Topic: CB750 K7 Still Doesn't Start (New Ignition/Carb Overhaul)

[AintNoEasyWay]

Having trouble bringing my bike to life. It's a CB750 K7 with PD41a carbs & 4 into 1 exhaust.

The bike won’t hold an idle. You can hear it try to start; it wants it. Yesterday my neighbor (a car mechanic & motorcycle rider) had it holding for about two seconds before it’d give out.

I know the ignition is set up correctly, new coils with great spark, new PAMCO electronic ignition with proper wiring (double checked yesterday). I haven’t checked compression, but the kickstart clearly has some. As far as fuel, carbs are clean with all new kits, including gaskets, O-rings, floats, jets & needles (but the jetting is for stock exhaust). Fuel is flowing fine from the tank. I drained the float bowl on carb #4 yesterday & got nothing but clean gas out of it. I de-rusted the tank during the carb overhaul. I cleaned out the airbox & replaced the filter.

Some things I can think of that may be issues (many of which I don't think would give the bike trouble starting, just keep it from running right):

1) I didn’t clean & fully inspect the accelerator pump on carb #2. I opted to just spray a good amount of carb cleaner down into the ball valve. Or maybe the accelerator pump is clean, but not set up correctly?

3) Maybe air screws are incorrect. I tried playing with them after originally setting them 1.5 turns out of their seats. My neighbor recommended trying .5 & even .25 turns out instead. I never tried anything greater than 1.5 turns out though.

4) Maybe float levels are incorrect? I can’t remember if I set them to 14.5 or 12.5 mm — the shop manual suggested one while the Clymer manual suggested the other. It’s one of the two though, so I doubt this is the issue.

5) Maybe the slides in the carbs are not set at the right height. I didn’t base their height on anything other than eyeing a photo of another set online. There not completely closed, but they’re not far from it.

6) Felt washers in the carbs may have melted off in Berryman's carb dip (I only put each carb in for an hour at most). Not sure if this would have that much of a difference; seems like some guys on the forum believe it’s a huge deal while others have run bikes easily without them. If mine got fried, it’s going to be a pain to try to make my own (they are not available anywhere as far as I know).

7) Maybe I need to re-jet for my 4 into 1 exhaust?

8 ) Something has lived in my exhaust pipe. Read where some guys tried everything before finally taking off exhaust & seeing some creature had made a nest inside. Once clean, they're bikes started fine.

Any thoughts? Wanted to get opinions before I take off the carbs again to recheck everything in there.

[Ericcb750]

Hey what's up Aint No,  I just recently finished a complete rebuild of my 78 CB750k and I was having the exact same problems when I first tried to start it. Just like yours it wanted to start I was getting spark and even now and then it would put a few seconds and then die. I pulled the carbs and cleaned them Multiple times . What I eneded up checking as the last thing (which I probably should have done earlier) is that the carb floats are set correctly. Mine were just a bit too short not allowing the pilot jets to suck up gas at idle. Once I did this It fired right up and would hold an idle. I'm still in the process of syncing the carbs as I just got my gauges last week.  I have the PD42 carbs so I don't know if they're different but I tell you that made a huge difference for me. I'm not expert and it's my first Cb750 but, Hopefully it helps. Good Luck man.

[AintNoEasyWay]

Thanks for the input man. Sounds like you had the same trouble (& this is my first CB as well). I believe the carbs are very similar. Do you remember what setting you used for your floats?

[LesterPiglet]

Rule out different jets for the exhaust, your bike will run even without headers on it. Air screws make a big difference. Dont' be scared about adjusting them an extra bit. I've turned mine out 1 turn more than recommended when fiddling. It may not help but it won't break your bike.

[lucky]

Have you checked to see if the accelerator pump spray nozzles are operating?
Take a small LED flashlight and look into the intake of each carb while you twist the throttle(ENGINE OFF).

You should see a small squirt of gas.
Also on the float levels.... See my photo that is posted here.
Just a coincidence!
When the float is set right it will be even with the top of the idle jet on the K7.

Are you using the stock air box with stock air filter? If not then it WILL need rejetting.
The slide needles would also need to be raised. #120 main jets.

If it almost starts but won't on this K7 a squirt of starter fluid will make it run for a minute or so.
But you will still need to remove the carbs and get these problems solved.

[AintNoEasyWay]

Thanks, Lucky. I actually remember coming across one of the original threads you posted that photo in & set the float accordingly. The problem I think that could come from just following that method is that the floats may sit in the bowls properly, matched up against the idle jet, but they may still hang at varying lengths. Maybe this is my issue.

I am using stock airbox with stock air filter.

I used a high E string to clean my idle jets, but very carefully. I don't think I did any damage to them (they are the only remaining original part of carb jetting/needles). If I did damage them despite my care, would this keep the bike from starting?

Thank you.

[Tetter]

My nephew had the same issue.... he mixed up the ignition wires...
1-4 was firing when 2-3 should and vise versa.

also sounded like it was going to start, but just didn't...
Worth the check 

[AintNoEasyWay]

My nephew had the same issue.... he mixed up the ignition wires...
1-4 was firing when 2-3 should and vise versa.

also sounded like it was going to start, but just didn't...
Worth the check 

Thanks Tetter. A friend recommended this to me today. I checked all the ignition wiring yesterday, but I didn't check the spark plug wires. I knew the original point system wasn't working when I bought the bike, so I went ahead & replaced it with PAMCO. It still wouldn't run, so I moved to cleaning the carbs (they definitely needed it).

So that said, it's been a while since I set up all of the ignition, & it's certainly worth checking. Would love if the problem was that easy of a fix!

[750K]

Did the bike run at all before you replaced your ignition and rebuilt the carbs?

Did you actually pull the press in idle jets or leave them in place while you cleaned the carb bodies? You need to pull them and clean them seperatly, don't just think you got the carbs clean. Clean them right the first time and make sure all the passage ways are clean, dipping is good but did you spray carb cleaner through the passageways and blow out with a compressor? Nothing sucks more than putting it all back together on the bike only to have to re clean it all.

Make sure the accelerator pump actually works, you should be able to see a strong squirt of gas into the carb throat from the little brass nozzles.

Did you do a proper bench sync and check and set up your fast idle cam?

Set the floats to factory spec, then with the carbs back together you need to verify float levels with the clear tube method. All of the above can make it a #$%* to start cold and idle. You most likely will have to play with jetting to account for the non stock exhaust. I did on my 77 cb750k and I'm running stock airbox and filter, stock ignition with a 4-1 and I went up in size on both idle and main jets. As well as raising the needle by dropping the needle clip a position.

I'm pretty sure this was my last carb set up, I've been keeping notes as I tune. I'm pretty bang on but need to tune out a mid range burp around 1/2 to 3/4 throttle.
PD42b bodies with PD42a internals
#40 pilot jets
#120 main jets
Needle clip in #3 position
Mixture screw 1 7/8 turns out
Floats set and checked with the clear tube

Once clean and set up a set of pd's is a joy to cold start a bike, twist the throttle twice to get the accelerator pump spraying, pull the choke and it should fire up and hold an idle around 2500rpm if the fast idle cam is set proper.
 
I'd also verify spark at the plug and once you get it to run make sure all four header tuber are getting hot.

[AintNoEasyWay]

Did the bike run at all before you replaced your ignition and rebuilt the carbs?

Did you actuall pull the press in idle jets or leave them in place while you cleaned the carb bodies? You need to pull them and clean them seperatly.

Make sure the accelerator pump actually works, you should be able to see a strong squirt of gas into the carb throat from the little brass nozzles.

Did you do a proper bench sync and check and set up your fast idle cam?

Set the floats to factory spec, then with the carbs back together you need to verify float levels with the clear tube method. All of the above can make it a #$%* to start cold and idle. You most likely will have to play with jetting to account for the non stock exhaust. I did on my 77 cb750k and I'm running stock airbox and filter, stock ignition with a 4-1 and I went up in size on both idle and main jets. As well as raising the needle by dropping the needle clip a position.

I'd also verify spark at the plug and once you get it to run make sure all four header tuber are getting hot.

I pulled the press in idle jets & cleaned them up until all holes were clear. I plan to check the accelerator pump this evening. Didn't do a bench sync. I'll search for what you mean by clear tube method. I just used a caliper to set the floats. I'm not sure how to set up the fast idle cam, what does that entail?

[750K]

Fast idle cam is easy, it's in your manual or clymer will cover it. You can see the adjuster nut between the #2 & 3 carb body, when set up right the choke mech will hit it when you pull the choke out. It raises the slides at cold start, on the older model carbs you did this manually by twisting the throttle and using the throttle lock screw.

[AintNoEasyWay]

Fast idle cam is easy, it's in your manual or clymer will cover it. You can see the adjuster nut between the #2 & 3 carb body, when set up right the choke mech will hit it when you pull the choke out. It raises the slides at cold start, on the older model carbs you did this manually by twisting the throttle and using the throttle lock screw.

Great. I think that's set correctly but I will re-check.

[flybox1]

Caliper setting of the floats will be fine. do a bench sync before those carbs go back on your bike.
if your slide heights are way different between each carb, you wont start.
and get on that accel pump.  a few squirts will really help with your starts.

[Roach Carver]

If this hasn't already been said, switch the wires for 1-4 and 2-3 coming out of the pamco. Its easy to mix up and IIRC they require trial and error. That's what happened to me.

[AintNoEasyWay]

If this hasn't already been said, switch the wires for 1-4 and 2-3 coming out of the pamco. Its easy to mix up and IIRC they require trial and error. That's what happened to me.

Switched these wires on Saturday. Wouldn't rev or even sound like it wanted to start before I switched them. Now It at least sounds like it's really trying to start. Careless beginner error, which is also why I'm wondering if something as simple as what Tetter recommended, wrong spark plug wiring, is the culprit.

Also, I don't know exactly how to do a bench sync, but I set the slides to all match the slide in carb #2. I thought I read somewhere on here that was the proper setting for the slides since you can't adjust slide #2?

[Tetter]

how about compression ?
do you have it ?

If not, or little has the valve timing and clearance been checked ?

[madmtnmotors]

Switched these wires on Saturday. Wouldn't rev or even sound like it wanted to start before I switched them. Now It at least sounds like it's really trying to start. Careless beginner error, which is also why I'm wondering if something as simple as what Tetter recommended, wrong spark plug wiring, is the culprit.

This is something I do not recommend doing by ear. Hook up a timing light and VERIFY that 1-4 fires on the 1-4 marks and that 2-3 fires on the 2-3 marks...

You need to know for sure before moving on to the next potential culprit.

[750K]

If this hasn't already been said, switch the wires for 1-4 and 2-3 coming out of the pamco. Its easy to mix up and IIRC they require trial and error. That's what happened to me.


Also, I don't know exactly how to do a bench sync, but I set the slides to all match the slide in carb #2. I thought I read somewhere on here that was the proper setting for the slides since you can't adjust slide #2?

Put a very small drill bit or welding rod wire under #2 slide, lower the 2 slide till you feel just the smallest bit if drag when moving the wire etc in and out. Now repeat with 1,3,4 slides, it's like setting valves with a feeler guage.more than close enough to get you in the ballpark for running. Make sure 1,3,4 slides are higher than the 2 slide before you bottom out the 2 over the wire or drill bit, if any of the other slides are lower you can't drop the 2 slide far enough to touch the dril bit or welding rod.

[AintNoEasyWay]

Switched these wires on Saturday. Wouldn't rev or even sound like it wanted to start before I switched them. Now It at least sounds like it's really trying to start. Careless beginner error, which is also why I'm wondering if something as simple as what Tetter recommended, wrong spark plug wiring, is the culprit.

This is something I do not recommend doing by ear. Hook up a timing light and VERIFY that 1-4 fires on the 1-4 marks and that 2-3 fires on the 2-3 marks...

You need to know for sure before moving on to the next potential culprit.

Thanks madmtn. We used a timing light to make sure the PAMCO ignition was set up properly.

[Ericcb750]

Hey Man, sorry been a crazy day. I actually used the method that Lucky posted a picture of. I used a straight edge (i don't have a tiny caliper/measuring) I just made sure that when all the pressure was off the float valves that the pilot jets would be submerged. The accel pump probably would help too. I had issues with mine, There should be an o'ring on the #2 bowl between the bowl and the main carb where the fuel goes up to the accel pump circuit (sorry probably confusing), I think that is a big part of making sure it works properly. If you give it a blip of the throttle just makes sure all 4 squirt gas out.  Also when installing the carbs I put a little WD-40 around the throats of the rubber boots and the carb throat, that helped get them all the way in. THey almost clicked in. I hope this helps man, Let us know how it turns out. Like I said don't take my word as gospel as this was my first but i took those darn things off so many times I could probably write a book about em'. Take it easy.

[AintNoEasyWay]

Did the bike run at all before you replaced your ignition and rebuilt the carbs?

Sorry I didn't see this question. No the bike was not running when I bought it. PO acknowledged that the points weren't working, that was it. He had left the bike sitting in a garage for three yrs while out of country. Tank had to be de-rusted & carbs needed a complete rebuild.

[Spanner 1]

The correct float level for your PD 41A carbs is 12.5 mm, not 14.5 mm. If set at 14.5 mm then the float bowl will not fill with enough fuel to reach the slow jet = no start. You must hold the carb sideways with the float pivot pin horizontal and topside to measure. The 'tab' on the float must just touch the valve but not compress the valve spring any. Make sure that the valve is indeed seated ( closed)...... can not measure with the carb upside-down   ...... also make sure the floats are not installed wrong way up ( lots of us have done that too  )..

[AintNoEasyWay]

The correct float level for your PD 41A carbs is 12.5 mm, not 14.5 mm. If set at 14.5 mm then the float bowl will not fill with enough fuel to reach the slow jet = no start. You must hold the carb sideways with the float pivot pin horizontal and topside to measure. The 'tab' on the float must just touch the valve but not compress the valve spring any. Make sure that the valve is indeed seated ( closed)...... can not measure with the carb upside-down   ...... also make sure the floats are not installed wrong way up ( lots of us have done that too  )..

All good info. Thanks Spanner. I know the floats are in properly, but I as a beginner the problem could be something as equally simple as that.

[Duanob]

If this hasn't been said already, you need to go back and do a comlete 3000 mile tune up and then onto the carbs. If the timing is off, the saprk plugs are shot, plug wire ends are bad, the valves aren't adjusted right, the air filter is clogged, etc, etc, etc, you will have problems starting your bike. If all that has been done and ruled out then go onto the carbs. completely clean, adjusted properly, and bench synced you should be able to at least get it started for fine tuning. 

oh yes and make sure there is enough gas in the tank, that has been overlooked before.

[lucky]

Thanks, Lucky. I actually remember coming across one of the original threads you posted that photo in & set the float accordingly. The problem I think that could come from just following that method is that the floats may sit in the bowls properly, matched up against the idle jet, but they may still hang at varying lengths. Maybe this is my issue.

I am using stock airbox with stock air filter.

I used a high E string to clean my idle jets, but very carefully. I don't think I did any damage to them (they are the only remaining original part of carb jetting/needles). If I did damage them despite my care, would this keep the bike from starting?

Thank you.

The high E guitar string would usually be .009,.0010.or.011 thousandths.
The idle jet is at least .013 thousandths.
BUT...You need to REMOVE that jet to clean it.
You need to run the wire through it AND clean it with carb spray and compressed air. It makes a difference because just poking a wire through that gooeey old gas does not make the hole large enough if goey junk is still stuck to the sides.


There is no reason the flat needle or floats would stick at different levels.
If the float needles are new and the floats set properly by MEASURING you will not have to worry.
The fact that the bottom of the plastic FLOAT is level with the idle jet when the carb is upside down does not mean you should not use the proper measuring tool.