Topic: cb400 4 engine compression

[kblackburn]

Hi Guy's,

just bought myself a 1977 4004.

It did run rather uneven at idle, making a rough rattle, the last owner, said that he had replaced the piston rings, since then it had smoked on start up, but also if left ticking over, so I bought it at the right price, incase I have to rebuild the engine.

so on further inspection today, I have found that the carbs were not fitted to the air box rubbers correctly, the air screws settings all over the place,

it will not run without choke, so my first thoughts are that the carbs need balancing, to start with, so I'll check the valve settings.


I have carried out a compression check on the outer cylinders (120 psi) I am unable to get my tester to number 2 and 3 so will have to get an adapter, or should I just pull the motor and rebore ?.

any thought please


regards

kev

[calj737]

Why rebore when the condition of the motor is unknown? If the PO installed the rings wrong, they will smoke. Or it could be the guides, or valve seals.

If its smoking terribly, then it should be using oil. Pull the spark plugs and inspect them, and perhaps unbolt the header and look at the exhaust ports for presence of oil.

[kblackburn]

calj737,

thanks for the reply, do you think that 120 psi is ok, if I get that across all cylinders, at least it would show even wear. I'll get a proper plug adapter
for my compression tester and do a full check.

cheers

[Deltarider]

I've never managed to do a compression test well on any CB Four. A mechanic tought me his simple trick. 1. Remove sparkplug 2. Press thumb over plughole 3. (Let helper) operate starter (full open throttle, choke open ofcourse). 4. Evaluate. When thumb is blown of the hole: compression is sufficient. When thumbs stays put over hole: compression is bad. Enjoy your riding!

[calj737]

calj737,

thanks for the reply, do you think that 120 psi is ok, if I get that across all cylinders, at least it would show even wear. I'll get a proper plug adapter
for my compression tester and do a full check.

cheers

Lots of variables to compression tests. Ultimately, you are looking for consistency between all 4 cylinders, say within 10%. The actual value is effected by: length of hose, throttle being wide open, strength of battery, etc. If a bike has sat a long while, it will usually be lower in compression, but may return to a healthier reading after several runnings.

A "thumb" test only tells you whether you have decent compression, but its not nearly accurate enough to tell you how much you have as a comparison. But compression tests are limited in their value anyway. If you have low compression, the question then is, why? A leak down test helps pinpoint causes better.

[Bodi]

The specific reading is not useful. If you get 20psi, well Ok that's probably bad... but the 102cc cylinder displacement is so small that the hose volume makes a big difference in reading for the 90%+ of gauges with their check valve at the gauge (it's way more convenient than having it at the adapter). For a car engine where the ratio of cylinder displacement to hose volume is much higher the gauge readings will be closer to reality - a typical 4L 6 has 667cc per hole.
You look for consistency between cylinders. So if you get 120-120-60-120, you can be pretty sure there's a problem with the 60 cylinder, but you can't say with any certainty that the 120 readings mean they are actually 120, the real pressure with plugs in will be higher. You just don't know how much higher.

[kblackburn]

Hi all,

carried out the compression test and I have 3 cylinders reading 120 psi and one at 110 psi, so I'm happy that there's no great difference, I will have a look at the ignition system, as number 1 cylinder is not fully firing, I have a wet spark plug, if I take the plug out and crank the engine a few times, swap plug with number 2 cylinder, it runs and fires on all 4, but as soon as I switch off and fire it up again, I have the slight mis on number 1,
I have seen comments on checking float level externally, how do you do that ?

keevin

[Bodi]

"I have seen comments on checking float level externally, how do you do that"
Easy on carbs that drain trough the nipple when you open the drain screw but the 400 carbs drain through the screw.
You need a longish screw with the same thread as the drain screws (not easy to find, btw - it's a rare-ish fine metric thread) then drill a hole lengthwise through it, attach a clear tube (ie aquarium airline) to the screw head, remove the stock drain screw, screw your new screw into a bowl with grease on the threads as a sealant, open petcock and hold the tube beside the carb. The fuel level in the tube is exactly the level inside the bowl.
This level will not be what you measured at the float although pretty close. Generally the float level should be around the bottom of the body flange where the bowl seats. I've found no advantage to this fuel level test beyond proving all four are matched... adjusting the float levels the "old" way works just fine.
Be careful with fuel spills. The atmosphere can become explosive in a closed room with not that large a spill. You acclimatize to gasoline smell and can't easily smell that it's getting stronger and stronger.

[kblackburn]

Bodi,

Thanks for the information, it might just give me an insite as the wether the fuel level is wat too high, on number one cylinder, I have thought about replacing the points ignition system with an electronic unit to make sure I have a reliable spark, the po said that he had replaced the points and condenser with new items but on inspection, I think not.

kevin

[Don R]

 The tiny carburetor jets plug easily. I got one reasonably priced because the PO thought the engine was toast. It was a plugged carb. If you have not cleaned the carbs do that and a normal 3,000 mile tune-up. A point file will dress the points, read up on checking the dwell I think it's a good check to the point gap. 

[paulbaker1954]

Hi all,

carried out the compression test and I have 3 cylinders reading 120 psi and one at 110 psi, so I'm happy that there's no great difference, I will have a look at the ignition system, as number 1 cylinder is not fully firing, I have a wet spark plug, if I take the plug out and crank the engine a few times, swap plug with number 2 cylinder, it runs and fires on all 4, but as soon as I switch off and fire it up again, I have the slight mis on number 1,
I have seen comments on checking float level externally, how do you do that ?

keevin

Regarding compression tests I posted this a while ago on the uk site

http://www.sohc.co.uk/index.php/topic,15915.0.html

[dave500]

I think 120 is low if the guage is any good,a long hose is ok so long as it has a valve at its spark plug end,this one has and a dump valve at the guage end.

[paulbaker1954]

I would concur that 120 is a bit low but not disastrous as reading depends on the quality of the gear you are using

If you have 120 across all pots I would leave well alone and see how the bike runs and if it uses oil etc

[bochnak]

Make sure you are not using the HF compression gauge. Otherwise those compression numbers seem pretty even, so I would recommend getting the carbs sorted out:

[TwoTired]

If you are seeking compression acuracy to address cylinder health, then use a leak down tester rather than a compression tester.

Cheers,