ET: Engine (Twin) How Much Compression Reading Should A Twin Have & Leak Down

[CarlHungness]

Carl, a leakdown test checks everything is sealing with engine stationary. Compression tests rely on the engine's ability to 'breathe'. ie, if the valve timing is adrift, the cylinders won't fill enough, especially by kickstart. However, reading your 16 pages of very trying times, my gut feeling is insufficient fuel in the float bowls. The clear tube check is essential, during my recent carb troubles there was a stage where one float was only lifting 1/8". The tap could not fill the bowl fast enough and the fuel level too low. Are you trying with both taps open, out of interest ?
Fuel height is also crucial.
I did'nt see a response to the blocked exhaust comment. It was a classic when I was a kid, stuffing a spud up someone's tailpipe . Symptoms way too similar !
I admire your persistence,
Regards , Terry.

I'm ready to do another test of the float level. How did you raise the float. On my 289's there is only a clip on the rod holding the float so don't see how it can be raised. Possibly a gasket under the top cap would allow some more movement of the rod up into the cap. But I sure wonder why it's all worked for over 20 years and then this massive hiccup I cannot find.

 

[fogrider]

As you say, it's been OK all these years - always worth checking though but do ensure the clear tube is against the banjo nut, not the float bowl. If the level is somewhere in the middle to upper part of the top band of the banjo, it passes the test. ( although many seem to accept even higher is fine too).
Take it from there, hope it proves to be OK, good luck !

 

[teunvandriel]

Hello Carl. The problem with ATD slipping I also had once, I was driving on the motorway, the bike was slowing down, I was trying to reach an exit and just before the exit a big thud, hole in a piston. Also lapped the ATD with valve polish. Very recently, 2 other Vincent friends had the same problem. Last year the Vin had starting problems, the problem was dirt in the carbs but first I had looked for the problem in the magneto. A friend had a new magneto that I could borrow to test, when fitted the ATD faltered, the cause was that the taper was only about 15 mm long (and the taper of my own magneto was 16 mm) so the ATD was stuck against the crankcase.

 

[teunvandriel]

Last weekend I did a float level test, applied extra gasket under the top cap and an extra thick gasket seal (2.5 mm, 1/10 " thick between the float chamber and carb

 

[CarlHungness]

Last weekend I did a float level test, applied extra gasket under the top cap and an extra thick gasket seal (2.5 mm, 1/10 " thick between the float chamber and carb

View attachment 60001 View attachment 60002 View attachment 60003

I just made a fitting that replaces the 1BA bolt in the float bowl arm and hope to test tomorrow. I don't have immediate access to an extra bottom banjo bolt as I see in your photos. Having a hard time reading the photos:
Upper left looks like the level is above the base nut, lower left appears as though your hand has lowered the tube itself and it appears as though the fuel level is at your marker point. Thus I'm not sure how to measure after looking at the top left photo. Thanks for posting the great looking photos and the advice on washers, but I don't know if I'll know what I'm doing when I hook up my line, so will give it a go and see if I can ascertain where the fuel level actually is. It seems to me the bike is running out of fuel once started, and haven't touched float bowls in years nor moved the position of the carbs...but at this point I'll try everything.

 

[greg brillus]

The tube should be held next to the mixture adjusting screw, and set just bellow the thread........Vincent carbs are usually quite level for and aft so checking as per the pics above will work fine as well.........Carbs that drip fuel generally after the engine is shut down is a good indication one of the fuel taps is leaking internally........And also if a side stand is used.........this is very common.

 

[Alyson]

I did a compression test on my Rapide. Throttle held wide open, snap on tools were used and do not use the type of gauge where you hold it over the hole. It needs to be screwed in. Mine screws in. Both plugs removed while performing the test. I did one cylinder at a time. Front was 135 p.s.i. and the rear was 145 p.s.i. I think I may have been able to squeeze a little more out of the front if I gave it a few more kicks. Can't remember but I'm pretty sure I have MK 1 cams. Hope this helps, Alyson

 

[Jim Bush]

Since you are having so much trouble setting the ATD in the full advance position, why don't you try setting at full retard at 4deg BTDC

 

[CarlHungness]

Since you are having so much trouble setting the ATD in the full advance position, why don't you try setting at full retard at 4deg BTDC

I did that, at 6 degrees. Didn't work, and now if I take the timing cover off and use just the bolt to snug up the ATD I can set it at full advance with basically a 5/16" bolt in-between them. It didn't start at 6 or 35 this time...BUT I DID
get it started at 35 degrees a couple of weeks ago, sounded normal, for about 60 seconds and died. I thought the ATD had slipped. It hadn't. Re-checked it this time and it hasn't slipped either. This is the most maddening circumstance I have ever encountered with an internal combustion engine.

 

[CarlHungness]

The tube should be held next to the mixture adjusting screw, and set just bellow the thread........Vincent carbs are usually quite level for and aft so checking as per the pics above will work fine as well.........Carbs that drip fuel generally after the engine is shut down is a good indication one of the fuel taps is leaking internally........And also if a side stand is used.........this is very common.

I made a fitting from brass tube that almost went into the threads, filed it until it could be barely screwed in
without knackering the threads, then filed it a bit more, put some Teflon tape on it, and it worked. The rear float seems to be as close as dammit to the photos here and just below the thread of the adjusting screw. Near as I can tell the front is a bit higher, but even my un-trained eye says I'm in the ballpark. I've ordered a couple of top cap gaskets, mine has none. Holding the camera and hose with my handicapped right thumb is a challenge and hope these show where my level is. So overall, I think I'm still in trouble..I may need some mod to the levels but not sure. It STILL seems like the engine was starving for fuel once I got it running...and then with the B-TH when I got it to run it was rough as can be.