Help me to find out what the heck it's wrong with my GS650G engine

[Guest]

Sometime ago I was asking for help here, it went well. Now I'm struggling diagnosis what it's wrong with my engine... here's the symptoms.


First of all, the engine has an overall low compression. Cylinder no1 measures just over 60 psi and the other 3 are between 80/85 psi. Suzuki says the minimum it's 100 and normal it's 142/199 psi. So, I'm guessing this it's my main problem.


The bike starts, and runs but not even closer to a good condition. I hear a tacking/ticking/slapping noise and I'm pretty sure that come from the timing chain. The chain is in spec, but the tensioner seems not to be working so well (I'm already order another).
So, the bike runs, and in a highway I'm not getting more than 130/140 km/h (80/90 mph). At those speeds the engine runs between 10/11k rpm and in 5th gear. It runs with the engine making so much effort.


Another problem/symptom starts a few days ago. After riding and warming the bike, if I stop, the engine suddenly stop working and it will no start again if I'm not letting it resting for some time. Today that happens again and I put a set of spark plugs (used, but working well), and the bike starts and run, but stops again after a few miles. The cyl 2 plug has a little wet fouling and the others seem to be a little more black (carbon) than normal.


My cylinder block and pistons are in spec and with new rings, so I'm hopping to not leaking from there. My best guess it's the cylinder head, valves, valve guides, etc.


What are you guesses on this? I'm missing something? what would you inspect to diagnose what it's wrong?


Thank you so, so much for your help.

PS. sorry for my English haha.

[gsrick]

Are the rings gaped to spec? Did you do a proper break in for the new rings with conventional oil, not synthetic? Valves adjusted to spec?

[gfpunto]

Yes, the rings are in spec. I did roughly 450km (280mi) and counting, at low/mid rpms. Valves are adjusted, but two or three of them has a very tight fit to the guide. I didn't know the oil thing, I put synthetic.

[gsrick]

Yes, the rings are in spec. I did roughly 450km (280mi) and counting, at low/mid rpms. Valves are adjusted, but two or three of them has a very tight fit to the guide. I didn't know the oil thing, I put synthetic.

Synthetic is too slick. The rings will never seat properly and that is more than likely your problem. Drain it and put in conventional oil, maybe ad some Lucas oil with zinc.

[Steve]

Low to mid RPM running will not break-in the engine. If anything, light useage will prevent proper break-in.

Good that the valves are adjusted, but how did you determine that two or three of them are tight to the guide?
If you had the valves out to replace the seals, did you clean the valve stems befor putting them back in the head?
If not, there might have been some debris on the stem that got dragged into the guide.
About the only other cause of a tight fit is a bent stem, but that would show up as NO compression, because the head would not seal.

It is my opinion that the use of synthetic oil will not prevent engine break-in. Others will have different experiences and opinions, but I have used synthetic on any engine for the last 20 years or so, with no problems.

.

[Nessism]

I don't think the problem relates to synthetic oil either. I'd double check the valve clearances and do a leak down test on the engine. That will tell you if the valves or rings are the problem.

BTW, what grit and type of hone did you use before installing the new rings?

[gsrick]

gfpunto, I have to disagree with Steve, synth is too slick and will not allow the rings and the cross hatch hone to rub each other to seat the rings properly. Do a Google search on using synthetic oil during ring seating and see what the expert engine builders have to say. They do this for a living and have reputations to uphold. We were told at MMI by multiple instructors from different manufactures, to never use synth during the break in period. You can use straight conventional oil, but adding some with extra zinc helps as long as you don't go overboard with the zinc. At the shop I work at, I don't have oil with extra zinc, so I just use conventional 10-40 and haven't had a problem yet. Since you have already coated the cylinder walls with the synthetic oil, if you don't want to tear down the engine and re hone the cylinder walls, I'd ad a quart of oil with extra zinc. Here is a good article you may want to read, but I recommend doing more research and see what others say such a piston and ring manufactures and engine builders suggest.

https://www.enginelabs.com/news/dispelling-engine-break-in-myths-with-high-performance-academy/

[Steve]

Myths about synthetic oil

Break-In Oils and Assembly Lube Needs
(scroll down about half-way down the page)

From roadcarvin.com

As I said, others will have other experiences and opinions, these links show that I am not alone with <my> opinion.

.

[gfpunto]

Wow thanks for that approach. By “conventional” you meant mineral?

[gsrick]

Wow thanks for that approach. By “conventional” you meant mineral?

Yes, good old dinosaur juice. Like I said, do your own research and make your own judgement.

[gsrick]

Myths about synthetic oil

Break-In Oils and Assembly Lube Needs
(scroll down about half-way down the page)

From roadcarvin.com

As I said, others will have other experiences and opinions, these links show that I am not alone with <my> opinion.

.

From the same site but written by JE Pistons.
How to Break-In Your Piston Rings, The Right Way!

Break-in oil should also only be used for initial engine run-in and then changed along with the filter and replaced with the engine oil you intend to run. On a street engine, this would mean less than 100 miles. Changing the break-in oil removes the impurities that will be present in the oil from the break-in period. This is especially true with race engines that use less restrictive filters. JE also recommends avoiding synthetics during break-in in order to take full advantage of establishing wear patterns. Synthetics sometimes can do too good a job of reducing friction so that the rings cannot seat properly.

[gfpunto]

Interesting discussion here. Thanks for helping me.

I did a little research, and yes, most of the guides says to do a break in with conventional oil. I don't bother taking the engine apart once again and re-honing the cylinders.

This compression problem could be the reason for the engine stop working after a while, and not starting unless I put new spark plugs?

[tom203]

"....The cyl 2 plug has a little wet fouling and the others seem to be a little more black (carbon) than normal."

While you're checking stuff, make sure your petcock is working properly and not dumping gasoline into crankcase via #2 carb throat.
I'm surprised this critter will hit 10k rpms in 5th with #1 running low

[Gorminrider]

First of all, the engine has an overall low compression. Cylinder no1 measures just over 60 psi and the other 3 are between 80/85 psi. Suzuki says the minimum it's 100 and normal it's 142/199 psi. So, I'm guessing this it's my main problem....My cylinder block and pistons are in spec and with new rings, so I'm hopping to not leaking from there. My best guess it's the cylinder head, valves, valve guides, etc.

.............I hear a tacking/ticking/slapping noise.............The cyl 2 plug has a little wet fouling ...
What are you guesses on this?

My best guess is the same as yours. nessism mentioned the leakdown test...You can make a kind of tester by gluing a tire-valve into a sparkplug and blowing air in from TDC with the crank held....but really,I think you'll hear a lot of hissing .... the head is going to have to come off if you want to fix the bike. Maybe bent valves...#2 especially,eh? (which are harder to spot than I thought, in case that helps you)

About the only other cause of a tight fit is a bent stem, but that would show up as NO compression, because the head would not seal.

well I'm not so sure of that. They might not seal on the bench with a leak test but a spinning motor can build up some compression inthe cylinder if it's turning fast enough(and run too)...it depends how badly they are leaking and how much bent. You can do a quick test with some grinding compound to see if the grind shows all-around the valve and seat. If there's gaps in the ground part, the valve is slightly bent.
That's the theory that gave me results, anyways.

[Nessism]

First of all, the engine has an overall low compression. Cylinder no1 measures just over 60 psi and the other 3 are between 80/85 psi.

There is zero chance that this poor compression is related to the oil. Something else is wrong. A leak down test will tell the story.