diagnosing carb issue

[fallous]

I've searched the forum trying to see if anyone else has had the same issue, and I'm sure they have but I haven't been able to stumble across it with the right keywords I guess.

I bought an '81 GS850G a couple of days ago in partially running condition. Guy I bought it from had recently acquired it from someone else in partially disassembled condition (tank and carb rack were off the bike), slapped it back together, said he fired it up and it ran fine. Next day, would only run on choke and pretty rough when it did.

Bike has stock exhaust, pods (alas, some previous owner tossed the airbox). Petcock seems to be working correctly, since it flows on PRI, doesn't flow on ON or RES. Bike has spark and new battery.

When I got it, it looked like it was only running on the 3 and 4 cylinder, since after monkeying it enough to run on choke I took a wet shop rag to each header pipe to see which had got hot.

Drained bowls on the carbs, and managed to get all four firing but it's still a choke-only bike. Took the pods off to check out the condition of the slides (seem fine, move easily at least) and managed to get bike running on choke again. This time I blocked the large air passage at the top of the #1 carb inlet, and that seemed to increase rpm of the bike.

That's about it for diagnostics thus far. I'm guessing the pilots are clogged from junk in the tank after sitting for awhile and then getting fuel, but the rpm increase threw me off a bit. I figure since it will only run on choke, and dies when you try and rev it from the throttle (occasionally once it's warm it'll halfway let you give it gas from throttle but it's pretty sketchy) that the pilots are clogged, and when you grab throttle the transition from choke to main jet feeding the motor causes it to fall down.

I'm also wondering if maybe I've got an air leak, since messing with the air passage made the bike rev. Manifolds look good on the bike and are still flexible, no cracks that I can see. Dollars to donuts the previous owner that tossed the airbox didn't rejet for the pods, but I wouldn't think the pods would throw it off so much that it simply would not run at all except using the choke.

Any ideas from the experienced crowd on this?

 

[rustybronco]

Dollars to donuts the previous owner that tossed the airbox didn't rejet for the pods, but I wouldn't think the pods would throw it off so much that it simply would not run at all except using the choke.

Any ideas from the experienced crowd on this?

Oh yes it does!
there may be other carb issues, but you have to re-jet or put that air box back on.

 

[robertbarr]

From Mr. bwringer:

Every GS850 has (or had) a set of well-known issues that MUST be addressed before you have a solid baseline for further troubleshooting.

It's a vintage bike, and it's quite common (as in, every single GS850 I have had contact with) that there are multiple problems that have crept up and slowly gotten worse over the years.

It's not like a newer vehicle, where there's generally one problem at a time.


These common issues are:

Intake O-rings (install NEW OEM or Viton only - common nitrile o-rings will quickly deteriorate from heat)

Intake Boots (install NEW -- these cannot be repaired)

Valve clearances (more important than most people think)

Carb/airbox boots

Airbox sealing

Air filter sealing

Petcock (install a NEW one)

On '79 models, install new points or Dyna electronic ignition (or at least verify that the old points are working correctly)

On all models, it's fairly common to have problems with the spark plug caps. These are $3 or $4 each, and often worth replacing if you're keeping the stock coils/wires.

Stock exhaust with NO leaks or holes -- good seals at the head and at the junctions underneath.


None of these things can be neglected, worked around, or half-assed in any way. Once they are squared away, you most likely have a bike that runs like a dream -- these issues account for a very large portion of running problems.

If not, then you can pursue more exotic problems. For example, I have found incorrect float heights a couple of times due to a common misunderstanding in how these are best measured. But you can't uncover other problems without making sure the intake/exhaust paths are leak-free, the valves are adjusted correctly, and you're getting good spark.

 

[fallous]

well I solved some of the initial problems. drained float bowls, sprayed the needles there with carb cleaner, then drained gas tank and put in fresh gas. After a couple of minutes of stumbling on choke only, the bike suddenly picked up and started running without choke. Ran for another 5-10 minutes or so and then shut it off. Let sit for an hour, and fired it up without choke just fine. Feels like it's got some stumbles off-idle, so pilots are probably varnished and crudded up but it's a damned sight better than I got it.

New fun is the top cam cover leaks, and the half-moon on the left side nearest carbs is moderately loose. I see permatex all over the place up there, so I'll yank the cover and clean it all up, then order a Real Gasket for that particular hilarity. One thing I'm wondering is if the half-moons can go bad, and if so where to order a full set since I might as well swap all rather than one at a time.

 

[rustybronco]

One thing I'm wondering is if the half-moons can go bad, and if so where to order a full set since I might as well swap all rather than one at a time.

see Basscliff's mega welcome thread for sources on where to purchase the half moons.

Additional parts/info links:

I would definitely double and triple the recommendations to use Cycle Recycle II and Z1 Enterprises as much as possible. These guys are priceless resources. Z1 tends to have slightly better prices, CRC2 has a wider range of goodies available. If you're near Indy and can bring in an old part to match, CRC2 has a vast inventory of used parts.I have used z1 and recommend
http://denniskirk.com - Put in your bike model and see what they have.
http://oldbikebarn.com - seems to be slowly regaining a decent reputation, but it's still caveat emptor. They don't have anything you can't get elsewhere at a better price anyway.
http://www.babbittsonline.com/ - Decent parts prices. Spendy shipping. Don't give you part numbers at all. Useful cross-reference if you obtain a part number elsewhere. Efficient service.I have used and recommend
http://bikebandit.com - Fastest. Middlin' prices. Uses their own parts numbering system to obfuscate price comparisons -- can be very confusing for large orders. Cheapest shipping, so total cost usually isn't too bad.
http://flatoutmotorcycles.com - Slow. Cheapest parts prices, crazy shipping costs. Don't expect progress updates or much communication. Real Suzuki part numbers.
http://alpha-sports.com - Exorbitant parts prices. Different type of fiche interface that's quite useful at times, especially with superceded part numbers. Real parts numbers. Shipping cost and speed unknown due to insane, unholy pricing.

Stainless Bolts, Viton o-rings, metric taps, dies, assorted hard-to-find supplies and materials, etc:

http://mcmaster.com - Fast, cheap shipping, good prices. No order minimum, but many items like bolts come in packs of 25 or 50. Excellent resource.
http://motorcycleseatcovers.com - Great quality, perfect fit (on original seat foam), and available for pretty much every bike ever made. Avoid the textured vinyl -- it's perforated.
http://newenough.com - You DO have riding gear, don't you? Great clearances, always outstanding prices and impeccable service.
***************End Quote**********************
Additional parts/info links:

GSR Forum member Mr. duaneage has great used upgraded Honda regulator/rectifiers for our bikes. Send him a PM.
New electrical parts:
http://stores.ebay.com/RMSTATOR or http://www.rmstator.com/
http://www.ricksmotorsportelectrics.com/index.php
http://www.electrosport.com/
For valve cover and breather cover gaskets, I recommend Real Gaskets (reusable silicon):
http://www.realgaskets.com
The Rice Paddy (salvage/used)
http://www.ricepaddymotorcycles.com
Carolina Cycle
http://www.carolinacycle.com
Ron Ayers Motorsports
http://www.ronayers.com
MR Cycles
http://www.mrcycles.comI have used and recommend
Moto Grid
http://www.motogrid.com
If all else fails, try this:
http://www.used-motorcycle-parts.org/

or your local dealer...