HELP! 78 CB750F won't run right

[ImDoingMyBest]

I’ve done just about everything at least twice at this point and I’m pulling my hair out trying to figure it out. The Kreme in my gas tank started coming off so rust got into my carb. Was running very well until that happened, and after it would run okay until something would get lodged in the carb and it would die, then I could get it running a few miles before it happened again. Fast forward to now, the bike seems to hesitate occasionally at idle, dies or hesitates when giving gas at idle, accelerates fine after 1200k RPM and prefers a little choke, all while stationary. Mixture screws seem to have little to no effect on idle. All the way in kills it, adjusting by half turns barely changes idle (doing half since 1/8 didn’t do anything). Adjusting gets the bike to a semi steady idle or kills it but no “fastest idle” circumstance. Went for a ride to get gas, it hesitated while accelerating, low power, backfired, stalled twice while just trying to go from a stop, and sounded almost like it was running only on a few cylinders on the way back? Occasionally sounds like it’s running on a few cylinders, but only after warming up and usually after riding for a bit before it stalls. Before I tried synching the carbs, it would start and ride, but hesitate and stutter past 5k RPM and hesitate at idle, lowish power throughout the range. I made it 30 mins away no problem, on the way back on the highway, same way I came, after going 75+ for a mile or so giving gas did nothing and it stalled as soon as I slowed down. Had to wait a bit before it would kick over again and then die a few miles down the road.

Here’s what I’ve done so far:
- cleaned the carbs in an ultrasonic too many times to count, including all jets and poked the holes with a small wire and sprayed cleaner through every orifice. Main jets, idle jets, accelerator jets, everything
- synced the carbs
- checked for vacuum leaks (no change to RPM when sprayed with cleaner)
- checked spark plugs, they’re sparky but seem to have caked on dielectric grease
- gas I’ve been using has been refreshed a few times, originally had sea foam in it, now has startron mixed in instead
- running petcock filter and in line paper filter too, checked and carbs are getting fuel

I haven’t checked air filter yet, I’ll probably do that next. last time I looked at it it was basically new but it’s been a while. (Running stock air box, Keihin carbs)

Any advice would be greatly appreciated, I’ve been at this rodeo since August and I’m going insane

 

[ImDoingMyBest]

I’ve done just about everything at least twice at this point and I’m pulling my hair out trying to figure it out. The Kreme in my gas tank started coming off so rust got into my carb. Was running very well until that happened, and after it would run okay until something would get lodged in the carb and it would die, then I could get it running a few miles before it happened again. Fast forward to now, the bike seems to hesitate occasionally at idle, dies or hesitates when giving gas at idle, accelerates fine after 1200k RPM and prefers a little choke, all while stationary. Mixture screws seem to have little to no effect on idle. All the way in kills it, adjusting by half turns barely changes idle (doing half since 1/8 didn’t do anything). Adjusting gets the bike to a semi steady idle or kills it but no “fastest idle” circumstance. Went for a ride to get gas, it hesitated while accelerating, low power, backfired, stalled twice while just trying to go from a stop, and sounded almost like it was running only on a few cylinders on the way back? Occasionally sounds like it’s running on a few cylinders, but only after warming up and usually after riding for a bit before it stalls. Before I tried synching the carbs, it would start and ride, but hesitate and stutter past 5k RPM and hesitate at idle, lowish power throughout the range. I made it 30 mins away no problem, on the way back on the highway, same way I came, after going 75+ for a mile or so giving gas did nothing and it stalled as soon as I slowed down. Had to wait a bit before it would kick over again and then die a few miles down the road.

Here’s what I’ve done so far:
- cleaned the carbs in an ultrasonic too many times to count, including all jets and poked the holes with a small wire and sprayed cleaner through every orifice. Main jets, idle jets, accelerator jets, everything
- synced the carbs
- checked for vacuum leaks (no change to RPM when sprayed with cleaner)
- checked spark plugs, they’re sparky but seem to have caked on dielectric grease
- gas I’ve been using has been refreshed a few times, originally had sea foam in it, now has startron mixed in instead
- running petcock filter and in line paper filter too, checked and carbs are getting fuel

I haven’t checked air filter yet, I’ll probably do that next. last time I looked at it it was basically new but it’s been a while. (Running stock air box, Keihin carbs)

Any advice would be greatly appreciated, I’ve been at this rodeo since August and I’m going insane

Also forgot to mention, used a carb rebuild kit so new gaskets and low jets but I used the old floats. No matter what I did I couldn’t get the new float heights set right.

 

[hinksy]

Had the same problem with the floats when I rebuilt my 77 F2 . Had the wrong ones apparantly . Are you 100 poocent sure your accel pumps are working ? Have you cannected a fuel supply and watched them pumping ? Bogging and dieing when revved could be them . Having to use the choke to make it run usually is always the pilot circut/jets blocked . Did you get all of the rust out of your tank , I had rust in mine and it took a long time to get it out . There always seemed to be a small amount left that I couldnt get out , no matter how many times I rinsed . Very poor design tanks I think

 

[ImDoingMyBest]

Had the same problem with the floats when I rebuilt my 77 F2 . Had the wrong ones apparantly . Are you 100 poocent sure your accel pumps are working ? Have you cannected a fuel supply and watched them pumping ? Bogging and dieing when revved could be them . Having to use the choke to make it run usually is always the pilot circut/jets blocked . Did you get all of the rust out of your tank , I had rust in mine and it took a long time to get it out . There always seemed to be a small amount left that I couldnt get out , no matter how many times I rinsed . Very poor design tanks I think

Yeah I think the rebuild kit had a cheap set of Chinese floats, so I put the ones back in from when it was running right.

I checked to make sure carb cleaner came out when I sprayed through the connector end of the accelerator jets and then when I put the carbs back on I made sure gas came out of them when I twisted the throttle without the airbox on. Although I didn’t check the accelerator pump adjustment after I put the new diaphragm in, so maybe the new one is shorter. I’ll check that too.

I’m using a donor tank that doesn’t have rust in it along with a petcock filter and in line paper filter while my tank is getting cleaned and sealed with caswell.

As for the pilot jets, I’ve cleaned them as described above countless times and sprayed through every orifice I could find. Not to say it can’t be that, but if there’s a part of the circuit that could be blocked, I’m not sure where it would be. I can’t say I’m an expert on carburetors, but I’ve made sure the idle jet is clean and sprayed carb cleaner through where the jet seats. I’m not sure if there’s another hole to check in that circuit.

 

[hinksy]

It is frustrating ! I am going through the same crap now . Had it all set up , carbs synced and tuned , motor running good . And bam , rusty tank because of the painter . So I have been cleaning and blowing out the tank and carbs on multiple times just to have the jets block up again . Hopefully this time I have it right . My clamps gave out , they are all stretched and one broke . So I had to wait awhile to get replacements . If you dont have it , here is the workshop manual on the fuel pump settings

 

[ImDoingMyBest]

Thank you! I actually have a paper copy and an PDF of the manual. And yeah, I tried cleaning the tank myself but after multiple weekends I couldn’t get the tank completely clean and gave it to a shop to do right.

Minor update:
- Took the bowls off and sprayed cleaner through the idle jets to make sure all connected circuits are cleared.
- Checked the air box and the filter is clean.
- Adjusted the accelerator pump so it’s good now.
- Also re-synched the carbs just to be sure. While synching it was running quite well, mixture screws had an effect on idle, idle was mostly steady, acceleration and power seemed strong, towards the end though there was a low almost hollow exhaust note, low power, and the bike wanted to stall unless I gave it gas. It was getting plenty of fuel in the fuel line, I checked.
- On the test ride, giving it gas did almost nothing, still the weird exhaust sound, stalled a few times, low power, poor throttle response.

I’m going to check all the spark plugs next to see if maybe some of them were in worse condition than I thought. I replaced them after I started having issues before the tear down last fall, but maybe running the way it was fouled them a bit.

@hinsky, are you running a petcock and in line paper filter? It’ll help at least. Not sure what your situation is, and I don’t mean to give unsolicited advice but you might want to acid etch or coat your tank. I cleaned my tank so many times but the rust just kept coming back. It was surface rust but eventually it got through all my filters and gummed the carbs again.

 

[hinksy]

Yes , Yes and yes , I used vinegar then baking soda and tank is still good mths later . But , still finding the odd bit of very fine rust that I was never able to get out after many rinses .

 

[ImDoingMyBest]

Props to you, I spent so many weekends trying to clean the tank. I used like two gallons of acetone for the tank liner, many baking soda and vinegar washes, and rust remover and still couldn’t get it clean. Didn’t help that I don’t have an air compressor to dry it out. Even the shop I have it to was taking longer than expected because of leftover liner

 

[kirky]

my tank looked ok in the top but down below was bad, i used apple cider vinegar and left it for a few days then shook it about and drained it. i kept filtering the vinegar and repeated this process for two weeks . then put some small nuts in the tank and added a fresh gallon of apple cider vinegar and kept shaking [ still shaking here boss ] and letting it stand for another few days. i then drained this and re filled it with a gallon of kerosene , paraffin in the uk, and shook it again with nuts in. after draining this and getting the nuts out i finished off with a gallon of petrol . the crud never seemed to want to stop but i kept at it and now the tank is good and hasn't given any problems with fuel contamination for 3 years ---- touch wood.

 

[ImDoingMyBest]

Okay I replaced the spark plugs because two were kinda sludgy possibly from when they overheated as the only two plugs firing when I first had issues. Also adjusted the points. But I’m still getting this weird growling exhaust sound that mostly is there but sometimes it sounds normal and I can’t figure out what it is

 

[drgnrdr750]

Is it possible that some of the rusty crap made it into the cylinder and caused a slight buildup on the exhaust valve?

 

[ImDoingMyBest]

Is it possible that some of the rusty crap made it into the cylinder and caused a slight buildup on the exhaust valve?

I suppose it’s possible. I have the carbs off now so I’ll check with a flashlight to see if anything looks off next time I’m wrenching. I found out some of the choke and throttle seals are missing or bad and spraying carb cleaner where they should be drops the RPM. I’m working on getting the supplies to make new felt seals so hopefully that improves and or fixes this issue

 

[Rat Rice]

Go to the Dollar Store and buy a bottle of toilet bowl cleaner, it's acid and use it to clean your tank. Make sure you rinse well after draining the acid. Take a hair dryer and dry it out and then spray WD40 in it so it doesn't surface rust (only do this if you are NOT going to Kreme your tank). If you aren't sealing your tank install and fill it with gas.

 

[brettp]

Haven't heard of toilet bowl cleaner. Definitely cheaper than evaporust, which works well.

 

[ImDoingMyBest]

Unfortunately I dropped the tank off at a shop and they went gung-ho when they shouldn’t have on coating the tank. They coated over some of the liner, a few screws, and some rust in clear caswell…wasn’t their finest hour, but I’m hoping the seal holds at the very least.

Update:
- replaced the seals, day after #2/3 pistons were not firing. Traced it back to the ignition coil (not points, those are 1/3 and 2/4). Both seem to have weak ish spark. Have a replacement
- while checking ignition coil, went to put spark plug back in and it goes in crooked because the engine head threads got messed up somehow. Got a tap coming to very, VERY, carefully rethread the first few threads. Gotta make sure no metal chips get in the cylinder

New plan:
- get spark plug In socket
- replace coils
- check carb sync
- adjust cam tensioner and valve clearanxe
- have a rideable bike hopefully?

 

[dirtdigger]

The points are also paired cylinder 1/4 and 2/3. More often then not spark issues are related to bad points or condensers and poor timing and dwell adjustments. Coils do go bad just not as common of an issue as the other ignition components.

 

[Rat Rice]

While some of the folks on here will disagree with me, one of the first things that I do is replace the dual points with an electronic ignition, usually a Dyna. It seems to give a better spark and I got tired of messing with points. Also there's no point float at high RPM.

 

[K B]

I’ve done just about everything at least twice at this point and I’m pulling my hair out trying to figure it out. The Kreme in my gas tank started coming off so rust got into my carb. Was running very well until that happened, and after it would run okay until something would get lodged in the carb and it would die, then I could get it running a few miles before it happened again. Fast forward to now, the bike seems to hesitate occasionally at idle, dies or hesitates when giving gas at idle, accelerates fine after 1200k RPM and prefers a little choke, all while stationary. Mixture screws seem to have little to no effect on idle. All the way in kills it, adjusting by half turns barely changes idle (doing half since 1/8 didn’t do anything). Adjusting gets the bike to a semi steady idle or kills it but no “fastest idle” circumstance. Went for a ride to get gas, it hesitated while accelerating, low power, backfired, stalled twice while just trying to go from a stop, and sounded almost like it was running only on a few cylinders on the way back? Occasionally sounds like it’s running on a few cylinders, but only after warming up and usually after riding for a bit before it stalls. Before I tried synching the carbs, it would start and ride, but hesitate and stutter past 5k RPM and hesitate at idle, lowish power throughout the range. I made it 30 mins away no problem, on the way back on the highway, same way I came, after going 75+ for a mile or so giving gas did nothing and it stalled as soon as I slowed down. Had to wait a bit before it would kick over again and then die a few miles down the road.

Here’s what I’ve done so far:
- cleaned the carbs in an ultrasonic too many times to count, including all jets and poked the holes with a small wire and sprayed cleaner through every orifice. Main jets, idle jets, accelerator jets, everything
- synced the carbs
- checked for vacuum leaks (no change to RPM when sprayed with cleaner)
- checked spark plugs, they’re sparky but seem to have caked on dielectric grease
- gas I’ve been using has been refreshed a few times, originally had sea foam in it, now has startron mixed in instead
- running petcock filter and in line paper filter too, checked and carbs are getting fuel

I haven’t checked air filter yet, I’ll probably do that next. last time I looked at it it was basically new but it’s been a while. (Running stock air box, Keihin carbs)

Any advice would be greatly appreciated, I’ve been at this rodeo since August and I’m going insane

As for gas tank issues, I've cleaned a few with vinegar by letting it soak and shaking once in a while. You can put BB's or even penny's in to help get corrosion out if really bad. Then after a day or two, stop the acidic effect of the vinegar with baking soda, rinse with water, dry thoroughly, and fill with new gas.

 

[brettp]

I don't use BBs or screws. I use an old piece of chandelier chain. That way I grab a single portion and pull the rest out.