Getting a Project Going- Compression

All the work you do in the workshop. Maintenance, repair, troubleshooting, customizing, etc.
Post Reply
efinke96
New to XJR
New to XJR
Posts: 4
Joined: Mon May 29, 2017 9:38 am
Region or State: MD
Motorcycles Owned Currently: 1994 Seca II

Getting a Project Going- Compression

Post by efinke96 » Tue May 30, 2017 12:09 pm

Hey everyone, new to the forum. This is my first time working on a bike like this, usually a car guy, so forgive any ignorance. Bought a non runner from someone who said a new fuel pump was needed for 900 bucks, seemed like a good deal. As far as I can tell everything he told me is true, but still running into some problems. My main concern is some slightly low compression.

First off, changed the oil and filter. Bike had been drained before storage by the previous owner.

Secondly, when i was pulling the spark plug on cylinder one and went to put in a new one, it revealed a cross threaded spark plug hole. Just a little at the top, but enough to stop progress until a back tap comes in. Absolute bummer, but it really is very minor so I'm not too worried about it. The other three are fine though, and all cylinders appear to have spark. All spark plugs were replaced.

On the other three cylinders, Compression was around 130-135 for all cylinders. I'm assuming the cylinder I wasn't able to test will be around the same, but obviously will have to wait and see. For the sake of this post let's assume it reads between 130 and 135 like the others. I know this is a little low, and based on what the PO told me, I believe it will be a head gasket and valve adjustment. I'm fully willing and able to do both jobs, but I'd love for the bike to run at least on starter fluid before I start tearing it down. Does that sound possible with these compression numbers? I just want to hear signs of life before diving into the engine.

I'll keep updating with progress on the bike. I appreciate any and all advice. Thank you in advance!

User avatar
Jimbo
XJ Enthusiast
XJ Enthusiast
Posts: 536
Joined: Thu Jan 05, 2012 7:24 am
Region or State: IL
Motorcycles Owned Currently: 1994 Yamaha Seca II
Location: Illinois

Re: Getting a Project Going- Compression

Post by Jimbo » Tue May 30, 2017 6:27 pm

When you did the compression check, did you also squirt in a little motor oil into the cylinders and then check again? I like doing both the dry check and then the wet oil check for comparative data. The reason being if the bike had been sitting a long time the oil drains off the cylinder walls and may stick so they don't seal as well. If adding a little oil raises the numbers noticeably (acts as a temporary piston ring seal) then your lower compression issue may be the rings. Not to get overly concerned and thinking a rebuild would be in order, remember because as stated before, since the bike has not been run in a while getting it running again may loosen up the rings so they seal again. Even with compression in the 130 range, the bike should still start & run. It usually isn't till compression drops to close to 100 psi that the bike may not start.

Last, welcome to the forum!

efinke96
New to XJR
New to XJR
Posts: 4
Joined: Mon May 29, 2017 9:38 am
Region or State: MD
Motorcycles Owned Currently: 1994 Seca II

Re: Getting a Project Going- Compression

Post by efinke96 » Wed May 31, 2017 12:07 pm

Jimbo wrote:
Tue May 30, 2017 6:27 pm
When you did the compression check, did you also squirt in a little motor oil into the cylinders and then check again? I like doing both the dry check and then the wet oil check for comparative data. The reason being if the bike had been sitting a long time the oil drains off the cylinder walls and may stick so they don't seal as well. If adding a little oil raises the numbers noticeably (acts as a temporary piston ring seal) then your lower compression issue may be the rings. Not to get overly concerned and thinking a rebuild would be in order, remember because as stated before, since the bike has not been run in a while getting it running again may loosen up the rings so they seal again. Even with compression in the 130 range, the bike should still start & run. It usually isn't till compression drops to close to 100 psi that the bike may not start.

Last, welcome to the forum!
Awesome Jimbo, I really appreciate the input, and i'm I'm glad to be on the forum! Unfortunately I didn't have time to do the wet test, as I was scrambling to wrap up before a Memorial Day Barbecue, but that is definitely on the list of things to do. Hopefully I get a few psi back after that. After reading more on the forum, I'm leaning towards an electrical cause. I saw Radare say that the oil light coming on while starting is a good indicator of a failed safety switch, and I have that every time time I crank the bike. A buddy and I are going to do a full electrical diagnosis via Radare's guide and will report back early next week, hopefully with good news.

User avatar
Jimbo
XJ Enthusiast
XJ Enthusiast
Posts: 536
Joined: Thu Jan 05, 2012 7:24 am
Region or State: IL
Motorcycles Owned Currently: 1994 Yamaha Seca II
Location: Illinois

Re: Getting a Project Going- Compression

Post by Jimbo » Thu Jun 01, 2017 5:33 am

You also may want to try spraying some starting fluid into the carbs to see if it will run briefly. If it does, then you know the electrical systems are good and you can focus on fuel delivery system. These bikes can actually run without a fuel pump if the tank is elevated above the carbs (gravity feed) and the petcock is switched to prime. The pump is mainly for when the fuel level drops to less than half tank when mounted on the bike where the fuel level is getting lower than the carbs (can't gravity feed anymore). The pumps are very low pressure vacuum operated pumps. I would focus on getting it running first than worry if the pump is good or bad and needs replacement.

I also find replacing the gas lines with clear fuel lines helps a lot. You can actually see the fuel flow and if any contamination is in the system. I am sure the original fuel lines are getting old and hard by now.

Post Reply

Who is online

Users browsing this forum: No registered users