Topic: Bogging down at 1/4- midrange CB750K4 -problem solved!!!! Spanner saves the day!

[andy750]

Bike - CB750K4 - all stock jets/airbox/K&N air filter, stock exhaust. Carbs have been cleaned and balanced.

Here are the symptoms:

Starting from cold the bike will bog down in the 1/4 - midrange for the first 4 -5 miles and then run great after this. Flipping the choke lever up makes it worse (no surprise there). Plugs look good across all 4-cylinders. Turning the air screws out from 1 1/8 to 1 1/4 turn makes no difference to bogging issue. Changing jets from 110 to 112.5 makes no difference to bogging issue. Changed out the air screws to old stock ones that I had (before this had replacement Keyster air screws) and this made no difference.

Thinking now maybe its a tank venting issue? Or pilot jets perhaps but how to explain that it runs well after 5+ miles and idles smoothly at indicated 1500 rpm after a long run? Runs great at top end.

Any other thoughts on what to try next? Searched on here and found a number of posts like this but they were all seemingly solved by air screws being turned out more. Stock settings for the K4 carbs call for 1  - 1 3/8 turn.

thanks
Andy

[DH]

Humidity? Dries out when warmed up? Old plug caps? (test 'em)
Just guesses. Ever try lightly misting the wires and caps with a spray bottle of water? If there's any issues in that area, that test will bring it out. Just throwin it out there... Seems like it'd do it at any throttle range tho if it were electrical. Hmmm......

[Spanner 1]

+1 on a bad plug cap ( 1 bad cap would of couse effect 2 cyls. ), maybe one has a crack in it ??

[brewsky]

Are you saying the bike doesn't like to take gas when it is cold and the choke is off? Then runs good after it warms up?

If so, that sounds normal to me.

[andy750]

Wires and caps were replaced last week as I was thinking the same thing. Plugs were also changed.

As to this being normal I dont think so as this has never happened on this bike before. My CB750s (K2 and K4) in the past have started up with choke and then 5 -10 secs later choke off and they are good to go. May run a tad on the rich side but not too much. The K4 developed this "issue" earlier this year and I cant figure it out yet. I am going to swap gas tanks and see if its a venting issue or a fuel line issue but fuel lines look ok and no inline filter.

This morning coming to work it did the same thing  - started up fine but then struggles for first few miles and then improves. It still feels though even once warmed up like its lacking power....although you can feel the powerband after 5K.

thanks for the suggestions.

Andy

[brewsky]

My K8 has always been cold-blooded, taking a few miles to accept gas at low-mid throttle without choke.

How long has it been since you cleaned the carbs, and has it been sitting for a while? Today's gas can go bad in a short time clogging up the small openings.

[andy750]

I agree with you that the K8s may be cold-blooded but they have different carbs to the earlier Ks. Carbs were cleaned about 3 weeks ago and since then has not been sitting so long that gas would go bad. Filled up this morning with some 93 (3.2 gallons) so if it was bad gas Ill soon know if its been diluted. Ill also check the pilot jets this weekend but that wouldnt explain how well it runs after warm up.

Been wondering the crankcase breather hose might be pinched or making a difference. No carb overflow tubes at the moment to get pinched. I am pretty sure its carb related...

thanks
Andy

[Spanner 1]

Sounds like a fuel delivery issue rather than electrical ( spark ) 'cos if the spark wasn't happening consistantly the plugs would be wet-fouled by 5 miles with normal fuel being delivered. I think it's slow fuel delivery with a low fuel level in at least 2 bowls which causes the fuel to drop below the level of the slow jet, which sits much higher in the carb bowl than the main jet.......... and so your sucking air when you open the throttle above idle.  I would take the petcock off and clean it's passages, think thats the culprit  .

[andy750]

Thanks for that Spanner. Ill switch the K2 gas tank on as that bike flies and see if that solves the issue. Good explanation in any case!

Ill post back with updates.

cheers
Andy

[andy750]

Problem solved!!! Spanner was correct - I owe you a 6-pack of Sam Adams

I switched gas tanks and used the K2 gas tank (the K2 runs amazingly well) - instantly could see a difference and once again the K4 is a pleasure to ride - pulls hard and fast. Very happy! After trying all the usual suspects - carbs, plugs, wires etc...turns out its the petcock which would explain the slow progression of the symptons.

Big thanks to Spanner for coming up with this. So much appreciated! Had a great ride through the Blue Hills near Boston last night.

Cheers
Andy

[Spanner 1]

Sometimes my 'off-the-wall' suggestions are right .... LOL, glad you got it fixed, Andy....... I'll be sure to name the next 6 beers I drink c/o Andy.... thanks 

[brewsky]

Glad you got it sorted out.
I'd have never guessed that since it cured itself when the engine came up to temp.
I'm startring to have some petcock issues on my K8, so better address that before re-tuning the carbs!