Low compression after rebuild

I'm not sure about this but I rebuild the FJ600 a while ago. And honestly I probably used some used piston rings that I found online. I went used because at the time I was trying to get by with spending very little to get it running. Obviously that's not the way to go. I'll never do that again because I think I caused a problem.

Anyway my harbor freight gauge reads about 70-80 on all cylinders. I tried to start it but even with starting fluid nothing happens. I think my coils are actually bad as I have power and ground to them and the trigger does cause the spark to do its thing. But its a weak looking spark, like very skinny and hard to see.

I'm guessing under compression it can't even make a spark and that may explain why it doesn't start. I'm also just wondering has anyone done a compression check and an engine that has never started. I'm not sure if the cylinders have to have a glaze to actually do anthing or what the deal is. I have a differential pressure gauge that I was going to put on but I've never tried this before on a fresh build.

 

Briefly explain how you are doing the compression test.

Gary H.

 

Pull all plugs, disable ignition, wot and just cranking until the needle stops. Admittedly after about 4 or 5 cranks its only around 40. I think im going to have to redo the engine again

 

Battery fully charged? Headlamp fuse removed? Tci disconnected? Air box lid removed with filter out? Throttle held to wide open position while cranking? All the plugs removed? Have you tried another gauge? Try this...place your thumb over any plug hole making sure your thumb seals the hole (shouldn't take much force). With the throttle wide open push the start button. Strong compression will easily push your thumb off the hole. Hope this helps.

Gary H.

 

I did the thumb thing. It doesnt feel like much of anything. What would pulling the headlight fuse have to do with compression. Its a fully charged brand new battery. I also have a smart charger i leave on it after i dork around with it.

I also pulled the pod filters and theres no change.

Anyway the question i asked to begin with. Has anyone done this or does it matter.

 

Describe how the rebuild was done. Was this a cheapie at-home job, or did you have a machinist clean up the cylinders and check the head?

 

Before i explain my process ive built quite a few motorcycle and car engines so i have precision tools and i have a college cert in engine rebuild and repair but i might have missed something.

The engine when i got it was laying in some guys yard. Carbs oxidized and number 3 cylinder was filled with rust and water. I broke it free and the pistons were ....well there was something wrong. I dont recall.

Anyway, i honed the cylinder and measured and were the rust spots were there was only .0002 inches depth. I have a cylinder bore gauge.

After measuring all used pistons and examing them everything was within tolerance and installed the rings and checked gaps. All of that was fine. Checked cylinder and head for true. One spot between cylinders was just under .0015 so that was ok. Not great but i dont see that causing an issue. I believe spec is 0 to .0012 or something.

Installed the head with a yamaha head gasket, took the head to the machine shop and cut the seats due to the cavataion of aluminum and had the valves lapped.

Ordered shims and all of those are within spec. Set cam chain tension and thats it.

The last time i did the differential reading it was leaking from the valves...wtf right. Nice job machine shop. After putting some 10w40 in it and retesting everything was fine then as far as clearance. I think the oil lubricated the tappets enough.

Ill try again later since im bored. Anything besides the one spot raise an eyebrow. Also number 3 actually has 110 psi if cranked enough times.

 

The engine ran at one time. If everything is in proper order and within spec it should run again. You rebuilt the top end. Are you certain there was no water or any other damage to the case? Did you break the rack when you cleaned the carbs? 110psi is max compression at #3 because that's where the gauge needle stops right? All of this is being asked with respect to your education and experience.

Gary H.

 

Alright, I'm officially throwing in the towel.

Cylinder's 1-4 70% leakage through the crank case and both ignition coils extremely weak. I've spent probably 2K getting this turd this far. I'm not spending anymore. I'll post the link on here when I do the craigslist.

I've got a newborn, not really making anything at this job my friend talked me into and I guess I'm meant to have a bike another time in life. Nice to have met you guys. Hopefully it ends up with one of you.

 

That is truly crappy. This might be worth investigating again. You may need to pull the head and lap the valves, but you should still at least be hearing the engine trying to run.

Primary coil resistance is 2.5 ohms +/- 10%.
Secondary is 21k ohms +/- 20% with the caps in place on the plug wires (measuring cap to cap).

 

I was going to ohm out the coils. I have a feeling they are garbage. They are the ones that I pulled out of the yard so I could see them being junk.

I also noticed a slue of engines on ebay for around 200 bucks or just over. I'm actually thinking of just waiting a bit and buying one of them and just stealing all the good parts off this engine.

I honestly don't want to spend anymore but its so hard to go this far and then just give up. I kinda wished I wouldn't have charged the battery cause I'm sure it'll be shot by the time I have the bike going.

 

I've been there. Projects alway take longer than we want them to. Bide your time and pinch your pennies; there are deals out there if you keep looking. The battery will be fine if you keep it on a tender, or remember to top it up once a month.

 

I had the good ol pace around the garage. I think I'm going to pull the coils tomorrow and change them if they are bad. The caps are pretty rough, the secondarys are ok but not great. If I replaced the coils it probably wouldn't be a bad idea. I gave an extra plug I had a gap of like .20" and cranked it and the arch was so skinny and barely visible. Reminded me of like blue sewing thread.

I'll try coils first if they are bad. If the engine still won't roar i'll just go with plan b. I really want to see this thing through. I'm sure by the end of this in less than a year from now i'll have a new job and probably a buell firebolt in the garage.

 

Your compression may come up with a good run in. Don't give up just yet.

 

And still riding the sh!t out of your fj. Sounds like you've picked up the towel, got your second wind and ready to get your $2k worth...cool.
Congrats on the newborn.

Gary H.

 

More like 3rd wind lol. It's a love hate thing. I had a basket case cx500 i rode for 4 years and bought another a year later. ...one thing led to another i sold it for 200 more than i bought. It was way worse than i thought.

Not quite groundhog day but close

 

Coils have 3.7ohms and 3.2 on primary. Secondary both are open or resistance i can't read on both leads on both coils. Then 1 out of 4 caps is good.....well good enough. 1 is 10.5k, 1 is 48k, one is 4.4k, and one is 1 meg ohm.

I flipped the coils over and one has two cracks where mud is shoved into and the other coil has a broken spot where the lead goes in and two more rather large cracks.

I needs some coils i reckon. Youd think i was working on a 3.0 Taurus the way i was examing the ignition coils.

 

nice. complete fun bikes.
had an XB which i miss a lot

 

Any recommendations or things to look out for. I haven't been big into American motorcycles for a while. I know the xb platform shares some things with the sportster but that its not nearly the same internals. I have a thing for unique bikes and rare ones and after reading about buell its just what I'm looking for. Fast, weird, intelligent thinking.... just cool all around in my opinion. I'm looking for possibly a firebolt, maybe a lightning, or possibly a cyclone. Firebolt is the one I want though.

 

I had a 2007 xb lightning. Most of them, mine included, have three things that go wrong: leaking oil from header gasket, fuel sensor due to rubbed wires and some years need to get wheel bearings replaced. But it's an easy and cheap fix if you can do it yourself. Easy to work on.
pm me and I can share some pics.

 

Didn't Buell make one model with a water cooled Rotax engine as well? Always wanted to see if there was a way to get that engine into a Sportster.

 

More than one. Their recently current bike was a cooperative effort between EBR and Rotax. So far the sale of EBR is looking promising in that it may resurface as a going concern.

The Rotax engine won't fit in a Sportster frame without serious modifications to the frame.

 

Plus, Waaaaaay too much tech for a HD frame! But that's why I want one... It's like a Skyline engine in a K-car. (Ok, not quite like that, but still...)