Topic: 1978 CB750F stalls with aggressive braking

[Vintovka]

Hello all I've currently painted/rebuilt a lot of major components on my 1978 CB750F and I've just recently got it back on the road.  The bike runs excellent from what I can tell the only problem is when I apply a lot of the brakes suddenly stopping the bike, the engine stutters and dies.  If I quickly release the brakes the engine will actually start right back up depending on how fast I release the brakes.  This doesn't happen with normal stopping only when I need to quickly stop the bike.  This sounds to me like the carbs/engine aren't getting fuel when the bike is stopped this way.  Any suggestions?  I couldn't find any answers searching these forums but maybe I missed it.

The PO installed Kerker exhaust system,  Pod filters, and 120 main jets on the carbs.  I have cleaned and dipped the carbs and blew cleaner through everything and Im even running SeaFoam product in the tank to further clean the system out.  Like I said the bike seems to be running great the plugs were fouled yesterday but I cleaned them up and adjusted the pilot jets to 3/4 turn out from seat from the previous 1 and 1/8th turn out from seat.  When I took the carbs off the PO had the pilots set to only 1/2 out from seat!  Sort of off topic but does that sound right to anyone? 

Ive also re synched the carbs myself 4 days ago so the vacuum levels are all correct.  Any help is greatly appreciated

[Gearheadgreg]

My 78f runs good at 1 1/2 out on the mixture screw has a better idle, stock air box stock 4 in 1 exhaust,, these pumper carbs are like my 45 HSR Mikuni,,If it is (Rich)  turning the mixture screw (OUT) will lean the Fuel mixture,,,unlike most carbs opposite effect.

[Vintovka]

Is that right?  Damn everything I've read has said different but maybe I was reading the wrong year of carbs.  I will try turning them out the shop manual says stock setting is 1 and 3/4 turn out from seat I imagine with bigger jets, exhaust, and pods maybe 2 to 2 and a 1/4 would be better.  Im also going to upgrade from NGK D8 plugs to hotter D7 plugs.   Maybe the problem with it dieing with aggressive braking is not lack of fuel but maybe its too rich and is flooding the engine.  Any thoughts?

[Gearheadgreg]

Could be loading up, you said your plugs where dark,, Check out the FAQ thread above this and look at the Carb area should help.,

[Vintovka]

Wow this makes perfect sense thanks a lot for the carb guide.  Im going to come out to 1 and 3/4 turn and see how it runs.  Ill post results but it definitely sounds like its flooding on heavy braking not to mention black plugs.  The carbs even sometimes make a sort of spurting noise not fluidy though but airish sounding sppttt sound.  I think thats a sign of it being too rich


Just finished turning the screws out from 3/4 to 1 and 1/2.  The bike will barely run now and has plenty of white smoke suddenly out of the pipes.  Im not sure what the problem is it sounds lean too lean to me.  The Pilot jet on my carbs is on the engine side not the airbox side so IN leans it and OUT richens according to the guide.  But then the white smoke was coming at 1 and 1/2.  This sound all effed up now Im not sure if these are even the correct year carbs or what.  They are keihin carbs and just above the bowl says PD

[Terry in Australia]

G'Day Mate, as the PO changed the the base-lines for your carbs by adding pods and bigger jets, freer flowing Kerker pipe etc, you can't assume that what works well for Greg with his stocker will work for you. I actually thought your bike should have been running pretty lean with your setup, and by leaning your mixture further I'm not surprised that it's not running well now.

The best way to adjust your mixture is to get your engine up to normal running temperature, set your idle at say, 1000 RPM, then turn your mixture screws in or out in small increments until you get the highest, smoothest idle.

PD carbs were designed to run lean to meet US EPA requirements, so adding pods will only exacerbate the situation. Sooty plugs/stalling can also be a symptom of a weak ignition system, so once you get your carbs sorted, check your ignition, particularly your points and your plug leads and caps, if they're original 33 year old OEM parts. NGK D7's are much better for our bikes than D8's, I run them in all of my CB750's. Cheers, Terry.    

[Vintovka]

Thats another problem I have is that when the bike is idling at 1000k RPM it doesnt seem to change RPMS at all what so ever when I adjust the pilot screws.  I have to actually seat the pilot screw to get the RPMS up which of course I dont leave it like that as its not receiving fuel so its currently at 1 turn out.  Its actually OUT richens and IN leans I'm not sure what carbs you guys are running but I even just got off the phone with the local motorcycle salvage yard and asked pilot jets in leans and out richens.

[Gearheadgreg]

If you have the pumper Carbs, you need to remove and clean your slow jets, i see so many people increase the Main jet and not the slow jet when 90% of cruising is in the idle circuit, I have shimmed even my F3 needles because of how LEAN mine was running from the Factory, Dose not make sense if you have Fouled plugs and still think it's lean.....Time to Open up those carbs and see what the PO did, check your Pilot Screws make sure They have 0 ring and washer + spring or will be impossible to fine tune, My 0 rings were crumbs after 32 years...2 where missing ?   good luck....btw with the Stock air box off my motor it would hardly run, way tooo lean...

[Vintovka]

My problem is the plugs are fouling after a short 20 to 30 mile ride.  Im trying to lean it out I have had the carbs all apart completely clean and clear.  On the slow jets though, 2 of them were missing the small rubber o ring and small washer.  I did not know they were that critical for getting the mixture right.  I'll look into carb kits to replace these and see if I can get a more precise tune and lean it out from its too rich setting.  I will also switch to hotter D7 plugs to combat the richness. 

I rather not switch from 120 main jets as it has kerker exhaust and pods on so I'm hoping with D7s like terry said and the o ring/washers on the slow jets that I'll be able to get it leaned out enough so it isn't fouling so much.  Thanks a lot guys

[Nikkisixx]

The bike runs excellent from what I can tell the only problem is when I apply a lot of the brakes suddenly stopping the bike, the engine stutters and dies.  If I quickly release the brakes the engine will actually start right back up depending on how fast I release the brakes.   

I had this exact problem-

then I learned to pull in the clutch   

OK, that wasn't helpfull at all     

[Gearheadgreg]

Hard to find those pesky O rings, hope you have them in your kit, make sure order is correct, spring-washer-O ring on the needle screw....also your (slow Jet) is inside the float bow next to the main jet, Pressed fit you can pull them yourself just be careful they will need to be cleaned, Do not confuse the slow Jet with the Idle mixture screw as the screw has the O ring not the Slow Jet.. hang in there you will get it right,most of the time Seafoam and Techtron will not free up the slow jet...Had 6 carbs on my Valkyrie they all are having problems with clogging slow jets try tuning 6 Carbs makes 4 seem easy..Is your Choke cable adjusted correct maybe check that, make sure it is open all the way..

[Vintovka]

Thanks a lot Greg can you recommend any kits to me?  Im having trouble finding any that seem to have them like you said.

[TwoTired]

The bike runs excellent from what I can tell the only problem is when I apply a lot of the brakes suddenly stopping the bike, the engine stutters and dies. 

Debris/sediment in the float bowls (how bad is the tank rusted?)  How's the fuel filter?

Or, gas level too high in the float bowls.


If it just quits, it could be a lean issue.  But, if it stutters first before dying, then it's getting too much fuel.

Cheers,

[Vintovka]

Like I said just got it back on the road after rebuilding a lot of things and cleaning up the carbs.  I boiled out the tank completely shiny with muratic acid and water then sealed it with POR15 tank sealent. Brand new fuel filter installed. Carbs were dipped in chem dip (minus the rubber parts) blown out all the orifices with carb cleaner spray until I could see it dripping from all holes and all the way through. 

The only problem that Greg pointed out is that Im missing an o ring and washer on two of the pilot screws I was under the impression that I could still tune it by RPM just that two might be slightly off compared to the other two with the correct o ring and washer.  The bike seems to run amazing until the plugs build carbon and I have to take them out and spray off all the black.  I havent had it over 40mph yet waiting on my Pirelli Sport demon tires I ordered to show up at my door then I can really take it out as the current tires are dry rotted.  Around the neighborhood its been speeds of 30 (40 when no one is looking).

[Gearheadgreg]

Thanks a lot Greg can you recommend any kits to me?  Im having trouble finding any that seem to have them like you said.

Two tired is thinking what i am thinking float level or needle valve leaking and bypass into carb,,,Here is info for carb Float needle- O ring- bowl gasket ect..ect..you can also go to your local Stealer (dealer) expect to pay more...> #48-1995 >http://www.vintagecb750.com/products/6/fuel-system

[Vintovka]

Wow thanks a lot great website.  I think ill go ahead and replace all 4 of them.  Ill get some pictures of the bike up on the site whenever its all said and done.  Really appreciate the help guys hopefully it gets sorted with some hotter plugs and new rebuild kits.