CB450 Carb Issue

T

TripleKing

Been Around the Block
I have an interesting situation with my son's 1970 CB450. The bike has Mikuni VM32 Carbs. They were pre-jetted for the 450 by Dime City and I richened them a bit to account for the Uni-filters and unbaffled exhaust (exhaust came that way when we bought it.)

When you jump on the throttle, the right cylinder will pop and sometimes give a backfire out the right exhaust. However, it you roll on the throttle all is well. The left side does not do this. Both carbs are set up identically. The only difference between cylinders I can see is that the left (which does not pop) has compression of about 168 PSI and the right (which does pop) has compression of about 162 PSI.

Plugs are fine and there is no sign of the right cylinder running lean (spark plugs, no pipe discoloration and no popping on decelerartion). I would appreciate any input.
 
I would check for exhaust leak at the collar. Then onto timing and advancer, 162 is pretty dang good compression, so maybe check exhuast valve clearance last, but I doubt the issue is there.
 
Timing is where I am heading as it is a bear to get that timing down. I know I have it close to dead on, but maybe not close enough.
 
i know you say the carbs are set up the same but>>>

swap the carbs from side to side

if the problem moves you have missed something in the carbs

if it doesnt move dont give the carbs any more attention concentrate on the right cylinder
 
You also have to realize that if your cylinder compression ratio's are not indentical then the draw factor on the carbs will be different and therefore the mixture settings and throttle positions will be different on each carb. I use a manometer on each carb and an infrared thermometer on the headers to make sure the settings between each cylinder is close. ;)
 
cqyqte said:
You also have to realize that if your cylinder compression ratio's are not indentical then the draw factor on the carbs will be different and therefore the mixture settings and throttle positions will be different on each carb. I use a manometer on each carb and an infrared thermometer on the headers to make sure the settings between each cylinder is close. ;)
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Thx. I do realize that different compression could require somewhat carburetor settings, that is why I mentioned them. However, adjusting the air screw and throttle position have had little benefit.
 
FWIW, 164 psi is rebuild territory on a 450 engine. Should be around 185 for a well running broken in engine possibly 200 for a new rebuild. I would suggest checking tappet clearance, make sure your valves are not leaking. You're lacking in power probably quite a bit.

Cam chain on these bikes is another thing to check BEFORE checking the timing, make sure your tensioner is working, take it off the engine, take it apart clean it, put it back in with the wheel compressed all the way back. When you loosen the set bolt it should snap against the chain with a nice thunk. DO make sure the engine is in the right part of rotation before removing it though.

Then set the timing. Better yet, get a Pamco.
 
Maybe swap the filters. one may have more oil in it than the other.

Have you synced them?

Are you using a strobe light for the timing?
 
frogman said:
FWIW, 164 psi is rebuild territory on a 450 engine. Should be around 185 for a well running broken in engine possibly 200 for a new rebuild. I would suggest checking tappet clearance, make sure your valves are not leaking. You're lacking in power probably quite a bit.

Cam chain on these bikes is another thing to check BEFORE checking the timing, make sure your tensioner is working, take it off the engine, take it apart clean it, put it back in with the wheel compressed all the way back. When you loosen the set bolt it should snap against the chain with a nice thunk. DO make sure the engine is in the right part of rotation before removing it though.

Then set the timing. Better yet, get a Pamco.
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Pamco is on my son's list of things to do. It is his bike, I just jump in to help. Valves were set and cam chain adjusted by a motorcycle shop familiar with these engines. The engine runs smoothly and quietly. Not a knock or tick in it.

Did tighten up the exhaust collars and that actually solved much of the problem.
 
frogman said:
FWIW, 164 psi is rebuild territory on a 450 engine. Should be around 185 for a well running broken in engine possibly 200 for a new rebuild. I would suggest checking tappet clearance, make sure your valves are not leaking. You're lacking in power probably quite a bit.

Cam chain on these bikes is another thing to check BEFORE checking the timing, make sure your tensioner is working, take it off the engine, take it apart clean it, put it back in with the wheel compressed all the way back. When you loosen the set bolt it should snap against the chain with a nice thunk. DO make sure the engine is in the right part of rotation before removing it though.

Then set the timing. Better yet, get a Pamco.
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You know what is funny. I have read many forum posts on a variety of forums, spoke with owners and mechanics and have found few 450 DOHC engines with a few miles on them coming anywhere near 185, or 180 or even 175 PSI. Most i have run across are in the 160 to 170 PSI range. This engine has 10,250 miles on it.

FWIW, this engine has sat for nearly a decade before late last summer. Compression has risen from the mid 150s when we got it to the 160s these days (after valve adjustments, running it a bit, etc.) I bet if I checked it to day, it would come in around 170 or so.

The engine runs very well. It pulls hard. No smoke, no leaks, etc. I am not pulling it apart while it is running this well. The engine has two problems. It doesn't like to be below 3,000 RPM and has a small miss between 7500 and 8000 at part throttle. That is either a Mikuni tuning or timing issue.
 
trek97 said:
Maybe swap the filters. one may have more oil in it than the other.

Have you synced them?

Are you using a strobe light for the timing?
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Carbs are synched. I re-synched them today. Filters look to to be oiled the same. I uses a test light and timed to the marks. I can use by strobe timing like, but am waiting for the Pamco conversion as the points plate screws are not in great shape. I set the points gap and timed to the marks and got as close as I could. Seemed to be about right. Starts and idles smoothly. It just does not like to be below 3,000 rpm. Slip the clutch until three and crack the throttle, she will haul a$$.
 
Sprayed Seafoam into the intake and coated the valves and let it soak for an hour. Ran until warm and my compression is about 5 psi higher. Think I had gunk on the valves as the engine was sitting for ten years. Sprayed Seafoam into cylinders and letting soak overnight to see if it will unstick any stuck rings.

This is my son's first bike. As long as it runs well, I don't care if it is down a few horses. He doesn't need to go ripping around. I personally had it to 75 mph and it did it effortlessly. I am thinking of rechecking the valves. Had them done by a shop, but who knows.
 
Get him to learn how to do the valves, its something that will have to happen every time you change the oil and its pretty easy to do for the most part. Common among old 450 guys is that the measurement in the manual, I forget what it is .0012" ? is a little hard to get to and is actually too tight. .0015" to .002" is more attainable and allows for a little more oil in between the follower and the camshaft. Might make them last longer.

I made the mistake on mine of installing a 17 tooth front sprocket. Couldn't figure out why it wouldn't hold 65 in 5th gear but run 85-90 no issues. Ordered up a 16 tooth now can comfortable cruise 70-75 all day long and still have enough power to run up to 80-85 and pass someone if needed.
 
He wants to wait until after the riding season. It is his bike. I was thinking it might be too tight. It certainly does not sound like the valves are too loose. It has the quietest valve train of my three four-stroke bikes. The right cylinder is around 170 PSI warm and the left is in the mid-170s, so not too bad. The more we clean and ride it, the better it gets. However, I will pass along your advice about not having the valve clearance too tight so as to let more oil get in there.

If it were my bike, the engine would be out and at a machine shop for a proper rebuild/build-up. ;)
 
170's are much better depending on what compression tester you are using you may even be higher. Loosen those rings up make sure those valves are set right and run it. I love my 450, it just goes and goes and goes. I can always trust to get on it and go pretty much whenever, summer winter hot or cold. I've wanted one since I was kid and am glad to finally own a complete one. Still have another one in pieces to put together.

Major issue is wearing out the cam followers, they are practically unobtainable anymore. They can be re-welded and reground but at pretty high cost. 450's are known to chew threw them if not taken care of, or having the oil changed at correct intervals or sooner. To use the saying, "Should sound like mad sewing machine" is about right for what it should sound like.

I don't like to add ideas but the next 450 I hope to build will look like this.

photo038.jpg
 
Speaking of Hansen, I think they have cam followers, etc. Probably not cheap.

Interestingly, my 450 does not sound at all like a sewing machine. Other than the exhaust it is very quiet. Maybe too quiet. My 79 CB400T makes more noise. The CB450 has just over 10,000 original miles. The CB400T just over 11K.

I know what you mean about being able to jump on bad go. My son likes to kick-start the bike because he thinks it is cool (after I sprung for a new battery last year) and it will often start on the first kick, even after sitting for a few days.
 
Not sure if this matters, but my carbs are not original. The original carbs were FUBAR so I went with Mikuni VM 32s. I did the compression test with the engine warm and the throttles wide open and air filters removed.
 
try going up one size on the pilot jets. BTW, where are the air screws set?

Old bikes with carbs are never good if the throttle is opened too fast, but that low speed miss and higher speed, low throttle miss suggest fueling is lean probably off idle. That suggests either the needle jet a a size too small or the slide cutaway is too large.

Sometimes making the pilot jet slightly richer that "perfect" is enough to mask that. Longer term the solution is probably richer needle jets or different cutaway, but fatter pilot jets may be enough for now.
 
Screws out one full turn. Only happens on the right side. There was a another carb problem. When I had the shop. The mechanic doing some electrical work acquired a substance problem. He fuel through it and then let the bike sit for more than a year. I thoroughly cleaned the carbs, but the pilot jets are stuck. They are clear, but I cannot unscrew them.
 
Rode the bike today after letting Seafoam soak in the right cylinder. I did not re-check the compression, but the engine was noticeably smoother and more powerful. At first I got a miss around 7500 RPM, but not as badly as before. I rode a while and took it up that high and had no miss!. I did notice that when the RPMs get to 8000 my tach needle goes nuts and drops to 6000. The engine keeps pulling, but the tach falls. Another project for the boy ;)
 
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