Topic: Clyinder #1 is not firing, I've been trouble shooting but no luck, 75 CB750F

[bradl45]

Hello All,

I've just bought a 1975 CB750F in ok condition with 20k miles.  The bike starts ok but sounds rough.  When I looked the bike over before I bought it, I felt that the #1 pipe was cold.  I checked compression on all cylinders to make sure it wasn't in need of a motor rebuild(Fingers crossed). 

Compression
#1 131 PSI Plug black soot and wet
#2 127 PSI Plug black and gray color Dry
#3 129 PSI Plug black and gray color Dry
#4 132 PSI Plug Black Dry

I first checked for spark by having the plug in the boot outside of the motor and holding near the motor, all clyinders have good spark.

I moved to timing, and was having trouble getting my timing light to flash when hooked either on the 1 or 4 cylinder, 2,3 worked great.  I have a couple extra sets of coils to test with, I took a spare set with long wires and installed on the 2 and 3 cylinders because they were working correctly.  This coil worked fine, so I installed that same coil on Cylinders 1 and 4.  Still #1's pipe is cold when I run the motor.  I reset the timing via my Haynes manual and a ohm meter to when the points were opening up, I'll get my bro's timing light for testing on Tuesday.  Still no fire on #1.  Point gap was rest aftereach time I moved the plate.

I checked valve clearances, all were close, and I moved the exhaust to .004, exhaust was a bit tricky, I had to bend up a feeler.  Cylinder 1 had .002 for intake, and .003 for exhaust, and I reset exhaust to .004

I waited for the carbs for last, I bought a KZ650 from a guy who swore it needed carb work, but it just needed timing, and it's a screamer now, no luck with my CB yet.
I took out the carbs and dropped all the bowls and made sure that the jets didn't have a blockage, there was a bit of small rust particlies in all the bowls, but didn't find anything special in any of the carbs, I didn't adjust anything on the carbs, haven't read that far here on the forum or in my Haynes manual(little on the light side).  Bike started in about 5 seconds after I got the carbs back in, and I was disapointed to find #1 still cold.  I have the stockbox installed with a new airfilter, airbox fittings are tight for all carbs, I did reinstall carb #1 with a different connector to try something new.
I didn't take the top end off the carbs, I guess I'll have to look there for some junk.

From here I plan to time using a timing light, and then my carb might need adjusting.  Points look like new, I think previous owner installed a new point plate setup trying to get #1 working

Anyone have a few pointers?  I've got this far mainly by reading this forum, tons of good stuff here, so I'll keep searching.

I haven't checked the condensers, I figured if one of these were bad, I'd have trouble with 1 and 4, but 4 is running rich.

What adjustments should I try on the carb?  The needle's were all in the same postion on all carbs.

I'm scratching my head a bit, thanks in advance for any tips you can pass along.

Good day

Brad

[DammitDan]

Just to throw one out there...


Is #1 plug wet with oil or gas?

[750goes]

Swap plugs on #1 & #4, is #1 now hot and #4 cold ??

[tsflstb]

Check the slow jets on your #1 carb.  There are tiny passages that control fuel delivery at idle both in the jet itself and in the carb body.  Take a small strand of copper wire (0.016 or less) and run it through the holes.  I had this same problem a while back, and like you I checked everything else first.  It ended up being some crud in the slow jet.  On my 400, those can be unscrewed and cleaned.  I blew it all out and it ran like a champ afterwards.

[bradl45]

Hello,

I've swaped 1 and 4 plugs, and still #1 is cold.  I put a new plug in #1 and the pipe did build a little heat, bike backfired some and ran at a high idle, then stalled.  New plug didn't foul up, but bike only ran for 2-3 minutes, and plug came out dry.

Before the swap, #1's plug smelled of gas.

I'm going to retime with a timing light, then check about adjusting the carbs, #1 and 4 are running rich, I'm hoping this is just timing, working with points is a pain, I'm thinking about a dyna S, and using stock coils.

When I had the carbs apart, I pulled the slow jets, I tried using a pin to stick through, it was to large, but I can barly see light on all 4 of the slow jets, and I was able to blow air through them, once I time again, I'll pull the carbs again, rechecking everything, and I think I'll clean the top end as well, then bench sync the carbs.

Thanks for the tips

Brad

[Jerry Rxman Griffin aka MuthaF'er]

#4 is firing = coil ok but how ok we don't know

#1 not firing = perhaps bad wire or spark plug cap. Different coil with different wire and assuming different cap with pretty much same results = carb probably (however caps do go bad and won't spark)

you have spark now you need gas & air. open carb drain. gas flow? yes - no? as much volume as other carbs? If no, it's obvious. If yes, why not turn off petcock and carefully pull the bowls and compare gas volume. Approximately same = good. Less = blockage. Put other 3 bowls back on. Turn on petcock and watch for gas flow. Hold float up and see if flow shuts off. You just half ass checked your float needle. No flow = no gas = blockage. Clean out blockage. Low flow = do same. Make sure your float height is set correctly. Remove the slow/pilot jets as mentioned. Run fine smooth wire through them and use spray carb/brake cleaner, repeat, repeat. Hole is small so you won't see much light. Do 1 at a time so you can compare. When you gas it back up and get it running get a can of fuel system cleaner such as Sea Foam or BG44K. I prefer the BG44K. My Advance Auto keeps it behind the counter ($20+). It will last many tanks of gas AND a very thorough complete crab cleaning with dip, ultrasonic is not a bad idea! Syncing is a must. Bench sync at least. Buy or borrow a set of gauges if you can. You will use them again. Good investment.

While you're at it check your tank for crud, junk, rust(you did mention rust), etc that may be blocking gas flow. Drain most gas - not all. Remove it. Lay it on it's side with petcock up. Remove petcock and check for junk. If the strainer screen is trash (you did mention rust in the bowls) then order a new one and replace it or at least add an inline filter to the fuel line. Remove strainer screen & clean. It might easily tear! Take petcock apart & clean it. Put your thumb over the outlet on the tank and slosh the remaining gas around then drain it to get any other junk out. Your fuel line from the tank could still have crud in it. If you attempt to blow this through without detaching it from the carbs you will force the crud into your carbs. Be carefull with this. You might want to seriously consider a tank liner such as Kreem or POR at this point. Don't wait for the pinhole leaks at the bottom seam. New tanks are not available.
   
The Dyna ignition, coils, and wires might be the best money you spend on your bike! Set em and forget em!
 
One suggestion I can offer is to NOT do multiple fixes at the same time. You won't know which fix actually fixed it. If you like your 75 750F like I do mine (bought new) then you will accumulate tools, spend much money and time working on it. Rule #1 - ask questions, ask more questions and be patient. Rule #2 - spend money (not much way around this!) and fix it. Rule #3 - ride it when you're not fixing it. It will seem like as soon as you get one problem taken care of there is another one waiting. refer to Rule #1.

ps #1 is not running rich if it is not running and it is dry.

We're here to help.

Jerry (SOHC4-aholic anonymous)       

[bradl45]

Thanks Jerry,

I'll work on that list.

Some steps I've ommited, like tank cleaning.  I did have some rust in the tank, and did an acid bath, and cleaned up the leaking petcock.

I'll retime, and it still having trouble, I'll dunk the carbs and do a rebuild kit.

Do you have a link where I can buy a good set of sync sticks?

I also sheared off a brass drain screw on a carb, any ideas where I can buy a bowl, and good source of rebuild kits?

I can't wait to ride my CB around town, I hate riding my K1200LT in the city, too big and top heavy.

Thanks  Again, I'll get busy

Brad

[Jerry Rxman Griffin aka MuthaF'er]

"Do you have a link where I can buy a good set of sync sticks?"

"I also sheared off a brass drain screw on a carb, any ideas where I can buy a bowl, and good source of rebuild kits?"

Not sure about Carb Sticks or the carb bowl. I'm sure one of the guys here can help with a spare bowl and can give some ideas on a source for carb sticks, etc. I bought mine a loooong time ago.

[flatblack]

Most people here, including myself will recommend these for syncing: http://www.carbtune.com/
Shipping is fast, no problems whatsoever on the transaction.  They work nice.

I'll second that. Elegant solution and a no-problem transaction.

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[DammitDan]

Or if you don't have the $$$ for it, buy a set of motion pro mercury carb stix from denniskirk.com (average ~$45)

[benny365]

Well theres spark, Air, And timing.  Now all you need is FUEL

Sticky fuel needle jet, plugged jet.  just sounds like that cylender is not getting fuel

[DammitDan]

Well theres spark, Air, And timing.  Now all you need is FUEL

Sticky fuel needle jet, plugged jet.  just sounds like that cylender is not getting fuel

But he's got a plug wet with gas...

Or is the new plug not getting wet anymore when you pull it?

[bradl45]

The story continues.

All four cylinders are now firing, but I have a very high Idle, around 4000 rpms.  I figure I have an air leak some where, but before I fix that I've decided to do a good cleaning of the Carbs, I'm going to pick up my berrymans carb cleaner today, I've found it at a local store, I'll take apart the carbs and dunk them.  I just need to find some wire small enough to fit into some of the small jets.

I also think my points are giving me some trouble, and instead of replacing them, I've ordered a Dyna s, and will use the stock coils for now.  While I'm waiting for this, I'll repack my wheel bearings, and get the carbs back into the bike.  That stock airbox sure is a pain to use, I'm tempted to go to pods, but then I think I'd need new needles, are these expensive?  Air box does have many holes drilled into it, I'm hoping that the stock needles and jets will be ok.  Any guess where my clip should be?  I'll post the numbers of my jets and needle when I take apart today.

I'll check out the plugs conditions when I get the carbs and Dyna S installed.  I'll test once the carbs are finished before I put in the Dyna S to do the one step at a time.  My last bottom cleaning of the carbs, I didn't set the floats, and now I have a carb overflowing,  I'll set these this time.

Thanks for the links for the sticks, Carbsticks look good, just over $100 USD, and I'll look at denniskirt.com at there sticks.  I'm going to a local motorcycle boneyard to try to find a replacement bowl, at least my carb isn't leaking where I twisted the screw off.

Thanks for your help, and this web site, I've come a long way and learned a bunch about getting my bike up and running.

[benny365]

Be sure to check the Idle adjustment screw that sets the idle on all 4 carbs,. maybe previous owner turned it waaay up because it wouldn't idle before.

[bradl45]

Thanks for all your help guys,

The bike is up and running strong, the carb dunking took me a whole day, just dunking 1 carb at a time for 90 minutes in Barrymans.

Spent a bit of time locating the sock that's in the petcock, PO had removed the small tubes and sock that was in the tank, I for sure didn't want to let any crud into my freshly cleaned carbs, so I got a whole new petcock from a boneyard for $10.  I've also got an inline fuel and new fuel lines.

I still need to sync with a set of sticks, I've not bought a set yet, soon, the bench sync will get me by in the short run.

My Dyan S hasn't arrived yet, can't wait to through that in and will leave my bad dreams about points behind.

I do hear some clutch noise when Idling in neutral, and my hand isn't on the clutch, and it goes away when I apply the clutch, I hope this isn't a bad thing, I'll have to poke around a bit.

Paint sometime this winter, along with new bearings for steering, and I'll be all set, I'm thinking Yellow.

Thanks again

Brad

[Bob Wessner]

I do hear some clutch noise when Idling in neutral, and my hand isn't on the clutch, and it goes away when I apply the clutch, I hope this isn't a bad thing, I'll have to poke around a bit.

Very normal, oddly enough it's called "clutch rattle"  . Syncing the carbs helps minimize it.

[mrbreeze]

Yeah...ditto what Bob says...once you properly synch the carbs(assuming everything else is done prior),the clutch(and/or) cam chain noise goes away and it will idle so much better.It is well worth the $ you spend buying it and the time it takes to do the actual work. Just remember to be patient and also run some fans in front of the engine to keep er' cooled down.