Low compression

[Christopher]

Ok Im recomissioning a GS850GN with 15k miles engine which had been standing for an unknown amount of time. It idles just but doesnt run very well. After doing alot of work on carbs and charging system it hasnt improved although now charging. I finally decided to test the compression and it was low all cylinders. I also tested after putting a little oil in cylinder. Results were 60psi all cylinders and oiled shown below 2nd number

1. 60 70 2. 60 64 3. 60 65 4. 60 60

Please advice me where to go from here. What are the possible causes of the low compression. Is the engine salvageable?. Thanks Guys

[bwringer]

9.99 times out of ten, testing compression leads you astray. There are so many ways to do it wrong, or to have trouble with the tester, and over and over and over again, we've seen riders (well, people who want to ride) doing unnecessary work and being led down a dead end rabbit hole based on compression readings.

In other words, be extremely suspicious of compression readings, and never condemn an engine based on just this one piece of almost universally faulty evidence..

What I see there is remarkably consistent compression, and little change with oil, so that is very encouraging. Your rings and bores are almost certainly just fine. Your problem is elsewhere.

For example, have you checked valve clearances? 15K and sitting a while is an absolute GS classic story; that's right when neglected valve checks make the bike very difficult to start and it won't run very well.

Check the Newbie Mistakes thread for many other things to check.

For example, running the bike without a complete, properly sealed airbox with snorkel in place. These things will hardly run at all without the airbox, believe it or not, and shaftie airboxen and air filters MUST be sealed such that air only gets through where it's supposed to. You have to seal the sides of the airbox and the top of the filter frame with weather stripping, and the rubber snorkel on the back MUST be in place.


What are some of the ways compression testing can go wrong?

1) You cannot get a good compression test on these engines with the carbs in place. Opening the throttle isn't enough; the carbs must be completely out of the way.

2) Compression testers, even quality brands, are at a very high risk for incorrect readings, leaky fittings or o-rings, etc.

3) Tight valve clearances are extremely common, far more common than bad rings or worn bores, and cause compression loss. You cannot get a good compression test if the valve clearances are not in spec.

4) These engines are air-cooled, so compression readings when cold can be quite low. This is completely normal; the pistons and rings are designed with room for expansion than in liquid-cooled engines. You cannot get a good compression test on a cold engine.

5) Exhaust restrictions are another way to go wrong, but are fairly rare. Usually exhaust damage to that degree is obvious.

6) Oh, and you need a strong battery and starter motor to achieve the cranking speed needed to get any useful info from a compression reading.

7) Remove ALL the spark plugs for a compression test, not just the one you're testing. You need maximum cranking speed.

[Christopher]

Bwringer thanks for the very informative reply. Ive done work on sealing the airbox. The compression tester is a very cheap £8 chinese ebay item. I did the test with the throttle open removing 1 plug at a time on a cold engine with carbs in place. Ive never tested valve clearances before so im preparing to dive into this task. At the moment the engine just starts after many attempts pushing the starter and on choke. It idles just on full choke. It revs up roughly when it gets warm with the odd misfire. When I try to ride it it doesnt really pull and just mostly dies. I really love this bike despite owning it for 2 years now and not yet getting it running right. Ive cleaned the carbs a few times but havent yet set float heights(i need to learn this) or balanced them(also need t learn this). I will next do the valve clearances. Do you think this is causing my issues? Also ive replaced the rubber boots to airbox and to engine. Thanks Chris

[Christopher]

Also Im not sure what part is the rubber snorkel which you said must be in place?

[Christopher]

Also the number 1 header is the only one which gets signicantly hot

[bwringer]

On the back of the airbox, there's a rectangular hole, which should have a rubber snorkel in it that sucks in air from just in front of the top of the battery. These vanish with depressing regularity. On many chain drive GS models, the airbox lids usually mysteriously vanished sometime in the '80s as well, making the bike run like crap for 40 years.

And suppose I should add a #7; you're supposed to remove all the spark plugs for a compression test, not just the one you're testing. You want maximum cranking speed.

In any case, I think it's clear that your bike's rings and bores are just fine; you tested compression incorrectly, but you were consistent, and the results were consistent. So put that out of your mind; the engine is mechanically sound so your problem lies elsewhere. I should say problems in the plural; with neglected vintage vehicles, there are always multiple overlapping issues, and you have a list to sort out, not just one issue at a time. It's a different sort of troubleshooting and mindset.

Fortunately, this path is well-trodden and well-marked. Start here:

https://www.thegsresources.com/_forum/forum/technical-forums/general-maintenance/46923-top-10-newbie-mistakes?t=171846

Anyway, some of my advice for where to start, and others will chime in with other thoughts...

1) Yes, check and set the valve clearances.

2) If # 1 is the only one getting hot, that points to problems with carbs, or perhaps ignition. Likely both. See what I mean?

Something is likely still clogged in the other carbs. I wasn't there when you cleaned them, so I can't be more specific, but review the link in the newbie mistakes thread for the only correct procedure and common mistakes like using the shonky parts from carb kits. And yes, float heights are important.

3) Also verify that you have strong, correctly timed spark in all four. Weak voltage to one coil or the other is common, along with many other potential issues in the ignition wires and caps. It's also quite common for the wires to get hooked up in the wrong order if you're used to car engines. The left coil fires # 1 and #4, the right coil # 2 and # 3 And if your bike still has points, that's a whole 'nuther raft of issues.

4) Every petcock leaks eventually, meaning that the vacuum line from the petcock to the carb slurps in added gas. If the petcock is original, it is or shortly will become a problem (one of many). The rebuild kits don't work. Luck with the cheap Chinese replacements is mixed.

There are more, but that's what comes to mind off the top of my head.

[93Bandit]

On the back of the airbox, there's a rectangular hole, which should have a rubber snorkel in it that sucks in air from just in front of the top of the battery.

And suppose I should add a #7; you're supposed to remove all the spark plugs for a compression test, not just the one you're testing. You want maximum cranking speed.

In any case, I think it's clear that your bike's rings and bores are just fine; you tested compression incorrectly, but you were consistent, and the results were consistent. So put that out of your mind; the engine is mechanically sound so your problem lies elsewhere. I should say problems in the plural; with neglected vintage vehicles, there are always multiple overlapping issues, and you have a list to sort out, not just one issue at a time. It's a different sort of troubleshooting and mindset.

Fortunately, this path is well-trodden and well-marked. Start here:

https://www.thegsresources.com/_forum/forum/technical-forums/general-maintenance/46923-top-10-newbie-mistakes?t=171846

Anyway, some of my advice for where to start, and others will chime in with other thoughts...

1) Yes, check and set the valve clearances.

2) If # 1 is the only one getting hot, that points to problems with carbs, or perhaps ignition. Likely both. See what I mean?

Something is likely still clogged in the other carbs. I wasn't there when you cleaned them, so I can't be more specific, but review the link in the newbie mistakes thread for the only correct procedure and common mistakes like using the shonky parts from carb kits. And yes, float heights are important.

3) Also verify that you have strong, correctly timed spark in all four. Weak voltage to one coil or the other is common, along with many other potential issues in the ignition wires and caps. It's also quite common for the wires to get hooked up in the wrong order if you're used to car engines. The left coil fires # 1 and #4, the right coil # 2 and # 3 And if your bike still has points, that's a whole 'nuther raft of issues.

4) Every petcock leaks eventually, meaning that the vacuum line from the petcock to the carb slurps in added gas. If the petcock is original, it is or shortly will become a problem (one of many). The rebuild kits don't work. Luck with the cheap Chinese replacements is mixed.

There are more, but that's what comes to mind off the top of my head.

Agreed with everything Bwringer said. He gave you a solid place to start. All of those items need to be addressed and sorted before any further diagnostics can be performed, if needed. I'd bet once you get all those items addressed the bike will run well.

I only disagree with you Brian on one thing, the petcock rebuild kits. Maybe I shouldn't say I disagree, maybe I should say I have an anecdotal caveat to that statement. I did successfully rebuild my 77 750 petcock with a K&L rebuild kit. I think it has less to do with the kit and more to do with the condition of the internal surfaces of the metal parts of the petcock. Mine was leaking, but when I disassembled it the metal was in good shape. No pitting or other damage on the sealing surfaces. I carefully cleaned it up with some 0000 steel wool, made sure all dust and debris was cleaned out and installed the K&L kit with some silicon grease on the rubber parts. It worked well and did not leak. Maybe I'm right or maybe I got lucky? Anyways, I guess my point is the rebuild kits and some 0000 steel wool is cheap, so might as well give it a shot IMO. After all, like you said there aren't many other options for an OEM style petcock.

[bwringer]

True, the petcock rebuild kits sometimes work. At best, they're a roll of the dice (even when the old petcock's metal bits are in good shape inside), but some folks have indeed gotten lucky.

And of course, some folks have opened up their old petcock only to find serious pitting, corrosion, etc. They're not always rebuildable, and you don't know until you're in there.

[Suzukian]

Funny you mention the valves, as at that mileage, if they were never done, the bike would either be brought in to get fixed, or parked and forgot about, a sad end, but an easy fix.
I doubt there's much wrong with your bike and can be fixed without any major surgery. The steps the other members have given you are the best method to by. One step at a time.

[KiwiAlfa156]

Hi, so GS850GN has a kick starter? And slide carbs (VM) - these have a black rubber bung on the side of the outer carbs near the top.

... At the moment the engine just starts after many attempts pushing the starter and on choke. It idles just on full choke.

OK, It 'idles' on full choke. Does it idle with the choke off?

It revs up roughly when it gets warm with the odd misfire. When I try to ride it it doesnt really pull and just mostly dies.

Once warm/choke off, can you keep it alive by blipping the throttle? Will it pick up revs on the throttle? Are all the exhaust headers hot after you try to ride the bike? Are the outer 2 (1 & 4) or inner 2 (2 & 3) hotter or colder than each other?

Ive cleaned the carbs a few times but havent yet set float heights(i need to learn this) or balanced them(also need t learn this).

Ok, when you cleaned the carbs did you remove any or all of the brass fittings including the screw next to the the float bowls on the underside of the carb that faces the cylinder head? Can you describe what you did to clean the carbs?

I will next do the valve clearances. Do you think this is causing my issues? Also ive replaced the rubber boots to airbox and to engine. Thanks Chris

The quick and dirty check for valve clearance is to see if you can spin the large flat shim that sits a top the bucket underneath the cam using a suitable tool like a screwdriver or dental pick. You do this when the lobe is not pushing on the shim (i.e. the cam lobe is pointing anywhere between 10 O'clock and 2 O'clock, relative to the shim surface at 6 O'clock). If you can spin the shim there is clearance and this valve clearance actually increases with heat (alloy (head) expands at twice the rate of steel (valves and cam). So if it spins it is safe, and its not holding the valve off the seat.

Also the number 1 header is the only one which gets signicantly hot

Is this after you start it on the choke? Or after is been 'warmed up'?


Apologies for all the questions, but your reply opened the possibility of various things being issues and often there can be multiple causes contributing for what you are describing. If you can video the bike running and post it to YouTube (or something similar) that would be a help too.

Trouble shooting is a hands-on thing, but the key is to systematically eliminate the most likely options first until you solve the issue.

If it were me, I'd start check that I was getting good spark on all plugs. If your getting that, next thing is fueling -which I suspect is where your problem lies regarding hard starting and poor running. Don't worry about the compression at this stage. If the bike has sat for a long time, the chances are the rings are gummed up and stuck and these often loosen themselves with use/heat cycles. BTW is it blowing oil smoke? Looking forward to your information.

[uk gs nut]

Myself I would put some marvel mystery oil down plug holes. While that is soaking check valve clearance using Suzuki manual method. Remove carbs and clean again, poke thin wire down all holes. Guitar string or fishing line works, you need all brass parts out. Get an O ring kit and gaskets as needed. On VM carbs I start with the settings for fuel mix screw, that's the one on bottom 1 turn from LIGHTLY seated and side air mix screw 1 and 3/4 out. Be sure the bottom fuel mix screws are very sharp and not snapped and broken with tip stuck in carb body. Bench sync the carbs will be close enough to start with. Check ignition is ok, good sparks ?. Re assemble and go from there. Good luck, and try to get a colortune and strobe lamp to set up with vacuum guages.

[gtem]

Based on your description of needing to use choke, only one cylinder getting hot etc, and based on overall experience with what does/doesn't go wrong with these bikes in general, my own best guess is that you might still have some clogged/partially clogged pilot circuits. Are all of the jets original Mikuni/Suzuki parts in the stock size? You really need to thoroughly clean that circuit and that pilot jet, those tiny passages are notorious for clogging when the bikes sit with gas in the carbs. I always thoroughly clean that passage out by pulling the pilot jets (be CAREFUL extracting that soft brass jet, presoak in carb cleaner if needed, apply some heat with a heat gun), then using a guitar string or thin wire clipped off a wire brush stuck into the end of a wooden matchstick to run all the way down that passage. I then follow up with some carb cleaner, blasted out with compressed air, watching the spray pattern out of the 4 small orifices in the carb throat (one for the idle mixture screw and then three more closer to the throttle plate, at least on CV carbs the VMs are a bit different it's been a minute since I've messed with VMs). I also "bench sync" the carbs before I put them on the bike. This is eyeballing the alignment of the throttle plates using one of those small pilot circuit orifices as a reference point. This gets me in the right ballpark when I put the carbs on the bike, finished with a proper vacuum sync with gauges.

[93Bandit]

True, the petcock rebuild kits sometimes work. At best, they're a roll of the dice (even when the old petcock's metal bits are in good shape inside), but some folks have indeed gotten lucky.

And of course, some folks have opened up their old petcock only to find serious pitting, corrosion, etc. They're not always rebuildable, and you don't know until you're in there.

Indeed it seems based of the GSR collective experience rebuilding an OEM petcock is a roll of the dice. I desperately wanted to keep the OEM style petcock and the only options to do so were to either roll the dice on a chiese ebay petcock, or attempt rebuilding the original. I think it was worth the ~$25 to roll the dice on the OEM rebuild vs ~$15 on chiese petcock. In my opinion it's worth attempting if the insides are in good shape.

[gtem]

In regard to rebuilding vacuum petcocks I was successful in rebuilding the original one on my GS1100E (a K&L kit as I recall), conversely I bought a brand new OEM Yamaha vacuum petcock for $100 on Ebay for my XS1100 and the damn thing lets fuel by in the "ON" position

[gustovh]

Before you buy a new peacock, take the old one apart, it’s not hard. Inspect the internals, the peacock diaphragm has an O ring that sits in a tapered seat. If that seat is pitted or the O ring is damaged the valve can leak. The valve surface can be resurfaced for a good seal. You can make a mandrel to do the job out of a pack frame pin or some other soft metal rod. I tapered one end to match the valves tapered surface, the other end in a drill. Apply a small amount of rubbing compound or equivalent to the surface to be refinished and lightly polish with the mandrel, valve grinding compound is too aggressive. Clean and inspect the valve surface and polish until pits are removed, install new O ring and reassemble. If there is not a hole in the diaphragm it should not leak and give many more years of surface.

So I guess that you don’t need a kit unless the diaphragm is bad.
p.s. Don’t mess with the little brass fitting that runs the prime function.

Just my 2 cents

V