Topic: Backyard mechanic needs help

[revcbl]

I'm trying to get my 76 cb750 k running better, and I'm in over my head here.
The problem: rough idle, running super rich, gulping down fuel and losing power at highway speed (you give it more gas, but just... nothing).
The setup: Here's where it gets complicated. The owner before me added a 4to1 kerker exhaust, aftermarket air box and new coils. I'm not sure if these changes would suggest a change in jetting, or if that would explain any of the above problems, or, for that matter, if the jets have been changed or altered.
My plan is to clean and synch the carbs and do all the normal tune-up stuff, but while I've got them off I'd like to address the jets as well, if that's a possible cause of the problems.
Any ideas or suggestions as to how I should proceed, including whether I should just leave the jets alone and concentrate on the less complicated possibilities for now, would be a big help. I've had this bike for under a year and it's way more complicated than my old 360 (but more fun at the same time).
Thanks,
Chris

[Lumbee]

...when u say aftermarket air box, what do u mean.  Pod filters, breadbox?  If it were me I'd pull the carbs, and see what jets you have installed.  That could easily be why its running rich.  Is the exhaust baffled?  I'm assuming it is, but you never know, if its not, that is somthing else to consider.  With the set up u'v described I'd say you should be jetted in the 110-115 range.

[revcbl]

Thanks great one,
The airbox is a mystery-- no maker on it and I can't find a similar one. It's all chrome, much smaller than the original, and lets air in through a perforated outer ring all the way around the sides top and bottom. The foam filter is held in place between the outer ring and a similar inner ring. It seems like it would, by its design, let in a lot more air than the original. I put in new foam (and oiled it) soon after I bought the bike.
Now whether a lot more air helps or hurts things in this situation, I'm not sure, but it doesn't seem like it would contribute to the running-rich problem.
I have a stock airbox that I could switch to, but I doubt that would help anything (and it might make it worse).
Thanks for the assistance,
Chris

[Lumbee]

...sounds like a breadbox to me...and yes, this would increase air flow.  As I said, I'd check out the jets, just do you know where you stand.  U'r on track by syncing the carbs, and check out the timing and point gap also.  If this is a new bike and its running crappy like you said, sounds like a top-down tune up would be in order...

..once you'v covered all your bases on a tune up, report back your results and how the bike is running...

[scondon]

Sounds like you need to do a FULL tune-up. Valve gaps, points gap, timing as outlined in the manual. What you are describing sounds like a very poorly tuned bike. No way to determine what main jets will work until you get the bike running properly, though what Lumbee suggests sounds like a good range to start. The jets can be changed easily, at any time, without removing the carbs.

    Do all your carb work after you are sure that timing and gaps are correct. New battery? Healthy spark to each plug? Gas in each float bowl and at proper height? How's your compression?

Try taking the foam filter out of the airbox and see if it runs better. Just my opinion, but I can't see how oiled foam could possibly allow enough air through and could certainly cause a rich condition. In fact disregard the first two paragraphs until you've tried running without the filter.

[Dave Wyatt]

How heavy did you oil the filter?  Too much oil will cause a restriction worse than a stuck choke.  You always want to squeeze out the excess oil.  The foam should be damp with oil, not dripping. 

Been down this road with a two stroke before.

[TwoTired]

With the intake and exhaust changes made to the bike, carburettor setting a jets are unlikely to be correct when set to Honda book values.

The carburettors are the last things to set AFTER all the major tuneup items are addressed, and the carbs synced.  With an unknown bike history, compression test numbers are good to have as a cylinder problem can't be fixed with carburettor jetting.  But, if you are confident of the cylinder mechanicals, don't bother.

I don't see how correct jetting can be accomplished without knowing the combustion conditions.  There are dynos and exhaust sniffers out there which can speed the process significantly.  But, the old fashion home style way is to read your spark plug deposits.  If you know if you are too rich or too lean for the throttle setting in question, then you know what part of the carbs to change and whether you need to go up or down in needle, or orifice settings.

It is certainly nice to know what you have before you change it.  Knowing that your carbs are clean, assembled correctly, buoyant floats, correctly set, with known undrilled jets sizes, may help a great deal in addressing the issue.

Lastly, how did you choose the foam replacement?  There are many different kinds and densities of foam.  Many of which are unsuitable for your intended purpose.  And, certainly too much oil can clog up the foam.

Cheers,

[scondon]

Well put, TT. Feel like I'm bangin' away on a stone tablet with a hammer and chisel next to that post

[Dennis]

Well put, TT. Feel like I'm bangin' away on a stone tablet with a hammer and chisel next to that post

LOL   

TT can almost always find a much more eloquent way of explaining a situation than most of the rest of us!   

[revcbl]

Thanks for the response everyone.
I did the point gap, timing and other stuff last year-- it improved the idle but not the fuel consumption.
Didn't do the valves, though, so that's a good thought.
The foam is Uni brand I think, whatever they had at the parts store that I could cut up and fit in there. Over-oiling, as embarrassing as it is to admit, is a possibility, so I'll deal with that first. I did synch the carbs last year as well, but it was my first attempt with mercury stix and all I can say for sure is they didn't sound any worse.
I took a look at the carbs today and they looked pretty spotless. Guess that stuff I sprayed in there did something. Cleaned 'em anyway and will finish tomorrow setting float heights, etc.
Now, the jets are 130, but I'm going to leave them in until I follow the rest of your advice and see what happens. Seems big, and the originals are (according to this web site) 105. I found the paperwork for the coils, which suggest a larger size (they don't specify which), but the information online about the exhaust suggests a size smaller than stock, so, no help there.
Thanks again, and I'm open to other suggestions too.
I can use all the help I can get.

[TwoTired]

You might like to know that the Honda owners manual states that a major tune up is regular maintenance at 3000 miles or 6 mos intervals whichever occurs first.  If you haven't yet established a complete major tuneup yet, that's a good place to start when addressing performance issues.

The Uni foam should work well for your application.  I also prefer to use the Uni branded spray oil.  This stuff leaves a very tacky residue, which in this case is actually a good and beneficial adjective.

Coils- larger size?  Are you saying the coil documentation says to increase the carburettor jet size?  This seems odd.
However, higher voltage coils could use a larger spark plug gap. Any tech details on the coils you could share?

Scondon and Dennis, Thanks for the kind words!  Nice to know some of my posts are read and appreciated.
Cheers,

[revcbl]

Right, and since I've had it for about six months, here I am doin' the tune-up. I did some small stuff before the winter, but, well, wanted to ride instead. Then it got cold.
They're Accel dual tower coils. I'm not sure of the model number offhand. All the documentation I have is a one-pager that tells me "A 'hotter' spark usually normally requires larger jet sizes in order to maximize power output and prevent a possible lean condition..."
It also tells me to set the plug gap to a minimum of .035, which I hadn't noticed before, so thanks for that.
All this newfangled stuff makes things complicated.
-Chris

[Dennis]

All this newfangled stuff makes things complicated.
-Chris

 
"NEWFANGLED" 
  .........seems an unusual word to use when describing items for a SOHC4     

[dusterdude]

tt,your input is always appreciated.

[Dave Wyatt]

Lumbee suggested 110-115 on the jets and you have 130.  Ususally an exhaust swap such as yours will create a lean condition, requiring more fuel.  Cars, bikes whatever, typically don't need to go up more than a few steps to acomplish it.  130 sounds like a huge step, but I'm not that familiar with bike tuning.  On a car however, that much would be way out there.  I would suggest checking the filter as previously mentioned, make sure the rest of the carb(s) look OK, then attack the jets.

Or drain the tank and switch to methanol which requires huge jet sizes: