Tornado
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Have you checked the tank vent hole in the cap and the petcock mesh screens for fouling, giving a fuel starvation issue at max demand?
Combat performance issue - RITA
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NickZ
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For those interested in my experience with the Rex's Replacement RITA circuit board,
I believe it performed the same as the original, other than the one anomaly with the static spark test reported in my post #82 here. This issue might be a function of my master ignition switch, which is not original.
I did some tests to see if the new unit is sensitive to input voltage and found that in my bench test setup, it continued to produce sparks at least down to -6V, as did original RITA unit.
I did not make any power consumption comparisons. Not sure how useful those would be because the bulk of the power is consumed by the coils. In fact, isn't it true that if the modern circuit takes advantage of switching transistors that have lower saturation voltage (or whatever it is called now) there would be more power consumed by the system because whatever that difference in transistor voltage is, it will show up across the coils, resulting in greater current and therefore power consumption. The thing that could change power consumption would be the duty cycle, which I don't know if Rex's modified. (I think not).
I believe it performed the same as the original, other than the one anomaly with the static spark test reported in my post #82 here. This issue might be a function of my master ignition switch, which is not original.
I did some tests to see if the new unit is sensitive to input voltage and found that in my bench test setup, it continued to produce sparks at least down to -6V, as did original RITA unit.
I did not make any power consumption comparisons. Not sure how useful those would be because the bulk of the power is consumed by the coils. In fact, isn't it true that if the modern circuit takes advantage of switching transistors that have lower saturation voltage (or whatever it is called now) there would be more power consumed by the system because whatever that difference in transistor voltage is, it will show up across the coils, resulting in greater current and therefore power consumption. The thing that could change power consumption would be the duty cycle, which I don't know if Rex's modified. (I think not).
NickZ
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That does sound like something that would cause what I am seeing, but I believe I verified fuel flow is not an issue. Ran taps for 1 minute into catch bottles. This resulted in at least 425 mL, which well exceeds the 300 mL mentioned in AN post. Interstate tank fuel level was about 1” below top of center cavity.
Tornado
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OK, but it may take more than one minute for a slow or plugged tank vent to make a difference. Easy to give it a blow through with some WD-40 etc to verify nice stream through. Or could do a speed run with the cap not fully latched down (with a low fuel volume to save risk of spillage).
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Nick
I checked tank vent, played with carb adjustments, switched electronic ignitions, checked battery condition, checked leak down and cylinder compression…etc. Like you I have struggled on this for years and no solution. It revs freely in all gears except 4th. 21T front sprocket. The last time it pulled strong in 4th it had a standard cam a previous owner installed. About six years ago I put a combat cam to increase the overlap hoping to reduce the 183psi cylinder pressure. Variables I have not looked at are available fuel, spark plug leads (using copper core wires and NGK resistor caps). I remember my first combat I bought in ‘73 that still had a 19T front sprocket. It got to 100 like a scalded monkey while trying to pull my arms out of socket.
So I am watching to see what you find.
I checked tank vent, played with carb adjustments, switched electronic ignitions, checked battery condition, checked leak down and cylinder compression…etc. Like you I have struggled on this for years and no solution. It revs freely in all gears except 4th. 21T front sprocket. The last time it pulled strong in 4th it had a standard cam a previous owner installed. About six years ago I put a combat cam to increase the overlap hoping to reduce the 183psi cylinder pressure. Variables I have not looked at are available fuel, spark plug leads (using copper core wires and NGK resistor caps). I remember my first combat I bought in ‘73 that still had a 19T front sprocket. It got to 100 like a scalded monkey while trying to pull my arms out of socket.
So I am watching to see what you find.
The Combat engine produces less power than the standard 750 in the midrange. This is likely in part due to the cam and in part due to the factory Combat port job, which Dynodave described as "would be considered ruined by modern standards" or words to that effect.
Is it possible that this reduced midrange power when combined with a 21 tooth sprocket is enough to prevent the bike from pulling full revs in top?
Will these bikes pull to full revs in 3rd gear?
Even using 6500 as redline will net 90 mph in 3rd with a 21 tooth gb.
sprocket.
A shift into 4th at that point would put the engine at 5300 rpm which should be well into the Combat cam power band. The engine should then keep pulling to well above 6,000.
If you haven't already done this, try taking it to 90 mph in third before doing the shift into 4th. If the bike then pulls on up to 110 mph or so that would be quite telling.
According to the big white book, 110 mph is 6500 rpm in 4th with the 21 tooth sprocket.
Glen
Is it possible that this reduced midrange power when combined with a 21 tooth sprocket is enough to prevent the bike from pulling full revs in top?
Will these bikes pull to full revs in 3rd gear?
Even using 6500 as redline will net 90 mph in 3rd with a 21 tooth gb.
sprocket.
A shift into 4th at that point would put the engine at 5300 rpm which should be well into the Combat cam power band. The engine should then keep pulling to well above 6,000.
If you haven't already done this, try taking it to 90 mph in third before doing the shift into 4th. If the bike then pulls on up to 110 mph or so that would be quite telling.
According to the big white book, 110 mph is 6500 rpm in 4th with the 21 tooth sprocket.
Glen
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That may mean 30 mm carbs would be big enough, or it may just mean you are testing at too low rpm.
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Hi Nick,
Any progress on this? I just ordered Andover Norton’s fuel level test kit. I will let you know if adjusting this makes a difference. If not I may try a 19T gearbox sprocket, although I really doubt the combat isn’t able to pull this gearing if it’s working correctly. Race bikes with similar tuning ran higher gearing.
Any progress on this? I just ordered Andover Norton’s fuel level test kit. I will let you know if adjusting this makes a difference. If not I may try a 19T gearbox sprocket, although I really doubt the combat isn’t able to pull this gearing if it’s working correctly. Race bikes with similar tuning ran higher gearing.
NickZ
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I have been distracted by many things since early in the year including a move, and have not been able to experiment further with this issue other that to change needle positions, which as expected did not make significant difference at WOT. Hopefully I will soon have the new shop organized well enough to continue.
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Me too. I’ve played with jets, needle position, cam timing, ignition timing…etc. I don’t think I did actual checking of fuel bowl level after putting the stayup floats in. That kit is on the way from Andover Norton. Will post any improvement.
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Hi Nick,
Greg Marsh suggested the combat cam profile may not have enough low end torque to push a 21T gearbox sprocket in 4th gear. I finally switched to a 19T and have taken it onto a country road. Mine was pulling very strong in 4th gear and I stopped at 85mph. I also notice the increased engine braking that I remember with my first Commando, a near new ‘72 combat with 19T gear that I bought in May ‘73. I’m satisfied now…this thing is back to being scarey. I love it!
NickZ
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Wow, that is interesting. I've been messing with everything else. Replaced RITA with Boyer. I was going to replace the MK2 Amals with MK1s but after I switched to Boyer I was able to get the MK2s behave better, better idle performance and synchronization. This has resulted in incremental improvement in 4th gear pull, but not where I feel it should be.
I don't have a 19T sprocket but I do have a spare 20T I could put on. Have you tried a 20T?
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I put a 21T on it in 1998 during the restoration. It had a standard camshaft in it then. With the advent of ethanol in the gas the engine went through a period of severe pinging, so around 15 years ago I put a combat double S cam in it…more overlap bleeds off some of the compression. From that point it had trouble in top gear. As a result of pushing it with the 19T to 85mph, now my head gasket is leaking oil on the left side onto my shoe
I’m getting an order together for Andover Norton that includes two flame ring head gaskets. Those always worked in the past for my high compression 750 engines. I agree the RITA even with the new electronic board does not run as smooth as a TriSpark I was using in the past.
ashman
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Have you retorqued the cooper head gasket, all it takes sometimes, I had more troubles with flame ring head gaskets and now only use cooper head gaskets with a spray of cooper seal, when a flame ring gasket blows they really blow, if a cooper gasket blows you can retention it, sometimes it stops leaking but if not, it will slow it down.
