Topic: help me reason this out - carbs - yes, I searched...

[KB02]

1978 CB750K. Bored out to 850. Stage II ported head. Stock carbs. Pod filters. 4 into 4 glasspack exhaust (fairly open).

here is what I have going on. Rebuilt the engine (you might have seen my build post). Finally got it running and on the road. At start up (cold), it will need most if not all of the choke to start. If you rev the engine as it warms up, all four cylinders will fire. If you walk away for a second, some of the cylinders will cut out (#3 noticably). As soon as you hit the throttle it comes back in. Riding down the road it runs fine until you hit to about 50 or so MPH. At this point the engine begins to try and cut out a bit, like it just doesn't have the fuel available to go any faster.

Carbs have the air screw set at 1 full turn out. I am running 120 mains and stock Idle jets. In the current gearing configuration, 50 MPH is running about half throttle (I screwed up in figuring out the gearing, but a fix is on it's way for that).

Here is what I am thinking - you can tell me where I'm wrong:
Overall the bike seems like it might be running a touch on the lean side. I haven't done any plug chops as of yet, but after about 20 miles on new plugs, the plugs are looking pretty good, if maybe a little white. Is it possible that my float bowl levels are too low and that the bike is runnign out of gas in the bowls and not able to refill quick enough? I am running an inline fuel filter. Might that be too restrictive? The inside of my tank is "okay," but to the point that I would be nervous to run it without an additional filter.

After work tonight, I plan to do some more test runs and a propper plug chop at the problem area on the throttle. I will also recheck the float level and take out the inline filter, just to rule those out as issues.

What else should I look for? Or have I got it pretty well covered?

I'll post some pictures when I get home (as my work computer does not like Photobucket) of the outside plugs after their 20 miles and the current float level to see what you think. I've got just under three weeks to get this beast ready to ride to a motorcycle rally. It needs to run great, soon!!!

Thanks all!

[Duke McDukiedook]

Lean, yes, you'll probably want to up the main and see how that treats you.
Definitely plug chop at the RPM's you have problems with, there is no clear cut answer on carbs once you go off the stock reservation.
Lots of experimentation and carb removals are in your future.

[Trav-i]

I had a simillar problem with my bike as well.  Although mine is stock i was running pods.  Low end was good, Idle was OK, mid-range around 50MPH or so was terrible, and I had a hard time getting past 4 or 5 thousand RPM.

What I figured out was I had serious mid range problem, after reasearching my problem I figured out I had a problem at the transition to the main jet from the slow jet.  I changed the slows to some "drilled out" slows I had and it fixed the problem, but man my fuel economy in the ground.

So I finally put the air box back on and rejetted for it, and I'm much happier now.  It just seems smoother, and my fuel economy is back up.

So I THINK some larger slow jets are in your future (although they're hard to find). But I've also have heard bigger motors need smaller slow jets, so I guess its going to be lots of trial and error. I'm not sure about the cylinder cut out problem but I'm willing to guess it's a fuel problem as well.

My 2 cents is to put the air box back on (if possible) and jet accordingly, just to see what happens.

Good Luck,

Travis

[seaweb11]

78K - pods- 4-1 Mac 138 main jets =  4+ years thousands of miles and it runs like a champ. 

As said before, try larger mains

[KB02]

After 20 miles of riding on new plugs (kind of blury...):


Float levels (floats are pretty even across the rack):


I have some 130 mains that I could try (might make it a little rich...). I had wanted some 125's when I had the carbs apart last but the local bike shop did not have any last time I was there.

[scondon]

1 turn out on the 78 carbs is too lean, even for stock. Honda states 1 3/4 turns for stock. Try 2 to 2 1/2 turns for starters. If you haven't raised the needles, I would suggest shimming them with washers.

50mph in what gear? If your in 3rd gear or above then I don't think your main jet has even come into play yet.

My set-up with 850cc, pods, OPEN exhaust, and '78 carbs has been 2 1/2 turns on the fuels screws, two washers shimming each needle, 125 mains. About 35 mpg, but it runs strong through all ranges. One washer under each needle if you want better fuel economy, but the engine will run a bit hot due to lean mixture.

[1080]

Check for loose main jets.
I forgot to tighten one once & it backed off very quickly.
It was 754's fault
I should of re-checked my work after getting off the phone with 754

[scrapvalue]

My bike was running 110 main jets stock.
All I did to the motor was installed pod filters and the exhaust I built is just 1 1/2" straight pipes.
They are a lot longer then most pipes because I stretched the swingarm 8" and run the exhaust back past the axle bolt.
The stock jets would idle o.k., but as soon as I gave it any gas, it would cut out.
I installed 150 mains and then it would run up to about 1/2 throttle and then cut out again.
I then installed 190 mains and it didn't cut out, but it would stumble and burble,(?),when you tried to cruise.
I then dropped to 170 mains and it runs like a raped ape.
It idles fine, revs very fast and cruises effortlessly.
I know this sounds crazy, but I have put 3750 miles on it and never fouled a plug. Plugs are medium dark brown.
Needle is still in the stock position and slow speed jets are also stock.

But one thing I highly recommend is to install an electronic ignition. Doesn't have to be the real expensive one.
I have one on my stone stock 72 and my modified 74.
Mis adjusted points can act a lot like carb issues.
And carb issues can act a lot like points issues.
Until you are positive you have one of them right, it makes it real hard to get either right.  

Basically, no 2 jetting situations will be the same. It always requires a couple,(or more) tries.

[eurban]

1978 CB750K. Bored out to 850. Stage II ported head. Stock carbs. Pod filters. 4 into 4 glasspack exhaust (fairly open).
Carbs have the air screw set at 1 full turn out. I am running 120 mains and stock Idle jets. I

PD carbs have idle mixture screws that control fuel not air.  Have you removed your pressed in pilots and cleaned them and the entire idle circuit with meticulous care?

[eurban]

1 turn out on the 78 carbs is too lean, even for stock. Honda states 1 3/4 turns for stock. Try 2 to 2 1/2 turns for starters. If you haven't raised the needles, I would suggest shimming them with washers.

50mph in what gear? If your in 3rd gear or above then I don't think your main jet has even come into play yet.

My set-up with 850cc, pods, OPEN exhaust, and '78 carbs has been 2 1/2 turns on the fuels screws, two washers shimming each needle, 125 mains. About 35 mpg, but it runs strong through all ranges. One washer under each needle if you want better fuel economy, but the engine will run a bit hot due to lean mixture.

My experience with my 78K (836, pods, stock exhaust, dyna III w 3 ohm coils) is that with the high compression big bore it actually pulls more fuel through the pilots than the engine did stock.  About 1 turn out on the screws and she is happiest.  I have 77 style adjustable main jet needles and they are set one notch richer than stock (for the 77K).  Mains are 125.  Fuel mileage is an honest 42mpg when driven briskly and over 50 on a steady ride.  The size of pilots and the setting of the idle mixture screws has a large impact on MPG.  Perhaps you aren't optimized?  

[scottly]

Since you have the 130's, give them a try, and see if it affects your higher speed issues. It might also be helpful to see what the other 2 plugs look like. Check your advance, too.

[Frostyboy]

Float levels (floats are pretty even across the rack):

Interesting.
Now I realise that these carbs may be different to my 550, however that pic with the fuel levels throws me a bit. Aren't they hooked up to the overflow tubes? I've seen the drain plug method used in other posts where the tube is connected to a modified drain plug but this has really confused me. Are all your tubes split to allow fuel into there? Where do you place em when you run your bike? There's been a couple of posts recently regarding the placement of the o/flow tubes affecting the way they run, so that's why I ask.
Cheers.
Red ring = o/flow
Green ring= Drain plug

[seaweb11]

The 78 carbs are a completely different animal that the 69-76
I think some of the advice being given is not from 78K owners.....just saying

Try the 130s

[scottly]

Now I realise that these carbs may be different to my 550, however that pic with the fuel levels throws me a bit. Aren't they hooked up to the overflow tubes? I've seen the drain plug method used in other posts where the tube is connected to a modified drain plug but this has really confused me. Are all your tubes split to allow fuel into there?
Cheers.
Red ring = o/flow
Green ring= Drain plug

Yeah Frosty, I have been confused on this issue, too. What I know is that the 750's have drain screws that open into the drain/vent tubes, and I have gathered that 550's have drain plugs that are independent of the vent tubes. Your pic makes that obvious. Thanks!!!

Edit: ooops!! just realized that was the OP's pic!!! Still, I think the drain screws/plugs work different on the 550's.   

[Frostyboy]

Aha. Thanks for that Scottly. Like I said, l wasn't sure if they were the same, but your explanation clears that up for me.
Sorry for interrupting fellas, carry on. 

[KeithB]

I have a 78K with a stock motor, pods and a 4 into 1 MAC exhaust.
After a lot of experimentation, I ended up with 160 mains and a single shim washer on the needles,
3 turns out on the screws and it starts, idles and runs very well.
It used to need full choke for about 2-3 minutes and now full choke to start and within 30 seconds it's off choke.
Still looks a tad lean on the plugs but doesn't seem to be a problem.
They seem to be a very lean running bike in stock form so intake and exhaust mods just add to that.
As for fuel economy, it's a Cafe so who cares

[KB02]

As for fuel economy, it's a Cafe so who cares

  If I could manage 40+ on a 30 year old bike with a hot rodded engine, I'll be pretty happy.
________
Yeah, the drain empties into the bottom of the overflow tube. Makes for an easy way to look at the fuel level.
________
Wasn't able to work on the bike last night. Hopefully tonight or tomorrow. BUT, I'll try to answer some of the questions:

Running the Cycle-X optical ignition. Supposed to e as "Set it and Forget it" as they come. As far as I an figure out, it is set correctly.

I am running in 5th gear (most of the time) at this speed due to having the bike geared too high. I've been able it run it up to at least 8k o the tach for a short stint (acceleration), but then I get that running-out-of-gas feeling and can't maintain. I've got a smaller rear sprocket ready to go, but I'm waiting for a parts order before I do. One of the sprocket bolts will not survive going off and on again, plus the chain is a little long with the current sprocket. The smaller one will require the loss of a couple links off the chain.
________
Another symptom - which may or may not be relate - After riding with the RPM's at a higher (than idle) level for a little bit (not long), the RPMs do not want to come down. It just stays revved up until I put some load on the engine (slipping out the clutch a little bit when in neutral or dragging the rear brake with clutch engaged) and that will bring the RPMs back down. Like I said, I have no idea if it is related at all, but if it makes someone go, "Ah Ha!" then it's out there.

[eurban]

If you rev the engine as it warms up, all four cylinders will fire. If you walk away for a second, some of the cylinders will cut out (#3 noticably). As soon as you hit the throttle it comes back in.

As I mentioned before . . .Have you properly cleaned your idle circuit??  These symptoms are likely caused by idle circuit impairment (or other fundamental carb issues) and if so, you are completely wasting your time trying to fine tune a set of malfunctioning carbs. . . .160 mains??? 170mains???  I guess there are a few ways to skin a cat but going up 30 steps from a stock jet size is probably a bit excessive!  Must be some sort of compensation

[eurban]

Another symptom - which may or may not be relate - After riding with the RPM's at a higher (than idle) level for a little bit (not long), the RPMs do not want to come down. It just stays revved up until I put some load on the engine (slipping out the clutch a little bit when in neutral or dragging the rear brake with clutch engaged) and that will bring the RPMs back down. Like I said, I have no idea if it is related at all, but if it makes someone go, "Ah Ha!" then it's out there.

Air leaks? Carb synchronization problems?  Idle mix way off? Problem is not likely related to main jet size or main jet needle position and would fall in to the category of something that needs to be sorted out before you "tune".

Stock fuel line properly routed and with proper fuel cap venting will flow plenty of fuel for your bikes needs.  An inline filter can cause you trouble particularly with a level installation. Problems often only show themselves at lower gas tank fuel levels.  Make sure you have a full tank or temporarily ditch the inline. Just make sure that the in petcock filter is present.


In stock form the PD carbed 750s got great gas mileage and honestly my 836 gets good 40+ mileage.

[scrapvalue]

The 78 carbs are a completely different animal that the 69-76
I think some of the advice being given is not from 78K owners.....just saying

Try the 130s

You are right, mine are different.
I was just trying to show the extremes you may have to go through to get it right.
Should have stated that in my previous post.

Just a question, but can you put the old style carbs on the 78?
If so has anyone done it?
Good or bad?

[KB02]

You can put the old style carbs on the newer bikes, yes. You just need the older style insulators.
____________________

And yes, I forgot to answer this question, the idle circuit is nice and clean.  When I got the bike, the carbs were seized up just like the engine. Fortunately, I was able to fix the cars with just a thorough cleaning. Complete disassembly of the rack, several bottles of carb cleaner, needles, wires, wife checking in on me due to the fumes... Carbs are clean (how many times have we heard that on these boards?  ).
____________________

Got to work on the bike last night. I'm getting better and getting the carbs on and off (good thing, right?). I raised the float level just a touch and made sure it was even across all carbs. I turned the idle screws out 1/4 turn each, put the 130's back in and took it for a ride. At first I thought I was having some major problem because it did not want to run at all... then I turned the gas on...  After that, it was running much better (I mean overall).

Holding steady at 50-55 MPH is no problem now. 65 is about the max that it wants to do before the lean feeling comes back. I actually have some 135's that I am going to try today (have the day off) and see how it feels from there. Then I plan to plug in my sync'er again make sure that they are still in sync.

I think the problem with the racing idle may just be the bike not being properly warmed up before an idle was set. Unfortunately the tach seems to be a little bit dry (needs some lube) so I don't have an accurate reading of the idle speed.

[Duke McDukiedook]

That would be accurate, you'll want it to operating temps before you fiddle with the idle.

[KB02]

Okay, so the bike is running much better. Up to red line on the tach without an issue. BUT, now if seems to run lean when holding stead throttle at the 65-70 MPH mark. A little twist of the right hand and the leanness goes away (as does the bike ). So it appears that I am a little lean in the mix area.
I'm up to 135 mains. The mix screw is 1-1/4 turn out.
Which should I do, adjust the mix screw out more to help the blend in the mid range, or shim the needle? Since this has the stock 78 PD42 carbs, the needles are not adjustable. What should I use for shims if I go this route?

[KB02]

Shimming the Needles was all it took. Just needed to clean up that mid/transition-range and I am good to go. Nice stead throttle and engine speed going down the highway at 70 and 80. Just need to get the smaller rear sprocket on and doing the TON will be no problem at all. 

[cb750-bobber]

KB, Great post/topic, I in with the similar ordeal, any pics of  the shimming process, or desricptive play by play of how to tear the PD41/PD42  carb style down to get this work done? Thanks in advance.

[KB02]

KB, Great post/topic, I in with the similar ordeal, any pics of  the shimming process, or desricptive play by play of how to tear the PD41/PD42  carb style down to get this work done? Thanks in advance.

All the info you requested, sir. 

[KB02]

I just realized that I never added the link - I've had baby-brain for a while.

http://forums.sohc4.net/index.php?topic=71245.0

[cb750-bobber]

Fantastic, thanks for the help bro, i'll be tearing into them this week, any secret stashes of main jets for the PD Style carbs?..if they are interchangable, where can I best access mains  for trial runs?

[KB02]

I just went right down to my local Honda shop. They pretty much had everything that I needed for jets.