GS750: long warm-up interval. Also, bogging

[Longitudinal]

Hello,

It doesn't bother me too much, but the '78 750 takes a year and a day to warm up well enough to idle without the choke. Temps are in the 60s right now.

Also, the engine bogs progressively worse beyond about half throttle. The bogging is worse still when the engine is cold and still on the choke.

I readily admit that the carbs probably still need adjustment, but have others experienced these problems?

 

[Nessism]

How long has it been since you installed new plugs, gapped/replaced the points and condenser, adjusted the valves, cleaned the air filter, cleaned the carbs and replaced the various O-rings?

 

[BassCliff]

Hi,

I concur. Here's a little reminder...

Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers.

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information.

As mentioned in the "mega-welcome", certain tasks must be performed in order to establish a baseline for further troubleshooting.


Thank you for your indulgence,

BassCliff

 

[Longitudinal]

How long has it been since you installed new plugs, gapped/replaced the points and condenser, adjusted the valves, cleaned the air filter, cleaned the carbs and replaced the various O-rings?

About 30 miles since new points, wires, plugs, carb rebuild, EMGO filters. Still need to check and probably adjust valve lash, but I need to order a cam cover gasket before I tear into that.

Which o-rings? On the intake boots? Keep in mind that this problem is noticeably worse when the engine is cold and that it gets worse with more throttle. More throttle means less vacuum. Less vacuum means that a vacuum/intake leak should become less significant.

BassCliff, thanks for the reminder.

 

[tkent02]

Also, the engine bogs progressively worse beyond about half throttle. The bogging is worse still when the engine is cold and still on the choke.

Worse when cold sounds too lean, did you re jet the carbs properly when the pods went on?
If you don't want to use new O rings, you probably don't want to re jet either.

 

[Big T]

Worse when cold sounds too lean, did you re jet the carbs properly when the pods went on?

Ditto

Either put the stock airbox back on, or start rejetting

 

[Longitudinal]

If you don't want to use new O rings, you probably don't want to re jet either.

I'm sorry. I must have missed the part where I said I didn't want to use new o-rings.

I bought the first up-size of jets when I placed my big order to Z-1 a few weeks ago, but was told that I would not need them until I upgraded the exhaust.

I agree that the symptoms are of a lean condition.

 

[tkent02]

I'm sorry. I must have missed the part where I said I didn't want to use new o-rings.

"Which o-rings? On the intake boots? Keep in mind that this problem is noticeably worse when the engine is cold and that it gets worse with more throttle. More throttle means less vacuum. Less vacuum means that a vacuum/intake leak should become less significant."

Sounds like you don't want to change them.

I bought the first up-size of jets when I placed my big order to Z-1 a few weeks ago, but was told that I would not need them until I upgrded the exhaust.

I agree that the symptoms are of a lean condition.

It's lean when the throttle is open. So try a bigger main jet. You may need to raise the needle too.

 

[Longitudinal]

Sounds like you don't want to change them

You're inferring too much. I don't mind checking anything and replacing what I need to. I'd be stupid not to. The underlying message of my post was that leaking o-rings at the boots isn't a very good match to the symptoms.

 

[Guest]

If it gets worse with choke it's a rich condition.

 

[rustybronco]

It might help, if you would post what has been done to the bike, up to this point.
http://www.thegsresources.com/_forum/showthread.php?p=1091548#post1091548

Yes, I tried rapping/tapping/cajoling the float into floating. The carb had at some point begun to drip instead of streaming out fuel. It continued to do that, though. My friend came over and found that he had installed something wrong with the float on that carb. The carb still drips when the bike is parked on the side stand, but seems not to when parked on the center stand.

1) does it still drip fuel? if it still does, your just chasing your tail.

2) where are the fuel levels at, when measured externally? the procedure is found in the shop manual, as linked by basscliff... http://www.mtsac.edu/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf

3) have you done any other modifications to the bike, such as the exhaust?
http://www.thegsresources.com/_forum/showpost.php?p=1091748&postcount=14

what has been done other than the pods? exhaust changed? was it re-jetted?

4) what size main jets were in the bike, as found? what size jets did you use to replace them?
(i.e. stock jetting was 100.0 and you replaced them with 105.0, or were they replaced with102.5's)

5) what position is the clip on the needle? (i.e. 5f21-3, stock is third from the top)
http://www.thegsresources.com/_forum/showpost.php?p=324909&postcount=43

6) check the condition of the carb intake boots; are they still soft and pliable, or are they hardening up with age?

7) replace the intake boot o-rings, if their condition is unknown.

8) adjust the valves

9) synch the carbs and adjust the pilot fuel/air screws

this should be a good starting point for you...

 

[almarconi]

The valves should be checked and/or adjusted BEFORE trying to adjust the carbs.

 

[Nessism]

About 30 miles since new points, wires, plugs, carb rebuild, EMGO filters. Still need to check and probably adjust valve lash, but I need to order a cam cover gasket before I tear into that.

Which o-rings? On the intake boots? Keep in mind that this problem is noticeably worse when the engine is cold and that it gets worse with more throttle. More throttle means less vacuum. Less vacuum means that a vacuum/intake leak should become less significant.

BassCliff, thanks for the reminder.

All the maintenance needs to be up to day or you are just guessing as to what the problem is.

Cycleorings.com sells carb O-ring kits, and O-rings for the intake boots. Never a waste of time to refresh these unless you have done it recently.

My guess is that the carbs need to be rejeted since you changed the air filters.

 

[Longitudinal]

If it gets worse with choke it's a rich condition.

Unless the choke isn't doing its job , in which case the engine is left to its already poor running and also has to deal with not getting enrichment from the choke.

 

[tkent02]

Unless the choke isn't doing its job , in which case the engine is left to its already poor running and also has to deal with not getting enrichment from the choke.

With the throttle open the choke is NOT doing it's job, it only enriches with the throttle nearly closed, when the pilot circuit is working. At wide open throttle it does nothing. If it runs better wide open when fully warmed up than when cold it is too lean.

 

[Longitudinal]

With the throttle open the choke is NOT doing it's job, it only enriches with the throttle nearly closed, when the pilot circuit is working. At wide open throttle it does nothing. If it runs better wide open when fully warmed up than when cold it is too lean.

So we agree. Very good.

 

[Speedo]

Longitude dude - I moved out here FROM columbus oh 5 yrs ago and I lived there for almost 20yrs so i know first hand there are some really good cycle dudes & shops near you. Have you tried "Rice Paddy"? If still in biz, those guys were awesome.

Anyway, here's my 2 cents:
1. Listen to all these guys. They are really, really (really) good.

2. If you went to airfilt pods from stock, you ABSOLUTELY will need to rejet. I crashed my first 78 gs750 back in '85 (described below is my 2nd) when i went to pods before i had a chance to get rejetted because it ran so wierd. It had a header, but that makes not difference. Do it. If there is a "real" improvement in "breathing" (i.e. exhaust like header, or intake like pods), you must rejet. More air needs more fuel. If you wouldn't have to rejet, then there would be NO improvement in breathing. Think about it.

Carb jetting is both a science and an art. Ideally, you (or whom u hire) know what you are doing, have patience (lots of it), and a dynamometer to empircally measure results of changes. I went with such a guy in Columbus back in 90's. It was perfect. All throttle positions. Totally worth whatever $$ I paid the dude. Unf, I know he closed his biz (took a job as a crew chief on some bike race team). I know this because when I went to the smoothbores (source: Rice Paddy), I knew i needed to rejet again, and I could not find someone as good. The bike still doesn't run as good as it did before. Perhaps I'll find such a shop/guy out here but meanwhile i'm dealing with some other problems...

3. Are you still running points? Am I reading that right? If so, I assume stock coils too? Both weak points and both make these bikes "cold hearted beoches" as far as warming up. Again, 30 yrs, 2 gs750s of experience speaking here. About the only other weak point on GS's is the charging system (see my other posts) and the clutch if u hotrod it.

Go Dyna S ignition and dyna coils. It will fire up and warm up like never before.

4. O rings / intake boots. If you can find them, by all means change. Once of my boots had a pinhole in it years ago. All O rings were brittle. The difference is like night an day. btw, when cleaning bike, hit the boots and any other rubber stuff (ex tires, NOT) with 303 protectant. Will last almost forever.

5. Last, as RustyBronco and Chef1366 and others suggest, before you jet/rejet, MAKE SURE (for real) that you have good spark and there are NO vacuum leaks and NO fuel leaks dripping fuel is usually float sticking btw) or you are just wasting your time solving the wrong problem at the wrong time.