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Compression Rate #733943 05/03/18 2:39 pm
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I recently rebuilt the engine on my 1962 Venom. Now that the bike is finished, it wont start. I am pretty sure I have the magneto timed correctly. Could someone please tell me what the fully advanced ignition setting should be in inches or millimeters that compares/equals the factory recommended setting of 38° BTDC?
Also, what should I expect as a healthy compression test reading of a new engine with new bore, piston, rings, valves guides, etc.? It seems to me the compression is not what it should be. I can kick it through relatively easy, without using the compression release. When I took the engine apart it had two compression relief plates under the cylinder. I thought this was odd. I couldn't get any definitive answers as to what it should have, so I split the difference and only put one of the plates back in, along with a paper gasket on each side of the relief plate. I am now thinking this was a mistake.
I am asking the Velocette community what their thoughts might be.
Thanks.
Fullminator

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Re: Compression Rate [Re: Fullminator] #734020 05/04/18 1:17 am
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I wish I could remember that advanced measurement (you did fully advance the mag before checking didn't you?)

For timing I use a degree wheel and a positive stop device made out of an old spark plug, both dead easy to make.

I find 'feeling' compression on the Venom rather difficult because of the very low ratio of the kickstarter that naturally makes it really easy to kick over TDC.
Also check that your decompression valvelifter is not holding the Exhaust open a bit.
Also if the rings are new they won't have fully bedded in yet so could cause leakage.

I recently did a 'Syncro-seat' valve-grind on my Clubman, and with nicely bedded-in rings and carefully assembled top end, it was still really easy to kick over TDC.

Really the only accurate way to check compression ratio is to do a proper swept volume/compression space check, measuring the internal volume at BDC and then at TDC with a burette.
Then you'll know the actual compression ratio.
Trouble is It means pulling the engine out so you can tilt it on the bench as you measure the fluid (I use kerosine).

Sorry if I'm stating the obvious.

Re: Compression Rate [Re: Fullminator] #734084 05/04/18 3:22 pm
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Jools, thanks for your reply. Success has smiled upon me. I was able to get the bike started last evening. I rode it around the block-about 5 miles.
Talking with Ed Gilkison in Washington state, he said if the totally retarded spark is occurring anywhere after top dead center the bike will not start. I had it set so the retarded spark occurred at exactly TDC. I moved it just a tad, used a new spark plug, and presto, it started first kick. At fully advanced I have it set between 10 and 11 mm BTDC, which is about 37°. I found out that 38° is 7/16" BTDC on the Venom.
I also think the compression is OK. Ed mentioned that because of the large cam overlap on the Venom, compression is hard to determine and a Venom can be kicked over relatively easily because of this.
Now my attention is directed toward the clutch. It is dragging. I am thinking the oil used in the primary will affect this. My first ride of 5 miles, the primary was dry-no oil.
Fullminator

Re: Compression Rate [Re: Fullminator] #734142 05/04/18 10:29 pm
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Excellent mate.
Re: the clutch, in 'Norm's Technicalities' there's an exhaustive coverage of all of the possible problems that can occur.
If they're new clutchplates they're likely to drag initially before bedding in. I'm sure you know this but everything has to be JUST right with the Velo clutch.

Re: Compression Rate [Re: Fullminator] #734185 05/05/18 8:50 am
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Which reminds me... The Velo clutch needs a clutch lever that has a 1 1/8" pull. That's more than most clutch levers. Worth checking.

Re: Compression Rate [Re: Joolstacho] #734399 05/07/18 2:41 pm
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Jools, I am aware of the requirement of a 1-1/8" clutch lever pull, and was sure to have one.
I adjusted the clutch to be more free and now the clutch slips and drags at the same time. When using the kick starter, and coming up to compression, the clutch will slip. I can continue to push down the kickstarter all the way to the bottom of its stroke without turning the engine over.
With the engine started and in gear, and pulling in the clutch lever, the engine continues to drive the rear wheel as if the clutch is totally engaged.
This has made for some interesting traffic light stops during the total of 15 miles I now have on the bike. It is impossible to get it into neutral in its current state, unless stopped and unrunning.
I am aware that everything about the clutch must be correct. I was careful to ensure it went together correctly, but something is not right. I now must take apart the primary again, and see if I can determine the issue.
Thanks for your help.
Fullminator

Re: Compression Rate [Re: Fullminator] #772962 05/07/19 10:37 pm
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Here I am, exactly one year to the day since my above reply. I now have about 70 miles on the bike. Yes, it has been sitting unfortunately. I think the clutch is better sorted now. At least it can be held at a stop at traffic lights. If I cant get it into neutral as I coast up to a stop, it is impossible to find if totally stopped. In fact it will sometimes die if stopped, if not in neutral.
After running about ten miles or so everything seems to be in order, except there is a fair amount of smoke coming through the generator cover. Out from the primary, from the main crankcase? This seems odd to me.
The bike is also devilishly hard to start. After reading my above posts, I am wondering if the timing has slipped to be firing after TDC again? I am proud of it, as it was a great amount of work to restore. It looks really good, I just want it to run well too. I'm sure a bit of fettling is still required.
Any thoughts?
Fullminator

Re: Compression Rate [Re: Fullminator] #772978 05/08/19 12:25 am
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A few thoughts (you probably know most of this anyway but...)
Velo riders NEVER ride (hold in) the clutch while stopped, that's the certain road to veloblivion. You just get the knack of shifting into neutral just before you stop. Clutch slip and drag? Later models (Thruxtons) had a 20 spring clutch spring carrier which gives more pressure, that might be worth a try. I'm sure you know Velos are very sensitive to the amount of clutch pushrod / 'frying pan' movement. Sounds like a bit of bedding in and careful setting up of your clutch is improving it though. But I reckon it needs more than 70 miles use.

Most people nowadays are recommending timing fully advanced at 36 deg (not 38), because modern fuel has a quicker burn rate. I only ever use about 1/4 to 1/3rd travel retard on my manual advance lever, and that's just for starting. (Is your magneto auto advance or manual?)
Also, it's nowhere near run-in with only 70 miles on it, -that won't be helping your starting issues. I assume you've read all about the starting techniques, - can get a bit arcane with a Velo!
Most well setup carbs will need no throttle at all starting from cold, and just a 'crack' when warm.

Smoke coming from the "generator cover" Eeeek! Do you mean from the actual dynamo end-cover (RH side)? or from the primary chain case or belt cover? You haven't got a slipping dynamo belt have you?
The engine 'breathes' into the PCC via the crankcase LH mainbearing, but I've never seen one smoking. There's a well-known issue with Velo crankcase breathing, which is fixed by adding a couple of breather holes in the timing cover and a tube back up to the oil tank. Remember that before a ride you might need to drain the crankcase of oil if you don't ride it regularly, - oil can seep past the oil tank ball valve into the crankcase, and that makes starting much harder, and also oil can seep into the PCC, overfilling it, and also contributing to clutch slip. This is made worse if the bike is left on it's sidestand leaning to the left.

Yes mate. You'll never stop fettling it !

Re: Compression Rate [Re: Fullminator] #773031 05/08/19 4:49 pm
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My advance is of the automatic variety. I am thinking that at 36 degrees of full advance, would put it very near or even past TDC when at full retard, which would contribute to its hard starting. I was warned about this from Ed Gilkison. The bike is very hard to start when cold.
The smoke is coming from the left side of the bike, not a lot, but enough that it is a concern. It seems to be crankcase pressure smoke coming through the generator belt cover area. I am sure it is not generator related. Seems to be more combustion leaking through the left side crankcase.

I also know it is better to not sit at idle holding the clutch in. Finding neutral as I coast up to a stop is proving to be challenging. After more break in miles, I hope the starting and the gearbox will become more natural.

Let the fettling begin in earnest!
Fullminator

Re: Compression Rate [Re: Fullminator] #773087 05/09/19 12:01 am
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The fully advanced to fully retarded range of most auto advance mags is between 11deg and 18deg max, so even at 36deg fully advanced that would take you nowhere near "past TDC" when fully retarded.

Re: Compression Rate [Re: Fullminator] #773129 05/09/19 8:36 am
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But that if that advance is through a mag connected to the cam and not directly to the crank then those degrees double at the crank, so 18 becomes 36.

Re: Compression Rate [Re: Fullminator] #773182 05/09/19 8:24 pm
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That is interesting, and is making sense to me. I seem to remember when I timed the ignition initially, I checked it at both full advanced and fully retarded. In the retarded position it was at or very near TDC. Upon getting the warning from Ed Gilkison I moved it a little more to the advanced side, and the bike started right up. It now is hard to start again, which makes me think it may have slipped a bit.
This whole thing makes me wish I had a manually, cable operated advance. I could adjust it at will. Or maybe even better still an electronic ignition that mounts on the magneto flange housing.
Jools & Kommando-thanks very much for your input.
Fullminator.

Re: Compression Rate [Re: Fullminator] #773190 05/09/19 10:50 pm
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Dead right Kommando of course. But auto advance units are very basic bits of engineering aren't they, essentially they're either advanced or retarded, but with a slightly 'ramped' action transitioning between the two. My understanding is that the only time the bobweights close down (retarded) is on the over-run, (when you don't want/need any advance anyway), in 'normal' riding they'll be out at full advanced pos all the time. That's IF the auto advance unit is working as it should. It is possible that your bobweight springs have been replaced with stronger ones, causing the bobweights pull in more, retarding the mag sooner than it should. A long-shot but not unknown.

Re: Compression Rate [Re: Fullminator] #773229 05/10/19 9:52 am
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I use a Digital Boyer on 2 bikes, it has a idle stabilisation curve whereby once the revs drop below 1000 rpm the curve advances the timing all the way down to 600 rpm, below this it reverts to retarded timing for kicking, makes pulling up at traffic lights a non event. When you read the BSA and Norton advice for kicking over their big singles with manual advance they both recommend the advance lever is set 1/2 way. So for good starting you need some advance, springs with a set ie loose will do that as even at kicking speeds it gives some advance.

You could reduce the 18 to say 15 by silver soldering the holes so they are shorter.

Re: Compression Rate [Re: kommando] #773230 05/10/19 9:58 am
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Originally Posted by kommando
When you read the BSA and Norton advice for kicking over their big singles with manual advance they both recommend the advance lever is set 1/2 way. So for good starting you need some advance.


And a strong spark, which you don’t get from a fully retarded manual magneto, which was probably BSA and Nortons’ main point.

But yes, timing too far retarded can itself make starting difficult.


Amateur Loctite enthusiast.
Re: Compression Rate [Re: Fullminator] #778280 07/10/19 4:43 pm
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A dive back into the Velocette workings has been very enlightening. I did find the timing had slipped a bit. Full retarded ignition was about 2-3 degrees after top dead center. The bike was almost impossible to start. I experimented, and set the fully advanced timing to 1/2" before top dead center. The bike then started at will, first kick every time. However it had pinking sooo bad.
I have found that setting the timing at the recommended 36-38 degrees BTDC, and with the auto advance unit I have on my magneto, the retarded ignition occurs at or just after top dead center, which makes starting a real pain. This presents a dilemma, should I try to fit a manual advance unit? I am not sure how easy that would be and what would be required to do it. Should I only run aviation fuel in the bike? This is not very convenient as aviation fuel isn't sold at every fuel station like the ethanol crap we are force fed by the tree huggers. Should I solder the slots in the advance unit to limit the retarded point? Not sure how easy this would be, and I spent a lot of effort in assembling the advance unit to the magneto drive gear. It is riveted on, and wasn't easy to do, especially to do and keep its smooth-linear movement. Should I just park the bike, display it for looks, and ride the Norton or the BSA's instead?
None of these choices appeal to me much.
Fullminator

Re: Compression Rate [Re: Fullminator] #778296 07/10/19 8:46 pm
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Is the generator belt slipping or burning up ?
Is your venom equipped with a Lucas K1F and the correct Lucas auto advance unit.?
Not unknown for some one to put a MAC unit on that has more advance movement than the Venom

Last edited by RGSROB; 07/10/19 8:51 pm.
Re: Compression Rate [Re: Fullminator] #778297 07/10/19 8:59 pm
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sorry about all these small posts. I am mystified by your lack of compression.
I can stand full weight on the KS of my venom same vintage as yours.
Only time I could push it over easily was when the original split skirt piston cracked across the crown.
I think i would be checking tappets and maybe even head tightness and rings.

Re: Compression Rate [Re: Fullminator] #778378 07/11/19 3:56 pm
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I now think the compression is good, however I cannot stand on the KS without turning the engine over. Keep in mind, I am not as svelte as I used to be. The compression wont hold my 220 lbs. I don't think it should.
My magneto is a Lucas K1F, however it has a BTH automatic advance unit. Perhaps this is part of the issue? Ed Gilkison riveted a new steel magneto gear to this advance and it works just as it should, but is the advance range different than the standard Lucas?
I know Velocette used both BTH and Lucas magnetos.
I also am over my concern about the smoke that seemed to be coming from the primary cover. It is still slightly there but I think it is a bit of combustion coming through and it isn't near as bad as it was. It is all part of the shakedown stuff, I think. I am positive it isn't related to the generator belt.
I am still interested in soldering up the advance slot holes to limit the amount of movement the advance has. Is this a common bodge others have done? And did they do it for this very reason?
Thanks.
Fullminator

Re: Compression Rate [Re: Fullminator] #778383 07/11/19 5:00 pm
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RW BURGESS in his Velocette workshop manual does warn specifically against putting a BTH auto advance on a Lucas mag.
Not sure why but is probably different advance amount and spring pressure.
I have a BTH and a Lucas unit on the bench . Oddly the Lucas unit advances in the opposite direction to the BTH.-Will have to look at that again!
The BTH was from a MAC , the Lucas from a Venom.
Interesting spec for timing the mac with BTH units was to set timing in retarded position 4 deg BTDC

Re: Compression Rate [Re: Fullminator] #778472 07/12/19 2:57 pm
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Perhaps I should look at ignition timing a little differently. Maybe I should time for the fully retarded position at just before TDC, and let the auto advance find the fully advanced position at whatever it comes out to be.
I suspect this will lead to heavy pinking as it will be too far advanced, at least with the magneto set up I currently have. Again, I am intrigued by the idea of soldering the advance slots.
I am quite surprised you found a comment from Burgess about the very set up I have-Lucas Mag with BTH advance. This is the way the bike came to me. I am wondering what his reasons were for the warning. Could it be because of the very issue I am fighting?
I wouldn't think the two magnetos/advances would be that different.
Fullminator

Re: Compression Rate [Re: Fullminator] #779847 07/27/19 3:29 pm
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some data for you on Lucas ATD fitted to velocettes.
Prior to 1955 they used a unit with 16.5 to 18.5 advance range which is what I suspect you have. As pointed out earlier this will provide the possibility of firing after TDC.
After 1955 for all models mac and Viper/ venom a unit with 11 to 13 degrees
(equals full retard about 26 degrees BTDC at the crank) was specified.
Different spring rate on the MAC.
You can dismantle your current unit and probably reduce the advance amount by welding to the stops.
Either way also fit the steel timing gear to any ATD- the fibre ones shed teeth frequently,


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