Author Topic: Ignition or carb issue?? Full choke only (Read 3954 times)

nixon · « **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@!!!

robvangulik · « **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....

nixon · « **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

Bobarama · « **Reply #3 on:** September 22, 2018, 02:15:30 pm »

Classic air leak.

nixon · « **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?

nixon · « **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?

nixon · « **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?

nixon · « **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>>?

scottly · « **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?

nixon · « **Reply #9 on:** September 22, 2018, 10:14:43 pm »

Quote from: scottly 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?

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?

scottly · « **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..

nixon · « **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

nixon · « **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.

nixon · « **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.

ekpent · « **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 ?

nixon · « **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!

ekpent · « **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 ?

nixon · « **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

nixon · « **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

ekpent · « **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

).

TwoTired · « **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,

ekpent · « **Reply #21 on:** October 21, 2018, 03:01:29 pm »

Reply #5 shows he is using some older round top carbs Lloyd.

nixon · « **Reply #22 on:** October 21, 2018, 06:40:40 pm »

Will try and test tonight all headers getting got, and rate

nixon · « **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

ekpent · « **Reply #24 on:** October 22, 2018, 03:37:48 am »

Make sure you have not got any fouled spark plugs.

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Author Topic: Ignition or carb issue?? Full choke only (Read 3954 times)

Bobarama