81 GS750E gets flutter @ 3-3.5k rpms okay otherwise

[twistedwankel]

Amended info: I was able to get compression readings #1=120, 2=90, 3=120, 4=125 best I could do with makeshift setup but I did them all the same way at least.

29,000miles on well maintained original factory equipped 81, 750E.

After hard accelerating into traffic a noticable flutter (stumble?) started occuring between 3-3.5k rpm while accelerating and also while holding that rpm range.

My first thought was carb malfunction. So put in some Seafoam and rode around awhile 20 miles.

I've pulled the plugs and they look fine. #1 cylinder was the worst but still firing good.

Trying to find an adapter for my Sears USA compression meter so I can do a test on all cylinders simply because I've never had to do one on this bike. Need to find a 12mm-1.25 adapter to 18mm somewhere??

This bike starts perfect. Up until two days ago it ran strong in all speeds and activities. Very noticable stutter (almost like a slow rev limiter) but isn't getting any worst or better as of today.

Replaced the aircleaner no difference. Replaced the inline fuel filter no difference.

Runs great up to 3k and under full throttle runs really strong.

I hope I don't have to attempt a cleaning/sync of all four carbs. I dread that day and local bike shops won't touch my '81 either.

Is this possibly an ignition issue? The wires look good with no breaks and the spark is okay and the plugs are only 2000 miles on them and look good. When I pulled #1 wire it idles slower but when I pulled #4 wire it quit immediately. That was weird. Guess I'll have to do that to 2 and 3 also when I get the tank off and can see every thing without burning my hand.

I came up empty on the search feature. Trying to avoid wasting money...I've got time. Hope it's not the valves either....argh.

All the ignition components are original factory transistors units.

[chef1366]

How's the battery?

[Steve]

You say "29,000miles on well maintained original factory equipped 81, 750E", so it sounds like you know the history of the bike. Any chance you are the original owner?

If you are the original owner, that automatically answers a couple of questions. 29,000 miles without a valve adjustment is waaaayyyyy too long. If it has been used somewhat regularly, cleaning the carbs should not be necessary, but a sync job certainly would not hurt. Neither one of these jobs is very hard, but can seem rather intimidating when you approach it for the first time. The fact that pulling one plug only slowed the engine, but pulling another one killed it shows you that they are not contributing equally. A carb sync and mixture adjustment can help.

Another prime suspect would be the o-rings between the intake tubes and the cylinder head. Removing the rack of carbs from the bike is not that bad. If you then use an impact driver to remove the screws that hold the intake tubes, they might come off relatively easy, too. You can get the o-rings and replacement socket-head screws from Cycle o-rings. The owner is a member of this forum.


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[twistedwankel]

How's the battery?

Battery and charging system okay.

[twistedwankel]

[quote=Steve;761599]You say "29,000miles on well maintained original factory equipped 81, 750E", so it sounds like you know the history of the bike. Any chance you are the original owner?
Second owner for last 4 years but know all the history.
If you are the original owner, that automatically answers a couple of questions. 29,000 miles without a valve adjustment is waaaayyyyy too long.
I agree too long but this bike was merely a practice bike when I bought it and put 10k on it in two years down south.
Amended info: I was able to get compression readings WOT #1=120, 2=90, 3=120, 4=125 best I could do with makeshift setup but I did them all the same way at least.
If it has been used somewhat regularly, cleaning the carbs should not be necessary, but a sync job certainly would not hurt. Neither one of these jobs is very hard, but can seem rather intimidating when you approach it for the first time. The fact that pulling one plug only slowed the engine, but pulling another one killed it shows you that they are not contributing equally. A carb sync and mixture adjustment can help.
Guess I'll have to bite the bullet and buy a sync - I've seen a few posts on that one.
Another prime suspect would be the o-rings between the intake tubes and the cylinder head.
I don't think so as spraying with accelerant doesn't raise rpms at all on any of them? But if I end up having to do the valves the o-rings will be pretty easy and I'll do it as a maintenance item.
Removing the rack of carbs from the bike is not that bad. If you then use an impact driver to remove the screws that hold the intake tubes, they might come off relatively easy, too. You can get the o-rings and replacement socket-head screws from Cycle o-rings. The owner is a member of this forum.

Thanks for your help. Now that you see the compression readings you can give me any really bad news?

[Dave8338]

Valves...most likely the reason for the 90psi reading. Set the valves and replace the o-rings on the intake tubes and ride it another 29,000 (with one more valve adjustment in-between). \\/

If setting the valves has little effect on the 90psi, someone may have run the bike with a bad petcock.

[Dogma]

If setting the valves has little effect on the 90psi, someone may have run the bike with a bad petcock.

Please educate... How would a bad petcock cause this?

[chef1366]

Fuel drips into the #2 cylinder from the petcock and washes out the rings and cylinder walls.

[twistedwankel]

Here's the latest info.

Got the valves adjusted and the carbs syncronized.

Runs much better now. Still wants to hesitate a bit 3-4k rpm but acceptable for now. I even took the inline filter out which is another story but it didn't appear to make any difference.

150psi compression at WOT on all Cylinders. So that is a relief. No valve noise and perfect idle with easy starting.

1,2,3 plugs are perfect. #4 is black and therefore rich. Guess this could have been from before 150mi. so maybe I should change it out? I did switch 3 and 4 plugs for the moment.

Any suggestions on how to adjust #4 carb? Is there a specific screw to turn?

Exactly where are these alledged o-rings on the hoses? I don't have anything looking like a flange on there? Just big hoses clamped onto the carbs both sides which I've never taken off.

[posplayr]

Just been reading the link

The o-rings are for the older 8 valve engines. The TSCC 16 valve engines dont have the same setup.

Also I was going to mention holding open the throttle (real easy to forget if you are not use to it) when doing the compression check. Sounds like the engine is is much better heath than previously expected.

#4 carb, look for some one Else's response.

Posplayr

[twistedwankel]

Finally got it and a Home Run at that.

Got the valves adjusted and the carbs syncronized.

Runs much better now. Still wants to hesitate a bit 3-4k rpm but acceptable for now. I even took the inline filter out which is another story but it didn't appear to make any difference.

150psi compression at WOT on all Cylinders. So that is a relief. No valve noise and perfect idle with easy starting.

1,2,3 plugs are perfect. #4 is black and therefore rich. Guess this could have been from before 150mi. so maybe I should change it out? I did switch 3 and 4 plugs for the moment.

Any suggestions on how to adjust #4 carb? Is there a specific screw to turn?

Quote:
Originally Posted by focus frenzy
hmmmm just a idea here, take the diaphram cap off the carb and see if the return spring is where it is suspose to be or is coiled up on top.
I have seen this a couple times and one of those times the owner and given up and thought that one cylinder was toast and parted out the bike.
(I got the carbs on E-bay and discovered his mistake.

the other time my buddy was pulling his hair out on his Kat as it would not run right, it now runs like a scalded cat.


Okay. All I did was charge the battery overnite which really didn't need it, change all the spark plugs that probably didn't need it and take the top off the #4 carb and release the tension on the spring then put it back on. Totally different bike just that quick.

I took it on a 50 mile test run and it's better than ever. Can hardly hold it back. When I got home I even put the inline fuel filter back on and tried it again and it was still the same awesome power that comes on with the least throttle over the whole power band.

Everything I did needed doing but I believe the homerun was the tension spring on the diaphragm controlling the slide air valve. All hale Focus Frenzy\\/ That was a 5 minute deal. I found his advise on a totally different thread. Of course I only spent $200 for a professional to do the important adjustments but I am totally happy that I didn't have to clean the carbs and go thru that nightmare. That could have been very expensive for me.

Thanks to all the rest of you GSers as everyone had a helpful hint as to what was the problem(s) here and I tried them all. I learned a couple totally different things about this bike thru all this.

Happy motoring to all of you. SOG is back on the road with flying colours.

[Escobarclan]

I know this is after you found the problem, but this might come in handy sooner than you think.

I had a very similar problem with my 82 1000 kat. Turns out it was a stuck ring after storage, got it loose but the bike still ran funny. I took the pods off my bike and peeked in the carbs, I reached in and pushed each slide valve up and noticed one took a little more effort than the rest. It had a little crud on it, barely noticable but there. Cleaned it and she ran fine.

So, maybe that'll help too.

[twistedwankel]

It happened again but final resolution to issue.

The sputtering happened again after some 1500 miles of happy riding.

I never thought to pull the carb drain plugs. So I did into a white paint cap so I could see. Lots of small particles. Every bowl I filled and drained 3 times before they cleared up. Then I noticed the actual bolts were rusted and had provided the rust particles that got into the rest of the carbs only when I really got on it - figures greater flow and getting down into the bowl.

Anyhow a friend convinced me to get the o-ring kit and he would go thru the carbs. BUT guess what...in the week it took me to get the parts I replaced the bolts and seals with new ones and as quick as a rabbit the bike ran good as new.

Professional biker friend dropped the bowls and they were clean inside.

Moral to story? Guess one should routinely pull the drain plugs when they buy an old used bike. Especially before running Seafoam thru it. I will continue to use a precarb gravity filter now due to the hard rust in the tank but I have no reason to think there will be anymore issues with the carbs and we went on a 600mi trip with it last weekend.

At any rate I still have a complete set of o-rings if needed