Bike problems...

[Phatty5BMW]

Hey guys/gals, I have a question for ya......

Does anyone know anything about old hondas? I got a 81 CB750c I need guidence on and the shop won't do squat for less than 900$ lol. It's mostly carb issues. If anyone could help I would be greatly appreciated :)

Thanks,
Pat

[Barney Paull-Edwards]

If they are Mikuni`s, take them to bits, clean old petrol out, build carefully and balance with 4 pipe Davia balancer, remember not to lock up connectors until balanced! 9 times out of 10 problems are dirt or old petrol due to lack of use.

[Barney Paull-Edwards]

Oh, I forgot, then attach a chain to frame above engine, throw into lake and voila, one good mooring.

[Phatty5BMW]

Oh, I forgot, then attach a chain to frame above engine, throw into lake and voila, one good mooring.

Naw they are the kei-hin or something. I'm kinda pissed now, I bought another set of carbs off the bay, they have different part numbers... The ending digits on my original end it SI the others end in RE...

I couldnt check what mine were because the bike was being held hostage for more than its worth lol.. "if i only had a float...."

-Pat

[DaveVoorhis]

Whether Keihin or Mikuni carbs, the process is the same: Remove carbs, disassemble completely (keeping each carb's bits separate from the others), throw away all the rubber bits, clean thoroughly with strong carb cleaner until they're shiny inside like new, replace the jets because they'll be clogged with gunge, install all new rubber bits and gaskets, then reassemble and adjust the float levels according to the shop manual or (as a last restort) a Chiltons or Haynes manual. And replace the intake boots -- they're probably cracked, and de-rust the fuel tank and replace the filter screen. Check the compression, too -- you may have sticking or burned valves.

[DaveVoorhis]

Ah yes: Synchronise the carbs when you're done using a set of Carb Stix.

[Phatty5BMW]

Well original carbs has a cracked float. Float levels are non-adjustable. Not sure about taking float off new carbs and putting on old.

Problem with old, wouldnt rev past 4k, took apart cleaned, sat 4 years....putting those carbs on is a bitch..... 1 intake boot is cracked, but that shouldnt cause the bike to not start at all.

New carbs were off of a running bike. Carb 3 is a little different as its missing what looks like to be a choke needle or something but it has a nipple on the engine side of the carb. The original carbs have this.

What I am not sure of is whether the float itself is interchangeable with the original carbs being the float level is non-adjustable.

From all the information I can find, the years of production of the bike 79-82 from what I understand is that they are all teh same carbs.

The fuel distributor that feeds from the tank, has that extra tube on it, as the new carbs installed has no where to hook too. I don't know wtf to do...

Sorry for bugging you about this but its bugging the hell outta me and i know you guys are very resourceful :)

Thanks
-pat

[DaveVoorhis]

It's been over twenty years since I worked on Honda in-line fours, and almost ten years since I've owned one, so I'm really stretching my dodgy memory here -- you might be better off checking on a Honda forum. Anyway... As I recall, Carb 3 is the reference carb for synching the carbs, so it won't have an adjuster. The others are adjusted to match it. The extra tube on the tank fixture should be for the vacuum-operated petcock valve -- there should be a nipple on the intake (possibly on Carb 3???) where it connects. If the bike's not starting, it could be because there's no vacuum to operate the petcock.

To check for float interchangeability, simply eyeball it: If it looks identical, it is. Are you sure they're not adjustable? There's usually a tang that can be bent to adjust them.

If it's not starting at all, check the usual suspects: Verify you've got spark and pull a plug to verify fuel delivery to the cylinder(s). Old, nasty carbs and a rusty tank can be so plugged up that no fuel gets through, but I'd suspect the petcock first.

[whiskychaser]

Well original carbs has a cracked float. Float levels are non-adjustable. Not sure about taking float off new carbs and putting on old.

Problem with old, wouldnt rev past 4k, took apart cleaned, sat 4 years....putting those carbs on is a bitch..... 1 intake boot is cracked, but that shouldnt cause the bike to not start at all.


Thanks
-pat
So your problem is 1. You got a cracked float 2. Wont rev over 4k 3. Wont start now? Got a couple of mates run a bike shop for years. FFS if you only need a float and a bit of info no promises but will see what I can do

[Phatty5BMW]

Ok, Well pulled the carbs apart. Same float. I changed the floats over, made sure all my floats work correctly. Carb #2 is the primary carb. I checked all the bowls, carb 2 seems to have the most fuel and carb 4 has the least. I am going to pull them off again and blow everything out again. I am going to clean the plugs, they get spark but hey its a possiblitly.

The one thing i am not sure of is that automatic fuel cock. Its got 2 main gas and 2 vaccum. Kind of stumped, i know the one obviously goes to the tank and the carbs, but the vacuum. air only blows one way. so would the air flow towards the plug on carb 2 or the main rail in between the carbs.

If it wasn't 4 years since i took this apart, I could remember.

Since i have been cranking this thing over and over, out of all 4 exhaust tubes, seems as if cyl 2 is getting hot and not the others. I primed cyl 4 and it fired for a split second and exhaust came out so i know there is spark atleast.

I think it is defenitly a fuel issue, I am kind of leaning towards the fuel rail clogged or this fuel diaphram just isnt working (or i hooked it up wrong)

I have tried honda forums too by the way. When looking for forums you just can't find a place like this anywhere... not even on google.. all others is non-useful junk.. This has to be the best board on the net for sure.

Thanks for the help guys!!!!!
-Pat

[Phatty5BMW]

Ok, hot wired the fuel rail (just hooked up vacuums) supplied the tank straight to carbs and walaa... a bike running on cyl's 1&2. Pulled carbs, double checked all float needles, all other jets n everything is clean.

Put the carbs back on, choke cable stuck on me but I got 1-2-4 cyl's running. Cyl # 3 seems to not be firing... I played with the low speed idle screw which sets all the carbs, Now i can't even get the bike to start up again, i got to pull the carbs off and re-attach this choke.

I'm kind of dumb founded now. Before pulling the carbs off this last time, we dumped a bunch of sea-foam in the carb fuel intake and ran better but still didn't do what I wanted it to do... Still running like **** if it even starts at all.
Acts like its going to, then doesnt, then and/or backfires blowing nice flames out of the exhaust :)

Any insights would be great guys!!!

Thanks again
-Pat

[DaveVoorhis]

Assuming the carbs are clean, in working order and are correctly adjusted etc., and you're getting proper fuel flow to all of them, and they're not so badly synched that some are delivering fuel/air mixture and some aren't, then check for spark on all four pots. You might have a bad coil(s), plug wires, or spark plugs. Check compression too, in case you've got sticky valves.

[Phatty5BMW]

Since taking the carbs off to double check all jets and everything is clean, the only adjustment is on the bottom of the carbs. I belive this is the low idle adjustment's per carb then the other big idle adjustment runs all 4. From what the repair manual says, is tighten then back out 1 1/4 to 1 3/4 turns. I did this on all carbs and seems like not a difference.

I haven't checked the resistance on the coil wires, but the looks of it, they are getting plenty of spark at the plugs. After putting the carbs on for the last time, i had the 3 cyl's running, i had it running smoothly till 9k rpm, but the backfire and hesitation didn't sound good.

Seems like when it starts, if i can get it to start, won't idle below 3k and wont rev above 6 with the popping and sputtering and flames rolling out the exhaust lol.

Oh ya, havent check for compression, I don't have a tester. Bike has 13000 miles and basicly been sitting since 1992. So yes its possible but the cyl's that weren't running were intermittingly firing, say 1 fire per 5-10 strokes?

I just don't want to take the bike in and spend more than its worth. I don't like to pay people for something I can do myself. I belive i can do everything but sync these carbs, and maybe that is just my main problem. I just can not find any information on anything about these issues at all. Its either they run and run like **** or they dont run at all, But when they do run they run like the m30..... Thanks again!!!

-Pat

[bsell]

Is this a points system or electronically fired ignition? Have you checked your ignition timing yet?

Have you checked your altenator/battery to ensure the ignition system is getting 14 volts? If your coil is running undervoltage this can make any minor fuel mixture wanderings affect the engine's running much worse.

I ask the above as the OP, nor anyone else, has brought up the basics yet so I figured I would.:D

Have you been using the bike's starter motor to start this thing? When the motor turns over but doesn't fire, how even does the compression sound? While it would be nice to know your compression readings, as long as the motor turns over evenly, you can assume an equally worn engine which shouldn't greatly impact the starting/runability. If it turns over lumpily, then you can assume valve or compression problems.

Brian

[DaveVoorhis]

Honda fours of that era are electronic ignition. It's possible there's a bad ignition sensor, but the ignition systems on those bikes were pretty reliable. On a bike that's sat since '92, carburetor troubles, fuel delivery problems, and sticky valves are likely. However, it sounds like it stopped being ridden because of problems at the time, so check everything.

And, yes, check your battery too. I've been assuming you've been powering the bike off a car battery, booster, or equivalent. If you're spinning the starter on a bike battery it'll lose energy quickly, to the point that the starter can still turn but won't power the ignition sufficiently to start the engine.

And buy or borrow a compression tester. I suspect (at least!) sticky valves in a head that hasn't seen a drop of oil in fifteen years, and then was subject to the full brutality of a start attempt. If it were my bike, I'd now be going over everything with a fine-tooth comb. Bikes often don't do well for sitting over one winter; they don't get any better sitting through fifteen of them.

[Phatty5BMW]

The battery on the bike is shot, haven't got a new one yet. I am using a battery charger that has jump feature, once i get the bike runnin i switch it over to 60 amp charge and it will read over 14 volts, if and when it starts.
Defenitly looses juice around 3k rpm. When the bike did run it would go from
3k rpm to about 7rpm with cyl's 3 and 4 firing intermittingly.

As for the sticky valve, how would you unstick them? Seems to be turning over evenly, compression sounds good while starting. I know the exhaust valve is functional as it blows flames out on all pipes. If it blows flames its getting fuel from intake valve...

As for the ignition. Havne't checked anything yet on there. I got 4 cyl's with spark so I haven't messed with it yet. I will have to check for that next.

Thanks for the help guys,

-Pat

[DaveVoorhis]

It'd be better to have a good battery in place. The battery doesn't just supply current for the starter. It also helps to filter out momentary variations in current. A booster-charger supplies DC obtained by rectifying 60hz AC with no filtering at all, i.e., it's *all* variations in current. I don't know how the electronic ignition will react to that, but it could be badly. I'd recommend hooking up a well-charged car battery while you're working on it.

A pair of cylinders firing intermittently, while the other pair runs fine, is usually an indication of a bad coil.

[632 Regal]

Dave is right, you need a good battery. when I ran bikes I would foul the NGK plugs like clockwork, the intermittant miss, everything you describe. I changed to champion plugs and never fouled again. Just an idea, your real close.

[bsell]

Dave is right, you need a good battery. when I ran bikes I would foul the NGK plugs like clockwork, the intermittant miss, everything you describe. I changed to champion plugs and never fouled again. Just an idea, your real close.

Go figure, we had Subarus come in with poor running conditions (within the first 5-10K miles) and they came from the factory with Champoin spark plugs. The factory fix was installing NGKs and that took care of the problem.

I don't know what it is, but some engines really love a certain brand of spark plug...

Brian