Author Topic: 836 modified CB750K5 - ignition/spark problems? Turned into carb questions. (Read 2568 times)

petersan · « **on:** March 30, 2006, 09:46:10 AM »

Have lurked around for some time now, never posted - but I've got some problems and am tired of beating my head against the wall.

I've looked around for similar problems, but none seem to match this - here goes, bare with me.

Background:
A year ago I was looking for a larger CB (bigger than my '76 550) and one fell in to my lap. Someone stopped me and said they had a 1975 CB750 bored to 836 - but they never took the time to get it going - so I took it. Not seized - compression o.k. (all around 160), electrical seemingly in good order - the guy also claimed the carbs were rebuilt with a stage 3 kit with pod filters (no longer had airbox).

After going through the bike - standard tune up stuff: cleaned carbs myself and jetting seems to be stock, new plugs and plug caps (trimmed wires by ~1/2" when putting caps on), dyna s ignition, new battery, cables, rebuilt brakes, etc.

Finally got bike started. Set timing (dynamically) - and balanced carbs.

The bike starts easy with one kick now - but after running for a few minutes - #2 cyl stops firing and then the bike runs terrible. Doesn't seem to get excessively fouled - seems like I got a batch of bad plugs and they get killed when installed.

1) is it possible that #2 runs TOO HOT and kills the plug? (but plug isn't white)

2) problem with coil or the wire from the coil?

3) a carb problem I'm too stupid to sort out? (I know there are carb problems, idle a bit high and bogs down above 1/4 throttle but seems to run better with choke slightly on - just don't know if it's causing dead sparkplug problem)

Happy to provide any other info you guys may need
I've learned tons from this site - hope you guys have some suggestions for a weekend wrencher.

thanks - - paul.

petersan · « **Reply #1 on:** March 30, 2006, 08:58:51 PM »

I'm surprised to see so many views and no replies - please help me breath life back into this beast.

STLrocker · « **Reply #2 on:** March 30, 2006, 09:26:38 PM »

an easy place to start is to switch the plug wires on the #2 and #3 cylinders. the coil fires both wires at the same time, so you can switch them without afffecting anything. if the problem moves to #3 then you have a bad plug wire.

have you swapped the #2 plug into a different cylinder to see if the problem moves with the plug?

if that dosent solve the problem, you may try shooting a little starting fluid or gas from a spray bottle into the #2 carb with the engine running. if it starts firing again for a few seconds, and then stops when you stop spraying fuel, you have a carb that needs attention. either cleaning or possibly float adjustment.

cd811 · « **Reply #3 on:** March 30, 2006, 09:37:52 PM »

really sounds like a carb problem...are you sure the pilots are clear?...what size are your jets?...float level right?

crazypj · « **Reply #4 on:** March 30, 2006, 09:50:00 PM »

If it has an 836 kit the standard jetting will be off. you want about 8% for big bore and another 10% for pods so roughly 20% larger main jets to start with.( even bigger if it has a loud exhaust) The coil could be failing as it warms up, one end fires ground to centre electrode the other centre to ground (to complete the circuit on a twin lead coil) it takes higher voltage to fire from ground. Its unlikely to be the points as 2&3 would go out. As suggested, switch 2&3 plug leads see if it changes cylinders. If not, get back into the carbs. Are you starting on choke and it cuts out when choke is off or does it start without choke and cut out a couple of minutes later? If the latter take a look at fuel flow to carbs, may be a blockage so when you empty a float bowl it starts to miss/cut out, particularly if float level is off on one carb.
PJ

MRieck · « **Reply #5 on:** March 31, 2006, 06:06:35 AM »

I've seen plug caps fail when warm. Check for manifold (carb boot) vacuum leaks too.

vames · « **Reply #6 on:** March 31, 2006, 06:23:37 AM »

I had a similar problem once on my 76 750F. Bike woud start, idle and run beautifully for about two minutes, then
#2 would bog then cut out completely. I eventually found that even with the float down in the #2 carb, gas was flowing in at only a trickle. Enough to fill the bowl over time and run for two minutes, but not enough to keep up with the needs of riding. I found that above the float needle seat, there is a little strainer that was full of rust and silt - it's not that obvious and the junk is trapped when you're cleaning out the carbs, so sometimes it's hard to see. Cleaned that and all the other ones out, and the bike ran great.

petersan · « **Reply #7 on:** March 31, 2006, 06:43:48 AM »

Wow! thanks for all the suggestions.

To clarify - I'll start bike with choke - starts great with new plug in #2. Then I cut the choke, bike still runs well. I'll take it out and enjoy the ride for ~10 min (aside from the flat spots in the throttle probably because of restricted jet size). Sometimes on the ride - when I was trying to diagnose carb problems - I'd close choke slightly to see if I was too rich or lean - and when all 4 cyls are firing - bike runs better with choke slightly closed (indicating as you guys mention - jetting is probably small for the set up). But with no choke, it starts to run poorly - like its "8-stroking" - I'll get back to the house and #2 header is slightly warm to the touch. Oh - the bike has a 4-1.

I got the carbs off last night after work in anticipation of possible recommendations - I'll definately look at the slow jets and the strainer above float needle. I set the float heights last time I had the carbs off the bike ~ 3 weeks ago, but I'll check them again. And I'll look around for kinks in the fuel line on reassembly.

I should mention I put 122.5 main jets in (had 115's in them) - I couldn't find what the stock size for the main jets were (110? so 20% above would be ~ 130's or so?) - I haven't been able to get the bike running good for long enough to do a plug chop at open throttle. BTW - I have the air idle screw turn nearly completely in - is there a recommendation for upping the size of the slow jet (think they're currently 40's).

About coil/wire problems - I got a bonus from Uncle Sam this year so I put in an order for Dyna coils and wires (from MPS racing - http://www.mpsracing.com/ --- I found them for 112!) - hopefully they'll come soon and I can eliminate the possibility they're the culprit.

Got new plug caps on the bike in Jan - so I would doubt they're behind the problem, but is there a way to check if they're failing when warm?

And when I do the above and get the carbs back on the bike - I'll check for vacuum leaks (first I'll search the site and figure out how to do it)

thanks again everyone - keep the suggestions coming - and I'll keep you posted.

petersan · « **Reply #8 on:** March 31, 2006, 07:59:06 PM »

A bit of an update - at least on the condition of the carbs.

All jets, main and pilot, are clear - as are the perforations on the side.

Float heights were all close (within 1.5mm) of the spec. height - btw, what is the tolerence for the float height?

pulled the float needle and holder - don't seem to have the mentioned strainer in my carbs.

ALSO - got the Dyna coils today, a surprise. So, I'll try to get everything on the bike tomorrow or sun (weather permitting) and hope that things are solved.

one last question - I see recommendations often for changing the size of the main jet depending on mods to the bike. Is it required or customary to change out the pilot jet for a larger size? would someone have a ballpark est. in my case (836, pod filters, 4-1). I'll run to a shop tomorrow and try to procure larger mains, but I'd like to get larger pilots if you guys think they're needed.

Thanks again

cd811 · « **Reply #9 on:** March 31, 2006, 09:21:32 PM »

i'm running 40 and 122.5 on my 811...breadbox,4-2 straight
after new head gasket, I checked plugs(way too rich)(april 2)...went down to 115 main...more better

mutters · « **Reply #10 on:** March 31, 2006, 09:49:25 PM »

I think you might have to look at the air screw setting again.My 836 runs stock there with no probs.

petersan · « **Reply #11 on:** March 31, 2006, 10:29:14 PM »

I have my air screws turned out 1/2 turn. 40 pilots and 122.5 mains. plugs look o.k. a bit hot, but have not done a proper 'plug chop'

mutters - what mod do you have? and jetting?

still wondering if I should go up on the size of the pilots - so I can run something closer to 1 turn out on the air screws. (as I understand it - turn in to richen, turn out to make leaner).

petersan · « **Reply #12 on:** April 02, 2006, 08:56:02 PM »

A bit of an update

Got the dyna coils and wires on the bike, the carbs back on, made sure fuel line was not kinking.

Bike started up and it seems to be running great.

Brought the old coils inside and checking the resistance of secondary and primary coils, and plug caps, and noticed that the end of #2 plug wire was quite frayed inside the insulation - and with little tension I could pull them out. So, I'm hoping that was the problem - now solved.

However, still have questions about slow jet size for engines with many modifications. - Should the slow jet size be increased? Maybe it's better left for a different thread.

cd811 · « **Reply #13 on:** April 02, 2006, 09:17:35 PM »

not sure what pilot jets ya got, I would suggest 40

750deepsouth · « **Reply #14 on:** April 03, 2006, 02:58:14 AM »

Same as another reply i've put up tonight, and in agreement with others -

Quote

All other things being equal - turn out the air screws on each carb, start with an extra 1/2 turn, even an extra full turn.
I went Through the same issues with my K2 and resolved it almost totally with leaning the idle mix by a full turn.
The problem I think is related to modern fuel, octane's etc, the recommended idle screw setting leaves the motor running wayyy too rich on the idle jet.
Even so I sometimes get #4 to foul up a little, but overall it starts on all 4 pots 99% of the time. I've found that when starting from cold, full choke but only a little throttle. Starts almost straight away.
It runs much more smoothly at lower rev ranges ie 2500-4500rpm. The bike used to run a little unevenly at around town speeds - now it is very smooth.

Check the air screw settings, I forget what the factory setting is, 1.5 turns or something - try them another full turn out and see how it runs.
"nearly in" will be way to rich.

Headers. These should burn your hand after a couple of minutes. If you can touch them then that pot isn't firing much.

mutters · « **Reply #15 on:** April 03, 2006, 06:57:19 AM »

Pete, mate, I'm a k1 836 wiseco kit, 4into 1,stage 2 cam,( bigger lumps than a stage 1 ?) ramflo air filters ,and a mild cleanup job on the head.
130 main jet and no change to the pilots. very low miles but it runs nice and behaves well

keep at it mate and let us know how it goes.

cd811 · « **Reply #16 on:** April 07, 2006, 02:50:05 PM »

mutters...what size are your pilots?

mrbreeze · « **Reply #17 on:** April 08, 2006, 12:16:50 AM »

I never had probs with my stock 40 pilots but played hell going back & forth with the mains.You don't mention what your altitude is or where your needles are set.Those two items fit into the picture as well.You also haven't mentioned if you synched your carbys.all of this #$%* is so important!!! Give us as much info as you can and you can be sure someone here will be able to shed some light.

crazypj · « **Reply #18 on:** April 08, 2006, 09:42:50 AM »

1/2 turn on air screw with 40 pilot jet suggests the jet is too small ( Honda spec is 1.5 turns out)What carbs and exhaust do you have ?
PJ

petersan · « **Reply #19 on:** April 08, 2006, 12:31:24 PM »

I have some more mains coming in the mail - but the new coils and plug wires have made a big difference.

I have balanced the carbs, I'm waiting to get larger mains in before I do it again (also have 42 pilot jets on order).

I think the carbs are the stock ones that came with the bike (Kehins) and the exhaust is some aftermarket 4-1, no baffle, needle e-clip set in the middle (just raised them one notch), and I'm close to sea level (Washington DC).

I'm sort of in a holding pattern till the other carb parts come - then I'll put them in, balance, and try to get a plug chop done.

Thanks for all the info and suggestions.

STLrocker · « **Reply #20 on:** April 08, 2006, 09:22:12 PM »

if you just change the main jets, you should not need to re-synch the carbs. this is not affected by changing the jets. if you're sure you got it synched right already, you can leave it alone.

when you say you raised the needle clips one notch, do you mean that you moved the clips from the middle to the second from the top notch, or did you move it to the second from the bottom? to raise the needle, you need to move the clip down to the second from the bottom notch. just trying to cllarify, cause i couldnt tell from how it was worded, and it can be confusing.

as far as the pilot jets go, i have 2 bikes running stock 40 pilots. they run fine with no issues with teh mixture set at stock, or vey close to it. one bike has unbaffled 4-1, pods, and 125 mains. needle in second from bottom notch. the other has unbaffled drag pipes, pods, 120 mains, needle in second from bottom notch.

mutters · « **Reply #21 on:** April 09, 2006, 12:05:57 AM »

Sorry I'm a bit slow on weekends, I don't know the size of my pilot jets ,I just left them were because it worked.
if I get a chance I'll have a look after the weekend (work ,y'know)

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Author Topic: 836 modified CB750K5 - ignition/spark problems? Turned into carb questions. (Read 2568 times)

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