tough to start, backfiring Cyl. 3

[SlayerFan]

'82 CB750C, K&N individual Filters, Cut off Stock Exhaust where headers meet Mufflers, Dynojet Stage 3 Jet Kit installed exactly as directed, Cleaned Carbs.

All that noted, this thing is still really hard to start, but once it is warmed up, it idles fine at around 1000RPM (+/-).  Cylinder 3 is Backfiring... worked with it, adjusting fuel mixture screw.  This didn't seem to help much. Think I am going to shim that Needle and see if it helps.

When I rev and release, the engine hangs up above idle, but I cannot find intake side air leak... used carb/choke cleaner, sprayed toward the intake boots to see if the bike would rev up from idle, to try to find if (which) one was leaking.  Used this method when in the industry if we couldn't find the leak.  Found no leak.

[SlayerFan]

Shimmed Cyl. 3 Needle.  Still backfires.  Going to un-shim.  When I rev the engine now, it hangs up around 2500RPM .... Pissing me off, lol... Can definitely tell this thing's female...pain in the ass!  As bad as my ex-!

[Lucky 1]

Are the idle jets clean??

Call Dyno Jet.

Some of those slide needles from Dyno Jet for  some bikes do not work.

[Lucky 1]

Idle hangs up when mixture is too lean.
If you did not increase the size of the idle jets it will never run right.
You cannot just change one carb. What ever you do all 4 carbs must be set up the same.

[Lucky 1]

I would need more facts.

What kind of intake system do you have??
That is very important.

By the way I just do not like those so called "Stage kits"
Sometimes they work and many times they do not work.

One other thing...
The vacuum slides cannot have any splits in the rubber diaphram or it will never idle right.

[SlayerFan]

Ok... Thanks for the info, Lucky 1...

I have completed a thorough carb cleaning on it, even going back and cleaning the Pilots a second time when I couldn't get it to start easily and idle.

As for the intake system:

K&N Individual Filters, new.

Stock Keihin Carburetors, modified per Dynojet Stage 3 instructions. Purchased this Kit according to Dynojet website specifically for the 79-83 Honda CB750C.

Exhaust, Stock, modified by cutting off the mufflers (stupid one piece system) to make 4-into-4 drag pipes.

I have already gone back and removed the shim from the #3, to be back where I started.

I guess I'll go back and pull the pilot jet and see about getting some that are a couple of sizes bigger and start changing them out to see where the bike starts and runs well.

Thanks again for your consideration...Have a great afternoon

BTW... That digger 750 you're working on looks bitchin' ... I am wanting to turn mine into a Cafe. (How original, huh? ;) I've just wanted a vintage-ish cafe since I was working on Triumphs and Nortons back when......

On , "Lucky 1 [via Honda CB750'S]" <[hidden email]> wrote:

>
>
> I would need more facts.
>
>
> What kind of intake system do you have??
>
> That is very important.
>
>
> By the way I just do not like those so called "Stage kits"
>
> Sometimes they work and many times they do not work.
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>
>
>
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>
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> Working on a CB750 Digger named Rover
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> If you reply to this email, your message will be added to the discussion below:
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> To unsubscribe from tough to start, backfiring Cyl. 3, click here.
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> NAML
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[shinyribs]

Hey SlayerFan,welcome. Has your bike been sitting,nonrunning for any length of time. I just had a similar issue with a sohc bike and never could find 'the leak'. When I took the air filters off and saw an actual flame pop back thru the carb( back fire at its truest!) I realized a valve wasnt sealing. Maybe several,this engine hadn't ran in 12 years. So,I took it out on the highway and ran it at about 65-70 mph ( 5k rpm on my bike) for about 4 or 5 miles. Idles like a sewing machine now. I think I just had some sticking valves/rusty seats.

Oh,and if you want a cafe,build one. All custom bikes are original in their own right. There has not been a 'new style' of bikes in...well...since the 60's or 70's? Seriously,everything cool has already been built by our Fathers and Grandfathers.We're just following their lead. Cafes are cooland good dohc cafe's are few and far between.

[Lucky 1]

Also make sure all mixture screws are set the exact same amount and the idle screws are all set the same.

With the Dynojet kit they gave you a different needle.
What else was given to you?

[SlayerFan]

Thanks for the 'Welcome' I appreciate it! After work tomorrow, I'll pull the K&Ns off and see if I have that sort of back firing. It has actually been sitting for probably a couple years or so, since my buddy got a new bike.
As for the screws, Lucky1, I got 'em dead on... I'll get if figured out! I'll have my buddy come over and bring his balancer again, and I can go through that again, just to be sure.

The Jet Kit came with new needles, shims, multiple sized main jets and a larger set of primaries in case shimming the needles doesn't richen it up enough (if that's needed). Unfortunately, it didn't come with new Pilots, otherwise I would have thrown them in by now! It also came with drill bits for drilling the slide air holes.
Basically, I followed the instructions to the "T" for aftermarket filters and exhaust ... I doubt they would have been expecting 'Shorty' Drag Pipes like I've got, but then I am only getting fire out of #3... the others seem to be fine.

Thanks again, Shinyribs, for the welcome, and Oh yes, I am building it! I was just hoping to get some riding in before the snow flies!

And Lucky, thanks as always for your thoughts on things...




On , "shinyribs [via Honda CB750'S]" <[hidden email]> wrote:

>
>
> Hey SlayerFan,welcome. Has your bike been sitting,nonrunning for any length of time. I just had a similar issue with a sohc bike and never could find 'the leak'. When I took the air filters off and saw an actual flame pop back thru the carb( back fire at its truest!) I realized a valve wasnt sealing. Maybe several,this engine hadn't ran in 12 years. So,I took it out on the highway and ran it at about 65-70 mph ( 5k rpm on my bike) for about 4 or 5 miles. Idles like a sewing machine now. I think I just had some sticking valves/rusty seats.
>
>
> Oh,and if you want a cafe,build one. All custom bikes are original in their own right. There has not been a 'new style' of bikes in...well...since the 60's or 70's? Seriously,everything cool has already been built by our Fathers and Grandfathers.We're just following their lead. Cafes are cooland good dohc cafe's are few and far between.
>
>
>
>
> Montvale,Virginia
>
>  Piute said it best:  "wheres my 10mm?"
>
> Hellbilly-"that is a great way to tell if you have come to your senses, when you start drilling random holes in things... "
>
>
> LukeM-"Scooters rock, especially when they're in posse formation. "
>
>
> Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
>
> History teaches us that men and nations behave wisely once they have exhausted all other alternatives. ~ Abba Eban
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>
>
>
>
>
> If you reply to this email, your message will be added to the discussion below:
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> To unsubscribe from tough to start, backfiring Cyl. 3, click here.
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[shinyribs]

I understand the need to ride before the cold hits! it def sounds like you know what you're doing,and your way around carbs. I keep reading your syptoms hoping something will jump out at me...hasnt yet. But since it is isolated to just the #3 carb ( or cylinder) chances are it MAY not be carb tune related. Hard to say for sure,but if the other three seem to like what you've done with the tune I might consider looking elsewhere for the vector. It's most always something very simple. A vacuum leak or a leaking plug wire can cause some backfire type symptoms sometimes too. Doesnt make sense why,but it has for me in the past.Good luck with it bud.

[SlayerFan]

Yeah... you're right... It has to be something else... I am going to be buying a valve cover gasket this paycheck and I am going to be sure that all the valves are in tolerance...

I was a bike mechanic for about 7 years... Triumph(new and old), Norton, BSA, Moto Guzzi, BMW, Ducati ... Some japs ... but I have been out of the industry longer than I was in, now... so I have lost a lot of my troubleshooting skills, esp. since I didn't have a bike to work on in the last (nearly) 10 years. So I do pretty well know my way around them, I'm just rusty!

Anyhow... You and Lucky1 have been great! You guys have no need to be my personal info resource, I just found the CB750 Forum and have been kind of using it as my personal log of work and troubles... The fact that any of you have tried to help me out is just fantastic.

Anyhow, it seems now I have to figure out whether my Rotor/Stator Assembly is fritzing or if it is my reg-rec. After a few hours of idling and tuning, my new battery has given up on me,... should have been plenty of power to keep it charged if those things were working properly... I know that they work best off Idle, when they're putting out about 14.5V, but it's been running enough to not have decimated the charge. Time to bust out the multimeter.

Getting a bike for "free" is always fraught with problems! lol Get this... the guy had 4 different foot pegs on the thing! The left driver peg looks like it's a right driver from a completely different model... the rubber isn't even facing up! It's alright by me since I want to drop those pegs and find some semi-rear-sets for it anyway... I want to make it a 2 seat Cafe... I gotta bring my wife along sometimes- she's more than cool enough!

On , "shinyribs [via Honda CB750'S]" <[hidden email]> wrote:

>
>
> I understand the need to ride before the cold hits! it def sounds like you know what you're doing,and your way around carbs. I keep reading your syptoms hoping something will jump out at me...hasnt yet. But since it is isolated to just the #3 carb ( or cylinder) chances are it MAY not be carb tune related. Hard to say for sure,but if the other three seem to like what you've done with the tune I might consider looking elsewhere for the vector. It's most always something very simple. A vacuum leak or a leaking plug wire can cause some backfire type symptoms sometimes too. Doesnt make sense why,but it has for me in the past.Good luck with it bud.
>
>
>
>
> Montvale,Virginia
>
>  Piute said it best:  "wheres my 10mm?"
>
> Hellbilly-"that is a great way to tell if you have come to your senses, when you start drilling random holes in things... "
>
>
> LukeM-"Scooters rock, especially when they're in posse formation. "
>
>
> Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
>
> History teaches us that men and nations behave wisely once they have exhausted all other alternatives. ~ Abba Eban
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> If you reply to this email, your message will be added to the discussion below:
>
> http://honda-cb750-s.456789.n3.nabble.com/tough-to-start-backfiring-Cyl-3-tp4029227p4029263.html
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> To unsubscribe from tough to start, backfiring Cyl. 3, click here.
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