Honda-SOHC

SOHC.co.uk Forums => CB750 => Topic started by: El__burro on February 26, 2016, 05:15:39 PM

Title: Carb synchronising issues
Post by: El__burro on February 26, 2016, 05:15:39 PM: Hello,
I've been trying to get the carbs balanced and after a while I could not get a better result than the one in the picture below.
What could I be doing wrong?
Does the carb 4 variance means something wrong with engine pressure or leak?

Thanks
el__burro

Sent from my GT-I9300 using Tapatalk
Title: Re: Carb synchronising issues
Post by: royhall on February 26, 2016, 07:02:03 PM: It means that cylinder four is drawing more air than the other 3. The carb slide need adjusting down to reduce it. Failure to be able to balance the carbs could be a sign of air leaks at the isolator rubbers. Try spraying carb cleaner on the rubbers one at a time, with the engine ticking over, and see if the engine speeds up at all. That shows the carb cleaner is being drawn into the engine and therefore you have a leak. How hard are the carb rubbers?
Title: Re: Carb synchronising issues
Post by: gtmdriver on February 26, 2016, 09:14:23 PM: Adjusting and balancing the carbs should be the last thing you do after all the other engine adjustments.

Are all the valve clearances OK?

Are the plugs gapped correctly and the ignition timing spot on.

It might be worth doing a compression check to see if all the cylinders are roughly the same. This test can pick up worn rings or burns valves.

If everything is fine then you could certainly be looking at an air leak on the intake side.
Title: Re: Carb synchronising issues
Post by: royhall on February 26, 2016, 10:07:40 PM: GTMdriver is right about carb balance being the last thing to do.When you do the compression check, do it with the throttle fully open. Ignition wont affect the flow through the carbs. If the cranks turning there should to be even air flow across the cylinders. If the compression is even, only the intake and valve clearances will affect this. I strongly suspect carb isolator rubber leaks.
Title: Carb synchronising issues
Post by: El__burro on February 26, 2016, 10:30:59 PM: Plugs are gapped correctly, carbs rubbers are quite hard but ok, timing is fine too.
i guess carb slide and valve clearance my next things to tackle.

ps, i have lost the vacuum screw and washer, any idea where to buy it cheap.
thanks a lot
el__burro
Title: Re: Carb synchronising issues
Post by: El__burro on February 27, 2016, 01:03:47 PM: Mmm.. tried to do a compression test but my equipment doesn't have the adaptor for the Honda cylinder. Can anyone tell me what size I need to look for and eventually where I can get one?
Thanks
El__burro

Sent from my GT-I9300 using Tapatalk
Title: Re: Carb synchronising issues
Post by: El__burro on February 28, 2016, 06:36:10 PM: I've been looking all over, in my front garden and online but this screw it's not easy to find.
DSS, wemoto and other suppliers have no stock.
Does anyone have any idea where I could get one (for under £5)?

Part number 16161590004
(http://images.tapatalk-cdn.com/16/02/28/fbfe0ef7bbf3e1e0f8beeb0d17d9cd60.jpg)
thanks
El__burro
Title: Re: Carb synchronising issues
Post by: Bryanj on February 28, 2016, 06:39:35 PM: I got some washers but not the screw, it should be a standard thread so try getting some allen bolts from a fastener supplier
Title: Re: Carb synchronising issues
Post by: mike the bike on February 28, 2016, 07:31:26 PM: It's a bog standard M5 x 0.8 screw

Search for 42401 in Screwfix
Title: Re: Carb synchronising issues
Post by: El__burro on February 28, 2016, 07:41:10 PM: Thanks to both of you.
iIl be looking in my spare screw box tomorrow.
Can anyone confirm for the spark plug adaptor (for the compression test adapter) is 12mm?

thanks
el__burro
Title: Re: Carb synchronising issues
Post by: mike the bike on February 28, 2016, 07:47:51 PM: Aye, 12mm. You can also get the rubber cone type
Title: Re: Carb synchronising issues
Post by: neat street on March 01, 2016, 09:56:51 PM: when doing the compression test, full open throttle to get proper reading. if one or more is out, put a small dose of oil into each pot and test again. if you get even readings this would be a piston/cylinder issue? if the same (one or more is out (by +- 10%) would point to valve issue?
Hope this helps
Title: Carb synchronising issues
Post by: El__burro on March 01, 2016, 11:02:45 PM: Thanks, i've been reading about the test. While i am waiting for the adapter to arrive I've cut 4 M5 bolts to replace the vacuum ones. (no washers)
what do you think? (http://images.tapatalk-cdn.com/16/03/01/0242cf9dc316452d8e677c5541de3d17.jpg)(http://images.tapatalk-cdn.com/16/03/01/1150ac9101ceafb0eaf3059dd4c04539.jpg)(http://images.tapatalk-cdn.com/16/03/01/f1c25df809991dbe20d2683397a9e64d.jpg)
Title: Re: Carb synchronising issues
Post by: K2-K6 on March 01, 2016, 11:25:13 PM: They look like theyll do the job you want but make sure you remove the burrs on the end of the threads first so they don't chew up the carbs when you put them in.

One way to help you do it is to put a nut onto the screws before you cut them, then when you wind it off the cut threads it helps to clean out the lead in to the threads.

Careful filing with a small taper on the end should get them fettled into good shape.
Title: Re: Carb synchronising issues
Post by: gtmdriver on March 02, 2016, 08:45:32 AM: Quote from: neat street on March 01, 2016, 09:56:51 PM
when doing the compression test, full open throttle to get proper reading. if one or more is out, put a small dose of oil into each pot and test again. if you get even readings this would be a piston/cylinder issue? if the same (one or more is out (by +- 10%) would point to valve issue?
Hope this helps

....and take ALL the plugs out before you start.

Keep the motor turning over with the throttle wide open till the reading stops rising. All the cylinders should be within 10% of each other.

If you can't get a 12mm adaptor you could, as someone has already mentioned, use a gauge with a tapered rubber adaptor. You just push these into the plug hole as hard as needed to get a seal.
Title: Re: Carb synchronising issues
Post by: Erling on March 02, 2016, 09:34:15 AM: For tapering bolt ends just press it into a rotating grinding stone at a slight downward angle rotated with the "screwdriver" kept in position through a hole in a piece of metal plate. Have done that a hundred times over the years I've been in the hobby world of RC Bikes. 20 25mm bolts costs the same as 6mm long ones. So just shorted to the needed length.
Erling.
Title: Carb synchronising issues
Post by: El__burro on March 02, 2016, 07:23:56 PM: Ok, now that's weird
i have removed all the spark plugs, connected the cable with the adaptor (figs 1 and 2) open the throttle, push the start button but... nothing, the dial on the gauge doesn't move from 0;
tried to kick start nothing, tried a different cylinder, different tester, nothing.
Tried with engine hot and cold, I've used two different sets of tester but unless the problem is with the adapter I have no idea what I am doing wrong. (http://images.tapatalk-cdn.com/16/03/02/bb89ee570a1004ee251c6d2343be26ae.jpg)(http://images.tapatalk-cdn.com/16/03/02/b707cd56eb090938fae43cec6ba8e84d.jpg).(http://images.tapatalk-cdn.com/16/03/02/e49d81ad4647d82121eadd8844f08333.jpg)

Thanks
el__burro
Title: Re: Carb synchronising issues
Post by: Gixxer-18 on March 02, 2016, 07:57:32 PM: Silly Question, have you closed the purge button on the gauge? If it has one...
HTH
Nigel.
Title: Re: Carb synchronising issues
Post by: Chris400F on March 02, 2016, 08:15:23 PM: Amazon reviewers definitely don't think much of the Am-Tech tester you have!
http://www.amazon.co.uk/AUTOMOTIVE-PETROL-ENGINE-COMPRESSION-TESTER/dp/B000XJM8XK
Title: Re: Carb synchronising issues
Post by: El__burro on March 02, 2016, 10:55:56 PM: Gixxer, I thought that question would come up, no, the button is not pressed.
Chris, I have 2 because the first one has an issue with the reset button, the second was used on another engine and gave a true reading.
Just to make sure it is still good, I will do a test on my BMW R80 tomorrow.
It would be a shame to have to buy a new one, the adapter alone costed me £10, a full set incl. 12mm adapter costs a few quid more.

Thanks
El__burro
Title: Re: Carb synchronising issues
Post by: royhall on March 03, 2016, 07:19:42 AM: Sounds to me like you have a serious leak issue on your gauge. Even a totally clapped out engine should be showing at the very least 60psi. And yours is running so should be way higher than that. See if somebody on here is close to you that could bring another gauge to help you out.
Title: Re: Carb synchronising issues
Post by: El__burro on March 03, 2016, 01:40:52 PM: I've used the tester on a BMWR80 (cold) and the gauge went up to 50/60.
Not sure if the reading of 50/60 is correct but at least it moves.
Tried again on the Cb750 but Zero.

Not sure what to try next.
Thanks
el__burro
Title: Re: Carb synchronising issues
Post by: Chris400F on March 03, 2016, 02:28:36 PM: A search via Google reveals a lot of people who have had problems with this piece of kit.
I came across one thread that suggested there was a problem with the Schrader valve in the end of the pipe from the gauge.
Don't know if it will help but the thread is here, with pictures of the problem part.
http://www.clubpolo.co.uk/forum/index.php?showtopic=245024
Failing that the only way forward has to be a different tester. As Roy has suggested maybe someone local to you could help.
Title: Re: Carb synchronising issues
Post by: El__burro on March 03, 2016, 04:02:20 PM: I'll try to source a new one. Any recommendations?
Thanks
el__burro
Title: Re: Carb synchronising issues
Post by: mike the bike on March 03, 2016, 05:27:45 PM: I had one like yours - bloody useless. I bought another one from eBay which works fine. Here's the link.

http://www.ebay.co.uk/itm/Automotive-Petrol-Engine-Compression-Tester-Test-Kit-Gauge-Car-Motorcycle-Tool-/401039480729?hash=item5d5fd0d799:g:QI4AAOSwNphWaI70

BTW, it's got adaptors for all the plug sizes, plus the conical rubber type, straight and bent.
Title: Re: Carb synchronising issues
Post by: gtmdriver on March 03, 2016, 06:20:22 PM: Fair enough. It may be an iffy gauge, but if it reads at all on the Beemer why isn't it reading on the Honda?

Have you tried just sticking your thumb over the plughole during cranking to see if there is any compression at all?
Title: Re: Carb synchronising issues
Post by: El__burro on March 03, 2016, 06:23:58 PM: I had the same thought, yes it has some pressure, i am trying a way to test the gauge, probably with an air compressor?
I am about to buy the tester that mikethebike suggested.
thanks
el__burro
Title: Carb synchronising issues
Post by: El__burro on March 03, 2016, 06:37:50 PM: I was looking at this video I shot the other day (out of frustration) and I have realised that the smoke from the exhaust is blueish.
After that I checked for oil leak and I've noticed that the tacho drive leaks from the screw is supposed to hold it in place but even worse I have oil just under exhaust one, can't tell if is the exhaust bearing or the gasket.
regards
el__burro
http://youtu.be/xfHjuVb1dyE
Title: Carb synchronising issues
Post by: El__burro on March 04, 2016, 12:44:14 PM: Have I found a way to test the compression gauge?
I took a compressor for tyre and attached it to the gauge.
At its best the numbers on both gauges matched. (sorry for the awful picture)
it was 75 PSI
Unfortunately i had to push the compressor pipe in the terminal without any extension otherwise it would not read anything.
Also the 2 compression tester gauges show that one is not working properly.
I have run another test with engine warm still ZERO.

el__burro
(http://images.tapatalk-cdn.com/16/03/04/1633eb23316524ad94b0db6dee5cd323.jpg)(http://images.tapatalk-cdn.com/16/03/04/31e5006452443209c39d42ecd348d6d8.jpg)
Title: Re: Carb synchronising issues
Post by: gtmdriver on March 04, 2016, 04:41:55 PM: So could it be the gauge extension that is causing the problem then?

I can't imagine that an engine could be so worn on every cylinder that it would have absolutely zero compression pressure.
Title: Re: Carb synchronising issues
Post by: mike the bike on March 04, 2016, 06:26:03 PM: The engine wouldn't fire without decent compression. When air is compressed it gets hot because the same amount of joules have to occupy a smaller space. More compression = more heat.
Title: Carb synchronising issues
Post by: El__burro on March 04, 2016, 08:49:57 PM: Thanks,
what about the smoke in the video, is it blue? does it mean a blown gasket?
Since the bike starts nicely, I guess the best option
is a brand new tester as suggested above by Mikethebike.
Thanks

el__burro
Title: Re: Carb synchronising issues
Post by: hairygit on March 04, 2016, 08:57:40 PM: Blue smoke is burning oil, most common causes being damaged/ worn bores/pistons/rings or valve stem seals/ guides worn :(
Title: Re: Carb synchronising issues
Post by: El__burro on March 05, 2016, 02:25:10 PM: thanks Hairygit, I guess that in any of the above cases the engine must be taken out of the frame?!
It's not good news!
Thanks
el__burro
Title: Re: Carb synchronising issues
Post by: hairygit on March 05, 2016, 03:09:24 PM: On a 750 I'm afraid so. But if you can get a decent compression tester to get a set of readings it will tell you which cylinder is causing the issue, or all of them if it's general wear. Don't despair though, they are easy enough to work on, the hardest part is getting the engine out and back in again if you're doing it alone. If you can get somebody half sensible to help lift it out and back in it's surprisingly easy!

Sent from my X5 using Tapatalk
Title: Re: Carb synchronising issues
Post by: MarkCR750 on March 05, 2016, 09:23:01 PM: And if it's not smoking badly during normal use don't do anything!, you were giving it plenty of revs on that video, cracking the throttle open etc , had it been stood for a while before the video?, anyway burning a bit of oil won't hurt anything, wait til it get worse then strip it down, my guess would be a bit of valve guide wear.
Title: Carb synchronising issues
Post by: El__burro on March 08, 2016, 01:19:30 PM: Good day,
not good news, I guess.
I've received and used the new compression tester,
tried a first test at cold
C1 and C2 = 8 bar (115 psi)
C3 and 4= 7.5 bar (105 psi)

then I started the engine and warmed it up for 5 minutes,
Could only measure C1 and C4 as they are the easiest.
Both measured 9 bar (125 psi)
I've read that it should be between 10-12 bar (150-175 psi).
Could this improve by running the bike for longer?

Any opinion?
I guess I need to start looking into new rings, a new full set of gasket and other issue that may come out.

Regards
El__burro
(http://images.tapatalk-cdn.com/16/03/08/23fc866a5a7f78d591e2babbd602df00.jpg)
Title: Re: Carb synchronising issues
Post by: Chris400F on March 08, 2016, 02:31:34 PM: Glad you now have a working compression tester.
Might be interesting to try it on your BMW and see what readings you get.
Title: Carb synchronising issues
Post by: El__burro on March 08, 2016, 08:08:32 PM: Thanks. I'll do that tomorrow.
Could it be anything to do with the exhaust copper gasket?
El__burro
Title: Re: Carb synchronising issues
Post by: mike the bike on March 08, 2016, 09:54:26 PM: That's better than the readings on my GS650; 35, 105, 100, 105. . Worn out with a capital F methinks.
Title: Re: Carb synchronising issues
Post by: El__burro on March 08, 2016, 09:57:42 PM: Quote from: mike the bike on March 08, 2016, 09:54:26 PM
That's better than the readings on my GS650; Worn out with a capital F methinks.
you reckon worn out bores?
Title: Re: Carb synchronising issues
Post by: El__burro on March 10, 2016, 10:16:32 AM: thanks, i am planning (once the weather gets better) to take the engine out and replace the gaskets and (eventually) the rings.
I've seen some piston rings set, my question is why some of them have 5 and some 3 rings?
Any recommendation?
(http://images.tapatalk-cdn.com/16/03/10/e78cc54cffa66cba858f46c34b82a00a.jpg)
Title: Re: Carb synchronising issues
Post by: Bryanj on March 11, 2016, 07:02:41 AM: Bad practice to buy ANY parts before stripping and examining carefully
Title: Carb synchronising issues
Post by: El__burro on March 11, 2016, 09:29:55 AM: Thanks Oddjob,
even in light of info like this out there, I will not buy any parts before realising that I really need them, yet, if that will be the case, i'd like to know the best option out there based on your experience.
For example I've bought some forks oil seals (Chinese stuff) and they are just rubbish.
I'm sure that DSS will be the most approved source on the web(?)
Regards
el__burro
Title: Re: Carb synchronising issues
Post by: El__burro on March 11, 2016, 01:40:25 PM: I guess, I have to convince myself to take on this engine job,
i have done another test, this time with a bit of oil into cylinder one and it came up much better.
Now, I just need to wait for a bit of spring to remove the engine and explore.

Thanks
el__burro(http://images.tapatalk-cdn.com/16/03/11/1dcc9519d5c50c9b41de2b5d0fe9dc7d.jpg)
Title: Re: Carb synchronising issues
Post by: MrDavo on March 11, 2016, 03:14:28 PM: Quote from: El__burro on March 11, 2016, 09:29:55 AM
I'm sure that DSS will be the most approved source on the web(?)

The point is that if you want stuff to be a guaranteed fit and do a perfect job, only put parts on your bike that come in a Honda bag, with the correct part number. This is what DSS sell, wherever possible.