High RPMs power problem

[endwrench]

Hey everyone,

New here. Picked up a 1981 CB750F about a month ago and have been wrenchin' on it some. I'll start a build thread later to document everything, but I have one persistant issue that's driving me crazy -- I just want to ride it for now.

So, main issue is: Engine runs perfect until about 5-6k RPMs, and then it starts crapping out, has no power, sounds like it might not be firing on all cylinders, but I'm not sure. The bike has pod filters (I know everyone hates these but I'm determined to make them work) and a 4-1 header with a Jardine can on it. When I first got the bike it would barely start and wouldn't idle right and had the same issues I'm having now with high RPMs but it was like that all the way from 2k. Noticed the pilot screws were way out of adjustment and so I pulled the carbs and cleaned them well and put things back to stock. The carbs had the stock 68 main jet but had a (weird?) 130 in the secondary mains. I also discovered that the carb intake boots were bad and sucking like mofos. I replaced the boots and it was immediately idling perfectly, that issue solved. I bought a jet kit and messed around with the main jets (tried 105, 110, 115, 120, 140 -- yes I pulled the carbs that many times) but jetting was still wrong. Plugs were black and fouled, too rich. Finally I remembered the weird secondary main jets and read somewhere that the stock secondary main jet is a 100, so I popped a 110 in the main and a 105 (closest I had to 100) in the secondary mains, as they looked to be the same jet screw. I also put in hotter plugs as I suspected it wasn't firing hot enough (D7EA instead of stock D8EA). I rode it, and for awhile it was the same thing -- pulls strong until 6k and then craps out -- but after about 10 miles of me winding on it I felt it start to clean out and eventually it was running perfect, pulling super hard and fast after about 7k and feeling great. I decided to go back to the regular plugs just to see and it didn't run as well as it did with the hotter ones. So I rode it for a few days and it was fine, but then I developed an electrical system issue, starter wouldn't turn over and battery wouldn't charge. I cleaned up everything under the alternator cover and did all the electrical tests in my Haynes and everything passed except the regulator/rectifier so I have a new one of those coming in the mail right now. I thought I fixed the electrical issue the other day (battery was fully charged and starter was working, volt meter read 13.5 volts at 5k rpm, so it was charging) but I rode it pretty far and eventually it started running crappy and then died and wouldn't start.

tl;dr: an unsure electrical charging system and an engine that runs crappy after about 6k rpms. Is it a carb issue or an ignition issue? (my plug wires could be bad, I'm thinking) I heard that you can buy CBR coils and make them work pretty easier for a hotter ignition.

Sorry for the freakin novel, and thanks in advance for the help!

[Lucky 1]

13.5 volts @5,000 rpm?  Were the lights on or off?

Anyway a 12 volt battery should be 13.2=13.8 volts at rest -engine off.
Each cell is 2.3 volts or more.
So 2.3x6 cells is 13.8 volts.

If the battery is low and it is at idle it should be charging at 14-16 volts.

By the way I would use standard spark plugs and adjust the carbs. Not the spark plugs.

[rrgunslinger]

I agree with Lucky,  Don't fix a fuel problem with a hotter plug. 

On Tue, Jun 26, 2012 at 8:09 AM, Lucky 1 [via Honda CB750'S] <[hidden email]> wrote:

13.5 volts @5,000 rpm?  Were the lights on or off?

Anyway a 12 volt battery should be 13.2=13.8 volts at rest -engine off.
Each cell is 2.3 volts or more.
So 2.3x6 cells is 13.8 volts.

If the battery is low and it is at idle it should be charging at 14-16 volts.

By the way I would use standard spark plugs and adjust the carbs. Not the spark plugs.

1978 CB750K Completely restored cafe style

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[seestheday]

Wait, are you saying that you are running a 100 in the primary main and 105 in the secondary main?  I think that is your problem.

Primary main stock is a 68, and stock secondary is 105 for my 1981 CB750K.  Why not try a 75 or something smaller in the primary?
Going from a 68 up to 100 is a huge jump.

The performance Jet kit for the DOHC from sirius comes with 75's: http://www.siriusconinc.com/pro-detail.php?pid=&product_id=180

I have a 4-1 with an unknown can and a 75/105 combo is working well for me.  I'm running iridium plugs.

[endwrench]

Update. So, I put the regular plugs back in. I also realized I made a stupid (rookie!) mistake reading a jetting chart and thought that 102 was the stock for the main jet, but anyway, I put the stock 68 back into the mains and then put 105's into the secondaries. I ordered 75's as a coupla people have said that 75/105 is usually a good setup. But it didn't solve my problem... it didn't really do anything. Motor still ran fine until 6-7k and then just got really crappy. I thought it might be because it was too lean, so I wrapped painter's tape halfway around all four pod filters in an attempt to richen things up. It worked for around five minutes, which is weird -- it ran strong all the way up to 9k for a few minutes, but was then back to the usual crappy after 6-7k. I'm stumped. I dunno what to do, I've tried so many jetting combinations. What worked the best so far was 110 mains and 105 secondaries -- it ran really hot all the way to 9k for one ride but after I went to ride it again after discharging the battery completely and recharging it while trying to solve electrical issues it was back to the usual.

Could it be coils and plug wires? I have a hunch my plug wires are bad. The head popped off the wires of one of the plugs but I was able to stick it back on and it seemed ok. Does anyone know anything about using coils from a CBR? I've heard it's a plug and play mod for a hotter ignition. Also, does anyone know anything about using an F2 rear wheel? I've heard that you only need a small spacer to make them fit.

[endwrench]

And I'm fairly sure that my electrical woes are the regulator/rectifier, which should be here in the mail in a few days.

Oh, and here are some pictures of the thing.

When I brought it home:


A few weeks later:

[SmashEmUp]

Any update on your issues here?

[Re-run]

I am a little late to the party here and don't know if you got your issues fixed.

Anyways, I wanted to clear up a bit of misinformation.
A lead-acid battery, meaning a flooded cell, maintenance free, or agm, should NEVER read over 13 volts at full charge. A cell puts out 2.1 volts and the highest I have ever seen has been 12.8 volts.
On charging,  you should NEVER hit 15 volts. Outgassing on a lead-acid starts at 14.4, higher than that and you can start overcharging.

When you tested 13.5 volts at 5000 rpm, your battery should not go flat but maybe on your bike, 13.5 isnt enough. Do you know the age of the battery?

Also, for plugs, the dohc might be different but if you have the owners manual, it will say for sure BUT it is generally OK to change plug heat ranges.
For example, in the sohc cb750, Honda recommends using a d9ea instead of the d8ea for long distance running. For cold climates and such, a d7ea is fine to use.  I will assume the DOHC is the same.
In the NGK world, A higher number = a colder plug and a lower number = a hotter plug.

[LukeM]

agreed.  Anything over 14.5v will cook the electrolyte, boiling off the water, and eventually killing the battery.  The regulator on my Shadow went out, and it was sending 15v or more to the battery.  Luckily I caught it in time, but my battery still needed about 8 oz of water to cover the plates.

Luke M

[sgtslag]

I don't mean to rain on your parade, but...  Pods do NOT work on the Keihin Constant Velocity (CV) carbs, period.  

It's been attempted by numerous experts on Honda's equipped with the CV carbs, and these folks won't even discuss it anymore, on the CB750C forum.  The CV carbs depend on the OEM air box for proper vacuum, to operate them.  It's a dead horse.

With regards to everything else, be sure to download the Factory Service Manual (FSM), from the Home Page of this site.  As someone else stated in another thread, here:  "Download it, print it off, and put it into a 3-ring binder."  Haynes and Clymer are good, but the FSM fills in spots where they are either lacking, or silent.  Cheers!

[Hellbilly429]

It's definately a charging issue, I am currently having the same problem, i changed reg/rec and it was fixed for a bit then stopped charging again, tested alternator rotor and a winding opened up in it and now rotor is junk. Everytime my bike stops charging it will not go over 5-6k rpms before spitting and sputtering and acting like it is going to die. Make sure you test the rotor and stator before you run that new reg/rec you ordered as you could possibly damage the new unit with a defective rotor or stator.

Problem is these bikes need more voltage at higher rpms it seems, the coils do not fire very hot for these bikes and if the voltage is not up or the amperage is down, it will not fire hot enough at higher rpms.

[endwrench]

Thanks for the replies. I went ahead and got a new battery, and I made new plug wires and put a set of NGK Iridium plugs in, and that completely solved my top end problems... but not my charging problems. I checked the alternator components according to my haynes a month ago and everything passed but the reg/rec, so replaced that. I'll check the rotor again, I have a feeling it might be that.

And yeah, I've given up on the CV carbs. I'm buying a stock airbox from a member on here and I'm also looking into buying a set of mikuni flatside carbs from a GSXR, people over on Custom Fighters have done it. If anyone's interested, here's a link to by build thread on a different website -- mostly just more pictures and stuff.

http://www.customfighters.com/forums/showthread.php?t=58787

[shinyribs]

Your bikes looks real nice.Good job What kind of muffler is that? I was curious how it sounds

[endwrench]

Thanks! It's a Jardine can. It's loud, but sounds pretty good.

[endwrench]

Alright, so I think it's the rotor but I'm not entirely sure. I have two ammeter things -- one is a Fluke that I borrowed from my dad who is an electrician, which he says is designed for really huge electrical stuff so is probably inaccurate for small stuff -- and when I test between the rings with it, it fluctuates crazily but usually settles down to like 1 - 3 ohms of resistance between the two rings, and infinity resistance between the rings and the ground. My other meter is a super old Micronta from the 80's, and it says there is 0 resistance between the two rings and infinity between rings and ground. Hayne's manual says there should be some resistance... if there is zero on one meter and a really low number on the other does that mean that the rotor is bad? The stator has continuity between all the wires and the reg/rec is brand new and it's definitely not charging at all (unplugged alternator while engine was running and tested wires) so I concur that it must be the rotor.

Anyone know a good place to get a rotor / rotor puller? How much should a used rotor cost?

[iamjaime]

A lead acid battery is 2.1 per cell. So the battery should be at 12.6. However, after charging or after running it will carry a top charge and be slightly higher

[Re-run]

Wow, thread resurrection!