Compression totally over the top

[mdnzz]

Well took the bike in to get the carbs balanced adn retuned and to get the leak checked out in the 2cyl. Turns out the compression for a start is 1-4 respectively, 200,20,25,250, mech reackons its clipped the valves in the middle cyls and is gonna cost me a fortune to fix. dont want her to turn into a garden ornament but wondering if its worth keepin her goin?
your views much appreciated
cheers guys

[mro]

Bike was running?
No smoke?

Pulling the head is not too hard.
May have some bent/burned valves in the middle cylinders and will have to remove the head to know.
Also will see why such high compression in one and four.

Cost will depend on what you find and if good used parts be available down under...


mro

[mdnzz]

bike was running till i noticed a fuel leak out the the 2cyl exhaust, took in they did the compression test and came back with that score. wasnt smoking before hand. it has had some mods already done to it, its out to 1200 has a thicker gasket bottom to compensate somewhat for the high compression, was told that it had domed pistons, but guess will find out when the heads off

[AlanB]

Hi Mdnzz,

I would look a bit further before I pulled the head; Take the cam cover off and slowly turn the motor over (remember these turn the "Wrong" way!) If you have any bent/clipped valves, that bucket will not be close to the base of the cam lobe.

It is also possible that some of the valve clearances have closed up, so the valves are not sealing; there should be SOME clearance between the base of the lobe and the shim. (at this stage, the actual value is not important; only that there is some!)

As you turn the engine over, it is also possible to WATCH the valves moving, and see what is going on; a lamp shone into a plug-hole will help, and a view into the ports can also help, if you get as far as pulling the carbs and exhaust.

You should then have a pretty good idea of what the problem is before you go in there.

AlanB

[planedick]

Be suspicious

I would really suspect that the readings of 250 are not right at all and that the mech made a serious mistake. I can't imagine a reading of 250 on these as they're not a high compression engine. You are talking 250 lbs per square inch?

[fredintoon]

Suspect numbers

Hi mdnzz,
if those compression numbers are in psi I'm with planedick. No effin' way will an XS11 cylinder read that high. But didn't NZ go metric years ago? Are the pressures measured in Kpa (Kilopascals)? Now 250Kpa is a tad over 36psi which, with your 200, 20, 25, 250 numbers, says that #1 & #4 are super low and #2 & #3 are totally f**ked. Or your tech misread his instrument. Metric or Imperial numbers, either way, get a second opinion before you subject the bike to major surgery.

[81xsproject]

Compression testers are very affordable and a great tool to have on hand. A must when buying ANY used vehicle. Get one and do the test yourself. Make sure you do it with the plugs out and while holding the throttle wide open.

[mdnzz]

little update

well got her back from the bike shop, fired up and got her running. backfired a few times 1-2 cyl, running bit rich 3-4 cyl, got our workshop fellow to do a simple compression test, final readings we got from just turning it over was between 110 and 120 all cyls. have since taken tank and carbs off, need to repair inlet boots due to usual cracks, petcocks continue to dump fuel no matter what position, can I really just do away with these and go aftermarket? they are not connected correctly. Actually had my eye on a set of old school brass taps they are threaded so guess some mods follow. have got a 4n1 to go on should change her tune a little. did notice though while we had her running that there was a tappetting type rattle in the top?? valves?? any advice most welcome..cheers guys

[CarolynS]

Tappet type rattle

I'll probably be sorry I did this, but I can't help but send a response halfway around the world....
I noticed in the manual that the cam chain needs to be adjusted every 3,000 milesOR when it begins to make noise . I can't help but think that, according to all the research I'm doing-here and other places-that that may be an issue. The numbers on the compression test are a little low, that is probably part of the problem-cam chain.
I also like what the "guys" are saying...according to them there's really nothing on these bikes that a relatively intelligent, somewhat mechanical person can't fix. Either do some other testing or get a second opinion. The work you're talking about is extremely expensive. It would be lousy to pay the money and have the bike be no better than before.
Hope you work it out.

[TopCatGr58]

Hey there MDNZZ,

What work was done at the BIKE SHOP??? Did they adjust the valves, or pull the head and replace any valves??

These engines are notorious for tappet noises in the top end!

Read the tech tips thoroughly about doing the cam chain tensioner adjustment, and follow them Xplicitly and it should help reduce the noise a little! IF they adjusted the valve clearances properly, then once you've done the tensioner, then what you hear is probably what you will continue to hear!!

Stock Comp is 140 + 14 psi at sea level, but this is a NEW ENGINE, so depending on years of wear, 120 may not be that bad at all! Was the "simple" comp test done with THROTTLE WIDE OPEN after the engine was warmed up??? IF not, then you might want to repeat it, have ALL plugs out to allow engine to spin easier, Fuel OFF, but throttles wide open!
T.C.

[mdnzz]

hey topcat,
they did nothing other than a compression test, dont think they like the old bikes at all. got bike back cleaned her up and did the test myself,
Note no great mechanic here.
However after several test I got 1-4 152.5, 132.5, 125, 130.5.
Now these numbers look a little more acceptable bearing in mind it has a wiesco (sp) kit in her as well so am expecting its compression to be above normal.
Found I was getting a high pitch whistle and on further examination found the number 4 boot had seperated itself. Need now to replace the lot, have been told XJ boots fit, but what about the 650's would they fit as well.
Have removed the octopus setup and gone with 2 aftermarket taps that are direct to the carbs so getting plenty of fuel.
Do I have to have taps feeding opposite carbs as somebody else posted?
At present they are link left to 1-2 cyls and right to 3-4 cyls
Did this as the old taps never turned off and I had fuel/oil mix.

[TopCatGr58]

Hey there MDnzz,

Well, those comp levels are a bit more even, but may also still be a bit low for a Big Bore kit engine!? However, did you do the test with the engine warmed up?? Also, after getting 1 set of readings, you can place a few drops of oil in each cylinder, rotate a few times and the repeat the test. If it raises the levels a lot, then it's a sign of worn rings, if it doesn't raise them, then could be indication of valves not sealing as well as could!?

My 'little' big bore of 1179 on the '81 engine/head yeilded comp. levels in the 180 range after breakin, and this was COLD!!

Dan Hodges and others say that the big bore kits are supposed to give a comp. ratio of 10.25:1 vs. the stock 9.2:1 for 78-79 and 9.0:1 for 80-82, but that value was calculated on the older 78-79 engines with smaller head/combustion chambers. The 80-82 heads used larger valves, had larger domes, and was supposed to yield lesser comp. ratios because of this, but 180 PSI is pretty darn good/high IMHO!!!!

The crossed fuel routing was just for ease of fitting and routing, if yours are doing okay without kinking, then they are fine the way they are!

If the engine has sat for a long time, there can be some surface rust in the cylinders etc, and folks have found that after their bikes have been ridden a few hundred miles, that the rings reseat, and their comp. values go up!! Keep at it, good luck!
T.C.

[egsols]

152.5 on 1 sounds a little higher than the rest.

Mine averages around 165 on a supposedly stock 79sf. (warm, throttle open) mid 120's cold.

Even if your petcocks are not shutting off, your float vlaves should hold the fuel back from overflowing the carbs and allowing the fuel in the oil...you might want to check your float heights and valves.....also if you are re-routing the lines do yourself a favour and get some inline filters. Like TC said.....crossing the fuel lines from left side to feed 3-4 gives the extra room....and this will allow for the filters.

If your are talking about the intake rubbers, I don't believe the 650's will fit, different hole alignment at head mount.

[Ivan]

another trick to check the rings is to squirt a few pumps of light oil in the cylinder and re do the compression test. If the numbers jump significantly, the compression rings are probably worn out. If they stay pretty close to the same, rings are probably good, and a leak down test is in order. There is a tool for this, but it can be done by putting air to the compression tester hose WITH THE SCHRADER VALVE OUT from the plug end of the hose. With the engine at TDC on that cylinder, you can listen for air rushing out the exhaust or carb. That would give you an idea of valve leakage. Kind of a pain, but lets you know what is going on without engine disassembly.

Hope this helps.

[egsols]

Originally post by Ivan

There is a tool for this, but it can be done by putting air to the compression tester hose WITH THE SCHRADER VALVE OUT from the plug end of the hose. With the engine at TDC on that cylinder, you can listen for air rushing out the exhaust or carb.

Never thought of that. Instead of listening to the exhaust/carbs wouldn't the compression gauge register the pressure in the cylinder? If it did and started dropping fast then you know the valves are leaking....right?