750 Combat top speed problem (2015)

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Back again. Lucas RITA sorted out and getting one timing mark...timing is set to 28 BTDC at 4000 RPM. No issue there. Thought maybe the compression rings were getting too worn so bought a leak down tester. Both sides show 33% leakage at 10 psi which on the tester is well within the green zone. Leakage is past the rings as I'm getting gurgling at the oil tank and no noise at all at the mufflers or carbs - head was recently redone.
Now I'm at the main jets. I had 230s in the sleeved Amal 932/19 & 932/20. Just put in 240s, but haven't had a chance to run it yet. I went that direction because I already had the 240s. If it doesn't work will try some 210s for affect (no 220s in the box right now).

You other combat owners, what jets are you using and under what non stock conditions?

To restate the problem - will not pull beyond 80mph in 4th and not getting that high RPM power surge associated with a combat tune. Another possibility is cam timing, but I checked that pretty closely with degree wheel and travel dial when I assembled the new double S cam. This is an original combat 750 engine and all stock except for the XS reed valve breather in line and a Lucas RITA ignition.
 
Might be cam lobe going away. 240 is reasonable rich go fast jet. My pure factory Combat is almost annoying responsive w 19T as wants to pull harder in 4th after 90 which is not realistic survival out here so hurts resisting its eagerness. Should be able to hang with normal thrilling sports bike joy rides that rarely get over 100 but don't slow don't much or often below that. My Trixie is down to crank for resealing and some cryo cam tempering but I'm pensive I won't end up like you - over my head again so paying attention to what you teach us.
 
illf8ed said:
Back again.
You other combat owners, what jets are you using and under what non stock conditions?

To restate the problem - will not pull beyond 80mph in 4th and not getting that high RPM power surge associated with a combat tune. Another possibility is cam timing, but I checked that pretty closely with degree wheel and travel dial when I assembled the new double S cam. This is an original combat 750 engine and all stock except for the XS reed valve breather in line and a Lucas RITA ignition.
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I think MPH is misleading whereas RPM may be more indicative of the proplems as far as trouble shooting is concerned.

When nothing much has changed except for THE CAM, this raises my eyebrow of concern.

My carb is non-relative being a single 40mm, but I do have an non stock cam and had issues as far as being able to go passed a certain RPM post install.
Dialing in is all well and good and I assumed that my JS1 was right on the money when I closed it up. On first run it fired right up but fell flat around 4000 to 4500rpm. After hours of fiddling with carbs and different ignitions, the only thing left was cam timing.

How could this be? Well rumors have it that perhaps the keyway may or may not have been off a little. 5 degrees advanced took me to 6000 and started hitting the wall again. Went to 10 degrees advanced and all is right with the world and as far and valve position is concerned, it is visually right on the money at TDC. I can now easily run up to 100 in third at 7200 (beehive springs).

So I am not saying that this is your issue but I might add 5 degrees, check for clash and see how it goes.
 
I've never run anything other than 220 in my combat...and that is with Armours stainless peashooters, They are absolutely empty cans. Lucas Rita of course...
I can almost hit 95 at the drags in third. Shifting to 4th slows the time down for the 1/4.
My floats are higher than when you adjust them for a triumph.
As mentioned cam chains stretch. Make sure the cam is a little advanced and not retarded at all.
I still have to admit it seemed to run stronger with the standard 28.5mm port head.
I'm at sea level.
 
dynodave said:
I've never run anything other than 220 in my combat...and that is with Armours stainless peashooters, They are absolutely empty cans. Lucas Rita of course...
I can almost hit 95 at the drags in third. Shifting to 4th slows the time down for the 1/4.
My floats are higher than when you adjust them for a triumph.
As mentioned cam chains stretch. Make sure the cam is a little advanced and not retarded at all.
I still have to admit it seemed to run stronger with the standard 28.5mm port head.
I'm at sea level.
Click to expand...

Thanks Dave....thought you didn't like to type :) Exactly what I'm trying to get mine to do and it should. If the jetting doesn't get results, cam timing is next - not something I wanted to get into again so holding that for last. I'm at 200ft above sea level.
 
Following on from what Hobot mentioned, if the cam has been in there a long time, maybe it would it be worth checking the lift at your valves vs lift of the same cam known to be in good condition? I am assuming here that someone on this forum would have reliable data for comparison?

Did you really mean 33% leakage at 10 psi, and it's going past the rings? That sounds like stripdown time. I would have thought max 10% leakage at whatever is the recommended pressure would be OK on a street engine in reasonable condition, but please correct me I have understood your figures wrongly. Have a look at Jim Comstock's leak down test, where he mentioned that 7% leakage was good: new-leakdown-results-t21149.html
 
Did you really mean 33% leakage at 10 psi, and it's going past the rings? That sounds like stripdown time. I would have thought max 10% leakage at whatever is the recommended pressure would be OK on a street engine in reasonable condition, but please correct me I have understood your figures wrongly. Have a look at Jim Comstock's leak down test, where he mentioned that 7% leakage was good: new-leakdown-results-t21149.html[/quote]

Going by the gauge and it's my first experience with leak down. Two gauges on the instrument. Connect compressed air at the input and open the valve until getting a pressure reading. At 10PSI the leak gauge reads 33% and in the green. At 15PSI the lead gauge indicates zero leakage. At any pressure less than 10PSI the percent leakage is much higher. Not sure if I'm doing it correctly.
 
illf8ed said:
Going by the gauge and it's my first experience with leak down. Two gauges on the instrument. Connect compressed air at the input and open the valve until getting a pressure reading. At 10PSI the leak gauge reads 33% and in the green. At 15PSI the lead gauge indicates zero leakage. At any pressure less than 10PSI the percent leakage is much higher. Not sure if I'm doing it correctly.
Click to expand...

If you are unsure, could you do a compression test? I don't have compression pressure figures off the top of my head, but they have been posted on the forum. What does the cylinder compression feel like when you kickstart it?
 
How far apart are the two strobes? Are we talking a degree or two, or 10 degrees? A degree or two would not stop it from making enough horsepower to get over 80 mph.

Ken
 
You have never said whether the motor is cutting out, missing or anything of the sort. Does it run steady on both cylinders? Have you done a simple plug chop to check main jets? Are the petcocks flowing enough fuel? Are the floats set correctly? See if someone in the club out there has a spare ignition set up you can borrow if you suspect the Rita is bad. What sprocket are you running on the gearbox? INPUT!
 
There has been no mention of air cleaners , if it has some, are they allowing the motor to breath correctly?

Regards Mike
 
milfordite said:
You have never said whether the motor is cutting out, missing or anything of the sort. Does it run steady on both cylinders? Have you done a simple plug chop to check main jets? Are the petcocks flowing enough fuel? Are the floats set correctly? See if someone in the club out there has a spare ignition set up you can borrow if you suspect the Rita is bad. What sprocket are you running on the gearbox? INPUT!
Click to expand...

Good questions.
We also have not heard what happens in 3rd gear, if it will rev out or not.
 
Nothing strange on the set up and runs great in all gears, but doesn't pull in 4th. Will try different main jets then go to cam timing recheck - what a pain. K&N air filter in ham can, new no sink floats in the Amals, float level is correct, fuel flowing fine to the carbs.
 
1up3down said:
possibly fuel starvation issue

partially plugged petcocks/screens

possibly breather hole in gas cap plugged.....
Click to expand...
I agree with this. I wouldn't even get serious about testing main jet size, until fuel flow can be verified.
Remove the drain plugs from both float bowls. Put catch cans under each bowl.
Turn on the fuel for one minute. You should have almost 300 ml of fuel in each can.

Technically, you could make 50 bhp and only consume 1/2 that amount of fuel. In practice, it won't work if the fuel flow was measured at the drain plug. Why? Because the float is fully dropped (needle fully open) during that measurement, which would mean a very low fuel level in the bowl to get that amount of flow. That low fuel level would be leaning the mixture.
 
So it will do more than 80 in 3rd ?
A lot more ??

You still haven't actually said what sprocket you are running on it....
 
illf8ed said:
Back again. Lucas RITA sorted out and getting one timing mark....
Click to expand...
Out of interest, what did you change to get the single timing mark??
 
quote="X-file"]...Why? Because the float is fully dropped (needle fully open) during that measurement, which would mean a very low fuel level in the bowl to get that amount of flow. That low fuel level would be leaning the mixture.[/quote]

Low fuel level in the float bowl doesn't lean the mixture UNTIL the main jet is exposed. As the float drops more fuel flows through the float needle seat to replace fuel consumed unless there is a restriction in the fuel delivery system. Putting a larger main jet in will not overcome a flow restriction problem.
Restriction in this system occurs at the float needle seat orifice, within the stupid banjo junction housing, a clogged stupid seive within the banjo, the hoses, the tap, a clogged tap strainer in the tank or the tank breather. Interestingly, the tap strainer mesh may be clogged toward the base where detritis builds up but still flow well if the tank is full.
Ta.
 
Might as well diddle float level a bit higher and if that seems to help then might also try this mod I liked in my Combat's Amals.
750 Combat top speed problem (2015)
 
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