Hi everyone, I recently bought an 82 cb750k and have been trying to get it running. I just cleaned the carbs (only thing I've done so far) and now it runs and idles pretty well. When I give it throttle, though (~2000 rpm) it starts popping and backfiring a little bit. I thought that since it idles fine, it could be an electrical issue (maybe when the alternator kicks in, it starts missing?) Any other thoughts? I know there's a bunch of other tune-up stuff I need to do, which might fix it, but anything to point me in the right direction would be great.
It's not that loud, just some scattered popping. It does it when it's revving up and sustained, but not revving down. Can't tell where exactly it's happening, I can check and get back to you.
On Mon, May 13, 2013 at 10:32 AM, seestheday [via Honda CB750'S] <[hidden email]> wrote:
Is it loudly backfiring, or just kind of popping?
Does it do it when it is revving up, revving down, or sustained at 2,000 RPM?
Can you tell if it is happening in the exhaust pipe or the carbs?
Could be lean/rich condition, but I'm leaning towards a timing issue.
The alternator does not "kick in" It supplies 1/2 wave AC and the rectifier/regulator converts it to regulated 12 volts.
You are jumping to unfounded conclusions.
You need to give more information.
What kind of exhaust does it have?
What kind of intake does it have?
Carbs come LAST in the tuning up process. All other tune up items must be
finished first, like setting points gap and checking the static timing.
Valve adjustment and cam chain tension.
You need a workshop manual FIRST. The Internet can help but some info
on the internet can be incorrect. But if you have a workshop
manual you will have the correct information to do the job.
Yeah sorry, my knowledge about the bike and its workings is mostly based off bits and pieces of info from manuals, forums, etc. I read somewhere that the battery starts charging around 1400-2000 rpm, so I thought that could be affecting the voltage going to the plugs. It's all stock (original airbox and filter, 4 into 4 exhaust, stock jets, etc). I have the Honda service manual that I found online, not sure if I need a Clymer manual or anything. But yeah, I'll probably go through the whole tune up process anyway, just wondering if there were any specific ideas right off the bat
On Mon, May 13, 2013 at 10:38 AM, Lucky 1 [via Honda CB750'S] <[hidden email]> wrote:
The alternator does not "kick in" It supplies 1/2 wave AC and the rectifier/regulator converts it to regulated 12 volts.
You are jumping to unfounded conclusions.
You need to give more information.
What kind of exhaust does it have?
What kind of intake does it have?
Carbs come LAST in the tuning up process. All other tune up items must be
finished first, like setting points gap and checking the static timing.
Valve adjustment and cam chain tension.
You need a workshop manual FIRST. The Internet can help but some info
on the internet can be incorrect. But if you have a workshop
manual you will have the correct information to do the job.
Working on a CB750 Digger named Rover
If you reply to this email, your message will be added to the discussion below:
Just an FYI, the Honda Factory Service Manuals for your bike are available for download free here. In the same section you posted this, I added newer links on the page 4 a while back in case the ones at the start of the thread do not work for you.
81 Honda CB750C - Current Project
67 BSA Spitfire MkIII - Next Up (Full Resto)
81 Honda GL1100 - Bob / Cafe´
80 Suzuki GS750L - Bratstyle
72 Honda CB450K5 - Basket Case
73 Honda CB350F Cafe' (Gone but not forgotten)
Don't wait for opportunity to knock... kick the door down and drag the old harlot in!
First things I would look at are your plugs. Pull each one and notice the state of them. Do they look like the cylinder is running to rich/lean? Compare them to pics in your manuals or online if you don't know how to tell. Simple adjustment. Give 'em a good cleaning or replacement based on that. Second of course would be timing. And, if you cleaned the carbs, I'm assuming you removed them and pulled them apart? Did you sync them after? If not, might be something you want to look in to even if it's not your problem. I had a simple popping on my 81 C, a quick plug cleaning and once I got the carbs balancedm, it went away.
Syncing the carbs can be a pain if you don't know what you're doing but, once you get the hang of it, it's not so bad. There are hundreds of youtube videos on how to do this. Actually I believe there is a link in this forum somewhere that has some great info on it. You will need some gauges for this. If you can get the mercury ones I would shoot for that but, with some practice the air ones do work.
Hope some of that helps.
*Edit... Also, when looking at your plugs, check the gap. The correct gap will be in the manual. Do you understand how to adjust fuel/air mixture?
It ain't a custom till you have customized it yourself.
1981 CB750c (current daily ride and build)
1980 GL500 (Stripped and rebuilding)
1981 CM450 (Stripped and rebuilding)
Tires - are they older than 3 years? If so, plan to throw them out and get new ones, doesn't matter how much tread is left - you REALLY don't want a blow-out at speed.
Oil/filter change - Rotella T 15W-40 is decent and cheap
Valve lash - get them between .004" and 0.006" (very important!)
Compression - Ideally at 170 psi and all 4 within 10 psi of eachother, but 150 is considered to be quite good. Bike shouldn't run at all if they get below 100 (most engines won't).
Spark Plugs - NGK DE8A (stock) or DR8EIX (Iridium upgrade). Don't use anything else. Replace them if they're fouled.
Fuel filter - make sure you use one (paper is best) and put it between the petcock and where the fuel goes into your carbs
Fuses - check them to make sure they're all in good condition
Battery - get it load tested at an auto parts store (every one I've been too will do it for free), or just plan to use a brand new one.
After all of this is done, you should then start to look at your carbs and check your timing.
Regarding the carbs - you mention that you jump from 2700 to 3100. Are you sure that you put the jets back in the right spot? Here is a quick video I made showing the locations:
Also, has the bike fully warmed up when you are hearing the popping, or is it almost cold?
IF the bike has been sitting for a period of time all that may be wrong is that the idle jets are clogged.
This is very common when a bike has been sitting around.
Thanks for the advice. The previous owner said he just replaced the battery, plugs, and oil, but I'll check them anyway. I just put in a new fuel filter, too. I'm planning on checking the valves and cam chain tension today, hopefully I'll down the rest of the list soon.
The bike pops when it's cold or warm. I've typically let it warm up for about 5 or 10 minutes, maybe it needs longer? Also, when I cleaned the carbs, I made sure all the jets were clean and put back in the right spot. I could check again, but I'd rather not take the carbs off if I don't have to.
On Tue, May 14, 2013 at 9:23 AM, seestheday [via Honda CB750'S] <[hidden email]> wrote:
Like Lucky said, you need to do a full tune-up.
Tires - are they older than 3 years? If so, plan to throw them out and get new ones, doesn't matter how much tread is left - you REALLY don't want a blow-out at speed.
Oil/filter change - Rotella T 15W-40 is decent and cheap
Valve lash - get them between .004" and 0.006" (very important!)
Compression - Ideally at 170 psi and all 4 within 10 psi of eachother, but 150 is considered to be quite good. Bike shouldn't run at all if they get below 100 (most engines won't).
Spark Plugs - NGK DE8A (stock) or DR8EIX (Iridium upgrade). Don't use anything else. Replace them if they're fouled.
Fuel filter - make sure you use one (paper is best) and put it between the petcock and where the fuel goes into your carbs
Fuses - check them to make sure they're all in good condition
Battery - get it load tested at an auto parts store (every one I've been too will do it for free), or just plan to use a brand new one.
After all of this is done, you should then start to look at your carbs and check your timing.
Regarding the carbs - you mention that you jump from 2700 to 3100. Are you sure that you put the jets back in the right spot? Here is a quick video I made showing the locations:
Also, has the bike fully warmed up when you are hearing the popping, or is it almost cold?
I removed and cleaned the emulsion tubes, but I didn't remove the idle jets. I thought they were pressed in, so I just shot some carb spray down each. I take it I should probably remove them and clean them better? Although I might wait to take the carbs out again until I do everything else that doesn't involve removing the carbs.
On Tue, May 14, 2013 at 11:00 AM, seestheday [via Honda CB750'S] <[hidden email]> wrote:
Just because the plugs are new, doesn't mean they're the right kind. Make sure that they're NGK D8EA or DR8EIX.
Since you have an 82, your slow speed (idle) jets should be removeable (my early 81 has a pressed in slow speed jet).
Did you remove them to clean it? It's down the tube that is plugged by the rubber plug. You would need a small flat blade screwdriver to get it out.
Did you also remove all of your emulsion tubes to clean them as well?
I also recommend buying a set of allen key bolts to replace those easily stripped Phillips ones. If you're splitting the bank I'd say that getting new choke plate screws is mandatory as the stock ones are flared and you will very likely cross thread them if you try to put them back in.
If you're going to split the bank, make sure to replace the o-rings on the fuel and accel pump cross tubes. Splitting them a second time because of fuel leaking is a PITA.
LOL. Oh, and after taking my carbs on and off so many times I'm now a pro at it. I can go from bike on the centre stand with the tank and seat mounted to the carbs out in 20-30 minutes. I could do it faster if I had an electric screwdriver.
I just checked the plugs, and they're NGK D7EA (cold weather plugs according to the FSM). They also looked a little fouled, there was a thin coat of carbon. I'm not really sure to tell how much fouling is OK. I'm guessing new plugs are in order? Are the iridium plugs worthwhile?
On Tue, May 14, 2013 at 12:46 PM, seestheday [via Honda CB750'S] <[hidden email]> wrote:
LOL. Oh, and after taking my carbs on and off so many times I'm now a pro at it. I can go from bike on the centre stand with the tank and seat mounted to the carbs out in 20-30 minutes. I could do it faster if I had an electric screwdriver.
For what it's worth, the black coating on the plugs rubbed off easily and felt like oil, rather than carbon.
On Tue, May 14, 2013 at 1:17 PM, Sam Kenny <[hidden email]> wrote:
I just checked the plugs, and they're NGK D7EA (cold weather plugs according to the FSM). They also looked a little fouled, there was a thin coat of carbon. I'm not really sure to tell how much fouling is OK. I'm guessing new plugs are in order? Are the iridium plugs worthwhile?
On Tue, May 14, 2013 at 12:46 PM, seestheday [via Honda CB750'S] <[hidden email]> wrote:
LOL. Oh, and after taking my carbs on and off so many times I'm now a pro at it. I can go from bike on the centre stand with the tank and seat mounted to the carbs out in 20-30 minutes. I could do it faster if I had an electric screwdriver.
The Iridium plugs will fire almost instantly -- like having new, much more powerful coils installed! Problem is that they are around $8 each... If they foul, you buy a replacement. Yes, they are worth the cost, if you have everything else in good working order. In cars, they will last up to 100,000 miles; in a bike, they should last 50,000 miles, as long as they don't foul due to carb/engine issues. I'd recommend 8's (regular temperature range), not 7's. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII
Locked
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