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Discussion Starter · #1 ·
Hello all,

Just figured I'd post a couple of photos of my 81 cm400 project that is nearing an end. I've been using this forum every step of the way, so I figured it was time to share some photos of the nearly finished project.

I pulled this bike out of a friend's barn last year, where it had been sitting and last run in the year 2000. There were no known issues with it. Fortunately it turned out to be in pretty decent shape once I got it cleaned up. From advice here, I proceeded with the following:

-Cleaned out the gas tank (tons of rust)
-New battery
-New plugs
-Oil in the plug holes to loosen up the cylinder
-Rebuilt the carbs (twice)
-New air filter
-Changed oil/filter
-Adjust valves
-Adjust cam chain

I was then able to hook everything up and get it running, which felt like a small victory. The engine had good compression so I proceeded with the restoration. I put new tires on (my first experience doing that at home - very interesting) and a new chain. Also, lots and lots of cleaning and polishing.

I just finished the MSF course and got my M stamp and I'm ready to get this thing on the road. It will be registered in a couple of days.

THE PROBLEM:

Although the bike starts and runs great, when I accelerate and hold it in the 3000 rpm range it starts "popping." My assumption is that this is a carb issue (unburnt fuel hitting the exhaust) and my plan was to just ask to get the carbs professionally sync'ed when they do the inspection.

Any other thoughts?

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IMG_20180430_181945.jpg
 

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Great job saving that bike from a potentially sad end!

I assume you rebuilt your carbs according the the Rebuilding VB carbs sticky? If you didn't use new air cut valves you should look into replacing those. Also check your mixture screw adjustments.
These carbs are very difficult to get completely clean, it might just need a little more.

Also here's a carb sync tutorial, same bike
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Your popping is at constant throttle it sounds like. Is it from the exhaust or from the carbs? If it's from the carbs then you're looking at a vacuum leak somewhere, likely the carb insulators or possibly the air cut valves. If it's the exhaust then likely to be the exhaust gaskets where the pipes meet the head. Those are copper crush gaskets that can be used twice at most. It's not uncommon the have 2 or more gaskets stuffed in there because they're hard to see and define so people just add one more.
 

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Discussion Starter · #6 ·
Your popping is at constant throttle it sounds like. Is it from the exhaust or from the carbs? If it's from the carbs then you're looking at a vacuum leak somewhere, likely the carb insulators or possibly the air cut valves. If it's the exhaust then likely to be the exhaust gaskets where the pipes meet the head. Those are copper crush gaskets that can be used twice at most. It's not uncommon the have 2 or more gaskets stuffed in there because they're hard to see and define so people just add one more.
Thanks for the feedback! I had not thought of the crush gaskets. The popping is definitely at the exhaust--if you put your hand at the end of the pipe, you can feel the bursts of air exploding outward. I assumed it was unburned fuel exploding in the tailpipes.

How would I check to see if it is an exhaust leak issue?
 

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It sounds like the carbs are not cleaned properly and this will cause you issues if the slow speed jet or any carb passages are still partially plugged. With the bike sitting that long the gas in the carbs would have really had time to solidify.
Did you completely clean the gas tank of all rust and the fuel strainer on the petcock and put an inline fuel filter into the gas line? If the fuel tank still had some rust particles inside then your once clean carbs are now full of rust again.
The slow speed jet has such a tiny hole that it is usually easier to replace with new if your not really familiar with the jets. You need to have an appropriate very fine wire probe for cleaning the slow speed jet opening.
 

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Are you noticing this only when the bike is parked or while riding? If only when parked and not riding then you can probably ignore it. Engines perform differently when not under load.
 

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Discussion Starter · #10 ·
Are you noticing this only when the bike is parked or while riding? If only when parked and not riding then you can probably ignore it. Engines perform differently when not under load.
You guys are awesome. I took it out for its first spin around the block last night, and I had no problems with popping-- it ran smoothly through acceleration and deceleration and with constant load. I'll let it get some miles on it with a nice fresh tank of ethanol-free fuel with some seafoam and let the engine work some of the cobwebs out.

Btw, what a blast of a ride!
 
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