Topic: Where do I start with power loss at high speed.

[nzdreamer55]

Hello everyone,

I have a '78 CB750F and it has been sitting for a while.  It starts and runs, however when I am accelerating, it hesitates and feels like it is running out of gas maybe.  I have cleaned the petcock and the fuel strainer from the tank, put in new gas with fuel stabilizer.  I am concerned that the carbs maybe plugged with some varnish?  Any ideas of how I can decide if the carbs are the problem without pulling them?

Thanks

-S

[tlbranth]

If it runs and idles then fuel is moving through all the right channels which makes it a candidate for cleaning I'd say. Most people on these bike forums seem to favor Sea Foam but I've had better luck with injector cleaner. Try a tankful of fuel with cleaner in it. See if it helps. Also, try using Chevron fuel for a while. I think their Techron additive works to clean the fuel system and backs of intake valves.
All this said, I reckon it's better to first run through the items that require no magic to fix: timing chain, valve clearance, point gap, timing. Then move on to the black art of carburetor massage. Good luck.

[nzdreamer55]

Thanks for the reply.  What type of injector cleaner?

[fmctm1sw]

Did those bikes have accelerator pumps?

[Stev-o]

Did those bikes have accelerator pumps?

Yes, if it has the stock PD carbs....

[nzdreamer55]

Yes.  I have stock everything.

[dave500]

check the fuel cap vent.

[nzdreamer55]

Good idea.  How do I check the cap vent?  Is it cleanable?

[750cafe]

The strongest fuel system cleaner that is available at NAPA and O'Reilly's is the Berryman Total Fuel System Cleaner in a 15oz. can for about $7.50 each.
Start by using a half can per full tank of fuel. Do not spill any on painted surfaces and DO NOT INHALE! It is VERY STRONG!!!


Eric

[vames]

There is no fuel additive that is equivalent to pulling the carbs and cleaning them. Don't cut corners. You said this bike has been sitting for a while -- I guarantee you that it can benefit from cleaning the carbs. Then do valve clearances, clean and set points, timing, ensure plug wires all have healthy connections, change the air filter, change oil and filter. Then synch the carbs. Your bike will thank you by running very well.

* oh yeah and checking the fuel system -- cap/vent/filters/etc

[BobbyR]

There is no fuel additive that is equivalent to pulling the carbs and cleaning them. Don't cut corners. You said this bike has been sitting for a while -- I guarantee you that it can benefit from cleaning the carbs. Then do valve clearances, clean and set points, timing, ensure plug wires all have healthy connections, change the air filter, change oil and filter. Then synch the carbs. Your bike will thank you by running very well.

* oh yeah and checking the fuel system -- cap/vent/filters/etc

This is your best advice. I have the same carbs. The accel pump holds such a small amount of gas, it varnishes quick. Once it plugs up no cleaner can get in there. A good carb cleaning along with the other tune up items will make you much happier with the bike.

[flybox1]

Hello everyone,

I have a '78 CB750F and it has been sitting for a while.  It starts and runs, however when I am accelerating, it hesitates and feels like it is running out of gas maybe.  I have cleaned the petcock and the fuel strainer from the tank, put in new gas with fuel stabilizer.  I am concerned that the carbs maybe plugged with some varnish?  Any ideas of how I can decide if the carbs are the problem without pulling them?

Thanks

-S

Yes, you'll have an accel pump with your PD carbs, but once you're familiar with the design of the pump, it only pumps fuel with the first 1/4 throttle twist, once you're past that, holding a throttle position or slowly accelerating, it does NOT continue to pump.  It only engages from ZERO throttle twist position.  Its a one time shot. 
During gear shifts, if your throttle position hits back at zero(revs still up, tho), it will engage again for that brief moment of accel.

Sounds like you just need a good cleaning of your carbs and jets, nzdreamer55.  main jet is twist out to get to the emulsion tube.  clean all its tiny holes. pilot jet is a press-fit jet.  just pull it out.  clean its holes, too.
separate the halves of your airbox first.  makes getting the airbox top half, and the carbs out, an easy task

[dave500]

Good idea.  How do I check the cap vent?  Is it cleanable?

try running it with the cap not latched,but not with a full tank,or when the trouble starts try un latching the cap and see if it clears up?

[ozpacman]

Don't forget to have a look at your float levels while you have the carbs off. 

I recently had a similar issue with my K7 where it wouldn't pull cleanly.  Pulled the carbs off and found that the float level on the number 4 carb was set a little low.  It now runs beautifully.

Cheers,

Russ

[MoMo]

Check air filter for mice nest


Sent from my iPhone using rapper talk

[nzdreamer55]

Thanks everyone.  The last post about mice is really funny because that happened to me when the bike was stored for a while.  Now it is all clean.  So built a little sync tool and got the cylinders as close as I could, however #2 (which cannot be adjusted) won't play well with the others.  I am not sure why.  I also wanted to know if the fuel routing is correct.  It seems a little hard on the hose angle.  Is this the correct petcock for this bike?  I have seen where the output points into the middle of the bike.

Thanks again for the help and ideas.

-S

[Bailgang]

You probably already know this but I'm saying it just to make sure you know but being the #2 carb can't be independently adjusted from the other 3 then you have to get the other 3 adjusted to whatever the #2 carb is reading. The #2 carb is the base carb so to speak so all the others have to be adjusted around it. I also built a DIY manometer like the one you have and can tell that although it works it can be a PITA, try to borrow a gauge type manometer from someone and try it instead. My kid bought a gauge type manometer for his bike and he let me try it on both my 78 cb750 and gl1100, I had the carbs on both bikes synced in a fraction of the time it would've taken me with my DIY manometer. 

[nzdreamer55]

Thanks.  I have a spring dial gauge carb sync tool (like in the manual) but it is difficult to read as the needles bounce around.  The home made is super smooth and moves very nicely.  I realize that the number 2 is the base carb and cannot be adjusted.  I cannot get the other carbs to match #2.  This is the best I can do.  I suspect that I will need to pull the carbs and go through them.

I have isolated the power issue to when I am at speed and holding the throttle.  Never when accelerating or idling.  I don't think it is a tank cap issue (tried riding with the cap off and still had the problem).

Any thoughts on the fuel line routing (was the OD of fuel line in the '70 smaller because the line I get now has an OD that is very tight putting on to the carbs) and the petcock.

Any links to new intake boots (replicas are ok) for a '77K (this is the engine and carbs that are on the bike) that won't melt?

Thanks

-S

[Stev-o]

Stock fuel line is 5mm, I use 3/16" and it's a tight fit, no clamps needed. 
Fuel line routing does look odd, but I don't know the F's.  Why do you have the filter? That can cause issues.

[nzdreamer55]

I like to know that I have fuel flowing.  I guess I could swap it out for a clear connector without a filter.  In the past it has never been an issue and I put on a new one before testing today.

[tlbranth]

Yessir those filters can be troublesome. It seems logical that as long as the bowls are below the tank, fuel will flow unimpeded. I had 2 filters like yours and had trouble with stalling, loss of power etc. 'til I removed the filters. I think they get an air bubble in them or something and it slows down fuel flow.

[BobbyR]

On my 78K I use Tygon which allows for better routing since it is more flexable and does not require clamps. I routed mine so the gas runs downhill and the the filter is actually betwen1 & 2 carb.

To do a proper carb synch you need to do a valve adjustment. Timing also. 

[nzdreamer55]

On my 78K I use Tygon which allows for better routing since it is more flexable and does not require clamps. I routed mine so the gas runs downhill and the the filter is actually between 1 & 2 carb.

To do a proper carb synch you need to do a valve adjustment. Timing also.

Cool.  So what type of Tygon tubing.  The name Tygon is a brand of tubing not a specific type.  What type of tygon tubing did you use?  It looks like tygon fuel line is yellow.  Are you saying you put yellow fuel line on your bike?

-S

[Bailgang]

Thanks.  I have a spring dial gauge carb sync tool (like in the manual) but it is difficult to read as the needles bounce around. 

It sounds like your dial gauges are missing the damping valves that I'm pointing to in the pic, without them the needle will bounce all over. However that's not the point, if you're pleased with how your homebuilt manometer works then by all means keep using it. My DIY manometer is a PITA because if there is ANY difference in vacuum between the carbs then it's going to show it making it damn near impossible to get all 4 carbs on the same level. I think I used too small of a dia tubing on mine which is why it's so sensitive.

[BobbyR]

On my 78K I use Tygon which allows for better routing since it is more flexable and does not require clamps. I routed mine so the gas runs downhill and the the filter is actually between 1 & 2 carb.

To do a proper carb synch you need to do a valve adjustment. Timing also.

Cool.  So what type of Tygon tubing.  The name Tygon is a brand of tubing not a specific type.  What type of tygon tubing did you use?  It looks like tygon fuel line is yellow.  Are you saying you put yellow fuel line on your bike?

-S

I usually get it in places that repair small engines, like Lawn tractors and snow blowers. It comes in a few colors. I have yellow on there now and you can't see it. It comes out of the petcock into a filter with a 90 degree bend that sits between # 1 & 2 Carb. It goes down hill in a fairly straight line.

[nzdreamer55]

On my 78K I use Tygon which allows for better routing since it is more flexable and does not require clamps. I routed mine so the gas runs downhill and the the filter is actually between 1 & 2 carb.

To do a proper carb synch you need to do a valve adjustment. Timing also.

Cool.  So what type of Tygon tubing.  The name Tygon is a brand of tubing not a specific type.  What type of tygon tubing did you use?  It looks like tygon fuel line is yellow.  Are you saying you put yellow fuel line on your bike?

-S

I usually get it in places that repair small engines, like Lawn tractors and snow blowers. It comes in a few colors. I have yellow on there now and you can't see it. It comes out of the petcock into a filter with a 90 degree bend that sits between # 1 & 2 Carb. It goes down hill in a fairly straight line.

Any chance you can attach a picture of this?


Also to everyone...

What order would you follow to tune up this system (valves, then timing, then idle adjustment, then sync?)  Any thing l else I forgot?

-S

[MoMo]

valves, cam chain, timing, carb sync,  has been my regimen for awhile

[nzdreamer55]

Thanks MoMo.

Checked and set the valves, did the cam chain adjustment, set the timing (wow it was really off) and tried to sync the carbs but am having some trouble.  It seems like the vacuum from #2 (the one I cannot adjust) is low.  No matter how I try to bring the others down (I turn the screw in on # 1,3&4) this pushed the needle and throttle valve down and should drop the vacuum in those carbs make it closer to the #2, but the engine starts to die and I have to turn the throttle set screw to keep her running.

Is there a neutral starting place where I can turn the arm set link screw (this is the screw you adjust to sync the carbs on this set of carbs ) and then go from there?  Like 2 turns from the bottom or make it flush with the locking nut and then adjust, etc.

Thanks

-S

[flybox1]

tygon yellow is the only one fuel rated.
This is motion pro blue fuel line.
http://www.amazon.com/Visu-Filter-FF707C-Visu-Filter-Element/sim/B000UKO3VW/2