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Messages - gp_st3

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1
CB750 / Re: CB750 K4 rebore
« on: July 24, 2020, 11:21:23 AM »
From personal experience I can recommend HT Howard in Slough...

2
CB750 / Re: CAN'T FIND NEUTRAL WHEN HOT
« on: March 17, 2020, 03:10:03 PM »
smooth steels - Lucas/TRW, made in Germany - bought on ebay.uk for about £40 (set of six) about a year ago ... why these work better for me is a mystery, so best try all other suggestions before shelling out for new plates that may not improve things for you.

3
CB750 / Re: CAN'T FIND NEUTRAL WHEN HOT
« on: March 17, 2020, 10:06:37 AM »
Difficulty finding neutral when stationary with a fully hot engine has been a problem for me most of the eight years I've had my K2. Basically, everything works perfectly when engine is cold / warming up but becomes next to impossible when the oil temp is above about 60C.
Type and age of oil makes a difference but the biggest improvement came after trying various options and finally changing to smooth rather than dimpled steels.
Current setup uses only five rather than seven square cut friction plates with stronger springs to reduce the tendency for slip when accelerating hard.
Finding neutral can still be a little hit and miss in hot weather but at least doesn't require so much effort it feels something might break!
We're not the only ones btw, there are several threads about this problem on the US forum with no real consensus on the cause(s)...

4
CB750 / Re: Too rich pilot?
« on: September 05, 2019, 02:45:01 AM »
p.s. just remembered also fitted new needle jets when I originally cleaned/refurbed the carbs as they were badly worn and causing plugs to foul.

5
CB750 / Re: Too rich pilot?
« on: September 05, 2019, 02:32:29 AM »
I also had this problem about a year ago (657A carbs with 110 mains / 40 pilot and needle 4th from top) ...sooty plugs, poor idle and stalling when puling away from standstill once the engine was hot (fine when cold).

Wasted a lot of time and effort adjusting pilot screw and needle positions; finally went for #38 jets and what a difference!! - all running great with good response from closed throttle and clean/tan plugs.

Incidentally, now need full choke for starting regardless of ambient temp, but switch off completely soon after the engine has fired up and maintain the revs using the throttle until it warms up to steady tickover.

I buy genuine Keihin Jets at reasonable prices from allensperformance.co.uk

6
Project Board / Re: CB750 Barn find project - First Timer
« on: January 01, 2019, 05:02:50 PM »
Got them back on the bike and synced with a carbtune pro which was pretty easy but then noticed I had a significant leak around the front sprocket cover. Whipped the sprocket off and seen this



The drive seal wasn’t sitting flat as it should and was leaking at the bottom where it protruded out slightly. Unsure whether it wasn’t lined up correctly when I closed the cases or not but it’s not leaked for the first 50 miles until now. I even put a smear of hondabond on it as recommended in hondamans book so perhaps this held back the oil until now.

Rightly or wrongly I carefully tapped it back in flush and it doesn’t seem to leak any more. Am unable to ride as the seats with the upholsterer but fingers crossed it’s fine

Well, from looking at the picture I would say that either your drive seal is fitted backwards (inside/out) or mine is... I have no leak but perhaps I've just been lucky!

7
CB750 / Re: Bearing Shells Question
« on: September 03, 2018, 07:45:01 AM »
I'm using the the Red 'UK' Plastigauge (0.025mm to 0.175mm) as unable to locate a uk source for the the Green 'US' Plastigage (0.25 to 0.76) but have now ordered some White PL-X (0.018 to 0.045) so will see what results that gives.

The rod shells' surfaces are all pretty smooth, although one does have a narrow polished ring while the rest have an even matt colour - for the main bearing shells it's a different story with those having a distinctly rough 'sand paper' feel on the forward face between 2 and 5 o'clock so to speak...

Given the oil staining on the inside of the cases I would not be confident that this engine has been very well looked after in the past but if the clearance measures with the new plastigauge are the same then near-zero wear would seem to be the only explanation.

8
CB750 / Bearing Shells Question
« on: September 01, 2018, 01:40:51 PM »
I have my CB750K4 engine apart, am going through the process of checking the bearing shells and could do with some advice.

First, I Plastigauged the four existing rod bearings and got between 0.05mm and 0.055mm clearance

Could not find any evidence of shell colour so working on the crank numbers (‘3333’) and rod numbers (all ‘1’), concluded that the standard shell would be Yellow

At this point, I also measured the crank pins with a micrometer and while finding it hard to get consistent readings due my lack of skill, these came out at around 35.99mm which matches the ‘3’s scratched on the crank

So, reading in the Mark Paris book that a reasonable limit for worn bearings is around 0.056mm (although Honda states 0.08mm as the max.) and with my assumption that wear is mainly on the shell rather than the crank pin I ordered a single pair of Yellow shells from DSS (Honda OEM) to check things out…

Now the confusing part - with the new Yellow shells I get exactly the same values as before i.e. 0.05mm, which is not what I expected!

It appears that the old shells are effectively not worn at all, which is odd given that I’ve done 10k miles on the bike and there is no evidence in the paperwork (or the state of the inside) that the shells were previously replaced. As an aside, the clocked mileage is not relevant as by the numbers it’s ’74 engine in a ’72 chassis.

The follow-on conclusion is that any wear has been on the crank pins and (presumably) the original oil clearance was at the top end of of the standard 0.02mm to 0.046mm in the Honda manual.

The question now is what to do now - leave things as they are or replace with one step thicker shells?

Your thoughts please...

9
CB750 / Re: 61mm to 65mm Over-bore cost estimate
« on: August 21, 2018, 12:03:48 PM »
Understood - on balance I'm going to accept the risk and should the worst happen then either find a replacement block or perhaps new liner(s) if feasible.
I will be using flat-top pistons (only small increase in compression) and the standard K4 cam so hopefully all will be well...

10
CB750 / Re: 61mm to 65mm Over-bore cost estimate
« on: August 21, 2018, 09:29:09 AM »
Thanks for the quick replies and guidance.
Trigger, you mention 'prone to cracking', is that something you've experienced as I've searched the SOHC forums and not some across any references so would like to quantify the risk if possible?

11
CB750 / 61mm to 65mm Over-bore cost estimate
« on: August 20, 2018, 07:17:32 PM »
Hi, would be interested to hear from the forum what is considered a 'fair' price for a 61mm (standard) to 65mm (836) over-bore in the London/SE area?
thanks.

12
CB750 / Re: Boyer Ignition cb750
« on: March 24, 2018, 10:34:42 AM »
Thanks for the first post welcome Phil, finally ’broken my duck’!

A bit more about the Boyer - in addition to not following the Honda recommendation for the full advance point by some margin, @1100 RPM tick-over the ignition fires about 2 to 3 degrees ahead of the ‘F’ static timing point, which is also a surprise.

Could be that Boyer’s engineers have optimised/improved on Honda’s probably conservative spec (itself constrained by the relatively crude mechanical advance) or perhaps the basic module and timing profile is sold by Boyer for a number of motorcycle models including the CB750 and is therefore a compromise to suit a range of specifications...

It would be good to hear what you think when you have road tested because although happy with the performance, I have only ridden one CB750 so have no basis for comparison!

13
CB750 / Re: Boyer Ignition cb750
« on: March 23, 2018, 07:09:44 PM »
I fitted the same kit about a year ago to my K4 engine and had the same problem - in the end I did away with the washer and that meant all except the last turn of the thread was covered.  Having checked that the cover to rotor clearance was about 2mm, I gave the nut a decent hand torque with some loctite and told myself at the time that I would go back and notch out the rotor to accommodate the dogs and increase the clearance 'later' but as with many such things, not got round to that yet!

It's actually a rather poor design IMO but the rotor is such a tight fit on the shaft (and has so little mass) that it'll never move.

The other thing I never did understand is why the instructions say to "time to the Full Advance Timing marks ... with the engine running at a steady 4500 RPM" when according to everything else I have read, full advance should be there by 3000 RPM (and the Honda Shop manual says 'above 2,500 RPM)

The thing is, if you set the Boyer to give full advance by 3000 RPM then at higher revs, the advance goes too far so it has to be done according to the instructions and to be fair, the performance is excellent so will remain a puzzle.

G.

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