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Pulls hard then dies out at 5500rpm

  • Thread starter Thread starter GSNewbie
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GSNewbie

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My (new to me) '81 gs650G pulls hard until about 5500 and then bogs and slows down. If I back off and then just tickle it up I can get it past that point in the revs but never past 7000rpm. If the bike is sitting in neutral, it doesn't seem to have an issue and revs very high (so it seems to be only under a load).
I have had the carbs out almost a half-dozen times. Pilot jet is clean. Main jet is clean. floats set correctly and needles look good. I've worked the diaphram up and down and they appear to be good as they make the suction sound and are not sudden when they drop... air filter is new and stock. All jets appear to be stock (at least in numbered size. They could be drilled).
Electrical has a .8v leak but I haven't been able to figure it out yet. Cam chain seems to be making noise but I have yet to diagnose how to fix it.

I don't know if this is a gas issue or a spark issue. Frankly, I'm stumped. Can anybody here help me?
 
Just a couple of suggestions as to what I would do.

1. I would take it up to 5500 rpm in 4th and when it bogs I would open the choke a little to see if the bogging clears or gets worse. This will tell me if it is bogging through richness or leanness. Then make changes as necessary (main jet & needle position). Another thing I would do is run the motor as above until it bogs at 5500rpm and hold it there as long as I could then declutch and pull over to the side of the road and pull a few plugs and look at their condition to see if they give any telltale signs if richness or leanness, or electrical issues.

2. I would also check the voltage at the coils to make sure I am getting a full 12v with a fully charged battery.

Try these things and report back.

Good luck.

P.S. Is the air cleaner clogged.
 
Your airbox/filter assembly should be identical to my "L" model- air filter cover is on right side? The air comes in thru slots in opposite side just behind battery- make sure wires/tools/etc aren't blocking these slots or you could be running out of intake air. Did you LIGHTLY oil airfilter element? Try this test- roll back filter element on cage slighty and ride it. You'll likely have poor idle (mine ran ok with torn element), but see if hits 7k.
Also if bike has sat, make sure mufflers haven't been plugged with debris.
When you had the carbs off, you should have investigated cam tensioner in case it has been installed/adjusted incorrectly.
 
Make sure that little vent hole in the fuel tank filler neck is open - did you recently coat the tank?

Are you using an in-line fuel filter? If so, remove it...
 
GSNewbie said:
My (new to me) '81 gs650G pulls hard until about 5500 and then bogs and slows down. If I back off and then just tickle it up I can get it past that point in the revs but never past 7000rpm. If the bike is sitting in neutral, it doesn't seem to have an issue and revs very high (so it seems to be only under a load).
I have had the carbs out almost a half-dozen times. Pilot jet is clean. Main jet is clean. floats set correctly and needles look good. I've worked the diaphram up and down and they appear to be good as they make the suction sound and are not sudden when they drop... air filter is new and stock. All jets appear to be stock (at least in numbered size. They could be drilled).
Electrical has a .8v leak but I haven't been able to figure it out yet. Cam chain seems to be making noise but I have yet to diagnose how to fix it.

I don't know if this is a gas issue or a spark issue. Frankly, I'm stumped. Can anybody here help me?
Click to expand...

Are the air jets and associated passages clear? Are the bowl vents clear as well?
 
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Greetings and Salutations!!

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Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", "Top 15 Tips For GS Happiness", the Carb Cleanup Series, and the Stator Papers. All of these tasks must be addressed in order to have a safe, reliable machine. Now let me roll out the welcome mat for you...

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Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike! :D

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Thank you for your indulgence,

BassCliff
 
How may volts at the coils? Does your ignition advancer move freely and open all the way?
 
Ok, thanks for the tips. I have checked the bowl vents and they were all clear. I didn't fully disassemble the air passages but the carbs had been cleaned before me and I found nothing in the parts I had gone through to suggest anything other than that they were in pristine condition and clean.

Yes, I did just re-coat the tank with red stuff at a local shop. (got me a little worried if the carb cleaner stuff I poured in the gas would eat away my new liner or not).
Yes, it came with (and still has the in-line cone-style gas filter) I could see where that might starve it out. I'd think the last guy would have pulled if off if that had caused a problem though. I'll fix them both pronto.

I checked the bowl vents and they were all clean.
Air cleaner is NEW and was lightly oiled by pj1 and myself (always a possibility there though). I've only oiled air filters for the last 30 years!
Needles, I read that there is only one clip position. Didn't take them out but were clean when I checked 'em.
Yes, airbox is stock. I have yet to take off the filter and see if it'd run as well. I did pull the choke to see if it was better and didn't seem to be (also hard to hold the choke at speed too. I lubed it too well and it doesn't stick in the place I want to keep it at).

Don't know:
I don't know how to check the cam chain to see if it needs a new one; check the tensioner to see if it's working (but it would explain the noise). Tried to check the coil but I think I did it wrong as I could only get it to show continuity not a number...
Have wondered about the ignition advancer (I read an article that mentioned that) but don't know how to check that (or not yet).

I guess that's the challenge of older bikes. I jump on my other bike(s) and don't have to know how everything works to ride. With this, I've been reading and reading and trying to figure the whole bike out to keep from going to the shop and spending money on it. This thing began as my "backup" bike which I traded for an even older bike that didn't run. It came in two boxes and was pushed off the guy's truck missing a lot of parts. I've built and repaired a few bikes but this one is was the most basket case of the ones I've worked on yet!
Compared to my 20+ year newer bikes, I think this is a much simpler bike to work on as there are less moving parts. The titanium seems to be non-existent on this model as well.
So far, I do love the shaft drive and disc brakes on this bike. And the big plus: I don't have to worry about it getting stolen:)

Off to chip away at a few of the suggestions. Thanks all!
 
Last edited:
A 650 in 2 boxes! ignition is fully advanced by about 3500 rpm ,so that's unlikely to be a problem here. No where did you mention checking valve clearances, so you can't put this off much longer unless you saved those boxes. Very small (.002 inch ) numbers involved here- read up at Basscliff's site. While you have cam cover off, ideal time to check mechanical timing ( camshaft to crankshaft) just to make sure that the noisy cam chain hasn't skipped a tooth. Unlikely, but never hurts to be sure.
 
GSNewbie said:
My (new to me) '81 gs650G pulls hard until about 5500 and then bogs and slows down. If I back off and then just tickle it up I can get it past that point in the revs but never past 7000rpm. If the bike is sitting in neutral, it doesn't seem to have an issue and revs very high (so it seems to be only under a load).
I have had the carbs out almost a half-dozen times. Pilot jet is clean. Main jet is clean. floats set correctly and needles look good. I've worked the diaphram up and down and they appear to be good as they make the suction sound and are not sudden when they drop... air filter is new and stock. All jets appear to be stock (at least in numbered size. They could be drilled).
Electrical has a .8v leak but I haven't been able to figure it out yet. Cam chain seems to be making noise but I have yet to diagnose how to fix it.

I don't know if this is a gas issue or a spark issue. Frankly, I'm stumped. Can anybody here help me?
Click to expand...

What's the condition of the exhaust system pertaining to an issue with backpressure?
 
Not sure on how to check exhaust... It is stock 4 into 2 and has 34k miles though.
 
So here's a list of what I've done so far (with no results yet, still dies off)
new gas line minus filter.
tried riding it with the gas cap off as well.
hooked up a timing light and rode down the road to see if the spark was dying off (looked the same through all the revs). Not terribly scientific or SAFE in the dark!

Checked the carb vacuum boots by blowing air through the top of the intake on each carb and all checked out fine.

checked valve timing (close to spec)

checked cam chain tensioner (it was tightened in the loose position. re-installed correctly.

rode with the air filter pulled back

rode it with more choke

Still working on it...
 
Last edited:
I had this exact problem, I was missing one of the plugs on the breathers inside the carbs. Did you replace them?
 
jwhelan65 said:
I had this exact problem, I was missing one of the plugs on the breathers inside the carbs. Did you replace them?
Click to expand...


If you are referring to the rubber pilot jet plugs then we are thinking the same thing. Without them the bike will run way too rich.
 
Still working on this problem.

Still working on this problem.

I just correctly ran a compression check and they are all within 5lbs. and at 130lbs with #1 at 125. (Hot w/ WOT).
Does this mean it needs a full rebuild or just the valve seals since it smokes a little at startup? It lists the compression new at 142 and I can't see 12 lbs low being a reason for rebuild...

I have not run it with the pipes off as there doesn't appear any way to get the cans themselves off of the head pipes and I can't see taking it down the road without any headers at all. They seem to be putting out a lot of exhaust and I can see no debris in them looking down them with a light.


The ignition advance appears to be working correctly. There is some resistance with the springs and turns 3/8" on the shaft without binding.

All the carb rubber plugs were intact and looked good in the carbs. I can pull them again and will soon if nothing else comes up as the problem.

Still stumped...
 
Did you actually disassemble the carbs and dip them and replace the orings?
Not just take them out and inspect them, but really follow the tutorial and clean them??
 
No, I have not completely dipped 'em. I got them disassembled and they were super clean, synced, (checked that). I saw nothing that would lead me to go through dipping them and looking for all the rebuild parts. After trying about everything else, I'm back thinking there must be a passageway in there that is not a jet, needle, plug, or or vaccume diaphragm that might still be the cause.
Now to find carb parts (money) and dig in...
 
GSNewbie said:
No, I have not completely dipped 'em. I got them disassembled and they were super clean, synced, (checked that). I saw nothing that would lead me to go through dipping them and looking for all the rebuild parts. After trying about everything else, I'm back thinking there must be a passageway in there that is not a jet, needle, plug, or or vaccume diaphragm that might still be the cause.
Now to find carb parts (money) and dig in...
Click to expand...

You gotta dip em bro. You can't see inside every passage. The orings must be replaced etc. This is essential stuff. For as many times as you've had the carbs out and such you could have stripped em and dipped em and likely been riding trouble free this whole time.

If your diaphragms have any pinholes or tiny tears they're trash. If you need em, I have a like new set for sale in the parts for sale section.
If you bought them new they're nearly $100 each...
 
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