Topic: I've got my Ratty CB750K2 running, but..................

[Terry in Australia]

Well After many years of sitting around gathering rust and poop, I'm happy to report that I've got it running, but it's got a little "Pfut" "pfut" in #3, and the pipe never gets more than warm. I did a compression check and it's running 175 psi in #1,2 and 4, and 150 in #3 (cold) which I thought wouldn't account for the miss, if it had a burned out valve it wouldn't hold any air at all, so I keep thinking it's ignition but I've timed it and the points are providing a reasonable spark, and I've done the valve clearances, so I'm close to stumped.

I also cleaned the carbs main and pilot jets (left the carbs on the bike) so unless there's some #$%*e in the carbs circuits, I'm at a bit of a loss. I might try switching coils and if that doesn't work I'll pop the carbs off and swap them with the ones on the K1, which I know to be good. I'll whip off the head and replace a suspected buggered valve if I really need to, but I don't wanna pull the engine if I don't have to. Anything I missed? Thanks guys, Cheers, Terry.  

[Bob Wessner]

The compression sounds good enough and if you have spark, I would vote for carbs. Hard to do a good cleaning still on the bike. Personally, I would remove them, strip them down and give them a good cleaning.

[Gordon]

Can't argue with the carb guru, Terry.  You're gonna have to pull them.  That's almost definitely a clogged idle circuit. 

[dusterdude]

yea terry listen to bob

[eldar]

It could be clogged or it could be feeding too much fuel as in float may be sticking open. You should know that you need to pull the plug and look!! I say this cause when I first replaced the float valves in my carbs, #2 did this. Just too much fuel.

[Raul CB750K1]

Seems like a clogged idle circuit? Some wool strands maybe....?


Raul

[cadler]

Is it possible you have a bad spark plug?

[Bob Wessner]

Is it possible you have a bad spark plug?

Good point. In rereading the post, Terry mentions checking at the points, but not the plugs. Might be worth grounding it (them) to make sure he is getting the spark he thinks he is before tearing into the carbs.. though I think they are next. 

[Jonesy]

If the pipe is getting noticably warm, my hunch is that it's getting spark, but it wouldn't hurt to check that out. You do have new plugs in there, right?

Does the bike act like it's running a flood out when you rev it up? My bike had the same issue. Try this: Start the bike up. Note the temp of the pipe that's only getting warm. After running at high RPM for a few moments, does the pipe get hotter? If so, I'd look into a stuck/sunk float or buggered idle jet (in my case the idle jet was drilled out to the equivalent of a 150! I had raw gasoline dribbling down the exhaust header!)

Now, if the pipe didn't get any warmer, remove the airbox and with the bike running at idle, partially block the throat of the carb on the offending cylinder with the palm of your hand to choke it. Does the pipe get hotter now? If so, somethings likely plugged up in the carb. (This was how I found a plugged idle jet on my CB360...)

Anyway, give it a try and see if you find anything. Let us know if you do.

[paulages]

if nothing else, switching the carbs with a good set always provides you with a clear reference point. i can't count how many times i've sat there scratching my head "knowing" that i cleaned the carbs well enough, only to have the bike run perfectly as soon as i dropped a good set in. but a have on occasion, done the same thing, and it wsan't the problem. i just left them in until it was running, then swapped them back out.

every experiment needs a control.

[mcpuffett]

hi terry, if you decide to break it  i would'nt mind bidding on those exhaust for my ko  mick

[Terry in Australia]

hi terry, if you decide to break it  i would'nt mind bidding on those exhaust for my ko  mick

G'Day Mick, well i've thought of selling it for bits but Tom (El Cheapo, a top bloke) has got a heap of parts for me to turn it back into some semblance of a proper bike again, my cunning plan is to make it look good enough to restore back to it's former glory, then flog it on Ebay. The pipes look a lot better from where you're sitting, than from here mate, when I fired it up the first time, I "fragged" the dog with rusty shrapnel, ha ha! Cheers, Terry.

[Bodain]

"Pfut" "pfut"    I'm just about convinced this is an old 70's Honda trademark.

I've got new piston and rings in my 550. I've been in and out of the carbs countless times. Still cylinder #2 continues to do this until it warms up.

Others have stated many of the old Honda's do this unit warm.

[Terry in Australia]

"Pfut" "pfut"    I'm just about convinced this is an old 70's Honda trademark.

I've got new piston and rings in my 550. I've been in and out of the carbs countless times. Still cylinder #2 continues to do this until it warms up.

Others have stated many of the old Honda's do this unit warm.

Yeah, well my F2 did this due to a blocked pilot jet, so I'm assuming that even though I've cleaned the jets I've got a blocked circuit in my carbs, and regardless of how warm the engine is, the situation doesn't change. You're right though, these old girls are reknowned for making different noises when cold, I've got 4 running CB750's (and my racer in bits) and they all have different "characters", ha ha! Cheers, Terry.

[scout18]

I read with great interest your tale of getting your ratty 750 running.  I just took mine for a spin around the block today for the first time. I am used to a 350f. Even running kind of bad it just blew me away at how much more power these 750s have. As to your trouble with #3  I vote try spark plug first. make sure that your plug boot is not leaking spark too.  Then take those babies off and hose them out again. Never time to do it right but always time to go back and do it again? Some times things work out like that. Good luck.

[mcpuffett]

bet the dog was pleased with you    not, cheers anyway the shipping would have been an arm and a leg   mick.

[HondaMan]

About those compression numbers: did you check compression with the throttle full open? Just asking: I never saw a variance over 10 PSI across the engine if the valves were good....also, how about the clearances on the rockers? The exhausts should be .003" (.004" if you like big throttle openings) and the intakes .003" (even though the manual says .002" intakes...). If one is tighter than .001", it will drop a cylinder compression a whole bunch at cranking speed.

[Terry in Australia]

About those compression numbers: did you check compression with the throttle full open? Just asking: I never saw a variance over 10 PSI across the engine if the valves were good....also, how about the clearances on the rockers? The exhausts should be .003" (.004" if you like big throttle openings) and the intakes .003" (even though the manual says .002" intakes...). If one is tighter than .001", it will drop a cylinder compression a whole bunch at cranking speed.

Thanks mate, "been there, done that", the compression test was done cold (I hadn't attempted to start the engine at that stage, and it had been sitting for many years) with the throttle wide open. I'd adjusted the valves, cam chain, points, cleaned the plugs, pulled the pilots and mains and cleaned them too.

Once I got it going I timed the engine with my strobe light, but I haven't touched it since I posted the original question, so I'll do another compression test tomorrow, when I attempt to get it running again.

I think I'll remove the carbs and clean them properly this time, and I'll make sure I smooth any ridges in the float arms, as I have a leaking "needle and seat" (fuel valve?) and I think the needle is sticking in a ridge in the float arm. Keep thinking mate, any help is good help! Cheers, Terry.