Author Topic: super rocket top end speed  (Read 2112 times)

Offline paul

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super rocket top end speed
« on: 06.05. 2010 18:43 »
hi all,
         my last question to you was which carb i should have on my super rocket and as advised, i bit the bullet and purchased a new 389 from amal with all of the correct spec. My bike goes very well and smooth under 50 mph, if you dont wack open the throttle to quickly, but when you reach 50/60 mph and want to give it a bit more, it seems to hold back, you can ease off on the trottle and ease her up more gently but even then when reaching toward top speed, she still seems to hold back, both plugs are burning a nice light grey/brown, so the mixture is about right, so should i now be looking at ignition timing. Please can you give me a pointer or two it would be much appreciated and may be save me some time.
CHEERS ALL PAUL

Offline brackenfel

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Re: super rocket top end speed
« Reply #1 on: 06.05. 2010 19:54 »
Hi Paul,
It might be ignition but as I hate electrickery I always look at carburation first.. Check the carb is the correct one, and that the main jet and throttle cutaway are the right size.. Depending on year the carbs changed periodically, what year is the bike?

Adrian
1961 A10 650 Golden Flash - Blue
1954 BSA B33
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Re: super rocket top end speed
« Reply #2 on: 06.05. 2010 20:13 »
G'day Paul,
               Sounds as if the mixture is a little weak on the mains or the spark needs to be advanced a couple of degrees. If you advance the spark keep an ear out for pinging under load. If it does back it back off a tad at a time till pinging stops, I have a test hill for that.
 The best way to read plugs is to do a plug chop. Get to where the problem is (3/4 - full throttle) and at the same time kill the motor and pull in the clutch. Now look at the plug, it should be a mid to dark tan.
 Keep going, your getting close.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline dpaddock

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Re: super rocket top end speed
« Reply #3 on: 06.05. 2010 22:26 »
First, index your throttle twist grip with a marker pen on the body and a white mark on the grip so you'll know what tuning range you're in.

Next, when you encounter this problem, close the air lever to enrichen the mixture, observing the throttle range. If the power increases, enrichen the appropriate carb setting. If the power falls off, you'll have to stop the bike, lean out the carb setting, and try it again. (My experience with reading plugs is poor; one has to kill the engine at the throttle setting of interest, pull the clutch, coast to a stop, pull the plugs then and there, read them, adjust the carb, install fresh plugs and try it again). For me, it's the performance difference that's meaningful, not plug color per se.

Ignition timing is important too, and you can fiddle with that via the 'bar lever, but only into the retard region. Opening the points gap one flat at a time (maximum two) will advance the timing slightly and is a means to see if advancing the timing is needed.

In any case, do one adjustment (and only one) at a time, or you'll chase the problem for ever.

David

David
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Offline cus

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Re: super rocket top end speed
« Reply #4 on: 07.05. 2010 03:45 »
G'day Paul,
A couple of simple things to try first.
I don't know what type of air filter you are running, but go for a run without it &
see if there's any diff., also check the breather is clear in your fuel cap.
Also check the manifold flange isn't overtightened & the "O" ring
is still in position. Also, is your bike manual adv./ret., make sure its fully adv. when
in 4th. Probably all stuff you already know, but sometimes the simple things
can be overlooked.
regards, Cus
56 G/Flash project

Offline paul

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Re: super rocket top end speed
« Reply #5 on: 07.05. 2010 20:31 »
thanks one and all for your advice, hoping for some decent weather and an understanding mrs, this weekend so i can fiddle (with mi bike, not the mrs, or may be both never know ya luck) i will be back with the results thanks again, have a good weekend , Cheers PAUL

Offline a101960

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Re: super rocket top end speed
« Reply #6 on: 08.05. 2010 10:42 »
It is interesting that you should raise this matter because I have exactly the same problem and no amount of fiddling seems to resolve it. I have played about with the timing and the carb. With the timing set at 3/8" (35 degrees BTDC) the bike goes like S**t off a chrome shovel, but will pink under load. At 5/16" (32 degrees BTDC) it does not go quite so well and there is till occasional pinking. I have recently changed the main jet from 420 to 430 and the results are inconclusive. I have N3C plugs fitted and the colour looks fine. Irrespective of how the bike is set up, some where around 75 mph seems to be as much as it wants to go. Clearly something is wrong, but as the bike is perfectly ridable I have more or less given up playing with it and just get on with riding it.

John

Offline MG

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Re: super rocket top end speed
« Reply #7 on: 08.05. 2010 12:13 »
As most here know I'm running my Flash on 9:1 CR with a 356 cam and a 30mm carbie, so about what is fitted to the Rockets usually (except the smaller valves maybe).

With this setup and ignition set to 34 deg she wouldn't exceed 85 miles after a decent running-in period of approx. 500-600 miles. At around 65-70mph it seemed like she'd lack power with not much further acceleration.
At closer inspection I found the silencers to be very restrictive. They have a perforated tube inside with a diameter of approx. 33-35mm. At both ends a baffle plate is fitted with a drilling of maybe 22-24mm  in diameter.
I enlarged the holes with an air grinder and a hard metal grinding tool with an extended shaft to match the inner diameter of the perforated tube.
Ever since she would reach a good 90-95 mph, is running cooler (and sounds better, although some would say she's loud as hell  ;D *whistle*)
The change in gas flow was so big I had to readjust the carb on idle/low rpm range, bu now at around 65 in 4th gear she gets a second wind and starts accelerating smoothly to 90, problem solved.

This whole procedure cost me my cheapo-air-grinder, but that was worth it  *smile*

Got no idea what silencers these are because they came with the bike and were in pretty good nick, so simply were kept.

I'm positively sure she will reach the magic ton once I have covered a few more miles.

I bought a very interesting book recently, written by Ernst Leverkus (aka Klacks), a famous German journalist who tested almost any bike produced in the 50s, 60s and 70s. He claimed that ANY British bike he ever tested from new needed at least 5000kms to reach full power and full speed. One of the very first tests he always did was taking the brand new bike on the highway and see what she would do. This was repeated after considerable mileage on public roads and the Nuerburgring (race circuit).

Cheers, Markus
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

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Re: super rocket top end speed
« Reply #8 on: 08.05. 2010 15:13 »
G'day all,
            Marcus has the right idea. Breathing (in & out) plays the most important role in performance. Try running around the block breathing through a straw ! The faster you can get rid of the old and in with the new (at the right mixture) the better she'll go.
 Most here know I'm a rev head and not shy to try different things to go faster. Whilst trying to jet in the pair of Kehin PWK30's I did experience Paul's problem. It did get better with a richer needle and main jet. I also shortened the inlet manifolds for more top end.
 On the exhaust side I always run straight through, on the cafe it's got 4" reverse cone megas, on the '51 A7 there's nothing in those nice stainless mufflers.
 The type of cam plays a big role too. A 356 gives good low/mid range but lacking up top. The 357 gives fair low, mild mid and great top end. So at 60 mph in top it takes a while to reach 80 mph then takes off to 110 in no time. But at 60 drop it into 3rd to get it on the cam and hold on, catch 4th at 80 and in no time it's over the ton.
 More is good, lots is better, too much is just right.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline mike667

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Re: super rocket top end speed
« Reply #9 on: 08.05. 2010 16:21 »
More is good, lots is better, too much is just right.
Cheers

yes!