750 Combat top speed problem (2015)

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ILLF8ED

ILLF8ED

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Checking in with the members. My 750 Combat has had an issue for the last couple of years not being able to pull above 80mph in top gear. I've worked through several things from carbs to camshaft - these items all check out fine. Lately I suspected timing although had set this to 28 degrees max a while back. Over the last weekend I put a strobe on it and I'm seeing two timing marks. That indicates the two cylinders are not firing at the same point. I have a Lucas RITA ignition and notice the reluctor has touched the coil contact at the pickup at some point (points location). Checking to see if anyone has experience with the RITA and common faults. Up to this point I would have said the RITA is bullet proof. Before spending $300 for a Tri Spark I want to be sure this is a cure.
 
David if it pulls good up to 80 then I'd suspect something messing with too much ignition advance from wear in cam drive complex or trigger plate area, which you've already clued in on. The carbs supplied as is should pull way over the ton as you likely know and hold it a few long seconds before innate float bowel filling can't keep up. BSA found solution for that but don't apply in your case. Its possible a cam lobe as worn down allowing jerk slack in drive that mess with the ignition timing. Check your oil pump snout as they can crack loose and give unexpected slack intervals in chain the mucks up timing. Dynodave may have some Rita parts to rescue its faults but pretty much shut down his servicing them with all the upgraded options now.
 
hobot said:
David if it pulls good up to 80 then I'd suspect something messing with too much ignition advance from wear in cam drive complex or trigger plate area, which you've already clued in on. The carbs supplied as is should pull way over the ton as you likely know and hold it a few long seconds before innate float bowel filling can't keep up. BSA found solution for that but don't apply in your case. Its possible a cam lobe as worn down allowing jerk slack in drive that mess with the ignition timing. Check your oil pump snout as they can crack loose and give unexpected slack intervals in chain the mucks up timing. Dynodave may have some Rita parts to rescue its faults but pretty much shut down his servicing them with all the upgraded options now.
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Yeah, I was thinking about Dynodave for a replacement reluctor and rotor. My plan of attack is to put the points system in and see if there is improvement. Wow, have never used points on this particular machine. Will have to go back to the service manual for setup. I remember ignition played a big difference in my first combat as after getting it back from the local Norton dealer with fresh tune, it ran like a rocket.
 
illf8ed said:
My 750 Combat has had an issue for the last couple of years not being able to pull above 80mph in top gear. .
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Unless its geared real low, it should pull well above 80 in 3rd.
What happens if you try in 3rd to get above the 5000 or so max rpm its only currently doing.
(Its a lot gentler trying it in 3rd than 4th.)

Broken or sagged valve springs can also limit revs, been near there lately ?
A compression test is a quick-n-dirty check on engine condition too.
If one or more cylinders isn't sealing too good, the performance goes away real fast.
And unsynchronised carbs can be a performance killer too.
hth
 
i am by far not an expert here nor am i a troubleshooter by trade. however, as the previous posts have indicated, there isn't any mention of rpms attained in the various gears. a similar problem came up a few years ago at bonneville with a 72 combat a single carb set up. it turned out to be a gas volume/flow problem. Don't know if that applies in this case, but I would think it's worth a look.
 
You might be able to put an engine analyser onto the plugs and check what the ignition system is doing that way. Also have a look at the plug colour - a change in the timing can make them look as though the mixture is too lean. It is possible to correct an error with an error, so I'd make sure the ignition timing is right by strobing a degree disc. I'd do a ring around a few bike shops and find out what test equipment they have. Changing from a pointless pick-up to points is introducing another variable. - I'd only change one thing at a time when trouble-shooting.
 
If you go back to points, make sure you strobe the timing to ensure it's correct on both cylinders. It won't be if you just fit as per the manual.
 
I think the clue here is I'm seeing two timing marks when strobing from the left plug wire. That indicates the two points on the reluctor are firing at different degrees while passing the coil on the pick up. That alone is a problem. Whether or not it's what is causing the lack of engine rev I'm not sure. Past experience with my first combat tells me this engine is very sensitive to ignition tune. My original question is does anyone have experience with this double timing mark situation?
 
I'm pretty sure the correct mark is the one opposite the keyway, but I don't think it would even work with the wrong one.
 
DogT said:
I'm pretty sure the correct mark is the one opposite the keyway, but I don't think it would even work with the wrong one.
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When strobing it's not a matter of the correct mark. The RITA fires both sides every time the pistons come up. The reluctor has two points that trigger the inductor coil to create the spark - two points are needed to fire every time the piston comes up because the cam is running at half the speed of the crankshaft. It shows by strobing two timing mark positions on the degree scale in the primary case port which says to me each piston is getting different timing at full advance. The only conclusion I see is the ignition triggering system is not functioning correctly. I can't conclude that's causing the engine not to pull in top gear, but it's a smoking gun.
 
possibly fuel starvation issue

partially plugged petcocks/screens

possibly breather hole in gas cap plugged.....
 
Don't forget that problems like this are often the result of a few small issues adding up to a lack of speed.

If you find one thing that's faulty don't assume it's the sole cause. It might be just part of it.
 
pommie john said:
Don't forget that problems like this are often the result of a few small issues adding up to a lack of speed.

If you find one thing that's faulty don't assume it's the sole cause. It might be just part of it.
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Ah, but when you find something that's obviously wrong....pursue that first to eliminate it.
 
If you strobe the engine at 3000rpm + and the bike is on the centre stand the timing marks will jump around a bit. Do it on it's wheels with a buddy operating the throttle (if you trust him/her) get down with ear defenders on and pray to the timing god.
 
illf8ed said:
pommie john said:
Don't forget that problems like this are often the result of a few small issues adding up to a lack of speed.

If you find one thing that's faulty don't assume it's the sole cause. It might be just part of it.
Click to expand...

Ah, but when you find something that's obviously wrong....pursue that first to eliminate it.
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Of course. Good luck and keep us posted .
 
illf8ed said:
I have a Lucas RITA ignition and notice the reluctor has touched the coil contact at the pickup at some point (points location). Checking to see if anyone has experience with the RITA and common faults. Up to this point I would have said the RITA is bullet proof. Before spending $300 for a Tri Spark I want to be sure this is a cure.
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I have a Lucas RITA on another bike and it has (touch wood) been reliable for 20 years plus. What air gap do you have between the reluctor and the pick-up? It should be between 0.008" and 0.012". Check the air gap with feeler gauges in both reluctor positions, especially since you say it has been rubbing against the pick up. The reluctor should be able to hang by its own weight from anything steel, to check that it has sufficient magnetism.

Brian Hopwood, of http://www.hoppybikes.co.uk used to sell Moira ignition boxes as replacements for the RITA. Not sure if he still sells them or if they are discontinued. You could email him.

Poor fuel flow will strangle a bike. The worst culprits are paper element fuel filters, which tend to absorb water and clog with debris. If you have them, bin them.

Is it revving out in 3rd gear? Missing/hesitation? Does it accelerate better when you close the throttle slightly? Can you do a plug chop?
 
Quote: The reluctor should be able to hang by its own weight from anything steel, to check that it has sufficient magnetism.

You make a good point about the reluctor losing magnetism. I need to look into how to remagnatize it. That would be a better first action than replacing with the points setup....easier.

All the other points as covered already. - Thanks
 
I had the name of someone that would re-magnetize, but I don't remember now, but it wasn't cheap, like over $50. Last I looked a new welded one was around $100, but that may have been years ago. I doubt if that's the problem, mine's weak and I use a Pazon and haven't had any problems using a battery, it even keeps it charged, but it won't keep up with all the lights and high beam on.
 
Apples and oranges being discussed here - the reluctor is for the Lucas Rita.

How did the gaps measure out at for each firing ?
Not knowing Ritas, but if the gaps are not the same, the firings won't be ??

Did you try and see if it would rev out in 3rd gear ?
 
I have been thinking about what I wrote above, and I may have got it slightly wrong, because it's a long time since I did this as the RITA is basically a fit and forget system. The magnet may be in the pick up rather than the reluctor, but it is important that there is sufficient magnetism - sorry to be vague on this point! Mine is still magnetised, and it is unlikely to be a problem with yours. The correct air gap is important for starting. It can be difficult to get the correct air gap because the feeler gauge blade sticks to the magnet!

Also, check that the leads to the pick up are not reversed because that could affect the performance. If you are unsure which wire connects to which, try swapping them around and see what happens.

I will see if I can find some checking data so you can assess the health of the system. This may come in useful for myself at some point.
 
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