SOHC.co.uk Forums > CB750
Top Speed
Voxonda:
Hey, in my humble opinion it is not the lack of top speed but the lack of mid-range to get there. I have seen 'tuned' engines who could not get past the 125-130 mark because the bike could not get into the 'redzone' in topgear.
Personally I think it's a waist of time and money to put a hot cam in a otherwise streetbike, because all you get is a bike that is 'harder' to ride and is more fragile. If topspeed is all you want, try to get hold of a good streamline, like a CR fairing, and see what it does. Believe it or not but I have seen a 10mph difference in a otherwise standard bike. In the bikes I am building at this moment the key word is 'mid-range'. 850cc kit, 10.5:1 compr, 29mm KeihinCR's, slighty porting of the intake-ports, better ignition, because the standard one is losing it at higher revs, and a standard(!) camshaft. With this type of tuning the engine will have no problem with a longer gearing, aka more topspeed, and will pull into the red in topgear. Most important is that it will stay in one piece. Aiming at a speed of 140mph, don't forget that drag is the biggest enemy.
Cheers, Rob
Post edited by: Voxonda, at: 2007/06/27 17:05
Post edited by: Voxonda, at: 2007/06/27 17:07
Spitfire:
Hi Jack,
When I was riding my 76 F1 regularly it had no great problems hitting an indicated 125 mph.
Your bike should not really be starting without choke, what is the state of your air filter, have you a standard airbox set up ?
The F1 carbs should have a 105 main jet, 40 slow jet, and the pilot air screw should be approx 1 turn out.
The carbs work on three basic setting at different throttle openings, see attached diagram which I got from another site.
You should check your ignition timing closely, is the advance unit free and not siezed up.
Cheers
Den
Jack:
I would just like to say that I am not a speed freak but I do take exception to being overtaken by lorrys.
Anyway, it seems to me that from a standing start it nips off quick enough and the first third of the throttle is fine. Once past that the whole thing falls flat. Looking at the chart I would have to say the main jets are a problem and I will certainly be replacing those after the summer holidays but the clip position (I'm assuming here that you refer to the clip that holds the needle to the slide) is something I had not thought of.
To my knowledge, they were set initially in the middle of 5 positions, is that right? If so, would altering the clip position make much of a difference, and if so, which way should it go ie. move the needle to higher in the slide or lower.:huh:
Voxonda:
Hi,
Altering the position of the needle can make a big difference. To make the mixture leaner, place the clip in a higher position so effectively the needle drops. To make the mixture richer put in in a lower position.
Good luck.
Spitfire:
Hi Jack,
I still do not know the set up of your bike but I would not become fixated on one thing if I was you, main jets do not really wear out, they do get dirty though. I would approach the problem logically, if the bike runs fine at low and medium speeds it could be a couple of things:
Retarded timing, not enough fuel flow, not enough air flow, spring to mind.
Ignition timing, check points gap, check timing, check advance and retard unit works correctly.
Carbs, check air filter, check fuel flow, from the tank to the carbs, check the fuel tap, fuel lines, breather on the tank, float valves, note that on the F1 these have filters, float height, check all jets are clean. Check jet sizes are correct and the needle is in the middle position.
Check valve clearances.
Check cam chain tension.
Check plugs and leads/caps, the CB750 HT leads tended to short out where they come out of the coils in wet weather.
Cheers
Den
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