Topic: Ok, now I'm no longer confused!

[tophe7d]

So as I posted earlier in the tips & tricks section, I changed the plugs in my bike and she ran like a bat outta hell. Well, now when she warms up, she runs like crap again??? Basically the bike starts out running great at first start up and as soon as it warms up it has a part throttle misfire. Once I get past 1/3 throttle it picks back up again, but as soon as I get back to cruising (about 4000 rpm or so) she drops a cylinder again???

It's a '75 CB750 four. I have the idle air screws adjusted at 3/4 out. The carbs were sync'd a long time ago. It's got 110 main jets. I've checked and rechecked the timing numerous times. The points and condensors could probably stand to be replaced. I did do a fifth gear plug chop and the plugs looked like brand new suggesting lean. It's got a 4 into 1 Mac exhaust with a stock airbox/cleaner. I'm guessing that I should bump the main jets to about 120's or so, but suggestions are welcome. Would jetting alone cause this problem or should I be looking elsewhere?

Also, another question: What kind of gas mileage are you guys getting with your 750's? Typically, I've been getting about 90 miles or so before having to switch to reserve. This seems a bit low to me. Is this normal?

Thanks in advance!

Chris Thornton

[andy750]

Id check you are getting spark to all 4 cylinders - try cutting back the spark plug wires to make sure you get good contact. Also check valve clearances, adjust camchain and after this sync those carbs again. Your symptons could be either electrical (condensers/worn points) or check the condition of your jets - are the clean? You can check this with carbs on the bike - just drop the bowls. I had recent running problems that were jet related and bumping up from 110 to 115 solved my issues but I wasnt getting throttle misfiring (just lack of power). Id do a thorough tune up first before trying to troubleshoot.

Mileage should be 140 -150miles before filling up so yes you have poor gas mileage.

Good luck and keep us posted on progress,
cheers
Andy

[Tom Stark]

Your backfiring when warm would indicate to me that it is not from the jets.  not saying you are not running lean, just the back fire is probably not from the jets.  If it were, it should be worse when cold, (when you need to be richer). 
So far as fuel consumption goes, how do you ride? what kind of riding do you do? (hard stop and go, normal stop and go, performance type, highway cruising, etc.).  And what final drive ratio do you have.  I am running 2.278 ratio and I am getting from a low of 46.112 to a high of something over 52, but I am still tuning the bike so the numbers are not final, but should be reasonably close.
You said the points may need replacement.  Go ahead and replace them, and do the other suggestions given.  Then see what happens.  If you still have trouble after that, I would do a compression test just to see the mechanical condition of the motor.
Tom

[tophe7d]

Thanks for the suggestions.

One thing I did notice is that my idle screws were not set at what I thought they were. Either I can't count the number of turns of a screw driver or they are moving from vibration. Anyone else ever heard of this? They were all unseated about a turn +. Could this have been the problem? I'm going to readjust them and road test the bike later this weekend.

Thanks again! 

[tophe7d]

Just wanted to update you all on the bike progress. I'm definitely no longer confused thanks to the wonderful 10 pages of posts found in this thread here:

http://www.sohc4.us/forums/index.php?topic=18547.0

I ended up pulling the carbs and gas tank off the bike. I completely cleaned and went through the carbs. There were a couple of passages that were clogged. One of the floats was getting hung up on the bowls when it was installed and not allowing any gas into the carb. I reassembled the carbs and blew into the inlet hose to test the needle and seat. I then turned on the gas line to fill the bowls and started draining them one by one into a jar as suggested in the above post. I found that the number 3 carb wasn't getting any fuel (from the above float hang-up problem) and that the number one carb was getting too much fuel. After spending about a half hour fine tuning the float heights the carbs were ready to go back onto the bike.

After assembling the rest of the bike I can happily say that I have remedied the problem and that she is running the best that she's ever run!!! She gets up and goes and has no hesitations whatsoever. The only things left to tune are the main jets and have the carbs sync'd. I did another 5th gear plug chop and she's still running on the lean side. The plugs are fairly white with just a hint of tan. As I said before it has 110's in it now and I'm thinking 115's or 120's will take care of it for sure. I am currently doing a gas mileage test to see if this will have a significant effect. So far from what Im seeing, I believe it is. 

Thanks again for the help... I love these forums!

Chris

[medic09]

Oh, THAT thread!  My shame comes back to haunt me...

[tophe7d]

There is no shame in that thread whatsoever!!! If you ask me, I think that is should be put up as a sticky for some of the noobs (like myself) to read before bothering to post. I started reading and reading and thinking "wow, I've got a lot of the same bull#$!@ going on".

Take heart in knowing that your shame helped others. Thanks soooo much for posting and thanks to those who offered solutions to the problems!

I'm off to ride...   

Chris Thornton

[medic09]

There is no shame in that thread whatsoever!!! If you ask me, I think that is should be put up as a sticky for some of the noobs (like myself) to read before bothering to post. I started reading and reading and thinking "wow, I've got a lot of the same bull#$!@ going on".

Take heart in knowing that your shame helped others. Thanks soooo much for posting and thanks to those who offered solutions to the problems!

I'm off to ride...   

Chris Thornton

Oh, I'm okay with it.  My threads are great that way...all the possible oversights and stupid mistakes wrapped up in one convenient package!   

Meanwhile, even with a few lingering problems, the wife and I had a good ride today.  The K8 is a great bike (despite my inept attempts at repair and maintenance). 

[mrbreeze]

Just wanted to update you all on the bike progress. I'm definitely no longer confused thanks to the wonderful 10 pages of posts found in this thread here:

http://www.sohc4.us/forums/index.php?topic=18547.0

I ended up pulling the carbs and gas tank off the bike. I completely cleaned and went through the carbs. There were a couple of passages that were clogged. One of the floats was getting hung up on the bowls when it was installed and not allowing any gas into the carb. I reassembled the carbs and blew into the inlet hose to test the needle and seat. I then turned on the gas line to fill the bowls and started draining them one by one into a jar as suggested in the above post. I found that the number 3 carb wasn't getting any fuel (from the above float hang-up problem) and that the number one carb was getting too much fuel. After spending about a half hour fine tuning the float heights the carbs were ready to go back onto the bike.

After assembling the rest of the bike I can happily say that I have remedied the problem and that she is running the best that she's ever run!!! She gets up and goes and has no hesitations whatsoever. The only things left to tune are the main jets and have the carbs sync'd. I did another 5th gear plug chop and she's still running on the lean side. The plugs are fairly white with just a hint of tan. As I said before it has 110's in it now and I'm thinking 115's or 120's will take care of it for sure. I am currently doing a gas mileage test to see if this will have a significant effect. So far from what Im seeing, I believe it is. 

Thanks again for the help... I love these forums!

Chris

Glad that advice helped you out there Chris. When my bike was acting up after carb overhaul, I had to do a bunch of detective work on my own and while following advice from others here,nobody mentioned checking bowl levels even though I had just adjusted float levels to spec. Having good manuals and everyone's help here don't mean squat if you aren't willing to spend the time to get in and do the work.A good amount of patience does wonders also!!!