Author Topic: My 1978 Cb750k is almost complete and then..... (Read 4714 times)

bollingball · « **Reply #25 on:** June 09, 2013, 01:01:38 PM »

Cam I have not read every post but have you tried the clear tube fuel level check on your #2?
Ken

flybox1 · « **Reply #26 on:** June 09, 2013, 01:15:19 PM »

ID =inside diameter, and you'd have to measure it....so we might suggets baffle inserts

Pull your carbs and set Ll to14.5mm, or right to the the same level as the tops of your pilot jets.

Cam · « **Reply #27 on:** June 10, 2013, 06:13:27 PM »

Alright guys! (and girls don't mean to offend) the carb leak has been solved! The float in #2 was a touch off and I flipped the gasket over per cheftuskey121 and no more leaks.

So I'm ready to hear this thing fire for the first time since I've owned it (2 years ago and it sat in a barn for all but the last 2 months) and nothing. I checked my fuel like to make sure gas was coming in and my fuel filter showed plenty of fuel. I even turned a drain screw to see if fuel was still getting down to the bowls and it is. So I sprayed a little help in the carbs, closed the butterflies and......nothing. It's giving me some noise from the fluid but once that burns off I'm back to nothing. I'm attaching a video of my endeavor so I hope it works! Help!!

Also, when I rebuilt my carbs I completely tore them down and tanked them. Sprayed carb cleaner through every passage I could find to be sure they were not blocked, and shined light through anywhere I'd be able to see light through. One idle jet put up a hell of a fight and the way I cleared it was a funny story but that's for another time.

78 cb750 first try start

Cam · « **Reply #28 on:** June 11, 2013, 05:57:19 AM »

Another question i have is I know the port on the back side of the engine above the carbs is for the oil line from the separator. What is the port under the carbs?

flybox1 · « **Reply #29 on:** June 11, 2013, 06:43:03 AM »

for starters, i bet you have a weak battery - tell us the voltage with the bike off, and with the key in the ON position.
fast idle cam/choke is not assisting with your start. tell us how your fast idle cam engages, and what it does.
accel pump circuit not working (helps if you twist throttle 2-3 times to prime the cylinders with fuel) does yours do this?

did you set timing and advance.
do you have known good plugs, caps, and plug wires.
have you set your valves.

Cam · « **Reply #30 on:** June 11, 2013, 05:29:02 PM »

She lives! Came home from work and made sure the accel pump was working and with a few turns of the throttle we had power. It's in need of some tuning but after a 2 year trip it runs. I'm letting the battery charge for the night and giving it a shot again tomorrow.

Thanks for all the help everyone!

btw Flybox I took a quick measurement of the ID of my header collector and I'm showing 2 and 3/8 inches. I assumed it would be 2.5. I'm all for some type of insert to create back pressure and help improve the engine's performance but I'd also like to keep it as obnoxious as possible

Any suggestions are more than welcome!

cheftuskey121 · « **Reply #31 on:** June 11, 2013, 05:48:38 PM »

congrats man. what a great feeling!

flybox1 · « **Reply #32 on:** June 11, 2013, 06:08:24 PM »

Search DB Killer on eBay. It's a UK part. An insert. I had one and liked it, but switched to the cone engineering slip on. I think it was about $40....

Stev-o · « **Reply #33 on:** June 11, 2013, 07:00:22 PM »

+1 on the Cone, looks great on Flys bike.

Cam · « **Reply #34 on:** June 11, 2013, 07:10:00 PM »

I will look into that Flybox thanks for the tip!

Anyone had problems with petcocks leaking from the bowl collector whatever you call it? I have a '74 tank on my bike and I bought a brand new petcock from J&P Cycles I believe. When I checked my bike earlier it appeared as though a small amount of fuel is coming out where that bowl screws in but I'm almost positive it isn't cross threaded. The screws they sent me weren't the right length and I had to reuse my old ones. I'm starting to question the integrity of the part in general!

Has anyone else had issues with petcocks from them?

flybox1 · « **Reply #35 on:** June 12, 2013, 08:32:46 AM »

your bowl might just need to be refitted.

http://www.ebay.com/itm/BAFFLE-DB-KILLER-TO-SUIT-2-5-60mm-STRAIGHT-EXHAUST-CAN-/270608502457

enough back pressure that i could run the same setup i have in my signature. a bit louder than my coneeng slip-on.
good noise reduction at low rpm's. pretty loud as you get on it.
good seller. they shipped right away.

lucky · « **Reply #36 on:** June 12, 2013, 08:45:55 AM »

Main jets?
First you need to correct that float problem and If I was you I would buy 4 new float needles and seats. Probably 40 year old parts?

To help you with main jets we would need to know what exhaust and intake set up you have.

lucky · « **Reply #37 on:** June 12, 2013, 08:48:22 AM »

Main jets?
First you need to correct that float problem and If I was you I would buy 4 new float needles and seats. Probably 40 year old parts?

To help you with main jets we would need to know what exhaust and intake set up you have.

lucky · « **Reply #38 on:** June 12, 2013, 09:19:12 AM »

TIP for you 1977-78 OWNERS.

When setting floats it just so happens that on this model you will notice this is what lines up when the floats are set correctly.
See photo.

If you do not believe me check it out yourself. It has been approved by this site TIPS section.

flybox1 · « **Reply #39 on:** June 12, 2013, 12:22:06 PM »

well...if its approved in the TIPS section

...i though we couldnt use it until it was 'Kid tested. Mother approved"

Cam · « **Reply #40 on:** June 12, 2013, 08:28:44 PM »

Thanks Lucky my carb issues seem to all be solved (for the time being). I had purchased all new floats, needles, gaskets, o rings, accel pump, etc. The one leak I had seems to be solved by flipping the gasket over on the #2 bowl. I'm only having one fuel leak issue now. The petcock.

So today I tore the petcock completely apart to check everything and put it all back together. It still leaks. I was told by a couple co workers today (I'm a tech at a car dealership) that teflon tape will hold up to fuel. I've never tried it before and thought maybe it wouldn't hold up against fuel. So I tried putting some on the threads of the bowl on the petcock. Still leaks. Another guy I work with who has built drag bikes and seems decently knowledgeable about bikes suggested if the bowl on the petcock is leaking use grey gasket sealer. He claimed he has before on fuel system components and had luck. I'm weary of that idea knowing what a pain that stuff can be to mess with. This is an Alfa petcock I purchased from Vintagecb750.com btw. So while I'm messing with the petcock I realize that even when the petcock is set to "stop" fuel still flows out of one of the valves. This didn't make sense to me so I tried an old Honda petcock I have laying around. It also pours fuel in the "stop" position.

Can anyone explain to me the mysteries of these petcocks as I'm clearly in the dark on their functions. Ideas on how to stop my leaking is also greatly appreciated.

flybox1 · « **Reply #41 on:** June 13, 2013, 08:05:54 AM »

honda did not use teflon tape or gasket sealer. you shouldnt need to either. keep working on it.
it might just be that its one that needs to be returned to the seller.

post up a picture of it and where you think the leak is coming from.

Cam · « **Reply #42 on:** June 13, 2013, 04:16:45 PM »

Petcock was leaking from the bowl. I tried taking it off and putting it back together over and over with no change. After inspecting I just think the cheap metal was leaking at the threads. I just said the hell with it and I'm sending the cheap Alfa p.o.s back and today I purchased a genuine Honda petcock.

For those in need of a new petcock I advise spending the extra on a good one. The Honda one I purchased today was only $15 more than the Alfa and believe me it's worth the money to not have to deal with an annoying leaky petcock!

Cam · « **Reply #43 on:** June 18, 2013, 05:45:13 PM »

Yesterday the new Honda petcock arrived and it doesn't leak a drop. So relieved to be over that hurdle of fuel on my garage floor. So now to new problems! haha I started the bike and heard a humming but it was hard to detect with how loud my bike is. And since I'm still waiting on the carb sync tool I bought, I decided to put on the mac muffler and air filters just to quiet it down and hear better and read into if the bike ran differently with these changes. There is certainly a hum to be heard as if cylinders aren't firing.

So first I checked the headers. #1 and #4 are cool to the touch even after a couple minutes of running. #3 gets pretty hot and #2 gets extremely hot. I pulled the plugs and found that all had a little color to them (slightly blacked with a brownish tint but not too black) but #2 was excessively sooty with the tip white. I assume a lot of this can contribute to the carbs not being dialed in yet but considering the correlation on cylinders and how I believe #1 and #4 fire together it makes me question electrical concerns. I popped the ignition cover off and found a Dyna S ignition. I've never messed with these before so I decided to post here first for diagnosis ideas on my issue. My bike will also not idle and will only start on full choke. I'm assuming my problems are carb related from them not being better synced yet but I know the best way to diagnose is start with electrical and do carbs last. So any advice on a pecking order for figuring out my problems? And if only two were firing and I do have an electrical problem, would that also cause the bike to not want to idle?

Also, as mentioned before I completely tanked and rebuilt my carbs and the bike has brand new NGK D8EA plugs.

scottly · « **Reply #44 on:** June 18, 2013, 09:50:23 PM »

Quote from: bollingball on June 09, 2013, 01:01:38 PM

Cam I have not read every post but have you tried the clear tube fuel level check on your #2?
Ken

After seeing the plugs, he needs to do the clear tube test on all four carbs.

Cam · « **Reply #45 on:** June 19, 2013, 04:50:45 PM »

Well today I came home and checked the plugs to see if they were arcing. As I suspected #1 and #4 weren't, #2 and #3 were. So I switched the coils and found the coil was bad. When they were switched #1 and #4 arced and #2 and #3 didn't. So I bought new coils. Once they get here I'm gonna try to really dial in the carbs and hopefully I can get this thing on the street!

iron_worker · « **Reply #46 on:** June 20, 2013, 10:52:19 AM »

You may have jumped the gun on the coil purchase... How did you "switch" the coils?

If you switched the points wires around and the problem changed from 2-3 over to 1-4 then that means your DYNA is not functioning correctly...

If you switched the points wires around and the 2-3 plugs still did not fire THEN you can assume it's in the coil/leads/caps and not the points.

If you look around here about reviews on DYNA ignitions there has been many people how have struggled with them dying and the bike only runs on 2 cylinders.

IW

Cam · « **Reply #47 on:** June 27, 2013, 10:44:25 AM »

Alright I probably jumped the gun on the coils but oh well knowing it's new and genuine Honda isn't the end of the world. I replaced the coils and the problem continues. So I know I have new plugs, new plug wires, and now new coils. I know nothing about Dyna S Ignitions and the previous owner installed it so any help on troubleshooting the ignition and my issue is greatly appreciated.

Again the bike is firing on #2 and #3 but not #1 or #4. I have two sets of plugs I've been checking them with to be sure it't not a plug and they both wont fire on those wires.

flybox1 · « **Reply #48 on:** June 27, 2013, 10:55:47 AM »

new caps installed, too?
get a $20 used OEM points plate off feebay and be up and running in 20 minutes

Cam · « **Reply #49 on:** June 27, 2013, 11:40:40 AM »

http://www.ebay.com/itm/1968-78-honda-cb750-cb-750-k0-k1-k2-k3-k4-k5-k6-CONTACT-POINTS-CONDENSER-PLATE-/121126620014?pt=Motorcycles_Parts_Accessories&hash=item1c33b58b6e&vxp=mtr

Flybox this you say? Is my problem really this easily solved?

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Author Topic: My 1978 Cb750k is almost complete and then..... (Read 4714 times)

bollingball