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Author Topic: Ignition or carb issue?? Full choke only  (Read 669 times)

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Offline nixon

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Ignition or carb issue?? Full choke only
« on: September 22, 2018, 03:48:27 am »
I was on a big ride ride on my cb750a. Have k4 carb and cam. Was running ok, no missing etc.  Running ram air over the rubber stacks.  Have dyna s. It seemed to go to 2,3 cyl and limped home.  Cleaned carbs,  raised clip 2nd from top.  Did valve clearances.  Checked ignition timing.  Replaced leads . New plugs.  Flow balanced carbs.

Now, it fires up on full choke...ONLY.I've cleaned the carbs twice. Tried air /flow 1 turn out , 3+ no difference.

Could it be faulty dyna s? 6000ml old. Coils 5ohm dyna , tested perfect. How to test a dyna s?

Or is it definitely carbs, what to do next?
Thanks@!!!

Offline robvangulik

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Re: Ignition or carb issue?? Full choke only
« Reply #1 on: September 22, 2018, 12:03:50 pm »
Does it RUN only on choke, or does it start only on full choke?
The first is not good, the second option isn't unusual.
And with raising the clips to the second notch you lower the needle and make it run leaner, that might be the cause if it only runs on choke....

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #2 on: September 22, 2018, 01:47:49 pm »
First one. Dies straight  away  after reviving high.  Used  to barely need choke, then could use throttle.  I forgot to add I had suspect loom to coil wires,  then I adjusted needles.  Guess I'm taking the carbs off.... again lol
« Last Edit: September 23, 2018, 05:27:15 am by nixon »

Offline Bobarama

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Re: Ignition or carb issue?? Full choke only
« Reply #3 on: September 22, 2018, 02:15:30 pm »
Classic air leak.

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #4 on: September 22, 2018, 02:32:39 pm »
Forgot to mention,  did put a new set of needles,seats in.  has also newish carb to motor rubbers, but good point I'll try finding an air leak there...anywhere else?
« Last Edit: September 22, 2018, 03:18:32 pm by nixon »

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #5 on: September 22, 2018, 06:38:31 pm »
Hmm.  So before I adjust the needles,  I noticed how open the slides are, with idle adjustment completely slack.  It was set by a pro mechanic who races old bikes.  Could that be a problem?

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #6 on: September 22, 2018, 06:57:48 pm »
If I shine a light from the intake side.. I can just see a chink of light on others side, if I wind the idle up a little. Otherwise they are all closed?

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #7 on: September 22, 2018, 08:09:53 pm »
well..i took out all the air screws..no washers at all? that cant be right?

do they sit under or above the spring? could that be having big impact>>?

Offline scottly

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Re: Ignition or carb issue?? Full choke only
« Reply #8 on: September 22, 2018, 10:05:52 pm »
Your initial symptoms might suggest a bad Dyna, but after you've #$%*ed around with the carbs the symptoms have changed?
Don't fix it if it ain't broke!
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Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #9 on: September 22, 2018, 10:14:43 pm »
Your initial symptoms might suggest a bad Dyna, but after you've #$%*ed around with the carbs the symptoms have changed?

yeah that fact it went really bad on a ride does point to dyna..but i had loose connections on the loom to coils?

 I think ill put needle clip back to mid.

plus I need to understand what the gap - if any - should be on engine side of slides? 

Offline scottly

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Re: Ignition or carb issue?? Full choke only
« Reply #10 on: September 22, 2018, 10:25:53 pm »
The gap relates to the idle speed; if the motor is only running on two cylinders, a greater gap will be required to maintain an idle speed. There is not enough information to diagnose your problem..   
Don't fix it if it ain't broke!
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Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #11 on: September 22, 2018, 11:19:38 pm »
your right - i need to confirm if the dyna igniton is right, otherwise wasting my time on carbs

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #12 on: September 23, 2018, 01:21:27 am »
Well ,sparks fine on all plug.
I think the original issue was either,
A) very weak coil connections. Wiring was frayed, not insulated.
B) air/fuel screws were all over the place, because no washers  in any.  Some could vibrate out I think.
C) needle height or main jet.

I need to get 4 x 6mm alu washers,  and try putting clip  down I think.

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #13 on: October 20, 2018, 09:52:21 am »
Well, still trying to ascertain why it only runs full choke.

It starts and revs higher and higher.  As soon as drop the choke, it dies,  throttle won't pick it up.

Brand new oem made in Japan carb rubbers. 

What should I try next?

How many turns out approx should the air screws be? More open mufflers and ram air filters over the stock velocity stacks.

Offline ekpent

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Re: Ignition or carb issue?? Full choke only
« Reply #14 on: October 20, 2018, 10:03:31 am »
 There are no washers in the air jet screw area on the sides of the carbs but there should be little springs.On a regular K bike 750 the baseline settings for the air screw is one turn out from a gentle seat. The fact the bike was running well before and suddenly went bad suggests to me more electrical or mechanical other then the carbs. The next time you fire it up check the exhaust head pipes and see if any are hot or cold.
  Is your gas tank and petcock good and clean and fuel lines routed well with no kinks or fuel filters ?
« Last Edit: October 20, 2018, 10:14:25 am by ekpent »

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #15 on: October 20, 2018, 12:15:41 pm »
Yeah I just don't know what to do next. Yes tank and line good.  All carb bowls get fuel.  Regarding washers, seems some have them,  I can get them out I guess.
Because it runs on full choke,  screaming,  I can confirm if all headers get hot next time.  But I've checked coils and new spark and new leads.  I'm very lost right now!

Offline ekpent

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Re: Ignition or carb issue?? Full choke only
« Reply #16 on: October 20, 2018, 03:37:19 pm »
 The flat washer #16 you see goes under a screw on the side of the carb that is basically a plug. When you remove that slot head screw and washer you hook a carb synch stem to the hole on all 4 carbs to balance them with a tool. The only other washer goes under the needle valve seat inside the carb.
  You mention the words run,choke on and SCREAMING.  Is the idle super high when the choke is on and it does run ? How high ?
« Last Edit: October 20, 2018, 03:42:07 pm by ekpent »

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #17 on: October 20, 2018, 05:49:17 pm »
Thanks! that makes sense re washer.  I saw a listing for one on sure with a washer with air screw.  Must have been confused! Yeah it seems to rev higher and higher with choke,  not just idle. Like 2500rpm.   With main idle adjusted ,low or high doesnt seem to impact at all

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #18 on: October 20, 2018, 08:13:51 pm »
more info
- while running on full choke, I shoved a rag over each intake stack and fuel squirted out of all of them into , still pulled and cleaned all small jets -- very clean

- check resistance of the replacement auto leads - from 3.6OHM to 2.8 OHM - non R plugs.
- the previous ones i had on =  dyna ones when i bought ignition - 5.7- 6.6ohm one i think 8.6?

really dont know what to try next - any ideas welcome

could it be the dyna s unit? tiny blue spark on all plugs resting on head
 

Offline ekpent

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Re: Ignition or carb issue?? Full choke only
« Reply #19 on: October 21, 2018, 05:29:16 am »
 Were all 4 exhaust head pipes getting hot,like hot enough to sizzle water when touched with a wet rag (my scientific test :D).

Offline TwoTired

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Re: Ignition or carb issue?? Full choke only
« Reply #20 on: October 21, 2018, 02:43:28 pm »
PD carbs, Right?

How did you prove that the pilot jets are clear and delivering?


Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
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Offline ekpent

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Re: Ignition or carb issue?? Full choke only
« Reply #21 on: October 21, 2018, 03:01:29 pm »
Reply #5 shows he is using some older round top carbs Lloyd.

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #22 on: October 21, 2018, 06:40:40 pm »
Will try and test tonight all headers getting got, and rate

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #23 on: October 22, 2018, 01:45:53 am »
Phew.  Getting somewhere.  Managed to get it idling with no choke.  Changed all the leads,  to old dyna ones, suppression,  was running on 3 only.  Blocked off  2nd  intake which wasn't running then all running.  Could barely crack the throttle though.  So I'll get new solid core leads and nhk 5ohm caps.  Should  I start trying more half turns out or in on air screws? Currently one turn out
« Last Edit: October 22, 2018, 01:49:10 am by nixon »

Offline ekpent

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Re: Ignition or carb issue?? Full choke only
« Reply #24 on: October 22, 2018, 03:37:48 am »
  Make sure you have not got any fouled spark plugs.

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #25 on: October 22, 2018, 06:27:18 pm »
  Make sure you have not got any fouled spark plugs.
Ok thanks... they were new nhk  when I tried to figure this out. So I've run 5 min on rich with choke.  Should I replace or clean?

Offline TwoTired

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Re: Ignition or carb issue?? Full choke only
« Reply #26 on: October 22, 2018, 08:30:42 pm »
If the center electrode insulator isn't coated with black conductive carbon, the plugs should fire properly.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
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Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #27 on: November 08, 2018, 01:20:38 am »
Ok back , kinda to square one.

I installed some new solid core copper leads with ngk 5phm caps..hoping slight or drastic change. Nothing. still only ran full choke. if reduced choke or could just get idling , barely.


Convinced its carbs, i wound in air screws..all the way. Fired up fine, all cylinders..and pretty soon i could wind on throttle getting to rev quite high, then return to idle. no choke required.

Ive rigged a funnel into fuel hose as painting tank, and 8pm so didnt want to annoy neighbours too much..

but please, any input on what i should do or try next.. ..needle... jets , main, slow...is that ok to ride with air screws closed? whats going on.. still a newbie with carbs!!

Offline TwoTired

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Re: Ignition or carb issue?? Full choke only
« Reply #28 on: November 08, 2018, 08:07:30 am »
What do the spark plug deposits tell you about combustion conditions?

Are all the head pipes equally hot?

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Thinking:  It's like reading with your own mind

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #29 on: November 08, 2018, 11:37:58 pm »
I'll clean them and see how it looks tomorrow.  No real issue having air screws completely closed? That says running lean on slow jet?

Offline TwoTired

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Re: Ignition or carb issue?? Full choke only
« Reply #30 on: November 09, 2018, 08:20:41 am »
I'll clean them and see how it looks tomorrow.
Cleaning off the deposits on the plugs removes the evidence about how the cylinders are combusting.  Information you want. Like is it running rich or lean or the same among the cylinders?

No real issue having air screws completely closed? That says running lean on slow jet?
The issue is, it is not in anyway normal.  That would concern me and lead me to investigate why.
Remove one of the pilot screws.  Is the tip hollow with a cross drill?  If so, you can't really turn the circuit off with that screw.  You can only set it to minimum by design.  Normally, by design, your off idle response under load will make the engine stumble with the screws all the way seated.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Thinking:  It's like reading with your own mind

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #31 on: November 09, 2018, 04:31:54 pm »
appreciate your input!

They were black sooty , dry before i cleaned. Cleaned up easily. I thought to  clean , as id only had it running with full choke?

so thought clean and see again how they looked now that i can run without using  choke .

I should have mentioned - its idling , revving but hanging a little - with no choke now..but only with the rubber velicity stacks. And air screws completely closed.

If I add the unifoam filters over the stacks , I cant get running. Choke or no choke. Unscrewing the screws 1/2, 1, 1.5,2 made no big difference.


Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #32 on: November 09, 2018, 04:46:16 pm »

[/quote]
The issue is, it is not in anyway normal.  That would concern me and lead me to investigate why.
Remove one of the pilot screws.  Is the tip hollow with a cross drill?  If so, you can't really turn the circuit off with that screw.  You can only set it to minimum by design.  Normally, by design, your off idle response under load will make the engine stumble with the screws all the way seated.

Cheers,
[/quote]
yes it does have hollow tip with cross drill.  And 100% yes stumble with the air screws closed. if im very gentle i can get it to rev more. But it was idling ok and that for me is a big deal! Pretty sad huh..

All headers equally hot , tested with water spray 

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #33 on: November 09, 2018, 05:27:36 pm »
Which of the following should I be checking, adjusting? what could cause it to run with air screws closed - no filters over stacks, but wont run with turnred out screws when foam filters over stacks?

- Main jet ?
-idle jet?
-needle height?
-float heights
- ( other reason, im open to checking rechecking anything - havent ridden this bike for 3+ months!  )

Has brand new oem honda intake rubbers to engine.

When i pulled plugs 1 and 4 after cleaning - 1 was dry sooty and 4 was much cleaner?
« Last Edit: November 09, 2018, 05:59:10 pm by nixon »

Offline TwoTired

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Re: Ignition or carb issue?? Full choke only
« Reply #34 on: November 09, 2018, 08:38:21 pm »
If the float bowl fuel level is too high, it will bleed extra fuel into the intake runners.  At any rate, all the carb metering components rely on a known good float bowl fuel level to be consistent in metering.
If the float components are still stock, probably can be confident with a float swing measurement.  If the components aren't originals, then the clear tube method for actual fuel level verification is recommended.

That's where I would start.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Thinking:  It's like reading with your own mind

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #35 on: November 10, 2018, 12:43:55 am »
Thanks! I'll pull the carbs and assess.

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #36 on: November 10, 2018, 05:01:34 pm »
If the float bowl fuel level is too high, it will bleed extra fuel into the intake runners.  At any rate, all the carb metering components rely on a known good float bowl fuel level to be consistent in metering.
If the float components are still stock, probably can be confident with a float swing measurement.  If the components aren't originals, then the clear tube method for actual fuel level verification is recommended.

That's where I would start.

Cheers,

Twotired, I owe you a beer or 5!!  I had one original brass float, 3 black plastic ones. Only one was on 26mm! one was out 3mm.  I found another plastic one,  and carefully set them all.

Now - with foam filters on..i could run with no choke..and it revved quite cleanly within a few minutes! I started adjusting air screws.. 1/2 turn seemed ok but ill adjust according..back out till stumbles then 1/4 turn in?


From here I really need to take it for a ride and see whats happening WOT etc once warm ... thanks for listening to my rambles ppl!




Offline TwoTired

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Re: Ignition or carb issue?? Full choke only
« Reply #37 on: November 10, 2018, 06:41:21 pm »
The early fours all normally ran a bit rich in the idle regime.  Part of this reason is because the carb had no accelerator pump with the mechanically actuated slides.
When the slide is raised from idle, the carb throat vacuum rapidly falls.  The throat vacuum is what draws fuel from the jets. Low vacuum = low flow.  So, the mixture goes lean and if the vacuum is allowed to fall too drastically, the mixture ratio goes too lean to even fire in the chamber.  This stumble or wheeze as I like to call it, is immediately corrected by backing off on the throttle, and the engine resumes operating cleanly as if it was just switched on.

With these carbs, you should never expect the engine to respond well to whacking the throttle wide open from idle position, with or without load.

My method for pilot air screw adjustment:
Mark your throttle with some temporary tape.  So you can tell at a glance what the 1/2 applied position is.
While the bike is moving, put it in top gear and have the engine operating at about 1000-1500 in cruise. Suddenly open the throttle to the 1/2 point. If it wheezes, you need to make the pilot screws richer.  If it takes throttle and slowly increases speed you have an over rich idle.  If you can give even more than 1/2 throttle and the engine picks up predictably, the idle is too rich, and prolonged idle will carbon up the plug insulators, which causes a whole other set of issues.  So, make sure your plugs are clean before starting this test.  Anyway, if you can give it more than 1/2 throttle with reliable pick up then the pilots screws need to leaned out until the test passes with 1/2 throttle travel and no more.

I'll admit, I never tried this method with velocity stacks, as I hate them on a street bike (fine for full race at the track).  But, this tuning makes for a predictable and long lasting state of tune for the pilot air screws and the spark plugs.

Of course the slide needle and main jet have their own procedure for fine tuning.

Cheers,


Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Thinking:  It's like reading with your own mind

Offline nixon

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Re: Ignition or carb issue?? Full choke only
« Reply #38 on: November 11, 2018, 12:03:59 am »
wow thanks for your input! really appreciate it.

I wasnt sure a stock air box will fit the auto version with round top carbs, so ( stupidly it now seems) I took easiest route with ram air filters over stacks. i should mention i have only the one arm so fiddly jobs i put off!

i have installed bungs in the 4 into 2 headers a o2 sensor so im gonna take it to a mate whose a great mechanic now i can hopefully ride it there

 

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