750 cc commando lack of performance

Hi i have a 750cc 1972 commando. I bought it 2yrs ago and in my opinion its never really performed like it should do. I was told when i bought the bike it had been re-built to combat spec. Since consulting a friend of my who agreed it is under powered and is not performing as it should do. My friend tried all he knows to correct this but with know joy. The bike runs lovely it starts in all weathers 1st or 2nd kick it idles at 1000rpm constant. Everything about the bike is a joy to own apart from this lack of performance. There are know knocks or rattles even from cold there is a very slight blue mist from the exhausts. I was getting a discharge from the oil breather pipe which was coffee coloured. I was told this was because i was starting the bike in the winter and not letting the bike get upto temperature and it was causing condensation which was then mixing with the oil. I have since changed the oil and it would appear to be ok now. I've also notice that the down pipes of which one is slightly coloured a faint yellow the other side is like knew. I've since had the carbs checked and i was told they were out of balance which as since been done the carb that was out of balance was the same side as the pipe that as not coloured up. That all i can tell you any ideas please can you help me thanks for reading.

Hi Northy,

Lack of performance is of course a very subjective and personal description.

I assume your bike just "feels" like it has no snap when you crack the throttle.

Let's assume your carbs for now are set and jetted stock, as the manual specifies?

So, the obvious and easy possible solutions are

1) Do you have stock points in the timing cover or is it electronic ignition?
Either way, have you, can you, verify the points gap and the put a strobe light on and check the timing?
You have a strobe light, you know how to do this?

2) How long ago did you pull the plugs out and check the valve clearances?
It is possible they have be set wrong and are not closing fully so you have loss of compression.

If, once you have verified the above and they check out, I think that I would get my hands on a simple compression tester and put it in the spark plug hole and open the throttle all the way and kick four times to get a reading.
Report back with the compression for each side.

Maybe your motor has really never been rebuilt since new some 40 years ago?
It may well need a rebore and new pistons, rings, and the head freshened up, valves, guides etc?

You might also check the valve lift to the stock spec's.
You may have a flat cam.
An engine can still idle fine and even start fine but will not perform in the higher rev's.
You'll need a dial indicator to place on the valve spring and measure lift.
The oils we have today lack the ZDDP needed to lube the cam and tappets.
I use VR-1 Valvoline which has the higher ZDDP needed

First, and foremost, do a compression test. All other discussion is pointless if that shows low. The blue smoke is a symptom. The compression test will help decide if the oil burning is valve guides, or flopped out bore/rings.

Other thing is, you can't compare these old wide band torque bikes to the modern bikes that take off like a rocket in a narrow band, not that the Commando doesn't but it just doesn't compare to a fuel injected 750. Now handling may be something else.

I'm sure hobot will have some words.

Dave
69S

Do the checks as listed with cam the most suspected bugger as no proper Combat will be thought timid even by modern sports riders, so yes its not yet a real Combat. Combats tend to scars me as their power band just pulls harder the faster ya go so hard not to let go soon enough. If you are hard hearted enough the big port head don't really kick in the 3rd piston hit till upper 6000's. Factory valves tend to float-clash in the low to mid 7000's. The electro brained curves tend to take some sharpness off points response but does not take away the top end power, so likely can strike ignition off with such otherwise good behavior. Bad gas? Clogged flow? Air leaks. Inhaled carb slide piece that beat a curl in the valve lips? Cam chain shot or tensioner sagged? Cam miss timed?

If the bike has a Combat cam and other modifications it really needs to be reved to get the best out of it. If you are riding it around below 4 or 5 thousand RPM it probably wont feel particularly powerful.

It certainly should feel powerful everywhere - not just high revs.

In my experience the original 2S Combat cam has rather a big hole in the powerband around the mid rev range, but comes into it's own above 5,000 RPM. The 4S cam seems to be better throughout the range and just as good at higher RPM.

Combats are just as good as standard in the low down grunt rpms - just they can keep building on that faster and faster. I had a lobe flatten and it took out the top end KaPow but was not that noticed below 5000 rpm. Them standards are no laggards either so it takes a mean hearted pilot to extract the Pow out of the Combat Bomb.

Hey Northy,
Check your timing, make sure it is timed at 31 Deg BTDC and not 28 Deg as some people think.
AC.

Combat meaner and eviler but runs kinda the same. Need a definition of poor performance. Mileage ? Battery age ?

Worn cam photo.
This came from a 1975 Mark 3. There was a problem with the hardening of the surface of the cams that year as I recall.
You get a nice and soft engine performance!
You can visually check by looking at the rocker motion.

A real Combat w/o damaged lobes will injure you on back fire kick offs if set much over 29' I've found on more than one occassion with both points and boyer. Be ware detonation on poor gas if it does start ok at 31'. If it don't hurt you or detonate at 31' on WOT from lower rpms, then ain't a real Combat yet.

This would appear to be a long inherent problem rather than a symptomatic issue.

I don't know why "Cam Timing" seems to jump out at me but it does. 11 rollers between the dots? If you can't get above 5000rpm you'll never feel that surge.

If motor, at least the top end, is fresh and the compression test is way low, then this may confirm cam timing retarded.

Hows about a bad advance unit.

Are your carbs 32mm? Even 30's on a Combat unit will jump.

Do the basics. Tappet adjustment, point gap, timing, and check the exhaust for obstruction.
Check float height, clear jets and replace needle and needle jets.
Check battery with a good load test and verify stator operation at the battery.
Check fuel flow from the tank with and without the fuel cap closed to compare.
Clean the strainers on the petcocks.
Check tension on primary chain and drive chain for proper slack.
Check for drag on the brakes and wheels.

You cannot trust what the PO said or did. You must see for yourself what you have or do not have.
I hope it turns out to be simple but if it's not, we will still get you through it.

Good luck and for christ sake, post some pictures.

Take the mufflers off

ACTUALLY , doing it for a blat will tell you a fair bit, comparitively .Some silencors kill the power .

Other than that & also , Got a Good Spark ? Throw 1/2 in & blue / white . Minimum. Really .

CARBS , are they 32 mm ( Combat size )

What fuel .

Most run 28 deg advance , I ran 31 , makes it a bit lumpy , but with 8.7 : 1 C.R. was liveable . More retarded wouldntve gone quite as well .

A measureing stick on the Valves , as above , to see if the cam lobes are there ,

BOTH fuel taps ON , when Full Wellie

Could be naff air filter too .

CARB SLIDES , both exit top of bore simultaeneously ' Balanced ' , then go from there .

ALSO check the twist grip posn full throttle , you maynt be winding it on enough . Fit a ' 1/4 turn twist grip ' ?

ALSO , itll take a FEW THOUSAND MILES to free off , if its overhauled , wont get near Max etc till past 1.000,
unless you prefer abuseing it .

I am not trying to come across as an expert, cuz I am not! I do have a bunch of questions.

When the engine was rebuilt to Combat spec did they change the cam? Change the carbs to 32mm? How about intake ports? Shave the head to increase compression? Did they measure the springs to accomadate the higher lift cam? Did they bolt on a Combat head?

There is a lot of data that needs to be collected to really start to figure this out. Could it just be that they put on too large of a counter shaft sprocket? What kind of top speeds can you make in each gear? Does the engine fail to reach max RPM? How are the plugs looking after a run? Do you have a stock ignition and are the points gapped correctly?

Have you tried a leak down test and/or a compression test?

You might end up going through most of the bike to sort it out unless something can be pinpointed.

Russ

I would like to thank everybody for there input i have since passed all your info over to a friend of mine who's looking at the bike for me. If and when i resolve the problem i will let you all know. If you have anymore ideas please let me know Email address is grahamnorth@live.com thanks again.

I dont know what you have for mufflers, but when I first put Dunstall replicars on my bike it lost all kinds of power untill I knocked a baffel plate out of the muffler center tube. If the mufflers are re- pops and not straight thru ,it may be your problem.

Strange how the hardening doe's not affect all the lobes?

Bob Z. said:

Worn cam photo.
This came from a 1975 Mark 3. There was a problem with the hardening of the surface of the cams that year as I recall.
You get a nice and soft engine performance!
You can visually check by looking at the rocker motion.

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