Head work

[jrsunt]

My well used and abused 2006 bike is ready for full engine rebuild. Con rod, piston, cylinder, cam chain and head work. 
The con rod is straight from a CRF 250 and will have to be aftermarket as a genuine Honda part is only available as a full crank set ££££

The piston, cylinder etc are fairly straight forward, but I’ve never delved in to a cylinder head. I’m presuming valves will be need to be replaced and the seats re cutting and profiling which I will be getting someone else to do as I don’t have the tools to do them, whilst the head is off and stripped I thought I’d get it gas flowed. Anyone have any thoughts or experience with this?

After reading the service manual, the valve stem and valve guides need measuring together and against new valves (which I have) to determine whether new valve guides are needed.
These valve guides DON’T exist in the parts list or drawings. Any thoughts on this?

The cylinder head, cam, valves and everything in between have not changed from the original 2005 250 engine right through to the current 301. I was surprised by that too, and again the valve guides don’t exist in any parts diagrams. 
Again, has anyone had any experience on cylinder head work on bikes in general and any thoughts on the allusive valve guides?

Thanks

 

[konrad]

I have a little experience with this.  As you know, replacing the guides is not typically a DIY job.  After installation, the ID needs to be reamed to size and the seats re-cut.

The US company Kibblewhite manufactures aftermarket valve guides (and other head components as well).  See: https://www.kpmi.us/

The potential problem I see with gas-flowing the ports is that the technique is generally used to improve WOT flow.  Unless the person doing the work has experience specifically with trials, the results may actually make the performance worse.

[jrsunt]

After a bit of time on the google, I found Ben Coyle Racing for the head and crank jobs. He was familiar with the 4rt and knowledge about trials. After a quick phone call, gas flowing was ruled out as the process moves power around. What you gain in one area you lose in another, but a head clean up and polish would help things. The valve guides wouldn’t be a problem either as they custom make them for race engines. 
I’ve nearly all the parts gathered up, just the con rod and big end to decide on. The dogs dangles of rods is the CP Carrillo at around £280, but they don’t supply the crank pin, bearing or washers. So another manufacturer needs to be considered for those. In my mind, it appears to be pointless to put best rod in and then have to compromise on the pin and bearing. The next best option seems to be Wossner or Pro X for the complete kit at £120ish

 

[aussiechris]

I'm a bit late to the party but I thought I'd mention that I used a rod kit from HotRods for repairing my 2015 4RT 260 and it worked perfectly.  Their part number is 8616.

Original rod on the left...

Specs are on the lines for CRF250R or CRF250X...

Sources might be: https://www.allballsracinggroup.com/8616-connecting-rod-kit

Or in the UK: https://www.motocrosspartsuk.com/honda-crf-250r-x-2004-2017-hot-rods-connecting-rod

 

[jrsunt]

Firstly the cylinder head had a full overhaul, the exhaust valves were very lipped, intakes not so bad. The valve guides were still well within spec so they’ll do another 20 years. The small end of the con rod was showing wear, which was giving me a slight knock when flat out. The big end, piston and cylinder were surprisingly good with the piston rings taking most of the wear. The top ring had lost almost 1/3 of it originally circumference! 

The bike is finally going back together and it’s been put on a diet to shed a few pounds whilst everything has been apart. ( I’m not getting any younger, bike time is limited and a few kg’s saved will help a lot… in my head at least).

My next port of call is to tackle the fuel tank and get rid of the hefty steel fuel pump set up. I’m aiming on using the pump from the 260’s which is readily available from the NSF250R race bike, the injector mount to take the new pump will need modifying and a new hose will need to be made up. I’m pretty sure all the models use the same injector. I have a spare injector set up which can be a donor if needed. 
What I don’t know though is if my ECU will be able to communicate or can be made to communicate with the new pump. I have the Programmable HRC ECU from the early bikes. Has anyone done or tried this before? I can’t find any information anywhere.

[konrad]

40 minutes ago, jrsunt said:

What I don’t know though is if my ECU will be able to communicate or can be made to communicate with the new pump.

Are you thinking the ECU does something more than just turn the pump on and off?  I suppose the ECU could PWM the pump to vary the flow, but I have never heard of that being done.

[jrsunt]

Is it that simple? To be honest I don’t have a clue. Is it the ECU and injector which controls the demand and supply of fuel? Does the pump just pump at a constant rate regardless of throttle input?

[konrad]

Yes.  The fuel pump uses a mechanical pressure regulator and runs at a constant pressure.  The ECU opens the injector for the appropriate amount of time to deliver the required fuel.

I've written extensively about the OSSA EFI system here: https://www.ossa-efi.com/home  There are tons of details scattered across the website, but I've never described the basic operating principles of EFI.  Perhaps I should?

 

 

[jrsunt]

Thanks Konrad, a gold mine of information. Maybe this job isn’t as bad as I initially thought….

[konrad]

When you replace the OE pump with something different, make sure it's wired such that the pump pushes fuel from the tank into the injector (instead of the other way around). 🙂

The new pump's injection pressure must the same as the OE pump.  Officially, the pressure regulators are not adjustable, but it just uses a preloaded spring and may be possible to modify.

[sportsawyer]

The old style pump assembly (2005 - 2015) uses an external vacuum operated pressure regulator so there's a constant differential pressure on the injector. The new pump (2016+) provides constant absolute pressure to the injector inlet, and the ECU adjusts spray duration based on vacuum seen by the MAP sensor. Since the new system ECU is compensating for the varying differential pressure on the injector, I beleive you will need a 2016+ ECU. If you can find a Honda Common Service Manual (2012 or newer printing), look at the fuel injection chapter, pages 9-19 & 9-20 in the 2012 printing, where the difference between these two fuel delivery types (and ECU compensation for absolute pressure delivery) is explained.

You'd also need a new (2016+) tank because the newer big o-ring pump mounting is completely different. Not sure about the injector assembly, but the new one has the pressure regulator removed and capped. The vacuum port on the head is not longer needed and must be capped. And there's no fuel return line with the new setup. A new fuel pump, tank, and ECU will likely be pricey, and then there's all the small parts that will pile up. So it may cost less to try and find an old style Mitani / Future Trial / DDM or other lightweight base or fuel pump assembly if you wish to save weight there.

Edited 23 hours ago by sportsawyer
Removed repeated word.

[konrad]

I feel stupid now!  Thanks for setting the record straight.  The 2005 - 2015 system sounds like the type of pressure regulator I've seen on turbocharged motors.