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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 04-29-2009, 01:58 PM   #1 (permalink)
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Low compression after rebuild????

I have been dogged by compression problems for ages.
1. I have fitted new barrels.
2. New pistons and rings.
3. The head was fitted with new guides and valves (not by me).
4. New small ends
5. The head was carefully torqued-up.
6. The tappets were gapped and re-checked.
7. Oil is flowing well, new oil pump and release valve.

I did a compression test on the right cylinder...60psi.
Did it again 105psi...great things are looking up!
Repeated the test on both cylinders...doom and gloom-50psi.
I went to my uncles garage and asked if the compression gauge could be checked against another one.
He said, "after a rebuild like that, low compression is not unusual...sometimes we have to tow them to start them. I bet if you start it up and run it for half an hour, it will be OK. Do you think you broke the rings?". No.
"Put the plugs in, if its leaking via the head gasket, you will be able to detect it".

Your wisdom would be appreciated.
I.E. should I start her up or strip it down again?
Several thoughts occur...did I break the rings while fitting the barrel. Is the head/barrel warped. What did I do wrong?
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Old 04-29-2009, 02:59 PM   #2 (permalink)
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firstly I would run the bike a bit ( gently), But by dropping a teaspoon of oil in the bores you will seal the rings temporarily. Try this, if it does not make much difference, then your problem is likely to be valve seal. might be head to barrel joint, but they usually 'squeak'. More likely to be tappets set too tight.

You do have the throttles wide open dont you? manually make sure the carbs are fully open.

After that you looking at checking if all the rings are in the right slots and right way up. By then you will alos be checking there aren't any broken ones.

New bores will not seal well, that is what running in is for. Do it progressivly unless you have hard (nikasil) barrels.
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Old 04-29-2009, 05:42 PM   #3 (permalink)
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eya Caulky!

need more info.

how many miles/hours on this set of rings?

hows it actually running?

a few thoughts, was the hone on the new cylinders done right? i used to try for 45 degrees on the crosshatching. even so i never had probs with new cylinders but this sorta sounds like a weirdie prob.

usually to check ring probs you squirt 40 weight oil in the cylinders and check compression with a good tester (did unk actually check your compression tester?)

low compression at that time and any backfiring indicates a valve or ignition prob or both.

the way i used to check a head after someone else did it or when i first get them is to pour a little solvent down the ports and see if i get any seepage at the heads. any seepage at all and the valves come out for a re-lap, if that does'nt result in a good looking valve and seat surface and no leaks it's time to see what the real prob is. could be anything. not all valve jobs are equal eh?
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Old 04-29-2009, 06:02 PM   #4 (permalink)
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Quote:
Originally Posted by Caulky View Post
I did a compression test on the right cylinder...60psi.
Did it again 105psi...great things are looking up!
Repeated the test on both cylinders...doom and gloom-50psi.
This is a problem ... you really have NO idea what your compression is. You need to get some accurate results ... several tests with (at least) very similar readings.

I don't know how the 650s act, but I know the 500s don't like to seat rings. Mine had almost no compression after going through the top end. Late in my break-in, I actually felt it start developing reasonable compression.

As it stands, though ... does it run? if so, does it run well? Work on getting consistent compression readings and move from there.
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Old 04-29-2009, 06:54 PM   #5 (permalink)
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Sorry, perhaps I didn`t explain clearly enough. It has not run in this state. At present I have no idea whether it will run or not.
The engine is at the state where I have fitted the rocker box oil connections (domed nuts). No exhaust pipes or carbs.
Just the engine components.
I`m rather worried about fitting the carbs, startintg her up and doing the timing again. Last time lots of smoke and inability to correct, neighbours disdain.
The compression test was done in this state. Thought I`d check that there was the recommended compression. One online site said, "I wouldn`t even contemplate working on a bike with less than 100psi in both cylinders". "Waste of my time and the customers".
I think I`ll probably fit the carbs, set them up and see what happens.
Dont really fancy stripping-down again. (third time)
Thanks for the reassurance with low compresssion, after all the new bits.
I`m pretty sure that the push-rods are seated properly...a bit of grease makes the cups `stick` to the tappet balls and you can `feel` the suction.
Update later.
Thanks for your input.
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Old 04-29-2009, 10:11 PM   #6 (permalink)
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yup!

".a bit of grease makes the cups `stick` to the tappet balls and you can `feel` the suction."

one of my favorite tricks.

pretty normal for unseated rings to have lower compression than after a nice break in.

hope you gapped the rings properly.

you did'nt answer all my questions so theres no way to go further with the diagnosis without more info.

good luck on the start up though.

personally with any potential carb or timing/setting probs i'd rope in the most savvy triumph person i could find for assistance no matter what it cost.
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Old 04-30-2009, 09:59 AM   #7 (permalink)
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I would assume that you assembled the pistons to the jugs with some oil. If you built them dry, I would not expect much in the way of compression. To the point others have made, the rings are not seated yet. I tend to think what we have going on here is you have too much info. I've never checked compression on a rebuilt engine that was not fired up yet. There is no point to it. So you did and it is making you believe you have a problem. I don't think you do if you put it together right. Do you think you were rough fitting the rings into the bore, or did you use a good ring compressor and the pistons simply slippped into place? If they slipped in, why worry that you broke the rings?
I think you are over-thinking this because you are nervous about your rebuild. I can understand if this is your first.

When I was about 12, I helped my Dad rebuild a 265 Chevy V8. By help I mean I cleaned parts, took stuff off with the impact wrench and did a lot of watching. When it was done, the engine would not start. My Dad was beside himself. We are talking about a top notch mechanic with lots of experience including aircraft. He had never not had an engine start after a rebuild. A friend of his and former racing engine builder told him to squirt motor oil down each spark plug hole and try again. He did and it fired right up. Of all the rebuilds he'd done, this was the first where compression was insufficient for the engine to start. Once it started, the rings seated and he drove that car every day for at least 3 or more years. I never forgot that experience. So, if I were in your shoes, I would not give your low readings a second thought. Quite frankly, I would have never even taken the readings until I knew the rings were seated in the jugs.
If it ain't broke, don't fix it. Concentrate on the build and not on thinking up ways for what you've completed to be wrong.
Good luck,
Rob
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Old 04-30-2009, 01:15 PM   #8 (permalink)
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Before you go any further always check the piston to valve clearance, you don't want your valves hitting the pistons.
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