Topic: # 4 cylinder is hotter than others-1971 CB750

[alabama1971]

Thanks for all the great posts!! as a new owner I did a tuneup adjust valves/changed points /plugs set dwell and timing with the kind help i found here. The # 4 cylinder seems to get real hot at idle when i touch the head/ spark plug cap its very hot and i even saw smoke coming from the inlet side of carb where it mates up to air filter box after i shut it down, during road test it didnt seem any hotter than the other cylinder heads to the touch only after idling and during the time for checking the dwell/time, the bike was bored to 900(PO told me so it may be 8XX) has Yoshi cam he said.
Should i adjust the timing-retard- because of the mods?- is this an indicator that cylinder is running lean on fuel with it being so hot? My next project is to tackle carbs investigate jet sizing- clean/sync as it runs but seems to use lots of fuel- number two plug gets gas fouled so it may be pouring too much it seems - i dont see fuel running out the overflow tubes but i do see it seeping around the bowl gasket if i leave the petcock on when not running, all input is appreciated thanks

[Rigid]

You are definately on the right track.  It sounds like a good candidate for a float level check, jet size check and synch.  That extra hot cylinder could easily be a lean mixture on that cylinder with the others running richer.  Something is different, and it is unique to that cylinder, like valve settings, sparkplug, cap, lead, float setting, restricted jets, intake leak on that rubber coupler, etc. It shares a coil with #1 however. 

[alabama1971]

thanks Rigid for the input what is your opinion on the timing, i set it exact, but wonder should the fact of the mods(bored out-yoshi cam-4-4 straight out pipes but has stock air filter box) that are on that bike give me any reason to retard it a little bit? Also #4 only gets hotter than the others at idle thats why I was leaning to fuel being lean, Have you ever seen one so hot it smokes where the carb mates to the air filter box? Thanks

[TommyT]

When you pull that plug does it look like it's running leaner than the other cylinders? 

[Rigid]

Could that smoke be oil that is burning off or unburnt fuel ? Intake heat like that points to the intake valve not sealing, and that would be worse at idle as the flow would help cool it as it increased.  I had one spitting fuel out the intake one time, and it only did it at idle.  What is the compression on that cylinder?  As far as the timing, I have never dviated from stock no matter how radical the set-up with pipes, pods, or 836 kits.  Others will know more on that.

[Hondawggie]

If the cams are not stock, you may discover that the Honda standard valve adjustment procedure is altered a bit for the cam.  You really need to know how to tune the bike up and to do so, at least for the sake of knowing *for sure,* you can get info from the prior owner about what the cams are -- then look on the web for "your cam" and "valve adust" or "tuneup."   Hopefully not much has changed -- but if you have non-stock engine bits, the stock tuneup may not be effective.

I ran into the same problem with another bike.  The prior owner did the mods so long ago and he was in his late-70s in age he didn't remember the details, so I got into the motor to try to get some id from the cams but no luck - yet.   The standard valve adjust really has this particular bike down on power but I had nothing else to go on.   The bike is in my garage -- I may just put stock cams and ignition back in if I can't find out what the cams are.   Makes it difficult.

If you have 'detonation,' ie. when the plug fires too early, you can hole the piston and the spark plug will be whitish with specks of metal on it -- running lean will also make it run too hot.

If you can get a thermometer and check the header temps, you can pin down exactly how different the cylinders are heating up.  It won't fix the problem but it will clarify how big a problem you have.

I'd try to pin down the cam specs and then see if the Honda standard valve adjust procedure still flies.


http://www.harborfreight.com/catalogsearch/result?q=thermometer

During a road test the high temp problem might be masked by the airflow when you're at speed, and only when the bike is not being cooled is it evident.

[Don R]

A little of what I know about adjusting valves, my race car had an engine that allowed me to adjust the valves in the stock manner, I got #1 at TDC and adjusted the intake and exhaust, then rotate to the next cylinder in the firing order and repeat.
 My new engine requires me to watch the intake valve, when it is almost closed adjust the exhaust rocker, when the exhaust just starts to open adjust the intake lash, this gets the rocker arm or lifter on the base circle of the cam.
  If we could take the valve cover off, it would be easy on a sohc because we can see the cam.
  How it relates here, an intake valve not closing all the way could allow compression to blow back into the carb. an Exhaust without lash could allow unburnt fuel to escape the combustion process making the pipe hot also.

 Two things could affect exhaust temp, rich exhaust with fuel burning in the pipe or lean with a hot mixture exiting a very hot cylinder.

[alabama1971]

Thanks to all you kind gentlemen for your responses-heres todays update-#4 plug looked a little carbon fouled
I did recheck valves four exhaust was a little off- like 002-set them at stock, i will have to research the Yoshi cam specs as the PO is not around to advise so im lost there, new Number 1 plug was not firing-deadplug- tried against head and no spark- replaced with new plug and cylinder is back firing
Carbs are overfilling on 1/2 i can see that so ive got carb work to do,unburnt fuel in pipes, and my heart just sank as I checked my oil level just to check it since the oil change and the level is above fill line and its thin and smells like fuel is in the oil , thinking when the number 1  plug quit the unburnt fuel seeped into crankcase or from the other cylinders that are getting too much maybe?
Any advice or opinions on fuel in the oil? Should i immediatley drain and not run until i get carbs squared away?
Due to excellent advise on the forums-The only thing that went right before this was points install, static and strobe timing and timing and dwell is right on.
All input is appreciated as im just a country boy with a 40 year old 750 toy, shes a beauty though!
Thanks

[Rigid]

Drain the oil and replace new.  I am guessing you have at least one plugged overflow line and the bowl is draining into the engine past the intake valve and cylinder.  This will lead to wear and scoring over time.  Pull those carbs and do a complete. detailed, clean, repair, and adjust.  Set float heights and make sure the needles and bores for the float are clean and in good condition.  I always leak check off of the bike with a hose and gas can above the carbs for 10 minutes or so.  Saves many on and off sessions.  BTW, when you get the running and leaking issues all sorted out, that oil with gas in it can be reused in small quantities as the gas will evaporate in an operaing engine.  Fuel dilution was common before multiviscosity oils were invented to thin oil in winter months.  If it is dirty, or has some miles on it, don't bother. 

[alabama1971]

Thanks Rigid i drained the oil, will take your advice and work on the carbs next -question there are two tubes going up into the air filter box open ended one each from carbs two and three can you explain their function and how this may or may not work with the other two carbs?
 also i wonder about the fuel feeds off the petcock one is from the reserve position and the other from the on position where should those go so i can make sure they are correct as PO had no clamps anywhere so im wondering if their placement is correct.
The center stand is removed and the kick stand allows it to lean way low to left-16" rear wheel-shocks removed and struts in also contribute to the low left side lean- is there a chance that while its in this position and i have it idling the fuel delivery will be altered to the carbs due to the lean of bike and gravity flow?
thanks for the help!! I appreciate you!