Honda-SOHC

SOHC.co.uk Forums => CB750 => Topic started by: onethumb on February 26, 2017, 04:31:13 PM

Title: Compression numbers.
Post by: onethumb on February 26, 2017, 04:31:13 PM: About to take the compression numbers and wanted to know what kind of figures are about right. I'll be using the kick start?
Thanks again. Steve

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Title: Re: Compression numbers.
Post by: onethumb on February 26, 2017, 04:58:33 PM: Here's what I got.
No 1. 60psi
No 2. 137
No 3. 148
No 4. 125.
I knew about no1 so that's not an issue. What about the other numbers?
Thanks. Steve

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Title: Re: Compression numbers.
Post by: steff750 on February 26, 2017, 05:11:16 PM: ;) I would be a lot happier if they all read between150 psi -170psi
just my take on it and yes no1 is an issue lol
Title: Re: Compression numbers.
Post by: royhall on February 26, 2017, 05:14:29 PM: Just a thought. You did do the tests with the throttle wide open?
Title: Re: Compression numbers.
Post by: Trigger on February 26, 2017, 07:08:13 PM: With the throttle wide open and some oil down the bores but, number one is not sealing.
Title: Re: Compression numbers.
Post by: onethumb on February 26, 2017, 07:40:28 PM: I did it with no carbs on and some 3 in 1 oil in the bores.

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Title: Re: Compression numbers.
Post by: onethumb on February 26, 2017, 07:41:44 PM: Engine is out now and on the deck ready for stripping tomorrow.

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Title: Re: Compression numbers.
Post by: onethumb on February 26, 2017, 09:22:25 PM: Just did them again with a bit of engine oil in this time. New numbers. After 3 kicks. After 4 kicks
No 1. 30. 50
No 2. 150. 175
No 3. 150. 175
No 4. 125. 147

How do these look, do I need to look at doing the rings?

Thanks. Steve.

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Title: Re: Compression numbers.
Post by: Trigger on February 26, 2017, 10:06:13 PM: If your in the top end, I would check everything ;)
Title: Re: Compression numbers.
Post by: onethumb on February 26, 2017, 10:48:33 PM: Rightio trigger.
I've just ordered a Moore and Wright micrometer and a clutch tool so the checkover can begin.
It has had a rebuild at some time in its life so I don't know if that's a good thing or bad.
What do you think of the numbers?

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Title: Re: Compression numbers.
Post by: MrDavo on February 27, 2017, 08:40:40 PM: Factory manual says 175psi on a new one.

Whatever happens you know that #1 is well and truly up the pictures, the cause should be obvious once you're in there. Four doesn't seem too clever, if it were only normal wear and tear you'd expect them to be similar if low.
Title: Re: Compression numbers.
Post by: onethumb on February 27, 2017, 09:31:02 PM: Update. Right took the head off and the good news is there is no damage on the top of the pistons and the bores look good. Soaked the top of the pistons with a carb cleaner and wd40 mix for 2hrs and lost none.
Stripped the head but left the valves and springs in placed tipped it over filled the chamber with wd40 and lost it all though the inlet valve in no1 in about 5 seconds, lost most of it out of no4 in about 1 minute though both valves. Now I know I've got issues with no1 and 4 but I think I might get all of them redone.

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Title: Re: Compression numbers.
Post by: Trigger on February 28, 2017, 08:12:01 AM: Number 1&4 will have pitted valve faces and seats or bent valves. If pitted only, this is due to the tappets being too tight.
Title: Re: Compression numbers.
Post by: onethumb on February 28, 2017, 12:18:38 PM: Thanks trigger and mrdavo for the input. I'll leave it to the engineers to give me more info on what to do with the reconditioning.

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Title: Re: Compression numbers.
Post by: onethumb on April 09, 2017, 12:21:47 PM: Well trigger you were right. So the head went to the shop and he found pitting on number 1 along with a bent valve and two of the exhaust guides were gone. So it's been machined, new guides a new valve and the seats and valves re cut. Looks amazing.
Now the issue is: I've built the engine back and have done a test before putting it back in the frame and the numbers are weird. So before I was getting 1-4 40-150-150-125 now I'm getting 1-4 110-110-115-125. No plugs no carbs and a squirt of thin oil in the bores.
I did notice that the new head gasket that was from DS sat a little funny when on but flattened with the head in place. Could this be the issue? I'm getting a bit stressed over this. Help please.
(https://uploads.tapatalk-cdn.com/20170409/a04f6af5014f35fd0594b0cd08aebb82.jpg)(https://uploads.tapatalk-cdn.com/20170409/5f0d92f05fd1d538e05beaf21839c9c1.jpg)(https://uploads.tapatalk-cdn.com/20170409/21d3fa2a764ca36f8ae823bf4fedacdf.jpg)(https://uploads.tapatalk-cdn.com/20170409/94f5db25301927381a4b202844c67b28.jpg)

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Title: Re: Compression numbers.
Post by: Trigger on April 09, 2017, 05:44:42 PM: Have you done anything with the bores, new rings, X hatched ?
Title: Re: Compression numbers.
Post by: onethumb on April 09, 2017, 07:06:42 PM: Hi trigger. No the bores were really good. But I've just took it all apart and you can see that the new gasket has not sealed around 3-2-1. Has any one else had issues with the gasket kit from DS. Any recommendations for a new head gasket? (https://uploads.tapatalk-cdn.com/20170409/a6dbc484404b6f80e26537868e307a6f.jpg)(https://uploads.tapatalk-cdn.com/20170409/206c24dea40ee6410d279f321f1e5ba2.jpg)

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Title: Re: Compression numbers.
Post by: hairygit on April 09, 2017, 07:28:05 PM: You haven't fitted it back to front have you? (Seen it done!)
Title: Re: Compression numbers.
Post by: onethumb on April 09, 2017, 07:30:28 PM: Nope. On the f2 you can't. Luckily for me.

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Title: Re: Compression numbers.
Post by: yozzer74 on April 10, 2017, 05:42:50 PM: The 4 notch pins that the rubber seals go round are they sitting lower than the gasket ?? Just a thought .
Title: Re: Compression numbers.
Post by: yozzer74 on April 10, 2017, 05:45:05 PM: And David silvers has got 2 genuine head gaskets in stock for the F2
Title: Re: Compression numbers.
Post by: onethumb on April 10, 2017, 06:17:53 PM: Quote from: yozzer74 on April 10, 2017, 05:42:50 PM
The 4 notch pins that the rubber seals go round are they sitting lower than the gasket ?? Just a thought .
Hi Yozzer, I had them seated level with the top of the pins then pushed in to the hole till they bottomed out. Does that sound right? Those rubbers are not the best design I've ever seen.

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Title: Re: Compression numbers.
Post by: onethumb on April 10, 2017, 06:19:00 PM: Quote from: yozzer74 on April 10, 2017, 05:45:05 PM
And David silvers has got 2 genuine head gaskets in stock for the F2
Has he. I'm on the net.
Thanks dude.

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Title: Re: Compression numbers.
Post by: onethumb on April 10, 2017, 07:16:27 PM: Yozzer have you got a part number for the gasket as I can't find them on the site? Thanks

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Title: Re: Compression numbers.
Post by: yozzer74 on April 10, 2017, 07:39:25 PM: 12251410000 Part number I ment are then pins flush with barreles and not raised above them
Title: Re: Compression numbers.
Post by: onethumb on April 10, 2017, 07:41:54 PM: Thanks.
The pins are flush the rubbers sit high though.

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Title: Re: Compression numbers.
Post by: yozzer74 on April 10, 2017, 07:48:22 PM: http://forums.sohc4.net/index.php/topic,107040.msg1197785.html#msg1197785 It just a thought as I had seen it on this thread before . Be interesting to see what lads on here think to this thread how to rebuild top end
Title: Re: Compression numbers.
Post by: onethumb on April 10, 2017, 08:43:17 PM: Thanks dude just read it. I'll be doing some checking tomorrow.
Has anyone else left the 8 rubber seals/washers out.

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Title: Re: Compression numbers.
Post by: hairygit on April 10, 2017, 09:00:53 PM: If you're talking about the little rubber sleeves that fit round the dowels, F2 engines should only use 4, seems like your gasket kit was generic aftermarket, as F1's and K7's use all 8.
Title: Re: Compression numbers.
Post by: onethumb on April 10, 2017, 09:20:09 PM: Quote from: hairygit on April 10, 2017, 09:00:53 PM
If you're talking about the little rubber sleeves that fit round the dowels, F2 engines should only use 4, seems like your gasket kit was generic aftermarket, as F1's and K7's use all 8.
Yep hairygit I did use 8. So it's just the 4 that go on the pins that I need then? When I stripped it the other build had used 8. Reading the link that Yozzer pinned I'm thinking of ditching all 4/8.
Any thoughts. .?

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Title: Re: Compression numbers.
Post by: hairygit on April 10, 2017, 09:23:43 PM: Never risked it, and don't think I would, too much hassle if it leaks oil once it's up and running, engine out again etc.....
Title: Re: Compression numbers.
Post by: onethumb on April 10, 2017, 09:28:13 PM: Yep I hear that. I don't want to take it out again. Although I've got a good system now and can drop a head in 15 minutes. Bet trigger can do it in 5!

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