little compression on #3

[werdna80]

So my dad just bought a 79E, didn't take a compression tester with, Bike ran horrible on idle, but as soon as you got above 1500ish rpm it took off and ran great! We get the bike home start taking it apart to clean the carb check compression and find little to no compression on #3. PO had also crossthreaded the plug on #3.

why the heck would it seem to run great above 1500rpm with little to no compression on#3?

we are pulling it down on Friday to see what happened. Valve clearance was .015" on #3 intake.

[Bonz]

.006"-.008" is spec on intake, .015" is way loose but you wouldn't lose compresssion.

Put some engine oil (tablespoon or so) in the #3 spark plug hole, let it sit enough to put the tester in and check compression. Does it get better?

If yes, you may have bad ring seal or stuck ring(s) in #3. First thought with this is after doing the "wet" compression test (if it gets better with the oil) is put a good bit of Marvel Mystery Oil (MMO) in that cylinder through the plug hole and let it sit for a day. Start up, run the engine, repeat. See if you gain some compression back after a few cycles of that. This will loosen up a stuck ring and maybe get back the compression, but won't fix a bad ring.

[MPittma100]

Leakage

Run a cylinder leakage test and see what is leaking - valve(s), rings, head gasket? Compression test is to be run with throttle wide open. If carbs are off of engine, disregard.

[skids]

So my dad just bought a 79E, didn't take a compression tester with, Bike ran horrible on idle, but as soon as you got above 1500ish rpm it took off and ran great! We get the bike home start taking it apart to clean the carb check compression and find little to no compression on #3. PO had also crossthreaded the plug on #3.

why the heck would it seem to run great above 1500rpm with little to no compression on#3?

we are pulling it down on Friday to see what happened. Valve clearance was .015" on #3 intake.

Spark plug hole inserts can be done. I even did one once!. There is a way to hook up a shop vac (I had 4 into 1 exhaust) while the exhaust valve is open while you tap out the hole. Look up the thread if you do it.

[werdna80]

So after taking the head apart and the jugs off, we found that the PO had installed 1196 pistons in it. as far as the lack of compression on #3, we found a mouse nest in the air filter, and lots of carbon in the intakes. No valves are bent, just needs a lot of cleaning.
Also does anyone know of another brand of 1196 piston other than Wiseco? I'm attaching pictures shortly.

[Bonz]

Yep. Here's another possible source for that Big Bore Kit.

https://www.ebay.com/p/?iid=14224267...pp=true&chn=ps

[werdna80]

This is what we pulled out.

[MPittma100]

Mouse Nest

Sorry, but mouse nests in the air filter do not cause low compression. If you did not source the cause of the low compression prior to teardown, you will be lucky to have fixed it when putting back together.

So after taking the head apart and the jugs off, we found that the PO had installed 1196 pistons in it. as far as the lack of compression on #3, we found a mouse nest in the air filter, and lots of carbon in the intakes. No valves are bent, just needs a lot of cleaning.
Also does anyone know of another brand of 1196 piston other than Wiseco? I'm attaching pictures shortly.

[werdna80]

Sorry, but mouse nests in the air filter do not cause low compression.

I am aware that that would not cause low compression.
however is it possible that a mouse nest would restrict air flow and cause it to run rich? At this point with no bent valves, carbs that need cleaning, and a lot of carbon build up in the intake for #3 that's the theory I'm going with.

[TopCatGr58]

Well, the rat's nest in the air filter could have contributed to LOW compression values, but would think it would have affected ALL of the cylinders...unless the nest was specificially in the velocity stack tube part of the air filter housing for the #3 cylinder vs. the others....THEN it could cause less air flow for that cylinder, overly rich and also low compression values due to same affect as not opening the throttle all the way during the comp. test.

I did the 1179 kit, but it's been SO LONG ago, and I didn't photodocument how they looked, or whether I could see any wiseco name/logo on the underside of the piston, but I do REMEMBER that the top of the piston didn't have such a prominent dome or such recessed valve areas! SO...those may not be Wiseco, but they may be RC Engineering, I think I remember someone posting some photos that may have looked similar.

The CruzinImage pistons are also NOT forged, but cast...just like OEM's, and so their profiles are also very similar to STOCK. I know Wiseco are Forged/machined, I don't know but would think that RC Engineering are probably also forged/machined.

Also extra carbon ON the pistons can actually contribute to increased compression due to taking up more combustion chamber space so possibly increased ratio!

Okay, you may not be able to go back with the same head gasket....we know folks have reused OEM head gaskets. But the one that came with my kit was different than OEM, metallic/?Asbestos or such instead of cardboard material...and you'll need them with the larger 74mm bore size anyways. You can get them from Wiseco, but also aftermarket sources...COMETIC is the brand I remember.

With that high of piston domes, you will definitely want to ensure precise valve clearance setting, not too loose. Okay, good luck!

T.C.

[MPittma100]

Piston

Any other piston than Wiseco will be less quality. I would keep those Wiseco pistons. Just clean them up and install new rings if needed. Turn the head upside down before installing and pour a liquid into each chamber with valves closed, spark plug installed, and check for leaks in the ports. Your compression went somewhere.

[DiverRay]

That looks like an RC Engineering piston. They were about the same bore as the Wiseco, but more compression. Special Ed had them installed in his bike from a donor. VERY quick bike after that.
No scratches on the skirt, so clean it up and go.