Topic: Engine Blowby? - 1978 CB750F

[cowboycletus]

Hey Guys,

My 750 starts and idles like a champ. After a mile or so of conservative city riding it starts to sputter a little and bog down like engine braking, even with the throttle held open. It is firing on all 4. I previously had an issue where cylinders 2&3 were intermittent and found that to be a bad condenser. I had hoped the new condenser would fix this problem but it has not. I’m a complete amateur and learning as I go but my first thoughts were that this could be:

Engine Blowby
Something with the spark advancer
Something Exhaust related

Any/all advice is appreciated!

[ekpent]

 If its not smoking or blowing oil out the vents I would not worry about blowby per say but look for a tuning issue of some kind. Later model F's do get leaky valve guides sometimes though. Peek at the spark plugs is a good starter.

[cowboycletus]

If its not smking or blowing oil out the vents I would not worry about blowby per say but look for a tuning issue of some kind. Later model F's do get leaky valves sometimes though. Peek at the spark plugs is a good starter.

Weather permitting I’m going to set valve clearances tomorrow. I’ll be sure to take a look at the plugs then and post a photo. I did change the #4 spark plug today. When I pulled the old one it appeared to be very oil fouled.

[cowboycletus]

Photos of my spark plugs attached below. I did valve clearance adjustment today (.003in and .004in) and all the exhaust valves were very tight. One I couldn’t even get the feeler gauge in.

Spark Plug #1 is the oldest and has 300mi on it.
Spark Plug #2 and #3 were changed about 30mi ago diagnosing an ignition problem.
Spark Plug #4 is the newest and was changed yesterday. It has 3mi on it.

I also did a compression test today and there’s definitely a problem with #2. My measurements were:

#1 - 110
#2 - 90
#3 - 100
#4 - 110

Any help interpreting this information and advice on what my next steps are is greatly appreciated. Thanks Guys!

Edit:

Engine was cold and I was NOT holding the throttle open. What level of difference would have this made? Could I maybe (hopefully) get within 10% this way?

[Jerry Rxman Griffin aka MuthaF'er]

What's your sitting battery voltage? Have you checking your voltage through the rpm ranges?

[cowboycletus]

What's your sitting battery voltage? Have you checking your voltage through the rpm ranges?

I have. The battery with the key off is 13.4

It measures the same at 5k RPM

[bryanj]

Any blue smoke or oil consumption, F2 are notorious for wearing out exhaust valve guides

[cowboycletus]

Any blue smoke or oil consumption, F2 are notorious for wearing out exhaust valve guides

No blue smoke and I am uncertain how to otherwise diagnose oil consumption. I had a pretty nasty oil leak from the gear shift cover but I’ve replaced the gasket and it seems to be holding steady now.


I realized last night that I made a mistake and did my compression test with only the spark plug for the cylinder I was checking removed. I’m gonna let the engine warm up today, remove all spark plugs, and then test again with the throttle open and see what difference that makes.

[HondaMan]

Chances are 90% it is another bad condensor. For the last several years we are being subjected to Daiichi condensors, which are usually bad either out-of-the-box or within 100 miles. I went through 5 of them before I bought an entire points plate assembly DIRECTLY from Honda (South Sound Honda) to get some good TEC units. Any of the individual condensors you buy right now ARE Daiichi, and are also junk. The TEC points plate completely fixes the problem.

Some clues:
1. If there is NO marking on the condensor, it is Daiichi.
2. If there is a little 3-leaf clover-like marking, it is Daiichi.
3. If it is poorly stamped "Daiichi" it is from someon who is actually knocking off [fake] Daiich parts.
--if any of the above, it is trash, or needs to be.
4. If it has a "TEC" stamped on the baseplate, partly obscured under the condensor itself because it was stamped before the baseplate was spot-welded to the condensor, it is TEC. TEC is Tokyo Electric Corporation, a longtime Honda supplier.

[cowboycletus]

Chances are 90% it is another bad condensor. For the last several years we are being subjected to Daiichi condensors, which are usually bad either out-of-the-box or within 100 miles. I went through 5 of them before I bought an entire points plate assembly DIRECTLY from Honda (South Sound Honda) to get some good TEC units. Any of the individual condensors you buy right now ARE Daiichi, and are also junk. The TEC points plate completely fixes the problem.

Some clues:
1. If there is NO marking on the condensor, it is Daiichi.
2. If there is a little 3-leaf clover-like marking, it is Daiichi.
3. If it is poorly stamped "Daiichi" it is from someon who is actually knocking off [fake] Daiich parts.
--if any of the above, it is trash, or needs to be.
4. If it has a "TEC" stamped on the baseplate, partly obscured under the condensor itself because it was stamped before the baseplate was spot-welded to the condensor, it is TEC. TEC is Tokyo Electric Corporation, a longtime Honda supplier.

Thanks for that! I tried to order a points assembly plate from South Sound but the order never shipped. When I called to inquire about it they said that points plate had been discontinued and they didn’t have any in stock. I’ll call them again Tuesday and try again. Worst case scenario I end up with some good condensers.

[cowboycletus]

So I did another compression test today with the engine warm, throttle held open, and ALL spark plugs removed while testing.

#1 - 140
#2 - 100
#3 - 140
#4 - 140

I’m also attaching a video for reference. There was an odd clicking sound while the bike was warming up. I assume this is connected to the valve adjustment I did yesterday. It’s definitely (I say with extreme caution) coming from the left side of the engine.

[Jerry Rxman Griffin aka MuthaF'er]

Those numbers look better EXCEPT #2. Lower compression on both tests. Double check your valve adjustments on that cylinder. If the adjustments are good then perhaps it's time for a leak down test.

[cowboycletus]

Those numbers look better EXCEPT #2. Lower compression on both tests. Double check your valve adjustments on that cylinder. If the adjustments are good then perhaps it's time for a leak down test.

Will do. I’m letting the engine cool now and then I’ll re-install the spark plugs and double check the valves on #2. Being that I am a literal shade tree (but without the tree) mechanic I’m hoping this is something that can be fixed without pulling the engine. I’ll likely go to a shop at that point.

[cowboycletus]

Update. So I just rode 8 miles with no performance problems. A small tick developed from the right side of the engine about halfway into the ride and I’m assuming that it’s another valve clearance issue. This is the first time I’ve adjusted valves and now that I have better feel for it I’ll redo them all tomorrow.

As for the poor throttle/engine boggling issue I was having...that’s completely disappeared and I believe I know why. The small hose that goes from the top of the air box to the #2 carb (I assume for the accelerator pump?) was disconnected. The union between the hose from the air box and the hose from carb was actually broken but there was enough left on one end that I was able to reconnect it. I noticed this as I was putting my gas tank back on after adjusting the “ticky” #2 valve. That being said, is it a reasonable thought that this hose being disconnected could explain the low compression on #2? I certainly hope so. I’ll be doing another compression tomorrow and will report back!

Thanks guys.

[bryanj]

Not sure what that hose is but it wont affect the accelerator pump

[cowboycletus]

Not sure what that hose is but it wont affect the accelerator pump

So I did another compression test this morning and surprise surprise #2 is still low. Aside from that the bike runs great. Is casual/conservative riding safe for the engine in this condition?
I guess I’ll be investing in the tools to do a leak down test. What diagnostic information can I expect from that?

Also if anybody has any clue what that tiny hose from the top of the air box does I’d love to know!

Thanks guys for all the help so far!

Also should I change the subject of this thread to be about a compression issue? Not sure what’s considered proper etiquette here.

[seanbarney41]

If it runs great, ride on!

[Jerry Rxman Griffin aka MuthaF'er]

Also if anybody has any clue what that tiny hose from the top of the air box does I’d love to know!

Basically a re-burn system of fumes

[jukku]

Ventilation from nr.2 carb float chamber to aircleaner box?

[HondaMan]

Also if anybody has any clue what that tiny hose from the top of the air box does I’d love to know!

Basically a re-burn system of fumes

Vacuum source for the under-battery PCV contraption.
If it is still there? If not, plug the hoses on the end. It is the #2 carb on most of the F2/3 bikes. This will let that one's exhaust valve get a little more carbon collected on it and lower compression temporarily, but it will probably surprise you after 100-200 miles of riding (after it is sorted out) by re-seating the valve's face and cleaning it off auto-magically. These engines do that.

[cowboycletus]

Also if anybody has any clue what that tiny hose from the top of the air box does I’d love to know!

Basically a re-burn system of fumes

Vacuum source for the under-battery PCV contraption.
If it is still there? If not, plug the hoses on the end. It is the #2 carb on most of the F2/3 bikes. This will let that one's exhaust valve get a little more carbon collected on it and lower compression temporarily, but it will probably surprise you after 100-200 miles of riding (after it is sorted out) by re-seating the valve's face and cleaning it off auto-magically. These engines do that.

Nice! That certainly restores some optimism.

There is a PCV looking device under the battery. Does  this need routine maintenance of some kind? I know draining the crankcase breather is a part of regular checks/tuneup but the hose on my bike (right side by the rear tire) doesn’t have a cap. Should it?