750 Combat top speed problem (2015)

Hi hobot.
Australian Design Rules (ADRs) now require accuracy at + 0, - x %.
U.S. and others will have similar statutory requirements.
Ta.

needing said:

Hi hobot.
Australian Design Rules (ADRs) now require accuracy at + 0, - x %.
U.S. and others will have similar statutory requirements.

Click to expand...

Not so.
ADRs fall into line with Europe.
Speedo can be within 10%.

But now MUST not allow a vehicle to do more than the speedo shows...

1st google result.
http://www.racq.com.au/cars-and-driving ... o-accuracy

Note that this is for NEW vehicles.
There is nothing stipulated about how accurate the speedo has to be, allowing for wear and tear....

Hi rohan.
You are correct but my point is that current legislation does not allow under-reading although it did once.
An incorrect speedo is still no excuse under law so recalibration as + OR - will allow under-reading of the actual road speed.

Current instrumentation ADR 18/03 states:
"...5.3.               The speed indicated shall not be less than the true speed of the vehicle.  At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1 ) and the true speed (V2).0 £ (V1 - V2) £ 0.1 V2 + 4 km/h..."
http://www.comlaw.gov.au/Details/F2006L01392
Ta.

needing said:

Hi rohan.
You are correct but my point is that current legislation does not allow under-reading although it did once.
An incorrect speedo is still no excuse under law so recalibration as + OR - will allow under-reading.

Current instrumentation ADR 18/03 states:
"...5.3.               The speed indicated shall not be less than the true speed of the vehicle.  At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1 ) and the true speed (V2).0 £ (V1 - V2) £ 0.1 V2 + 4 km/h..."
http://www.comlaw.gov.au/Details/F2006L01392
Ta.

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That's some mighty fine Googling there. Needing. Hey Rohan, you been served!

Hi pete v.
Re your: "...That's some mighty fine Googling there. Needing. Hey Rohan, you been served!...".

I consider it 'supporting evidence' in preference to your line of puerile input.
You win. I'm out.
Ta.

pete.v said:

Hey Rohan, you been served!

Click to expand...

needing said:

An incorrect speedo is still no excuse under law

Click to expand...

My Commando speedo was sold on the basis it only had to be accurate to +/- 10%.

As more than a few have pointed out, it probably only needs a class action against governments
to have some of the current speed 'laws' thrown out, since new laws can't override previous law re transport vehicle requirements.

If your bike was sold new without a speedo, or a headlamp, or even a brake light, it cannot legally be required to have one.
These were all later additions to law...

As it should be, or we'd have 1902 Darraq's requiring seat belts, plastic bumpers, fuel injection, rollover protection, etc etc etc.
Or 1852 Fowler steam engines the same....

needing said:

I consider it 'supporting evidence' in preference to your line of puerile input.
You win. I'm out.

Click to expand...

You seem mighty quick with the insults, for what seems a light hearted comment
Still trolling ?

needing said:

You win. I'm out.
Ta.

Click to expand...

Now if you could just expand that on a more global scale, we'd really have something.

speaking of speed....

I have a book that has all the road tests of the various Commandos through the years as they came onto the market.

The tests routinely report that the Smith's speedometers were off by as much as 5 mph at certain speeds.

As far as top speed is concerned the 75 Mark3 and the Combats were the "slowest" being actual road tested at right around 108mph.
This makes sense as the Combats were limited by their 19 tooth sprockets.

The standard 750s topped around 110 and the 73-74 850s were the fastest at 112, likely due to the 21 tooth countershaft sprockets.

In terms of the quickest Commandos as measured by 1/4 mile times, the Mark3s at 15 seconds, standard 750s 13.4,
and the Combats and 73-74 850s pretty much consistently tied at 13 flat give or take a couple of tenths.with the tests reporting the 850s faster in the first 1/8th mile and the Combats catching up in last 1/8th, the quickest reported 1/4 time was by an 850 at 12.2 for the quarter.

Most road tests of that era stated that these quarter mile times depended more on the track condition of that particular day, along
with the wind speed and direction, the rest rider's own ability to launch at the start, and the presumed stock state of supplied tune.

MK3 s just needed to have the advertised "whisper quiet" mufflers removed, then the top speed and acceleration is much the same as the earlier 850s. I suppose the extra weight of the starter might slow acceleration a tenth or so in the quarter, that would be about it.
Quiet pipes absolutely kill top end. I have a Vincent Rapide that when I first received it would only do 85 MPH flat out with it's extremely quiet muffler. After a session with a three foot long 1" drill up the muffler, colonoscopy style, it managed 115 MPH, no other changes.

Glen

Normally we-me only use spdo to avoid citations. Regardless how disappointing most Cdo preform a good Smith can satisfy the 0-1% regulations, at least for a time. Meanwhile I wish those missing the ton mark, sweet solutions soon.

In regards to trying to run with, let alone delight in drawing the really fast ones out ahead of the squads of intense riders even a 70 hp Combat will not cut it after the very first open moderns can get traction, pashaw.

Still working on this, not solved yet. '72 750 combat engine with SX reed valve in the breather
Problem statement: struggles to pull past 80mph it top gear although it revs to redline in gears 1 to 3
21 tooth counter sprocket

What I've eliminated
1. Head is fresh with Kibblewhite black diamond valves, new guides and springs
2. New piston rings on +.020 piston that have around 30,000 miles
3. Leak down test at 5% leakage, nothing getting past the valves...a little sound in the oil tank
4. New TriSpark ignition and new coils
5. Timing at 4000rpm is 28 BTDC, may try advancing this a few degrees but this ethanol gas doesn't work well with too much advance in a combat engine
6. Cam timing checked with dial indicator and degree wheel and is where it is supposed to be

What I'm working
1. Changed to NGK 5000 ohm resistor plug caps, no chance to take it out yet
2. Fuel flow - air breather plugged in the gas cap, this is low probability as it is not affecting lower gears
3. Getting some oil leak from the head, but I think this is the stainless rock feed pipe as I notice it leaks from the screw together joints. The stock black plastic never leaked.

Change main jets ? What do the plugs look like ? Whitish or dark ?

Cam chain one tooth out?

Plugs are tan and the cam timing is confirmed with travel dial and degree wheel. It's not something obvious. Have been plagued with this for three years and not found the solution yet.

It sounds like you have covered all the bases. I haven't gone all the way back through the thread to see if you have dealt with float levels. The Amals require quite a high level to supply full fuel flow. I saw one BSA service bulletin where they recommended an extra hole in the float bowl to allow more fuel to pass. I cant really imagine it would require that much. Revving to red line in 3rd should give over 100mph so that should also require the same amount of fuel.

my only other thought is a flattened cam lobe. On mine I have two different brands of Pea Shooters and it makes a difference side to side on the mixture. You have of course checked the air filter. This one is quite the mystery, especially since it will do it in 3rd but not in 4th. Maybe the transmission is binding up.
Dan.

Without having to re-read the entire thread, maybe the enrichener slides are partially lowered (cables too long)???

I have a friend who recently had the same problem with his combat

turned out that once he reduced the number of "resisters" to one only the problem resolved and his motor pulled strongly well over 100, appropriate for a 19 tooth

the adage of if one thing is good than many things must be better does not seem to work with electronic ignitions like the trispark

you only need one "resister" in the sequence, more can strangle ignition firing/power

you could have resister plugs, resister plug wires, etc - too damn many resisters

what do you think you have in your set up now?

A few pages back I posted my similar issue which turned out to be simply too large of a main jet. If you have float bowls with the drain plugs and a jet key, you can easily try a few different main jets without a lot of disassembly. I put locktite on the jet holder when I install it in the body of the carb so when I unscrew the jet it's able to be removed from the jet holder and be changed, rather than unscrew the whole jet holder which can not be removed without removing the whole floatbowl first.

I didn't look over the whole thread to see if you tried a leaner, or richer jet to see if it helped, but going from the 240 mains to 220 mains cured my issue which was exactly the same as you discribe.... My bike would hit about 80mph in 4th gear and twisting the throttle more gave zero response... The bike seemed fine in all the other gears... Sound familiar?? As I said, my bike's issue was simply a minor change in main jet size (240 to 220) and it was fixed. HTH

1up3down said:

I have a friend who recently had the same problem with his combat

turned out that once he reduced the number of "resisters" to one only the problem resolved and his motor pulled strongly well over 100, appropriate for a 19 tooth

the adage of if one thing is good than many things must be better does not seem to work with electronic ignitions like the trispark

you only need one "resister" in the sequence, more can strangle ignition firing/power

you could have resister plugs, resister plug wires, etc - too damn many resisters

what do you think you have in your set up now?

Click to expand...

My "resistor" setup is NGK BP7ES plugs, non resistor wires and just added NGK 5K ohm resistor caps as recommended with the Tri Spark. The problem existed with my previous Lucas RITA ignition which eventually stopped working hence replacing with the TriSpark recently.

Enriching chokes are set up properly and open fully. Carbs are Mike Gaylord resleeved with the new no sink Amal floats and standard for combat 230 main jets. I have tried up and down on main jet sizes and the 230s are the best.

In the past my experience with this in ability to pull in top gear was due to no enough compression when using a hotter than normal cam profile. I don't think this is the case here as cam timing is confirmed and leak down is 5%.

I know it's very difficult to diagnose without being in front of the machine. Appreciate the feedback, was looking to see if anyone had some insight. I will continue and if I find the offending issue, will report back.