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I have a 82xj750 j maxim did compression test numbers yesterday 115 ish on all cylinders. Saw on the forums 128 was considered low. My question is how necessary is a ring job or other things to look at? Top end was rebuilt over winter new valve shims and seal. Therese no oil leaks or smoke out of exhaust. Any input would be appreciated-cheers-

 

How does it run? If compression is even across all four cylinders, then you can still tune and ride it. Just have to decide if the fuel economy, possible oil consumption, performance is acceptable to you, and if so, then keep riding it. I had similar compression numbers on my bike except two were lower than the others. This made it difficult to tune properly, no matter how much time I spent on the carbs, and I was getting approx 25MPG so I decided to overhaul the top end (oversize pistons and rings, full valve job).

I am expecting to see a marked improvement in performance after the break-in period, and hope to see MPGs in the mid-40s.

 

She runs strong. I’ve been working on dialing in carbs this season. Synced carbs and colortuned. My plans were to just ride and consider new rings this winter. I’ll keep an eye on the oil consumption

 

Did you do your test with the throttle wide open? Many newer gauges are terrible on accuracy, the fact that you don't have a lot of variance between cylinders and that it runs well and doesn't smoke I would just ride and not worry about it.

 

Yeah made sure throttle was open. Saw a lot about cheap gauge tools. Might buy a name brand one or rent and test again. I appreciate the quick response guys thank you

 

many an XJ keeps running strong with lower than spec compression numbers.

if your "low" numbers were all close to each other thats the important part of the test

 

Try a different gauge and if it runs good just ride.

 

I got about 125 on mine last I checked. But I'm also at 6000 feet. I get about 52 MPG on long rides - usually in the mountains. Around town I average about 38 MPG.

 

So you're actually at about 150psi, which is a very happy place for your engine:

XJ650 and XJ750 air-cooled engines:
Minimum: 128 psi
Standard: 156 psi
Maximum: 171 psi
Max. variance between lowest and highest: 14 psi


NOTE: for readings taken at locations that are above sea level (ASL), the following correction factors should be applied to the readings that your gauge obtains:

- 500' ASL, multiply your readings by 1.013 to get a "true" compression reading.
- 1000' ASL, multiply your readings by 1.029 to get a "true" compression reading.
- 1500' ASL, multiply your readings by 1.042 to get a "true" compression reading.
- 2000' ASL, multiply your readings by 1.060 to get a "true" compression reading.
- 2500' ASL, multiply your readings by 1.072 to get a "true" compression reading.
- 3000' ASL, multiply your readings by 1.093 to get a "true" compression reading.
- 3500' ASL, multiply your readings by 1.103 to get a "true" compression reading.
- 4000' ASL, multiply your readings by 1.126 to get a "true" compression reading.
- 4500' ASL, multiply your readings by 1.136 to get a "true" compression reading.
- 5000' ASL, multiply your readings by 1.160 to get a "true" compression reading.
- 5500' ASL, multiply your readings by 1.172 to get a "true" compression reading.
- 6000' ASL, multiply your readings by 1.196 to get a "true" compression reading.
- 7000' ASL, multiply your readings by 1.233 to get a "true" compression reading.
- 8000' ASL, multiply your readings by 1.272 to get a "true" compression reading.


Also, be aware that barometric pressure differences (during testing events separated by more than just a few hours) will cause changes in indicated compression readings. Atmospheric pressure can vary significantly over time at the same altitude, due to weather systems….you hear this on weather reports, where the forecaster is talking about “High” and “Low” pressure systems. Basically, higher atmospheric pressures translate into higher indicated compression pressures. Although it is rarely of major consequence, in order to be "dead-nuts" accurate you should also record your local barometric pressure at the time of each test. If you measured compression on a day when it was 1030mbar and observed 150psi, and then took a measurement later in the week/month/etc. when the ambient air pressure was only 990mbar, that’s an environmental air pressure difference of about 4%, and assuming nothing else had changed you would then read a 4% less compression pressure of only 144psi......

 

Went for a spin to the hardware store today. Last stop sign I noticed some smoke coming from front end. I didn’t think much of it til I made it home. Left bike running and inspected front of engine and saw oil leaking from valve cover gasket. Removed cover to replace and noticed this unfortunately. Is this part interchangeable?

 

Well, depends what you mean by "interchangeable"................also, can't tell which cap it is. Those "short" caps can either be a bearing cap or a non-bearing cap. Non-bearing caps can be swapped between engines with no issues; bearing caps are, theoretically, "married" to the cylinder head that they came on, because the cap and its saddle (in the head) are bored together and thus are "mates". However, in reality, you can (and have no choice but to) substitute a cap from another engine, and then check clearances with plastiguage, etc. to make sure that there are no problems, and if there are problems, that they aren't going to be of the semi-catastrophic kind.

 

How can I tell if the cap is bearing or non bearing? There are two caps that this has happened to. Both are the smaller ones located on the right side of both cam roller sprockets. Was considering replacing the entire head. Was able to track one down that comes with matching cams. Was just curious if this could be a quick fix i guess. Whats the worst case scenario of swapping caps?

 

Sorry forgot to ask what tolerances should i be looking for with a plastigauge? I will remove them tomorrow and post some pictures for more information thank you!

 

I believe this is the info you're looking for... https://www.xjcd.org/node/664

Should also be in your manual if you have one.

 

Having cold start issues. There’s spark on all plugs. Once bike is warmed up no starting issues. Thoughts ?

 

Have you cleaned those carbs yourself? Broke them down sense you have owned the bike? Most of the bikes that we pick up even if they are running, need to have the carbs run though by the current owner.

 

enrichment circuit may be plugged , the enrichment wells in bowls may be plugged. or pilot jet circuit may be clogged

 

HCP22924 Aftermarket camshaft journal NON-BEARING CAM CAP. On many engines, on the right side of the cam tunnel opening there is a non-bearing camshaft cap. Unlike all of the other cam caps, which actually hold the camshaft down to the head and actually have smooth bearing surfaces……the only purpose for this cap is to supply a threaded hole to accept the valvecover hold-down bolt. This same cap is used on both the intake and exhaust cams, and is on the right side of the cam tunnel opening. NOTE: on the left side of the cam tunnel opening, there is a very similar-looking cap (also with a valvecover bolt acceptance hole), but that cap is an actual bearing cap. This replacement non-bearing cap, although similar, cannot be used in place of that actual bearing cap. This non-bearing cap is easily cracked and broken if its valvecover bolt is over-tightened. And unlike the original cast aluminum cap, this cap is machined from a billet of high-strength T6061 aluminum and features a much stronger profile than the original cap. Fits all of the following engines on both the intake and exhaust camshafts: XJ550, 1984-85 FJ600, 1986-88 FZ600, 1986-90 YX600 Radian, 1984-91 XJ600 Euro, all 1982-83 XJ650 Maxim (YICS) engines, all XJ700 air-cooled, all XJ750 air-cooled, and all XJ900RK, RL, N/FN, and F engines. NOTE: XJ900 engines actually use 4 of these non-bearing caps, located on both sides of the cam chain tunnel. Each:

 

I’ve already replaced caps and checked tolerance with plastic gauge. Took them awhile to find at autozone lol. I’ve done lots of cleaning on these carbs. There was one pilot jet with partcial blockage. I removed air fuel screws and discovered more partial blockages. Bike starts better on cold start. Idles way better. I synced the carbs were off slightly. Still curious cause the bike would start right up. There has been some small electrical issues. If the shifter is slightly touched lights dim. The voltage regulator seemed pretty warm after syncing carbs today. I replaced stator plug connectors because they were melted last year. New battery fully charged. Voltage tested good loaded and unloaded. Rpm test showed 13.25. Not real sure were to go from here? Maybe check startor brushes ?

 

voltage regulator gets warm it is normal.
low output voltage is typically the brushes minimum leangth is 11 mm.
this thread has a section on the charging system how to test
The Ultimate Relay, Switch, Sensor, and Diodes Guide