CB378 high compression *HELP*

MrMister

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Good day everyone.

It's been about a year since my 378 swap, and since I did, the compression has been high. I'm about ready to pull the engine again and get even thicker gaskets from Lani to get it perfect.
All i can think is that the head must have been shaved without my knowing it at some point and that didn't equate into my gasket thickness calculations. I'll get an exact compression number by the weekend.

I know that I can look it up, but spending 2 hours sifting through forums to get different numbers is never fun. My question is...
What's the general gasket/compression rule of thumb for thickness equals compression? (plus "?" thickness = minus "?" psi)
 

irk miller

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Compression and compression ratio are not the same thing. The compression ratio is a measure of volume of the cylinder and combustion chamber at the bottom of its stroke and the volume of the cylinder and combustion chamber at the top of its stroke. Cranking speed, altitude, temperature, wear on camshaft lobes, and the duration profile of camshafts factor in to your compression readings.
 

teazer

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Irk is right., but to add to that,
How high?
What is the cranking pressure?
Why do you think it's too high?

The math is simple enough to work through, but before we spend time working out a solution, let's define the problem in objective terms - if there is one.
 

MrMister

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Irk is right., but to add to that,
How high?
What is the cranking pressure?
Why do you think it's too high?

The math is simple enough to work through, but before we spend time working out a solution, let's define the problem in objective terms - if there is one.
So. It cranks very heavily, Kick starting is heavy (Think Harley) and the starter motor wants to die if I even think of using it. Adjusting the alternator to TDC is almost impossible because it builds up too much pressure in the chamber for the piston to stay that high.

Unfortunately I only have an amazon special pressure gauge (kind of like harbor freight), so I can't rely on the numbers.
Firstly, Yes, engine hot. Yes throttle open. Yes choke open.

Both read with the crappy needle bouncing around 100-120. But again... I definitely don't trust this thing.
 

trek97

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If you got the factory carbs installed "Throttle open" won't fully raise the slides while kicking or cranking, as they are only vacuum operated.
You will need to pull the carbs and maybe exhaust to read more accurately.

Also 100-120 w a vacuum gauge is pretty low. Both Rachels CB400 Hondamatic and my CL100 are 180.
 
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teazer

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120 is not very high. Sounds like something is binding or seized inside. Could be the camshaft run without oil for example.

Pull both plugs and see how hard it is to spin over.
 

MrMister

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120 is not very high. Sounds like something is binding or seized inside. Could be the camshaft run without oil for example.

Pull both plugs and see how hard it is to spin over.
Definitely compression related. With the plugs out, it cranks normally. I ordered a proper compression tester. But it will only arrive next Friday/Saturday. We'll see.
 

teazer

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Try popping the carbs off and crank it or kick it until the compression pressure stabilizes - probably 5-6 kicks.
 

MrMister

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Try popping the carbs off and crank it or kick it until the compression pressure stabilizes - probably 5-6 kicks.
It’s been like this for about 60 miles (since the rebuild). It runs. It just has that issue. Does this sound like a compression or compression ratio issue?


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teazer

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Possibly but that's unusual. That's why we need to know what the real cranking pressure is with an accurate gauge. I tried a few cheap ebay compression testers and they were all inaccurate. I went back to a good Motion Pro gauge which reads consistently.

120psi is quite low and it should be easy peasy to kick over at 120p. That may be an inaccurate gauge or done with carbs not fully open (best to remove CV carbs) and crank it over 5-6 times to see the pressure build after each kick. Eventually it will stop building and that's when you read it.

Just as an aside, are you sure that the cam is correctly timed in relation to the crank and that ignition timing is close enough to spot on?
 

MrMister

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Possibly but that's unusual. That's why we need to know what the real cranking pressure is with an accurate gauge. I tried a few cheap ebay compression testers and they were all inaccurate. I went back to a good Motion Pro gauge which reads consistently.

120psi is quite low and it should be easy peasy to kick over at 120p. That may be an inaccurate gauge or done with carbs not fully open (best to remove CV carbs) and crank it over 5-6 times to see the pressure build after each kick. Eventually it will stop building and that's when you read it.

Just as an aside, are you sure that the cam is correctly timed in relation to the crank and that ignition timing is close enough to spot on?
Yeah I’ll have to wait for the compression tester.
Cam position is good. I checked timing again this morning (Pamco EI) Both plugs fire on the nose at 1k rpm. But giving gas 2,700+ it doesn’t quite make it in between the advanced marks. It settles on the first mark.


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teazer

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Should be close enough. Let's see what a good compression tester reveals - when it arrives.
 

irk miller

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Have you ruled out battery and/or connections to the starter? Has the battery gone through any deep discharges? I just want to rule out that the plates haven't built up sulfur or lost lead, which will reduce cranking amps. It can still reach proper recharge when this happens, just without the amps.
 

MrMister

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Should be close enough. Let's see what a good compression tester reveals - when it arrives.
Christmas came early (the new tester arrived). I just did a cold test and both sides read 195psi.


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MrMister

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Have you ruled out battery and/or connections to the starter? Has the battery gone through any deep discharges? I just want to rule out that the plates haven't built up sulfur or lost lead, which will reduce cranking amps. It can still reach proper recharge when this happens, just without the amps.
Hi Irk.
The build is fairly new. I have a 8 cell lithium bat.
I’m less bothered by the starter motor screaming like a mad pig than I am about the kick being heavy and feeling the compression being tough while setting tdc on the stator. I’ll ride around the block tomorrow then do the compression test again (hot), and see what the changes are.


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Maritime

Over 10,000 Posts
cold 195 is pushing high, hot may show higher again and be the reason. My GL1000 is 170 cold and I've broken the stater bolt off turning it over without removing the plugs and so I can understand how 195 would feel with plugs in trying to kick or turn over.
 

trek97

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This is interesting, as it seems a thicker gasket would help...
but then you are also adding more total volume of a/f getting squished at the same time???
Not to mention I have everything needed to bolt up the 378 mod sitting on the shelf...collecting dust.
 

irk miller

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Thicker gasket will lower compression, in the same way that carbon buildup on the top of the piston can (technically) raise it. My OG post was meant to clarify that there are several factors that needed to be checked to narrow down the culprit. It's also a pretty general number, but there's typically a 10% difference in compression from hot to cold.
 

Sonreir

Oregon
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So long as you're not pinging, high compression would be a good thing. Harder to kick over, sure, but a stronger engine is something for which people usually pay extra. :p
 

teazer

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Sonrier: Agreed. Higher CR makes for better street performance as long as the fuel octane level is high enough.

At least you have a real number now to confirm that the compression is quite high

Let's assume you would like to reduce cranking pressure from 195 (nominally 13.2:1) to say 180psi or 12.2:1, that would require an increase in clearance volume from 15.5cc to 16.8cc which is an increase of 1.3cc and with a 70mm bore that amounts to around 14thou (0.014") spacer or a gasket .013-.015" thicker than you have now.

Somebody should check those calculation though.
 

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