Exhaust Compatibility

[nateweisiger02]

Hey there, I have a 1978 CB750k. I had the stock exhaust on mine but had a rust hole in it, so I chopped it right below the foot peg…. Bad idea and wish I never had. Haven’t ridden it much at all, barely had any torque after that, now the bike will barely pull its own weight. Thinking I burnt the rings so I am about to break the engine open. First question, would a 4-1 exhaust give me enough back pressure to keep this from happening again? I’m not feeling spending over $1000 for a used stock exhaust that came on it. Second question, is there any type of compatibility in the years and models as far as exhaust? Does F or K matter as far as exhaust and which years can use the same? Thank you very much!

 

[jpdevol]

Hey there, I have a 1978 CB750k. I had the stock exhaust on mine but had a rust hole in it, so I chopped it right below the foot peg…. Bad idea and wish I never had. Haven’t ridden it much at all, barely had any torque after that, now the bike will barely pull its own weight. Thinking I burnt the rings so I am about to break the engine open. First question, would a 4-1 exhaust give me enough back pressure to keep this from happening again? I’m not feeling spending over $1000 for a used stock exhaust that came on it. Second question, is there any type of compatibility in the years and models as far as exhaust? Does F or K matter as far as exhaust and which years can use the same? Thank you very much!

Nice pipes at a value price: https://delkevic.com/complete-systems/honda/1978-models/cb750k-sohc/

IMO, cutting the headers at the footpeg area did not cause engine damage in itself. It may have caused the fuel air mix to be slightly leaner, but there had to be other things going on for damage to result

 

[nateweisiger02]

Nice pipes at a value price: https://delkevic.com/complete-systems/honda/1978-models/cb750k-sohc/

IMO, cutting the headers at the footpeg area did not cause engine damage in itself. It may have caused the fuel air mix to be slightly leaner, but there had to be other things going on for damage to result

Hmm possibly. I could drive it around when I first chopped it, then it slowly dwindled down to now it’s having a hard time pulling it’s own weight. Compression is at 80,85,90, and 70.

 

[jpdevol]

Yeah, that's really low compression that'll need corrected

 

[nateweisiger02]

Yeah, that's really low compression that'll need corrected

I just replaced the valves, gaskets, and rings about two months ago. Thinking the rings are burnt from having no back pressure. But other people have driven around with a chopped exhaust, so I’m not sure why mine is so different.

 

[nateweisiger02]

Now I’m planning on replacing the rings again, as well as new gaskets for the whole engine

 

[nateweisiger02]

Update: Just poured a little oil in the spark plug holes, and retested the compression. Nothing has changed, still within 5 psi of what they were before across the board

 

[dirtdigger]

Your compression tester has to be faulty or inaccurate. That compression is too low to run on. Did you have the seats cut when you replaced the valves? Did you hone the cylinders when you replaced the rings? Have you rechecked valve clearances? If you just threw valves on old seats you probably beat the valve face to where the valve clearance closed up. The exhaust did not cause your issues. Backpressure has nothing to do with hurting rings....too short of exhaust can hurt the exhaust valves if you are under heavy decel after a hard pull. Backpressure in an engine is a back thing anyway. You want an efficient exhaust that draws vacuum(scavenging) not backpressure....backpressure robs power because that means you are forcing exhaust out of the engine. A good exhaust will help draw exhaust out of the engine.

 

[Old_Crow]

Yeah, if your compression only went up 5 psi when you did a wet check (adding oil to the cyl) then your problem is in the valves. You were holding the throttle wide open while cranking the motor, right?

 

[nateweisiger02]

Your compression tester has to be faulty or inaccurate. That compression is too low to run on. Did you have the seats cut when you replaced the valves? Did you hone the cylinders when you replaced the rings? Have you rechecked valve clearances? If you just threw valves on old seats you probably beat the valve face to where the valve clearance closed up. The exhaust did not cause your issues. Backpressure has nothing to do with hurting rings....too short of exhaust can hurt the exhaust valves if you are under heavy decel after a hard pull. Backpressure in an engine is a back thing anyway. You want an efficient exhaust that draws vacuum(scavenging) not backpressure....backpressure robs power because that means you are forcing exhaust out of the engine. A good exhaust will help draw exhaust out of the engine.

Dirt digger, thank you very much for your response! The cylinders have been honed, but you nailed it right on the head. I put brand new valves in the old seats without lapping or cutting them across all four cylinders. I did do a leak test though, and nothing seaped through. But I guess all it take is a micrometer of a difference to keep the bike from pulling its own weight. I will also check the compression gauge on my car and see what that reads to double check.

 

[nateweisiger02]

Yeah, if your compression only went up 5 psi when you did a wet check (adding oil to the cyl) then your problem is in the valves. You were holding the throttle wide open while cranking the motor, right?

Word, I didn’t think to cut the valve seats when doing the rebuild. And yep throttle was wide open while cranking.

 

[nateweisiger02]

Your compression tester has to be faulty or inaccurate. That compression is too low to run on. Did you have the seats cut when you replaced the valves? Did you hone the cylinders when you replaced the rings? Have you rechecked valve clearances? If you just threw valves on old seats you probably beat the valve face to where the valve clearance closed up. The exhaust did not cause your issues. Backpressure has nothing to do with hurting rings....too short of exhaust can hurt the exhaust valves if you are under heavy decel after a hard pull. Backpressure in an engine is a back thing anyway. You want an efficient exhaust that draws vacuum(scavenging) not backpressure....backpressure robs power because that means you are forcing exhaust out of the engine. A good exhaust will help draw exhaust out of the engine.

So what should be my next approach from here? Adjust the valve clearance to make up for the valve face being beat and see how it responds?

 

[dirtdigger]

Try a good compression tester, engines typically wont run on what your numbers are. Good compression is 170 to 180 psi. 120 is the lowest that they still run ok at. Check valve clearances see if they are held open causing low compression first. .002" intake, .003" exhaust.

 

[nateweisiger02]

Try a good compression tester, engines typically wont run on what your numbers are. Good compression is 170 to 180 psi. 120 is the lowest that they still run ok at. Check valve clearances see if they are held open causing low compression first. .002" intake, .003" exhaust.

Okay I will do that, then give an update. Thank you!

 

[nateweisiger02]

Try a good compression tester, engines typically wont run on what your numbers are. Good compression is 170 to 180 psi. 120 is the lowest that they still run ok at. Check valve clearances see if they are held open causing low compression first. .002" intake, .003" exhaust.

Alright, readjusted the clearances and now they are all set. Compression didn't change, Still I'm with you, my gauge is wrong so I will get another and retest to see what the actual psi is. I would like to add, started the bike up and it was idling pretty funny and sounded like it was misfiring. Not very responsive to the throttle either. Thought it may have something to do with the carbs being out of sync. Went to do that and saw that the carbs were all balanced, but they all had 5 inches of vacuum. Not sure what this means or could be a result of but it may be why the bike is not wanting to pull its own weight. Checked the spark plugs and they had some smut on them, replacing those tomorrow.

 

[nateweisiger02]

All carbs were matched to number two not too long ago. Should I bench sync, then see how off the other carbs are from number two, or if they are all affected the same?