Topic: hot pipes

[Sandcast 2356]

I have got my bike up n going, and I noticed that when run at higher rpms all 4 pipes are the same temp. However, if I just put around in-town (30-35 mph) #1 and 3 pipes are the same temp. But, #2 is freakin burning, and #4 is almost cold. (#1 being on the far left if sitting on bike). All 4 cylinders are firing, but it does backfire occasionally out of the right pipe only when run at these low speeds in around 2nd or 3rd gear. I'm assuming it's in the carbs, but I have them all adjusted as balanced as possible without any equipment (college student). Only other thing I can think of is the points. Being #2 and #4 are on the same point, and that point I found to be bent to constant contact and bent it back to a running state. I have ordered new points (being the old ones are barely doing the job) and was wondering if that might fix the problem. Any input would be helpful as this is my first bike.

Also, I was wondering if the frame has a certain level it has to run at in order to keep the engine cool. The #$%@&* that had it before I did put longer forks on it and didn't bother changing the fork angle. I have adjusted them a little lower being it was rediculous to drive before but it is still a littl high in front. (bout 1" to 1-1/2" difference from tail to engine.

[Bob Wessner]

Well, I knew the redline value on the '69 which is why I replied, but in rereading the rest of your post, have you checked for manifold leaks on the cyclinder that gets hot at low RPM's? Maybe at higher RPM's you are still able to draw enough fuel volume to offset the leak somewhat, but not at lower RPM's. The number #4 sounds like maybe a plugged or partially plugged idle circuit?? Just a couple of thoughts from an inexperienced wrench.

[Sandcast 2356]

the heads on 1 and 2 are loose from the other guy, but I thought I had um seated pretty good, I'll run through a check. Thanks for the advice.

[Dennis]

2 & 4 DO NOT run on the same point set.
1&4 are together and 2&3.
also check spark plug caps

[Sandcast 2356]

huh, you learn something new everyday, and did you mean gaps??

[AndreRA3]

Nathan,
plug caps refer to the sparkplug wire boots, or connectors.  If the boots (or caps, or connectors) are toast, they will have higher resistance and as a result the spark will be weaker (if it sparks at all).  Same goes for wires and coils.  But if only certain cylinders are doing that, wires and boots are the best guesses (changing the wires might mean replacing the coils, though).

As for your problems, I would clean the carbs with Yamaha carb cleaner, with the carbs on the bike.  Just drain the bowls, fill them with a 3:1 ratio of gas and yamaha carb cleaner, kick twice with the ignition off, and wait 30 minutes.  Drain bowls, repeat until the liquid you drain comes out clear.  I did this on my bike and the bowls are shinny on the inside (I later opened them up to check the jets).  Kicking the bike will draw some cleaner up the jets, but DO NOT RUN THE BIKE, as the cleaner has a very low octane rating (I think).  One of the guys on the old forum (were I learned most of what I'm telling you) dipped his carbs in the yamaha cleaner and they came out shinny inside and out.  Inside only was good enough for me, though  .

Checking your sparks might be a good idea, as upgrading my ignition helped a lot.

These are all guesses, by the way.  It's definately (probably? maybe?)  a combustion issue.  Wether it's fuel related or spark related, that's a different story...

Good luck

[heffay]

you say you assume that it is in the carbs even tho you have them balanced to the best of your ability... i would still think it is in your carbs too.  what bob said about the idle circuit might be a good place to start.  BUT, i was unaware of the points being situated in that way too so they might also be onto something there. 

[Sandcast 2356]

ah, now you're speaking my language. I have tested the spark and all seemed well, the carb cleaner thing sounds interesting, any particular reason I should use Yamaha carb cleaner? Can I rule out the points (being they are mechanically flawed, refer to open forum under "My Bike")

[AndreRA3]

Nathan, Yamaha carb cleaner is good because it does not damage the seals and gaskets in the carb (which is why it can be used without dissasemblying the carb).  It's not too expensive, I paid 12 bucks for one at the local yamaha dealership, and it's used at a 25% concentration with gasoline.  Replacing your points (and setting a correct ignition timing)  sounds like the first step here anyway...

Good luck again, man.

[eldar]

well I ran into something like this. I would not worry too much about points but a couple of quick tests you can do would be to check the wire coming from the coils. If there are cracks then it would be time for new wires and coils possibly. Next look at the plug CAP. measure its resistance with a ohm meter, you can get them for cheap about $15 or so. These are not the most accurate but work enough on a limited budget. The plug cap should be UNDER 10000 ohms around about 5000 is normal I think. Also change your plugs if you have not done that. If you have try moving them to different cylinders and see what happens. Otherwise I would have to say that your #4 is running too rich at idle and even at higher speeds. When do the rough sync, use a smooth wire and adjust all slides to that wire gap as a start. Also you may want to see about your pilot screws.

[AndreRA3]

I used an ear syringe (found at any pharmacy or walgreens type place for a couple of bucks) and pumped the mixture into the fuel lines 10cc at a time.  Takes some time, but it's doable.  If you can find a bigger syringe, take it...

[Dennis]

about the resistance of the plug caps ................
the OEM Honda caps should actuatlly be around 10K, and +/- 20% or so is all right also. Many times they are higher because of aging and those should be replaced.
The most commonly used NGK replacement is 5K. It is marked right on the caps if you have the newer ones.

[Harry]

As for your problems, I would clean the carbs with Yamaha carb cleaner, with the carbs on the bike.  Just drain the bowls, fill them with a 3:1 ratio of gas and yamaha carb cleaner, kick twice with the ignition off, and wait 30 minutes.  Drain bowls, repeat until the liquid you drain comes out clear.  I did this on my bike and the bowls are shinny on the inside (I later opened them up to check the jets).  Kicking the bike will draw some cleaner up the jets, but DO NOT RUN THE BIKE, as the cleaner has a very low octane rating (I think).  One of the guys on the old forum (were I learned most of what I'm telling you) dipped his carbs in the yamaha cleaner and they came out shinny inside and out.  Inside only was good enough for me, though  .

I need to do this, as well. I picked up some Yamaha carb cleaner, but how do I fill the bowls after draining them? Do I need a turkey baster type thing to get in through the bottom, or is there an easier way. (Total newbie to this sort of work.)

('77 CB750f Supersport.)

Take fuel line off tank, add cleaner through fuel line.

[lawrence]

Seriously here's what I'd do, learned to me from guy's that pour a quart of transmission fluid down their carbs on their small block chevy's to clean out the carbon on the valves, decreasing visibility on my street to  0 feet, the same guys that wedge a breaker bar to the ground on their crankshaft and bump the starter to free the crank shaft bolt (aka The Redneck method). I cannot be responsible for what I am about to suggest. As performed on Skippy's cb750 Ratbike.
I'd unscrew all the spark plug caps from the wires, trim the wires down with a good pair of side cutters 1/8 - 1/4" squirt a good helping of dialectric grease in each of the wires, and screw the caps back on, get me a set of new D8EA s' , check the gaps, hit one end with anti seize compound (the treaded side) the other with dialectric grease, put them all in. Then I'd go to float bowl #4  pull the pin, pull the float, making sure to catch the needle/stop valve, check the valve for wear, then I'd get me an 8mm socket and unscew the brass fitting that the valve seats in, then I'd find me a good phillips screw driver(one that"s not all chewed up) stick it in the brass fitting (square it up so the screw driver is straight in there, like lapping valves) making sure not to apply pressure then slowly twist the screw driver around with my finger and thumb to try and create a new seat in the brass for the stop valve. Reinstall then I'd pull down the float just a pinch and or tweak the tang up a little. #4 sounds like it's just plain flooding. 


I thought I mentioned this before, then I remembered that I did,  but I guess carbon monoxide and beer failed to beseech my recall and now I am left to plagiarize myself like a doddering pundit with gin blossoms. http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=00AkoB.

[Glenn Stauffer]

I need to do this, as well. I picked up some Yamaha carb cleaner, but how do I fill the bowls after draining them? Do I need a turkey baster type thing to get in through the bottom, or is there an easier way.

Disconnect the fuel line at the petcock and use a small funnel to fill the carbs with the cleaner/gas mix through the fuel line.

--Glenn

[Alan F.]

Definitely use that ohm meter to check all of your coil windings, primaries and secondaries, measure cold,  it'd be a shame to find out later on that one of your coils had an intermittant condition. And also be certain to unscrew the plug caps and trim back the wires a bit then screw the caps back on.

[Sandcast 2356]

Thanks a ton for all yall's help, yet, I still wait on my new points, I really don't want to mess with my carbs until I get some working points in there. BTW, how does one adjust the float height??? Sorry, but I'm new to all this stuff as I have said many many times before.

[CHUNG]

At WFO throttle/high speed the carbs are all the same, wide open. But at lower throttle settings, the carbs will show that they are outta balance. That's probably you prob? Set the timing on the points perfect, gap... close. Get the carbs to lift together. You can do it with them in your hand and get them really close if you practice, that is the highest maintainance except for the chain ;-(
I never run any engine above 7krpm for a long time but I have flogged many a 750 well past 11k for a gear or two ;-)/ They say that they won't live long above 8500 and I've never held that rpm for any length of time on a 750 (I have on a 350k though, great bikes) but I have floated the points and valves at 10k or so on a 750. Bent 5 on a 55mph downshift to 2nd once. LOLLOLLOL Gotta love thoise Hondas, ehh?

[Sandcast 2356]

so....I can adjust the float heights by twisting the thread of the shaft of the carbeurator piston-looking thingys???

[Bob Wessner]

No, no! With the bowl off the carb(s) you'll notice the floats pivot between two posts. Coming off the float assembly is a flat metal tab, or tang some call it. This extends out to the float valve needle. This tab presses against the float needle and seats it as the fuel level in the bowl increases, which closes the float valve and prevents more fuel from entering from the fuel lines. How far the tops of the floats are from the edge of the carb body is the float height. On your bike the float height is 26mm. If the height needs to be adjusted, it is done by gently bending the metal tab one way or the other depending on whether you want to necrease the height, or decrease the height. When doing the measurement, the carb body should be held at an angle so the metal tab from the float is just barely touching the tip of the float needle. The reason for the 'just barely toucching' is the float needle contains a spring mechanism and if you allow the tab to rest too heavily on the end of the needle while trying to measure/adjust the float height, you will compress the spring inside the needle and get an inaccurate float height.