1980 CB750 Bobber Sputters while riding - Need Advice

[Black956]

Hey Everyone.. .

I recently picked up a 1980 cb750 that the PO made into a bobber/hard tail. The bike fires right up and idles well, but when riding down the road (15mph - 50mph) it sputters pretty bad and looses power when giving it throttle. It almost seems like the carbs are intermittently getting too much, or not enough, fuel.

The PO put pod filters on the bike which I have since removed and reinstalled a stock air box (although i had to cut down the size of the stock box to fit it). I also had the carbs professionally cleaned.....

Time for a carb rebuild? Any thoughts or advice would be appreciated!!

 

[amc49]

When was the last time the valves had clearances set? The major problem with these and not done to lose compression and the exact same result as carbs messed up and why so many mess up carbs further trying to fix it. Compression test to make sure you aren't wasting your time.

There is no such thing as a 'professional cleaning' if there was no rebuild done at the same time. Cleaning is a waste of time if carbs are not dimensionally right part wise.

You don't say how airbox modded, depending on what done there you could easily have turned it into the same thing as pods.

 

[Black956]

When was the last time the valves had clearances set? The major problem with these and not done to lose compression and the exact same result as carbs messed up and why so many mess up carbs further trying to fix it. Compression test to make sure you aren't wasting your time.

There is no such thing as a 'professional cleaning' if there was no rebuild done at the same time. Cleaning is a waste of time if carbs are not dimensionally right part wise.

You don't say how airbox modded, depending on what done there you could easily have turned it into the same thing as pods.

I had them cleaned by a mechanic; that's what i meant by professionally cleaned. Not sure about the compression so I will def check that out.

I pretty much cut the back portion of the airbox off; the section where the snorkel is. I'm going to fiberglass the section that is now exposed to reduce he opening and reattach the snorkel.

Thanks for the feedback!!

 

[amc49]

As long as the chamber after the air filter is OK you should be fine. You can open the back up before filter anyway you want but engine likes the OEM type paper filter, it helps the carbs open slides better.

Common valve effect is bike seems to run Ok until you get up in rpm a bit say fast cruise then seems like engine drops out of sync, that is, you hear plainly obvious uneven running like one carb not in sync with the others, a 'burble' type noise similar to pods with too much oil in them. Unlike the pods which suck off most of the oil to run fine later though, the off running does NOT go away. If due to valves then no carb work on the planet will fix it.

Most shops quickly checked all the valves or maybe even part of them to pronounce the valves as 'set' then get the money. Bikes hardly ever get adjusted shims like they should be and commonly on almost every one you find out there.

 

[Black956]

As long as the chamber after the air filter is OK you should be fine. You can open the back up before filter anyway you want but engine likes the OEM type paper filter, it helps the carbs open slides better.

Common valve effect is bike seems to run Ok until you get up in rpm a bit say fast cruise then seems like engine drops out of sync, that is, you hear plainly obvious uneven running like one carb not in sync with the others, a 'burble' type noise similar to pods with too much oil in them. Unlike the pods which suck off most of the oil to run fine later though, the off running does NOT go away. If due to valves then no carb work on the planet will fix it.

Most shops quickly checked all the valves or maybe even part of them to pronounce the valves as 'set' then get the money. Bikes hardly ever get adjusted shims like they should be and commonly on almost every one you find out there.

Thanks for the info! Very helpful!!