1982 Suzuki GSX400L low compression... What's next?

[Phelicks]

Hey guys, being working alot on a GSX400 lately, but can't make it run right... I did all the usual (carbs, oil, Spark plugs, etc) and it starts just fine, can ride it okay, but no top end, there's nothing there. Played with the carbs, but always feel like there is something else.

So decided to do a compression test, got 90 psi on both cylinders, soooo i'm out of spec there. Did it hot, cold, with oil, always stays at 90 psi. Thinking it can be valves that's where I am.

Did some research for problems on this model (8 valves, 2 cylinders) and people say that this model is prone to cracked heads around the spark plug hole to exhaust valve. So what should I do? Get it to a shop and ask to make the motor fresh or just ride it slow?

Btw bike got 42 000km and everything seem's to be original on it, maybe it was neglected who knows 

[chris900f]

It is odd that both cylinders came out exactly the same, did you hold the throttle wide open when testing?

The valve-clearance adjustment has probably been neglected, so I would start there.

[Phelicks]

Yaaaa that's the thing I don't like, seems fishy to get the exact same reading, might be my luck day  . I did the valve adjustment prior to the compression test, set in & ex to 0.08mm so I don't think it's the issue and kept the throttle wide open on all my tests. Another thing I say was sputter of oil on the exhaust valves when I turned the bike over with oil in the cylinder. Could it be bad valves not closing properly due to carbon build up? Something to think about because of the mileage and overall condition of the bike  .

[chris900f]

If the rings are worn the oil should have increased the compression. Someone else here had the same experience and it
turned out to be the compression gauge. Try the rubber press-down fitting for the spark-plug hole if your set has one.
That fixed it for the other member.

[Bluesmudge]

Someone else here had the same experience and it turned out to be the compression gauge. T

I agree with chris900f that both cylinders testing exactly the same under all the conditions you listed is pretty much impossible.

If you have seen the wonderful HBO miniseries "Chernobyl," this reminds me of the "3.6 Roentgen....not great, not terrible." scene. 3.6 was as high as the radiation readers would go but it continued to get reported as the actual measurement, leading everyone to believe the problem was much different than the reality. The true measurement was something like 15,000. You can't always trust the measuring device.

[Phelicks]

Yes kudos to chris, got 120 psi on bot sides with the rubber ends, gonna take that like a pass and try to do a leak down test soon.

Still I don't seem to find the cause of the lack of power at higher rpm's. It get's worse the higher gear i'm in, I can get to around 80-90 km/h in sixth gear and any more throttle it does nothing and hits a invisible limiter.

Fyi i'm running pods homemade which is a crossover between a pod filter and stock air box might add a picture. Anyways, tried adding restriction and makes the issue worse.

Right now i'm waiting for a new petcock to make sure it's not a fuel issue and just because I don't know how good it is. Tried pulling the choke when bugging and did not help, releasing throttle seem to help for a sec then slowing down. Really don't know if the diaphragms are good, I assume they are because I cant see any rips and it seem to hold vaccum when lifting slide and plugging air hole.

Let me know what you guy's think!

[Bluesmudge]

Did you ever have it running right with the stock airbox?

In general these GS bikes with CV carbs like to have the increase air velocity created by the stock air boxes.
The fact that it seems to get worse the higher gear your in is probably just because the extra load on the engine from the air resistance makes the lack of power more obvious. I would guess its actually RPM related, aka fuel/air related. Possibly because of the airbox. Unless you have access to a dyno to look at your air/fuel ratio, its always best to set everything back to 100% stock and get it running right before messing with aftermarket items like pod filters or exhausts.

[Phelicks]

Sadly the airbox was in bad shape, so never was able to try the stock form and exhaust was in bad shape so had to replace everything pretty much... It's a trial and error process which is not so nice, but at least the bike starts and works okay. Just wanting to be able to ride on the highway without overheating 

[chris900f]

On some old CV carbs the slides will not lift without the constant low-pressure vacuum created by the air box.

If that's not the problem, the next obvious thing is jetting. You seem to be describing a lean condition. What
main jets are you running? are they the stock size?

[Bluesmudge]

I think it would be worth searching high and low and long and far on eBay for a used OEM airbox. On my GS1000, even leaving one of the side covers off the airbox makes it run like crap. I had a buddy with a GS550 that never got it running right with pods. Knowing those anecdotes and watching this forum for 15 years and seeing nothing but problems with pods on the GS500 makes me think the Suzuki GS bikes just don't work with pods filters. People have had much more luck with the "lunchbox" style K&N filters, but even those result in lots of posts with jetting problems. Meanwhile, people with stock airboxes are just out riding their bikes.

[mr72]

+1 try to get an airbox.

Thing is, and I am really not trying to come off as a jerk, but the fact that you are having to ask the forum how to sort out jetting issues with pod filters is evidence you are not really prepared for the challenges of running pod filters. If you just want it to work, get the airbox back. Exhaust will make minimal difference in jetting but airbox will make a lot of difference.

IMHO.

[The Buddha]

Actually If you have a free flowing pipe in addition to those pods, it will likely follow the same formula as a GS. I'd not touch the pilot, and go +8 or 10 (meaning 2.5X10 - 10 sizes, so 125 becomes 150, not 135) on the mains with 1 washer for the needle.

Some of these older bike had such restrictive exhausts because they were built during the oil shock era and that was a way to get them to use less fuel while still be nice running and torquey.

Post a pic of your carbs preferable the float bowl and top parts open, I'll tell you if you can use the GS jetting method.

Cool.
Buddha.

[Phelicks]

Yes, the stock air box would be the best option, but it's such a pain to work on carbs with it that I don't want to go that route. 

Anyways decided to take a chance and change my diaphragms and wow, now the bikes feels like a fun thing to drive! Probably had too much faith in the old 40 years old rubber.

Now i'm able to rev to 7-8k in every gear with a 125 main jet and 45 pilot and the plugs are white, telling me it need's a little more fuel. Taking in consideration Buddha's advice, for my model (original main = 117.5) the mains I should run should be more around 137.5 to 142.5. Time to order some jets!

Here's a picture of the bike  I installed some emgo pipes and the pipe inside also got a baffle to quiet the beast down. Sounded so loud before, I thought I was in the Bosozoku gang!

Kepping you guys up to date, thanks for the help!!

[The Buddha]

Slow down a sec there tiger. Plugs are white at what throttle position ? Remember 5 seconds of steady state in that throttle position will color the plug.
If I were you, and jets were the same cost as they are in the US, I'd buy every size there is from where you are to what is 10X and maybe even 12x. Going to the dealer to get them costs more than the jets sometimes. So buy 20 and you will spend only 1/4th that on gas.

Cool.
Buddha.