Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: RGP750 on June 20, 2017, 09:40:45 PM
-
Hi All
KO 180 miles since rebuild, first 120 miles running great.
Last week went for a ride and stopped running on number 4, plug wet so thinking plug.
Tonight swapped number 4 plug with number 1, initially ran well but after 40 miles number 4 off again.
I got home and checked exhaust temperatures. Number 2 was 20deg the hottert but i seem to remember
this can be due to heat from the front disc???.
number 3 was next then 1, and 4 way behind, but still hot enough 300mm from head to make me think it is still firing
although not fully.
any ideas welcome please.
oh new coils and leads.
-
Check compression and then valve clearance on number 4 to make sure nothing obviously untoward.
-
Thought i would try that next, just don't want to know compression down after all of the re build.
I'll let you know the outcome when i have done it.
cheers mate.
-
Yes you'd hope not after just putting it together, sounded as sweet as anything when you ran it at Ardingly.
Something else I do that is quick and no cost, run it in as dark lighting conditions as you can find and have a look at the HT lead to see if there are any tracking leaks on no 4. I've had things like that run me ragged before.
-
You've tried swapping the plug, next time unscrew and swap the plug cap from the lead with another cylinder and see if the problem moves with it.
Pre rebuild I kept fouling number 2 plug, a resistance test on all 4 plug caps with a multimeter showed a massive resistance on number 2 compared to the other three, a new cap solved the problem.
-
I'll give that a go as well thanks.
The coils were new from US ,the brass terminal screws in the cap broke on 2 of the caps
when i screwed them on the Ht leads ,think i'll scrap the caps and get NGK >:(
Checked valve clearance and all spot on.
Compression test when it gets here.
Cheers Rich
-
I always insist on NGK caps and plugs from an official NGK distributor - and always check impedance of the caps before fitting. There's a lot of fake NGK parts in circulation...
-
Cheers James , how do i check if the new ones are impotent?? :D :D
Seriously how do i check?
-
Get a multimeter, set it to the 20 kOhm setting and meter across the ht caps individually (removed from the HT lead). Should read 5 kOhms.
Then meter the ht leads for each pair of coils with the caps removed (stick the probe into the end of the ht leads ). So you'll be measuring 1&4 as a pair and 2&3 as a pair. Should read 15 kOhms.
Ring me if you get stuck or want to talk it through.
J
-
Thanks James.
Mick is lending me some caps tonight until my new ones turn up.
Rich
-
Check my Coil refurb writeup Rich.
if you find anything amiss I will refurb them for you for free .. least i can do ...you do plenty for me and others on here.
http://info.sohc.co.uk/download/general/Ignition-coil-fix.pdf
https://www.youtube.com/watch?v=qxWtR_m80Fc&feature=youtu.be
-
Thanks Ash
I'll give that a go as well.
I haven't forgotten your rim etc just over run at work and not stripped it yet .
Not forgotten the evening out with the other forum member yet either.
Son getting married July 1st and hope to sort it after that.
R
-
Results so far
150 psi on all 4,so happy with that.
Nicked the caps off of my 500 NGK all 5 ohms
checked the ones on the 750 the one i'm having trouble is 15 ohms number 1 is 7 ohms and 2 and 3 are 5.ohms
coils are 17 1 and 4 and 2 and3 is also17ohms.
3 plugs seem rich, there seems to be conflict ion on forums re weather the screw should screw in or out to richen/weaken
mixture your findings welcome please.
Hope for a ride after work tomorrow :)
-
Rich, looks like the mechanical stuff is ok. I always keep a close look at valve clearance on newly built engines just in case they have closed at all so that you can catch it with no harm done.
Honda give a routine for adjusting the slow speed circuit in their manuals, this is in their F2 book so for PD carbs.
Warm the engine on a 10 minute ride after setting idle screws one and a half out from closed, set tickover stop to give 1000 rpm +/- 100 then on no2 carb adjust the idle screw in or out to give highest rpm then turn it in until you get a 100rpm drop. Reset the main tickover stop to bring overall rpm back to 1000rpm again and then do each of the other cylinders the same method. This should set the idle mixtures correctly if everything is functioning as it should.
Is yours four cables straight into the carbs? Not sure how you set overall tickover on that type if so.
-
Morning K2-K6
Valve clearances were all as they should be .
Mick (Kettle) set the carbs up for me he is a bit of a wizzo with the guages.
Each carb is set individually to achieve tick over.
I'm seeing him tomorrow at a local jumble and have a word with him.
Cheers
-
Thinking about this last night and to pick up on those resistance readings.
From my view it looks like with that 17ohm reading on 4 and because that coil is at the same time firing cylinder 1 it could result in the following effect; I know when a plug is fired under pressure ( compression stroke) it has an effective increase in resistance up until the point where ionisation occurs ( the start of the spark jumping across the plug gap) so if when no4 is required to fire and the wasted spark accompanying it in no1 is under no cylinder pressure, then the combined difference in resistance would lead to most of the coil energy effectively exiting from the combined coil down to no1 plug. This would lead to a very compromised ignition of the no4 cylinder, and to the effects you have experienced. Perhaps Ash, with his electronics hat on, may offer a more technical view of this.
The question about the idle adjust screw (raised on here a fee weeks ago with reference to a thread on the USA site) I believe is as follows; the idle air passage when the throttle slides are closed will be under vacuum, this will pull fuel up through the idle jet into main air stream. The air screw if backed out will allow that vacuum to pull more air from front side of carb and LESS fuel up from the float chambers. If you wind the screw in it will restrict the air bleeding into the passage from carb front side, causing the vacuum to assert greater pull via the fuel jet, so INCREASING fuel delivery.
If you get out of range with the air screw it would indicate you have a idle jet too small or large to work with the setup you have (not the case in your bike) but for those with modifications perhaps.
I think this subject causes debates because people think of throttle slide which as the needle jet moves with the air slide, then as you increase air it also increases fuel at the same time. The idle circuit is a bit arse about face in that you have a fixed vacuum from the engine tickover drawing through the idle passage which uses the air screw as a proportioning valve to bias that pull toward or away from the fuel jet.if that makes sense.
After all that going round in my head I had to go and think of Francoise Hardy (poster girl of CB750) to be able to get some sleep. ;)
-
Primaries should be around 4.5 Ohms Secondaries 15k (15,000) Ohms for original Tec coils. You must use a decent multimeter on low Ohms range for primaries though. Main thing is to connect meter to secondaries and wiggle the HT wire to make sure inner conductor is not fractured. On most of my 10 resto's I will be replacing the HT leads, as in my write-up. US bikes (like yours) are the worst for suffering from HT insulation hardening and internal conductor core fatigue. Primaries are very reliable and not usually the cause of any problems.
-
Thanks you two.
Results look good in the bench, I have all 4 running now after fitting new NGK plug caps.
Ash the coils are new ones i got from the US as my originals were hardened as you say and i assume shot ,at least the ht leads.
as no OHMS reading across the leads.
I have backed out the mixture screws 1/4 turn. The US site is where the screw in or out issue arose.
Off for a ride to a bike show in 10 mins so report back when i get back this afternoon.
cheers rich
-
Great ride today and a bonus of a 1st at the show ;D
Bike running fine now and i have learnt about resistance in plug caps '
Thanks all for your input
Rich
-
Well done Rich. You will need to make a new Trophy cabinet soon 😊😊 x
-
+1
-
It was great to speak with you today. The bike deserved that 1st, it is stunning!
Sent from my iPhone using Tapatalk
-
Thanks Al
Nice to meet you as well
Good to put another face to the forum
-
Another K0 steels the show ;D ;D
Me jealous na not me :P
-
Well never entered a bike show before this year,
3 shows two first and a second Happy days ;D ;D
Not entered anymore this year, just going to put some miles on it.
-
That's what I like to here give her a good thrashing
-
+1
Must be nice to own a bike that is so good it wins prizes, but even better to own a bike that gets ridden regularly, regardless of its condition.
-
It's too nice to ride just to leave it in the Garage. I'm not a pot hunter but nice to have some
others appreciating the work i put in.
The second i got at the last show i was beaten by a massively over blinged US 750 that the owner doesn't ride
Wheeled out of a van and got first and wheeled it back in, it may not have even run for all anyone know.
Happy riding ;)
-
I have always said that the winning bikes should have to be fired up before the prizes are given out. Seen many show winners that dont get used. One Triumph T100 I spotted at a show had a fuel pipe that stopped just under the tank and wasn't connected to anything. Once you put petrol in them they start to rot you know.
Question for Ash. Checking the coils, is that just for Honda's or would it work for other makes. I have a similar problem on my GS1000.
-
Well done Rich ... so all of that pain was worth it in the end.
Roy: not sure but I will check.
The only non-Honda manual I have is for Kwak H1 500 and H2 750 and that's a CDI coil so not the same. I would have thought they would be similar though.
Man is that Kwak manual detailed ..much better than the Honda ones IMHO ..well in the electrical section anyway. it even gives you both DC resistance and inductance values for the coil. Marvelous for geeks like me !
It's here ;-
https://www.dropbox.com/s/2jhu87j38ejtd7p/h_manual.pdf?dl=0
EDIT
Roy someone here talking about GS1000 coil fix and 4.5 Ohm primary (same as CB350/400/500/550/750 'Tec' coils) .
https://kzrider.com/forum/4-electrical/7480-coil-resistance