Author Topic: CR750 Set Up  (Read 937 times)

Offline philward

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CR750 Set Up
« on: September 23, 2018, 06:44:45 PM »
As you are probably aware (from previous posts) I’ve got the CR up and running on the road and am trying to fine tune the bike – just need some opinions from the more technically astute members.
To recap, this is the set up
836 standard compression kit, moonpie 4 – 4 race exhaust with KO rear baffles fitted, Ramair foam pod filters, Boyer Micro system inc coils (new rubber head to carb rubbers by the way)
Set up so far. Started with standard carb settings and Boyer set up statically - as didn’t want to set up with strobe at 4500 as per Boyer instructions on initial tight engine. It run as per the following Youtube - https://www.youtube.com/watch?v=YmL8YYWeUMo (rev counter reading double by the ways – Smiths still trying to match odd Boyer signal to rev counter)
As can be heard, sounds ok (but if you run to the end of the video, it cuts out as I blip the throttle at the end) but when on road, short of fuel further up revs and when kickstarting, kicks back and spits back through carbs occasionally when kicking. Starts on 2nd/3rd kick).
After a few gentle miles, changed main jets to 135 and changed needle to second to bottom clip notch (2nd richest). Tried to time to full advance as per Boyer instructions (used a spare standard rev counter) but strobe shows tickover timing ok but doesn’t get near full advance (see Boyer response below). Tried air screw at 1 turn and 1 ¾ (either side of standard?)
Boyer Tech Dept Response:- ‘Thank you for your enquiry, when strobing try reversing the clamp on the strobe gun, also don't use a strobe that has a timing adjustment on it. We think the strobe is flashing on the coil energising and not switch off, this is why it runs ok but shows not to be on full advance’ – Haven’t tried this yet.
Result:- Now revs clearly through mid range (obviously not revved further up scale yet) – cured fuelling issue. On half hour run at mid revs range, plugs around about grey/sand colour. Still kicks back and spits back through carbs occasionally when kicking. Ticks over fine though.
So, what’s your view on the kick back/spitting back issue?
Thanks in advance
Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Honda CB350K0 (1969)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Going on eBay ASAP)

Offline SteveD CB500K0

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Re: CR750 Set Up
« Reply #1 on: September 23, 2018, 07:14:01 PM »
moonpie 4 – 4 race exhaust

We have a member on here called lordmoonpie (Simon Tappin). I wonder if its the same guy? He hasn't been onlne for a couple of years...
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Offline philward

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Re: CR750 Set Up
« Reply #2 on: September 23, 2018, 07:20:25 PM »
I bought the exhausts and a few other bits off Simon Tappin (Moonpie) Steve. I've spoke to him previously but his experience is with the race CR carbs
Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Honda CB350K0 (1969)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Going on eBay ASAP)

Offline K2-K6

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Re: CR750 Set Up
« Reply #3 on: September 23, 2018, 07:30:52 PM »
I thought the Boyer system fires all of the plugs, all of the time.  Confirmed,  I think, by your tacho receiving twice the signals normally associated with a four cylinder engine.

If so,  as an example when no1 is at tdc and firing,  then one of the centre cylinders is at bdc on intake stroke,  but also firing. At normal dynamics when tickover speed is reached it won't be seen as an effect,  but at low revolutions it could be that which is spitting back from a single cylinder.

Just a thought,  also on that basis you've effectively got the pulse rate of a V8 for each rev so if you have a tach with that option to configure,  it could read correctly when set that way.

Offline philward

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Re: CR750 Set Up
« Reply #4 on: September 23, 2018, 08:48:55 PM »
I've also been in touch with Boyer (and copied Smiths in) - here's their reply (which I think supports your view K2 - K6)

'The tacho should be calibrated for two pulses per rev and capable of working with a current pulse ignition system, with this the current pulse is varied by the micro processor depending on rpm. The peek voltage spike is around 200 volts on the primary of the ignition coil.
The tacho needs to trigger on this and ignore everything after it.
If you get stuck we do a tacho recalibration service, as long as we can get at the electronics.
For cost and inquiries contact us at sales@boyerbransden.com'

Smiths have been aware of the Boyer firing sequence but have had a go at the tacho twice (great service really) - Smiths have been given the above and are sending another re-programmed tacho this week to try - Fingers crossed!

Whats your thoughts on the kick back/spit back? (Timing retarded?)
Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Honda CB350K0 (1969)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Going on eBay ASAP)

Offline hairygit

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Re: CR750 Set Up
« Reply #5 on: September 23, 2018, 09:17:10 PM »
If you're certain the carb rubbers are good, and tight, check by spraying carb cleaner on the joints of the suspect cylinder, any change in revs will show a leak. After that, possibly a weak or broken valve spring or dodgy sealing of an inlet valve seat.

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Offline K2-K6

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Re: CR750 Set Up
« Reply #6 on: September 23, 2018, 09:29:17 PM »
Sounds like you've got a reasonable base setting with the jetting so shouldn't be giving you anything out of the ordinary.

Spitting back through carbs has to have some pre-conditons.  Inlet valve has to be open to allow the combustion to go back out that route, this is usually achieved by ignition being too advanced but you've got yours ok at tickover so it shouldn't be a concern.

But as far as I can see,  that firing of all cylinders will give this condition.  If you look at no1 cylinder coming up to compression it will fire something like 10 degrees before top with both valves closed,  giving the power stroke.  At exactly the same point, no3 will be on its intake stroke with piston 10degree before bottom and just the inlet valve open.  This should fire that mixture straight back out the intake IF it ignites, giving you spitting.

What Boyer won't disclose is the strategy around this,  is there a lock out for all plugs firing at the same time? If so is it based on a rev floor limit?  Or could it be set on time basis ie if you switch ignition on will it give a time base of just firing plugs conventionally (say 30 secs) after which it expects to see the engine above a certain rpm (perhaps 700 rpm) and return to multiple firing?
These are some of the things you'd find in even fairly old ecu controlled systems, so perfectly possible.

You could try turning on and immediately starting it to see if it spits at you,  or pose the question to them

Offline philward

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Re: CR750 Set Up
« Reply #7 on: September 23, 2018, 09:37:22 PM »
Hairy - I think that the kick back/spit back is on multi cylinders and quite pronounced - not just a little 'puff' (kick back through the kickstart is quite severe sometimes
Also, forgot to mention, when hot it pops through exhaust on de-acceleration at low revs (below 3k ish)
Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Honda CB350K0 (1969)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Going on eBay ASAP)

Offline philward

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Re: CR750 Set Up
« Reply #8 on: September 23, 2018, 09:58:02 PM »
Sounds like you've got a reasonable base setting with the jetting so shouldn't be giving you anything out of the ordinary.

Spitting back through carbs has to have some pre-conditons.  Inlet valve has to be open to allow the combustion to go back out that route, this is usually achieved by ignition being too advanced but you've got yours ok at tickover so it shouldn't be a concern.

But as far as I can see,  that firing of all cylinders will give this condition.  If you look at no1 cylinder coming up to compression it will fire something like 10 degrees before top with both valves closed,  giving the power stroke.  At exactly the same point, no3 will be on its intake stroke with piston 10degree before bottom and just the inlet valve open.  This should fire that mixture straight back out the intake IF it ignites, giving you spitting.

What Boyer won't disclose is the strategy around this,  is there a lock out for all plugs firing at the same time? If so is it based on a rev floor limit?  Or could it be set on time basis ie if you switch ignition on will it give a time base of just firing plugs conventionally (say 30 secs) after which it expects to see the engine above a certain rpm (perhaps 700 rpm) and return to multiple firing?
These are some of the things you'd find in even fairly old ecu controlled systems, so perfectly possible.

You could try turning on and immediately starting it to see if it spits at you,  or pose the question to them

That makes sense K2 - K6.  I bought the Boyer system based on having done trouble free miles over about 40k miles in the 70s - TBH, I wish I'd gone for another make. I'll put the question to them
Thanks



Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Honda CB350K0 (1969)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Going on eBay ASAP)

Offline philward

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Re: CR750 Set Up
« Reply #9 on: September 23, 2018, 10:23:57 PM »
Sounds like you've got a reasonable base setting with the jetting so shouldn't be giving you anything out of the ordinary.

Spitting back through carbs has to have some pre-conditons.  Inlet valve has to be open to allow the combustion to go back out that route, this is usually achieved by ignition being too advanced but you've got yours ok at tickover so it shouldn't be a concern.

But as far as I can see,  that firing of all cylinders will give this condition.  If you look at no1 cylinder coming up to compression it will fire something like 10 degrees before top with both valves closed,  giving the power stroke.  At exactly the same point, no3 will be on its intake stroke with piston 10degree before bottom and just the inlet valve open.  This should fire that mixture straight back out the intake IF it ignites, giving you spitting.

Have emailed Boyer based on your point - thanks again K2 - K6
What Boyer won't disclose is the strategy around this,  is there a lock out for all plugs firing at the same time? If so is it based on a rev floor limit?  Or could it be set on time basis ie if you switch ignition on will it give a time base of just firing plugs conventionally (say 30 secs) after which it expects to see the engine above a certain rpm (perhaps 700 rpm) and return to multiple firing?
These are some of the things you'd find in even fairly old ecu controlled systems, so perfectly possible.

You could try turning on and immediately starting it to see if it spits at you,  or pose the question to them
Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Honda CB350K0 (1969)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Going on eBay ASAP)

Offline UK Pete

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Re: CR750 Set Up
« Reply #10 on: September 24, 2018, 08:09:42 AM »
I say revert back to points , well set up points system is going to be trouble free

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Offline K2-K6

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Re: CR750 Set Up
« Reply #11 on: September 24, 2018, 03:05:49 PM »
I was thinking along the same line's as Pete as you'd see if the kickback problem went away.

But been reading more about their system and realised that it's not clear where the initial timing is set (against that which I thought in above post)  as all the timing mark hardware is replaced.
It became clear ( he says with some confidence  :) )  what they're doing in asking you to set the timing with a check at 4,500rpm is to give the hard set point that is safe for the engine and above which the logic won't pass.
The advance curve fitting in under this is more fluid and exists as a national set of parameters within their logic. They state it can advance the timing even at less that tickover speed to give an anti stall effect and try to stabilise the tickover.
I'd interpret that as having a target for rpm as eg 950rpm and will adjust timing to aim for that figure.  If you advance it, then it will speed up.  If you retard it,  then it will slow down.

If it reads slow rpm as you attempt to start it then it maybe trying to advance it to stop it stalling,  even though it hasn't started, if that makes sense.

But it does also point to the initial setup as being already too far advanced,  so it's probably worth retarding your initial setting by a couple of degrees to try it and see if it improves.  Then a couple more if it seems better in a suck it and see situation to find improvement.

Offline K2-K6

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Re: CR750 Set Up
« Reply #12 on: September 24, 2018, 03:16:00 PM »
Forgot to add,  you need to get the use of a strobe that will verify that total advance position to ultimately set it.

Aside from the tacho issue it's probably worth running it,  just a bit of a pain to get there perhaps.

Offline philward

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Re: CR750 Set Up
« Reply #13 on: September 24, 2018, 10:13:03 PM »
Lots of info to take in there! thanks for all your research.
Ref the timing marks - the system retains the advance & retard bob weight carrier (with cam, springs & weights removed) so all timing marks are retained. I'm leaning towards the timing as you suggest.
Got a response from Boyer:-'Thank you for your enquiry, you should not get any kicking back, popping out of the carburetors is normally week mixture or having the throttle to wide when starting.  Check the magnetic rotor is not touching the solder pips on the stator plate.
Check the spark plug gaps are around 0.8mm, 0.032 inch'. Regards Tech Dept.
I have my plugs at 0.025inch so that's one thing I can change.
Reference air screws - I've read so much about setting them which contradicts quite often - air screw in for more or less air? 
Current Bikes:-
Honda CB750K2 (1975)
Honda CB500K2 (12/1972)
Honda CR750 Replica (1972)
Honda CB350K0 (1969)
Kawasaki ZZR1100D3 (1995)
Kawasaki ZZR250 (1990) Project (Going on eBay ASAP)

Offline K2-K6

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Re: CR750 Set Up
« Reply #14 on: September 24, 2018, 10:53:07 PM »
Turning the airscrew in will change the idle mixture to richer.  It restricts the air bleed to the idle circuit so a given vacuum in the carb venturi will draw more fuel as you close the air passage down.

Likewise screw out will open the air passage and the same vacuum cannot then draw up the same amount of fuel from the float chambers as it'll just pull more air past the screw so making the idle mixture leaner.

If you warm it then, during tickover, adjust one air screw,  you should here that cylinder go faster or slower. If the airscrew has to travel more than half turn from initial setting,  then it would indicate that you need an idle jet size change. Too far out it wants a smaller jet, too far in it wants a bigger jet.

 

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