Topic: Wits end- 1971 750

[Sethpat]

Hey guys, looking for some advice here. I can’t get my 750 to run to save my life and it’s driving me absolutely insane. I’ve been working on it for the past few years on and off, and usually give up once I get super frustrated (like I am today) however I’m finally determined to get this thing going, or just take it to the dump and move on with my life lol.
Here is the set up:
2nd over piston kit and top end overhaul
Power Arc ignition kit that I got from cycle x
I also sent my carbs to cycle x to be cleaned and based tuned for my set up. 120 mains, honestly not sure what everything else is set to, however I also had a buddy who is an older Honda tech look at them and he said they looked good.
Pod filters (I know I know but it’s all I have)
4 into 2 open exhaust with no baffles.
The boots from the carbs to intake are new

I’ve tried running smaller jets, bigger jets, I’ve reset the power arc at least 5 different times just to verify I had it in right, cleaned the carbs, changed plugs, checked and re-checked all of my electrical and now I’m completely lost.

Sometimes it will run for a few minutes if I stay on the throttle and keep it around 1000 rpm however won’t stay running long or well enough for me to be able to get my gauges on to try and tune it ,  other days (like today) it won’t start at all and will just give a massive backfire like a damn cannon going off. If anyone has any suggestions as to where I should start before taking a sledgehammer to it I would sincerely appreciate it.

Thanks for your time

[ekpent]

 First thing to check is if it is firing on all 4 cylinders. Doesn't sound like it. Start it up sometime and let it run for a few minutes and then compare the temp of the head pipes of the exhaust close to the head. A few minutes of run time stationary should make a wet rag sizzle. K1's came stock with 120 mains from the factory. Even with pods it should run good enough to at least get around a little so I would be starting with electrical /spark plugs and going on to carbs from there.

[Mr. Mike]

So one issue is your bike won’t idle properly. Pods, rejetting can be tricky, and I’m not the one to offer suggestions if you want to go that route.  The timing has been checked? Post your location and who knows...maybe there’s someone near by who can come and see what you’ve got going on.

[Sethpat]

Thanks a lot for posting guys. Yeah I’m pretty positive that it is only running on three cylinders. I know for sure that I installed the electronic ignition correctly so timing shouldn’t be an issue, but just for the hell of it I cycled through all of the possible options. I pulled my plugs and grounded them, they’re all sparking, and they’re also all wet after trying to start for a few minutes.

Maybe my electronic ignition is faulty ?

I’m in Pensacola, FL.

Thanks for everyone’s time,
Seth

[jgger]

Easy mistake that folks make is coil wires should go to 1&4 from one coil and 2&3 from the other coil. Plugs will still spark fine, just at the wrong time.

[Sethpat]

Just double checked, the orientation is indeed correct. I was really hoping for that though; thanks for the thought. Here is a photo of my plugs 1-4 from bottom to top.

[BenelliSEI]

What type of plugs are you using? Try 4 X new D8EA. I’ve had some real head scratching brought on by NGK plugs that sparked laying on the cam cover, but the second the bike fires, they fail and spark intermittently........

[Mr. Mike]

[QUOTE
“Thanks a lot for posting guys. Yeah I’m pretty positive that it is only running on three cylinders. I 
I pulled my plugs and grounded them, they’re all sparking, and they’re also all wet after trying to start for a few minutes.” QUOTE]

Spark on all four, but all wet too. In your frustrations, you might be over throttling it when the plugs are already fouled out. New plugs as Benelli suggested (OEM part#too) would be (my)  first.
Then see how it cold starts without manipulating the throttle too much until it’s (hopefully) running. Double check the two clamps on the trouble carb insulator boot for tightness/air leaks.
Oh- carbs were bench sync’d by Cycle-X do you know?

Keep us posted.

[Honda3]

Just double checked, the orientation is indeed correct. I was really hoping for that though; thanks for the thought. Here is a photo of my plugs 1-4 from bottom to top.

I cant remember the guys name, but he did a "bible" on airpods and jets and its what ppl live by. Its somewhere on the net. I'll see if I cant find it.

With what youre running you need big mains ext. I had a 77 Yamaha XS650 with pods and open exhaust and it was hell to tune.

Holy #$%*! I stepped away while typing a reply and found it. Here it is:

http://www.xs11.com/forum/showthread.php?t=382

"Jetting Recommendations
by Denny Zander

Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.

1 jet size for custom 4 into 2 exhaust

2 jet sizes for 4 into 1 exhaust

1 jet size for K&N filter (single inside airbox)

1 jet size for drilling out the bottom of the airbox

3 jet sizes for individual filters

2 jet sizes for no muffler (open header)

1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size)

Under a mismatch condition, like individual filters and stock exhaust or 4 into 1 header with stock filter and air box, subtract an additional 1 jet size.

Check plug color often, sync carbs after each jet change, make sure the floats are set correctly, and seriously consider purchasing a Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).

Make sure your carbs are in perfect working order before making jet changes.

Example from my '79 XS1100 F:
Stock main jets: 137.5

Stock pilot jets: 42.5

2 sizes for 4:1 exhaust (Jardine)

1 size for single K&N

1 size for drilled air box

4 (main sizes) - 1 = 3 or 137.5 + (2.5 * 3) = 145.0
1 (pilot jet size) or 42.5 + (2.5 * 1) = 45.0

With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong exceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 76K mile on it, of which I have put 44K with this jet configuration.

I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs. "

[Scott S]

 There's no "cure all" for pods or else everyone would be running them. It takes trial and error, a dyno, and making multiple changes until you get it right.

 I would be looking at the tune up. Are you sure valves are adjusted correctly? Was cam installed and timed correctly?
 You say you have plug wires routed correctly but do you have the "points" wires TO the coils correct? The 1/4 wire going to the correct coil and not to the 2/3 coil?

[Honda3]

Hey guys, looking for some advice here. I can’t get my 750 to run to save my life and it’s driving me absolutely insane. I’ve been working on it for the past few years on and off, and usually give up once I get super frustrated (like I am today) however I’m finally determined to get this thing going, or just take it to the dump and move on with my life lol.
Here is the set up:
2nd over piston kit and top end overhaul
Power Arc ignition kit that I got from cycle x
I also sent my carbs to cycle x to be cleaned and based tuned for my set up. 120 mains, honestly not sure what everything else is set to, however I also had a buddy who is an older Honda tech look at them and he said they looked good.
Pod filters (I know I know but it’s all I have)
4 into 2 open exhaust with no baffles.
The boots from the carbs to intake are new

I’ve tried running smaller jets, bigger jets, I’ve reset the power arc at least 5 different times just to verify I had it in right, cleaned the carbs, changed plugs, checked and re-checked all of my electrical and now I’m completely lost.

Sometimes it will run for a few minutes if I stay on the throttle and keep it around 1000 rpm however won’t stay running long or well enough for me to be able to get my gauges on to try and tune it ,  other days (like today) it won’t start at all and will just give a massive backfire like a damn cannon going off. If anyone has any suggestions as to where I should start before taking a sledgehammer to it I would sincerely appreciate it.

Thanks for your time

The backfiring is pretty indicative of a fuel/air carb issue since you said you checked the timing, etc. I experienced this with my Yamaha XS650. Have a look again at the carb/airpod bible I linked and let me know how it turns out. That link is revered by many many ppl including skeptics who tried it, and it worked. Thats why its looked to all over the net and posted in almost every existing forum.

[ekpent]

 Make sure your not trying to run it with the choke on    Lever up is on,lever down is off and yes get some regular new plugs to begin the troubleshoot. Make sure the battery has a good charge also.

[1976cb750f836]

Forget everything you've already done, pretend u just brought it home, do a major tune up, check and set everything.
Check all connections in wiring, don't forget grounds,unplug and replug All bullets.
Check timing on the chain.
Check chain tensioner.
Adj valves, leave them a tad on loose side.(Only temperialy until it runs right then reset).
Set both sides of ignition, get them the same.
Remove carbs, remove bowls, remove jets,float and needle,air screw,carb clean straw through all passages, followed by compressed air, reassemble,check float adj.
Check bench sync with drill bit.
If it still don't run, check compression?
I do this to all bikes when I bring them home.

[Scott S]

The backfiring is pretty indicative of a fuel/air carb issue since you said you checked the timing, etc. I experienced this with my Yamaha XS650. Have a look again at the carb/airpod bible I linked and let me know how it turns out. That link is revered by many many ppl including skeptics who tried it, and it worked. Thats why its looked to all over the net and posted in almost every existing forum.

 How'd it work on the XS650? So well that you sold it with running problems?

 It's on the internet so it must be true. Yeah.... right. He  never even mentions needle position or mixture screws.  Let alone carb sync.
 And serious racers even change emulsion tubes.

 It backfired once, which was unburned gas in the exhaust igniting.
 Popping at idle or on deceleration is indicative of carb tuning and fuel/air mixture.

 Improper timing, either electrically by the ignition system being installed wrong, or mechanically by the cam timing being off is what I'd be looking at.  Since you've had the carbs off several times and changed jets.
 Did you adjust the valves while the engine was on the wrong stroke? Improper valve adjustment not operating the valves at the correct time?

[Bankerdanny]

Electronic ignition can be very sensitive to battery voltage. Make sure your battery is fully charged.

[scottly]

Your plugs are fouled; any symptoms with fouled plugs are null and void. Either replace with new, or, as I do in situations like this where the trouble-shooting process might cost a small fortune in new plugs before a solution is found, burn off the black with a propane torch.
What sort of "pods" do you have? All pods are not created equal.
Do you have the original ignition points plate, advancer and coils? (I'm not a fan of the Power Arc system)

[Honda3]

The backfiring is pretty indicative of a fuel/air carb issue since you said you checked the timing, etc. I experienced this with my Yamaha XS650. Have a look again at the carb/airpod bible I linked and let me know how it turns out. That link is revered by many many ppl including skeptics who tried it, and it worked. Thats why its looked to all over the net and posted in almost every existing forum.

 How'd it work on the XS650? So well that you sold it with running problems?

 It's on the internet so it must be true. Yeah.... right. He  never even mentions needle position or mixture screws.  Let alone carb sync.
 And serious racers even change emulsion tubes.

 It backfired once, which was unburned gas in the exhaust igniting.
 Popping at idle or on deceleration is indicative of carb tuning and fuel/air mixture.

 Improper timing, either electrically by the ignition system being installed wrong, or mechanically by the cam timing being off is what I'd be looking at.  Since you've had the carbs off several times and changed jets.
 Did you adjust the valves while the engine was on the wrong stroke? Improper valve adjustment not operating the valves at the correct time?

Sorry, that bike wasnt actually sold. Deal never happened. I gave that bike to a friend of mine who was down on his luck with bikes. He made a lengthy instagram post about it and how much he appreciated it (he has over 20 thousand followers and is a "name" in the biking world (and hes a model too) so it made a bit of a splash. He hasnt had a problem with it in the year since I gave it to him, and it runs amazingly (he and I have gone out on a few rides, me on my 1982 Honda CB900f and him on the Yamaha).

The "carb bible" is celebrated because a great number of people who followed his advice no longer had carb problems. Its not conjecture, its tried and true enough to be quantifiably useful.

The original poster has a lot of things going on with the carbs and exhaust so its a good idea to start there. All information is useful and the carb bible is hallowed. People rave about it.

[DaveBarbier]

The backfiring is pretty indicative of a fuel/air carb issue since you said you checked the timing, etc. I experienced this with my Yamaha XS650. Have a look again at the carb/airpod bible I linked and let me know how it turns out. That link is revered by many many ppl including skeptics who tried it, and it worked. Thats why its looked to all over the net and posted in almost every existing forum.

 How'd it work on the XS650? So well that you sold it with running problems?

 It's on the internet so it must be true. Yeah.... right. He  never even mentions needle position or mixture screws.  Let alone carb sync.
 And serious racers even change emulsion tubes.

 It backfired once, which was unburned gas in the exhaust igniting.
 Popping at idle or on deceleration is indicative of carb tuning and fuel/air mixture.

 Improper timing, either electrically by the ignition system being installed wrong, or mechanically by the cam timing being off is what I'd be looking at.  Since you've had the carbs off several times and changed jets.
 Did you adjust the valves while the engine was on the wrong stroke? Improper valve adjustment not operating the valves at the correct time?

Sorry, that bike wasnt actually sold. Deal never happened. I gave that bike to a friend of mine who was down on his luck with bikes. He made a lengthy instagram post about it and how much he appreciated it (he has over 20 thousand followers and is a "name" in the biking world (and hes a model too) so it made a bit of a splash. He hasnt had a problem with it in the year since I gave it to him, and it runs amazingly (he and I have gone out on a few rides, me on my 1982 Honda CB900f and him on the Yamaha).

The "carb bible" is celebrated because a great number of people who followed his advice no longer had carb problems. Its not conjecture, its tried and true enough to be quantifiably useful.

The original poster has a lot of things going on with the carbs and exhaust so its a good idea to start there. All information is useful and the carb bible is hallowed. People rave about it.

I’ve never heard of this carb bible. Sync carbs after every jet change? Colortune? No needle clip talk? He does say that it MIGHT get you in a good STARTING point. That’s the important part. You can get close-ish following advice from other people but to really get it tuned right you need either a dyno or to do plug chops at various throttle positions.

I disagree that a good idea is to start on carbs. That’s the last thing until you’re positive all else is done.

To the PO: Start at the beginning. Strong battery, good grounds, tune up including valves, cam chain, plug gap, everything in the manual for the 3k mile maintenance interval.

The PowerArc is the same as the C5 ignition which I had and eventually got rid of because it failed and gave me a hell of a headache. The trigger module was overheating and slowly cutting out all cylinders after about an hour ride. I would let it cool for 10 minutes and then it would run perfectly. Don’t let that scare you though . Go through these troubleshooting steps to make sure your unit is good. But even if it passes these test you might still have an intermittent issue.
http://www.c5ignitions.com/troubleshooting.html

Are you using their coils and plug wires?

Are you using the included springs inside the boots for the coils? The right orientation?

Do you have points to swap in for a test?

One thing I don’t like about the PowerArc/C5 is that they really want you to run power and ground to the coils and trigger module directly from the battery with no relays or anything. Also they really stress not having more than 14.2 volts max going to the unit. Even a Rick’s reg/rec (for my 650 at least) went up to 14.8. They do have a new unit that’s made for LiFe batteries that maxes at 14.2.

[WhyNot2]

Hey guys, looking for some advice here. I can’t get my 750 to run to save my life and it’s driving me absolutely insane. I’ve been working on it for the past few years on and off, and usually give up once I get super frustrated (like I am today) however I’m finally determined to get this thing going, or just take it to the dump and move on with my life lol.
Here is the set up:
2nd over piston kit and top end overhaul
Power Arc ignition kit that I got from cycle x
I also sent my carbs to cycle x to be cleaned and based tuned for my set up. 120 mains, honestly not sure what everything else is set to, however I also had a buddy who is an older Honda tech look at them and he said they looked good.
Pod filters (I know I know but it’s all I have)
4 into 2 open exhaust with no baffles.
The boots from the carbs to intake are new

I’ve tried running smaller jets, bigger jets, I’ve reset the power arc at least 5 different times just to verify I had it in right, cleaned the carbs, changed plugs, checked and re-checked all of my electrical and now I’m completely lost.

Sometimes it will run for a few minutes if I stay on the throttle and keep it around 1000 rpm however won’t stay running long or well enough for me to be able to get my gauges on to try and tune it ,  other days (like today) it won’t start at all and will just give a massive backfire like a damn cannon going off. If anyone has any suggestions as to where I should start before taking a sledgehammer to it I would sincerely appreciate it.

Thanks for your time

Trust me.......feel your pain.

Although, I'm a lot lazier than you..........I only have 4-1 exhaust, ran then quit, killed the battery, fried the relay, now nothing, just spins.

Fire pit here I come.

[Sethpat]

Gotta say, thank you all so much for all of the advice and input. I was so damn frustrated with this thing... the other day I was out there for six hours straight, cleaning plugs, checking connections, pulling my carbs, cleaning, checking floats, swapping mains (multiple times), resetting the ignition and every time still getting the same result.

Being so pissed off, I just needed that reminder to keep it simple, stupid. I put in a set of the regular NGK stock style plugs that were recommended (had iridium plugs?), re bench synced my carbs and it fired right up. First time in about 30 years or so. Got the carbs vacuum synced and now it’s nice and smooth. Hopefully the power arc holds up, I probably should’ve done more research before, but hell, live and learn.

Thank you so much everyone,
Seth

[DaveBarbier]

Awesome, glad to hear it. I just looked back at post #6 and I can now totally see that you have iridium plugs. Wish I zoomed in at the time. That’s a no-no for these bikes. Stock plugs and stock plug gap. Now go ride your heart out.

[Mr. Mike]

YAAAAY!
Now let's go riding!

[seanbarney41]

Now that the easy stuff is figured out.  I can't figure out how to work instant grams and I nor none of my friends are models...so should I read that carb bible?

[BenelliSEI]

Is it just me noticing how often “change the plugs” is the solution.......?
NGK used to be the best you could get. Not so much anymore.

[Johnie]

I don't know...all I use is stock NGK and they are performing perfect for thousands of miles.