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Author Topic: Is low compression the culprit?  (Read 3633 times)

Offline SoEazy

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Is low compression the culprit?
« on: February 11, 2014, 05:55:11 AM »
Hey guys. I've been having a problem with my Bandit ever since I acquired it. First, it is hard to start. It usually requires me to push start in order to get it going. Second, the engine will lose power somewhere after the engine gets warm. The engine will continue to run but will only rev to 1k or so RPM at wide open throttle.

 I've replaced nearly all electronics including CDI and cleaned and rebuilt carbs. After running the plugs always come out black and sooty. The air screw is set at 1.5 turns out.

I decided to do a compression test as a last resort. The cylinders measured 140 psi on 3 cylinders and 135 on the #1 cylinder. I'm guessing the compression is too low to get a complete and clean combustion? Also, the test was done on a cold engine.

The manual says compression should be 142-206 psi. Am I too low to run correctly? I'm just trying to figure out if it's finally time to surrender and part the ole' girl out. Any feedback is much appreciated.
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Offline tomacGTi

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Re: Is low compression the culprit?
« Reply #1 on: February 11, 2014, 02:24:12 PM »
Compression test should be done with the engine hot, not just warm. You should also have 10% loss between cylinders, which you do.

If the numbers are down when the motor is lot, a little motor oil in the spark plug hole will rule out the rings, then it will be the valves. Perhaps a valve adjustment is in order. If you can get a leakdown test, it would be better as it would be able to tell you if it's coming out of the intake, exhaust or the rings.

Something tells me that it is more carb related than anything. The ignition system on these bikes are fairly straightforward: they either run or they don't. Another thing: you sure you have the plug leads to the right cylinders?

Start with the basics: make sure there are no vacuum leaks, make sure that every orifice and jet in the carbs are clean, make sure your choke isn't hanging up and the plungers fully return.

Offline ventYl

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Re: Is low compression the culprit?
« Reply #2 on: February 11, 2014, 04:22:51 PM »
You did not specify which engine you have, but I have one experience with B6 engine whose highest compression pressure was 123 psi and it was able to rev to limiter and otherwise ran good. So I don't think that compression is your biggest problem now.

Is oil in your engine ok? Isn't engine seized or losing oil pressure?

edit: I just found that this is posted in B4 section. Mine engine has similar compression pressures and runs fine. You have most probably some other problem.
« Last Edit: February 11, 2014, 04:24:37 PM by ventYl »
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Offline andrewsw

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Re: Is low compression the culprit?
« Reply #3 on: February 18, 2014, 01:20:03 PM »
I agree w/ tomacGTi, it's mostly likely a carb problem. It's likely not compression, as mine runs just fine on compression tested values of 105-115 psi  :yikes:. Black sooty plugs point to a too rich fuel system.  Often that can be an issue with the float needles sticking open. Other culprits can be a heavily warn needle or needle jet making it too rich when it comes off the pilot jet.

Offline SoEazy

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Re: Is low compression the culprit?
« Reply #4 on: February 19, 2014, 11:44:22 AM »
I thought I had found the problem... I had added liquid gasket to the rubber seal on the choke to create a better seal when I rebuilt them, but realized it was probably slightly opening the choke and causing the richness, so I removed it all.

That still didn't fix the problem.  :banghead: It idled beautifully for 15 mins. or so, then started to rev up and down. After test riding for around 20 minutes the engine quit revving again. It would only run 1-2K rpms at wide open throttle.

Any other ideas of things to check?
I've heard it's a good idea to check for air leaks with carburetor cleaner around the intake boots. I checked the coils which were good cold, I'm going to recheck them when hot to see if a weak spark is causing the stalling problem.

I assume the needles and slides in the carbs should be fine because the engine is fairly low mileage. It has approx. 10.5K miles on the clock.
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Offline andrewsw

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Re: Is low compression the culprit?
« Reply #5 on: February 19, 2014, 01:47:37 PM »
I assume the needles and slides in the carbs should be fine because the engine is fairly low mileage. It has approx. 10.5K miles on the clock.

you know what they say about assuming...

if the slide are worn at all, the needle jets can easily get out of round causing all sorts of problems, but I suspect this is not the case here.

Have you inspected the float needles? That's the first place I'd look for this kind of problem.

Offline SoEazy

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Re: Is low compression the culprit?
« Reply #6 on: February 19, 2014, 05:15:59 PM »
After almost a year of work on this bike, I will admit assuming ANYTHING is stupid. I am going to completely disassemble the carbs and give everything a thorough check. Including checking to see if the needles and jets are stock sizes. After all, it was given to me from a guy who got it from a guy.  :duh: Anyways, I am hoping the problem will lie within the carbs and I'll be riding this spring... finally.

I post results as I go. Thanks for the input gents.
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Offline Sean

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Re: Is low compression the culprit?
« Reply #7 on: February 20, 2014, 03:36:52 AM »
How clean is your gas tank? I had similar issue when I was using the tank I got off my parts bike and had freshly painted which also happened to be rusty, much rustier than I thought. Bike ran great sitting still (and sitting in the garage) but once I stirred the sediment up a bit riding around it got all anemic on me.

Offline SoEazy

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Re: Is low compression the culprit?
« Reply #8 on: March 03, 2014, 09:33:00 AM »
Gas tank is clean. I've cleaned it, plus ran probably a dozen tanks of gas through. Currently the carbs are off again. I'm hoping I've found the culprit of a couple of damaged o-rings in the pilot screws. I'm waiting on the new ones to come before I can retest. I'll post the results when I get 'em in.
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Offline SoEazy

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A possible discovery...
« Reply #9 on: March 11, 2014, 11:26:30 AM »
I don't know why I didn't think of this sooner....
When I was given the bike, I was also given a box of assorted parts. In the box was a spare set of carbs. Since I had the original carbs out, I disassembled both sets and found the main jets, needles and springs to be different.

The needles I had originally been using are smooth tapered with different clip positions at the top. They have no numbers or markings on them at all. The needles I found in the spare set have one taper area at the bottom and another smooth taper the rest of the way. The needles are marked 5EZ74, which I found out to be stock.

The main jets in the carbs I had been using are #97.5, the main jets I found in the spare set are #102.5. The other difference I found is the slide springs. The set in the spare carburetors are much softer than the springs I had been using.

I swapped out the needles and main jets this morning. I had to use the stiff springs because one of the soft springs was damaged. I'm hoping this doesn't create a new problem since I'm mixing parts a bit.

With any luck this will solve the rich condition in my engine that continually fouls my plugs. I'll post pictures of the needles in case anyone may be able to ID them.

Thanks gents. 
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Offline SoEazy

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Re: Is low compression the culprit?
« Reply #10 on: March 12, 2014, 04:10:29 PM »
Whoop! Whoop! It looks like the needle/jet swap worked! I was able to ride for over an hour with no fouled plugs, stalling or bogging. I hesitate to say it, but I think it's fixed.

In case anyone is interested... here are some photos of the needles that were in my carbs and results on my plugs from using them. If anyone know what needles these are, please chime in, I'd love to know.




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Offline Squishy

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Re: Is low compression the culprit?
« Reply #11 on: March 18, 2014, 08:32:42 AM »
Yes, worn needles let through more fuel.
You could've tried adjust the clips to compensate..

Offline greg737

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Re: Is low compression the culprit?
« Reply #12 on: March 18, 2014, 02:31:17 PM »
Here's a little more thought with regard to your needle discovery...

I've seen pictures of the holes in the tops of the emulsion tubes that had been worn by the needles.  The wear had caused the holes to take on an oblong/oval shape rather than having the original perfectly round shape.  As a result of this wear the holes in the tops of the emulsion tubes were now larger than the original specification and could therefore admit more fuel than originally intended.

Here's a thought about your situation:  At some point in your Bandit's history somebody installed new needles that where without doubt part of the prior owner's effort to increase the bike's performance (the soft CV slide springs are another sign of a performance modification, what I call the "boy racer" setup that makes the CV slides slam wide open the moment the carbs develop even the slightest vacuum). 

The fact that these new performance oriented needles were still in the carbs when you got the bike suggests that the bike, at least initially, ran okay on those needles.  But if, after being installed by the prior owner, these new needles started to wear into the soft brass of the emulsion tubes, slowly widening the holes, they might have eventually created a un-rideable, over-rich situation.

So now you've discovered that putting the original, leaner-running needles (leaner-running as part of Suzuki's OEM setup for the purposes of meeting government emissions regulations) makes the bike run okay.  I'm just saying that you may have not really "solved" the carb's problem.  It might be worth taking a good look at the emulsion tubes.
« Last Edit: March 18, 2014, 02:37:45 PM by greg737 »

Offline SoEazy

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Re: Is low compression the culprit?
« Reply #13 on: April 01, 2014, 11:28:36 AM »
So now you've discovered that putting the original, leaner-running needles (leaner-running as part of Suzuki's OEM setup for the purposes of meeting government emissions regulations) makes the bike run okay.  I'm just saying that you may have not really "solved" the carb's problem.  It might be worth taking a good look at the emulsion tubes.

Unfortunately, I have found that you may be correct. I've been able to ride for around a week before fouling the plugs again. The air screws are set to two turns out, so that shouldn't be causing the rich problem. Also, I think the factory jetting from half throttle on is a bit lean with a Micron slip-on.

The engine will bog during idle and sometimes requires a good bit of revs before it will go. I checked the emulsion tubes and at first thought they were fine, until I looked a little deeper and noticed there is a ridge on the inner part that appears to be worn off on some of the tubes.

So, next question... anyone know where to get new ones? They aren't sold at Bike Bandit. Is there another bike that shares the carbs that maybe I could get them for?
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Offline bandit dk

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Re: Is low compression the culprit?
« Reply #14 on: April 01, 2014, 11:57:45 AM »
Cmsnl.com has allmost all oem parts at stock bud they are asking alot of money for it

 

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