Combat performance issue - RITA

NickZ said:
Thanks for checking, I have been a little slow on reporting progress.

I swapped out the stator and magnetic pick-up with a replacement unit that I have. Set the air gaps with the original reluctor. Set static timing at 28*. Strobed timing. Similar performance as before: 21* indicated at 2000rpm and 40 ish at 5000rpm. Not jumping around as much though. Retarded static timing to 15*. It now strobes 30* at 5000+rpm. Left it set there. Road test showed that rpm kept increasing past 4500 in 4th gear. Couldn't go past 5000rpm because of traffic.
I buttoned everything up with the modern replacement RITA electronics (Rexs), replacement stator and original reluctor.

Conclusion: I believe there were 2 issues. First, the Mistral RITA set up instructions result in way too much advance at higher rpms. This was easily resolved by setting up for the correct advance at high rpm, as was suggested by some folks earlier in the thread. The other is that it appears there was a problem with the original RITA stator & magnetic pick-up assembly that caused quite a bit of jitter at high rpms.

However, while the 4th gear performance is better than before, I still feel that it may be less than what I experience with my stock 71 750 Roadster. There is the difference in 21 vs 20 tooth sprocket, but come on man, this is supposed to be a Combat!

Now that I feel the ignition timing is ok, I have moved to carburation. The bike has 32mm Amal Mk II carbs jetted same as called for MK I on Combat engine. A WOT roll-off test indicated that it was lean. I swapped the 230 mains for 240 and there is now no surge on roll-off, but I also sense no difference when the throttle is moved from above 3/4 to fully open.

That is where I am now. I am going to try different needle positions. It is currently on center position. I'm going to move them 1 notch richer. I realize that shouldn't affect the WOT performance, but that is the next thing I can experiment with.
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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?
 
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).
 
Tornado said:
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?
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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.
 
NickZ said:
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.
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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).
 
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.
 
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
 
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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.
 
ILLF8ED said:
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.
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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.
 
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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