Loss of power 1979 CB750 - full throttle

[wassupitaly]

Hello guys,

I'm having an issue on my 79 CB750.

Here goes the scenario:
Turn the bike on (takes a while but no issue there I've been told they are a b....to start up)
Let it warm up. Once I get on the road it's fine until i reach 5th gear...if I give it full throttle the bike start missing and sputtering and eventually shuts off. Bike sounds like it's starving for fuel.
After about 10 minutes bike restart. I try to get home and everything is fine until again too much throttle and shuts off. Same thing as before, I get it running and finally reach my destination. Can anybody identify what could be going wrong here? Any help is appreciated. Thank you much in advance!

[rrgunslinger]

Is it stock?

On Fri, Jun 8, 2012 at 12:24 PM, LOLO [via Honda CB750'S] <[hidden email]> wrote:

Hello guys,

I'm having an issue on my 79 CB750.

Here goes the scenario:
Turn the bike on (takes a while but no issue there I've been told they are a b....to start up)
Let it warm up. Once I get on the road it's fine until i reach 5th gear...if I give it full throttle the bike start missing and sputtering and eventually shuts off. Bike sounds like it's starving for fuel.
After about 10 minutes bike restart. I try to get home and everything is fine until again too much throttle and shuts off. Same thing as before, I get it running and finally reach my destination. Can anybody identify what could be going wrong here? Any help is appreciated. Thank you much in advance!

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[Hellbilly429]

Have you checked the charging system? I am having the same issues with my bike because my charging system currently doesn't work, seems like it misses at the higher rpms because it needs more voltage. Also can pull the spark plugs out to check and see if you're running too rich or too lean.

[sgtslag]

Actually, no, they are not difficult to start up, if they are properly tuned.  You have other issues going on with the full throttle problem, but the hard starting is separate.

The full throttle problem could be a number different things:  pods are a challenge, under the best of circumstances, to make work properly -- stock air box works quite well, even better with a K&N permanent air filter; main jets plugged; plugged fuel filter?

With regards to hard starting, the least expensive solution would be to install NGK Iridium spark plugs, at around $8 per plug.  I installed a set, several years ago, and my bike starts very easily now -- the weak OEM coils can throw a spark through the Iridium plugs very reliably, making starting much faster.  Check the operation of the choke, also, to be sure it is working properly.  Cheers!

[wassupitaly]

As far as the cold starting - it cranks with choke on but wants to shut off immediately. What I usually do is turn the idle knob on bottom a little just to turn it on without choke. Keep it running that way for couple minutes and as soon as the RPMS start increasing because it's warming up I turn it down back to 1200 RPM.

I have stock air box on it. I will buy a new air filter ASAP just to eliminate that variable.

How do I remove the fuel filter? I bought a new petcock valve and came with the fuel filter but I did not remove the one that was already in my tank as it seemed like it was tight in there and clean.

I just put new plugs in NGK 8DEA. I don't think those were iridium plugs as they only were 2$ a piece. What is the code for the NGK iridium spark plugs that will fit the '79 CB750?

Thanks.

[rrgunslinger]

Sure sounds like it is running our of gas.

On Fri, Jun 8, 2012 at 2:53 PM, wassupitaly [via Honda CB750'S] <[hidden email]> wrote:

As far as the cold starting - it cranks with choke on but wants to shut off immediately. What I usually do is turn the idle knob on bottom a little just to turn it on without choke. Keep it running that way for couple minutes and as soon as the RPMS start increasing because it's warming up I turn it down back to 1200 RPM.

I have stock air box on it. I will buy a new air filter ASAP just to eliminate that variable.

How do I remove the fuel filter? I bought a new petcock valve and came with the fuel filter but I did not remove the one that was already in my tank as it seemed like it was tight in there and clean.

I just put new plugs in NGK 8DEA. I don't think those were iridium plugs as they only were 2$ a piece. What is the code for the NGK iridium spark plugs that will fit the '79 CB750?

Thanks.

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[seestheday]

I eliminated my starting issues with a combination of iridium spark plugs and pulling the auto-petcock off.  Right now I can basically breathe on the starter button and it'll fire up strong.

[shinyribs]

Hellbilly is on to something. A poorly charged battery will not start the bike very well. The two issues may be related. Do you have an inline fuel filter,or are you just running with the in-tank petcock screen? That screen is more for protecting the seals inside the petcock from big hunks of rust or what not,it doesn't double so hot as a fuel filter. That being the case,you could have a fuel starvation issue like was mentioned. Hard to tell until to dive in,but the guys have told you the two best things to start checking IMO.

[TOOLS1]

Check the breather in your gas cap. It may be plugged. As, for the starting, it seems like your pilot jets are plugged.
TOOLS

[wassupitaly]

Ribs,

What do you use for an inline fuel filter? I guess as you described it I don't really have a fuel filter at the moment except for the in-tank one.

Got the spark plugs ordered and should be in tomorrow. 7$ for each at the local auto store. I will be changing those and checking some other stuff you guys have mentioned.

Thanks for the help guys. I really appreciate it.

[Jeremy]

Check charging system as said before.
Clean jets
Make sure carb air passages are clean
Gas cap vent
Spark plugs

Basically everything the other guys have said.

Start with the cheap stuff first

[sli750f]

ok i have a 1980 758f, and im having the same issue bike cranks and ran fine up till yesterday, yesterday i peged it out and now it acking as your is but when i get it up to 4grand is when it starts the same i have the k&n air filter were are the jets located and how can u tell if ur gas cap vent is working truble n texas any thang will help

[Piute]

how can u tell if ur gas cap vent is working truble n texas any thang will help  

suck n blow

[sli750f]

ok tha cap seems to b working its rusted but it sucking and blowing air

[sli750f]

ok tha cap seems to b working its rusted but it sucking and blowing air

[shinyribs]

Any paper element filter that will fit your fuel line. The paper elements filter the best .Any almart or autoparts store should carry them. You can find them in the lawn mower section.

[sli750f]

there is a filter between the petcock and carbs it looks like a con shap looks like a willow brush n it

[shinyribs]

Sorry.I dont know what a willow brush is.

[sli750f]

im not sure but i will bye a new filter for it and new spark plugs thanks for all the help i will keep u posted

[sli750f]

one more thang what would be the worst thang it could be?