Modern Vespa

I have searched and found a few links to this specific issue I have done most of the suggestions but I am still finding a very bad low compression.

Rally 180
Piston 2nd Oversize still running in on first tank of Gas about 40km
New Exhaust about 200km
Current compression 600kPA = 86.297453949 PSI no matter how many decimal points I add this is far too few PSI.

I say it is the Cylinder head because if I take for a short run I find drips of unburnt oil and fuel sprayed onto the exhaust as well as dripping from the bottom of the cylinder head. also when I remove the Head there is a distinct burn pattern at the bottom Inside stud closest to the Bike body.

While the cylinder and piston was in for bore I sent the head to get checked for warping or other. From my mechanic he seems to think it all looks okay.

Yesterday I added as recommended by my mechanic some Shellac to try resolve this issue. But it seems to still be low compression (no change)

Please every one I am now getting tired of being left at every traffic light by P125's and his dog.

To me that sounds like a warped head.. tell your mechanic to look at it once more.

I don't know what shellac is, but I can't imaging how adding that (to the fuel, I presume?) should resolve anyting.

Lap it.

I will send it for Lapping again, I did lap it myself on wet dry paper on glass. Will send it back to him maybe it is still not true.

As for Shellac aka Indian Head Shellac, its a gasket compound gummy brown stuff painted on both barrel and Head allowed to dry to tacky and then mated. it's a way of making a gasket with out giving too much thickness.

I will take the head and barrel off tonight and send it back to my Engineering shop and mechanic

Take head and cylinder off, double check cylinder studs are in tight and not moving.

decoke head + cylinder, make sure rings arent stuck.

put thin smear of high temp silicone on case where cylinder goes, then new base gasket then another thin smear over gasket and put cylinder on.

lap the head, or buy new one, put thin smear of silicone on cylinder top and head itself, stick head on.

make sure you have the washers on first then then spring washers on, put a dab of super glue on before you put the nuts on and torque up in criss cross pattern. (thin its 18nm's? i always do 25 tho)

if your head still leaks after that i'll eat my hat..............

I did this same thing but used Yamabond with great results: no leaking head.

I have double checked the studs and the head was decoked, I did install the barrel with automotive silicone *(thin smears) and the gasket. I will re apply the indian head Shellac tonight and allow to dry over night and try again tomorrow.

Thanks for the posts this far and I will give feedback. I will also try find Yamabond will call a few Yamaha dealers here in South Africa Today.

I think you REALLY ought to lap the machined lower surface of the head nice and flat before you try any sealant. It's usually not needed & just makes more work later. There's a place for that stuff, but maybe not here.

Just lap as posted or search for more info. I did it with good results, almost anyone can. It's not hard & won't take long. Bonus: you'll see right away how much warpage you have.

As I said I have lapped the head once my self with Wet /dry 600 on the head only and also sent the Barrel and head to the Engineering shop and they worked it a little and said it should be true. I wish I did not have to use sealant, or perhaps I should send head and Barrel to a different machine shop and get a second opinion

Not saying this is it, just thinking out loud.............

Too much piston/cylinder clearance on the recent rebore?

Too much ring end gap?

Try and get a 3rd oversize set of rings and try them in your 2nd oversize piston and rebore. If you can get them to fit, assemble the topend, run it at idle on the centrestand for a few minutes then do a compression check and see what you get then.

I had the same problem before. I decoked the head, changed studs etc etc. Compression was good but some dripping from the low part of the cylinder head didn't stop. I examined the oil contamination pattern and found out that it was not only under the cylinder and the pattern I saw couldn't be produced by that dripping even with a help of a flywheel.
I found the problem in another department: I took the airfilter off and found out mild wetness on the bottom of the carb box. I took a torque wrench and, yes, the bolts had to be tightened. I took Vespa for one energetic ride, parked in the garage, put a paper towel under the engine. I checked it next morning- the "cylinder head" leak stopped...
Also, few years earlier I had problems- happened to be an air leak under the carburetor.
If you are sure the plug is tight and threads not stripped, you can put brand new cylinder head and see if it makes any difference. If not- you are looking at the wrong place.
Add few more points in your list of differential diagnoses, it can help you to narrow the problem. It is better to check simple things first rather than focus on complicated issues from the beginning. What if you take the engine apart, found nothing , get frustrated, freaked out, sell the bike and all the new owner had to do is to tighter two bolts?
I always remind myself not to be obsessed with mild oil leaks on shifters, it is part of the game. If the function is preserved, the bike runs, the plug is in a good shape, other systems work, then relax. "The best" is an enemy of "good".

mine has leaked slightly at the head joint ever since I bought and its never been a problem. I have been told by many many mechanics that it is normal.

I think your problem is elsewhere. rings likely. they dont last forever.

Rings and piston are new not even done their first full tank of fuel yet.

The only reason I am still blaming a leak on the head is the fact there is a distinct burn pattern on the head through the seating surface and a little on the cooling fins

And I am not worried about the oil leak I own a Land Rover defender Oil leaks confirm there is oil I just believe that because there is oil there is insufficient seal from the head to the Barrel and thats where I am loosing the compression. I will photograph the leak and also the head when I remove it this evening.

Hey Mod... take a can of starting fluid and spray it directly into the head-to-cylinder connection.. if you have a leak it'll die.. I have an airleak right now on the polini kit I just put on (I used the old crummy base gasket) and I found the leak at the Base gasket.. Bike is idling, one spray directly at that connection and it Kills it instantly.. Be careful, don't spray it all over or aything (just direcly at the mating point) and if you have a leak there this should let you know right quick. If your head and cylinder are flat you shouldn't have the leak (I know you know this) if you get no result from the spray there, move to the base gasket and see what happens. In my case the cylinder is on and torqued well but one spritz and it's light out, so however tight I thought the connection.. it's not air tight. (I've got a new gasket on the way and I'll check and clean the mating surfaces, and dab a touch of yamabond or permatex on there. Worth a try... Good luck.

Thank you for this

This is what would help to at alleviate different issues thank you. I do understand that a flat mating point should not leak but I took the advice from the mechanic so who am I to tell him he is wrong.

I will try this starting fluid thing, am I right to say this is the same stuff normally sprayed into the carb to try get a start. We have what is called Quick start here in South Africa I am sure its the same stuff.

Also do I just to short burst sprays of the stuff at the point I think the leak is at

BTW do you know why it just dies then. is it due to a change in Oxygen fuel mix

Yes, same stuff used to start motor, and yes just spray a burst (about 1/2 to 1 second of spray) directly where you suspect the air leak... I did it several times and Every time it kiled the motor. (stopped running).
Cheers

"...when I remove the Head there is a distinct burn pattern at the bottom Inside stud closest to the Bike body."
Assuming properly torqued(with a torque wrench), you should NOT see this if it's lapped flat right, unless the mating cylinder surface is warped. Check this as well as the head surface. I recall once seeing a guy lap the head directly to the cylinder end, not a Vespa & dunno if recommended, but worked for him.


If you have the piece of glass & a fresh full sheet of sandpaper, lap it wet just a little & then take a pic. From your description it sounds like the mechanic only looked at the head & may have also measured it for flatness. No mention of lapping & it does sound as if a proper lapping & the rest of Benji's advice will fix you up to PASS Vader's test with motor still running. I like your idea of getting a second opinion if you still have problems.

Okay I will strip the lot tonight and try work it tomorrow. I do have 2 questions regarding lapping.

1. do I lap both the head and barrel on the Wet paper
2. I have searched and found that cars you use a grinding compound (Paste) but motorbikes and scooters use water papers WHY??? and why can valve grinding compound not be used.

I still may just remove them both and take them to a different machine shop to get them true. will advise on what route I went.

You can use sand paper on the head. It is aluminum and after a few strokes on the glass and wet sand paper you will notice what material is being removed if you look at the face if the head being treated. That will dtermine how many more strokes you will need give or take a few.

Generally; from what i understand, the cylinder does not need to be faced because it is steel and more resistant to warping. When i git my oversize rebore, i asked the machinist to face the cylinder just to see what would happen. It was noticealbe that he ran a tool to face it but removed very little material.


Now...i beleive it not uncommon for heads to allow a little seapage even though they were designed to seal tightly without a gasket, but your compression number is really very low. Have you checked your ring end gap?

If this Piston is new and the barrel has recently been bored would the could this end gap still be my problem

what is your end gap?

you should lap it in a figure 8 pattern at a minimum or rotate it, not slide it back and forth on the paper.

Compound will embed in aluminum more quickly than steel. I use paper as high as 1500 or 2k if I have it.

I use dykem on the part but you can usually tell by the sanding pattern how far you need to go.

I suspect the head wasn't torqued correctly. You should use oil on the washers so they don't bind. Best to use new, high-quality washers and a proper wrench. You should also retorque after riding a while.

The Mod,
I had the same distinctive pattern. I decoked the cylinder, put new studs and tightened the nuts. I didn't polish the head. The engine started working like new. I suspect minimal oil leak but I don't see any symptoms of air leak therefore I decided not to investigate till next decoking or upgrade.
My air leak was under the carburetor.
Happy thanksgiving tinkering !

Went home yesterday armed with my git 600 water paper heavy glass (we use for steam valve seat grinding at the office and the tin of Starting fluid.

started the scooter and while running I did the start fluid test (Thanks Vader) seems to be fairly air tight except when I get to the cylinder head and as you explained my old lady died. then tried again in different points around the head seems there is not only this one place i have a leak as originally thought.

stripped the head and barrel on visual inspection the head looks flat but I do think the Barrel is a little warpy. took them both to the Water paper for just a few figure 8's just to get an idea on the severity of the issue.

The head looked very good only after about 6 turns the entire seating surface was a nice matt finish. the barrel on the other hand was not as nice.


I sent the Barrel and Head in this morning to my engineering shop (different to my scooter mechanic) for them to confirm (in his words) "The Barrel mating surface looked like the ocean it was so wavey." he has run it under the surface grinder so Hopefully this afternoon I will have a fully functioning Rally 180.

I will confirm after the weekend if this worked or not. But I believe we are on the money this time maybe I can get this compression up to a very respectable pressure now. can I confirm I should be in about the 110psi range is that correct.

Wow, damaged barrel...Usually it is a head... BUT: you have awesome excuse to install bigger cylinder!!!

can someone explain to me why the starter fluid test works so well? It seems contrary to the purpose of the fluid...

I dont know but from what I can assume in my very limited knowledge I think it floods the engine with more combustibles and not enough oxygen to keep it alight

not sure though.

On a different note seems all my bogging issues are now resolved I think the head was my main problem BUT I still seem to be very low on compression. but thank goodness the leaking head is now fixed

Bump.

Hey TCC, I am very sorry I don't know why I never saw this post the 1st time. we don't have much in terms of decent Vespa spares shops here in South Africa so to get hold of a set of 3rd over size is not that easy but what I will do is bring the scooter to work one of these days and take it to a engineering shop near me and ask them if they think it was bored too big or not.

Thanks I never thought about that.