thru the tutorials.

https://www.thegsresources.com/_forum/showthread.php?196338-New-Members-CLICK-HERE!!


What you really need to do is slow down and do some reading. First, go to the GS Owners forum and introduce yourself and your bike
Then, edit your profile to include the year and model of your bike

Read thru the link above and especially every link there, including the BassCliff website and bookmark that page

While we believe we have the best technical information available anywhere, we are also a community of like minded bike nuts

You probably want to start a thread in Projects/Rebuilds for your bike. You are already posting in the Technical forums

 

[Rich82GS750TZ]

thank you for the response and help!

@rich82gs750tz just read your top 10 newbie mistakes linked in your signature - definitely really helpful! Yes, we still have a lot of work to do - will need all the help from everyone so i'll try my best to update everyone with every details possible.

I cannot lay claim to this wonderful collection of collective wisdom, but have referenced it from time to time.

have a question regarding point #3 on your newbie mistakes - how do i adjust the valves and what does "checking shim" mean? We've never adjusted engine valves before.

Many references here and on bikecliffs website on how to do this. Steve has an offer shown in his signature to email him for his spreadsheet. Take the guesswork out of what sizes you need and what you have and may be able to swap some around. Please read up on the procedure before attempting. All steps very important. Like “don’t turn the engine without a shim (or at least a quarter)in all buckets.” doing so could damage the surface of a cam lobe scraping over an empty bucket. I have never done this as my engine had a different method of setting valve clearance. Proper valve clearance is important to establish so as not to ruin valves and other things in the engine. You also cannot tune the carbs until this is done.

founded tip #7 to be extremely helpful as well - didn't know that suzuki botched themselves by wiring one of the wires to the left handlebar. Good thing that we connected the three r/r wires directly to the stator. Have a question regarding fuses - the original wiring diagram calls for 15a fuse, should i use 20a instead as recommended by your tips (assuming that we are using 14 awg wires for ignition system)?

Sorry, can’t advise on fuse size, not an electrical engineer, but there are some here that would be happy to answer that question. I aren’t smart enough. I do know that all GS would benefit from a better RR than suzuki gave them. A Shindengen SH775 is what I and many others have upgraded to. My charging system story in my signature is just like many others.

on point #4, is there a calculator/formula for knowing what kind of rejetting is needed? How do we check if a carburetor has the right jet?

Don’t know about a calculator but if you’re running stock exhaust or any but the most free flowing exhaust and you’re running the stock air box and OEM filter, there should be no need to rejet. Most believe that the gss were lean from factory and a slightly larger jet that factory would richer the mixture nicely. I can’t speak to this as all is stock on my bike.

This link is to a page found on BikeCliff’s Website (there that is again, must be a great place. Hint: It is.) where you can look up the stock jetting from factory for your or any GS.

And as Big T said, put your bike info in your signature, don’t rush this. Start a thread in GS Owners forum. Take one thing at a time. If I were you, Since you pretty much know it’ll run, don’t risk running it again til you KNOW valve clearances are in spec, pipes are on, and airbox/filter are on.

Get it starting, running, charging properly, lighting and signals working.

Then brakes and tires.

Forget how It looks until the important stuff is done.
....................

 

[schwalbewurger]

Pods allow a lot more air through the carbs than the relatively restrictive airbox, therefore you must increase the amount of fuel to bring the mixture back into harmony.

@allojohn If our carbs have pods rather than airbox, how do I know which jet I should change to adjust the amount of fuel (like do I go up one size or go down one size)?

 

[schwalbewurger]

You need to go to the BassCliff site, download a factory service manual, and read thru the tutorials.

https://www.thegsresources.com/_forum/showthread.php?196338-New-Members-CLICK-HERE!!


What you really need to do is slow down and do some reading. First, go to the GS Owners forum and introduce yourself and your bike
Then, edit your profile to include the year and model of your bike

Read thru the link above and especially every link there, including the BassCliff website and bookmark that page

While we believe we have the best technical information available anywhere, we are also a community of like minded bike nuts

You probably want to start a thread in Projects/Rebuilds for your bike. You are already posting in the Technical forums

I think that sounds like a good idea. I have founded the service manual since we started fixing this bike, however there's much stuffs that we haven't read in the service manual (I mostly used the service manual to rewire the bike). Will follow your advice and go to the GS Owners forum to introduce myself and the bike. Will start a thread in Projects/Rebuilds for my bike as well.

 

[schwalbewurger]

And as Big T said, put your bike info in your signature, don?t rush this. Start a thread in GS Owners forum. Take one thing at a time. If I were you, Since you pretty much know it?ll run, don?t risk running it again til you KNOW valve clearances are in spec, pipes are on, and airbox/filter are on.

Get it starting, running, charging properly, lighting and signals working.

Then brakes and tires.

Forget how It looks until the important stuff is done.
....................

@Rich82GS750TZ Thank you for the info! Will try to start a thread in the GS Owners forum. Will not try to run it again until the valve clearance are in spec, pipes are on, and the pods are on. I have a question though - if we are using pods for air intake instead of an airbox (because the previous owner didn't left an airbox on the bike but instead a pods on it), does that mean we have to rejet the carbs? I've emailed Steve for the spreadsheet and will let y'all know the process in rejetting it.

 

[Rich82GS750TZ]

@Rich82GS750TZ - if we are using pods for air intake instead of an airbox (because the previous owner didn't left an airbox on the bike but instead a pods on it), does that mean we have to rejet the carbs?

Yes. That’s as good an answer as I can give. I’ll bet there are hundreds of threads on the forum about rejetting for pods. You’ll see recommendations in those threads or from others chiming in here with experience. I have none. Depends on if you have VM or CV type carbs, and what kind of pods. All pods are not created equal. Cheap ones provide nearly no restriction and may actually block important ports on the intake side of the carbs. Again, others with experience will have to advise you on specific jetting with specific pods, on specific carbs. No easy answer here.

Big tip on using the site’s search function:
always used Advanced.
always be specific with your search words
always search titles only. If you don’t, you won’t get good search results at all.

 

[allojohn]

@allojohn If our carbs have pods rather than airbox, how do I know which jet I should change to adjust the amount of fuel (like do I go up one size or go down one size)?

You need to do a lot of reading on the forum and the workshop manual which is available on the BassCliff site you've already been directed to.
The carb cleaning pdf has images of the carbs broken down to their components - study it.
Also, see my sig.... I have a '78 750 with pods. I, and a bunch of others on here have done the donkey work for you already. A bleeder jet (pilot) of 15 (stock) and 117.5 main jet will work - full stop. BTW, you have four carbs so you need four main jets.....

Now, as some others have said there's probably many other things you need to fix on this bike to make it safe, reliable and smooth running. Learn about your bike and figure out what's important and what ain't. Very few people buy a 42 year old bike and not have to spend dozens of hours and lot of $$$$ making it road worthy.

 

[schwalbewurger]

Yes. That?s as good an answer as I can give. I?ll bet there are hundreds of threads on the forum about rejetting for pods. You?ll see recommendations in those threads or from others chiming in here with experience. I have none. Depends on if you have VM or CV type carbs, and what kind of pods. All pods are not created equal. Cheap ones provide nearly no restriction and may actually block important ports on the intake side of the carbs. Again, others with experience will have to advise you on specific jetting with specific pods, on specific carbs. No easy answer here.

Big tip on using the site?s search function:
always used Advanced.
always be specific with your search words
always search titles only. If you don?t, you won?t get good search results at all.

Will do! Thank you for your the info and advice in the search function.

 

[schwalbewurger]

You need to do a lot of reading on the forum and the workshop manual which is available on the BassCliff site you've already been directed to.
The carb cleaning pdf has images of the carbs broken down to their components - study it.
Also, see my sig.... I have a '78 750 with pods. I, and a bunch of others on here have done the donkey work for you already. A bleeder jet (pilot) of 15 (stock) and 117.5 main jet will work - full stop. BTW, you have four carbs so you need four main jets.....

Now, as some others have said there's probably many other things you need to fix on this bike to make it safe, reliable and smooth running. Learn about your bike and figure out what's important and what ain't. Very few people buy a 42 year old bike and not have to spend dozens of hours and lot of $$$$ making it road worthy.

For sure! Thank you for your info and experience. Will definitely follow your advice. Also will update with more pictures of the bike.

 

[schwalbewurger]

Carbs of the bike with pods