Topic: This bike is crazy

[mustang2003]

Ok guys heres the thing I have a 78 honda cb 750 f and I just put on newly rebuilt carbs, they are doing the same thing as the other carbs, fouling my plugs and backfiring, It seems when I look in the carbs they  don't seem to be getting any gas, but somehow the cylinders are getting too much, any suggestions ?? HELP I'm going INSANE ~~~~

[eldar]

You are funny!

Ok now, what is your tune up history?

have you adjusted cam chain? set valve lash

done timing and made sure points were good?

Where did these carbs come from and what has been done to them?

Have your cleaned your plugs or gotten new ones?

have you set you accel pump correctly?

[mustang2003]

Funny thing is she'll run with the choke on but soon as you start to push it in she'll quit, and no this bike has been sitting for a while and I did change the plugs, and the acc pump is new, but other then that I didn't do anything else. Am I doing something wrong here?? I thought alright I got me a good set of rebuilt carbs, Now this baby's going to run, well I think she had another way of thinking.

[Duck]

compression, timing, spark, fuel

[jdpas29]

is the airbox on the bike?

[mustang2003]

No I don't have the airbox on, I 'm sorry guys but I'm new to this motorcycle repair, I have always worked on cars, but does it matter that much if the airbox isn't on the bike??

[comcrx]

yes, it matters a ton. my k6 will idle ok with no airbox, but the second I try to give it some gas it dies out.

try putting the airbox on to see if that helps

[mustang2003]

Ok thanks guys I'll try that in the morning

[750k5]

If you replaced the carbs, and it is still doing the same thing, obviously the carbs are not the problem. Carbs almost never cause a backfire, work on the ignition, plugs and points, and you will be headed the right way

[eldar]

Well You will certainly want to at least clean and regap your points. Then set your timing, Do the cam chain also. And like cars, this is easiest with the plugs out of the bike to lose the compression.

Now, set your FUEL screws in front of each fuel bowl, 1.5 turns out as a start.  What plugs are you running anyways?

The 750 uses D8EA as standard. If you have D9, those can foul easier. Make sure there is no R in the plug number, this denotes a resistor plug. You already have resistor caps.

Look in the back of your arbs and you should see the cut away part of the slide, if you do not see the cutaway, they are not assembled correctly. This is assuming your carbs have this which they should if they are 78 carbs. Put on your airfilter, it makes a BIG difference.

Clean your plugs and make sure they are gapped between .024 - .028 I think. I forget the exact gap sometimes thats why we have books!

That reminds me. Unless you have a dial connection, download the shop manual avalable right here on this site.

here is the url:  www.sohc4.us/forums/index.php?topic=1570.0

You see, everything is tied together on these bikes more than it is on cars. All these little adjustments affect the whole.

So start at the beginning. Your problem may not even be with your carbs.

[Duck]

#1 Check the compression. If it's not right you're pissin into the wind with other stuff

#2 When the compression is right check the ignition for a good spark. It's wasted spark so you test between the two ends of the coil with a spark tester. Cheap to buy, easy to make. If you want a 3 pt you'll have to make it.

#3 When you have a good spark set timing and dwell.

It will run now if the mix is correct.
If not, now it's time to mess with your carbs.

[eldar]

In order to get good spark though, your points and condensers need to be gapped properly though. If they are not, no matter what else you do, spark will be poor. Your points are what saturate your coils with energy or maybe Or should call them the pathway. If they have poor contact points, are worn out, or just not set right, you are just as dead in the water as if you had bad comp.

[Duck]

Yes, but comp is the easiest thing to check and tells you if you need to look at rings, valves, or valve timing.

Your doc doesn't go prescribing this or that to see if it works OR cutting holes in you for a look around until he checks out the easy cheap stuff. Working on the bike benefits from the same approach.

Sure, no spark and no fuel and you won't run, but why spend all the effing time and money when you don't KNOW that something that can be checked in 5 minutes with a $9 tool is not right. Especially if that thing can make it look like you have a fuel or ignition problem. $60 for carb kits on a four. 2 hours to go through carbs. $20 for points and caps. Gotta put them in and set the dwell and timing. Still not running, is it low voltage at the coils? Are my coils bad? Is a fuel line pinched?Did I miss something in going through the carbs?

Shot  gun approach will work but it wastes a lot of shot.

-Duck

[Lumbee]

the fact you said it won't idle makes me think the slow/idle jets are blocked. You said the carbs are rebuilt, did u rebuild them or are you trusing someone elses work?  If you cleaned them, did you remove the slow jets, and could you see light through them before you reinstalled them?  You need to be able to see the light my brotha!!!

[eldar]

Never said not to check the comp but comp in most cases will not cause a backfire. Poor timing and poor spark are the most common culprits. I went thru this same thing with my 78k. Had absolutely nothing to do with comp at all. It was a combo of timing, poorly adjusted points and float valves not working correctly. Besides, if your timing is off enough, it can cause incorrect readings when trying to test comp. That is why most of us agree that you start with cam chain and timing, then valves before doing comp at all. Also if you look, he says he is getting too much gas, now that is either too much fuel from the carbs or incorrect timing or poor spark and almost every case.

He does not have to sync before doing comp but he will have to anyways sooner or later, so he will still need the vacuum guages. He had the carbs rebuilt so we can assume that there were new seals and such put in. one thing he should find out is if there were different main jets installed. Stock on the 78F model is 105.

[eldar]

To add on to this, if the points are old, they should most certainly be dressed at the very least. If they are pitted, you mught be able to smooth them out but might just be better getting new ones. If they were new but sat, then dressing should be fine.

[scout18]

Seems to me that if it won't run off choke you need to see the light first. Then do all the other stuff as lined out by the master.

[mustang2003]

I put the air box back on, set the points, set the carbs, now she will run but she won't idle, and I got to keep feeding her gas, every once in a while she will backfire, she also sounds a little muffled when I rev her up, I think my one set of points are no good because even when I loosen them they won't touch together, so I guess they are worn, but Thanks for everyone's help, I greatly appreciate it, I guess my next step is to get a set of points, Any other suggestions, maybe why she sounds muffled?

[TwoTired]

Seems a common issue for the 77-78 bikes that have been sitting for awhile is plugged slow jets.  These are pressed into the carb bodies, and cleaning can be difficult and thorough cleaning nearly impossible.  The slow jet size is only 0.014 inch or so, it plugs easily with gum or debris, and unless you yank them out for cleaning, difficult to determine if they are completely clear.
 The plugged jet makes the idle mixture very lean, spit back through the carb and idles poorly.

Are you certain these jets are clear and clean?

Cheers,

[Duck]

It could be the muffler bearing cage. Most agree that this is a problem on these bikes. Especially if PO was ET. Accepted protocol is to send muffler bearing cages, sans bearings, to Jumpin Joe. Jumpin is the acknowledged expert on care, feeding, resurrection, and spell casting when it comes to muffler bearing cages for these particular model years. I would not trouble Jumpin Joe with parts from any other year. He has been known to hunt down and hack to bits with a cane knife, anyone who fails to securely pack the cages prior to shipping to his abode in Kigali. You've been warned. Turn around isn't too bad these days now that the Hutu's and Tutsis have thinned themselves out a bit. Hint: Include a fifth of Black & White for prompt service. Whatever you do, don't mention Nadia!

[eldar]

TT does bring up a good suggestion. I assumed that since you got rebuilt carbs, that they would be clean. But many never pull out the slows and clean them.

Well no matter what, poins can close. You have to adjust the points to correct gap so that when you purn the points cam, they will close. I would detail it but am not at home and so cant describe it.

Have gotten to download the manual yet? I guess there was pdf problem but it has been solved.
I would not worry about muffler cages though, I doubt it applies to your problem. I think thats more of a crisis problem.

[Duck]

Yep. Fractious Duck. Learned to save time with analytical vs shotgun approach and frustrated because he's not good at sharing. I suspect I've been as annoying as a reformed smoker at a smoking club. As you were.

[eldar]

Well, I dont know what your issue is buddy. I have a 78K, I had his issue. I can assure you that there is a bout a .05% chance of it being compression. Sure it may be a freak thing, but not likely.
You call it a shotgun but I call it making sure everything is working properly. You like to do as little as possible, fine, it is your bike and your ass.
the prefered method of all the experts here, I don't consider myself an expert yet, is to do a full tune up first if you do not know the maintenence history of the bike. It is a good base to start from and many times, problems can be solved just by doing this. Not only that, it helps keep the bike running in top form.

So you can keep chasing after your compression ghost, I will keep giving peoplethe same advice which has helped many times before.
Besides, tune-ups do not always require money. Condensers do not go bad very often on our bikes and points can last 4000 and sometimes 6000 miles with a couple of dressings.

So good day there,  little brother rubber.

[Duck]

Issue: an important question that is in dispute and must be settled

I got it the first time. 'Most agree with you'. I don't.

Please understand that not doing things that don't need to be done is not the same as not doing things that need to be done.

I'm suggesting to the guy with the problem that he might want to spend 5 min to learn what he has to work with before spending a lot of time and money trying 'this and that' because 'this and that' solved the problem for other people who's bikes exhibited similar symptoms. On thing that does not need to be done is spend a bunch of money and time 'chasing ghosts' on a bike that needs a top end overhaul.

Scenario.
Unknown bike. Unknown history. Not running right. What to do with it. Have already made preliminary check of gear changing and clutch by rolling it around or maybe it was running well enough for a short ride. Even better because handling can be checked.  It passed. What next.

Option one. Follow all major tune up steps and see if it runs. If not then follow diagnostic flow charts in manual. It's one path to success.
The major drawback to this approach comes when you get to the step about. 'check compression' and learn that the next step is 'tear down top end for inspection, replace out of spec components'. I can't afford to take this approach. I want to know if the bike is going to become a rider or sit in the 'waiting for top end parts' corner.

Option two. Take the plugs out for a look and check the compression. 5 minutes. If pass great. If no pass then find out why by checking wet. If wet says rings then tear down is required. If wet says valves then check clearances and timing chain for slop and correct if necessary. While checking clearances apply air pressure to each cylinder and listen for leaky valves.  Valve leak indicates teardown required. If no leak and compression is still no good then check the mechanical timing. Maybe someone had it apart before.

If it didn't get rolled over to the tear down aisle at one of the above steps then the next step is to pass fail the ignition. A $10 home made tool does the pass fail. Kettering ignition is pretty simple to diagnose and repair and there are no major expense surprises associated. Parts are usually required. So, pass or fail, the carbs get inspected next. If any carb parts are required then everything can be ordered at once.

I like option two because it saves time, money, and frustration. A lot of guys buy these old bikes because they want to ride and the bikes appear affordable. For someone who doesn't want to tear the top end down, paying someone else to do it will often cost more than the bike did. Better to sell it or keep it for parts and use the knowledge gained in running diagnostics to select a 'good' bike on the next go-around.

IMO it's a good idea to bring a compression gauge when inspecting a bike prior to purchase. For the guy who knows what else to look/listen for, low compression is a negotiating point. For the guy who doesn't, it's a sign that he might want to look at some other bikes. If bike has no kicker, bring a set of jumper cables and a charged motorcycle battery too. Some folks freak out when they see a car battery connected to a bike, especially theirs.

-Duck 

Mood: Fractious

[eldar]

So then what if comp is fine? What if you dont have a comp tester? What if you dont know anyone who does? Even that method still costs money. See mustang already HAS the bike. He is not checking it out to buy. If I was going to buy a bike, then sure I would test comp. But for an issue that is usually carb related(fouling plugs and backfiring) You start with the basics.

Are you timed correctly, Dont need a timing light for this. So no money spent there. Check points, dress first. Only requires sandpaper which almost everyone has around their house and if they dont, it is much cheaper than a comp tester and only requires a screwdriver to pull off the points cover. If you are timed and the points are clean and not all pitted, You move on. This step takes about 5 minutes, I did it on my lunch break and went for a ride, ON my lunch break AFTER eating lunch. $0 spent doing it.

Clean your plugs and make sure they are the right ones. Are they D8EA, which is the standard plug. Does the plug have an R in its designation? This indicates a resistor plug, which is incorrect and will most certainly foul. So clean plugs and run engine till hot and do a series of plug chops at different throttle ranges. Note which ones put more deposits on the plugs. Most likely, his idle circuit is not working correctly. He replaced the accel pump but is the actuating arm adjusted correctly? If not, fuel will not be fed to the carbs correctly.

How are the low speed mix screw adjusted? Are they turned out more than 2 turns? If so then in most cases, that is too rich.

All comp checks for is comp, nothing else. Besides that, You will get different readings if your engine is warm or cold. SO to do comp CORRECTLY, you need to take readings on ALL 4 cylinders dry, then with a tablespoon of oil in each, COLD.
Then you need to do the EXACT same thing with the engine warmed up. When the engine is warmed, the metal is going to have expanded and reading will be different. So to do all that basically 4 times, requires much more than 5 minutes, Yo uare looking at about an hour.

Unfortunatly, mustang did not say if his engine behaved this way through-out the rpm range or just idle.

Not only that, but spark is DEPENDENT ON TIMING AND DWELL!!! NOT the other way.

Yes the kettering is simple and reliable and it is POINT and DWELL dependent.

Final point on this is that if you buy one of these bikes, unless you find a rare jem that needs no work(very few) you WILL have to do a tune up anyways.

Not only this, but I had the EXACT same issue mustang is having. FROM EXPERIENCE, it is 80% likely to be the carbs, 19% something with ignition, 1% with comp.

Sorry but that is just the way it is. Remember this question was about a bike already in ownership, NOT being looked at.

Not like mustang is still looking at this. He has probably lost hope on this thread. Sorry stang, ask again and I will leave it and see what others come up with. Doubt it will be comp, imo.