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Constellation top speed

Posted: Wed Apr 27, 2022 4:22 pm
by Jakub
I am trying two different carburetors, different needles, different jets and still can’t get past 75 MPH.

I have an electronic gizmo for checking ignition for magnetos, it’s about 2-16mm which is well within factory range.

The valve clearance supposedly should be nil, since setting and defining nil is pretty difficult I’ve set clearance with the smallest blade on feeler gauge.

Not far from my home I have about 1,5 mile of dual carriageway which was bypassed by new expressway hence there is almost no traffic there. I do run both ways and check speed with GPS recorder.

Today I was trying 320 jet for Amal Monobloc 5/32. For the first time I have also tried to check top speed on third. To my surprise it was 80 MPH. So now I am really baffled. Top speed on third is higher than on fourth. It’s like top gearing is wrong. Like the engine cannot get into revs where the power is.

I have also installed electronic rev counter, it’s cheap and wildly inaccurate but with top speed on third it indicated about 6000 RPM which I suppose is in the ballpark.
Also with 35 MPH on fourth it shows about 2000RPM, again it’s more or less right. (17,5MPH for 1000RPM on fourth).

One of previous owners installed 21T sprocket. I understand there should be 20T sprocket, could one teeth make such a difference?

It’s not like I have need for speed but it would be nice to know it’s there.

Re: Constellation top speed

Posted: Thu Apr 28, 2022 1:38 pm
by Vince2
Hi, many years ago I brought my first connie, it had the same problem. Tried all the usual tricks until I discovered it was a meteor minor engine, therefore gross overgearing worth checking. Alternatively bear in mind the magneto run in the reverse direction, are you sure your timing it advanced not retarded. Vince

Re: Constellation top speed

Posted: Thu Apr 28, 2022 1:56 pm
by Jakub
Thanks Vince,

I have done cylinders rebore, pistons are 70mm but I guess Minor has same diameter. No easy answer here.
Did not checked the stroke. Are there any other differences in between Minor and Constellation engines?

As for magneto I do check with fourth gear on and moving back wheel slightly. Everything is possible but I am pretty sure the ignition is on the piston going up. Also the color and appearance of the spark plug is immaculate.

The gearing for Minor is slightly different, 15MPH with 1000RPM in fourth versus 17,5 MPH. Would it be possible to mix Minor engine with Constellation drivetrain?
If so it would get proper gearing but not enough power.

I will try to check the stroke.

Mystery continues.

Re: Constellation top speed

Posted: Thu Apr 28, 2022 3:57 pm
by Jools G
You can tell Connie barrels from those of a Meteor Minor at a glance - the MM barrels' lowermost fin is right at the very base, right on top of the crankcase. Those on a Connie have an extra (cast-in) spacer underneath the lowermost fin, so it's the spacer that sits next to the crankcase.

The Meteor Minor engine is a very tame thing compared to a Connie. I surprised the heck out of my mate (and myself) when cruising up the M1 to Donnington Park many years ago. I thought I'd been holding a nice steady 70mph, but his Kawasaki Z750 told a different story - my 70mph was 95mph in reality! I found I could also hold on to him when he was riding my T150V, where a second handful of throttle was required. All done with a single 1 1/8" monobloc and Super Meteor spec cams (in fact I've never seen an original cam that had the correct grind - the supposed 'R' markings mean stuff all!).

Constellations were always very fast - don't forget that Bob Macintyre was hauling in Mike Hailwood's 650 Triumph Thunderbird at over 3 seconds a lap in the Thruxton 500 of 1959 after he had to pit for emergency repairs. That's seriously quick!

I suspect that the timing is out on your engine if you can't get past 75mph - either that or you've got an air filter fitted which is suffocating the breathing?

Re: Constellation top speed

Posted: Thu Apr 28, 2022 4:45 pm
by Jakub
I do have pretty substantial spacer below fins. Also I have certificate from RE Owners Club listing frame and engine number, unless someone bothered himself with replacing engine number its matching vehicle.

Ignition timing or valve timing?

I did try different ignition advance on the move (manual lever), most advanced seems the best. Makes sense since modern fuels usually are of better quality and burn slower requiring more advanced ignition. It’s about 16,5mm or about 9/16 as opposed to 7/16 advised by manual.

Valve timing – don’t know, haven’t taken right cover yet but it starts, drives and accelerates pretty nicely. Not sure about it.

No air filter present.

Re: Constellation top speed

Posted: Thu Apr 28, 2022 5:12 pm
by Vince2
Hi, this could be overgearing, check the engine sprocket it should be 29t. 33 and 32t were available but with a smaller gearbox sprocket fitted. Correct sprockets from front 29,56,20,46. Looking at the magneto does the manual advance cable go into mag at the front or back? Vince

Re: Constellation top speed

Posted: Thu Apr 28, 2022 5:16 pm
by Jakub
cable to magneto is from the back,
clutch gearing don't know, back chain is 21/46

Re: Constellation top speed

Posted: Thu Apr 28, 2022 5:22 pm
by Jakub
crankshaft sprocket is 29 I think,

Re: Constellation top speed

Posted: Thu Apr 28, 2022 9:59 pm
by Vince2
Hi, do you close lever to advance ignition or open? Vince

Re: Constellation top speed

Posted: Thu Apr 28, 2022 10:10 pm
by Jools G
Early (1958) Connies were fitted with a 33T engine sprocket, this being replaced by the 29T item in all subsequent production years. The bigger sprocket should produce a higher top speed in theory, if all other aspects of the transmission remain the same. However, it would hamper acceleration.

You really do need to spend a couple of hours getting the ignition timing absolutely spot-on - it really will make a tremendous difference in how the engine starts and goes. Setting the magneto (K2F) such that the spark timing occurs precisely at the correct point BTDC both for full retard and full advance isn't a 5-minute job. On the contrary, it's fiddly and needs checking multiple times before you can finally tighten the central screw which holds the points tapered boss to the armature.

Being a single set of points, it is of course set correctly for one cylinder only (the LH one), there always being an error from one cylinder to the other. You'll need to check for the advance on the RH cylinder too in order to ensure that the discrepancy between the two isn't too large - any serious difference is best shared between the cylinders - you'll have a vibratory engine otherwise.