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Tuning for no air box and chopped exhaust

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nateweisiger02

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I have a 1978 cb750k. I took the air box off when I first bought the bike, and it ran without it. Not great, but it would run and I could drive it around. Now, I just chopped the muffler off of the exhaust and it will run for about 3-5 seconds then cuts off. When I give it any gas, it immediately puts out and cuts off. I have seen other bikes with the same setup so I didn't think it would be a problem. Does anyone have any idea if I need to mess with the idle or mixture screws? Or what I need to do to get it running and driving again?
 
Go through all the tune up items first. clean/check/adjust points, set timing, valve clearance, compression, then probably needs carbs cleaned then you can sync them. then you can continue with jetting. If everything is good tune up and carbs clean it should run even if the jetting is off.
 
dirtdigger said:
Go through all the tune up items first. clean/check/adjust points, set timing, valve clearance, compression, then probably needs carbs cleaned then you can sync them. then you can continue with jetting. If everything is good tune up and carbs clean it should run even if the jetting is off.
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Could the chopped exhaust mess with the compression enough for it not to run?
 
I would like to note: when I first got the bike, the engine was locked up from sitting for 15 years. Could have something to do with the rings not operating properly
 
It shouldnt even run with those compression numbers...anything under 100psi normally wont fire. Normal compression on these is 170ish, I consider the engine damaged or worn out when compression is under 120. If those numbers are accurate you will never get it to run good or even decent without a rebuild.
 
dirtdigger said:
It shouldnt even run with those compression numbers...anything under 100psi normally wont fire. Normal compression on these is 170ish, I consider the engine damaged or worn out when compression is under 120. If those numbers are accurate you will never get it to run good or even decent without a rebuild.
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So I first got it and it had a locked up engine, got it freed, got spark again the it ran for about a week until I tore it down for paint and then chopped the exhaust. What do you suggest? New piston rings and head gasket to increase compression? Or a whole engine rebuild?
 
Could be rings or valve/seat issue, wont know till a person gets it apart. I am sure the cylinders are rust pitted and need bored, valves could be pitted as well.
 
dirtdigger said:
Could be rings or valve/seat issue, wont know till a person gets it apart. I am sure the cylinders are rust pitted and need bored, valves could be pitted as well.
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That would make sense, I'm debating having it bored to 836cc. Is it just as simple as take the block somewhere to get it bored, put the new pistons and rings in, put it all back together and ride? Or will I need to do a lot more if I'm planning on having it bored to that cc?
 
836 is a simple bore job. Send to a motorcycle shop though, I suggest cyclex or APE. Cycle x normally has kits where you exchange your cylinder for an already machined cylinder with matching pistons. I always suggest doing other upgrades as well to fix some of the weak points in the engine. I am betting the cylinder head is due for a rebuild and probably has some rust damage, again send to cycle x or APE they know what they are doing unlike automotive machine shops.
 
dirtdigger said:
836 is a simple bore job. Send to a motorcycle shop though, I suggest cyclex or APE. Cycle x normally has kits where you exchange your cylinder for an already machined cylinder with matching pistons. I always suggest doing other upgrades as well to fix some of the weak points in the engine. I am betting the cylinder head is due for a rebuild and probably has some rust damage, again send to cycle x or APE they know what they are doing unlike automotive machine shops.
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Took the engine out, the pistons look pretty rough but the cylinder walls look good so I don't think I need a bore. The valve on cylinder 2 was also cracked, so that explains such low compression on cyl 2. As well as the head gasket not completely sealing on almost all of the cylinders. When ordering new pistons, do any of the other years share the same size pistons with the same compression ratio?
 
The problem with just putting new pistons in is that you dont know if the running clearance is going to be correct for the new pistons. Different brands can and have run different cylinder clearance. Unless the old pistons are relatively new and of the same brand as the new ones then you cant assume it will be correct. Also you would need to have the cylinders honed if they do check out ok for the new rings. I would honestly find a different cylinder and have them bored since you are getting new pistons anyway or go to cycle x and do the cylinder exchange with them.

I have never seen a valve crack...ever. does it look like it hit the piston or some other object? where is it cracked?

The cylinder studs more than likely relaxed causing the leaking head gasket, I never assemble a 750 without heavy duty studs, the stock ones stretch. But you should also inspect and make sure the top of the cylinder and the cylinder head are flat.
 
dirtdigger said:
The cylinder studs more than likely relaxed causing the leaking head gasket, I never assemble a 750 without heavy duty studs, the stock ones stretch. But you should also inspect and make sure the top of the cylinder and the cylinder head are flat.
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I know it's OT, but do you have a source for heavy duty studs for a DOHC motor?
 
Try APE. I dont build dohc motors so dont know whats available.
 
dirtdigger said:
The problem with just putting new pistons in is that you dont know if the running clearance is going to be correct for the new pistons. Different brands can and have run different cylinder clearance. Unless the old pistons are relatively new and of the same brand as the new ones then you cant assume it will be correct. Also you would need to have the cylinders honed if they do check out ok for the new rings. I would honestly find a different cylinder and have them bored since you are getting new pistons anyway or go to cycle x and do the cylinder exchange with them.

I have never seen a valve crack...ever. does it look like it hit the piston or some other object? where is it cracked?

The cylinder studs more than likely relaxed causing the leaking head gasket, I never assemble a 750 without heavy duty studs, the stock ones stretch. But you should also inspect and make sure the top of the cylinder and the cylinder head are flat.
Click to expand...
Okay, sweet thank you. It doesn’t look like the piston hit the valve. And I didn’t find anything that could’ve blown through it. I couldn’t upload my own picture but it looks almost identical to the valve in the link.
Nd9GcR1iS7JrvwkanZGwp4b_6vP9fN8A31pU9Z6lg&usqp=CAU.jpg
 
I'm surprised no one has said anything about the main jet in the carb. You don't say which SOHC carbs your using, but you will have to got to 125 - 130 on the main jets. I run a 1976 with open velocity stacks & a Kerker style 4 into 1 exhaust. I also went to #45 for pilot jets for the low end. Pre "77" carbs are easier to tune, but 77, 78 can be done.
 
Like shown above no amount of tuning or jet changes will fix low compression which is why I sent them in that direction. After that is fixed yes they will definitely need to do some jetting.
 
That valve looks over heated to me, from running lean. I'ld say burnt over cracked, may have started as a crack. I assume you don't have a bore gauge handy. If you can get one I'ld check bore diameters & out of round (more wear 90 degrees from the wrist pin (front to back for your motor)) & if it has a belly spot. there's more wear where the rings ride over time. This wear can be measured (inside micrometer). If it's straight & true, it's OK to use your cylinders. A new cross hatch (hone) may set you right up. If not & you need to bore. The sleeves can be removed from the SOHC. Stick 'em in the oven @ 350 degrees setting on the sleeve bottoms & wait for the bang. (Aluminium fins falling down). Remove o-rings around sleeve base first though. My bore shop finds it easier to do, if the sleeves are out of the jugs. Lastly the are many high performance part available for the engines, including a 6 speed transmission. Folks still race these things. 836cc kits are pretty bulletproof, 900cc - 1100cc kits make the sleeves too thin in my opinion for a street legal bike. I have been told that the CB350 stock piston & rings can be used to make an 836 kit, but I haven't done it myself yet.
 
I run a 970 on the street no problem. Its in the bike in my profile pic. It all depends on the sleeves you use. The 970 I built has a thicker sleeve thickness then 836 kits. The cb350 pistons indead had been used for the old big bore kits but it is pretty much a moot point now days with all of the aftermarket kits available today that are sooo much better in every aspect.
 

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