Topic: '76 750F Jetting

[American Locomotive]

So the engine on my bike is more or less all stock. Stock 4-1 exhaust, factory air box with standard paper filter, etc... The bike always felt lacking under high load and high RPM situations. Basically at ~6500 RPM and above it doesn't feel like there's any real difference in power between 11/16-3/4 throttle and full throttle. You turn the throttle open more, and the air box gets louder but there's no significant difference in power. Bike has 150 PSI of compression on all 4 cylinders, electronic ignition, I've verified the base timing and made sure the mechanical advance functions.

I had read that the stock 105 main jets on the 750F K1s were a little lean, and that switching them to the 110 mains the other bikes used would result in a big improvement. All kinds of people saying rejetting them lets them pull the front wheel right up in 1st and shifting into 2nd gear hard will get it to lift up and all kinds of other stuff.

So I went ahead and ordered some 110 jets, put them in....and no real change. If anything I'd say it softened the mid-range a little bit, and MAYBE slightly more top-end power, but it's hard to tell. I wouldn't call it "slow", but this thing doesn't even come close to picking the front wheel up in any gear. My CRF450R with the high compression piston and '02 Cam will readily pick the front end up in 1st, 2nd and even 3rd gear. So I know what a bike that wants to wheelie feels like. My 750F is not one of them.

So what's the deal here? Is there actually nothing wrong at all, and maybe I'm just expecting too much from a ~40 year old bike? Perhaps all people telling these stories of these events that happened 20-30 years ago have gradually embellished those memories over time?

[seanbarney41]

the only stock cb750 I have ridden that acted like it wanted to lift the front wheel even the slightest was my 750f3 and you had to get rolling about 10 mph, pull the clutch in, rev to about 7500rpm and pop the clutch...or do a full drag racing launch.  Non of 'em are gonna just do power wheelies.  Consider that your CRF is about half the weight of a cb750 with the same or more power and is balanced VERY different

or how about 15f 52r tooth sprockets?

seriously, try a K and N stock replacement air filter in the box and some 110-115 mains...also 17 tooth front 48 tooth rear really helps if you don't have those already

[Jerry Rxman Griffin aka MuthaF'er]

My 75F ran GREAT with stock 105/40. It only pulled the wheel once when I banged 2nd right after it's first tune up. I had 2 buddies with Kaw H1's that picked on K's. They BOTH tried it with me and I smoked them. Will your bike pull up to the top of the redline?

[American Locomotive]

the only stock cb750 I have ridden that acted like it wanted to lift the front wheel even the slightest was my 750f3 and you had to get rolling about 10 mph, pull the clutch in, rev to about 7500rpm and pop the clutch...or do a full drag racing launch.  Non of 'em are gonna just do power wheelies.  Consider that your CRF is about half the weight of a cb750 with the same or more power and is balanced VERY different

I wasn't trying to compare the performance of the two bikes, as they are of course very different. I guess I was more or less saying I know what a bike that can wheelie feels like to ride, and my 750F doesn't even come remotely close to that. So I have a hard time believing some of the wheelie stories unless something is wrong with my bike.

seriously, try a K and N stock replacement air filter in the box and some 110-115 mains...also 17 tooth front 48 tooth rear really helps if you don't have those already

I have the 110 mains in it right now. For kicks I tried running it with no air filter earlier today, and it ran like complete garbage above quarter throttle, even with the 110 mains.

My 75F ran GREAT with stock 105/40. It only pulled the wheel once when I banged 2nd right after it's first tune up. I had 2 buddies with Kaw H1's that picked on K's. They BOTH tried it with me and I smoked them. Will your bike pull up to the top of the redline?

Absolutely no possible way my bike would do a wheelie shifting into second. I've even done no-lift power shifts right into 2nd staying on the throttle and there's just no way. My bike will pull to 9500 RPM pretty well in 1st, less so in 2nd; by 3rd gear it really starts to struggle and starts to really fall off around 8000 RPM. I'm guessing the "issue" exists in 1st gear as well, but it's just masked by the lower gear ratio.

[scottly]

I have the 110 mains in it right now. For kicks I tried running it with no air filter earlier today, and it ran like complete garbage above quarter throttle, even with the 110 mains.

Check your fuel levels in the float bowls with the clear tube method; if removing the filter makes the bike run worse, it indicates a lean condition. Since you have larger than stock main jets, it may be due to incorrect float settings.

[American Locomotive]

Check your fuel levels in the float bowls with the clear tube method; if removing the filter makes the bike run worse, it indicates a lean condition. Since you have larger than stock main jets, it may be due to incorrect float settings.

I've already done that. Float levels are perfect.

[Jerry Rxman Griffin aka MuthaF'er]

If you're not pulling rpms in 3rd then there is an issue. But then again I weigh in around 160.

10,000 rpm in 3rd should get you to 100mph but that was with my 812 and cam kit. Do you think the engine is stock? Mine never, except for that one time, pulled any substantial wheelies but then I never really tried. It only has 2 wheels and I like to keep both on the pavement. When I power shift my weight is forward and I'm low on the tank.

[seanbarney41]

the only stock cb750 I have ridden that acted like it wanted to lift the front wheel even the slightest was my 750f3 and you had to get rolling about 10 mph, pull the clutch in, rev to about 7500rpm and pop the clutch...or do a full drag racing launch.  Non of 'em are gonna just do power wheelies.  Consider that your CRF is about half the weight of a cb750 with the same or more power and is balanced VERY different

I wasn't trying to compare the performance of the two bikes, as they are of course very different. I guess I was more or less saying I know what a bike that can wheelie feels like to ride, and my 750F doesn't even come remotely close to that. So I have a hard time believing some of the wheelie stories unless something is wrong with my bike.


yeah, I hear ya, what I am saying is these stock bikes really do not want to lift the front wheel, anyone that told you they do, is tellin' some real "cool stories bro"   Not that it's impossible, but it takes same serious brutality to make it happen.

It sounds like you may have some too big expectations of what these bikes can do or you got some kind of hidden tuning problem.  What sprockets do you have on the bike?

[scottly]

Check your fuel levels in the float bowls with the clear tube method; if removing the filter makes the bike run worse, it indicates a lean condition. Since you have larger than stock main jets, it may be due to incorrect float settings.

I've already done that. Float levels are perfect.

What do you consider perfect??

[American Locomotive]

It sounds like you may have some too big expectations of what these bikes can do or you got some kind of hidden tuning problem.  What sprockets do you have on the bike?

I really had no expectations when I first bought the bike. Seemed adequate enough to me, but then I read stories about people doing certain things on their bikes, when there's no way my bike will come close to doing those things.

If you're not pulling rpms in 3rd then there is an issue. But then again I weigh in around 160.

10,000 rpm in 3rd should get you to 100mph but that was with my 812 and cam kit. Do you think the engine is stock? Mine never, except for that one time, pulled any substantial wheelies but then I never really tried. It only has 2 wheels and I like to keep both on the pavement. When I power shift my weight is forward and I'm low on the tank.

I'm really not sure my bike can do 100 MPH in 3rd gearing wise. I'll have to go out and check the sprockets, but my bike is geared extremely low. I want to say it's around 4700 RPM at 60 MPH in 5th, but I'd have to double check

Also don't get me wrong. I'm not trying to do wheelies, but just stating my bike is utterly incapable of doing one. Really the only way I could conceivably do one would be a high RPM clutch dump in 1st from a slow roll while pulling up on the bars. But geeze, I could wheelie a dump truck doing that.

What do you consider perfect??

All of the levels within less than 1mm of each other, to the spec that's plastered all over this forum.

[scottly]

WHAT IS THE DISTANCE FROM THE FUEL TO THE TOP OF THE FLOAT BOWL?
Edit: OK, I went back through your posts, and found that you did set the levels to 2-3 mm below the top. That was all you needed to say.
I also found this:
Someone coated my tank, and it didn't really need it either. Now every once in a while I get bits of coating that come off and plug up my fuel strainer.

[American Locomotive]

I have also resolved that issue.

[seanbarney41]

for the 3rd time...What is your front and rear sprocket tooth count?

[Jerry Rxman Griffin aka MuthaF'er]

4700 RPM at 60 MPH in 5th - yeah double check. I'd EXPECT wheelies with that gearing. How about 5000 @ 70 17:48 stock - 75mph @ 18:48.

[American Locomotive]

for the 3rd time...What is your front and rear sprocket tooth count?

It's barely been a day man. I'm not going out into the pouring rain to uncover my bike and count sprocket teeth.

Anyways, they're 17:48, which is stock I believe. So I guess any discrepancies on my speedo/tach are likely due to them being 42 years old. My tach needle does bounce a round a bit at times, but the speedometer matches my GPS close enough.