77 cb750k bogs down when throttling cold

MotoMechDog87

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Hello everyone, first post here. Please be gentle.

So a friend gave me a 750 (for free!) over the winter. I spent about 3 months putting it together and prepping it for start-up. fast forward to this month and I’ve been riding it for about 200 miles now.

It starts up first try and has no problem idling albeit being a little funky while it warms up. I gotta give it time to warm up good and hot before I can take off. When I try to open the throttle while it’s cold, it wants to die off. after it warms up, it’s a little rough on take off but rides great once it decides to take in the fuel and rev high. Then, no issues for the rest of the time.

I rebuilt the carbs but didn’t change the needles as I don’t have a sync gauge yet. Plan to get one very soon. Accelerator pump is also rebuilt and seems to work fine. i did a compression test but I don’t trust the gauge very much. Cylinders 1-3 showed 95psi and cylinder 4 was at 30…not good. I did a valve lash adjustment and did another compression test. It showed 90-95 psi across the board But I also did it with the engine warm this time. My tester wasn’t very expensive and I’ve read the cheap ones read lower than the better ones. So I’m just not sure. Regardless, the engine runs and starts right up first try so I don’t think the compression is truly the issue. Maybe part of it? I plan on a top end rebuild over next winter, maybe sooner.

yesterday, I adjusted the point gaps and ignition timing. It was a bit off, for sure. Runs a bit better now but still has the cold bog down issue. it has a 4 into 1 exhaust (came with it. I’d prefer stock but those prices!) and stock airbox and intake boots. Ignition coils seem to be older, could this be the culprit? The #4 cylinder plug is starting to foul and it was a brand new plug. The others look good. Exhaust temps read similarity except for #4 which is significantly lower.

I feel like I’m close to the solution but just don’t have enough experience to diagnose exactly what it is. Oh, I have an inline fuel filter with a see through glass chamber. The gas tank was in bad shape so I re sealed it and I wanted to view the condition of the gas coming out. This makes the fuel line a bit longer and doesn’t quite feed directly down To the carbs. should I remove this? Perhaps it’s making the #4 carb lack fuel?

sorry for the long post. Please feel free to ask me any questions and thank you in advance for any help/suggestions.
 

dirtdigger

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Sounds to me like you have a dead hole compression wise and an overall unhealthy engine. That was your low compression cylinder and the one with cold exhaust. 30psi is dead and will not fire, 90 to 95 is barely enough to make it fire......I consider them dead as well. 150psi is minimum I consider good, anything below that and there is an issue somewhere. Pull all plugs, hold throttle wide open when doing compression test, make sure battery is fully charged so you get consistent cranking speed. If compression is no good there is no point going any further trying to get it to run good because it never will until you have a healthy engine.
 

MotoMechDog87

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Sounds to me like you have a dead hole compression wise and an overall unhealthy engine. That was your low compression cylinder and the one with cold exhaust. 30psi is dead and will not fire, 90 to 95 is barely enough to make it fire......I consider them dead as well. 150psi is minimum I consider good, anything below that and there is an issue somewhere. Pull all plugs, hold throttle wide open when doing compression test, make sure battery is fully charged so you get consistent cranking speed. If compression is no good there is no point going any further trying to get it to run good because it never will until you have a healthy engine.
I totally agree and a rebuild is certainly on the way. I didn't realize I should have held the throttle wide open. And remove ALL spark plugs. I'll do the test again.

The number 4 cylinder does seem to fire at least to a certain extent. When I pull the plug wire there is a noticeable difference in engine performance. Not much but it's there.

I rode around yesterday for 50+ miles without any significant problem. Its probably not as powerful as it should be but it still is holding its own. I'm thinking I'll ride it until the winter and do a complete rebuild then, at this point.
 

Saranden

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Hello everyone, first post here. Please be gentle.

So a friend gave me a 750 (for free!) over the winter. I spent about 3 months putting it together and prepping it for start-up. fast forward to this month and I’ve been riding it for about 200 miles now.

It starts up first try and has no problem idling albeit being a little funky while it warms up. I gotta give it time to warm up good and hot before I can take off. When I try to open the throttle while it’s cold, it wants to die off. after it warms up, it’s a little rough on take off but rides great once it decides to take in the fuel and rev high. Then, no issues for the rest of the time.

I rebuilt the carbs but didn’t change the needles as I don’t have a sync gauge yet. Plan to get one very soon. Accelerator pump is also rebuilt and seems to work fine. i did a compression test but I don’t trust the gauge very much. Cylinders 1-3 showed 95psi and cylinder 4 was at 30…not good. I did a valve lash adjustment and did another compression test. It showed 90-95 psi across the board But I also did it with the engine warm this time. My tester wasn’t very expensive and I’ve read the cheap ones read lower than the better ones. So I’m just not sure. Regardless, the engine runs and starts right up first try so I don’t think the compression is truly the issue. Maybe part of it? I plan on a top end rebuild over next winter, maybe sooner.

yesterday, I adjusted the point gaps and ignition timing. It was a bit off, for sure. Runs a bit better now but still has the cold bog down issue. it has a 4 into 1 exhaust (came with it. I’d prefer stock but those prices!) and stock airbox and intake boots. Ignition coils seem to be older, could this be the culprit? The #4 cylinder plug is starting to foul and it was a brand new plug. The others look good. Exhaust temps read similarity except for #4 which is significantly lower.

I feel like I’m close to the solution but just don’t have enough experience to diagnose exactly what it is. Oh, I have an inline fuel filter with a see through glass chamber. The gas tank was in bad shape so I re sealed it and I wanted to view the condition of the gas coming out. This makes the fuel line a bit longer and doesn’t quite feed directly down To the carbs. should I remove this? Perhaps it’s making the #4 carb lack fuel?

sorry for the long post. Please feel free to ask me any questions and thank you in advance for any help/suggestions.
Just something I have just done but may not be anything. I stripped my carbs down, new jets all round, put the cut lip on centre groove even though the old one was on top groove, started and a ran lovely. However it did bog down at aroun 4000rpm. I noticed pulling the choke on just about a 1/4 up cured it completely. What I noticed was, when I put my choke cable back in and pulled up the choke fully, it wasn’t pumping the pump on carb 2, so once I’d set the cable so it did actuate that pump when pulled up full, everything is running great. Just a thought.
 
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