750 Combat top speed problem (2015)
- Thread starter ILLF8ED
- Start date
- Status
- Joined
- Dec 30, 2003
- Messages
- 3,658
Fast Eddie said:
Checking static compression is quite easy:
1. Get a cylinder at TDC on the compression stroke.
2. Lean the bike over so that the spark plug to that cylinder is as vertical as possible and support it (it'll be over a long way)
3. Using a syringe or similar, carefully measure how much oil it takes to fill the combustion chamber. I was taught to fill up to the first two threads of the plug thread to allow for the hollow part of th plug.
4. The calculation is to determine static compression ratio is: (swept volume of one cylinder + combustion chamber volume) / combustion chamber volume.
If your Combat motor is on a stock bore, and it is supposed to be 10:1 CR then the combustion chamber volume should be around 41 - 42 cc (if my maths is correct).[/quote]
Cranking compression and compression ratio are not the same thing. Cranking compression can indicate wear. I'm not in question on the compression ratio. Leak down is a bit different as it indicates wear and where it is. Will assume CR is as designed. Currently +.020" bore.
I may be getting somewhere after rechecking fuel level and adjusting it up. No chance to give it a good run yet.
Thanks
Fast Eddie
VIP MEMBER
- Joined
- Oct 4, 2013
- Messages
- 20,681
illf8ed said:
Cranking compression and compression ratio are not the same thing. Cranking compression can indicate wear. I'm not in question on the compression ratio. Leak down is a bit different as it indicates wear and where it is. Will assume CR is as designed. Currently +.020" bore.
I may be getting somewhere after rechecking fuel level and adjusting it up. No chance to give it a good run yet.
Thanks[/quote]
Fair enough David. I had thought you were trying to establish the compression ration though. You say its a Combat motor, but do you know it has the correct head and thus CR? I was wondering if the Combat cam with a low CR could be your issue?
- Joined
- Dec 30, 2003
- Messages
- 3,658
Checking static compression is quite easy:
1. Get a cylinder at TDC on the compression stroke.
2. Lean the bike over so that the spark plug to that cylinder is as vertical as possible and support it (it'll be over a long way)
3. Using a syringe or similar, carefully measure how much oil it takes to fill the combustion chamber. I was taught to fill up to the first two threads of the plug thread to allow for the hollow part of th plug.
4. The calculation is to determine static compression ratio is: (swept volume of one cylinder + combustion chamber volume) / combustion chamber volume.
If your Combat motor is on a stock bore, and it is supposed to be 10:1 CR then the combustion chamber volume should be around 41 - 42 cc (if my maths is correct).[/quote]
Cranking compression and compression ratio are not the same thing. Cranking compression can indicate wear. I'm not in question on the compression ratio. Leak down is a bit different as it indicates wear and where it is. Will assume CR is as designed. Currently +.020" bore.
I may be getting somewhere after rechecking fuel level and adjusting it up. No chance to give it a good run yet.
Thanks[/quote]
Fair enough David. I had thought you were trying to establish the compression ration though. You say its a Combat motor, but do you know it has the correct head and thus CR? I was wondering if the Combat cam with a low CR could be your issue?[/quote]
My combat is mechanically as built by Norton Villiers. Low compression due to cylinder or piston wear is one of the considerations I have worked on, but at this point eliminated with good leak down results. Working the fuel starvation angle now. Eliminated gas cap breather, it isn't obstructed. L.A.B. comment that carb bowl float level adjustment is top of bowl to fuel level was helpful. Supposed to be at a range of .17" to .25". I set both bowls at .19". They were more than .25". Wish me luck and Merry Christmas to all.
1. Get a cylinder at TDC on the compression stroke.
2. Lean the bike over so that the spark plug to that cylinder is as vertical as possible and support it (it'll be over a long way)
3. Using a syringe or similar, carefully measure how much oil it takes to fill the combustion chamber. I was taught to fill up to the first two threads of the plug thread to allow for the hollow part of th plug.
4. The calculation is to determine static compression ratio is: (swept volume of one cylinder + combustion chamber volume) / combustion chamber volume.
If your Combat motor is on a stock bore, and it is supposed to be 10:1 CR then the combustion chamber volume should be around 41 - 42 cc (if my maths is correct).[/quote]
Cranking compression and compression ratio are not the same thing. Cranking compression can indicate wear. I'm not in question on the compression ratio. Leak down is a bit different as it indicates wear and where it is. Will assume CR is as designed. Currently +.020" bore.
I may be getting somewhere after rechecking fuel level and adjusting it up. No chance to give it a good run yet.
Thanks[/quote]
Fair enough David. I had thought you were trying to establish the compression ration though. You say its a Combat motor, but do you know it has the correct head and thus CR? I was wondering if the Combat cam with a low CR could be your issue?[/quote]
My combat is mechanically as built by Norton Villiers. Low compression due to cylinder or piston wear is one of the considerations I have worked on, but at this point eliminated with good leak down results. Working the fuel starvation angle now. Eliminated gas cap breather, it isn't obstructed. L.A.B. comment that carb bowl float level adjustment is top of bowl to fuel level was helpful. Supposed to be at a range of .17" to .25". I set both bowls at .19". They were more than .25". Wish me luck and Merry Christmas to all.
Fast Eddie
VIP MEMBER
- Joined
- Oct 4, 2013
- Messages
- 20,681
I do wish you well with it David, and I suspect you MUST get there soon, simply through process of elimination!
However, with respect, your bike is not as built by NVT. It is on a plus .020" bore, so has clearly had a top end rebuild. So it is possible that some did something (change the head, fit a comprsession plate, etc) that means you do not have the 10.1CR you should have.
Jus' sayin' ...
However, with respect, your bike is not as built by NVT. It is on a plus .020" bore, so has clearly had a top end rebuild. So it is possible that some did something (change the head, fit a comprsession plate, etc) that means you do not have the 10.1CR you should have.
Jus' sayin' ...
- Joined
- Dec 30, 2003
- Messages
- 3,658
Fast Eddie said:
There are no past owner mysteries on this machine. I'm the "someone". Have owned this bike since 1997 and it sat for 20 years before I put it back on the road starting 20,470 on the odemeter, now 55,844. The oversize bore was me in '98. I have had the engine apart many times over my period of ownership. No decompression plates and used a flame ring head gasket last year, didn't like the copper type.
Fast Eddie
VIP MEMBER
- Joined
- Oct 4, 2013
- Messages
- 20,681
illf8ed said:
Fairy Nuff David...
Seems likely that the compression ratio is not the culprit. Another suspect ruled out !
I didn't realise you'd had it so long. Can you help us understand the problem history / timeline a little more? Did it used to run well? If so, when did the troubles start and what had changed prior? Why did you strip and rebuild the motor? Has it ran well since the rebuild? Etc.
- Joined
- Feb 13, 2013
- Messages
- 505
My money is on the carbs / fuel delivery. They were recently re-sleeved I recall.
Is the slide cutaway correct? New stay-up floats too low? Slides rising fully clear in the bores? Correct spray tubes?
Fuel filters in banjos clean?
Is the slide cutaway correct? New stay-up floats too low? Slides rising fully clear in the bores? Correct spray tubes?
Fuel filters in banjos clean?
- Joined
- Dec 28, 2009
- Messages
- 5,134
Yes it's not too hard to simply pull the timing cover and ign. to count the sprockets /camchain spacings. I've seen this before. Sloppy assembly . Seen the red spacer paint marks also magically fade/vanish too.
- Joined
- Apr 21, 2015
- Messages
- 322
Whilst probably NOT your problem....
Once had a Dommy that would not pull and in changing the (hah ha) silencers from the when I bought the bike genuine Gold Star shove a broomstick in one end and see it appear the other to pattern ones in an attempt to stop the police taking an interest from 5 miles away I lost 1,000 rpm and it would NOT pull above 6,000 where as before at 7,000 it would still give a kick in the butt. Later I learnt that Norton twin required a correctly designed inlet and exhaust system if they were to perform well AND that it pays to do a simple fuel check before all else........shove the tank on a coupole of seats and with a pint pot beneath turn on the fues tap...I roughly calculated that flat out IF at 120mph and covering 30 miles per gallon whilst doing so I required something like one pint every 2 minutes ..... i was getting one pint every 2 mins 40 seconds. At full throttle the tank would run the carbs weak and the longer it occured the weaker the mixture became.I was up to 340 main jets in an effort to cure the problem before I used ny brain and thought of the obvious. The fitting of a nice big bore ex Beck Arnley Enots chromed brass fuel tap solved the problem instantly. Tis worth checking the simple things first.....something many of us learn later in life.....
Many years later when we were racing a so called 'classic' Atlas we had two problems..one was eventually found to be the neoprene seal in the fuel tap had expanded and had partly blocked off and reduced fuel flow and the other was that the rubber tube mounting the carbs to the head had swelled due to the crap they call fuel these days and was partially blocking off a port... BOTH were so simple we failed to think of them but many are the old car owners whose engines would stop and then fire up again a few minutes later.....the problem being the fuel cap breather hole was blocked and fuel useage created a vacume in the tank......one learnt the first thing to do when it occured was to rush round the car and open the fuel cap and if you heard the sound of air rushing in CLEAN out the breater hole!! I bet many have had car engines apart in bits when the problem was this!!!
Probaly will NOT apply these days but we still use coils ..Once had my old Morris Minor stall every time I applied the brakes heavily.....I EVENTUALLY traced the problem to the coil which had sprung an unnoticed leak...as I applied the brakes the remaining insulating oil in the coil must of rushed to one end and the coil shorted down to earth..............a cheapo good second hand coil from my local breaker instantly cured the problem.
Once had a Dommy that would not pull and in changing the (hah ha) silencers from the when I bought the bike genuine Gold Star shove a broomstick in one end and see it appear the other to pattern ones in an attempt to stop the police taking an interest from 5 miles away I lost 1,000 rpm and it would NOT pull above 6,000 where as before at 7,000 it would still give a kick in the butt. Later I learnt that Norton twin required a correctly designed inlet and exhaust system if they were to perform well AND that it pays to do a simple fuel check before all else........shove the tank on a coupole of seats and with a pint pot beneath turn on the fues tap...I roughly calculated that flat out IF at 120mph and covering 30 miles per gallon whilst doing so I required something like one pint every 2 minutes ..... i was getting one pint every 2 mins 40 seconds. At full throttle the tank would run the carbs weak and the longer it occured the weaker the mixture became.I was up to 340 main jets in an effort to cure the problem before I used ny brain and thought of the obvious. The fitting of a nice big bore ex Beck Arnley Enots chromed brass fuel tap solved the problem instantly. Tis worth checking the simple things first.....something many of us learn later in life.....
Many years later when we were racing a so called 'classic' Atlas we had two problems..one was eventually found to be the neoprene seal in the fuel tap had expanded and had partly blocked off and reduced fuel flow and the other was that the rubber tube mounting the carbs to the head had swelled due to the crap they call fuel these days and was partially blocking off a port... BOTH were so simple we failed to think of them but many are the old car owners whose engines would stop and then fire up again a few minutes later.....the problem being the fuel cap breather hole was blocked and fuel useage created a vacume in the tank......one learnt the first thing to do when it occured was to rush round the car and open the fuel cap and if you heard the sound of air rushing in CLEAN out the breater hole!! I bet many have had car engines apart in bits when the problem was this!!!
Probaly will NOT apply these days but we still use coils ..Once had my old Morris Minor stall every time I applied the brakes heavily.....I EVENTUALLY traced the problem to the coil which had sprung an unnoticed leak...as I applied the brakes the remaining insulating oil in the coil must of rushed to one end and the coil shorted down to earth..............a cheapo good second hand coil from my local breaker instantly cured the problem.
- Joined
- Dec 30, 2003
- Messages
- 3,658
Rohan said:
The current problem started after I removed a standard cam a previous owner had done and put the correct "SS" stamped cam in. Your next question will be why did I do that. The engine suffered horrible pinging that got worse over the time of more ethanol in the fuel available. When my compression guage was still working, the reading on both cylinders was 183psi. Recently cam timing checked and confirmed using a degree wheel and travel dial. Even with that I played around with advancing the cam timing and found setting to the factory marks gave the best result.
- Joined
- Aug 8, 2005
- Messages
- 2,688
mschmitz57 said:
I have found with my experience regarding a set of re-sleeved carbs (850 using original 932's ) is that I had to go up one main jet size to get it to pull beyond 80 MPH. Ride your bike on a long slight uphill road and get it on WOT. If it is bogging roll the throttle off slightly and see if the engine runs better. If so try and up the jet size Note: If pinging is at all prevalent do not continue until you resolve that issue first...e.g. change your gas station
Cheers,
Thomas
CNN
- Joined
- Aug 26, 2010
- Messages
- 8,215
With much difficulty. !
I'm not aware of any (comprehensive) published figures for a Combat cam,
using the Norton cam followers and valve lifts.
Not that I have looked for any, not owning a Combat cam.
But I was curious how you were so certain that it is accurate...
(And we might ask, how that cam was loose and not in a bike ?)
I'm not aware of any (comprehensive) published figures for a Combat cam,
using the Norton cam followers and valve lifts.
Not that I have looked for any, not owning a Combat cam.
But I was curious how you were so certain that it is accurate...
(And we might ask, how that cam was loose and not in a bike ?)
- Joined
- Dec 30, 2003
- Messages
- 3,658
Rohan said:
I first measured the new camshaft to make sure cam lobe maximum lift was to spec. With the camshaft in the engine set up a degree wheel on the drive side of the crankshaft setting TDC determined by dial indicator on the piston crown. Max lift should be halfway between spec open and close degrees. Set up the travel dial on the valve tappet and rotate the crankshaft forward until max lift occurs. Read the degree wheel to see if it is indeed half way through the open/close spec. Assume the cam is ground to spec, so didn't check the profile. Cam timing is verified. When taking the degree readings go back and again forward crankshaft rotation to confirm. Only take readings from a forward crankshaft rotation. It's really only necessary to do one lobe as the lobes are fixed pisition in relation to each other.
- Status