Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Jack on June 26, 2007, 05:56:22 PM
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Hi all, seems my bike is not performing as well as it should, can someone please tell me what top speed could be expected from an F1 engine in a normal state of tune. Mine struggles to reach 80 mph and I was wondering what would be a realistic figure to expect.
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Jack wrote:
Hi all, seems my bike is not performing as well as it should, can someone please tell me what top speed could be expected from an F1 engine in a normal state of tune. Mine struggles to reach 80 mph and I was wondering what would be a realistic figure to expect.
Hey Jack,
A normal good F1 did reach 120mph, or at least should have. But a 30 odd years engine, like it's drivers, won't be so fast anymore. But should still be able to pull 100-110 'no problem'. If it does not it need some handywork.
Cheers from Holland.
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Many thanks for that, looks like I need to do some more tinkering.;)
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beforeI had my carbs cleaned and balanced I was geting similar performance, I can get to just over a ton now, but GOD it's hairy, it's not like riding a modern superbike where it's a smooth ride all the way up.
Up to 70/80 it's fine after that It shakes, rattles and is very un happy.
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I rode the bike for the first time when I took it for it's test, which involved a ride down the A1. It rides fine up to 65-70 mph but then just seems to fizzle out. I must admit that the old bike was not filling me with the desire to push it much harder anyway, it was starting to feel decidedly vague in the handling department and I'm sure that from 70 mph the braking distance could be measured in minutes not meters ( and that is despite having three discs ).
I was planning to sort the carbs with a good clean out and then new floats and float valves, aswell as new main and idle jets. The ignition is of the points type but is all brand new, as are the spark plugs.
I'm pretty sure though that it is running far to rich - it will start from cold with no choke!!! ( If I put the choke on at all it promptly floods ). I have the mixture screws set at 1 1/4 turns out but it doesn't seem to make much difference what I set them at to to be honest. The chokes themselves are all operating as they should as are the slides, so all in all the carbs are the prime suspects.
Even after taking into account the handling and brakes and any number of other little quirks that the bike displays, it still puts a smile on my face, and I did notice that the sport bike brigade took much more intrest in my old bike than I did in their shiney new machines. To that end I just want to try to get it running as good as I can.
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I rode the bike for the first time when I took it for it's test, which involved a ride down the A1. It rides fine up to 65-70 mph but then just seems to fizzle out. I must admit that the old bike was not filling me with the desire to push it much harder anyway, it was starting to feel decidedly vague in the handling department and I'm sure that from 70 mph the braking distance could be measured in minutes not meters ( and that is despite having three discs ).
I was planning to sort the carbs with a good clean out and then new floats and float valves, aswell as new main and idle jets. The ignition is of the points type but is all brand new, as are the spark plugs.
I'm pretty sure though that it is running far to rich - it will start from cold with no choke!!! ( If I put the choke on at all it promptly floods ). I have the mixture screws set at 1 1/4 turns out but it doesn't seem to make much difference what I set them at to to be honest. The chokes themselves are all operating as they should as are the slides, so all in all the carbs are the prime suspects.
Even after taking into account the handling and brakes and any number of other little quirks that the bike displays, it still puts a smile on my face, and I did notice that the sport bike brigade took much more intrest in my old bike than I did in their shiney new machines. To that end I just want to try to get it running as good as I can.
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Sorry all my computer just freaked out on me and posted twice.:blink:
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had mine to 122mph. scared the hell out of myself
so i backed off
:evil:
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had mine to 122mph. scared the hell out of myself
so i backed off
:evil:
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:) Mmmmmmmm thats more like it! So much for a so called Classic Bike comic claiming 112 mph was about top whack! \"eat my jet fumes\"My first CB750K2 which was bought \"secondhand\" topped 125mph with a \"standard\" engine My first & only Brand New bike CB750F1 also did about the same in \"standard trim\" although the ecceleration was far better.My current F1 has a lot of tuning/go faster engine bits & pieces inc sprocket/ & gearing changes I tried it when I first recommisioned it after a 10 year lay up & it was totally \"standard trim it went over the \"ton\" quite easily I,m hoping for a whole lot more from the engine mods I have made otherwise I,m going to be very disappointed :)regards Asimo
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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
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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 (http://www.honda-sohc.co.uk/components/com_joomlaboard/uploaded/images/jetgraph_onpage.jpg)
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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:
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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.
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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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Thanks everyone, I've now got lots to go on. The bike seems to be running a little better every time I take it out aswell.
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I agree with Rob, I'm riding my CB650 a lot at the moment, (Between down pours) and it's got tons of mid range umph, it's great cruising up behind a car at 55-60, then just winding open the throttle to overtake without \"Cogging Down\".
Different engine, I know from past experience mid range power and handeling is a lot more useful than a fast top end.
The Important thing is to enjoy the ride.....
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Hi all, glad the site is back up. Stripped my carbs last week and main jets had potatoes growing in them, also the clip position was running them too rich. Putting them back together tomorrow, then I'll be of down the road for a roadtest before it starts raining again. 8)