HONDA CB750 1981 - PROGRESS, BUT NOT PERFECT... HELP APPRECIATED!

Hey guys - thanks for stopping by...

Here is my very first bike build - my HONDA CB750 DOHC 1981.

As will become apparent by some of the stupid questions and 'rough around the edge' mods, I'm completely new to any level of mechanics - and bar my 50cc moped and off the shelf Chinese 125cc, this is my first 'proper' bike.

I've posted on here a couple of times, so thanks to those that helped out back then.
I still need some help (likely always will!), so plan on using this thread as a build series and a helpline to keep things interesting and on track.

Feel free to fire any suggestions or thoughts on the build - would love to hear what you think!

Thanks.
BP

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To kick things off right, here's a picture of the bike when it arrived last summer...



And, here's where I'm at now...






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I'll break this down from day 1 soon enough, but for now, I've got more problems - this time with it running (now that I've finally managed to get it to start!).

I can't get it to idle properly with choke off at all - and with that, when throttling to slowly raise the revs, it just doesn't sound all that healthy. I'll post up a vid and anyone that can help, I'd really appreciate it...

Here's the links to the x2 Videos - https://www.dropbox.com/sh/brdho1gn5b709n2/AAAhdDP-9qOZ-p1t9sPcdslna?dl=0
5281) The bike running on choke, showing when choke is released, it begins to die (pod filters were removed at this point).
5282) This is with air pods on, the Fuel / Air screws released a little further than originally and choke on - then released -- the bike seems to be making a real spluttering noise here?

Hope you guys can give some guidance, even in Scotland it's nearly bike riding weather!!!

BP...

Have you cleaned the carbs? Thoroughly? And again? There are carb rebuild instructions on several of the DOHC forums. They are ace. BTW, they will not help you with getting pods to run right on those forums as the CV carbs are real finicky with anything but the stock airbox. They can be eventually tuned for pods, but it really isn't something that you should try without a lot of patience. What most report is that they run out of revs at about 5000. My stock, ratty '79 750F pulls all the way to 9000 (as far as I've taken it so far). BTW, Honda fours usually need a LOT of warmup to get off of the choke. Oh, and if you can get non-alcohol fuel there, use only that.

Great looking ride you get there mate, nicely done. You're asking the right questions in the right place - I can't be much help I'm sorry to say, did you ever run the bike without the pods (with the stock airbox) for a baseline?

What did you do with the original exhaust by the way? Asking for a friend :

pull one of your pods and have a look at the lip on the inside if the lip protrudes into the air flow area they will block the slide sensor port

post a picture of the inside if you want to know

yours look like the ones that cause the issue

pidjones said:

Have you cleaned the carbs? Thoroughly? And again? There are carb rebuild instructions on several of the DOHC forums. They are ace. BTW, they will not help you with getting pods to run right on those forums as the CV carbs are real finicky with anything but the stock airbox. They can be eventually tuned for pods, but it really isn't something that you should try without a lot of patience. What most report is that they run out of revs at about 5000. My stock, ratty '79 750F pulls all the way to 9000 (as far as I've taken it so far). BTW, Honda fours usually need a LOT of warmup to get off of the choke. Oh, and if you can get non-alcohol fuel there, use only that.

Click to expand...

Hey Pid - Cleaned them once over pretty good and it seemed to do the job.
They're pretty damn clean in there - even down to float seats etc.

I'm aware of the Pod issues that are bound to be inevitable - but I'm determined to get this right at some point....

Thanks for your guidance!
BP

The Jimbonaut said:

Great looking ride you get there mate, nicely done. You're asking the right questions in the right place - I can't be much help I'm sorry to say, did you ever run the bike without the pods (with the stock airbox) for a baseline?

What did you do with the original exhaust by the way? Asking for a friend :

Click to expand...

Thanks Jimbo - Good to hear from you again - big thanks for your help previously!

Question for you - did you ever run a Carb Sync?
I've not run that yet, but figure it may be some of my woes in this carb situation.

RE The Exhaust - Sold it unfortunately mate - threw it on eBay and someone got themselves a pretty good deal - I was just happy for some garage space!

How's your ride running - had the chance to get out this year yet!?

Thanks,
BP

cxman said:

pull one of your pods and have a look at the lip on the inside if the lip protrudes into the air flow area they will block the slide sensor port

post a picture of the inside if you want to know

yours look like the ones that cause the issue

Click to expand...

Hey Cxman - first up, thanks for trying to help out!

I'm not near the bike right now, but managed to grab an image from the site where I purchased them -


I hadn't spotted anything on the forums etc about that being an issue - other than Pods were reaaaaaal hard to get to work right.
Does that image show anything being an issue?

If so - is there a fix, or do I need to go buy something else? (I'm already more than a little over budget ;D)

Thanks man!
BP

Yeah man, no problem!

Pretty sure a carb synch is an absolute must. Think of it as tuning a guitar - if your strings are all out of whack then that righteous Stairway To Heaven intro is gonna sound like shit. There are two carbs synchs to do, one on your bench (before you install the carbs on your bike) and the other's a vacuum synch (with the carbs hooked up to the engine and the engine running). If you google "bench synch carbs" you'll find plenty of vids on youtube, but pretty sure this is the one I watched -

https://www.youtube.com/watch?v=oVlLitzF38I

That'll give you a good baseline synch. Once you've done that, and the carbs are installed on the bike, then you do the vacuum synch. You'll need a special tool for this - it's four tubes which hook up to the four screws, one each above each intake boot on your engine. It looks like this -

https://fortnine.ca/en/emgo-carburator-synchronizer-tool-4-gauge-84-68594?gclid=CjwKCAiAkrTjBRAoEiwAXpf9CawZ-Wa5_i_uHOrkP0g7j-VSphj0XRLVXbvoirm0hB0Zq1T7sFL5-BoC7LAQAvD_BwE

Get that done mate and at least you'll know that carbs are synched well.

Mine's running well, at least it was last year - haven't got the old girl out yet this year (not allowed to until mid March, earliest...thanks Montreal and your snowy winters). Working on another 750 this winter so need to sell the other one - having a few bikes in the garage makes perfect sense to me, the missus less so!

those pods will cause you to have slide sensing and lift issues

you need a filter with nothing hanging down into the throat of the carb anywhere

also just to get the baseline

do a charging system test

check voltage at each coil or just so the coil relay mod

take each exhaust temp and see what is firing and what isnt a quick easy thing to do with a non contact infared thermometer 20 bucks or so

cylinders fire 1and 4 2 and 3 so if yur pipes are cold in one of these pairs it may be pulse gen or igniter or coil related

a lot of dohc carb woes are electrical by nature

cxman said:

those pods will cause you to have slide sensing and lift issues

you need a filter with nothing hanging down into the throat of the carb anywhere

Click to expand...

Hey Cxman,

Just found a useful post to back this up online too - is there a fix to my pods to save purchasing new ones?

Alternatively - if I need to purchase new, do you recommend pods or velocity stacks?

pods for the street

there is no good fix for those cheap pods

Hi guys...

PREVIOUS MESSAGES :

Thanks Cxman for your guidance before - finally got the chance to check over the Pods in person.
Confirmed I have 'Cheap Pods', but they seem to be a bit of an upgrade on a lot of what I've come across in reviews / youtube breakdowns as the really basic options. As such, they DON'T block any of the passages or gaps for the slides etc and seem to be suitable as per recommendations - so I'm going to run with them for now.

MOVING ON - LATEST UPDATE :

The focus is getting this bike running for one - then running right.
I wondered if you guys could take a look at a few vids to see if there's anything you can pick out to help point me in the right direction, which I'll try explain below:

Vids Link is dropbox : https://www.dropbox.com/sh/udhujtob1s4ztdx/AAB0zHPJ1GPxiTb6SDXRWZSqa?dl=0
They're listed in order and with a description!

The Situ...

I was having issues getting the bike to really run at all before, so hit the internet and decided to up my Secondary Main Jet to 80 (from 72 - now running 80/118) and shimmed my Jet Needles with x2 Keihin Shim Washers. FYI - F/A Mixture Screws are 3 turns out.

Gave the bike a turn over today and SHIT, the thing fired up almost right away with Choke and sat idling pretty nicely.
From there, I gave it a few revs to check that side and first time round, which also sounded on the money.
This seemed to be all good for a minute or so, but right at the end of the vid you'll hear that when I stop throttling for revs the bike just drops off on idle and it cut itself out. [Video 1 & Video 2]

So I figured no biggie - maybe just needed to warm up right and really, the bike has barely been running for more than a few minutes in nearly a year...

Tried starting it up again (Choke on) and giving it a rev...
This is where I'm stuck -
It seemed to hold on to the revs pretty highly for a prolonged period after revving (definitely not the throttle sticking), and then dropped back down. When it drops down it sounds fairly patchy and then goes weak - and this happens over and over for a few rounds of revving up and letting it drop back to idle. [Video 3 & Video 4]

It eventually cut itself out again when it dropped down, so gave it another few tries and some seemed almost perfect and some with this really weird revving (and sometimes abnormally high as if it was almost WOT). This wasn't always just from throttle, sometimes on start it would just whip high and for fear it wouldn't stop itself I'd cut Choke or just turn it off and try again. [Video 5]

FYI - I also gave it a try altering the Choke level a few times whilst running to see if that helped after a few minutes - but it wasn't really distinguishable any particular way to make a positive difference.

For next steps :
I have a Carb Sync tester tool which I need to run and Im aware playing around with Fuel mix or adding / reducing on those Shims could alter my current results - but as before, would be great if anyone can offer up any immediate guidance based on the current sounds...

Hope you guys can help - it's nearly bike riding weather, even in Scotland!

Thanks a mill.
BP

Slow return to idling speed is a classic symptom of a carb sync issue. I'd wait for your tool to arrive and see how that pans out.

Could be one of a few things. As Sonrier said above, sync your carbs. IF that doesn't fix it, then take some starter fluid or carb cleaner or anything flammable and spray it around the intake boots, both around the carb side and the head side. Listen for any change in idle at all (usually it will rise, but sometimes bog) which will mean you have a bad boot that's leaking vacuum.

Almost guaranteed one of those will be your issue, classic causes of high/abnormal idle.

A hanging idle is usually indicative of lean.


Sent from my iPhone using DO THE TON

Is that rear set up not causing you clearance problems?
I cant tell if you lowered the rear suspension or that rear tire is really tall?