Topic: Carb jetting (Sorry)

[Blaize]

I know this has been covered over and over again, and yes I did search and do some looking, but I still have questions. So here we go.....I have a 74 cb750. I am finishing up the winter rebuild/cafe chop and I know there will be carb tuning in my future. I have what I think is MAC exhaust, 4 into 1. and pod filters. otherwise it is all stock (the engine internals I mean). It has got 120 mains in it now and ran fine with the 4-1 exhaust and the stock airbox before I tore it apart. I did once in the past take the baffle out and buzz it around the block and it ran like crap (but sounded fantastic) but of course I dont want to go deaf so the baffle will be in it.

 My understanding is that I will need around a 130 jet and move the needles up one notch to start with. this sound right? what size jets should I buy to have on hand? normal sources will have them (Z1 ect)? I am in seattle (at or near sea level) and I will be using it for primarily in town riding in dry weather (so thats about 20 min. per year). I will be moving to England in 6 months and taking it with me, but I suspect the riding will be much the same.

 I am frankly ashamed to admit it but I am a factory trained Porsche tech, but as they have not used a carb since before I was born, I really am clueless about it all. In the past I have actually kept cars stock just so that I didn't have to dick with the carbs. Seems to me it is about bloody time I figured this out.

 So any advise?

[750goes]

do yourself a favour and see how it runs when you put it back together - the only change you have made is to put on the pods as per your descriptrion, you may well get away with only adjusting needle position or even tuning the carbs a bit more by richening the fuel mix...
do small changes at a time and not wholesale changes of every variable then trying to work out what is going wrong..

just a thought..............

[Andoo]

Moving the clip up will lean the mixture while lowering it will richen it. Before you do anything do a plug chop and make sure your need to richen the mixture. Many times you don’t need to. If you do I would leave the main jets stock and lower the clip one slot. Also turn the air screws out a tad to richen up the idle mixture.

However unless you notice a performance loss or your plugs indicate running rich I wouldn’t touch it.

Don’t just take my advise though, let some other guys chime in, I could be wrong.

I have a highly modified exhaust, 836cc kit, and pods. I have stock idle jets (40) near stock main jets (122) and lowest clip position plus one washer.
If you try and run 130 jets you will foul the plugs very quick. That’s WAY too rich! Trust me, I learned the hard way.

[Blaize]

Thats what I was initially thinking, that I could use the std jets, but all the reading I did was giving me the impression I may need to go bigger. I won't bother buyng any extras then and just see if I can adjust it the way it is. Thanks

[Blaize]

Well now that it is on the road I am pretty sure it is in fact lean. once it is warmed up, when riding alomg at around 3k rpm, if I suddenly twist the throttle all the way and hold it, it loses all power. Same as if I hit the kill switch. As I unwind the throttle it will suddenly get a strong pull and then settle into normal acceleration.

 So far I have lowered the needle clips to the lowest slot, and adjusted the float height. I have got 125 mains and 40 slows. I have yet to fiddle with the screws but it runs fine down low so I dont think the trouble is there. I have also found that if I just slowly roll into it I can rev it all the way to redline and it is pretty bloody fast. I think adjusting the needles (they were one posit. up from all the way) made a difference but it may be in my head. This bogging is indicative of a lean condition right?

 As always thanks for any ideas/help

[Andoo]

Many times a lean mixture and a rich mixture feels the same. best bet is to do a plug chop. Also the standard carb synch and do adjust the idle screws.

The more you play around with the carbs you will get a better idea of what the bike is doing. It can be very frustrating 

[Gordon]

best bet is to do a plug chop.

Agreed.  You're not going to make much progress by guessing at the condition.  Do the plug chop and find out for sure.

[Blaize]

So, you mean plug chop as in wind out 5th gear and then hit the kill switch? or can I get readable results by doing the same thing in 2nd. I live at the top of a hill so I can do it in 2nd pretty easy but steaming along at 110 mph is going to be tough in my neighborhood

[TwoTired]

For a moment, think about how your carbs work.
When the piston moves down on the intake stroke it creates a partial vaccum in the intake runner.  This draws air through the carburetor.  The vacuum and the venturi effect through the carb draws fuel through the fuel jets.  The flow through the jets is determined by the amount of vaccum and the size of the fuel jet orifice.

The air filter on the inlet side of the carb provides some restriction to the air flow, which acts to deepen the vaccum in the carb throat and draw more fuel though the fuel jet orifice.  The amount of restriction, therefore effects the fuel mixture at any throttle setting.

The slides are mechanically linked to the throttle.  At idle or low throttle settings, the slides block the inlet passages which result in high vacuum present on the fuel orifices.  When the slides are suddently opened, the high vacuum drops to near non-existant levels, which curtails flow from the fuel jets and results in no power from the engine.

Many carburetors with direct linked slides have accelerator pumps to enrich the mixture with sudden openings of the throttle.  The 74 CB750 carbs do not.  It relies (in part) on an over rich idle mixture to provide throttle pickup.  The maginitude to the overrich idle condition is a trade off between having sooty plugs and poor idle fuel economy and how much throttle opening is allowed before the wheeze response is effected.

Filter pods generally have very lax filtering requirements compared to the stock air filter, leading to less restriction at the carb inlets.  The reduced restriction causes the idle mixture to become lean and this effects the throttle twist response.  The idle mixture can be enriched by turning in the air screws, or enlarging the slow, idle, or pilot jet.  How much change is needed depends on your filter pods.  There may not be enough adjustment range on the air screws, and larger slow jets will be required to acheive the same throttle response that the stock air filter provided.  Or, you will have to adjust to turning the throttle only part way to achieve the engine pickup you desire.

I assume you already know that the throttle valve needles, and main jet are also similarly effected by filter pods.

Cheers,

[Blaize]

So I have been working out my rearsets all month and not really worrying about the jets, but now I am back to it. Thanks for the great info two tired. I hadn't really thought it thru but what you say makes alot of sense. Perhaps I am closer than I think.

 So as to the plug chop, I really do want to know how the rest of you are doing it. Do I really need to wind out top gear? if so, why? Wouldnt 2nd or 3rd give the same results? I cannot imagine all of you are really bombing around at top spead then pulling over to check plugs next to the scene of the crime.

[TwoTired]

This is why Dynos were invented.
Fuel requirements vary with the engine load and HP output.  A dyno can adjust the rear wheel drag to fully load the engine in any gear.  Lower gears give a mechanical advantage, so the engine doesn't have to work as hard.  Less energy is required in the fuel mix to achieve the required power.

On pavement, the engine has the highest load in top gear fully loaded, with air friction drag and at a throttle setting that is intended to give max power.  If you can arrange an uphill climb, so much the better to increase the load on the engine.  These conditions will require the highest energy content from the fuel and the best air/fuel ratio.  And, this you can read from your plugs deposits.

Cheers,