1982 CB750F...Better Devil

Jimbonaut

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Borrowed a set of coils from a friend known to be in good working order, didn't change a thing. 45 min ride, idle bogged and engine stalled. Same thing every time now. Not a vacuum leak - sprayed brake cleaner around each carb insulator with no increase in rpm. Charging system on the bike seems tip top - the new electronic speedo/tach gauge I've installed has a volt meter - shows 14.4v at 1500rpms.

If it's the carbs man, shit. I've pulled those things out god knows how many times and cleaned the bejeezus out of them. Hard to believe they're dirty. Float heights are all within spec (15.5mm). I'm missing something, but I'll be fucked if I know what it is.
 

Maritime

Over 10,000 Posts
Is it just taking that long to truly warm up? If you do your carb setting at not full operating temp they are not correct. How soon does the choke need to come off? If you ride it for 40 min then re-do your pilot and throttle adjustments does it then run fine? I'd try that and see what happens. You my then need to choke it longer after a startup to get it ridable but you won't have the shit run conditions at full temps.
 

pidjones

Over 1,000 Posts
Is it just taking that long to truly warm up? If you do your carb setting at not full operating temp they are not correct. How soon does the choke need to come off? If you ride it for 40 min then re-do your pilot and throttle adjustments does it then run fine? I'd try that and see what happens. You my then need to choke it longer after a startup to get it ridable but you won't have the shit run conditions at full temps.
I agree. My '79 CB750F was terribly cold shouldered. For the first dozen or so rides I would carry a driver to idle it down after 10 miles or so, but had to adjust again after about 40 miles. Finally learned to keep it on the idle-up ramp of the choke for much longer. Additionally, the valves are noted for needing one or two thousandths more than Honda spec to keep them happy.
 

Maritime

Over 10,000 Posts
I had a terrible time with Williams scooter. it was multiple learning curves but one big one was getting it to "Full" operating temp which took a 3-4 KM WOT ride to accomplish. Idling for even 20 mins never did it. Once I figured that out I would head away from the house 1.5 KM turn around and come back and as I hit the yard it would be full operating. Let it idle and did the pilot and idle stop adjustments then went out and did runs to set the needle clip etc. now it runs right, he just fires it up and off he goes and I don't get calls about it stalling out half way to school etc.
 

Maritime

Over 10,000 Posts
You'll find the GL the same. It takes 10 KM of riding to get to full operating temp and on a cold day 10 C it probably won't get all the way to full warm. On a 20-25 C day it will and over 30 C the fan will kick in a lot.
 

doc_rot

Oh the usual... I bowl, I drive around...
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I would start looking very carefully at the carb feeding the pot that is fouling, something is a miss there.
 

Jimbonaut

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Thanks as always gents for your suggestions and insight. I rely on help 100% when I find myself in these situations, and remain hugely grateful for all of it. My ambition far exceeds my skillset, but the learning curve is priceless.

Started off yesterday with a list, checked the thing twice and had at it

BsVpK4Rh.jpg


Sorted out the KLR and GL and moved on to the Better Devil. Checked the float levels with a trick I'd just been told about. Attach a clear hose (I used an old battery breather hose) to the overflow nipple of each carb in turn, open the drain screw and you can see the fuel level in the bowl matched by the fuel in the hose

Carb 1 -

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Carb 2 -

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Carb 3 -

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Carb 4 -

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Carb 2 may be a mm or two lower but otherwise they're all right on the button. Hooked up an auxiliary tank direct to the fuel rail on the carb (bypassing the vacuum fuel pump on the carbs), started the bike, removed the manifold from the carb to the air box on carb 1 and cracked the throttle to see if any gas was spraying out of the nozzle inside the carb throat - it wasn't

NainISOh.jpg


and gas is pissing out of carb 1 overflow tube

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So, have found at least one smoking gun. Or two. Namely, float problem in carb 1 - maybe the seat. Also, no gas spraying out of brass jet inside carb throat in at least carb 1, so I'll need to check where the block is and get it cleared.

Also, had me a brainwave. I'm going to need to re-do the fork seals (no idea how they're blown already, they were brand new) but anyway. Getting the front wheel off the ground on these bikes is a headache of no small proportions without a centre stand, but I'd removed it way back when. Had the thing sitting around so cleaned it up and re-attached it (minus all the springs) and oh yeah, once I get the bike on the table and weigh down the rear end, getting the front end off should be much easier. Centre stands. Don't want to live with them, but damn they have their moments

bCBBofjh.jpg
 
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irk miller

You've been mostly-dead all day.
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That nozzle is your accelerator pump nozzle. Check the diaphragm and your actuator, as well as for clogs. As far as fork seals, have you trailered the bike? Pretty common for people to blow the seals in the forks when they tighten the front straps too much. It helps (if you have the room) to block between the fender (or tire) and the bottom triple. Or strap it at the top of the fork lowers instead of the handlebars.
 

Jimbonaut

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Pretty sure the accelerator pump assembly is working well but maybe there's a blockage somewhere in the nozzles. I know #1 is borked now, not sure about the others but I do remember checking when I rebuilt the things and I'm pretty sure it was all fine then. I'll pull the thing - again - and see what gives.

Definitely the bike's been trailered, and also strapped down tight on the table. So that fucked the seals. Damn. Now I know. Does that mean they're shot, or just need re-seating? I already have in hand another set just in cas
 

chickenStripCharlie

Coast to Coast
Did you check the floats themselves for cracks and if they're letting gas in?

Bad luck with these carbs. Mine got cleaned and then sat dry for like a year. So far so good. Knock on wood.

Also, what mirrors are those?
 

Jimbonaut

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Did you check the floats themselves for cracks and if they're letting gas in?

Bad luck with these carbs. Mine got cleaned and then sat dry for like a year. So far so good. Knock on wood.

Also, what mirrors are those?
Only visually. I'll try submerging the questionable float in gas and see if any bubbles escape. I'll also try swapping out known good float assembly components in carb 2, 3 or 4 with the leaky set-up in carb 1 and see if I can narrow down the buggered part be it the float, needle or seat. It's a learning curve and no mistake. Thankfully the KLR is keeping me roadworthy while the CB gives me fits.

The mirrors are Highsider
 

Maritime

Over 10,000 Posts
Jim, Teazer told me to test float for leaks to use hot water, sink the float and if they leak air bubbles come out as the hot water heats the air inside. If they bubble pull them out as after all the air goes out they will fill with water otherwise. You can also shake them and hear the sloshing gas first. That's how I figured out my CB125 one was bad.
 

Jimbonaut

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Thanks Mike, I'll do that. I know when I eyeballed the floats before there were no signs of cracks and no gas inside. But maybe it's hairline, and a tiny amount is getting it without me seeing it. I'll try that technique and see what happens. Are the floats repairable? The new ones I have sit higher in the bowls than the originals and throw everything out of whack
 

Maritime

Over 10,000 Posts
You can solder them if they are brass, if plastic you could use a fuel tank repair epoxy. Teazer said if you do the solder it has to be quick to not over heat the float. If they get too hot and air escapes and you seal them after they will collapse and distort and be f'ed up beyond fixing.
 

Maritime

Over 10,000 Posts
I was going to try the fuel safe epoxy on the ones I have to use until the new arrived but the fact they got stepped on put a no on that plan.
 

doc_rot

Oh the usual... I bowl, I drive around...
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I don't know what the specs are for this bike but usually the float level is lightly lower than the top of the bowl. also you can polish the needle seat by putting a cue tip in a drill with some polish. seems to help when using old float needles
 

Jimbonaut

Over 1,000 Posts
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I don't know what the specs are for this bike but usually the float level is lightly lower than the top of the bowl. also you can polish the needle seat by putting a cue tip in a drill with some polish. seems to help when using old float needles
Polished the bejeezus out of the seats with Autosol. Things are shi-nee
 

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